DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 373: Prompt turn-off of ECCD when no longer required or detrimental for NTM stabilization | ||
| Name: | Francesco Volpe ( |
Affiliation: | ORAU |
| Research Area: | NTM Stabilization | Presentation time: | Requested | Co-Author(s): | -- |
| Description: | PCS-control of ECCD timing, to turn it off as soon as the NTM stabilization is complete (according to Mirnov signals). | ||
| Experimental Approach/Plan: | The experimental approach is the same as in CW ECCD stabilization of 3/2 NTM, including alignment procedures. Once a reference stabilization discharge is obtained, the discharge should be repeated with a shorter ECCD pulse. In particular, ECCD should be turned off at the time at which stabilization was complete in the reference shot. If encouraging, one might automate the process by using BDOTEVENAMPL or similar trace in the PCS for a "dud detector" that would switch off the gyrotrons as soon as the mode is suppressed. | ||
| Background: | ECRH and co-ECCD are known to make Delta' more negative. Co-ECCD also replaces the "missing" Bootstrap current in a neoclassical island. The two effects stabilize NTMs, for deposition at the exact island centre. In reality, for full suppression it is sufficient to shrink the island below its marginal width (which relaxes the requirement on the alignment between ECRH/ECCD and the island), then the mode will automatically disappear, as shown at DIII-D. If well-aligned, continuous application of ECRH/ECCD after that moment is equivalent to pre-emptive ECCD, which has been shown to prevent the mode (re)appearance. If, on the other hand, the misalignment exceeds the marginal width, the island will be initially mitigated and its width will decrease but, eventually, it will become comparable with the misalignment and, at that point, a significant amount of current will be driven outside the island and have a destabilizing effect.
Note that the marginal width (hence, the alignment requirements) change with beta and tend to become more stringent as a result of NTM stabilization and of the accompanying increase in beta. Consequently, an initially good alignment might not be satisfactory later in the discharge, and the aforementioned destabilizing effect might take place even under conditions of initially good alignment. Moreover, the technique would readily make ECRH/ECCD resources available for other tasks, such as q-profile tailoring in hybrid and AT plasmas. This is particularly appealing in devices like DIII-D and ITER equipped with steerable launchers. | ||
| Resource Requirements: | -- | ||
| Diagnostic Requirements: | -- | ||
| Analysis Requirements: | -- | ||
| Other Requirements: | -- | ||
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