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

A new five-pin probe design called the vorticity probe is presented that explicitly measures the vorticity in the ExB flow from floating potentials, independent of any absolute calibration errors. The five Tantalum probe tips are arranged in a diamond pattern with 5 mm tip spacing. The fluctuating floating potential at each tip is measured and used to compute a finite-difference approximation of the ExB vorticity. The probe is tested in the large plasma device (LAPD) [W. Gekelman , Rev. Sci. Instrum. 62, 2875 (1991)], operated with a variable bias between the anode and the chamber wall that creates a sharply localized E-r profile at 30 cm from the axis of the 100 cm 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 15-30 kHz matches the theoretical prediction from the measured dE(r)/dr gradient that reaches 17 kV/m(2) in the B=0.075 T axial magnetic field. (C) 2005 American Institute of Physics.