DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 71: Well Aligned Current Drive for Sustaining High qmin | ||
| Name: | C. Craig Petty ( |
Affiliation: | General Atomics |
| Research Area: | Core Integration (Steady-State Scenario) | Presentation time: | Not requested | Co-Author(s): | -- |
| Description: | Sustain the high qmin target profile produced by combined Ip and BT ramps by better aligning the sources of noninductive current drive and reducing the plasma density. The central current over-drive will be eliminated by using a more balanced beam injection. To increase the effectiveness of off-axis ECCD, the lower divertor cryopump will be used for density control in this high triangularity plasma shape. (The higher electron temperature resulting from the lower density should also slow the undesired current evolution.) The off-axis location of the ECCD will be determined by code analysis to give a flat loop voltage profile with the desired high qmin current profile. With the current profile better decoupled from the heating profile, high beta value can be possible obtained because the q profile will not be adversely affected by increasing the NBI power. | ||
| Experimental Approach/Plan: | Two days are required for this experiment, with high power ECCD only being required on the second day. The first day will concentrate on density control in the "flat shear" scenario with combined Ip and BT ramps. The mix of co and counter NBI will also be adjusted to eliminate the central current over-drive. Between shot analysis of the loop voltage profile will guide the experiment. After the first experimental day, off-line analysis of the current profile evolution will determine the optimal location for the ECCD to achieve a flap loop voltage profile which will be close to zero for 100% noninductive current drive. The choice of which BT value to end the toroidal field ramping is part of this consideration. On the second experimental day, the off-axis ECCD will be added to sustain the desired current profile with high qmin. Again between shot loop voltage analysis will help guide the experiment. High beta will help increase the bootstrap current for more off-axis current drive. Since the tayloring of the noninductive current profile will better decouple the heating profile from the q profile, a push to high beta values in the second day of this experiment will probably be more successful than previous attempts. | ||
| Background: | Recent analysis of the "flat shear" experiments in Thrust 8, which include the effect of ramping the toroidal field on Ohm's law, showed that the sources of noninductive current drive were not well aligned with the target current profile. The loop voltage profile was negative near the axis and positive near the edge, which indicates that there was too much central current drive and not enough off-axis current drive. As a result, even though the integrated noninductive current fraction was close to 100%, the current profile continued to evolve until a q=2 surface appeared in the plasma which lowered the beta limit. If a better aligned noninductive current profile is utilized, then the targer high qmin profile can be sustained. The central NBCD overdrive can be easily corrected in 2006 by substituting some counter NBI. To increase the off-axis current drive, it will first be necessary to lower the plasma density. The previous experiments were essentially unpumped because of the high triangularity plasma shape; however, the modification to the lower divertor will now make it possible pump on these plasmas. (The optimal direction of BT for optimal pumping will need to be determined in separate experiments.) Using six long-pulse gyrotrons will then allow increased off-axis current drive to be obtained. This proposal is related to ITPA Joint Experiment SSO-1.1. | ||
| Resource Requirements: | NBI: At least 5 sources.
EC: For 2nd day of experiment, need 5 gyrotrons. | ||
| Diagnostic Requirements: | MSE is critical. | ||
| Analysis Requirements: | -- | ||
| Other Requirements: | -- | ||
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