Novel technique for direct measurement of the plasma diffusion coefficient in magnetized plasma

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 B tor. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.