J.A.C. Cabral

Engineering aspects of the tokamak ISTTOK

AbstractThe first Portuguese magnetic confinement experiment, the tokamak ISTTOK, has been in operation since 1993. This tokamak device is described and the main technological features, as well as the novel techniques of its diagnostics and control and data acquisition system, are reported. A synopsis of the experimental activity is also presented.

Enhancement of the ISTTOK plasma confinement and stability by negative limiter biasing

Experimental results concerning the plasma response to the biasing of the tokamak ISTTOK localized limiters, on a strong flat-top plasma current reference discharge, are reported. Modifications of central beta as well as of energy confinement time are determined through time-resolved measurements of the line-averaged plasma density, electron density profile, electron temperature and ohmic power. Gross particle confinement variations are confirmed by the associated changes of the ratio between the line-averaged electron density and the radiation level. Plasma stability modifications are analysed by measurements of the plasma column transverse displacement, plasma poloidal rotation frequency and sliding fast Fourier transform spectra of both the magnetic and the electron density fluctuations. The evolution of the amplitude as well as the frequency of the most important tearing modes is determined. Negative bias leads to better particle and energy confinement, and improved stability. Positive bias reduces both confinement and stability, causing a significant transitory vertical displacement of the plasma column as well as of its current axis.

Influence of electrode and limiter biasing on the ISTTOK plasma

In this contribution, limiter and electrode biasing experiments are compared, in particular with regard to their effects on the edge plasma parameters. For a positive electrode bias (EB), the plasma potential profile is strongly modified in the region between the electrode and the limiter. Both the edge Er and its shear increase significantly (values of Er larger than 10 kV m−1 have been measured). The modification in the edge parameters is accompanied by a substantial increase in the average plasma density and an improvement in the gross particle confinement. For moderate/high plasma densities no steady-state discharges have been achieved with EB since a degradation in confinement is observed a few milliseconds after the bias is applied. For negative bias (−250 < Vbias < 0 V), no significant modification in either the global or the edge parameters is observed since the electrode current drawn is too low to modify the plasma parameters. The general discharge behaviour for positive limiter biasing is similar to that observed with EB. However, the larger increase in radiation losses and the smaller modification of the edge electric field observed when bias is applied leads to almost no modification in particle confinement and to a stronger and faster degradation in confinement a few milliseconds later.

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.

The heavy ion beam diagnostic for the tokamak ISTTOK

In this paper we describe the heavy ion beam diagnostic for the tokamak ISTTOK, which has been designed to determine the temporal evolution of the plasma density, poloidal magnetic field and plasma potential profiles. This diagnostic makes use of a new type of high density caesium plasma source, a multiple cell detector and a fast data acquisition system. We describe the numerical code for trajectory and beam attenuation simulations, a method for the experimental determination of the poloidal field profile, the ion gun and the detection, control and data acquisition systems. Calibration tests and the first experimental results are presented. >

Operation of the tokamak ISTOKK in a multicycle alternating flat-top plasma current regime

Operation of the tokamak ISTTOK in a multicycle alternating square wave plasma current regime is reported. Discharges with seven half-cycles without dwell times, over a total time span of about five times the maximum duration of a single DC discharge, were obtained by feeding the primary of the transformer with an electrolytic capacitor bank switched in polarity by a fast insulated gate bipolar transistor (IGBT) H bridge

Engineering aspects of the ISTTOK operation in a multicycle alternating flat-top plasma current regime

Abstract The main engineering aspects of the tokamak ISTTOK operation in a multicycle alternating flat-top plasma current regime are presented. AC discharges have been obtained feeding the ohmic and vertical magnetic field circuits with a specially designed alternating power supply, based on a single electrolytic capacitor bank and a fast insulated gate bipolar transistor (IGBT) H-bridge, feedback controlled by a discreet power DAC. The horizontal magnetic field has been created by an independent and pre-programmable DC power supply. The optimization of AC operation has also implied the installation of a new set of symmetric and more external windings for the vertical B-field and of an adequate gas puffing system. Discharges with seven half-cycles and flat-top plasma currents of about ±4 kA without dwell times were already achieved, in a total time span of 240 ms approximately equal to five times the maximum duration of a single DC discharge.

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.

Development of a new type of Cs plasma ion gun for application in a heavy ion beam tokamak diagnostic

A new type of high-density caesium plasma ion source is described, which is capable of producing Cs+ beam currents of up to 300 mu A. The ion emitting surface in the caesium plasma conforms automatically to the accelerator geometry to give perveance match, providing that the ion density is correctly adjusted with respect to the required beam energy. Ready operation of the source at different energies with constant perveance is facilitated in this way. Consequently, the beam divergence is practically independent of the beam energy. The plasma density is maintained at the required level by controlling the temperature of the source chamber walls on which atomic caesium is deposited. The ion source and the beam transport line have been developed at Culham and deliver a low-divergence 1.5 mu A Cs+ beam at the gun output, with an energy of up to 25 keV. The apparatus now serves as the injection part of the heavy ion beam deflection analyser of the ISTTOK tokamak in Lisbon.