Thermal stress analysis of 1 MW gyrotron collector

During 2004 and 2005, collectors on three of the gyrotrons used at DIII-D failed due to stress cracks. In order to investigate reasons for these failures, a nonlinear elastic plastic thermal stress analysis of the collector/design was undertaken. The thermal stress analysis results indicated that the effective strain for oxygen free high conductivity copper material under the operating conditions limited the cycle life of the collector due to fatigue, resulting in failures.The desired service life of more than 105 thermal cycles can be obtained by: (1) operational changes, such as increasing the frequency and amplitude of sweeping to reduce the effective heat flux, (2) design changes, such as: increasing the height and/or diameter of collector, enhancing the heat transfer coefficient by roughening the coolant channel walls, or (3) changing the material of the collector to dispersion strengthened copper such as Glidcop. The analysis and conclusions are presented.