Observation of cross-field plasma rotation in CDX-U using a simple Mach probe

Abstract

Evidence is presented that a simple, relatively nonperturbing Mach probe can measure plasma flow across a magnetic field. Measurements of poloidal flow in the CDX-U plasma made by a simply constructed Mach probe are presented.

Poloidal plasma rotation and its effect on fluctuations in a toroidal plasma

“…Recently, a Mach probe similar to the one that we have in BETA was used to measure the poloidal flow in the pure toroidal plasma of the Current Drive Experiment-Upgrade (CDX-U) machine [25]. The plasma production mechanism and the equilibrium maintenance in CDX-U are similar to those in the BETA device.…”

“…The radially movable Mach probe consists of a cylindrical insulator housing with two 7.5 mm diameter discs mounted in it back to backone looking at the upstream flow of the poloidal rotation and the other at the downstream flow. The asymmetry in the ion saturation current collected by these two discs of the Mach probe represents the poloidal plasma flow [25]. The geometry of the Mach probe is such that the parallel flow will not interfere with the measurements unless the pitch angle of the magnetic field lines is >45O (implying a toroidal current offew kA, too large to be produced by biasing or any asymmetries).…”

Generation of Radial Electric Field with Electrode Biasing

“…16 The value of the poloidal ow velocity cannot bedecided directly, but it is proportional C s , where C s is the ion acoustic velocity, = (I u ; I d )=(I u + I d ) is the ratio of the collective ion saturation currents up and down stream along the poloidal direction. 17 The standard two-point cross-correlation technique 18 was used to obtain the spectral feature of uctuations, including the turbulence induced particle ux ; f . The proles were measured with a shot-to-shot scan, and the measurements at each radial position were carried out in several reproducible discharges.…”