High-Gradient Test of a 3 GHz Single-Cell Cavity
ProÂton and carÂbon ion beams preÂsent adÂvanÂtaÂgeous depth-dose disÂtriÂbuÂtions with reÂspect to X-rays. CarÂbon ions allow a betÂter conÂtrol of "raÂdioreÂsisÂtant" tuÂmours due to their highÂer biÂoÂlogÂiÂcal reÂsponse. For deep-seatÂed tuÂmours proÂton and carÂbon ion beams of some nA and enÂerÂgies of about 200 MeV and 400 MeV/u reÂspecÂtiveÂly are needÂed. For these apÂpliÂcaÂtions TERA proÂposed the "cyÂclinac": a high-freÂquenÂcy linac which boosts the hadrons acÂcelÂerÂatÂed by a cyÂclotron. The diÂmenÂsions of the comÂplex can be reÂduced if highÂer acÂcelÂerÂatÂing graÂdiÂents are achieved in the linac. To test the maxÂiÂmum achievÂable fields, a 3 GHz cavÂiÂty has been built by TERA. The 19 mm-long cell is foreÂseen to be exÂcitÂed at 200 Hz by 3 us RF pulsÂes and should reach a 40 MV/m acÂcelÂerÂatÂing graÂdiÂent, which corÂreÂsponds to a peak surÂface elecÂtric field Es of 260 MV/m. In a first high-powÂer test perÂformed at CTF3 the cell was opÂerÂatÂed at 50 Hz with a maxÂiÂmum peak power of 1 MW. The maxÂiÂmum Es achieved was above 350 MV/m. The breakÂdown rate at these field valÂues was around 10-1 bpp/m. The maxÂiÂmum value of the modÂiÂfied PoyntÂing vecÂtor is close to the best valÂues achieved by high graÂdiÂent strucÂtures at 12 and 30 GHz.
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