DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 321: RWM feedback control in ITER-like low rotation plasmas | ||
| Name: | Ted Strait ( |
Affiliation: | General Atomics |
| Research Area: | RWM Physics | Presentation time: | Requested | Co-Author(s): | -- |
| Description: | The goal of this experiment is to determine whether active RWM feedback control improves the stability of low-rotation plasmas above the no-wall beta limit. In contrast to previous attempts at this goal, the starting point here will be a plasma with active feedback. The key issues to be addressed are
(1) does the stability become poorer when feedback is turned off? (2) what instabilities are encountered with and without feedback: ideal or resistive? | ||
| Experimental Approach/Plan: | Use a low-triangularity LSN plasma with a low no-wall beta limit. Begin with I-coil feedback turned on, using feedback parameters based on previous experience.
With beta above the no-wall limit, reduce the rotation at constant beta using counter NBI until omega_E is zero (derate the toroidal field a little, if necessary). Look for the onset of an RWM or other instability at low rotation. Document the rotation profile at onset, and the radial structure of the mode. Repeat with feedback turned off. Also compare with intermediate values of feedback gain. | ||
| Background: | RWM feedback experiments in 2007 reinforced the need for active feedback control in high-beta AT plasmas to provide stability during ELMs and other transient events. These experiments also gave us confidence that the feedback system, with its present configuration and gain settings, is indeed capable of stabilizing RWMs.
However, the need and efficacy of active feedback at low rotation remains an open question. This issue has been clouded by two questions: (1) Is feedback actually required in a quiescent plasma, or do kinetic effects stabilize the RWM down to zero rotation? (2) At low rotation, are we still addressing the ideal-kink RWM, or is there a larger coupling to non-rotating islands which are less amenable to feedback control? A separate proposal addresses stability at low rotation without feedback. Here we attempt to address the role of feedback at low rotation. This issue is highly relevant to ITER design. ITER's anticipated strategy is to reserve space for "port plug" coils for RWM control. However, to make this design change credible and to keep the space from being reclaimed for other uses, we need a clear demonstration of RWM feedback control in an ITER-relevant low-rotation regime. | ||
| Resource Requirements: | I-coils and 24 audio amplifiers. | ||
| Diagnostic Requirements: | All available diagnostics should be used to determine whether the observed non-rotating modes have an ideal kink or island structure: SXR toroidal array, ECE, Thomson scattering, CER (Ti profile), MSE, maybe BES.
CER rotation measurement is also critical. | ||
| Analysis Requirements: | -- | ||
| Other Requirements: | -- | ||
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