Electron cyclotron resonance heating (ECRH) or current drive (ECCD) is often applied for control of plasma profiles or instabilities, like neoclassical tearing modes or sawteeth. Such applications typically require precise localization of the ECRH power deposition. On the TEXTOR tokamak a dedicated electron cyclotron emission (ECE) diagnostic has been implemented for control of the ECRH power deposition on the basis of the "same line-ofsight" principle. It is based around an ECE-receiver located in the ECRH transmission line,and looking along the sight-line of the ECRH beam. This allows detection of structures in the plasma (for example, a tearing mode or sawtooth inversion radius) using the same optical path that along which the ECRH power is launched. Through steering of the ECRH/ECE wave beam, a structure then only needs to be localized in the ECE spectrum at the gyrotron frequency (or just above or below) in order to deposit the ECRH power exactly on top of it (or on its high or low field side). The major challenge is the separation of the ECE power (of the order of a few nW) from the ECRH power (1 MW). The design, construction, and tests ofthe Line-of-Sight receiver are presented together with first ECE measurements during high power ECRH. The key, frequency selective element of the system is a 25.75 mm thick quartz plate (Infrasil301) acting as Fabry-Perot filter. At an angle of 22.5o it is transparent to the gyrotron radiation and has maxima in reflection at the selected ECE frequencies. Low power tests of the reflections from the quartz plate are in agreement with theoretical expectationsbased on the material properties of the plate. In high power (up to 400 kW) tests a maximum ECRH pulse length of 2 s was achieved on the plasma resulting in a temperature rise in the quartz plate of 43 K, consistent with predictions. During 400 kW ECRH pulses, the gyrotron stray power level at the position of the radiometer horn is below 1 mW, which is effectively filtered by an 80 dB notch filter included in the radiometer. ECE measurements both duringhigh power ECRH as well as without ECRH demonstrate the capability of the system to localize rotating magnetic islands or the sawtooth inversion.
|Title of host publication||35th EPS Conference on Plasma Physics, Greece, Hersonissos, Crete|
|Publication status||Published - 2008|