Vorticity probes and the characterization of vortices in the Kelvin–Helmholtz instability in the large plasma device experiment

Abstract

A new five-pin probe design called the vorticity probe is presented that explicitly measures the vorticity in the E×B flow from floating potentials, independent of any absolute calibration errors. The five Tantalum probe tips are arranged in a diamond pattern with 5mm tip spacing. The fluctuating floating potential at each tip is measured and used to compute a finite-difference approximation of the E×B vorticity. The probe is tested in the large plasma device (LAPD) [W. Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)], operated with a variable bias between the anode and the chamber wall that creates a sharply localized Er profile at 30cm from the axis of the 100cm diameter chamber. The fluctuations are peaked in the shear flow layer and are correlated with theoretical calculations of the Kelvin–Helmholtz instability for this plasma. The spectrum at 15–30kHz matches the theoretical prediction from the measured dEr∕dr gradient that reaches 17kV∕m2 in the B=0.075T axial magnetic field.