I. S. Nedzelskiy

Improved confinement events triggered by emissive electrode biasing on the tokamak ISTTOK

Edge polarization experiments were carried out on ISTTOK using an emissive electrode. The dependence of the bias current on the radial electric field, measured by a radial array of Langmuir probes, is in qualitative agreement with the expression derived by Stringer for the transitional region between plateau and collisional transport regimes (Stringer T.E. 1993 Nucl. Fusion 33 1249). Improved particle confinement is clearly observed for negative bias associated with a large radial electric field. In some cases, a double-peaked profile of the radial electric field is observed just after biasing, evolving afterwards to a single-peaked profile as confinement improves. These types of profiles may be related to the multiple solutions of the non-linear equation for the radial electric field, recently put forward by Kasuya et al (2003 Nucl. Fusion 43 244). Above a certain threshold of the bias current stronger improved confinement events are observed, during short periods, for both bias polarities.

Upgrade of the lithium beam diagnostic at JET

A 60 kV neutral Li beam is injected into the edge plasma of JET to measure the electron density. The beam observation system has been improved by replacing a Czerny-Turner spectrometer with a high-resolution transmission-grating spectrometer and a fast back-illuminated frame-transfer camera. The larger throughput of the spectrometer, the increased sensitivity, and the faster readout of the new camera allow inter-ELM (edge localized mode) measurements (frame rate of 100 Hz). The calibration of the setup, as well as an improved spectral fitting technique in the presence of carbon background radiation, is discussed in detail. The density calculation is based on a statistical analysis method. Results are presented for different plasma scenarios.

Control of the edge turbulent transport by emissive electrode biasing on the tokamak ISTTOK

In this paper results are presented on the changes induced by emissive electrode biasing in the ISTTOK edge transport. The boundary plasma is characterized with focus on the relation between E × B sheared flows and particle transport. We suggest that the distinct behaviour of the particle confinement for positive and negative bias observed in ISTTOK is related to the low E × B shear induced by positive bias in the core periphery region associated with the appearance of large amplitude fluctuations. In addition, the effect of the electrode bias on the edge turbulent transport has been investigated identifying the changes induced in the fluctuations frequency spectrum and probability density function. We have shown that negative electrode bias reduces the propagation of large-scale events, making the fluctuations distribution more Gaussian and resulting in low amplitude fluctuations across most of the edge plasma region. For positive bias, large amplitude broad spectrum fluctuations appear in the core periphery, which increase the cross-field diffusion and contribute to the observed asymmetry in particle transport with the bias polarity.

Limiter Biasing Experiments on the Tokamak ISTTOK

The influence of the polarization of a localized limiter on the global and edge parameters is described. In particular, the effect of varying the limiter position and the bias voltage is investigated. Improvement in confinement has been obtained for both positive and negative limiter bias as a result of the creation of large radial electric fields in the region just inside the fixed limiter. Furthermore, it has been observed that the modification in the floating potential due to limiter biasing is proportional to the collected current.

Measurements of the plasma potential by HIBP using a TOF method

A heavy ion beam diagnostic is being developed for the tokamak ISTTOK (R=0.46 m, a=0.085 m, Bt≅0.5 T, Ip=6–9 kA), based on multiple cell array detectors, aimed at the study of the time variation of the plasma density, electron temperature, poloidal magnetic field, and plasma potential radial profiles. In this article we describe an alternative method to the traditional electrostatic energy analyzers for measurements of the plasma potential based on the time-of-flight technique. The initial results of the measurements of the changes on the average plasma potential during tokamak discharges with minor disruptions are also presented. These results have been obtained measuring the time-of-flight of the ions of a pulsed primary beam from the electrostatic plates of the ion gun to the primary detector.

The first operation of the advanced heavy ion beam probing diagnostic on the TJ-II flexible heliac

An advanced heavy ion beam probing diagnostic (HIBP) based on simultaneous utilization of two different systems for the secondary ions detection (a multiple cell array detector and 30° Proca-green electrostatic energy analyzer) has been developed for the TJ-II flexible heliac. This innovative design aims at enlarging the HIBP capabilities in order to allow the instantaneous measurements of electron density and plasma potential profiles together with their respective fluctuations. This paper presents the detailed description of the main parts of the HIBP equipment and their characteristics obtained during the first operation on TJ-II. Special attention is employed to the control and data acquisition system built on two VME controllers. The results of the diagnostic beam carrying through the magnetic structure of TJ-II into secondary beam line are presented and compared with the trajectory calculations. The problems of plasma loading of the detection system and HX-ray excitement by the probing beam are considered together with the ways of their solution.

New detection system and signal processing for the tokamak ISTTOK heavy ion beam diagnostic

The tokamak ISTTOK havy ion beam diagnostic (HIBD) operates with a multiple cell array detector (MCAD) that allows for the plasma density and the plasma density fluctuations measurements simultaneously at different sampling volumes across the plasma. To improve the capability of the plasma density fluctuations investigations, a new detection system and new signal conditioning amplifier have been designed and tested. The improvements in MCAD design are presented which allow for nearly complete suppression of the spurious plasma background signal by applying a biasing potential onto special electrodes incorporated into MCAD. The new low cost and small size transimpedance amplifiers are described with the parameters of 400 kHz, 10(7) V/A, 0.4 nA of RMS noise, adequate for the plasma density fluctuations measurements.

Radial profile measurements of plasma pressure-like fluctuations with the heavy ion beam diagnostic on the tokamak ISTTOKa)

The Heavy Ion Beam Diagnostic (HIBD) on the tokamak ISTTOK (Instituto Superior Técnico TOKamak) has been modified, in terms of signal conditioning, to measure the local fluctuations of the neσ1,2(Te) product (plasma density times the effective ionization cross-section) along the tokamak minor diameter, in 12 sample volumes in the range of -0.7a < r < 0.7a, with a maximum delay time of 1 μs. The corresponding signals show high correlation with the magnetic Mirnov coils in the characteristic MHD frequency range of ISTTOK plasmas and enable the identification of tearing modes. This paper describes the HIBD signal conditioning system and presents a preliminary analysis of the radial profile measurements of local neσ1,2(Te) fluctuations.

Simultaneous measurements of the parallel and perpendicular ion temperature with a pinhole probe in the scrape-off-layer of the tokamak ISTTOK

A pinhole probe (PHP) for the simultaneous measurement of the parallel, T(parallel), and perpendicular, T(perpendicular), ion temperature has been designed and tested in the scrape-off-layer (SOL) plasma of the tokamak ISTTOK. The PHP consists of a tunnel immersed into the plasma parallel to magnetic field and an ion collector. One end of the tunnel is covered with a thin foil that has a pinhole sampling ions from the plasma. The other end of the tunnel (close to the negatively biased collector) is covered with a fine-mesh screen. The possibility of performing an analytical description of the PHP current-to-voltage characteristics obtained on the collector when biasing the tunnel simplifies the interpretation of the results. The PHP operation has been previously tested in T(parallel), T(perpendicular) measurements in low temperature weekly magnetized plasma [H. Mase, T. Honzava, and G. Miyamoto, J. Appl. Phys. 49(10), 5171 (1978)]. In this paper, the PHP operation in the SOL of the tokamak ISTTOK is described, and the first results of T(parallel) and T(perpendicular) measurements are presented. The obtained results demonstrate strong (~30%) variation of T(parallel) and T(perpendicular) on a time scale of 0.5 ms, and general predominance of T(parallel) > T(perpendicular) anisotropy (T(parallel mean)/T(perpendicular mean) ~ 1.5) during plasma shot.

Modified biased split detectors for the HIBP electrostatic energy analyzer

In electrostatic energy analyzers of heavy ion beam probes the required accuracy of plasma potential measurements is provided by differential detection of the ion beam on the split detector. Secondary electrons created on the detector surface by the analyzed ions and ultraviolet plasma radiation can strongly disturb the measurements. In this article we consider the influence on analyzer operation of secondary electrons emitted from the detector, describe the relation for corresponding errors in the plasma potential measurements, and present two modified biased split detectors which avoid these errors.