L. Ruchko

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.

Turbulence and transport in the scrape-off layer TCABR tokamak

We study the scrape-off layer turbulence and transport in Tokamak Chauffage Alfven Bresilien (TCABR) before and during the launching of radio frequency excited waves that interact with the boundary plasma. We present the main characteristics of the background electrostatic fluctuations and the intermittent bursts. During the perturbation, the plasma turbulence intensity increases and the average phase velocity reverses its direction. The total turbulence driven transport and the transport radially convected by the intermittent fluctuation bursts also increase with the perturbation. The probability distributions of the bursts have long tails, resembling those typical of flight distributions with a decay parameter increasing with the wave excitation.

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.

Advanced antenna system for Alfvén wave plasma heating and current drive in TCABR tokamak

An advanced antenna system that has been developed for investigation of Alfven wave plasma heating and current drive in the TCABR tokamak is described. The main goal was the development of such a system that could insure the excitation of travelling single helicity modes with predefined wave mode numbers M and N. The system consists of four similar modules with poloidal windings. The required spatial spectrum is formed by proper phasing of the RF feeding currents. The impedance matching of the antenna with the four-phase oscillator is accomplished by resonant circuits which form one assembly unit with the RF feeders. The characteristics of the antenna system design with respect to the antenna-plasma coupling and plasma wave excitation, for different phasing of the feeding currents, are summarised. The antenna complex impedance Z ZR ZI is calculated taking into account both the plasma response to resonant excitation of fast Alfven waves and the nonresonant excitation of vacuum magnetic fields in conducting shell. The matching of the RF generator with the antenna system during plasma heating is simulated numerically, modelling the plasma response with mutually coupled effective inductances with corresponding active ZR and reactive ZI impedances. The results of the numerical simulation of the RF system performance, including both the RF magnetic field spectrum analysis and the modelling of the RF generator operation with plasma load, are presented.

Influence of conducting side limiters on the excitation of Alfvén waves in tokamak plasmas

In recent experiments carried out on the Phaedrus tokamak, the conducting plates that protect both sides of a poloidal antenna set have been replaced by non-conducting boron nitride plates. With this modification, direct heating and current drive by Alfven waves were demonstrated. An analysis proves that elimination of poloidal conducting protectors plays a crucial role in antenna performance improvement. The effect of the conducting side limiters on the wave spectrum and antenna impedance is calculated numerically for a cylindrical plasma column and model antenna current. The results demonstrate that conducting side limiters cause the excitation of additional modes that are deleterious for the Alfven heating and current drive scheme

Identification of local Alfvén wave resonances with reflectometry as a diagnostic tool in tokamaks

Local Alfven wave (LAW) resonances are excited in tokamaks by an externally driven electro-magnetic field, below the ion cyclotron frequency. Based on TCABR (Tokamak Chauffage Alfven Bresilien) experiments and numerical calculations, it is shown that a combination of small power deposition in LAW resonances, swept by plasma density variation or scanned by varying generator frequencies, in combination with detection of the density fluctuations in the LAW resonances by reflectometry, can serve as a diagnostic tool for identification of the effective ion mass number Aef and q-profile in tokamaks. The idea is based on the simultaneous detection of the position of m = ±1 LAW resonances, which are excited by M/N = ±1/±2 antenna modes and m = 0 generated by the poloidal mode coupling effect in tokamaks. The m = 0 resonance depends only on the effective ion mass number and not on the q-profile, so that the mass number can be determined unambiguously. Then, we can determine q-factor at the position of m = ±1 LAW resonances. Using the multifluid ALTOK code, we identify mass number in TCABR experiments and demonstrate the possibility of applying this method in the Joint European Torus.

Electron density measurements from right-hand cutoff of ECE in the TCABR tokamak

In tokamak machines with low toroidal magnetic fields and high plasma densities, the accessibility conditions impose restrictions to the detection of the Electron Cyclotron Emission (ECE). In these machines, the righthand cutoff condition can be used as an independent method to determine the local electron density from the ECE data in thermal discharges. In this paper is shown the results obatined from the detection of ECE radiation in the TCABR tokamak, in operation at the Institute of Physics of University of Sao Paulo. The effect of the ECE radiation cutoff was observed for different radial positions of the plasma column. To reach the ECE cutoff condition, the electron density was increased monotonically by the use of an external gas puffing system. For sufficient high densities, the emission at some frequencies is cutoff and the first and the last frequency to be cutoff depends on the shape of the density profile. These measurements do not require the plasma to be optically thick. It was observed that, for a toroidal field BO = 1:14T, the first cutoff of ECE occurs for a radial position r @ 5 cm. From these measurements the radial electron density was determined. For a symmetric parabolic profile ne = neo [1 - (r/a)2]a , values of a between 0.86 and 0.97 were experimentally obtained. A good agreement of these values with those obtained from the microwave interferometer measurements (a »0:85) was found. Therefore, the ECE right-hand cutoff constitutes an independent method to obtain information about the electron density profile.

Description and characterization of a ECR plasma device developed for thin film deposition

The design, construction, and characterization of an electron-cyclotron-resonance (ECR) plasma device and its utilization for growing AlN polycrystals are described in detail. The plasma density and electron temperature were measured by two types of Langmuir probes under different conditions of magnetic configuration and RF substrate polarization. For the investigated nitrogen plasmas, the electron temperature increases towards substrate holder and decreases with pressure. The magnetic configuration and plasma parameters required for successful growth of polycrystal aluminum nitride have been determined.

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...