PBFA-II Vacuum Insulator Stack Failure Mechanisms

摘要

The PBFA-II accelerator includes a large-radius, vertical-axis vacuum insulator stack. The possible failure of the acrylic rings in the stack from electron- or gamma-induced charge buildup is being evaluated. The induced static charges could remain for many hours, and either type of irradiation might cause dendrites to form. Aluminum grading rings sandwiched between the acrylic affect charge accumulation; the acrylic would preferentially break down to these grading rings. The charge buildup and the bremsstrahlung dose could depend critically upon the directionality and position of the electron loss. This paper concerns the effects of electron loss that occurs in the vicinity of the ion diode, where the electrons have energies of about 30 MeV. Monte Carlo electron-photon transport calculations indicate that the bremsstrahlung dose expected in an acrylic ring once diode experiments begin in 1986 could be as much as 5 krads per shot, with roughly half of the photon energy above 5 MeV. Moreover, the calculations indicate that the charge deposition in an individual acrylic ring might exceed 2 x 10 exp 11 electrons/cm exp 2 . (ERA citation 10:035777)