DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 186: Role of error fields in resistive wall mode stability | ||
| Name: | Ted Strait ( |
Affiliation: | General Atomics |
| Research Area: | RWM Physics | Presentation time: | Requested | Co-Author(s): | -- |
| Description: | The goals of this experiment are to:
(1) demonstrate the tradeoff between torque and error field correction required for RWM stability (2) test the hypothesis that the apparent critical velocity for RWM stability is in most cases a result of error field penetration (3) if possible, determine the "true" critical velocity in the absence of error fields -- This experiment could also be considered under the Rotation task force. | ||
| Experimental Approach/Plan: | In a plasma with beta above the no-wall stability limit, vary the error field shot by shot, and find the "critical rotation velocity"in each case.
(a) Use feedback to determine the optimal error field correction with combined C-coils and I-coils. (b) Ramp the neutral beam torque downward at constant beta. Look for the error field amplitude where the rotation locks and/or the RWM becomes unstable. If necessary, reduce Bt slightly to permit reaching zero torque at constant beta. (c) Program the C-coil and I-coil current to match the feedback output of the previous shot. Add a small n=1 error field at the beginning of the torque rampdown. Look for the critical torque and rotation as the error field is varied. The anticipated results relative the three goals described above are: (1) Critical neutral beam torque varies proportional to the error field. (2) The rotation threshold for RWM instability is half of the initial velocity (as observed previously). (3) If a non-zero critical velocity exists that does not depend on error field, the scaling of critical torque proportional to error field described in (1) should break down at small values of the error field. | ||
| Background: | This experiment will help to establish some key physics for ITER: the tolerance for error fields at high beta and low rotation, and the possible need for RWM feedback control coils. The decision on "port plug" coils for RWM control in ITER will be made in 1-2 years.
Some of the effects described above have been observed in a database covering multiple days and experimental configurations, and also in more systematic error field ramp experiments conducted early in 2007. This experiment is distinguished from the previous data in at least two ways: the error field is held constant in each discharge, as it would be in an ITER discharge, and the focus is on the behavior at small error field and torque. | ||
| Resource Requirements: | I-coils | ||
| 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.
CER rotation profiles are also critical. | ||
| Analysis Requirements: | -- | ||
| Other Requirements: | -- | ||
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