Cryogenic radiation effects on electric insulators

Abstract

The radiation allowable at the superconducting magnets of a fusion reactor may be limited by the radiation tolerance of organic and composite electrical insulators. The allowable dose at the magnets will in turn influence the thickness of shielding necessary and therefore the size of the magnets. The response of organic substances to radiation at 4 K cannot be confidently predicted from irradiations at higher temperatures. Therefore, epoxy-based, polyamide, and polyimide insulators chosen from superconducting magnet practice were irradiated in liquid helium to a total gamma dose of 20 MGy (2 × 109rd). Electrical resistivity was measured during and after irradiation and electrical breakdown was measured after irradiation. Mechanical properties measured at 77 K after irradiation included flexure strength, shear strength (of adheslves between copper), and compressive strength. Electrical properties were generally degraded, but the materials should still be useful as insulators. Mechanical strengths, however, were sufficiently decreased to require re-evaluation of some designs. Aluminized Mylar (superinsulation) was severely embrittled.