Electron Cyclotron Emission in the TCABR Tokamak: An Experimental Study.

Author(s)

António Manuel Marques Fonseca

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Física (IF/SBI)
• Defense date: 2005-08-15

Examining board members

Ruy Pepe da Silva; Luiz Ângelo Berni; Paulo Ricardo da Silva Rosa; Mário Ueda; Alvaro Vannucci

• Advisor: Ruy Pepe da Silva

Abstract

In this work, an experimental study of the Electron Cyclotron Emission (ECE) in the TCABR Tokamak, is described. An ECE sweeping heterodyne radiometer, operating in the 50-85 GHz frequency range, was installed, calibrated and used to detect ECE radiation in the second harmonic extraordinary mode from thermal plasma discharges. The noise temperature, amplitude and frequency stability of the radiometer were determined. The frequency bandwidth (horizontal space resolution) was measured. A Gaussian Antenna is used and the gaussian beam waist radius (W0 ~ 1 cm) and the focus position (d ~ 37 cm) were experimentally determined. The focus of the antenna was positioned close to the center of the vacuum vessel. The absolute calibration of the equipment was done and the effect of the diagnostic window was considered (reflections and absorptions). The system can operate in sweeping mode, in order to obtain the radial electron temperature profiles, and also in the single frequency mode to obtain temporal electron temperature profiles with high time resolution. Due to the TCABR accessibility conditions and also to avoid runaway electrons, most of the ECE measurements were obtained in discharges with line electron density between 1.0x10+19 m-3 and 1.5x1019 m-3. For ne > 1.5x1019 m-3 (with B0 = 1.14 T) the cutoff in the ECE radiation was observed. The ECE cutoff was used to determine the radial profile of the electron density and applied to three different situations: discharges with additional gas puffing, with the application of a biasing electrode and in discharges with radio-frequency wave injection. Using a parabolic profile it was observed that, 0.85 < alfa < 1.0 for discharges with additional gas injection or with RF injection, and alfa ~ 0.6 for the electrode biasing experiments. The electron temperature profiles and Mirnov oscillations (f ~ 11.7 kHz) were simultaneously measured in discharges with q (r=0) > 1. The position and the width of the magnetic island were therefore calculated. The results indicate the presence of a dominant tearing mode in rs ~ 9.5 cm and the magnetic island width W ~ 2.0-2.5 cm. These experimental results are in agreement with the theoretical results foreseen by models of heat transport in the presence of magnetic islands. It was also observed that, the position of the magnetic island coincided with the region where the temperature radial profile is approximately flat. In another plasma scenario, with q(r=0) < 1, sawteeth oscillations with a period ~ 0.44 ms, crash time ~ 0.12 ms, inversion radius r ~ 4 cm, were measured. Peculiarities in the Te radial profile associated with the amplitude of Te oscillations, |DeltaTe|, due to the sawtooth instability, were observed. It was verified that the minimum values of the |DeltaTe| profile coincided with the region where the temperature radial profile was approximately flat. These results together with the q(r) profile yielded the following mode numbers (m/n), positions (r) and widths (W) of the magnetic islands: m/n = 4/3 (r ~ 9 cm, W4/3 ~ 0.9 cm), m/n = 3/2 (r ~ 11.8 cm, W3/2 ~ 0.9 cm) and m/n = 2/1 (r ~ 13.7 cm, W2/1; ~ 1.4 cm). A novel method to determine the position and width of the magnetic islands, in discharges with the presence of sawtooth instability, is presented here. (AU)