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Ion cyclotron transmission spectroscopy in the Tokamak Fusion Test Reactor

The propagation of waves in the ion cyclotron range of frequencies has been investigated experimentally in the Tokamak Fusion Test Reactor (TFTR). A small, broadband, radiofrequency (RF) magnetic probe located outside the plasma limiter, at a major radius near that of the plasma centre, was excited with a low power, frequency swept source (1-200 MHz). Waves propagating to a distant location were detected with a second, identical, probe. The RF transmission spectrum revealed a region of attenuation over a band of frequencies for which the minority fundamental resonance was located between the outer plasma edge and tile major radius of the probe location. Distinct, non-overlapping attenuation bands were observed from hydrogen and 3He minority species; a distinct tritium band should be observed in future DT experiments. Rapid spectrum acquisition during a 3He gas puff experiment showed that the wave attenuation involved the plasma core and was not a surface effect. A model in which the received power varied exponentially with the minority density, averaged over the resonance region, fitted the time evolution of the probe signal relatively well. An estimation of the one dimensional tunnelling parameter from the experimental observations is discussed. Minority concentrations of less than 0.5% can be resolved with this measurement