Linear accelerator

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One of the longest linear accelerators in the world (3 km) at SLAC in California
A LINAC at the KEK in Japan. The rings (yellow) with spoke-like windings (brown) are quadrupole magnets for focusing the particle beam.

A linear accelerator or LINAC (from English linear accelerator ) is a particle accelerator that accelerates charged particles such as electrons , positrons or ions in a straight line.

This designation almost always refers to a system in which the acceleration is achieved using an alternating voltage of high frequency . The more precise, but rarely used term is therefore high-frequency linear accelerator ( English radio frequency linac ). DC voltage accelerators also have straight particle trajectories, but are usually not referred to as linear accelerators. Here the names of the individual types such as Van de Graaff accelerator , Cockcroft-Walton accelerator , Dynamitron etc. have established themselves.

Compared to the different types of ring accelerators , a linear accelerator is technically simpler and avoids energy losses of the particles due to synchrotron radiation due to the straight particle path . On the other hand, it needs considerably more acceleration elements because the particles only pass through each element once and not repeatedly. For high particle energies, construction lengths of many kilometers are required.

Linear accelerators are often used for particle physics experiments , either directly or as a pre-accelerator for research synchrotrons . The electron synchrotron systems for generating synchrotron radiation also usually contain a linear accelerator as a preliminary stage. Electron linear accelerator lower energy are mainly used for the generation of X-radiation used, such as for medical radiotherapy , radiographic inspection or radiation sterilization .

First developments: the Wideröe accelerator

Wideröe accelerator: The positive particles (red) are accelerated forward in the gap between the drift tubes. (The red arrow represents the direction of the electric field.)

In the first accelerators, the particles were accelerated by direct voltages , like in an electron gun . The particle energy corresponds directly to the operating voltage, which is technically limited by problems such as leakage currents , corona discharges or electric arcs . To get around this, Gustav Ising proposed an alternating voltage linear accelerator in 1924 . Such an accelerator was first built by Rolf Wideröe in 1928 at RWTH Aachen University . The basic idea is that the particle is subjected to the same accelerating voltage many times. Despite a relatively low voltage, the particle achieves a high kinetic energy in this way .

The Wideröe accelerator consists of many tubular electrodes , the drift tubes , which are arranged along a straight axis. These are alternately brought to a positive and negative electrical potential . Since the drift tube itself acts like a Faraday cage , there is no electrical field inside. The particle flies there at constant speed, unaffected by an applied voltage. It is different in the gap between two neighboring drift tubes; there the particle is influenced by the electric field. The polarity of the drift tubes must change so that the particle in the gap always finds the field direction suitable for forward acceleration. Such an alternating electric field is generated by an alternating voltage source of the appropriate frequency, an HF generator. In Wideroe's original experimental setup, the frequency was 1 megahertz . If the length of the tube is chosen so that the time of flight of the particle to the next slit is equal to half the period of the alternating voltage, the accelerating field direction is also found there. Since the particle speed increases with each acceleration step, but the time for polarity reversal remains the same, the length of the individual drift tubes must increase accordingly, as indicated in the figure.

A 1.2 m long Wideröe accelerator with 30 drift tubes was built by Lawrence in Berkeley in 1931 . It was operated with a voltage of 42 kilovolts at 7 megahertz and accelerated mercury ions to 1.26 MeV .

Wideröe accelerators are suitable for particle speeds up to about 5% of the speed of light . For example, this corresponds to an energy of 1.2 MeV for protons , and only 640 eV for electrons with their small mass . Accordingly, the Wideröe accelerator has no significance for electrons. Even at this speed, the drift tube must be several meters long at the practically usable frequencies, and the limited speed of propagation of the current is noticeable.

Modern linear accelerator concepts

The higher the frequency of the acceleration voltage selected, the more individual acceleration thrusts per path length a particle of a given speed experiences, and the shorter the accelerator can therefore be overall. That is why accelerator technology developed in the pursuit of higher particle energies, especially towards higher frequencies.

The linear accelerator concepts (often called accelerator structures in technical terms) used since around 1950 work with frequencies in the range from about 100 megahertz (MHz) to a few gigahertz (GHz) and use the electric field component of electromagnetic waves .

Standing waves and traveling waves

When it comes to energies of more than a few MeV, accelerators for ions differ from those for electrons. The reason for this is the large mass difference between the particles. At a few MeV electrons are close to the speed of light , the absolute speed limit; with further acceleration, as described by relativistic mechanics , almost only their energy and momentum increase . On the other hand, with ions of this energy range, the speed also increases considerably due to further acceleration.

The acceleration concepts used today for ions are always based on electromagnetic standing waves that are formed in suitable resonators . Depending on the type of particle, energy range and other parameters, very different types of resonators are used; the following sections only cover some of them. Electrons can also be accelerated with standing waves above a few MeV. However, an advantageous alternative here is a progressive wave, a traveling wave . The phase speed of the traveling wave must be roughly equal to the particle speed . Therefore, this technique is only suitable when the particles are almost at the speed of light, so that their speed only increases very little.

The development of high-frequency oscillators and power amplifiers from the 1940s, especially the klystron, was essential for these two acceleration techniques . The first larger linear accelerator with standing waves - for protons - was built in 1945/46 in the Berkeley Radiation Laboratory under the direction of Luis W. Alvarez . The frequency used was 200 MHz. The first electron accelerator with traveling waves of around 2 GHz (gigahertz) was developed a little later at Stanford University by WW Hansen and colleagues.

  • Principle of the acceleration of particle packets

  • by a standing wave

  • by a traveling wave

In the two diagrams, the curve and arrows indicate the force acting on the particles. Only at the points with the correct direction of the electric field vector, i.e. the correct direction of force, can particles absorb energy from the wave. (An increase in speed cannot be seen in the scale of these images.)

Focus

In the case of many linear accelerator structures, the particle beam must be focused (held together) along its path using special measures . For this purpose, quadrupole magnets and sextupole magnets are sometimes used, and sometimes solenoids through which the beam flies along their axis. The focusing elements are arranged alternately with the accelerating elements.

High frequency quadrupole accelerator

Electrodes of an RFQ resonator. The cross-section of each electrode is hyperbolic . Two electrodes are shown cut away to show the contour. The distance between the axis and the electrode varies sinusoidally. Where the axial spacing of the horizontal electrodes has the minimum value a, it is maximum for the vertical electrodes (b) and vice versa.

The Hochfrequenzquadrupol resonator, usually as RFQ (radio frequency quadrupole) - accelerator called, is suitable for the same speed range as the Wideroe accelerator, but has a much more compact design. It was proposed in 1969 by the Russian researchers Kapchinskiy and Teplyakov. It uses an electric quadrupole inside a cavity resonator . The four electrodes lie symmetrically around the particle beam, run parallel to it and are shaped in such a way that their distance from the beam axis varies in an undulating manner. This gives the alternating electric field of the standing wave a longitudinal component that alternately points in and against the beam direction. A continuous particle beam that is fed in is broken down into particle packets and these are accelerated. The transverse components of the field have a focusing effect on the beam. Unlike magnetic focusing, this electrical focusing is also effective for very slow ions, because their force does not depend on the particle speed.

RFQ resonators operate at frequencies of up to 500  MHz . They are often used as precursors for high-energy ion accelerators, but also in the MeV energy range instead of DC voltage accelerators.

Cylindrical symmetrical cavity resonators

functionality

More or less modified versions of the pillbox resonator can be used to accelerate ions into the MeV range and beyond . These cylinder-symmetrical cavity resonators are often called English cavity (dt. Cavity) or cavity . Their resonance frequencies are usually a few hundred megahertz. The particles move along the cylinder axis. The TM 010 mode is used from the various waveforms (modes) that are possible in such a resonator . The magnetic field lines run around the beam and the electric field lines run along the direction of the beam.

The acceleration sections in synchrotrons - also for electrons - are also mostly cavity resonators, possibly superconducting .

Single resonators

Single resonator. The directions of the electric (E) and magnetic (B) fields of the TM 010 wave used are indicated. Below the inductive high-frequency feed by means of a coupling loop

In the case of the single resonator, drift tubes are inserted into the end walls of the resonator so that the accelerating field is concentrated on the gap between their ends. The resonance frequency depends approximately only on the diameter and is inversely proportional to it; a resonator for 500 MHz has e.g. B. 46 cm inner diameter. A linear accelerator can be constructed from a number of individual resonators. In addition to the simplest type shown, there are also multi-cell resonators. In general, each resonator is supplied from its own high-frequency oscillator and amplifier; Depending on the design of the accelerator and the type of particle, the correct phase position of the oscillators must be ensured.

Alvarez accelerator

The Alvarez accelerator, named after Luis Alvarez, consists of a long cylindrical tube as a resonator. Inside, drift tubes are attached along the axis; in this respect it is reminiscent of the Wideröe construction. The drift tubes are attached to the tube wall by thin wires or stems. This acceleration structure can be understood as a chain of individual resonators in a row. The TM 010 shaft is also used here . The resonance frequency is usually 100 to 200 MHz. Small quadrupole magnets can be built into the drift tubes for focusing, the current and, if necessary, cooling water supply of which is routed through the fastening of the drift tubes. Alvarez accelerators work well for ions up to about 60% the speed of light. The eponymous first specimen in Berkeley was 12 m long and accelerated protons to 32 MeV. Alvarez structures are used, for example, as precursors for the large ion synchrotrons.

Linear accelerator for electrons

Traveling wave accelerator

A progressive wave (traveling wave) in a cylindrical waveguide is particularly suitable for accelerating electrons that are already almost at the speed of light ; the electrons then “surf” on the crest of the waves. The accelerating force therefore acts constantly and not just pulsating on the particle. The TM 01 mode is used. The phase speed of the wave, which in a smooth tube would be greater than the speed of light, is reduced to the necessary extent by regularly attached circular perforated diaphragms ("iris diaphragms"). Such an acceleration tube is also called a wrinkle tube . It can also be viewed as a series of pillbox resonators placed directly next to one another, the "bottoms" of which are pierced in the middle. Standing waves are avoided here by combining the distance between the diaphragms and the wavelength.

The traveling wave is created when the high-frequency power is fed in at the beginning of the pipe. The wave is dampened by the energy given to the particles (and inevitably also to the pipe wall) . At the end of the pipe - with larger accelerators at the end of a pipe section of a few meters at most - the residual power not absorbed by the particle beam and pipe wall is decoupled and absorbed in a load resistor without reflection. Larger traveling wave accelerators accordingly consist of several or many such sections, each with its own power supply.

The so far most energetic electron accelerator in the world (45 GeV, 3 km length) in the Stanford Linear Accelerator Center is a traveling wave accelerator, as are most of the compact electron linear accelerators for medical and industrial purposes with energies of around 5 to 50 MeV.

Superconducting cavity resonators

Superconducting cavity resonator made of niobium for accelerating electrons ( TESLA project). The nine-cell resonator 1.25 m in length has a resonance frequency of 1.3 GHz

A fundamental disadvantage of traveling wave acceleration is that the wave is dampened in its course; on the other hand, with individually fed resonators - with a correspondingly higher effort for generating the high-frequency power - the maximum possible field strength can be made available to the beam over the entire length of the accelerator . In systems for very high final energy, the highest possible increase in energy per meter length is crucial in order to minimize the overall length and thus the construction costs. Standing waves in cavity resonators are therefore also advantageous here for electrons, especially if one accepts the expense of superconducting components. The type of resonator for electrons shown on the right was developed and tested at DESY . The TM 010 shaft is also used here. With this type, with careful shaping and surface treatment, field strengths of up to about 35 MV per meter were achieved.

The same type of resonator is used, for example, in the linear accelerator of the ELBE facility, and a similar 20-cell type in the S-DALINAC linear accelerator . In these and similar systems for medium-high electron energies (below 100 MeV), the use of superconductivity serves less to achieve maximum energy gain per meter and more to save high-frequency power, so that smaller power amplifiers are sufficient.

Continuous wave and pulsed operation

Every AC voltage accelerator can basically only accelerate those particles that reach the acceleration distance with a suitable phase position of the AC voltage. The particle beam is therefore always divided into “packets”, ie pulsed and not continuous. This Mikropulsung is usually not referred to in practice as pulsing. Provided that the RF source is continuously working and a particle bunch (with each single shaft English bunch is accelerated), is of continuous wave (cw) spoken OPERATION or continuous wave mode. One speaks of pulsed operation or pulsed beam only when the high-frequency voltage is not constantly applied, but is regularly switched on and off ("keyed"), so that macro pulses arise.

Concepts in development

Various new concepts are currently (2015) in development. The primary goal is to make linear accelerators cheaper, with better focused beams, higher energy or higher beam current.

Induction linear accelerator

Induction linear accelerators use the electrical field induced by a time-varying magnetic field for acceleration - like the betatron . The particle beam passes through a series of ring-shaped ferrite cores standing one behind the other , which are magnetized by high-current pulses and in turn each generate an electrical field strength pulse along the axis of the beam direction. Induction linear accelerators are considered for short high-current pulses from electrons, but also from heavy ions . The concept goes back to the work of Nicholas Christofilos . Its realization is heavily dependent on advances in the development of suitable ferrite materials. Pulse currents of up to 5 kiloamps at energies of up to 5 MeV and pulse durations in the range of 20 to 300 nanoseconds were achieved with electrons.

Energy Recovery Linac

In previous electron linear accelerators, the accelerated particles are used only once and then fed into an absorber (beam dump) , in which their residual energy is converted into heat. In an Energy Recovery Linac (ERL; literally: "Energy recovery linear accelerator"), the electrons accelerated in resonators and used in undulators , for example, are returned through the accelerator with a phase shift of 180 degrees. They therefore pass through the resonators in the decelerating phase and thus return their remaining energy to the field. The concept is comparable to the hybrid drive of motor vehicles, where the kinetic energy released during braking can be used for the next acceleration by charging a battery.

The Brookhaven National Laboratory and the Helmholtz Center Berlin reported on the corresponding development work with the “bERLinPro” project. The Berlin experimental accelerator uses superconducting niobium cavity resonators of the type mentioned above. In 2014, three free-electron lasers based on Energy Recovery Linacs were in operation worldwide : in the Jefferson Lab (USA), in the Budker Institute for Nuclear Physics (Russia) and at JAEA (Japan). An ERL with the designation MESA is under construction at the University of Mainz and should (as of 2019) go into operation in 2022.

Compact linear collider

The concept of the Compact Linear Collider (CLIC) (original name CERN Linear Collider , with the same abbreviation) for electrons and positrons provides a traveling wave accelerator for energies of the order of 1 tera-electron volt (TeV). Instead of the numerous klystron amplifiers that are otherwise necessary to generate the acceleration power, a second, parallel electron linear accelerator with lower energy is to be used, which works with superconducting cavities in which standing waves are formed. High-frequency power is extracted from it at regular intervals and transmitted to the main accelerator. In this way, the very high acceleration field strength of 80 MV / m is to be achieved.

Kielfeld accelerator

In the case of cavity resonators, the dielectric strength limits the maximum acceleration that can be achieved within a certain distance. This limit can be circumvented in Kielfeld accelerators : A laser or particle beam excites an oscillation in a plasma , which is associated with very strong electric field strengths. This means that significantly more compact linear accelerators can possibly be built.

commitment

Ion linear accelerator

In basic physical research, linear accelerators are generally used for the same purposes as ring accelerators . With linear ion accelerators based on standing waves, for example, the proton energy of 800 MeV is achieved at a current strength (time average) of the beam of 1 mA ( LANSCE accelerator in Los Alamos National Laboratory , USA). Systems for lower proton energies, for example, routinely reach 25 mA. For purposes of applied research, even higher currents may be necessary: ​​the prototype (under construction, as of 2012) of a linear accelerator for the IFMIF project is supposed to deliver a cw- deuteron beam of 125 mA.

Practically every ion synchrotron facility uses a linear accelerator as a preliminary stage.

Electron linear accelerator

Medical linear electron accelerator for cancer therapy at the UKSH Campus Kiel

With electrons, linear accelerators have the advantage over ring accelerators that the loss of energy due to synchrotron radiation is avoided. This is why colliders for electrons with very high energy are also built with linear accelerators. The planned International Linear Collider provides for two linear accelerators facing each other, each about 15 km in length and an end energy of up to 500 GeV per particle.

The existing SLC electron accelerator in Stanford, part of a collider system, uses traveling wave technology to generate a beam of 45 GeV and a time-averaged 670 nA (nanoampere). The SAME accelerator provides with the above superconducting resonators almost 40 MeV a cw beam of 1.6 mA, but in short pulses up to several hundred amps.

Some free-electron lasers work with a linear accelerator, for example the FLASH system in the DESY research center. In racetrack microtrons , too , the actual acceleration takes place using an electron linac.

There are practical applications above all for shorter electron linear accelerators. They are mostly used to generate X-rays , mainly in medical devices for radiation therapy . Many of these devices are set up for alternating use of both types of radiation, electron beam and X-ray radiation. Typical for medical devices are beam currents of about 1 mA averaged over time for X-ray generation or 1 µA for direct electron irradiation.

However, electron linear accelerators are also increasingly used in industry. As for other types of accelerators, they can be used for a wide range of applications, ranging from radiographic testing of thick-walled components and x- raying freight containers to radiation sterilization or food irradiation .

literature

General:

About induction linear accelerators:

Individual evidence

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See also

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