Influence of swift heavy ion beams and protons on the dielectric strength of polyimide

The breakdown voltage of radiation damaged polyimide is investigated. Motivated by the application of polyimide as insulator used in ion accelerator magnets of the future Facility for Antiproton and Ion Research (FAIR), two different kinds of polyimides were irradiated with different high energetic ion beams (p, C, Ni, Ru and Au) and with gamma radiation from a Co-60 source. Breakdown voltage measurements showed that the dielectric strength of irradiated polyimide was found to decrease as a function of the irradiation dose. The rate of decrease was found to be dependent on i) the type of radiation and ii) the angle of incident beam. Gamma and proton radiation leads only to minor changes in the observed dose regime while heavy ion irradiation drastically decreases the dielectric strength at even low doses. For heavy ion irradiation the rate of decrease is found to be dependent on the energy loss of the used particle beams. Furthermore an increase in incident beam angle, i.e. closer to the surface normal gives a lower decrease in the dielectric strength suggesting that the breakdown follows the path length of the produced ion tracks. Weibull analysis was used on a selected data set to discuss failure expectations for the later FAIR magnets.