The multi-frequency Electron Cyclotron Heating (ECRH) system at the ASDEX Upgrade tokamak employs depressed collector gyrotrons, step-tunable in the range 105-140 GHz. The system is equipped with a fast steerable launcher allowing for remote steering of the ECRH RF beam during the plasma discharge. The gyrotrons and the mirrors are fully integrated in the discharge control system. The polarization can be controlled in a feed-forward mode. 3 Sniffer probes for millimeter wave stray radiation detection have been installed. Key words:Electron cyclotron resonance heating, step-tunable gyrotron, fast steerable launcher. IntroductionElectron cyclotron resonance heating (ECRH) and current drive (ECCD) experiments at the ASDEX Upgrade tokamak first started in 1996 with a single frequency (140 GHz) system that comprises 4 gyrotrons with 0.5 MW / 2 sec each (optionally 0.7 MW / 1 sec) and 4 independent transmission lines and launchers [1]. A new multi-frequency ECRH system is currently under construction that employs depressed collector gyrotrons, step-tunable in the range 105-140 GHz with 1 MW output power at 140 GHz and slightly reduced power (~800 kW) at lower frequencies [2]. The pulse length of the system is 10 s, corresponding to the maximum flat top time of ASDEX Upgrade plasma discharges. In its final stage it will consist of 4 gyrotrons, where the first 3 gyrotrons are two-frequency gyrotrons, operating at 105 and 140 GHz. For the fourth gyrotron a broadband output window is under development that will allow operation also at intermediate frequencies. The transmission line consists of a quasi-optical Matching Optics Unit (MOU) and of non-evacuated corrugated HE 11 waveguides with an inner diameter of 87 mm and a total length of approximately 70 m. The launchers of the new system have a poloidal fast steering capability that allows for a change of the deposition location during the discharge without changing the toroidal magnetic field. The ultimate goal is to have a very flexible system for localized plasma heating and current drive that allows for feedback control of neoclassical tearing modes, pressure profile and transport [3].Since 2007 ASDEX Upgrade operates with fully tungsten covered plasma facing components. Central heating with ECRH plays a key role in suppressing tungsten accumulation in the plasma center. Up to now the standard operation mode is the extraordinary mode at the second harmonic, X2-mode, because of its full single pass absorption. In order to extend the applicability of central ECRH to a wider range of magnetic field and plasma current, schemes with reduced single-pass absorption have been implemented: X3-mode heating allows to reduce the magnetic field by 30 %, such that the first H-modes with an ITER-like safety factor of q 95 =3 could be run. Heating with the second harmonic ordinary mode, O2-mode, increases the plasma cutoff density for the ECRH by a factor of 2 and therefore allows to access higher plasma currents and triangularities [4].
Abstract. The ASDEX Upgrade (AUG) ECRH system now delivers a total of 3.9 MW to the plasma at 140 GHz. Three new units are capable of 2-frequency operation and may heat the plasma alternatively with 2.1 MW at 105 GHz. The system is routinely used with X2, O2, and X3 schemes. For B t = 3.2 T also an ITER-like O1-scheme can be run using 105 GHz. The new launchers are capable of fast poloidal movements necessary for real-time control of the location of power deposition. Here real-time control of NTMs is summarized, which requires a fast analysis of massive data streams (ECE and Mirnov correlation) and extensive calculations (equilibria, ray-tracing). These were implemented at AUG using a modular concept of standardized real-time diagnostics. The new realtime capabilities have also been used during O2 heating to keep the first reflection of the non-absorbed beam fraction on the holographic reflector tile which ensures a well defined second pass of the beam through the central plasma. Sensors for the beam position are fast thermocouples at the edge of the reflector tile. The enhanced ECRH power was used for several physics studies related to the unique feature of pure electron heating without fueling and without momentum input. As an example the effect of the variation of the heating mix in moderately heated H-modes is demonstrated using the three available heating systems, i.e. ECRH, ICRH and NBI. Keeping the total input power constant, strong effects are seen on the rotation, but none on the pedestal parameters. Also global quantities as the stored energy are hardly modified. Still it is found that the central ion temperature drops as the ECRH fraction exceeds a certain threshold.
In this paper, we present an FFT based method for fast calculation in a mode converting overmoded circular waveguide with adaptive surface perturbations. The results of this method are compared with those of an existing integral equation approach.
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