A. N. Fagundes

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.

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.

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.

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.

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.