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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.

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.

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.

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.

A novel probe and approach to the direct measurements of the plasma potential in a strong magnetic field is suggested. The principle of this method is to reduce the electron saturation current to the same magnitude as that of the ion saturation current. In this case, the floating potential of the probe becomes identical to the plasma potential. This goal is attained by a shield, which screens off an adjustable part of the electron current from the probe collector due to the much smaller gyro-radius of the electrons. First systematic measurements have been performed in the CASTOR tokamak.

A new probe head, composed of two electro-emissive probes and two Langmuir probes has been designed to measure edge fluctuations in the CASTOR tokamak. In this contribution, only one Langmuir and one electron-emissive probe are used to measure the temperature and potential fluctuations and the phase angle between them.

A fundamental role in fusion plasmas transport processes is played by electrostatic turbulence. In the edge region of toroidal experiments, the use of Langmuir probes allows a detailed investigation of the temporal and spatial structure of this turbulence. In recent years, the issue of self-similarity and its violation, termed intermittency, has received substantial attention due to its implications in terms of universality of turbulence behaviour. In particular, a general trend has been identified, by which the probability distribution function (PDF) of the fluctuations is almost gaussian at large scales, and develops strong tails at small scales. this behaviour, which looks independent from the magnetic configuration of the plasma, is due to strong events which occur at small scales more frequently than in the gaussian case. In the RFX reversed field pinch experiment, by the use of specific detection algorithms and of conditional averaging, it has been possible to associate the intermittent events to coherent structures present in the turbulence. It has also been demonstrated that these structures play an important role in determining the overall turbulence-driven transport. In the CASTOR tokamak, a similar kind of behaviour is observed. Furthermore, the use of one and two-dimensional arrays of probes has allowed to perform an imaging of the turbulence. This approach has revealed the presence of large scale coherent structures with a very long lifetime.