DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 83: Is the Reduced L-H Threshold With Counter Beams Due to Lost Ions, or Velocity per se? | ||
| Name: | John Degrassie ( |
Affiliation: | General Atomics |
| Research Area: | Rotation Physics | Presentation time: | Not requested | Co-Author(s): | G.McKee |
| Description: | *Measure the threshold power reduction as a function of target density. Fast ion loss from the counter beams is not dominated by prompt, first orbit loss, but rather by fast ions being lost before thermalization within the plasma. This loss tends to create a negative radial E field in the edge region. Reducing the fast ion slowing time in the target plasma, most readily with higher density, will reduce the number of counter flowing ions lost before thermalization.
*When the density is changed there will presumably be a different power threshold. If the toroidal velocity at the transition is the same, then we may not have separated the importance of fast ion loss versus the target velocity profile. If the velocity is different at the transition, then we have made the separation. | ||
| Experimental Approach/Plan: | *Start with the scan done in 2007 on L-H transition power with counter beams mixed in. Take a full co-beam case and one with added counter beams for a two point torque scan. Then raise the target density and repeat the two point torque scan for threshold power. Given the result, either raise, or lower the target density from the first level for a third density target.
*If it is clear that either lost fast ions, or the velocity per se is the controlling parameter, then devise a scan to reinforce this conclusion. | ||
| Background: | *Adding counter-Ip beams to the NBI mix results in a lower L-H transition power level. This could be due to the larger fraction of fast ions lost in the edge with counter injection as compared with co-injection, thereby promoting an edge negative E field and getting a head start on the H-mode negative E-well. Or there could be some reason that the lower target co-toroidal velocity itself is the driving factor in reducing the H-mode threshold. | ||
| Resource Requirements: | Same as for last years L-H power threshold scans. | ||
| Diagnostic Requirements: | -- | ||
| Analysis Requirements: | -- | ||
| Other Requirements: | -- | ||
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