Particle Accelerators

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Particle accelerators are an incredibly useful piece of technology and a diverse set of artifacts produced for many purposes. While particle accelerators are often invoked as weapons of war in science fiction, and have captured the public attention as giant research systems the size of cities, they have an incredible widespread range of applications. There are also many, many ways to build a particle accelerator, depending on what effects you need to achieve and what circumstances you work under.

This article aims to be a general primer on particle accelerators, their physics and a selection of technologies and construction styles. In detail, all particle accelerators are high-tech engineering customized to their circumstances in many, many aspects.

Further pages covering application fields are in the work.

Accelerator physics

The simple explanation is that we use electric fields to accelerate charged particles. But from this basic principle of leveraging the electromagnetic forces springs an entire set of accelerator technologies with different performances, trade-offs and characteristics. The most basic particle accelerators are just a big high-voltage source and can still be found in old CRT TV’s and dentist x-ray machines around the world. Things rapidly get more complex from there, but first we need to lay out some terms so we can all have a common conversation.

The first thing to keep in mind with particle accelerators is that we are talking about charged beams here, even if we neutralize them later. So no neutrons, photons, or other innately neutral particles. Your choices range from the simple electron up to anti-uranium and more, but they all must be charged. The particle of choice determines just about everything else about the accelerator and any potential uses. Really heavy particles (like large ions) don’t accelerate easily and take a lot of energy to get relativistic enough for time dilation tricks to take into place. Really light particles (like electrons) accelerate super easily, but can have other radiation problems to worry about.

The second most important thing to know about a given accelerator is the particle energy. This is a measurement of the kinetic energy of any individual electron, ion, whatever in the beam and is measured in a unit called the electron-volt or eV for short. The lower bound for any worthwhile particle accelerator is in the MeV (or million eV) range and we typically want a lot more! Inside of the accelerator we tend to talk about the electric field doing the accelerating (measured in Volts/meter). For singly charged particles (like electrons) this gets really to figure out the energy because of how the eV is defined. We just take the electric field (also called the accelerating gradient) and multiply by the length of the machine! Boom, now you have particle energy! For more complex particles you have to do some scaling based on mass and charge state.

Also a quick trick to help you with deciding if something is relativistic or not is to divide the particle energy $E$ by the mass energy $M$ of the accelerated particle (0.511 MeV for electrons/positrons and about 1 GeV for a proton) and add one. The resulting number is called the Lorentz gamma value and is used all over in accelerator engineering.

$\gamma =1+{\frac {E}{M}}$

Once you get a value much larger than 1, you have relativistic particles. Once that number is in the hundreds you get an ultra-relativistic beam. Figure 1 helps show the relationship between Lorentz gamma and speed of the particle.

The third most important feature of a particle accelerator is the emittance which is just a fancy term for divergence of the beam and can be thought of similar to the wavelength of lasers. No beam is made perfect and they will naturally want to expand outwards. The smaller the emittance, the longer the beam will stay in a tight spot. Emittance can be altered with a wide variety of beam engineering techniques that take entire books to discuss, but one other cheating thing you can do is drive the beam energy higher. This naturally lowers the emittance by a factor of Lorentz gamma due to the cheating effect of special relativity! Thank god for Einstein!

Acceleration forces

Every accelerator tries to build an electric gradient appropriate for the charge of the particles it is accelerating. The greater the electric gradient, the bigger the amount of work that can be done, the more energy is imparted on a particle as it travels through the gradient, and thus the higher the acceleration. The simplest way to build up a gradient is simply two charged electrodes with a strong potential between them. Many other accelerator methods use electromagnetic fields, in the form of waves confined in electrically conductive structures. In this case we exploit that electromagnetic fields have both an electric and a magnetic component. The magnetic component cannot be used to accelerate particles (only bend their trajectory - which can be exploited for effects like deflection or focusing) but as the electromagnetic waves oscillate, so does an electric field. This naturally creates a charge difference across half of the phase of an electromagnetic wave - this electrical gradient can accelerate particles. Both of these technologies encounter problems with the physical materials asked to contain the electric or electromagnetic energy. Eventually the fields get so intense that solid materials get broken down and an arc is formed. As such, various advanced concepts use clouds of charged particles and plasmas, which are not subject to the material limitations of solid matter.

The strong and weak nuclear force as well as gravity (which are all of the forces we know of at the time of writing) are not suitable for accelerating particles. This is related to the power and range of these forces. The strong and weak nuclear force have extremely short ranges - even if we had a means to artificially shape them like with electromagnets, their range would be too short. Gravity has the range but is incredibly weak. To generate a measly 1 G of acceleration, 9.81 m/s², requires the mass-energy of the entire earth, some 5.97e24 kg of mass, to do the job! The acceleration involved in particle accelerators is much, much, much larger (on the order of 10^13 times larger or more).

Interestingly, one source of very high-energy particles are giant gravity wells. Black holes generate conditions around them that can eject particles with enormous energies. The jets of particles that shoot from the north and south poles of many such stellar entities are particle accelerators.

If you want to use a force other than electromagnetism to accelerate particles, it’ll have to be a fictional one. The forcefields of various sci-fi properties could be considered, since they can often apply forces to particles. If you propagate a wave of such forcefields down a tube, you could have something quite a lot like many particle accelerator systems!

The particle bunch

Essentially all high energy particle beams are not constant streams of particles, but a time sequence of discrete groups of particles, commonly referred to as “bunches”. These bunches can be described both in their distribution in lateral space to the traveling direction (x and y axis) and longitude (z axis) along the traveling direction, anchored on an origin point (0,0,0) that moves in time.

Bunches can have various profiles. Generally they are somewhat distributed in the lateral direction just because the charged particles have slightly different starting positions and lateral velocities (also called transverse emittance). In longitudinal space, differing acceleration experienced by individual particles as well as dynamics within the bunch scatter them out, with “noses” and “tails” that contain fewer particles. The end results is that most beam profiles follow a gaussian distribution, though there are exceptions.

This bunching behavior allows us to analyze other properties of particle beams very similar to lasers. There is a “pulse frequency” given in Hertz. Each bunch contains a certain energy, which can be large enough that we give it in Joules and not electronvolts. There is an average output energy over a given timespan, given in Watts. We have a beam peak power, given by how quickly a bunch arrives at a target and deposits its energy, which we give in Watts. Related to this we also have a related peak intensity depending on how much area/volume the beam is deposited into, with units of either W/m² or W/m³. If we have charged particles being accelerated, we get a beam voltage, the same way as in a wire. Beams can also have a temperature, the same way as any other group of particles with random motion from their energy, given in Kelvin or eV.

The systems that generate particle beams of course have efficiencies, with the wall plug efficiency (power inserted into the entire system versus power deposited in the particle beam) being the most interesting for first-order engineering. They also have an operating temperature.

Particles for acceleration

With the constraint that our particles must have an electric charge in order to be accelerated with anything but fantastic technology, we can discuss quite a large family of particles, from the conventional to the exotic. They differ in the applications they can be interesting for. Some can be similar, but no two particles behave entirely alike.

Electrons

The ubiquitous electron is the most common particle to be accelerated and the first one ever pushed up near the speed of light by humans. Due to its excellent charge to mass ratio (the best possible due to the extremely low mass and charge of 1e) electrons are very easy to get up to relativistic speeds. Electron accelerators for radiography and sterilization are often portable by a single person and not much larger than a laptop in size (although they can be much bigger for power output reasons). The ease in which electrons reach relativistic speeds means that electron RF accelerators can be designed with no speed changes assumed (since the electrons are already moving at effectively light speed) and thus tend to be highly efficient and have the highest acceleration gradients achievable. For similar reasons electrons are also able to be accelerated in plasma based accelerators, allowing for even higher acceleration gradients and smaller machines for a given energy. The major downside of electrons is that due to their large charge to mass ratio they will radiate energy like crazy when bent by a magnetic field. Thus ring shaped accelerators are of limited use for electrons since you have to make the ring extremely large to not just lose all of your energy as beams of x-rays coming out of the machine! This very trick is used to make high quality x-ray sources for industrial and research applications.

When electrons hit matter they tend to release their energy as photons in a process called bremsstrahlung, or braking radiation. These extremely high energy photons (roughly ~⅓ the peak energy of the electron on average) then proceed to make electron/positron pairs that then make photons that then make pairs that then make photons… This whole thing is a massive cascade of radiation that ionizes, heats and can even damage on a nuclear level! At high enough energies you will generate every particle known and might even make some new ones!

Proton

Protons are the next most common particle to be accelerated and are currently used by the absolute largest particle accelerator ever made (the Large Hadron Collider or LHC) as the primary particle of choice. Since they have ~2000X lower charge to mass ratio than electrons, protons are about ~2000X harder to get up to speed. Unlike electrons they are not promptly at light speed so we have to send protons through a series of various accelerators depending on the energy of choice. For example the protons in the LHC the protons go through 5 different particle accelerators plus the extremely low energy stage that first makes the beam of ions! This difficulty in reaching highly relativistic speeds has two other bad side effects. Firstly, if we want to transmit our protons through empty space we will probably need to neutralize them with a beam of electrons.

Note: When shooting extremely high current (kA), relatively low energy (not highly relativistic or in some cases relativistic at all) beams in the atmosphere the higher mass and positive charge of protons gives them many beam propagation advantages over electrons if the beams can be made. In this case no neutralization is required as the atmosphere itself becomes a plasma and neutralizes the beam.

Luckily this is not very hard and will add almost nothing to your comparatively large proton accelerator, but may increase your beam spread a little bit via some neutralization physics. This does mean your beam is technically plasma, which is cool! Second, we effectively cannot use those neat plasma based accelerators for protons since we would need nearly TeV class proton beams to inject into the plasma accelerator and by the time you reach those energies you have basically already done all of the work. Maybe in the future there will be ways around this, but as of right now we are limited! There is one major benefit to this poor charge to mass ratio though, and that is the lack of synchrotron radiation! Ring shaped accelerators are ideal for protons, and basically all ions.

When protons hit matter they can interact with all of the four fundamental forces, although only two of them (Strong Nuclear and Electromagnetism) tend to play big roles. Protons barrel on in causing huge ionization cascades and can also release bremsstrahlung photons at high enough energies. They are also ideal at knocking neutrons, protons, deuterons and more out of the target nuclei which just adds to the fun! Protons also come with a neat trick known as the Bragg peak, where they deposit a large portion of their energy right at the end of their trajectory through a target material. This is a very useful trick for medicine and means you can “tune” the beam to almost ignore large portions of material and primarily heat, ionize, and destroy the area of interest. You can imagine how useful this is for cancer therapy or more nefarious things!

Light ions

Light ions are a rather fuzzy term but here we will use the definition of any ion heavier than a proton but lighter cobalt. The fuzzy reasoning here is that you get on the other end of the nuclear binding curve (i.e. can’t get net energy from fusion) at iron, so cobalt will then no longer be “light” and has to be made in a different process. Feel free to pick your own cut off! Historically beams up to neon have been experimented with for nuclear fission, nuclear fusion and medical therapy applications. Light ions have even worse charge to mass ratios than protons (Z/A times worse) and typically you will find light ions have about ½ the charge to mass ratio of a proton on average, although the isotope 3He is a notable exception. This means they are even slower to accelerate, although often the exact same machines as protons can be used with minor tweaks. Light ions have similar requirements for neutralization as protons, although their heavier mass can help fight beam bloom for a similar beam speed (not energy since the increased mass means the particles are higher energy for a given speed). The decreased charge to mass ratio also means that synchrotron radiation is even lower so beams can be driven to higher speeds before radiating. Light ions have similar target interaction physics as protons, although the ion itself will break up at very low speeds and add to the radiation environment.

Heavy ions

Heavy ions are being defined here as cobalt up to whatever unstable element you can jam into your accelerator! Typically heavy ion beams are made of elements like lead (used in some runs on the LHC), gold (used in the Relativistic Heavy Ion Collider) and even californium (used in some runs on the Argonne Tandem Linac Accelerator System). These beams tend to be for pure nuclear physics research, but there has been a long standing interest in heavy ion beams for inertial confinement fusion research and as potential methods of propelling spacecraft. These extremely heavy ions will be even harder to accelerate to ultra high speeds than light ions and much slower for a given energy, but they do provide a lot of momentum to make up for that! They have the same neutralization requirements and same potential upsides for beam bloom as light ions, but will most likely be orders of magnitude slower reaching our targets than the lighter beams. Heavy ions can also be used in the same machines as protons with small modifications and generate almost no synchrotron radiation as a result of their much smaller charge to mass ratio. They also have similar interaction physics to protons and light ions, but will stop in much thinner layers of materials. While electron, proton and light ion beams tend to both mechanically destroy and irradiate targets through-out, heavy ion beams will act much more like lasers and burn in from the outside of the target on in. There will still be a spray of ionizing radiation, but far less of one than an equivalent energy electron, proton, or light ion beam. The much higher momentum per particle, lower radiation concerns and ease of deflection makes heavy ion beams very interesting for beamed propulsion purposes.

Macrons

Macrons are typically defined as macroscopic particles, such as extremely large molecules or even grains of sand, that are charged up and accelerated via similar technologies as traditional particle accelerators. There are many difficulties with getting macrons up to appreciable speeds due to the extremely low charge to mass ratio, engineering difficulties with getting consistent and good charging, phase matching with accelerating cavities and lack of effective focusing optics. There are so called “dust accelerators” in existence that utilize the first stage of ion accelerators to generate their “beams” and proposals have been made to create better versions using updated technology. Macrons will interact just like a very small and very fast bullet unless they are designed to carry a nuclear payload to boost this output. Antimatter is an assured option for getting energy gain from a macron impacting a target, although there are potentially options for fusion, fission and fission/fusion given the correct sizes (mm scale or bigger), clever target design and the right impact speeds (most likely >>100 km/sec). The various energy gains versus speeds can be seen in Figure X.

It can be seen that antimatter proves to be worth it up near the speed of light, but both fission and fusion stop providing much gain well below the relativistic regime. This also helps explain why heavy ion uranium beams are not going to provide some incredible advantage over any other ion.

Antimatter

Antimatter is the evil cousin of normal matter and whenever the two meet a release of energy equivalent to their combined mass energy follows. Now this does sound like an amazing option for beams but there are two downsides. First, antimatter is energetically expensive to make! Positrons (antimatter electrons) are the “cheapest” at 1.02 MeV of input energy at a theoretical best, but they only give you that much energy back when hitting a target. Since a typical positron beam can be expected to be equivalent to an electron beam, there is little gain in adding 1.02 MeV per particle to a multiple GeV (or even TeV) beam. You can see this explicitly in Figure X below.

Note: Positrons can potentially fill an interesting niche either as a co-propagating neutralizing beam for electrons or perhaps as a higher performance atmospheric beam than electrons due to the positive charge. In this case the annihilation properties are no what is of interest, rather the positive charge and relative ease of acceleration compared to ions. Since positrons share all properties with electrons except for charge, they can be accelerated just as easily and even co-accelerated with electrons in the same machine.

For anti-protons the math becomes more favorable, but creating these particles is much more difficult and energy consuming. At best anti-protons require ~2 GeV of input energy to create, but in practice >400 GeV per anti-proton has been needed. Luckily significant energy gain is had up to several GeV of beam energy, so if anti-protons can be made at a central facility of some kind and easily stored (neither being easy things) then there may be some advantage to such a beam. Heavier ions are significantly harder to make since fusion with antimatter must be performed to generate those ions. Barring a naturally occurring source being found, antimatter heavier than a proton is most likely not worth using.

Muons

Muons are an interesting cousin of electrons and positrons. They are unstable and heavy cousins of electrons, massing about 200X more than electrons and living about 2 microseconds in the lab frame before decaying into an electron (or positron) and a neutrino. If we accelerate them up to relativistic speeds (about 10X easier than protons and the muons are typically born relativistically thanks to how we make them) then the muons can be made to “live” far longer. Muons have long been of interest to accelerator physicists due to their position as an almost ideal intermediary between electrons and protons. The requirement to make them on site has always been an impediment though and muons are currently only used for certain niche experiments. During the planning stages for a potential muon collider it was also noted that a muon collider would also have a serious issue with neutrino radiation! This is due to how neutrinos are more likely to interact with matter at high energies and a muon collider would be generating a LOT of really high energy neutrinos in a ring around the machine. This has been proposed as a sort of super weapon even!

Muons should interact with matter in a similar fashion to electron beams and the beams will probably be treated in a similar manner. They can propagate through the atmosphere quite well in theory and should outperform electron and proton beams for a given beam energy, although at quite a cost… Muons are typically generated with proton beams impacting targets and will create an equal number of positive and negative muons at high energies that must be carefully captured. This would allow for the easy creation of a neutral muon beam though, which may make for a great giant doom beam! Do note that negative muons also can help catalyze nuclear fusion so a very high power beam hitting a fusion fuel tank could produce some rather… Interesting results!

Other exotic options

There are a wide variety of odd particles that could also be chosen for use in beams given a specific need for them, although generation and utilization of these particles will be quite difficult! Extremely short lived particles like pions, kaons and taus can be generated and accelerated up to extreme speeds to help propagate them far enough to matter, although that will need some very extreme speeds. At the upper end of extreme there are even proposals for accelerating tiny charged blackholes!

Accelerator technologies

There are a wide variety of accelerator technologies and here we try to summarize them as best as possible. Different accelerator types may be sequenced one after another. Some particle accelerator technologies depend on the particles fulfilling certain requirements of speed (thus energy) and hence different systems may be employed in sequence. Accelerator systems are described in terms of their efficiency of turning input power into particle energy, their acceleration gradient measured in electric field which translates to volts per meter, and the mass per length meter. An interesting secondary parameter is their operating temperature. Some accelerators can operate well at high temperatures. Others require cold or cryogenic conditions to work well.

Core accelerator assembly

We define the core accelerator assembly as the structure where energy is imparted onto the particles.

DC Accelerators

For very low beam energies we can utilize large DC electric fields between plates or grids to accelerate particles up to speed. This is the same technique as is used in ion drives and is the most common industrial accelerator used. Every dentist's office and old CRT television uses this acceleration technique. Ion accelerators often also use a DC accelerator to generate the initial beam of ions, while electron beams utilize other techniques. These machines are limited by material breakdown to about 0.5 MV/m accelerating gradient and typically no more than a couple of MeV total particle energy for extremely large machines. The highest energy DC accelerators can reach 40 MeV for proton beams, but this is for machines massing in the 100s of tons and at low total beam powers. Very large DC accelerators are currently the only tested method for accelerating macrons up to 10’s of km/sec.These accelerators are limited by the risk of arcs jumping between the electrodes at too high voltage differentials.

Radiofrequency Quadrupole

A more advanced first stage accelerator for ion beams is the radiofrequency quadrupole or RFQ. These machines utilize low frequency radiowaves and carefully designed conductive bar structures to efficiently generate high current ion beams up to energies of ~7 MeV in a couple of meters. RFQs were a revolution in ion accelerator physics and caused quite a stir when the USSR first showed them off as they provided a much higher power and higher quality ion source in a smaller package than anything the USA had built! RFQs have also been proposed as methods of efficiently accelerating macrons up to >100 km/sec. These machines can be normal or superconducting and can operate with efficiencies as high as RF LINACs in theory.

Cyclotrons and Betatrons

One of the earliest ways to accelerate particles up to relativistic speeds is with circular accelerators known as cyclotrons (for ions) and betatrons (for electrons). In Cyclotrons the accelerator is a round chamber which houses two half-circle electrodes roughly shaped like a D (and thus called “dees”) separated by a gap, sandwiched between the poles of a large electromagnet. The magnet creates a static magnetic field which bends the path of charged particles into a circular motion. An oscillating alternative voltage is applied to the electrodes (what we often call RF or radiofrequency) to accelerate the ions. This radiofrequency field’s oscillation is matched so that one cycle of the voltage matches one circuit of the particles. As a consequence when crossing the gap from one electrode to the other, the charged ions experience a voltage gradient and are accelerated. As the particles gain energy and speed, their radius in the magnetic field gets larger. The result is an overall spiraling trajectory from an injection point in the center to an exit point at the rim where the charged particles are bent out of the cyclotron. Cyclotrons use the same voltage gradient many times over, defeating the electrostatic breakdown that limits the single-time electrostatic accelerators. As a consequence, proton energies of up to 70 MeV may be reached. Cyclotrons energies can also be increased by switching from normal magnets and RF resonators to superconductive components. This eliminates various losses from electric resistance and absorption and also allows for much stronger bending magnets, which allows the cyclotron to contain particles up to a higher energy if desired. Superconducting cyclotrons have achieved up to 520 MeV. Still, cyclotrons suffer from poor beam current, pulse rate, and beam quality issues that make them a less than ideal choice. The large magnet required (measured in meters) also does not lend itself to a lightweight weapon and as such, cyclotrons are regulated to niche medical and physics uses.

A close cousin is the betatron, which is used to accelerate electrons. In this case the magnetic field strength is increased as the electrons speed up, which prevents any spiraling trajectories of the particles. Betatrons have been operated up to electrons energies of 300 MeV, but are more typically in the ~10 MeV range. In principle a betatron could also be made superconducting, although magnet ramping may prove to be difficult. Other types of ring shaped accelerators may prove to be more useful, although electrons are heavily limited by synchrotron radiation and it is often better to use linear accelerators for them. For now betatrons remain limited to small (sometimes portable) accelerators for nuclear physics, industry and security work.

Radiofrequency Linear Accelerator

The real work-horse of the particle accelerator world is the RF linear accelerator. These babies use the same alternating field trick that cyclotrons use, but in a straight line rather than needing to bend the beam around in a circle. This provides a couple of nice advantages over the more traditional cyclotron. First and foremost you get to leave that large, bulky magnet behind which saves you a ton of mass, cooling and power consumption! You also get to now have much higher RF frequencies (1-10 GHz right now compared to the MHz used in cyclotrons), which means higher accelerating gradients. To generate these powerful, high frequency RF waves (also known as microwaves) that drive the accelerator we typically use military radar power supplies or (in more advanced cases) even lower energy particle accelerators! State of the art RF LINAC technology is well into the 100 MV/m range and have no upper limit on beam energy! The Stanford Linear Accelerator reaches 50 GeV and was built back in the 1960’s! There have been traditional RF LINAC designs for machines reaching up to 1 TeV, with the only limit being how much space you have to put these monsters. They do operate best (i.e. with high accelerating gradients and efficiencies) with particles already moving at relativistic speeds, so any ions fed in need to use a series of lower gradients LINACs or other pre-accelerators. Electrons and positrons can naturally be used in LINACs since they accelerate up to relativistic speeds so quickly.

LINACs also have been shown to operate at high average powers (MW scale is now in use) and high pulse energy (10’s of kJ pulses fired at high rep rate) while also maintaining a high quality beam that stays well focused. The two newest advancements have been to push the frequency ever higher for higher acceleration gradients (300+ MV/m has been shown and there are plans for even more) and to switch to superconducting RF accelerating cavities (the name of the actual piece of equipment that does the accelerating) which then dramatically increases the electrical to beam efficiency (80-90% has been shown and even copper waveguides have been operated at 70%). All in all, LINACs beat the tar out of cyclotrons which is why we don’t build cyclotrons anymore. To quote an old Fermilab engineer “cyclotrons are an expensive and bulky way to get a really crappy beam or a cheap way to get a giant chunk of lightly radioactive steel that can never leave your lab”.

LINACs work in detail by shaping the radiofrequency energy in a confining waveguide. The oscillations of the RF energy may be held in space, or allowed to travel down the waveguide over time. Radiofrequency waves have connected magnetic and electric components, which oscillate.

Particles of the right charge in the right space of the RF wave experience an acceleration, being drawn to the opposite electric charge. If they land in the other region, they are instead decelerated. Inbetween, particles experience less to no acceleration. This behavior naturally separates a stream of particles into discrete bunches.

It also creates an implicit limit to what can be effectively accelerated using a specific radiofrequency accelerator. The particles driven through it have to keep pace with the oscillations of the RF waves, reaching the next correct region as the wave flips (and potentially travels down the waveguide). If they are too slow to keep inside the accelerating regions, the accelerating action will be ineffective. This requires RF cavities to be matched to the particle's velocity gain under energy imparture and their velocity when entering, being inside, and exiting the accelerator. For electrons and positrons, this isn’t that complicated. Since these lightweight particles accelerate readily, they have few problems “staying in the pocket”. Particles that are slower to gain speed, such as ions and even larger particles, must however be accomodated. This in turn impacts how much acceleration and energy per meter can be fitted into the accelerator in the first place. The acceleration of heavy particles requires specifically designed and sequenced RF accelerators for specific regions of particle speed. This in turn complicates the design of the larger accelerator infrastructure, with multiple discrete stages getting the particles to the energy required so that the next stage in sequence can take over.

RF cavity accelerators are limited in their eV/m capability by the critical current at the containing waveguide. To increase the acceleration provided, more intense electrical fields must be built up. Eventually the electrical charge of the RF waves becomes so strong it can rip electrons out of the cavity walls, causing an arc to form and the cavity material to break down physically.

Advanced Accelerator technologies

Present accelerator technology caps out at around 250 MeV/m for the most overdriven designs. This poses issues for a variety of applications, where higher energies in shorter distances are desired. In science, "desktop" and "chipset" accelerators are of interest. When militarizing particle beams, some effects only become available at very high energies - but you want something compact enough to turret.

urrent and even near future accelerator isn’t really ready to supply that sort of scale quite yet, so we have to move on to the cutting edge of accelerator physics. There are three big options with sub technologies within these broad fields. We have extreme RF cavities using much higher frequencies, plasma accelerators using lasers for power and plasma accelerators using particle beams for power. Each technology provides a unique series of advantages and disadvantages, but fundamentally they are all a way to make linear accelerators that are much smaller.

Dielectric Wakefield Accelerator

LINACs using much higher frequencies than traditional LINACs have two fundamental problems to fight. The first is electrical breakdown and the second is generation of those mm-wave (10-100 GHz) or even THz (100-1000+ GHz) sources at sufficient power and efficiency to power a particle accelerator. As the frequency of the driving power goes up the electric field climbs up, which means the accelerating gradient gets larger (which is good) but also means that we run a higher risk of a huge arc forming in the accelerator and breaking it (which is bad). This is often fought by changing the accelerator cavity material from a metal (or superconductor) to a dielectric material like ceramic, or by using a coating of dielectric, which can sometimes earn them the moniker of Dielectric Wake Field Accelerator (DWFA). The issue of creating powerful and efficient enough sources of this higher frequency drive power is being actively pursued for a variety of fields (5G cell signals, beamed power, etc) but is still quite immature compared to the microwave RF sources of traditional LINACs. There are options using essentially "over-clocked" microwave sources, low energy particle beams, and even laser-plasmas. There is still a lot to be done here, but these sorts of problems are being actively worked on there are working accelerators using this technology. There is potential for GV/m or greater acceleration gradiatens and all of the same advantages of traditional RF LINACs (i.e. high powers, high fire rates, high energy pulses, quality beams) should stay the same. The wall plug efficiency is liable to be a bit lower, but that will be highly dependent on the efficiency of the source of these high frequency waves.

Laser-Plasma Accelerator

Plasma accelerators using lasers for power are a whole world in and of themselves. There are a wide variety of ways to accelerate particles with lasers and plasmas, but we will focus on the most studied technique known as Laser Plasma Accelerators (LPA). LPA accelerators utilize ultrashort (<1psec) and ultraintense (>10^18 W/cm^2) lasers to drive a plasma wake (thus sometimes earning them the name laser wakefield accelerator) and accelerate particles through the resulting charge differential. They have only shown the ability to accelerate electrons and positrons, but any ions that could be injected at the right time and at extremely relativistic speeds could conceivably be accelerated this way. In the operation of a typical LPA the ultraintense laser is fired into a jet of gas (turning it into plasma) or a channel of lightly pre-ionized plasma. The laser then blasts through this plasma like a boat through water and leaves a void behind it (the wake). This void ends up having a HUGE accelerating gradient (100 GV/m) and suffers no risk of electrical breakdown since the plasma is already broken down! A pre-accelerated bunch of particles can be injected into that void at the right time (called the witness bunch) for a BIG acceleration or you can rely on the wake itself spewing electrons out into the void and make a beam out of the plasma itself. The gas or plasma then quickly is reformed and the laser fires as soon as its cooling and charging allows. Keep in mind that even though the beams can hit incredible energies in short distances, the laser itself is not a pocket sized device. These lasers are room sized at minimum and not easy to shrink. The pulse lengths required are also much shorter than most of our hypothetical weapons-grade pulse lasers that may operate in the nanoseconds regime - dual-use may prove difficult. The intensities reached also pose significant issues for the laser hardware. If you don’t spread the energies out, it becomes quite possible you will break parts of your laser from the produced intensities.

Current LPA systems suffer from a couple of problems that are being actively worked on. First and foremost they are incredibly efficiency limited by the laser tech being used. There are plans to move on to much more efficient fiber lasers and eventually make LPA machines that operate in the 25% or greater wall plug efficiency, but that is not currently doable. These lasers also need to be fired at higher rates than the current ~1 Hz, but that is much more of a limit of laser engineering than accelerating engineering. LPA’s also have yet to link more than one accelerator together and the lasers are limited in just how much plasma they can accelerate through right now (~20 cm max) so the beam energy runs into a wall quite quickly. Aside from R&D on linking LPA’s together in a way similar to RF LINACs there are two other options. The acceleration gradient is higher the denser the plasma, so a move to denser plasmas and shorter wavelength lasers (to penetrate the plasma) could provide higher beam energies in similar distances. The other option is to use clever optical techniques like the so-called “flying focus” to allow for the laser to go through much longer plasmas. This could in theory allow for more efficient LWFA’s and single stage machines that provide ~1 TeV electron beams in meters! The final issue is beam quality. Even the best LPAs will not make beams that have as tight of a beam spot, energy spread, or emittance as a normal RF LINAC. This means they will end up needing some beam “cleaning” if you want to get a lot of range out of them.

LPAs are of engineering interest for very short wavelength X-ray free-electron lasers in a compact format. Currently these facilities require linear electron accelerators many kilometers in length. With an LPA as the electron accelerator, an XFEL able to image molecular structures and chemical reactions as they happen might be shrunken down to a much more compact format. (Sometimes this is discussed under the ultimate promise of the “benchtop” or even “chip-format”(sic!) XFEL)

Beam-Plasma Accelerator

Plasma accelerators using particle beams for power are very similar to LPAs in principle but use a particle beam rather than a laser. We typically just call these things Beam Plasma Accelerators (BPA) for short. The concept is very similar to the LPA in that a high intensity beam comes in and blasts a void into a plasma, which particles can then be accelerated in. BPA provides some interesting advantages over LPA in that you can use highly efficient superconducting RF LINACs (or synchrotrons if you want) to provide the particles at very high efficiency. They also have little in the way of plasma density limits and the accelerated beam takes on the characteristics of the beam driving the BPA. Thus little if any beam cleaning will be needed after acceleration. The big downside is the need for a relativistic particle beam to drive the effect, and typically one in the 100’s of MeV to low GeV range, depending on the particle used. Thus your final accelerator may be short, but you still won’t be making a handheld particle beam device.

Accelerator Shapes

The accelerator technologies discussed above are the means by which we create situations for particles to gain energy. The accelerators become embedded in a larger structure that produces, directs, accelerates, even stores the particles until they are directed towards their final use.

There are two principle shapes for accelerator assemblies. Ones where the beampath is not closed onto itself, and designs where the beam path is closed on itself in part of the assembly, forming a loop.

Linear accelerator

Everything from electron generation to acceleration to conditioning is built in a straight line, one after another. This is the Linear Accelerator or LINAC. The main catch here is that there is only one single passage for the particles through the entire accelerator. Re-using accelerator hardware to drive the same particle to higher energies by repeated passage is out. The particles have to reach their desired energy in a single pass. Practically this puts limitations on the energy achievable with heavy particles in a LINAC. The length required gets impractically large. Even for electrons, lengths of hundreds of meters to kilometers may be required, unless advanced accelerators with high gradients are used. This said, every accelerator contains linear sections. Since so much high-performance accelerator technology depends on particles having a minimum velocity, ions must be “pre-conditioned” in a set of linear accelerators before the re-use of an accelerator through looped beam paths can be considered.

LINACs of low energy may be used in many applications. Medicinal they find use in radiation therapy and radiosurgery.

Synchrotrons

The other shape which has some interesting properties are accelerators where beampaths are looped onto themselves. These are called synchrotrons. Since beam paths are closed onto themselves, particles can pass through the same accelerator unit repeatedly, gaining more energy on every passage. This is advantageous for heavy particles which do not gain much speed on any single passage. They can pass through the same accelerator thousands or even millions of times instead. This is obviously advantageous for facility bulk and mass and allows us to reach energies that would otherwise be impractical. The accelerated beam bunches can also be held in storage, looping like the rotating mass of a flywheel, holding energy. The LHC can store some 700 Megajoules of energy in the recirculating beam after full speed has been reached.

That said, the devil is in the details. When charged particles are bent in circles they emit electromagnetic radiation which scales very rapidly (to the fourth power!) with relativistic gamma factor and with bending radius (inversely to the second power). This phenomenon is called synchrotron radiation. Due to the scaling, high energy particles emit more synchrotron radiation than lower energy ones and lighter particles (like electrons) at those high energies emit much more than heavier particles. In addition, the bigger you can make the bending radius of the synchrotron (often called “the ring”) the less synchrotron radiation you get.

Although these numbers look small, keep in mind that this is PER PARTICLE. A typical pulse in an electron accelerator will have more than more than a billion electrons in it! Large synchrotrons like the LHC will have over 300 trillion ions circulating through it! Synchrotron radiation necessarily has to be managed to reduce radiation shielding, the radiation load (and possibility of hardware failure) on the accelerator, and to reach high energies in the first place - you might lose all the energy your accelerators are adding to synchrotron radiation!

Deflecting the beam also requires the application of magnetic fields, and higher degrees of deflection, in a smaller length, at higher particle energies, require stronger and stronger magnetic fields. Eventually multi-Tesla fields from superconductors will be required. And due to action and reaction, those magnets need to be braced against the pressure the beam exerts as it is deflected. Ultimately this leads to the same limitations as with flywheels - the mechanical strength of the braces and static connections that keep the magnets together as a larger structure is the accelerator’s limitation on J/kg held in storage. A further complication: The magnet strength has to be matched to particle energy. As a beam is accelerated, the deflectors have to ramp up their power to keep the degree of deflection constant.

Sometimes, synchrotron radiation from electrons is intentionally induced in a ring-shaped accelerator. Since synchrotron radiation of high-energy electrons tends to be hard x-rays emitted along narrow vectors, it’s a way to get x-rays used in various imaging techniques in material science and molecular physics.

Synchrotrons used for ions also need to respect the limitations of the involved accelerators. Synchrotrons often use radiofrequency accelerators. Since radiofrequency accelerators must match the speed and speed gain of their particles, an ion synchrotron cannot be made up of a single accelerator loop. Instead there are sequential loops connected in sequence, each designed to boost the particle through a specific velocity and energy region so that the next accelerator takes over. A designed example is the LHC. Particles headed for the main LHC actually circulate in two other synchrotrons beforehand, gaining sufficient energy so that the LHCs RF cavities can take over. (Also, before being injected into the first synchrotron ring, the ions must first go through multiple linear accelerators using different technologies to reach a base energy.)

An interesting design consideration: Synchrotrons don’t necessarily have to take the form of a (near-ideal) ring. Funamentally any dipole represents a single point of deflection of a straight path - they're polygons! Any geometric shape that contains a loop, and where any single bending curve keeps within the limits of the synchrotron radiation you are willing to tolerate, works. As such, complicated two- or three-dimensional “folded-up” shapes are also possible formats for a synchrotron.

Storage Rings

Since synchrotrons have to match their deflection magnet strength to the energy of the particles being accelerated, a single synchrotron track cannot hold more than one small section of beam bunches in storage at a time, and becomes unavailable to accelerate further bunches until it discharges the stored beam. This is disadvantageous for various applications where we may want to store a fully accelerated particle beam “bolt” for a longer time.

Storage rings are separate synchrotrons designed to hold multiple sets of bunches in circulation at a target energy at a time. The bunches are delivered from an accelerating synchrotron (sometimes called a booster ring) and held in circulation in the storage ring. Since there are synchrotron radiation losses, a few accelerator sections must compensate for these.

Storage rings can also be used to condition a stored beam. A common thing to do in storage rings is to “cool” a beam, draining unwanted movement energy out of the particles. Beam cooling may take significant time, which makes it ideal to conduct in a storage ring. The result is a beam with less tendency to drift apart, advantageous for many applications.

Other parts

Particle sources

Particle sources are assemblies which supply particles to accelerators. Here we can group them into three categories: electron sources, ion sources, and exotic particle sources. The later involve the production of particles by means of gathering them from collision reactions (or the handwavium blackbox of your choice) and feeding them into another accelerator for further use. Positrons for example may be manufactured in beam collisions.

Electron sources require the production of a free stream of electrons. The oldest method here are thermionic sources. A hot piece of material ejects free electrons into a vacuum, where they can be drawn in using electric differentials and forwarded to the accelerator.

Field emission electron sources instead use a strong potential between a needle-like cathode and an anode plate with a hole. The electrons are drawn from the needle and through the anode due to the high charge differential. This method produces a “cooler” beam with less random divergence than thermionic sources.

A further improvement still are photoemission sources. Here the precise application of laser light onto a photosensitive material energizes electrons in the photocathode until they break free. This process is thermodynamically very precise and due to the nature of the laser light, also very controllable. The result is a low-emittance stream of electrons. These may be accelerated using electrostatic attraction with an anode at one end of the vacuum chamber, or directly using an RF field.

Ion beam sources are more complicated. The physics of ion formation are complex, which opens many design choices when creating an ion beam. Generally a plasma must be produced, from which the ions can be extracted.

The plasma can be formed in a variety of ways. Electrons from a local cathode-anode array, plasma frequency heating, the injection of electron beams, the application of laser light. The end results can be as diverse. Some methods only produce weakly charged ions and on many vectors, so with high emittance. Other methods offer greater ion charges or reduced emittance.

Magnets

While acceleration of charged particles is effected with electric fields, magnetic fields can be used to bend the trajectories of particle beams. As such, electromagnets can be used to bend the trajectories of charged particles in various ways, and in a controllable fashion.

Dipole Magnets

Dipole electromagnet are used to steer the entire particle beam in one axis. (To steer in two axes, we must either rotate one dipole about the beam axis, or couple two dipoles in sequence.) They can be used to direct a beam inside a particle accelerator, inject and extract beams out of looping constructs by switching on and off at the right moment, and point a beam at a target.

In a dipole magnet, a positive and negative coil oppose each other, embedded in a “choke” assembly often made of steel. Charged particles moving through the magnetic field between the coils experience a deflection in one direction specified by the polarization of the field, and a degree of deflection specific to the field strength.

Quadrupole Magnet

Dipoles can steer, but they influence all particles equally. As such, they cannot be used to manipulate the beam's shape. The first type of magnet able to influence a particle beam’s shape are quadrupole magnets.

They are assembled from four coils at 45 degree angles, with coils of the same polarity opposing each other. The passing beam is compressed on one axis, and expanded on the other. To focus the beam overall, at least two quadpoles must be arrayed in sequence.

Quadrupoles are the basic and essential “magnetic lens” of particle beams. Without them, charged beams would inevitably defocus and run havoc.

Hexapole Magnets

Hexapole magnets exist to address one problem of quadrupoles - they focus differently depending on the energy of the particle passing through the magnetic field. The effect is likened by particle accelerator physicists to the different diffraction of light in a prism - and hence the effect is called “chromatic aberration”. Hexapoles are used to correct for this, stronger deflecting higher-energy beams.

Integrated optics

As the field of particle accelerator science and engineering advances, more complex designs are proposed to handle more complex phenomena inside accelerators. An upcoming field is “integrated optics” - complex magnetic systems with nonlinear behaviors. These could provide more advanced manipulation of electron beams, building upon existing capabilities and adding new ones for shaping a beam.

Plasma lenses

Plasma lenses are an up-and-coming technology, where very high currents in plasma are used to create very strong magnetic fields. Similar to plasma-based accelerator technology, the fact that charges are already free in plasma is used to circumvent conventional charge breakdown and critical current limitations on solid-matter devices. The resulting magnetic fields can be much stronger and bend beams over shorter distances. Furthermore, by creating complicated profiles in the plasma, elaborate beam manipulations could be performed, more akin to sophisticated optical elements. As well, the charges in the plasma neutralize the particle beam in the region of the plasma lense - affecting its propagation behavior further and in a positive manner, as self-repulsion is neutralized.

The catch is that this is plasma - that means you are dealing with all the complications of plasma physics. Heating of the plasma has been an early challenge to successfully forming and maintaining plasma lenses.

Plasma lenses require an appropriate chamber, possibly active containment to keep the plasma from losing energy to the walls or leaking into other accelerator parts, and the necessary plasma sources, maintenance heating devices and then lense shapers and control sensors. In high-powered plasma lenses there will also be a need to cool the plasma as it heats up from waste heat. Potentially we may manage this by open-cycle cooling - venting the plasma controlled in one direction into open space, rather than have a closed-cycle solution for reconditioning the plasma. This could cause some very cool visuals!

Plasma lenses for particle beams should not be confused with optical plasma lenses for lasers! Those exist as well, but are their own topic.

Chicanes

Chicanes are assemblies of multiple dipole magnets used to manage the longitudinal dispersion of a particle beam due to different particle energies. Since particles with higher energy move faster, they drift ahead in the beam distribution over time; and particles with less energy increasingly lag behind.

Chicanes deflect high-energy particles onto an outward curving trajectory. Since traversing the curve takes longer, the high-energy particles are effectively delayed, allowing lower-energy particles to catch up. By the end of the chicane assembly, the bunch has been compressed in the longitudinal direction.

Chicanes however, do not remove energy from too-fast particles. That is the responsibility of other structures.

Undulators and wigglers

Undulators and wigglers are magnetic assemblies used to transform particle energy into electromagnetic emissions, with different properties from the specific implementation of a shared basic principle.

Assembled from stacks of generally dipole magnets (other configurations exist), both undulators and wigglers force charged particles onto “wiggling” trajectories by bending their path left and right. Wigglers force very strong arcs; undulators less so. In both cases, the rapid deflection of the charged particles causes them to shed parts of their energy as electromagnetic emissions. Due to the energy of the particles, these photons are generally highly energetic x-rays.

Undulators may be used to build free-electron laser systems.

They can also be used as a means of cooling the beam. The removal of energy from particles can also suppress unwanted movement modes, allowing accelerator structures to re-add energy only to desired movement modes. Of course, this means the particle beam actively loses energy, and produces x-rays that have to be absorbed (and cooled away.)

Beam coolers

Beam coolers are a collection of systems and strategies that are used to remove unwanted movements from a particle beam. Undulators and wigglers are one way to do this.

One method set of beam cooling relies on finding particles who are very “hot” with random motion, and either removing the particles from the beam using a kick device, or applying a precise counter-force that acts against the unwanted motion.

If one is dealing with ions, one clever trick is to inject a beam of electrons. Due to the thermodynamics within the particle population, the lighter electrons absorb heat from the heavier ions, taking this unwanted motion energy with them when the electron beam is bent out.

As well, lasers can be used (what can’t lasers be used for!) to apply counter-force on ions and remove unwanted movement in this manner.

Beam coolers are generally not instantaneous devices. They work over time to make a beam better. Often they involve the removal of energy or particles from the stream, requiring accelerators and sources to make up the loss and creating waste heat.

Beam recovery

Some accelerator systems can actually be run in reverse - they can turn beam energy back into some other energy format and then electricity. This can be used to recover some to most of the energy from a particle beam before disposing of it. This is advantageous both for your energy bills and making a very high-energy beam liable to spray radiation everywhere, easier to absorb in a beam dump.

Beam dump

A beam dump is a construction where a particle beam can be routed in case it has to be disposed. Beam dumps are designed to safely absorb the particle's energy without leakage of dangeorus radiation or activated materials to the outside enviroment. Common materials are carbon and metals, embedded in an actively cooled assembly.

Propagation physics

Energy loss in matter

- ionization

 -- electron cascades

- brehmsstrahlung

 -- electron-gamma showers

- electron capture and screening - phonons and displacement

 -- displacement cascades

Operational Considerations

Logistics

The logistics of particle beam can be a mixed bag. By itself, any particle accelerator is a complicated piece of machinery full of delicate parts that have to deliver high performance. This may require cryogenic temperatures with associated challenges on the materials and mountings used (insulation, and distortions under temperature changes). When working, particle accelerators invariably generate some radiation across the whole assembly, which irradiates and activates the structure. Some parts may be exposed to significantly higher radiation flux due to their working principles. Often the particle beam is a dynamic system that must be managed with active, closed control loops, involving sensors, modeling and control software, actuators, and the computing and communication hardware that brings it all together. This poses engineering challenges across all disciplines.

A practical particle beam is inherently a counterintuitively sturdy device. If it was too delicate, it would break too much to be useful, though of course this is a sliding scale. Still, while sturdy, this doesn’t eliminate complexity, it manages complexity. The result is a system that is the product of a high-tech civilization. A superior machine shop and well-trained engineers are a must to maintain a particle beam system. Some parts of a particle accelerator may be mundane - others may be custom-machined parts that require their own dedicated industries to produce. That said, the manifold utility of particle accelerators is on your side here. Since there are so many practical applications, it is economic to produce parts for industrial demand. (that said, these may not be the kind of components you want in specific high-performance applications.)

The main thing a particle accelerator requires the most is electricity. Any mature power production facility can supply the power.

However, all particle beams require at the least a source of neutralizing mass. The projection of a charged beam will leave the projecting object oppositely charged. In an atmosphere the natural plasma formed by the beam will ground everything out, but in space the diffuse vacuum will not do the job.

Furthermore particle beams using ions also require a feedstock for their ions to produce from, and in an appropriate format. This can be a gas, a fluid or an easily ionized solid. It can be the feedstock species in its pure form or in an easily split molecule, though every complexity adds to the amount of preparation work that has to be done before the feedstock enters the particle beam.

Don’t worry about the mass of your source or neutralizing ions though. Large particle accelerators, like the LHC in Switzerland, only consume tiny gas bottles worth of feed gas every couple years. The mass of a single 5.56 NATO bullet worth of hydrogen could feed a 10 GW, 10 GeV ion beam for nearly 4 days of firing! You will not be running out of beam feedstock easily.

Cooling

Cooling a particle accelerator can be a serious challenge. Various bits of a particle accelerator love to be supercooled for efficiency or basic working principle - any bit of superconductor used in the accelerator, or magnets, or EM generators, will require cooling to its generally supercool operating temperature.

Even if Standard Temperature and Pressure Superconductors (273 Kelvin, 100 Kilopascal) are available, present superconductors have demonstrated superior performance by being supercooled. That means to achieve even stronger fields, even STP superconductors may be cooled to cryogenic conditions.

Cooling cryogenic systems sucks. Thermodynamics shows itself from its worst side. You can forget running cryogenic radiators - the Stefan-Boltzman Law penalizes the direct radiative cooling of anything cryogenic severely as much as it empowers very high-temperature radiators. This only leaves stages of heat pumps to provide cryogenic fluids that can in turn, cool cryogenic systems. Heat pumps draw power, and for such a giant temperature gradient as we can have here, they will draw a lot of power.

However, we have other options. Cryogenic systems also tend to be highly efficient, so while they do heat up, they do so very slowly and there is little overall energy absorbed. This provides the option of running off of bunkered cryogenic fluid in dewars for the duration of operating the equipment and not running any heat pumps all the time.

Other bits of a particle accelerator system aren’t as required, but really nothing is a fan of high temperatures. Going higher than room temperature can be treated as an oddity. High-performance lasers may not require cryogenic conditions, but they certainly like it cool. Radiofrequency producers of any kind generally fare worse with rising temperatures. That means cooling overall remains a challenge - 300 Kelvin is still not that great of a hot loop temperature for radiators.

Safety

Ionizing Radiation

There are surprisingly many sources of ionizing radiation in a particle accelerator, besides the obvious threat of the output beam itself. Not all particles are accelerated correctly or in the correct direction. When such particles interact with the structure of the accelerator, they release a shower of secondary ionizing radiation. In electron accelerators and sources, stray electrons may be accelerated and release x-rays from bremsstrahlung as they collide with surrounding matter. The resulting doses can be significant and dangerous. Also, such collisions leave behind residual activation, making the accelerator a radiation hazard for some time even after it has been shut down. Generally, people should not work near the particle accelerator while it is functioning, and radiation barriers isolate the core accelerator from adjacent facilities. (In real-life facilities, you will often see accelerators inside larger buildings being cocooned inside doped concrete plates to shield the rest of the room.)

Radiofrequency

RF energy is used in many kinds of particle accelerators. This energy may leak from the structures it is generated, contained and used in due to damages or incorrect assembly. As microwave radiation and above is used, the radiation can be readily absorbed by bodies, hydrogen-rich materials or other objects. The end result of this can be rapid heating and thermal effects. Especially in accelerators designed to throughput a lot of energy (say a GW or more), this may become a serious issue if a waveguide is damaged. It’s not recommendable to stand in front of a multi-MW radar either!

High-powered electrical and electronic systems

The operation of the magnetic systems and accelerators often involves high-powered electrical and electronic systems. High voltages and/or amperes are involved. This poses risks like shock hazards, electrical arcs, and electrical fires. Some electrical systems such as capacitors or inductors may have combustive or explosive failure modes.

Flammables

Parts of an accelerator or its infrastructure may be manufactured from flammable substances such as hydrocarbons. In the case of an ignition source such as a high-powered beam cutting into a physical substance or an electrical fault, these may catch fire.

Lasers

Some accelerator systems use high-powered lasers. Laser plasma accelerators use very high-powered pulse lasers to provide pumping power. The safety concerns of lasers for blinding, physical damage, and possible exposure to ionizing radiation as a secondary effect of the beam impact, apply. Some high-powered lasers can also drive electrons which generate strong EMPs, which may damage or destroy other systems in turn. Parts of a laser assembly or laser beam guide may shatter if exposed to overloading conditions.

Magnetic systems

Parts of a particle accelerator may use strong magnets, up to superconductive magnets with multi-Tesla fields. While the magnets are integrated into chokes that shape the magnetic fields, fringe fields invariably extend beyond the core magnet installation.

Multi-Tesla transient electromagnetic field can affect the human nervous system temporarily, inducing nausea, vertigo, metallic taste and optical phenomena, if the person moves in relation to the magnetic field. Otherwise, the risk of exposure is low. That said, electronic systems that are not shielded properly, such as say, implants, could be adversely affected by the magnetic fields.

The biggest physical risk are reactions of other magnetic objects with the magnets. You can look up cases where metallic objects interacted with the multi-Tesla fields of MRI scanners. Such parts may flie across the room towards the magnet at increasing speeds, and damage anything or injure anyone in their path, possibly ending with the object or person squashed (or impaled!) to the magnet.

Also, large superconductive magnets store not insignificant amounts of energy. If the superconductive property breaks down, the magnet quenches and becomes a resistor. The resistance site heats up as current tries to flow through it, expanding the quench and possibly being heated until it sublimates into gas. This could possibly happen explosively. In such a case, up to many GJ of energy might be unleashed in a conflagration or explosion (subsonic or supersonic), damaging surrounding systems and ejecting shrapnell.

Cryogenic cooling systems

Particle accelerators often involve cryogenically cooled systems. This means cryogenic fluids like nitrogen and helium are present in the accelerator infrastructure. Such gasses have both a very low temperature and a high ratio of gaseous volume at high temperatures to their liquid volume.

Exposure to the leaked fluid can lead to cryogenic cold damage, injury, and death. Furthermore such gasses are efficient at displacing ambient breathing gasses such as oxygen, so they pose an invisible, and rapidly spreading, suffocation hazard.

Also, if insulation is damaged, ambient heat may leak into the cryogenic circuits, and cryogenic fluid may begin evaporating. This can lead to dramatic rises in internal pressure. Relief valves should trigger to safely direct such pressure spikes away from important places and keep the system from failing catastrophically, but otherwise, pipes and cryogenic storage devices may fail, up to explosively.

Beam excursions

If the magnets responsible for manipulating the beam break down, the beam may excurse from its designed paths. The beam may become defocused, or maintain its focus. The beam begins cutting into the accelerator structure, evaporating material and depositing energy. This will definitely foul the ultra-high vacuum inside the accelerator, leading to secondary radiation production as the beam collides with gas inside the accelerator. Some sensitive facilities requiring ultra-high vacuum such as some RF cavities may “crash” and become ruined. Further systems may become damaged or destroyed from either physical stress or the radiation pulse. If the beam bunches contain a lot of energy and maybe are heavy particles, the deposition may lead to an energetic local explosion. Alternatively the beam may escape and cut into structure beyond the particle accelerator itself, damaging and destroying equipment and seriously hurting or killing personnel.

In the worst cases, a beam excursion on a very powerful beam may lead to an explosion that destroys the accelerator utterly. Even in a less bad case, the radiation pulse might damage or destroy critical equipment around the accelerator and take the entire facility offline.

Beam dumping

If any other systems fail, the accelerator probably has to be shut down. This poses a problem - what do you do with the beam energy that is still in the accelerator system?

Sometimes, the beam is designed to leave the accelerator immediately, and shutting down the beam is easy. But in the case of cyclotrons and storage rings, the beam may circulate for a while. What do you do then? You have to dump the beam from the accelerator. Special beam stops are used on research and industrial accelerators, large blocks of dense material able to absorb the beam. During this process the blocks both heat up and become radioactive. For particle beams storing very high energies, this may become an increasing challenge. How do you safely dumb a beam containing many GJ or more? In some cases you may have no other choice but to dump the beam out into the environment. Where and when can you do this safely without threatening third parties?

Application areas

(This is a skeletal overview summary) Particle accelerators have many, many, many areas of application.

In science, the big particle accelerators like the LHC are common knowledge. But electron microscopes also use components of particle accelerators. Many particle accelerators worldwide are used to produce high-energy x-rays in synchrotrons or linear accelerators, imaging samples for material science, chemistry, biochemistry and other areas.

In manufacture, particle beams can be used to cut material, but also weld material. Precision beams can break down gaseous feedstocks to deposit metal in micrometer-sized structures. Directed ion guns are used to implant traces of elements into surfaces for the manufacture of electronic devices.

And in medicine, particle accelerators of ever-greater sophistication find increasing use in cancer treatment and beyond, destroying life-threatening tissues with ever-reduced doses to surrounding healthy tissue.

Sources

Authors: Sevoris Doe, Gerrit Bruhaug. Check reading by Luke Campbell.