Control of plasma profiles or instabilities, like neoclassical tearing modes or sawteeth, is envisaged by means of localized heating or current drive. Electron cyclotron resonance heating (ECRH) or current drive (ECCD) is recognized as providing an ideally localized source for such applications. In particular, control of MHD instabilities requires precise localization of the ECRH power deposition. A typical feedback control loop then must deal both with the identification of the instability and its position in the plasma, as well as with the steering of the ECRH wave beam in order to deposit the power at a specific location and generate the desired effect. An overview is provided of different elements entering the general control loop and of the specific choices in case of the TEXTOR system for MHD control by ECRH. Plasma diagnostics like electron cyclotron emission (ECE) and or Mirnov coils are used to identify the occurrence of tearing modes or sawteeth. Generally, detailed knowledge of the magnetic equilibrium is required in order to localize the instability. The steering of the ECRH wave beam is affected by a fast steerable mechanical launcher. A dedicated controller is developed for the TEXTOR ECRH launcher, on the basis of its mechanical properties as obtained from Frequency Response Function measurements. On TEXTOR, the tasks of instability identification and localization and of the steering of the ECRH wave beam are combined in a single system using the principle of "same sight-line" electron cyclotron emission (ECE): when the ECE spectrum is viewed along the sight-line of the ECRH beam, steering of the combined ECRH/ECE wave beam allows to localize a given structure in the ECE spectrum relative to the ECRH frequency and thereby automatically to position the ECRH power relative to this structure A major challenge is the separation of the high power ECRH (~1 MW) from the low power ECE (of the order of a few nW) coming from the plasma. In the case of the TEXTOR bi-directional ECRH/ECE launcher this is achieved with a Fabry-Pérot etalon First ECE measurements demonstrate the capability of the system to localize rotating magnetic islands or the sawtooth inversion.
|Title of host publication||Control for Nuclear Fusion : Workshop, Eindhoven, Netherlands, 2008-05-07|
|Publication status||Published - 2008|