O. C. Usuriaga

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.

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.

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.

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.

Recombinative plasma in electron runaway discharge

Cold recombinative plasma is the basic feature of the new regime of runaway discharges recently discovered in the Tokamak Chauffage Alfven Bresilien tokamak [R. M. O. Galvao et al., Plasma Phys. Controlled Fusion 43, 1181 (2001)]. With low plasma temperature, the resistive plasma current and primary Dreicer process of runaway generation are strongly suppressed at the stationary phase of the discharge. In this case, the runaway avalanche, which has been recently recognized as a novel important mechanism for runaway electron generation in large tokamaks, such as International Thermonuclear Experimental Reactor, during disruptions, and for electric breakdown in matter, is the only mechanism responsible for toroidal current generation and can be easily observed. The measurement of plasma temperature by the usual methods is a difficult task in fully runaway discharges. In the present work, various indirect evidences for low-temperature recombinative plasma are presented. The direct observation of recombinative...

Analysis of the electron temperature measurement in TCABR tokamak by Electron Cyclotron Emission and Infrared Thomson scattering diagnostics

This work presents the experimental analysis of the central electron temperature measured by the electron cyclotron emission (ECE) radiometer and the infrared Thomson Scattering (ITS) diagnostic. The detection of the ECE radiation is done by a heterodyne scanning radiometer that works at the second harmonic extraordinary mode, in frequency range from 50 to 85GHz, which allows measurement of the radial profile of electron temperature with good spatial and temporal resolutions. The ITS diagnostic uses a Neodymium Glass laser (wavelength 1.054 μm). This ITS diagnostic measures the electron temperature in the center of plasma column one time during plasma shot. Results also show a discrepancy between the two diagnostics in the electron temperature measurement in the presence of Magnetohydrodynamics activity that gives an explanation for this apparent inconsistency.

Comparative electron temperature measurements of Thomson scattering and electron cyclotron emission diagnostics in TCABR plasmas.

We present the first simultaneous measurements of the Thomson scattering and electron cyclotron emission radiometer diagnostics performed at TCABR tokamak with Alfvén wave heating. The Thomson scattering diagnostic is an upgraded version of the one previously installed at the ISTTOK tokamak, while the electron cyclotron emission radiometer employs a heterodyne sweeping radiometer. For purely Ohmic discharges, the electron temperature measurements from both diagnostics are in good agreement. Additional Alfvén wave heating does not affect the capability of the Thomson scattering diagnostic to measure the instantaneous electron temperature, whereas measurements from the electron cyclotron emission radiometer become underestimates of the actual temperature values.

Density Limit in TCABR Plasmas With Alfvén Wave Heating

Alfven Waves (AW) were launched in tokamak (TCABR) density limit plasmas for the first time. Experimental evidence of plasma heating is backed up by calculations from an 1‐D numerical cylindrical code, based on the toroidal electric field diffusion. Simultaneously, increase in the density limit and plasma pressure with negligible impurities level launched by the AW antennas were also observed, without major appearance of a resistive disruption. The increase in the density limit and the heating might be related to the expected edge and off‐axis AW power deposition, respectively, in agreement with the calculation performed by an 1‐D numerical code linked to ASTRA.

Error analysis in the electron temperature measurements in TCABR

An analytical method is proposed to evaluate the experimental uncertainty in the electron temperature measurements in the TCABR tokamak. Solving the integral equation resulting from the convolution of two functions, one representing, the scattered light and the other the spectral apparatus function, i.e., the polychromator, an analytical expression for the electron temperature is obtained, from which the uncertainty in the measured value is readily evaluated. The results show that the major contribution to the error comes from the noise in the signal; the uncertainties in the filters parameters do not contribute significantly to the total error.