Instrumental aspects of extraordinary mode scattering on TFTR

Abstract

Initial results from extraordinary mode microwave scattering on TFTR show that the magnitude of density fluctuations approximately follows the mixing length prediction in both beam-heated and ohmic discharges and that the most important part of the k spectrum falls below k⊥ρs=0.5 at a scale unresolved in these measurements. These results are consistent with most scattering measurements done in the ordinary mode on smaller devices. We observe only electron diamagnetic spectral shifts in ohmic discharges while beam-heated discharges display a shift which depends on the sign of the toroidal flow velocity. The spatial resolution of the system inferred from ray tracing is consistent with the measured toroidal velocity profiles and measured spectral shapes. There are some limitations on our measurements caused by a relatively large ‘‘ghost’’ component in the low-frequency spectrum which cannot be identified with simple scattering from the crossing region of the incident and scattering antenna patterns. This component has some of the average features of scattering in the central region but does not have the frequency offset expected for drift waves and does not exhibit a Doppler shift when the plasma is rotating. We show that this ghost feature can be explained by non-Gaussian wings on the transmitting and receiving antennas which have a response to the high fluctuation levels at low values of k.