E. Martines

A novel approach to direct measurement of the plasma potential

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 indentical 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 perfomred in the CASTOR tokamak.

Dynamics of turbulent transport in the scrape-off layer of the CASTOR tokamak

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 et al., 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 z...

Emissive probe measurements of plasma potential fluctuations in the edge plasma regions of tokamaks

The plasma potential Φpl and its fluctuations Φpl were measured by electron emissive probes in the edge plasma regions of two fusion experiments: the Instituto Superior Tecnico Tokamak (ISTTOK) (Lisbon, Portugal), and the Czech Academy of Sciences Torus (CASTOR) tokamak (Prague, Czech Republic). Into ISTTOK, three emissive probes were inserted outside the last closed flux surface (LCFS) on different minor radii. In CASTOR, two emissive probes, poloidally separated, and two cold cylindrical probes, mounted on the same shaft, were used, which could be radially shifted outside and inside the LCFS. The advantages of a sufficiently emissive probe are that in principle Φpl and Φpl can be measured directly, without being affected by electron temperature fluctuations or drifting electrons.

Formation of convective cells during scrape-off layer biasing in the CASTOR tokamak

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.

Tunnel probes for measurements of the electron and ion temperature in fusion plasmas

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 Te. 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.

New insights into MHD dynamics of magnetically confined plasmas from experiments in RFX

The experimental and theoretical activity performed in the RFX device has allowed a deeper insight into the MHD properties of the reversed field pinch (RFP) configuration. A set of successful experiments has demonstrated the possibility of influencing both the amplitude and the spectrum of the magnetic fluctuations which characterize the RFP configuration. A new regime (quasi-single-helicity states) where the dynamo mechanism works in a nearly laminar way and a helical core plasma is produced has been investigated. With these studies a reduction of magnetic chaos has been obtained. The continuous rotation of wall locked resistive tearing modes has been obtained by an m = 0 rotating perturbation. This perturbation induces rotation of m = 1 non-linearly coupled modes.

Multi-machine studies of the role of turbulence and electric fields in the establishment of improved confinement in tokamak plasmas

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.

Transport mechanisms and enhanced confinement studies in RFX

The results of an extensive study on transport mechanisms and on improved confinement scenarios in RFX are reported. The scaling of the thermal conductivity in the core with the Lundquist number indicates that the magnetic field in this region is not fully stochastic, as proved by the existence of thermal barriers observed in single helicity configurations. The electrostatic transport at the edge has been proved to depend on the highly sheared E × B flow, which has been interpreted using fluid and Monte Carlo models. Regimes of improved confinement have been obtained in the core by poloidal current drive techniques, and the electrostatic transport has been reduced at the edge by biasing experiments. A radiation mantle caused by impurity seeding has been found to successfully reduce the local plasma-wall interaction without causing a significant deterioration in the plasma performance.