Feedback control of ECRH polarization on LHD

Abstract

Abstract. The polarization of ECRH waves, set by the orientation of a pair of corrugated mirror polarizers in the transmission line, determines the degree of coupling to O-and X-modes in the plasma and has an important effect on the first-pass absorption. Existing methods for determining the required polarization have been found adequate in most experiments. However, as the pulse length is increased it becomes increasingly important to maximize the first-pass absorption while the plasma or injection conditions change or when there can be significant O-to Xmode power coupling during propagation, particularly in the edge plasma region of a stellarator. This has motivated the development of a dedicated feedback control system which is able to adjust the polarizers' angles settings during the discharge in order to maintain the highest possible absorption. An extremum seeking controller is shown to successfully recover the optimum polarization setting during long-pulse ECRH experiments on LHD.

Optimization of Incident EC Wave Polarization in Real-Time Polarization Scan Experiments on LHD

“…The operation scenario of the ECRH injection system optimally preset before a discharge can result in a high performance plasma [4]. In addition, feedback control of the ECRH injection system set to an optimal target in real time can create a desired power deposition and reduce the stray radiation in the vessel during a long-pulse discharge [5].…”

Fast polarizers installation for ECRH and ECE in TCV

“…The TCV ECRH system transmission line uses HE 11 , 63.5 mm diameter, evacuated waveguides from General Atomics (GA), while the ObECE launcher antenna is connected to the same waveguide type (non-evacuated). Studies [8,9,12] indicate that the polarizer angle requirements in some typical experiments are of the order 4-10 • . To be able to perform experiments within TCV 2 s long shot, a bandwidth for polarization change between 8 Hz and 10 Hz is thus required.…”

“…Each driver is set to velocity mode and is programmed to convert a feed-back control voltage signal, sent to one of its analog inputs, to a direction and velocity. In the TCV studies using the RTM operation of the FP, polarization values are not pre-defined in the control algorithms: the polarizer mirrors are requested to rotate in specific directions at velocities proportional to the feed-back control signal value [8,9]. During an experiment, the polarizer angular positions are acquired by the TCV data acquisition system.…”

“…non-coupled) power is, in general, of little consequence, whereas in long pulsed fusion machines with more powerful ECRH systems, like ITER, such a situation might lead to damage of invessel components and/or diagnostics. To avoid this, we need to design, install and test ECRH systems for which the polarization can be varied during the plasma discharge [8,9].…”

“…While installed in the ECRH system, they will be used for different studies, one being the testing of methods for realtime feedback control of the polarization of the injected beams. A difficulty in such a control system is to choose a reliable feedback signal that estimates the absorbed power in the intended mode [8,9]. The possibility of using the stray power as this signal will be investigated, for example, by studying its response to the variation of the mode purity of the injected beam (using the FP to change its polarization), as in [13].…”