Development of a multi-channel capacitive probe for electric field measurements with fine spatial resolution*

A capacitive is one of a few diagnostics that is directly sensitive to the plasma potential. A Multi-Channel Linear Capacitive Probe (MLCP) for turbulence measurements has been developed. The MLCP has 10 spatial channels and provides 9 points of radial electric field measurements simultaneously with the spatial separation of 7 mm. The electrodes are stainless steel cylinders 2 mm long and 11 mm in diameter. These 10 electrodes are separated by 5 mm resulting into 5.7 mm of radial sensitivity to plasma potential. A BN cylinder with 2 mm wall thickness is the dielectric. The time response of this probe has been improved by a new readout circuit. A correction technique for low frequency attenuation is also developed to achieve the required time resolution. The performance of the MLCP is tested using reversed field pinch plasma; and to confirm that the MLCP resolves sub-centimeter structures and plasma potential fluctuations up to 680 kHz. Two of these probes, separated by about 5 m, were used for zonal flow study, long wave length structure in plasma potential. Probe design, read out circuit, spatial resolution, and gain calibration will be presented. Spectra of electric field fluctuation in standard and improved confinement, PPCD plasma will be presented.