E. A. Lerche

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.

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.

Application of microwave reflectometry to register Alfvén wave resonances in the TCABR tokamak

Application of microwave reflectometry to study Alfven wave resonances in the TCABR tokamak is described. A microwave reflectometer was used to register plasma density oscillations driven by the excited Alfven waves, under the condition of the spectrum scanned by a controlled plasma density rise. It is shown that when the position of the local Alfven resonance rA, which is defined by the relation ω=k∥(rA)CA(rA), is close to the plasma zone where the microwave signal is reflected, the high-frequency modulation of the output signal of the reflectometer at the rf generator frequency increases. This method can give information about the localization of the rf power deposition zone in Alfven wave plasma heating and current drive experiments. It also allows finding the plasma current profile in the region of the rf power deposition.

Recent Results of Alfvén Wave Studies in TCABR

The results on comparative studies of Alfven wave plasma heating by two different antenna types in TCABR are presented. Emphasis is placed on the excited wave spectra and parasitic coupling with the edge plasma. The antenna modules have two groups of RF current‐carrying straps separated by a toroidal angle of approximately 22°. In type I antenna, each group consists of two circular loops that are cut in two half‐turn windings. The feeders of each loop pair are rotated 90° in the poloidal direction with respect to each other, to decrease the mutual coupling between them and make it possible to excite single helicity plasma modes (M=+1 or M=−1). In type II antenna, each group consists of two poloidal straps located at the low‐magnetic‐field side of the vacuum chamber. The poloidal extension of each strap is around 90° and the angle between straps is also of the same value. In both antenna types, the straps have side protectors of boron nitride. Initial experiments indicate that the parasitic interaction wit...

Results of localized Alfvén wave heating in TCABR

en Br´ (TCABR) plasmas. The m=N =1/2, 0/2 and 0/3 modes are dominant in the observed wave heating, due to Alfv´ en wave continuum absorption. In particular, the resonant absorption of the m = 0 sideband harmonic plays an important role in the central core and peripheral heating. The increase of the electron temperature during the radio frequency pulse was accompanied by a local density rise that produced a cut-off of the ECE emission. The observed heating profile is also consistent with the reflectometer measurements of the density

Identification of the Alfvén wave resonances in the TCABR tokamak by the microwave reflectometry

The experimental results on the Alfven mode structure identification by the microwave reflectometry in TCABR tokamak are presented. The knowledge of the spatial spectrum of the excited waves is crucial for optimization of Alfven wave plasma heating and noninductive current drive scenarios in tokamak plasmas. No less important is the possibility to use the Alfven wave excitation for diagnostic purposes. A microwave reflectometer with fixed frequency was used to register plasma density oscillations driven by the excited Alfven waves, under the condition of the spectrum scanned by a controlled plasma density rise. It is shown that when the position of the local Alfven resonance, rA, which is defined by relation w = k||(rA)CA(rA), is close to the plasma zone where the microwave signal is reflected, the high-frequency modulation of the output signal of the reflectometer at the RF generator frequency increases. The possibility to use the observed effect for finding the plasma current profile in tokamaks is discussed.

Runaway discharges in TCABR

A new regime of high-density runaway discharges has been found in TCABR (Tokamak Chauffage Alfven in Brazil). This regime is obtained by initiating the discharge with low filling pressure and, after the initial current rise, maintaining a large filling rate. The line density keeps a constant value, ∼1019 m−3, during the current ramp-up phase and then drops by a factor around 3 or 4 in the quasi-stationary phase of the discharge, when the new regime is formed. The average value of the line density is then kept approximately constant, almost independent of the intensity of the neutral gas injection. The Hα emission increases substantially and shows strong spikes, of the same type observed in the H-mode ELMs, possibly triggered by the relaxation instability typical of runaway discharges. As the filling rate increases, the spike period decreases and its behavior changes as in ELM type I to type III transition observed in experiments in large tokamaks with detached plasmas. Spikes well correlated with the Hα o...

Preliminary results on Alfvén wave system in the TCABR tokamak

A brief review of the Alfven Wave Excitation System (AWES) designed for the TCABR tokamak and the first experimental results on RF plasma heating are presented. One of four antenna modules has been completely installed in the vacuum chamber and the initial experiments were carried out in the low power regime using the four-phase RF generator. The main objectives were the antenna tuning according to the typical plasma parameters of TCABR and the evaluation of the antenna parasitic loading, as well as the calibration of the RF diagnostic tools in real discharge conditions. The first results have been obtained with standard diagnostics and with the RF signals measured using high sampling rate digital oscilloscopes. They showed that daily antenna cleaning and correct wave helicity excitation reduce significantly the parasitic loading and are crucial for efficient plasma coupling.