DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 177: Compatibility of the radiative divertor with high performance plasma operation | ||
| Name: | Thomas W. Petrie ( |
Affiliation: | General Atomics |
| Research Area: | Core-Edge Integration | Presentation time: | Requested | Co-Author(s): | -- |
| Description: | This study will combine ALL the essential elements for making the first real test of a radiating divertor concept in an AT/hybrid DN (or near-DN) plasma, using realistic high triangular shape and particle exhaust configuration anticipated for high performance tokamaks. PRESENTLY, ONLY DIII-D HAS THE CAPABILITY TO MAKE THIS TEST. Argon is injected into the private flux region near the upper outer divertor separatrix target. Enhanced deuterium plasma flow toward the divertor in the low field SOL is enhanced by a combination of deuterium gas injected upstream of both outer divertor targets and active cryo-pumping from both outer divertor locations. Locating the optimum value of magnetic balance (dRsep) that results in a significant reduction in divertor heat flux while still maintaining good AT-properties is the focus of this experiment. | ||
| Experimental Approach/Plan: | The plasmas are DN and near-DN AT, and the upper inner- and both outer divertor cryo-pumps are at liquid helium temperature. The gradB-ion drift direction is downward. This experiment is probably best done in as follows:
* First establish the sensitivity of AT plasmas to deuterium gas injection at dRsep = +1.5 cm. Scan the deuterium gas puff rate, i.e., establish operational limit to how much D2 gas injection the AT plasma can accommodate before plasma degradation results; trace argon is injected into the private flux region of the upper divertor, * Repeat for dRsep values closer to DN. Establish the minimum dRsep for which AT/hybrid conditions can be maintained during significant gas puffing, and * Scan of the argon injection rate at a reasonable D2 injection rate at this minimum dRsep at the best D2 injection. Important measurables from this experiment are the changes in the (poloidal) radiated power distribution and heat flux values, changes in the density and electron temperature at the divertor targets, and the accumulation of argon in the core and divertor plasmas. | ||
| Background: | High performance AT and hybrid plasmas in the double-null (DN) and near-DN configurations are attractive for future power reactor operation due to their high toroidal beta and energy confinement properties. However, for futuristic AT-machines (like ARIES-AT), there can be severe divertor power loading problems. A possible way of reducing excessive power loading at the divertor target(s) is to radiate significant power outside the main plasma, mainly in the divertor (hence, radiative divertor). But the resulting divertor cooling may also lead to a cooling of the upstream (core) plasma, which, in turn, may result in a marked degradation in AT-edge properties (e.g., bootstrap current). The expected increase in the argon presence in the pedestal can also be expected to affect the AT-pedestal adversely.
Previous work with radiating divertor H-mode DN plasmas has shown that the �??balanced�?� DN results in overly rapid accumulation of the seeded impurity (argon) in the core plasma. Two important reasons for this are (1) the relatively easy penetration of an impurity specie from the high field side into the core plasma of the DN and (2) the particle drifts in the scrape-off layer plasma in one of the divertors that always assist in the escape of injected impurities from the divertor region to the vulnerable high field side SOL [NF2007(submitted),APS2007]. On the other hand, the radiating divertor was shown to be effective in magnetically unbalanced DNs (dRsep=+1 cm) for reducing divertor heat flux while still maintaining good (hybrid) H-mode properties. Clearly, there is an optimal value of dRsep between 0 and +1 (with gradB drift down) for which the divertor heat flux reduction is attained with minimal loss of good H-mode properties. Since the heat flux scrape-off width for this class of H-mode is about 0.5-0.6 cm, the �??optimal�?� dRsep for heat flux reduction AND maintaining good H-mode plasmas would likely fall between +0.5 cm and +1.0 cm. This suggests the best chance for compatibility between good AT-class AND heat flux reduction would also fall in this range in magnetic balance. | ||
| Resource Requirements: | Machine time: 1 day, dome- and both outer baffle cryo-pumps cold, minimum 6 beam sources. | ||
| Diagnostic Requirements: | Asdex gauges inside the dome and inside both outer baffles, core Thomson scattering, upper divertor and centerpost fixed Langmuir probes, upper divertor IRTV, and CER. | ||
| Analysis Requirements: | UEDGE, MIST, ONETWO | ||
| Other Requirements: | -- | ||
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