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Title 172: Is the Radiating Divertor Scenario Compatible With ELM Suppression?
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 third in a series experiments for determining whether the I-coil method of ELM suppression is compatible with radiating divertor (i.e., puff and pump) scenarios. This experiment provides a side-by-side comparison of how a standard ELMing plasma and an ELM-free plasma (with I-coil) respond to a puff-and-pump scenario with a perturbing amount of argon as the injected impurity. DIII-D IS UNIQUELY CONFIGURED TO DO THIS EXPERIMENT. We focus on addressing the following questions:

(1) Is there a significant change in the argon accumulation in the plasma core under I-coil operation?

(2) How does argon entrainment in the divertor change when the I-coil is activated?

(3) How does the ratio of divertor-to-core radiated power change when the I-coil is activated?
Experimental Approach/Plan: The plasma is maintained in a standard ELMing H-mode regime (e.g., Ip =1.2 MA, Bt = -1.7 T, dRsep = +1.0 cm, and upper SN). This setup is similar to the one that was shown previously (IAEA2006) to perform robustly under the puff and pump scenario. Significant deuterium gas puffing, which is needed to raise the SOL plasma flow into the divertor, will also raise the density. A previous experiment will have shown that ELMs are suppressed under these conditions by a pre-selected I-coil current: Comparison of impurity screening between ELMing and ELM suppressed plasmas�. Hence, it is possible that adjustments to the I-coil settings may be necessary if the D2 injection rate is changed, as it is in this experiment.

After this prep work is done, 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. A radiating divertor plasma in an 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 between ELMing H-mode and ELM-free H-mode under similar plasma conditions.

The argon injection rate with a fixed D2 injection rate is scanned; then the D2 injection rate with a fixed argon injection rate is scanned. Important measurables are the changes in the radiated power distribution and heat flux values, the accumulation of argon in the core and divertor plasmas, and the density and temperature conditions at both divertor targets.
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 the peak power loading at the divertor targets without concomitant degradation of the ELMing H-mode plasma properties [IAEA2006 and PSI2006]. However, in combining the I-coil approach with such puff and pump scenarios, it is by no means clear that the injected impurities can be prevented from building up the main plasma as effectively as in the ELMing H-mode cases.
Resource Requirements: Machine time: 0.5 day, I-coil, dome- and upper baffle cryo-pumps cold, minimum 6 beams.
Diagnostic Requirements: Asdex gauges (in particular, in the dome and upper baffle locations), Penning gauge, core Thomson scattering, upper divertor and centerpost fixed Langmuir probes, and CER.
Analysis Requirements: UEDGE, ONETWO.
Other Requirements: I-coil