A New Multi-Channel Capacitive Probe for Measuring 3D Electrostatic Field

A new multichannel hybrid capacitive probe that combines excellent spatial and time resolution has been developed for turbulence measurements [RSI, 88, 023502 92017], [RS, 89, 10J118(2018)]. The probe consists of four stalks, each with four capacitive steel rings, separated radially by 7mm, and two magnetic coils used for alignment purposes. Each ring measures the plasma potential and forms the corner of a square grid that repeated at each radial location results in 16 independent measurements of plasma potential. These 16 measurements allow the determination of the curl of the electric field at the center of the three cubes, verifying that the probe is measuring the plasma potential. Measurements of electric fields provide critical information to understand transport in magnetized plasmas. In Reversed Field Pinch (RFP) plasma, magnetic fields are stochastic in the core, a positive Er develops to reduce the electron transport to the level of the ion transport. The probe was calibrated with an aluminum calibration tool slid over the four probe stalks and connected to a signal generator providing 24V and we observed the ring measurements through an oscilloscope at a frequency range of 5kHz to 200kHz. Which successfully showed the gain was independent of frequency in the range of interest. Details of the probe design, the initial measurement of plasma potential and local radial profile of the electric field vector will be presented.