Novel Technique for Direct Measurement of the Plasma Diffusion Coefficient in Magnetized Plasma

Abstract

The diffusion coefficient in the edge plasma of fusion devices is measured by a new technique. The method is based on studying the decay of the plasma fluctuation spectrum inside the so‐called “Ball‐pen probe”: a small ceramic tube having its mouth flush with a magnetic surface and a movable electrode inside the tube. When the electrode protrudes from the tube, the measured signal shows the floating potential fluctuations of the plasma or the ion saturation current outside the tube. Retracting the electrode into the tube, the power spectrum of the signal displays an exponential decay that is different for different frequencies. Assuming a mainly diffusive behaviour of the plasma inside the tube, the spectrum decay length can be used to derive the value of the diffusion coefficient. The measurements were performed at several radial positions in the CASTOR edge region for three different values of Btor. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Interpretation of fast measurements of plasma potential, temperature and density in SOL of ASDEX Upgrade

“…• We tested whether the T e drop may come from the published fear [26] of the BPP's general insensitivity to high frequencies. For shallow insertion of the BPP collector inside its hole (as is the case here) this fear was, however, later disconfirmed in [13,14] using the ESEL simulation.…”

Application of the Ball‐Pen Probe in Two Low‐Temperature Mag‐netised Plasma Devices and in Torsatron TJ‐K

“…The quantity R = I − sat /I + sat expresses the ratio of the electron and ion saturation current, respectively. Figures, a detailed description and experiments performed thus far can be found in our previous works [1,[3][4][5][6][7][8][9][10]. If this is achieved, the floating potential of the probe is equal to the plasma potential, as follows from equation (1).…”

“…The ball-pen probe consists of a metallic collector, which is shielded by an insulating tube; the probe head itself must be oriented perpendicular to magnetic field lines. Figures, a detailed description and experiments performed thus far can be found in our previous works [1,[3][4][5][6][7][8][9][10]. For its simplicity and rugged construction the ball-pen probe is subject to intensive 3D PIC modeling [11,12].…”