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EFFECT OF LOCALISED ELECTRON CYCLOTRON HEATING ON ENERGY CONFINEMENT AND MHD IN TCV

Introduction - This paper reports the effects of localised power deposition on the heating, MHD activity, and confinement in TCV. Initial ECRH studies concentrated on low elongation plasmas, including positive and negative triangularities: this is a necessary step before investigating more strongly shaped plasmas. ECRH power of up to 1 MW at 82.7 GHz (second harmonic X2) was injected, representing up to 14 times the Ohmic power during the heating pulse [1]. When complete, the ECW system will provide 3 MW second harmonic and 1.5 MW third harmonic at pulse lengths of 2 s [2]. For TCV (R = 0.89 m, a = 0.25 m, I p < 1.2 MA), the nominal field of B = 1.44 T and the frequency of 82.7 GHz place the resonance slightly off-axis on the high-field side (HFS) of the magnetic axis ( Δρ=-0.16 to -0.2). This allows us to scan the power deposition location on the HFS, above and below the magnetic axis, on a shot-to-shot basis; or, to sweep it during one shot. Sweeping is done using 1) steerable launcher mirrors (primarily the poloidal angle θ), 2) vertical plasma translation in the TCV vessel and 3) radial displacement of the cyclotron resonance by changing the magnetic field. The power deposition location was determined using the soft X-ray response at the EC shut-off (plus Thomson electron temperature profiles); when not coincident with a sawtooth crash, which perturbed the measurement. The maximum of the power deposition location coincides with the ray tracing prediction to within a few percent in normalised radius ρ, as shown in the example of Fig. 1. The low sampling rate of 10 kHz causes the measured deposition profile to appear wider due to the large time window needed for the measurement. The location of the maximum, however, is not affected by the window choice. Absorption location - The absorption location of the EC power strongly affects the resulting central temperature, confinement and sawtooth behaviour, as shown in Fig. 2, depicting a B-field scan (0.5 MW). The deposition is swept across the sawtooth inversion