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The ball-pen probe (BPP) is an innovative electric probe for direct measurements of the plasma potential. This probe was developed in IPP Prague and it is based on the Katsumata probe concept. Combined measurements of the plasma potential by a BPP and floating potential by a Langmuir probe provide also the value of the electron temperature. First test of the BPP on the RFX-mod reversed field pinch in Padova has been performed in November 2006. The BPP head, made of boron nitride, is equipped with four graphite collectors, which are positioned at four different radial positions h inside four shafts hollow into the probe head. The radial profile of the plasma potential and also the electron temperature were measured.

The GOLEM tokamak was operated remotely via Internet connection during the 6th International Workshop and Summer School on Plasma Physics. Performances of hydrogen and helium discharges are compared in this paper. It is found, at similar vacuum conditions, that helium discharges are shorter but the breakdown of the working gas can be quite easily achieved at almost the same loop voltage. The plasma current in helium discharges is slightly lower than in the case of hydrogen. Turbulent fluctuations of the floating potential measured by means of an array of Langmuir probes reveal a noticeably different character in the two discharges.

Simultaneous measurements of the ion temperature by the segmented tunnel and Katsumata probe in the CASTOR tokamak are reported. The segmented tunnel probe measures two ion saturation currents Iseg1, Iseg2 towards two segments of the tunnel. The axis of the tunnel is aligned with the total magnetic field. The value of the ratio of these two currents combined with the results of the PIC modeling provides the ion temperature. Design and experimental arrangement of the Katsumata probe enable us to measure the pure ion current by sweeping the applied voltage. The ion temperature is then estimated from the exponential part of the measured I - V characteristic. Simultaneous measurements of the ion temperature by the segmented tunnel and Katsumata probe agree within a few eV. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.

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.

Experimental observations on edge plasma biasing on the CASTOR tokamak are summarized. Three regimes differing in the electrode position and in the biasing voltage are identified. Characteristic features observed at the plasma edge are studied by a set of electric probes and impact of biasing on the edge plasma as well as on global confinement is described.

An extensive (INTAS) research programme started in 2002 to investigate the correlations between, on the one hand, the occurrence of transport barriers and improved confinement in the medium-size tokamaks TEXTOR and T-10 and on the smaller tokamaks FT-2, TUMAN-3M and CASTOR, and on the other hand, electric fields, modified magnetic shear and electrostatic and magnetic turbulence using advanced diagnostics with high spatial and temporal resolution, and various active means to externally control plasma transport. It also requires one to characterize fluctuations of various important plasma parameters inside and outside transport barriers (TBs) and pedestal regions with high spatial and temporal resolution using advanced diagnostics, and to elucidate the role of turbulence driving and damping mechanisms, including the role of the plasma edge properties. Furthermore, one needs to determine the cross-field transport from the measurements and compare this with available theoretical models. This has been done in a strongly coordinated way, exploiting the complementarity of TEXTOR and T-10 and the backup potential of the three other tokamaks, which together have all the relevant experimental tools and theoretical expertise. Physical mechanisms of several TBs have been studied: electron internal transport barriers in T-10 and TEXTOR, ergodization-induced TB in TEXTOR, TB in ohmic discharges in TUMAN-3M, periodic bias-induced TBs in CASTOR. Geodesic acoustic modes (GAM) have been investigated in T-10, TEXTOR and TUMAN-3M. Core turbulence has been characterized in T-10, and small-scale turbulence has been revealed in FT-2. © 2007 IOP Publishing Ltd.

By taking advantage of the inherent difference between the gyro-radii of ions and electrons in a magnetised plasma, we developed a method to measure the plasma potential directly. The principle is based on the concept of the Katsumata probe. The probe collector is hidden inside a tube which screens an adjustable fraction of the electron flux whereas it lets pass most of the ions. In this paper an arrangement of three Katsumata-type probes with different diameters is used in the edge plasma region of the CASTOR tokamak to investigate the influence of the dimension of the probe. The results show almost no influence of the probe diameter on the values of the plasma potential. © Institute of Physics, Academy of Sciences of Czech Republic 2006.

In this paper, the propagation of turbulent events along radial and poloidal probe arrays in the scrape-off layer (SOL) of the tokamak CASTOR [M. Hron, Czech. J. Phys. 49, 181 (1999)] is observed using a novel correlation technique. The typical turbulent structures appear as dipoles on the floating potential signals and as positive bursts on the density. It is found that both dipoles and density bursts propagate over long radial distances in the SOL while propagating poloidally. A comparison of the measured velocities of the potential and density fluctuations shows that the density structures move radially faster than the potential ones. This allows us to understand how the density is convected by the turbulence. The density moves radially along the potential valleys created by the poloidal dipoles without modifying them. In the framework of an interchange turbulence, this property indicates that charge exchange collisions with neutrals in the SOL of CASTOR lead to a high viscosity that damps the zonal flows. © 2006 American Institute of Physics.

A new method for measuring the diffusion coefficient in the edge plasma of fusion devices is presented. The method is based on studying the decay of the plasma fluctuation spectrum inside a small ceramic tube having its mouth flush with a magnetic surface and its axis aligned along the radial direction. The plasma fluctuations are detected by an electrode, radially movable inside the tube. In the experiment described herein, which was performed in the edge region of the CASTOR tokamak, the electrode measured the floating potential. The experimental arrangement is the same used for the direct plasma potential measurements according to the "Ball-pen probe" [1], the design of which is based on the Katsumata probe principle. When the electrode protrudes from the tube, the measured signal shows the floating potential fluctuations of the plasma. Retracting the electrode into the tube, the signal power spectrum displays a decay. This decay is different for different frequencies, and is exponential. Assuming a mainly diffusive behaviour of the plasma inside the tube, the spectrum decay length can be used to derive a value of the diffusion coefficient. The measurement were performed at different radial positions in the CASTOR edge region, so that a radial profile of the diffusion coefficient was obtained. Typical values of D are of (2-3) m2/s, consistent with expectations from the global particle balance. The radial profile shows a tendency of the diffusion coefficient to increase going deeper into the plasma. © Institute of Physics, Academy of Sciences of Czech Republic 2006.

We describe experiments with a biased electrode inserted into the scrape-off layer (SOL) of the CASTOR tokamak. The resulting radial and poloidal electric field and plasma density modification are measured by means of Langmuir probe arrays with high temporal and spatial resolutions. Poloidally and radially localized stationary structures of the electric field (convective cells) are identified and a related significant modification of the particle transport in the SOL is observed. © 2005 IOP Publishing Ltd.

A novel approach to the direct measurement of the plasma potential in magnetized plasmas, using the so-called "ball-pen probe", was recently tested in the CASTOR tokamak. Comparison with the standard technique of plasma potential measurement using the emissive probe is reported. It is found that the plasma potential determined by the emissive probe is systematically lower than that measured by the ball-pen probe. The difference is of the of the of kT e/e. A possible reason of this difference is the space charge occurring in the proximity of the emissive probe.

The poloidal distribution of turbulence in the scrape-off layer (SOL) of the CASTOR tokamak is studied by means of a ring of 32 electric probes covering the whole perimeter of the poloidal cross section. Analysis of floating potential fluctuations in a SOL created in the top part of the machine by shifting the plasma downwards reveals a dominant periodic structure that propagates poloidally in the direction of the Er × B drift. Its poloidal mode number is found to be equal to the local safety factor q. Correlation and pulse propagation analyses show that this high m mode is a signature of a single long flute-like structure that is aligned with the magnetic field and snakes around the torus several times before terminating on the limiter.

In this paper we present the first experimental results obtained in the CASTOR tokamak by using a segmented biasing electrode, which is composed of five segments radially separated by 3 mm. The basic idea of choosing such configuration was to allow a spatial distribution of the biasing voltage in the radial direction. In the described experiments, one or more selected segments can be biased up to +300V, while the remaining segments can be either grounded or floating. Two rake probes measure the edge radial profiles of the floating potential and the ion saturation current at the top and low field side of the torus. A Gundestrup probe, located at the top of the torus, measures the parallel and perpendicular Mach numbers together with the local electron temperature and density. A clear and reproducible transition to a regime with improved particle confinement is routinely observed, if the electrode is inserted deep enough into the plasma (r/a ≈ 0.5) and if one or two segments are biased up to +250V (the remaining segments are floating). The steepening of the radial profiles of the plasma density and potential demonstrate the formation of a transport barrier just inside the last closed flux surface. Fast relaxations of the edge profiles, with a frequency of about 10 kHz or higher, are observed when the value of the average radial electric field within the barrier reaches values of about 20 kV/m and the density gradient increases up to a factor 3 with respect to the ohmic phase. A detailed analysis of the spatial-temporal behavior of these relaxations is presented.

We have developed tunnel probes for localized measurements of the electron and ion temperature in the edge plasma region of smaller tokamaks and stellarators. A normal tunnel probe for Te measurements consists of a metallic tunnel of 5 mm diameter and 5 mm length, with the axis parallel to the magnetic field. One side is open, the other side is closed by a metallic backplate, isolated from the tunnel. If both electrodes are biased negatively, ions flow into the office, and their current is distributed between the tunnel and the backplate. The ratio of the two ion currents is a function of T e. With an additional diaphragm in front of the orifice, the probe becomes ion sensitive, since the electrons are prevented from the tunnel because of their smaller gyroradius, but ions can still reach it. In this way, the perpendicular ion temperature can be derived. By segmenting the tunnel axially into two parts, also an approximate measure for the parallel ion temperature can be found.