R. M. O. Galvão

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.

Electromagnetic ion-beam instabilities in a cold plasma

We study the stability of the cold-plasma dispersion relation for circularly polarized waves in a plasma composed of an ion background and an ion beam. The presence of the beam introduces a resonant branch into the dispersion relation for right-hand-polarized waves propagating in the direction of the external magnetic field, which, for V > V ? , has negative energy (here V is the beam velocity and V ? is the wave phase velocity). Therefore this branch may give rise to explosive instabilities when the waves experience parametric decays. It is shown graphically that resonant right-hand-polarized and nonresonant left-hand-polarized waves, propagating parallel to the external magnetic field, can be unstable. It is also shown that the instability region can extend to large frequencies and wavenumbers, and that the instability regions have a band structure. The parametric dependence of instability thresholds and marginal modes is also studied.

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.

Zonal flows generated by small-scale drift-Alfvén modes

The generation of zonal flows by small-scale drift-Alfven (SSDA) modes is investigated. It is shown that these zonal flows can be generated by a monochromatic wave packet of SSDA modes propagating in the ion diamagnetic drift direction. The corresponding zonal-flow instability resembles a hydrodynamic one. Its growth rate depends on the spectrum purity of the wave packet; it decreases for relatively weak spectrum broadening and the instability turns into a resonant one, and eventually is suppressed, as the broadening increases. A general conclusion of this work is that the SSDA modes are less effective for driving zonal flows than standard drift modes.

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).

Runaway discharges in TCABR

It is found in experiments carried out in Tokamak Chauffage Alfven Bresilien (TCABR) that two regimes of runaway discharges (RADs) with very different characteristics are possible. The RAD-I regime, which is similar to that observed in other tokamaks, can be obtained by a gradual transfer from a normal resistive to a RAD by decreasing the plasma density. This regime can be well understood using the Dreicer theory of runaway generation. The total toroidal current contains a substantial resistive component and the discharge retains some features of standard tokamak discharges. The second runaway regime, RAD-II, was recently discovered in the TCABR tokamak (Galvao R.M.O. et al 2001 Plasma Phys. Control. Fusion 43 1181). The RAD-II regime starts just from the beginning of the discharge, provided that certain initial conditions are fulfilled and, in this case, the runaway tail carries almost the full toroidal current. The background plasma is cold and detached from the limiter due to the recombination process. The primary Dreicer process is suppressed in the RAD-II and the secondary avalanche process dominates, even at the start-up phase, in the generation of the toroidal current. It is possible to trigger a transition from the RAD-I to the RAD-II regime using plasma cooling by gas puffing. The experimental results are shown to be in reasonable agreement with theoretical predictions based on the runaway avalanche process.

Nonaxisymmetric magnetorotational instability in ideal and viscous plasmas

The excitation of magnetorotational instability (MRI) in rotating laboratory plasmas is investigated. In contrast to astrophysical plasmas, in which gravitation plays an important role, in laboratory plasmas it can be neglected and the plasma rotation is equilibrated by the pressure gradient. The analysis is restricted to the simple model of a magnetic confinement configuration with cylindrical symmetry, in which nonaxisymmetric perturbations are investigated using the local approximation. Starting from the simplest case of an ideal plasma, the corresponding dispersion relations are derived for more complicated models including the physical effects of parallel and perpendicular viscosities. The Friemann–Rotenberg approach used for ideal plasmas is generalized for the viscous model and an analytical expression for the instability boundary is obtained. It is shown that, in addition to the standard effect of radial derivative of the rotation frequency (the Velikhov effect), which can be destabilizing or stab...

Tokamak research at University of São Paulo

The main results obtained in the small tokamak TBR-1 of University of São Paulo (USP) are reviewed. The main effort has been concentrated on the characterization and external control of MHD activity, plasma edge phenomena and diagnostic development. The design of a small-aspectratio tokamak, TBR-E, and the research program to be carried out in TCA, to be transferred from Lausanne to São Paulo, are also briefly described.

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.

Dust-induced instability in a rotating plasma

The effect of immobile dust on stability of a magnetized rotating plasma is analyzed. In the presence of dust, a term containing an electric field appears in the one-fluid equation of plasma motion. This electric field leads to an instability of the magnetized rotating plasma called the dust-induced rotational instability (DRI). The DRI is related to the charge imbalance between plasma ions and electrons introduced by the presence of charged dust. In contrast to the well-known magnetorotational instability requiring the decreasing radial profile of the plasma rotation frequency, the DRI can appear for an increasing rotation frequency profile.