DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 154: Tearing Mode Beta Limits and Error Fields in Low Rotation Baseline Plasmas | ||
| Name: | Richard Buttery
( |
Affiliation: | UKAEA |
| Research Area: | Rotation Physics | Presentation time: | Requested | Co-Author(s): | R J La Haye, E Strait, D Brennan |
| Description: | A proposal is made to explore NTM thresholds at low rotation with error field ramps. In this way the sensitivity of ITER baseline plasmas to error fields can be determined to address: (i) whether error sensitivity is increased at low rotation and medium betan values; (ii) how broad the effect is in rotation value; (iii) what the scaling of the effect is for extrapolation to larger devices; and (iv) shed further light on NTM physics via the perturbative effects of the error field. These questions can be answered with a simple set of reliable shots taking about a day or so. Results will be used to determine error field correction and rotation requirements for mainstream scenarios in future large devices such as ITER. | ||
| Experimental Approach/Plan: | A simple and efficient scan is proposed: dial up specific torque and beta values, then apply error field ramps. Scan would explore a small grid in betan (1.5, 1.8) and torque (+/-50%, 25%, 12%, 0). At some chosen optimal point (low positive rotation, betan~1.8) 1d density and TF scans would then be performed to allow dimensional scaling to be obtained (using a dimensional constraint to infer machine size dependence, as previously used for Ohmic error field studies and confirmed by CMOD). Some additional shots with beta ramps at fixed error field and torque may augment the data set. A single phase scan would also checkout the intrinsic error role. Conditions: standard DIII-D single null ITER-like shape, q95=4.3 for good stability/disruption avoidance. Shots would be based on 2006 experiments where a robust shot design was arrived at. I coils would be used to correct intrinsic error or apply additional error, as these give cleanest and best understood error field spectra effects. | ||
| Background: | Recent DIII-D studies [Buttery EPS 2007] have shown that 2/1 tearing mode thresholds can fall significantly as net torque is removed, to betan values ~2. These studies also gave indications that beta thresholds were further lowered when error fields were applied, though data was limited to 3 points at medium (+~30%) torque. Certainly at high torque, an effect of error fields lowering NTM beta limits has been clearly observed [Buttery EPS05], while both experimentally and theoretically error field sensitivity is found/expected to be increased in low rotation plasmas [Buttery NF 2000, Fitzpatrick PP 1994].
Thus it is now time to systematically address this issue of error field sensitivity and impact for future devices such as ITER, with specific scans to assess the error field thresholds as a function of beta and rotation to determine the severity and breadth of the effect, and further scans in density and temperature, to infer scalings. The action of the error fields, in locally braking q=2 rotation is also a powerful tool to understand NTM physics, via the detailed observation of the chain of events involving plasma rotation profiles and mode onset (locked/rotating) behaviour. [ITPA MDC 3]. | ||
| Resource Requirements: | 2 co and 2 counter beams, plus 30L and 330L blips. I coils. ~1MA, 1.6T. Lower pump. | ||
| Diagnostic Requirements: | CER (all channels needed), ECE, magnetics, TS. | ||
| Analysis Requirements: | Results should be largely self evident, with some modest processing to obtain scalings, and standard DIII-D codes to obtain error field values. | ||
| Other Requirements: | None. | ||
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