DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 169: Can the RMP coils eliminate ELMs from SNs with B x gradB out of the divertor? | ||
| Name: | Thomas W. Petrie ( |
Affiliation: | General Atomics |
| Research Area: | ELM Control & Pedestal Physics | Presentation time: | Not requested | Co-Author(s): | T. Evans, M. Fenstermacher, and M. Schaffer |
| Description: | This experiment is the first necessary step for determining whether the ELM suppression method developed here at DIII-D is compatible with radiating divertor scenarios. Previous DIII-D studies focused on the effect that particle drifts in the SOL/divertor had on fueling, particle pumping, and radiating divertor behavior. We concluded that the most promising (only?) way to successfully employ a radiating divertor in order to reduce heat flux at the divertor targets with a minimal cost to plasma core H-mode properties was to use a SN plasma characterized by having the gradB ion drift directed OUT of the divertor. Presently, however, it is unclear whether ELM suppression in SNs using the RMP coils is attainable, if the gradB ion drift is directed out of the divertor. In this experiment, we investigate if it is possible to suppress ELMs of a SN plasma with the gradB drift direction out of the divertor. Once demonstrating the feasibility of eliminating ELMs under these conditions, we are then ready to move onto the next step in demonstrating the feasibility of RMP ELM suppression in a radiating divertor environment. | ||
| Experimental Approach/Plan: | The upper SN plasma is maintained in a standard ELMing H-mode regime (i.e., Ip =1.2 MA, Bt = -1.75 T, dRsep = +1.0 cm, q95=4.2, and Pinj = 6 MW). These parameters yielded the best of the radiating divertor results, but the resulting q95 may (or may not) be optimal for ELM suppression with the I-coil. To identify the range in q95 that yields the best prospects for ELM suppression, q95 is reduced during the shot by reducing Bt while the I-coil current is set to maximum. Once this q95 range is identified, choose value of q95 in the middle of that range and run successive shots with increasingly lower I-coil current. This is done to identify the minimum coil current, so as to minimize the perturbing effect of the RMP on the pedestal region. | ||
| Background: | Eliminating ELMs from H-mode plasmas using the I-coil approach presents an interesting possibility for resolving the ELM-issue in ITER and future highly powered tokamaks. Yet, even if the damage to the divertor structure from ELMs pulses were eliminated via the I-coil approach, steady peak power loading at the divertor targets could still be unacceptably high. A radiating divertor solution, whereby an impurity gas is injected into a pumped divertor with simultaneous deuterium gas puffing upstream of the divertor, has shown promise as a way to reduce peak power loading at the divertor targets without concomitant degradation of the ELMing H-mode plasma properties [IAEA 2006, PSI2006]. However, in combining the I-coil approach with such puff and pump scenarios while still maintaining favorable H-mode operation, the injected impurity must still be prevented from escaping the divertor and contaminating the main plasma.
The most promising radiating divertor scenario involves using a SN divertor with the gradB directed out of the divertor. However, it has not been demonstrated that a SN with the gradB out of the divertor is itself compatible to ELM suppression with the RMP coils. We suspect it is, because (1) SN plasmas run at typically lower density than corresponding plasmas with the gradB drift directed into the divertor and (2) SN plasmas with the gradB drift out of the divertor have a pumping rate on the outer separatrix that is about 50% greater than for a corresponding SN with the gradB drift into the divertor. Both these factors should result in lower collisionality in the pedestal in the gradB OUT case and thus better ELM suppression. Lower collisionality in the pedestal is helpful in ELM suppression with the RMP coils. As a result, the collisionality in the gradB out of the divertor cases will be lower than in the more standard gradB INTO cases, and the former would be expected to tolerate the higher gas puff rates needed to impede the escape of the impurities from the divertor. | ||
| Resource Requirements: | Machine time: 0.5 day (in forward Bt), I-coil, dome- and upper baffle cryo-pumps cold, minimum 6 beams. | ||
| Diagnostic Requirements: | Asdex gauges (in particular, dome and upper baffle locations), core Thomson scattering, upper divertor and centerpost fixed Langmuir probes, and CER. | ||
| Analysis Requirements: | UEDGE, with ONETWO | ||
| Other Requirements: | I-coil | ||
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