MHD control and ECCD in Compass-D
1993
Electron cyclotron current drive experiments have been carried out in Compass-D using a power of approximately 500 kW at a frequency of 60 GHz. The fundamental resonance was used and the waves were launched from the high field side of the torus through four mirror-antennae. Significant asymmetry in the loop voltage behaviour is observed when comparing co and counter current drive cases, suggesting a driven current approximately 15 kA (in a plasma current of 130 kA). The BANDIT-3D Fokker Planck code has been used to model these discharges revealing a similar value for the predicted driven current at zero loop voltage but strong synergistic effects with the toroidal electric field at the approximately 0.4 Volts/turn typical of the experiments. This synergy appears to be sufficient to explain the inferred net 'co' current drive in most of the 'counter' current drive shots. In H-mode discharges featuring a strongly asymmetric single null separatrix configuration, discrete ELM's sometimes produce sufficient relaxation of the current profile to cause loss of vertical control, leading to vertical displacement events. Application of resonant magnetic perturbations (RMPs) of predominantly n=1 (with various low m values) is seen reliably to increase the frequency of the ELM's, although the exact mechanism underlying this effect has still to be elucidated. A real-time electronic neural network has been used for the first time for feedback control of the equilibrium in a tokamak. The neural network was used for feedback control of the plasma elongation throughout the discharge, while simultaneously monitoring the plasma vertical and horizontal position.
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