Abstract
Highly localized deposition of ECRH/ECCD is particularly suited for MHD control, in particular when combined with real-time beam orientation and power control capabilities. The powerful (4.5MW) and flexible (7 steerable launcher) EC system on TCV has recently been complemented by an equally flexible digital real-time control system with the aim of developing and testing integrated MHD control methods [1]. Sawtooth pacing is one such method [2]. The crash time of stabilized sawteeth can be precisely controled by removing the EC power at a given time after the last sawtooth crash, causing the crash to occur at a short and reproducible time thereafter. This control strategy is combined with efficient neoclassical tearing mode (NTM) preemption by depositing power at the mode rational surfaces only during a short time synchronized with the island-seeding sawtooth crash. If an NTM appears nevertheless, full power is applied to stabilize the mode. The real-time steerable launchers have also been employed to stabilize fully saturated NTMs and to investigate the precise requirements for deposition localization for full island stabilization. Finally, though ELM dynamics is markedly different, recent results show that ELM pacing is possible using a similar control technique as used for sawtooth pacing. In this case, edge EC power is removed after each ELM, and is reapplied after a programmable time interval. The ELM period can be real-time controlled by adjusting the length of this interval. While the overall trend conforms to the increase of ELM frequency with increasing power, this technique provides a means to significantly regularize the ELM cycle.
Highlights
ECRH/ECCD constitutes a prime surgical tool for controlling plasma MHD mode activity
Individual sawtooth crashes and Edge Localized Mode (ELM) events have been shown to be precisely controllable by suitably timed EC power
neoclassical tearing mode (NTM) stabilization is combined with sawtooth pacing in a preemptive NTM suppression strategy, where power is added just at the sawtooth crash time to preempt a possible seed island from growing into a fully saturated NTM
Summary
ECRH/ECCD constitutes a prime surgical tool for controlling plasma MHD mode activity. Use not yet decided from 6 gyrotrons powered by two independent power supplies (3 gyrotrons per power supply [3]) These powerful heating and current drive actuators receive commands from the recently commissioned digital control system [4]. This system processes information from serveral multi-chord diagnostics and provides command references to the poloidal steering mirrors and/or gyrotron power supplies The first node contains a real-time sawtooth detector based on x-ray signals, as well as the real-time decision-logic algorithms to actuate the various phases of the discharge This node operates at a clock rate of 10kHz in these experiments. The second node treats signals from magnetics and photodiodes at 50kHz and runs a digital PLL (Phase Locked Loop) for NTM detection and/or an ELM detector algorithm based on a threshold of the photodiode (Hα) emissions
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