DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 164: Catastrophic MHD-affiliated non-axisymmetric fields | ||
| Name: | Yongkyoon In ( |
Affiliation: | FARTECH, Inc. |
| Research Area: | Error Fields | Presentation time: | Requested | Co-Author(s): | M. Okabayashi, E. Strait |
| Description: | The proposal is to investigate the 'unknown' inherent non-axisymmetric fields in DIII-D plasmas that would readily interact with other catastrophic MHD activity (e.g. RWM, ELMs, NTM or locked modes). If successful, we may be able to not only characterize these error fields but also help to establish the control algorithm to either mitigate or divert the catastrophic influences, though it would be ideal to eradicate catastrophic MHDs. | ||
| Experimental Approach/Plan: | Identify a set of plasma conditions that would be influenced by the non-axisymmetric error fields (n = 1, 2 and 3). For example, three plasma sets would be listed;
1) high beta, high torque plasmas (for ELM-induced RWM), 2) high beta, low torque plasmas (for NTM or RWM-prone plasmas), 3) low density plasmas near locked mode condition Once the plasma conditions are repeatable, sweep the externally stimulated non-axisymmetric field (e.g. n= 1, 2, 3) toroidally to observe the plasma responses. Since the relevant waveforms might not be the same for different types of plasmas, it would be better to investigate each case (which is expected to require half a day) in separate run days. Regardless of the types of the runs, the 'unknown' field that is inherently combining the vacuum and plasma non-axisymmetric field would be measurable. If a certain toroidal angle has stronger plasma responses for all the n spectra, the error field source might reside externally. If no preferable toroidal angle is found, it would suggest that no dominant non-axisymmetric fields are present. | ||
| Background: | Recent experiments showed that RWM could be triggered by ELMs in high torque plasmas, where the plasma rotation supposedly exceeded the RWM rotational thresholds. Although the mechanism needs to be thoroughly investigated, one could imagine that a wall stabilized RWM would interact with ELM-induced non-axisymmetric components and then the resonant field would be amplified enough to drag the plasma rotation below the effective rotation threshold, resulting in unstable RWM. Similar observations were made even with externally stimulated n=1 pulses, which are supposed to mimic the ELM-induced n=1 error fields. Interestingly, during a toroidal sweep of such external n=1 pulses, there was stronger plasma response in a certain toroidal angle than in the other toroidal angles. This indicates that even the 'unknown' error field, which is likely to be a sum of all the residual error fields from machine and plasmas, would have a certain preferred toroidal angle. Considering that the goal of the error field study is ultimately relevant to understand the plasma responses in various plasma conditions, it would be more relevant and practical to apply various waveforms of externally controlled non-axisymmetri fields (n=1, 2, and 3) and then investigate the 'unknown' fields. | ||
| Resource Requirements: | -- | ||
| Diagnostic Requirements: | -- | ||
| Analysis Requirements: | -- | ||
| Other Requirements: | fully independent I-coil control is desirable | ||
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