DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 170: Comparison of impurity screening between ELMing and ELM-suppressed plasmas? | ||
| 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 presents the second step in determining whether the ELM suppression method developed here at DIII-D is compatible with radiating divertor scenarios. This experiment, which uses non-perturbing (trace) argon under puff-and-pump scenarios, provides a side-by-side comparison of how well the injected impurity is screened in ELMing H-mode plasmas and in ELM-free H-mode plasma (with I-coil). DIII-D IS UNIQUELY CONFIGURED TO DO THIS EXPERIMENT. We focus on addressing the following questions: (1) Is there a significant difference in the argon accumulation in the plasma core under I-coil operation? (2) How does the exhaust enrichment change between the ELMing- and the ELM-free (I-coil) cases? | ||
| Experimental Approach/Plan: | The plasma is maintained in a standard ELMing H-mode regime under the conditions established in the lead-in experiment (e.g., Ip =1.2 MA, Bt = -1.75 T, dRsep = +1.0 cm, and upper SN). A trace amount of argon is injected into the private flux region of the upper divertor, while deuterium plasma flow toward the divertor is enhanced by a combination of deuterium gas injected from the bottom of the vessel and active cryo-pumping from both upper divertor locations. Trace argon is injected at a steady but trace level from t = 3.0 s to 7.0 s. A standard ELMing H-mode regime is established previous to t = 4.5 s of the discharge. At t = 4.5 s, the I-coil is activated and the ELMs are eliminated. This provides a direct comparison of the trace argon behavior between ELMing H-mode and ELM-free H-mode (I-coil) under similar conditions. [Note that the q95 and the coil current selected have been determined from the lead-in experiment: Can the RMP coils eliminate ELMs from SNs with the gradB OUT of the divertor�]
The deuterium injection rate is scanned. The key measurables are the accumulation of argon in the core and divertor plasmas. | ||
| Background: | Eliminating ELMs from H-mode plasmas using the I-coil approach presents an interesting possibility for resolving the ELM-issue in ITER. 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 maintaining favorable H-mode operation, the injected impurity must still be prevented from contaminating the main plasma. This is by no means assured, due to the ergodic nature of the pedestal. In this experiment, we compare the dynamics of impurity screening between ELMing H-mode plasmas and ELM-free H-mode (plus I-coil) plasmas. | ||
| Resource Requirements: | Machine time: < 0.5 day (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), Penning gauge, core Thomson scattering, upper divertor and centerpost fixed Langmuir probes, and CER. | ||
| Analysis Requirements: | UEDGE and ONETWO | ||
| Other Requirements: | I-coil | ||
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