Modulation of the intensity of suprathermal electron cyclotron second harmonic radiation by magnetic islands in a tokamak

Observations were made of the modulation of the intensity of suprathermal electron cyclotron second harmonic radiation from runaway electrons by m,n = 4,1 magnetic island present during the current rise in the PRETEXT tokamak. This modulation was investigated via a calibrated tunable radiometer, toroidal and poloidal arrays of Mirnov coils, and a calibrated internal Mirnov coil. Frequency scans of the intensity of the radiation indicated that during the MHD activity the radiating runaway population was of very low energy (less than or equal to30 keV). Spectral correlations of the B/sub pol/ fluctuations and the 2omega/sub ce/ intensity modulations were used to localize the modulation radially and poloidally within the magnetic island structure. Calculations of free fall acceleration implied a runaway confinement time of 0.5 msec and intense radial transport (D/sub perpendicular/ approx. 2 x 105 cm2/sec). The observed radiation was modeled with contributions from thermal and runaway electron populations. The model runaway population was of low energy (less than or equal to 30 keV) and carried less than 3 KA. Runaway transport and expected runaway guiding center orbits were considered as possible physical mechanisms for the observed modulation