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An '''electrostatic particle accelerator''' is one of the two main types of [[particle accelerator]]s, in which [[charged particle]]s are accelerated to a high energy by passing through a static [[high voltage]] potential. This contrasts with the other category of particle accelerator, [[Particle accelerator#Oscillating field particle accelerators|oscillating field particle accelerators]], in which the particles are accelerated by passing successively through multiple voltage drops created by oscillating voltages on electrodes. Owing to their simpler design, historically electrostatic types were the first particle accelerators. The two main types are the [[Van de Graaf generator]] invented by [[Robert Van de Graaff]] in 1929, and the [[Cockcroft-Walton accelerator]] invented by [[John Cockcroft]] and [[Ernest Walton]] in 1932. The maximum particle energy produced by electrostatic accelerators is limited by the accelerating voltage on the machine, which is 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.
However, these machines have advantages such as lower cost, the ability to produce continuous beams and higher beam currents that make them useful to industry, so they are by far the most widely used particle accelerators. They are used in industrial irradiating 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. More powerful accelerators usually incorporate an electrostatic machine as their first stage, to accelerate particles to a high enough velocity to inject into the main accelerator.
== Details ==
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== Single-ended machines ==
Using a [[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-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
Once the platform can be electrically charged by one of the above means, some [[Ion 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 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|SF<sub>6</sub>]] which has dielectric constant roughly 2.5 times that of air. However, even in a tank of SF<sub>6</sub> the maximum attainable voltage is around 30 MV. There could be other gases with even better insulating powers, but SF<sub>6</sub> 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.
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=== Confusion with linear accelerators ===
Electrostatic accelerators are often confused with [[linear accelerator]]s (linacs) simply because they can (but do not always) accelerate particles in a straight line, as a linear accelerator does. The difference between them is that an electrostatic accelerator accelerates a charged particle by passing it through a single DC potential difference between two electrodes, while a linear accelerator accelerates a particle by passing it successively through multiple voltage drops created
== Particle energy ==
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