Electrostatic particle accelerator: Difference between revisions

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{{Short description|Accelerates particles with a static electric field}}

{{additional citations|date=September 2024}}

[[Image:Westinghouse Van de Graaff atom smasher - cutaway.png|thumb|upright=1.5|The [[Westinghouse Atom Smasher]], an early [[Van de Graaff accelerator]] built 1937 at the Westinghouse Research Center in Forest Hills, Pennsylvania. The cutaway shows the fabric belts that carry charge up to the mushroom-shaped high -voltage electrode. To improve insulation the machine was enclosed in a 65  ft. pressure vessel which was pressurized to 120  psi during operation. The high pressure air increased the voltage on the machine from 1  MV to 5  MV.]]

[[File:KEK Cockcroft-Walton Accelerator (1).jpg|thumb|upright=1.5|750 keV [[Cockcroft-WaltonCockcroft–Walton accelerator]] initial stage of the [[KEK]] accelerator in Tsukuba, Japan. The high -voltage generator is right, the ion source and beam tube is at left]]

An '''electrostatic particle accelerator''' is a [[particle accelerator]] in which [[charged particle]]s are accelerated to a high energy by a static [[high voltage|high-voltage]] potential. The reason that only charged particles can be accelerated is that only charged particles are influenced by an electric field, according to the formula F=qE, which causes them to move. This contrasts with the other major category of particle accelerator, [[Particle accelerator#Electrodynamic (electromagnetic) particle accelerators|oscillating field particle accelerators]], in which the particles are accelerated by oscillating electric fields.

Owing to their simpler design, electrostatic types were the first particle accelerators. The two most common types are the [[Van de Graaf generator]] invented by [[Robert Van de Graaff]] in 1929, and the [[Cockcroft-WaltonCockcroft–Walton accelerator]] invented by [[John Cockcroft]] and [[Ernest Walton]] in 1932. The maximum particle energy produced by electrostatic accelerators is limited by the maximum voltage which can be achieved the machine. This is in turn limited by [[electrical breakdown|insulation breakdown]] to a few [[volt|megavolts]]. Oscillating accelerators do not have this limitation, so they can achieve higher particle energies than electrostatic machines.

The advantages of electrostatic accelerators over oscillating field machines include lower cost, the ability to produce continuous beams, and higher beam currents that make them useful to industry. As such, they are by far the most widely used particle accelerators, with industrial applications such as plastic [[shrink wrap]] production, high power [[X-ray machine]]s, [[radiation therapy]] in medicine, [[radioisotope]] production, [[ion implanter]]s in semiconductor production, and sterilization. Many universities worldwide have electrostatic accelerators for research purposes. High -energy oscillating -field accelerators usually incorporate an electrostatic machine as their first stage, to accelerate particles to a high enough velocity to inject into the main accelerator.

== Applications ==

Electrostatic accelerators have a wide array of applications in science and industry. In the realm of fundamental research, they are used to provide beams of [[atomic nuclei]] for research at energies up to several hundreds of [[electron volt|MeV]].

In industry and [[materials science]] they are used to produce ion beams for materials modification, including ion implantation and ion beam mixing. There are also a number of materials analysis techniques based on electrostatic acceleration of heavy ions, including [[Rutherford backscattering spectrometry]] (RBS), [[particle-induced X-ray emission]] (PIXE), [[accelerator mass spectrometry]] (AMS), [[Elastic recoil detection]] (ERD), and others.

Although these machines primarily accelerate [[atomic nuclei]], there are a number of compact machines used to accelerate [[electronselectron]]s for industrial purposes including sterilization of medical instruments, x-ray production, and silicon wafer production.<ref name="hinterberger">{{cite web |last1=Hinterberger |first1=F |title=Electrostatic Accelerators |url=https://cds.cern.ch/record/1005042/files/p95.pdf |website=[[CERN]] |access-date=10 May 2022}}</ref>

A special application of electrostatic particle accelerator are dust accelerators in which nanometer to micrometer sized electrically charged dust particles are accelerated to speeds up to 100&nbsp;km/s.<ref>{{cite journal |last1=Mocker |first1=A. |last2=Bugiel |first2=S. |last3=Auer |first3=S. |last4=Baust |first4=G. |last5=Collette |first5=A. |last6=Drake |first6=K. |last7=Fiege |first7=K. |last8=Grün |first8=E. |last9=Heckmann |first9=F. |last10=Helfert |first10=S. |last11=Hillier |first11=J. |last12=Kempf |first12=S. |last13=Matt |first13=G. |last14=Mellert |first14=T. |last15=Munsat |first15=T. |last16=Otto |first16=K. |last17=Postberg |first17=F. |last18=Röser |first18=H. P. |last19=Shu |first19=A. |last20=Strernovski |first20=Z. |last21=Srama |first21=R. |title=A 2 MV Van de Graaff accelerator as a tool for planetary and impact physics research |journal=Review of Scientific Instruments |date=September 2011 |volume=82 |issue=9 |page=95111-95111-8 |doi=10.1063/1.3637461 |url=https://ui.adsabs.harvard.edu/abs/2011RScI...82i5111M/abstract |access-date=27 April 2022 |bibcode=2011RScI...82i5111M}}</ref> Dust accelerators are used for impact cratering studies,<ref>{{cite journal |last1=Neukun |first1=G. |last2=Mehl |first2=A. |last3=Fechtig |first3=H. |last4=Zähringer |first4=J. |title=Impact phenomena of micrometeorites on lunar surface material |journal=Earth and Planetary Science Letters |date=March 1970 |volume=9 |issue=1 |page=31 |doi=10.1016/0012-821X(70)90095-6 |url=https://ui.adsabs.harvard.edu/abs/1970E%26PSL...8...31N/abstract |access-date=27 April 2022 |bibcode=1970E&PSL...8...31N}}</ref> calibration of [[impact ionization]] dust detectors,<ref>{{cite journal |last1=Grün |first1=E. |last2=Fechtig |first2=H. |last3=Hanner |first3=M. |last4=Kissel |first4=J. |last5=Lindblad |first5=B.A. |last6=Linkert |first6=D. |last7=Maas |first7=D. |last8=Morfill |first8=G.E. |last9=Zook |first9=H. |title=The Galileo Dust Detector |journal=Space Science Reviews |date=May 1992 |volume=60 |issue=1-4 |pages=317–340 |doi=10.1007/BF00216860 |url=https://ui.adsabs.harvard.edu/abs/1992SSRv...60..317G/abstract |access-date=11 February 2022 |bibcode=1992SSRv...60..317G}}</ref> and meteor studies.<ref>{{cite journal |last1=Thomas |first1=E. |last2=Simolka |first2=J. |last3=DeLuca |first3=M. |last4=Horanyi |first4=M. |last5=Janches |first5=D. |last6=Marshall |first6=R |last7=Munsat |first7=T. |last8=Plane |first8=J. |last9=Sternovski |first9=Z. |title=Experimental setup for the laboratory investigation of micrometeoroid ablation using a dust accelerator |journal=Review of Scientific Instruments |date=March 2017 |volume=88 |issue=3 |page=id.034501 |doi=10.1063/1.4977832 |url=https://ui.adsabs.harvard.edu/abs/2017RScI...88c4501T/abstract |access-date=27 April 2022 |bibcode=2017RScI...88c4501T}}</ref>

== Single-ended machines ==

Using a [[high voltage|high-voltage]] terminal kept at a static potential on the order of millions of volts, [[charged particle]]s can be accelerated. In simple language, an [[electrostatic generator]] is basically a giant [[capacitor]] (although lacking plates). The high voltage is achieved either using the methods of [[Cockcroft-WaltonCockcroft–Walton generator|Cockcroft & Walton]] or [[Van de Graaff generator|Van de Graaff]], with the accelerators often being named after these inventors. Van de Graaff's [[Van de Graaff generator|original design]] places electrons on an insulating sheet, or belt, with a metal comb, and then the sheet physically transports the immobilized electrons to the terminal. Although at high voltage, the terminal is a conductor, and there is a corresponding comb inside the conductor which can pick up the electrons off the sheet; owing to [[Gauss's law]], there is no electric field inside a conductor, so the electrons are not repulsed by the platform once they are inside. The belt is similar in style to a [[Conveyor belt|conventional conveyor belt]], with one major exception: it is seamless. Thus, if the belt is broken, the accelerator must be disassembled to some degree in order to replace the belt, which, owing to its constant rotation and being made typically of a [[rubber]], is not a particularly uncommon occurrence. The practical difficulty with belts led to a different medium for physically transporting the charges: a chain of pellets. Unlike a normal chain, this one is non-conducting from one end to the other, as both insulators and conductors are used in its construction. These types of accelerators are usually called [[Pelletron]]s.

Once the platform can be electrically charged by one of the above means, some [[Ionion source|source of positive ions]] is placed on the platform at the end of the beam line, which is why it's called the terminal. However, as the ion source is kept at a high potential, one cannot access the ion source for control or maintenance directly. Thus, methods such as plastic rods connected to various levers inside the terminal can branch out and be toggled remotely. Omitting practical problems, if the platform is positively charged, it will repel the ions of the same electric polarity, accelerating them. As E=qV, where E is the emerging energy, q is the ionic charge, and V is the terminal voltage, the maximum energy of particles accelerated in this manner is practically limited by the discharge limit of the high -voltage platform, about 12 &nbsp;MV under ambient atmospheric conditions. This limit can be increased, for example, by keeping the HV platform in a tank of an [[insulating gas]] with a higher [[dielectric constant]] than air, such as [[Sulfur hexafluoride|SF₆]] which has dielectric constant roughly 2.5 times that of air. However, even in a tank of SF₆ the maximum attainable voltage is around 30 &nbsp;MV. There could be other gases with even better insulating powers, but SF₆ is also chemically [[Chemically inert|inert]] and non-[[Toxicity|toxic]]. To increase the maximum acceleration energy further, the [[tandem]] concept was invented to use the same high voltage twice.

== Tandem accelerators ==

[[File:Nuclear accelerator in NCSR Demokritos.jpg|thumb|Electrostatic [[Van de Graaff generator#Tandem accelerators|Van de Graaff]] Tandem nuclear accelerator at [[National Centre of Scientific Research "Demokritos"|NCSRD]] in Greece]]

Conventionally, positively charged ions are accelerated because this is the polarity of the atomic nucleus. However, if one wants to use the same [[Static electricity|static electric]] potential twice to accelerate ions[[ion]]s, then the polarity of the ions' charge must change from anions to cations or vice versa while they are inside the conductor where they will feel no electric force. It turns out to be simple to remove, or strip, electrons from an energetic ion. One of the properties of ion interaction with matter is the exchange of electrons, which is a way the ion can lose energy by depositing it within the matter, something we should intuitively expect of a projectile shot at a solid. However, as the target becomes thinner or the projectile becomes more energetic, the amount of energy deposited in the foil becomes less and less.

Tandems locate the ion source outside the terminal, which means that accessing the ion source while the terminal is at high voltage is significantly less difficult, especially if the terminal is inside a gas tank. So then an anion beam from a [[sputter]]ing ion source is injected from a relatively lower voltage platform towards the high -voltage terminal. Inside the terminal, the beam impinges on a thin foil (on the order of micrograms per square centimeter), often [[carbon]] or [[beryllium]], stripping electrons from the ion beam so that they become cations. As it is difficult to make anions of more than -1 charge state, then the energy of particles emerging from a tandem is E=(q+1)V, where we have added the second acceleration potential from that anion to the positive charge state q emerging from the stripper foil; we are adding these different charge signs together because we are increasing the energy of the nucleus in each phase. In this sense, we can see clearly that a tandem can double the maximum energy of a proton beam, whose maximum charge state is merely +1, but the advantage gained by a tandem has diminishing returns as we go to higher mass, as, for example, one might easily get a 6+ charge state of a [[silicon]] beam.

It is not possible to make every element into an anion easily, so it is very rare for tandems to accelerate any [[noble gas]]es heavier than [[helium]], although KrF⁻ and XeF⁻ have been successfully produced and accelerated with a tandem.<ref>{{cite journal | journal=Nuclear Instruments and Methods in Physics Research Section B | volume=5 | issue=2 | pages=217 | year=1984 |author1=Minehara, Eisuke |author2=Abe, Shinichi |author3=Yoshida, Tadashi |author4=Sato, Yutaka |author5=Kanda, Mamoru |author6=Kobayashi, Chiaki |author7=Hanashima, Susumu | title= On the production of the KrF- and XeF- Ion beams for the tandem electrostatic accelerators | doi = 10.1016/0168-583X(84)90513-5 |bibcode = 1984NIMPB...5..217M }}</ref> It is not uncommon to make compounds in order to get anions, however, and [[titanium hydride|TiH₂]] might be extracted as TiH⁻ and used to produce a proton beam, because these simple, and often weakly bound chemicals, will be broken apart at the terminal stripper foil. Anion ion beam production was a major subject of study for tandem accelerator application, and one can find recipes and yields for most elements in the Negative Ion Cookbook.<ref>Middleton, R: ''A Negative Ion Cookbook'', University of Pennsylvania, unpublished, 1989 [http://www.pelletron.com/cookbook.pdf Online pdf]</ref> Tandems can also be operated in terminal mode, where they function like a single-ended electrostatic accelerator, which is a more common and practical way to make beams of noble gases.

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== Geometry ==

One trick which has to be considered with electrostatic accelerators is that usually vacuum beam lines are made of steel. However, one cannot very well connect a conducting pipe of steel from the high -voltage terminal to the ground. Thus, many rings of a strong glass, like [[Pyrex]], are assembled together in such a manner that their interface is a vacuum seal, like a copper [[gasket]]; a single long glass tube could implode under vacuum or fracture supporting its own weight. Importantly for the physics, these inter-spaced conducting rings help to make a more uniform electric field along the accelerating column. This beam line of glass rings is simply supported by compression at either end of the terminal. As the glass is non-conducting, it could be supported from the ground, but such supports near the terminal could induce a discharge of the terminal, depending on the design. Sometimes the compression is not sufficient, and the entire beam line may collapse and shatter. This idea is especially important to the design of tandems, because they naturally have longer beam lines, and the beam line must run through the terminal.

Most often electrostatic accelerators are arranged in a horizontal line. However, some tandems may have a "U" shape, and in principle the beam can be turned to any direction with a magnetic dipole at the terminal. Some electrostatic accelerators are arranged vertically, where either the ion source or, in the case of a U-shaped vertical tandem, the terminal, is at the top of a tower. A tower arrangement can be a way to save space, and also the beam line connecting to the terminal made of glass rings can take some advantage of gravity as a natural source of compression.