Electrostatic particle accelerator: Difference between revisions

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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 | url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TJN-470P206-8H&_user=107951&_coverDate=11%2F30%2F1984&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_acct=C000008318&_version=1&_urlVersion=0&_userid=107951&md5=73a7003196ba8915a2830c37adcd4e4f| archive-url=https://archive.today/20130202104530/http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TJN-470P206-8H&_user=107951&_coverDate=11/30/1984&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_acct=C000008318&_version=1&_urlVersion=0&_userid=107951&md5=73a7003196ba8915a2830c37adcd4e4f| dead-url-status=yesdead| archive-date=2013-02-02|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.

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.