Thermal distribution of a carbon foil used to extract a 30-MeV proton beam

Abstract

Modern cyclotron technology employs negative hydrogen (H⁻) ion acceleration and positive hydrogen (H⁺) ion (proton) extraction using a carbon foil, which strips two electrons. The stripping efficiency depends on the thickness of the carbon foils. Typically, a 100-µg/cm² carbon foil extracting 30-MeV protons at beam currents up to 250 µ A has a lifetime of ∼120 h (∼30,000 µAh accumulated current). However, the lifetime was found to be short when the cooling was poor during beam irradiation. Therefore, the high thermal power dissipated in the foil by charged particles (both protons and electrons) will be a serious problem for high current extractions during long-run operation. In this research, the thermal distribution of carbon foils for both protons and electrons is analyzed for extracting proton currents up to 1 mA. An optimized carbon foil will be discussed for the sake of extending lifetime expected ∼100 hours or longer at a 1-mA extraction. Thickness ranges of the strippers from 10 to 1,000 µg/cm² are investigated by estimating the peak temperatures reached on the foils during proton-beam extraction. Reducing the peak temperatures calculated by modeling may be possible by decreasing the foil thickness, enlarging the beam size, and effectively reducing the ion-induced heat.