Tandem accelerator

Conventionally, positively charged ions are accelerated because this is the polarity of the atomic nucleus. However, if one wants to use the same static electric potential twice to accelerate ions, 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. However, as the target becomes thinner or the projectile becomes more energetic, the amount of energy deposited in the conductor foil becomes less and less.

Tandem accelerators 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 (with negative charge) from a sputtering ion source is injected from a relatively lower voltage platform towards the high voltage terminal with a positive voltage. Inside the terminal, the beam impinges on a thin metal 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 V is the voltage, and q is the positive charge of ions emerging from the stripper foil. These different charge signs add together because the energy of the ion increases in each stage. A tandem can double the maximum energy of a proton beam, which starts with charge −1, and whose maximum charge state is merely +1. The advantage gained by a tandem has diminishing returns with higher atomic weights, as, for example, one might easily get a 6+ charge state in a silicon ion beam.

It is not possible to make every element into an anion easily, so it is very rare for tandems to accelerate any noble gases, although KrF− and XeF− have been successfully produced and accelerated with a tandem.[2] It is not uncommon to make compounds in order to get anions, however, and 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.^[1][3] 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.

The name 'tandem' originates from this dual-use of the same high voltage, although tandems may also be named in the same style of conventional electrostatic accelerators based on the method of charging the terminal.

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An outline of the tandem accelerator system, with animation

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