J. H. F. Severo

Plasma confinement using biased electrode in the TCABR tokamak

Experimental data obtained on the TCABR tokamak (R = 0.61?m, a = 0.18?m) with an electrically polarized electrode, placed at r = 0.16?m, is reported in this paper. The experiment was performed with plasma current of 90?kA (q = 3.1) and hydrogen gas injection adjusted for keeping the electron density at 1.0 ? 1019?m?3 without bias. Time evolution and radial profiles of plasma parameters with and without bias were measured. The comparison of the profiles shows an increase of the central line-averaged density, up to a maximum factor of 2.6, while H? hydrogen spectral line intensity decreases and the C?III impurity stays on the same level. The analysis of temporal behaviour and radial profiles of plasma parameters indicates that the confined plasma enters the H-mode regime. The data analysis shows a?maximum enhanced energy confinement factor of 1.95, decaying to 1.5 at the maximum of the density, in comparison with predicted Neo?Alcator scaling law values. Indications of transient increase of the density gradient near the plasma edge were obtained with measurements of density profiles. Calculations of turbulence and transport at the Scrape-Off-Layer, using measured floating potentials and ion saturation currents, show a strong decrease in the power spectra and transport. Bifurcation was not observed and the decrease in the saturation current occurs in 50??s.

Suppression and excitation of MHD activity with an electrically polarized electrode at the TCABR tokamak plasma edge

Two reproducible regimes of tokamak operation, with excitation or suppression of MHD activity can be obtained using a voltage-biased electrode inside the edge of the TCABR tokamak. The experiment was carried out adjusting the tokamak parameters to obtain two types of discharges: with strong or weak MHD activity, without biasing in both cases. The plasma current was adjusted to cover a range of safety factor from 2.9 up to 3.5, so that when biasing was applied the magnetic island (3,1) could interact with the edge barrier. The application of biasing in subsequent discharges of each type resulted in excitation or suppression of the MHD activity. The results show that the dominant modes are m = 2, n = 1 and m = 3, n = 1 for excitation and partial suppression, respectively. In both regimes a strong decrease in the radial electric field is detected with destruction of the transport barrier and of the improved confinement caused by different mechanisms. The measurements include temporal behaviour of edge transport, turbulence, poloidal electric and magnetic fields, edge density, radial electric fields and radial profile of Hα line intensity. The explanation of the excitation and suppression processes is discussed in the paper.

Plasma residual rotation in the TCABR tokamak

This paper reports the first results on the measurement of the radial profiles of plasma poloidal and toroidal rotation performed on the TCABR tokamak, in the collisional regime (Pfirsch–Schluter), using Doppler shift of carbon spectral lines, measured with a high precision optical spectrometer. The results for poloidal rotation show a maximum velocity of (4.5 ± 1.0) × 103 m s−1 at , (a—limiter radius), in the direction of the diamagnetic electron drift. Within the error limits, reasonable agreement is obtained with calculations using the neoclassical theory for a collisional plasma, except near the plasma edge, as expected. For toroidal rotation, the radial profile shows that the velocity decreases from a counter-current value of (20 ± 1) × 103 m s−1, at the plasma core, to a co-current value of (2.0 ± 0.9) × 103 m s−1 near the limiter. An agreement within a factor 2, for the plasma core rotation, is obtained with calculations using the model proposed by Kim, Diamond and Groebner (1991 Phys. Fluids B 3 2050).

Alfvén wave heating and runaway discharges maintained by the avalanche effect in TCABR

Recent results of Alfven wave heating experiments and the characteristics of a new regime of runaway discharges found in Tokamak Chauffage Alfven Bresilien (TCABR) are discussed. (1) Wave excitation was carried out with one module of the antenna system, with and without a Faraday screen. Evidence of plasma heating was obtained in both cases, for coupled wave powers up to half of the Ohmic power, approximately, without uncontrollable density rise during the RF pulse. The antenna coupling with the plasma seems to have increased when the Faraday screen was removed. (2) The new regime of runaway discharges is produced by initiating the main plasma breakdown without pre-ionization and strongly increasing the neutral gas fuelling at the end of the current ramp-up phase. Consequently, the plasma cools down substantially and switches to a runaway mode in conditions under which the primary (Dreicer) mechanism is strongly suppressed. This new regime of runaway discharges is characterized by strong enhancement of the relaxation oscillations, which are seen in the H α and ECE emissions, coupled with large spikes in the line density, loop voltage, bolometer, and other diagnostic signals.

Magnetic islands and plasma rotation in the Tokamak Chauffage Alfven Bresilien tokamak

Collisional plasma rotation in the Tokamak Chauffage Alfven Bresilien (TCABR) tokamak [J. H. F. Severo, I. C. Nascimento, V. S. Tsypin, and R. M. O. Galvao, Nucl. Fusion 43, 1047 (2003)] has been experimentally studied. It was found that the measured plasma poloidal rotation velocity agrees within error limits with neoclassical theoretical predictions, and toroidal velocity with experimental results obtained in analogous tokamaks, almost everywhere along the minor radius r, except for measurements at r/a≃0.56 and r/a≃0.89 (the minor radius of TCABR tokamak a=18 cm). For the first point, the measured plasma rotation velocities are higher than the velocity of the background plasma, respectively ∼30% and ∼10% for the poloidal and toroidal rotation velocities. Using a set of 22 Mirnov coils displaced poloidally, magnetic field perturbations were measured in shots adjusted to reproduce the ones of the previous plasma rotation measurements, and the results confirm that in the region r/a≃0.89 the plasma rotates ...

Temporal behaviour of toroidal rotation velocity in the TCABR tokamak

A new method for determining the temporal evolution of plasma rotation is reported in this work. The method is based upon the detection of two different portions of the spectral profile of a plasma impurity line, using a monochromator with two photomultipliers installed at the exit slits. The plasma rotation velocity is determined by the ratio of the two detected signals. The measured toroidal rotation velocities of C III (4647.4 A) and C VI (5290.6 A), at different radial positions in TCABR discharges, show good agreement, within experimental uncertainty, with previous results (Severo et al 2003 Nucl. Fusion 43 1047). In particular, they confirm that the plasma core rotates in the direction opposite to the plasma current, while near the plasma edge (r/a > 0.9) the rotation is in the same direction. This technique was also used to investigate the dependence of toroidal rotation on the poloidal position of gas puffing. The results show that there is no dependence for the plasma core, while for plasma edge (r/a > 0.9) some dependence is observed.

Overview of Recent Results of TCABR

An overview of recent results obtained in TCABR is presented. Experiments on Alfven wave heating have been carried out in both low and high density regimes. Controlling the density rise usually observed in Alfven heating experiments, it was possible to get a clear confirmation of electron temperature increase in low‐density discharges. In the high density regime, the Alfven wave absorption occurs at mode numbers quite different from those for low density. Detailed experiments have been carried out on the transition between low and high‐density confinement regimes, triggered by electrostatic polarization at the plasma edge. The results indicate that the flatness of the density profile and the decrease of edge recycling depend strongly on the level of MHD activity during transition. A preliminary analysis of the electromagnetic emission associated with the relaxation instability in the new regime of runaway discharges discovered in TCABR shows that the observations are coherent with theoretical models. The heat transport in the presence of large magnetic islands has been investigated, in the collisional regime, and found to be properly described by the Fitzpatrick model. Finally, two diagnostic techniques have been further improved, the determination of the position of the local Alfven resonance by microwave reflectometry and the determination of the temperature and density at the plasma edge by the method based upon the uniqueness of the particle confinement time, determined from the hydrogen Balmer series emission.

Plasma rotation measurement in small tokamaks using an optical spectrometer and a single photomultiplier as detector.

The method for plasma rotation measurement in the tokamak TCABR is reported in this article. During a discharge, an optical spectrometer is used to scan sequentially spectral lines of plasma impurities and spectral lines of a calibration lamp. Knowing the scanning velocity of the diffraction grating of the spectrometer with adequate precision, the Doppler shifts of impurity lines are determined. The photomultiplier output voltage signals are recorded with adequate sampling rate. With this method the residual poloidal and toroidal plasma rotation velocities were determined, assuming that they are the same as those of the impurity ions. The results show reasonable agreement with the neoclassical theory and with results from similar tokamaks.

The analysis of alfvén wave current drive and plasma heating in TCABR tokamak

The results of experiments on Alfven wave current drive and plasma heating in the TCABR tokamak are analyzed with the help of a numerical code for simulation of the diffusion of the toroidal electric field. It permits to find radial distributions of plasma current density and conductivity, which match the experimentally measured total plasma current and loop voltage changes, and thus to study the performance of the RF system during Alfven wave plasma heating and current drive experiments. Regimes with efficient RF power input in TCABR have been analyzed and revealed the possibility of noninductive current generation with magnitudes up to ~8 kA. The increase of plasma energy content due to RF power input is consistent with the diamagnetic measurements.

Report on recent results obtained in TCABR

Recent results of experimental work and theoretical modeling carried out in the TCABR tokamak are reported on characterization of MHD instabilities, improved diagnostics of rotation of the plasma column, excitation of Alven global modes, identification of GAMs, and the effect of rotation on their behavior. Detailed measurements of edge electrostatic perturbations and of magnetic island evolution and rotation indicate that the edge turbulence is substantially affected by the islands growth, leading to a strong modulation of the edge particle losses at the same frequency of the MHD activity. Measurements with spatial resolution also show that the growth of the MHD activity is due to nonlinear coupling of magnetic islands with different poloidal mode numbers, which increases the impurity influx. A new system of data acquisition and processing of the TCABR plasma rotation diagnostic was implemented. The system is based upon a single monochromator coupled with six photomultipliers tubes and allows one toroidal and two poloidal simultaneous rotation measurements. The excitation of Global Alfven Waves - GAW has been investigated, using a new type of radio frequency amplifier. The GAW resonances are searched either by a pre-programmed density variation, at fixed generator frequency, or through three RF frequency sweeps from 2 to 4.5 MHz, at stationary density. GAW resonances have been found and their somewhat new characteristics are presented. The investigation of the effect of poloidal and toroidal rotation on the characteristics of the geodesic acoustic mode has been investigated, both theoretically and experimentally. It is found that the assumption of isothermal flux surfaces gives rise to a third branch of this mode. Detailed predictions coupled with experimental measurements are currently being carried out to investigate this question.