DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 155: 2/1 NTM beta limits in modest to strong counter torque 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 complete a previous torque scan exploring NTM beta threshold scaling with rotation. This will answer the critical question of how the threshold scales with increasing counter torque, which goes to the heart of understanding the physics of the behaviour, and therefore of extrapolations of NTM onset physics to future large devices. A simple scan in counter torque is proposed, which should take about half a day or so. | ||
| Experimental Approach/Plan: | Experiments would simply be an extension of the 2006 NTM rotation scan, with shots adapted for reverse field and current configuration. Points would be taken with up to 4 units of counter torque beams and down to balanced torque, to marry up with previous data. Shots would consist principally of beta ramps to access the 2/1 NTM, with some adjustment of net torque. Perhaps 7-8 data points are needed, though some work may be required to initially re-optimise shot in reverse field configuration, ensuring a robust front end (though a net counter start up to discharges was successfully devised in 2006). Conditions: standard DIII-D single null ITER-like shape, q95=4.3 for good stability/disruption avoidance. | ||
| 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. However the studies revealed a subtle dependence on rotation, with thresholds falling as co- torque is removed but then remaining low as counter torque and counter rotation was increased. Tantalisingly, the highest counter torque shots gave a slightly lower threshold still. Unfortunately this scan was limited by having only 2 beams in counter configuration. Further shots are needed with increased counter torque (by operating in reverse field configuration) to determine if this is the start of a trend towards even lower thresholds with more counter injection. Also more rotation data is needed for the counter rotation shots from 2006, where operational reasons (need to have a strong net counter torque) prevented obtaining as much CER data as we would have liked.
This is an important physics study to perform because it goes to the heart of understanding the physics, which is required for a prediction of the effect in future large devices such as ITER. The DIII-D results indicate that high magnitudes of rotation or rotation shear are not simply stabilising - stability depends on the sign of rotation as well. This opens up a number of other models that need to be tested with wider data scans, such ion polarisation currents, or changes in delta prime. The resolution of the counter torque behaviour will provide a key constraint on such models. [ITPA MDC 3]. | ||
| Resource Requirements: | 4 co and 2 counter beams, including 30L and 330L blips. I coils for error correction only. ~1MA, 1.6T. Lower pump. | ||
| Diagnostic Requirements: | CER (all channels needed), ECE, magnetics, TS. | ||
| Analysis Requirements: | Results should be largely self evident, but extensive CER analysis after wards to reconcile with physics models. | ||
| Other Requirements: | None. | ||
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