DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 174: What is the nature of the heat flux outside a slot divertor and are particle drifts important? | ||
| Name: | Thomas W. Petrie ( |
Affiliation: | General Atomics |
| Research Area: | Thermal Transport in the Plasma Boundary | Presentation time: | Not requested | Co-Author(s): | N. Brooks |
| Description: | Investigate the dependence of heating on the baffle top on particle drifts in the SOL and divertor. In the lower SN cases addressed in this experiment, we have found greater interaction of the plasma with the top of the lower baffle extension for the grad-Bt direction out of rather than into the lower divertor, if these particle drifts are important. We determine the differences in the scrapeoff widths in density, temperature, and heat flux for each Bt direction, in attached and detached cases. If the divertor scrape-off widths in ne, Te, and Qp are significantly different in the forward and reverse Bt cases, this will require two different divertor slot widths. This piece of information bears directly on an acceptable design of a slot divertor for ITER and beyond. | ||
| Experimental Approach/Plan: | The plasma is a high triangularity, lower SN, with dRsep = 3.0 cm. The toroidal field direction is forward, i.e., the direction of the gradB ion drift is downward. The outer lower cryopump is cold. (1) Use the D2 gas injector from locations upstream of the lower divertor (e.g., GasA). Scan the D2 gas injection rate up to and including the H-L back-transition. This range in density will include the partial detachment of the outer divertor separatrix. Reverse the Bt direction and repeat.
Important measurables from this experiment are the changes in the heat flux (IR) and particle flux (Langmuir probes) profiles in the slot and along the top of the lower baffle. Upstream electron density, electron temperature, and ion temperature measurements in the pedestal and near-scrapeoff layer (Thomson scattering and CER) are essential. Radial sweeps of the outer strike point inward are likely to be needed for a full radial profile in ne, Te, and Qp. | ||
| Background: | During the plasma shaping experiments from 2000 we noted that the heat flux on the horizontal section of the upper outer baffle shelf was NOT reduced during high density, radiating upper SN divertor operation, even though the heat flux inside the slot (i.e., between the dome and baffle pump opening) was reduced 3-5 times. Even under high density, detached conditions inside the slot, there was no reduction of heat flux outside the lip of the divertor "slot". Formation of a strong radiating region on the horizontal upper shelf with significant heating on the baffle top were observed. Energetic ions far out in the SOL, largely decoupled from the radiatively-cooled electrons, were thought to be responsible for this undiminished heating on the horizontal shelf of the upper outer baffle. Subsequent analysis with the UEDGE code has suggested that particle drifts (e.g., ExB and grad-Bt) may have been responsible for the spillover interaction the plasma with the baffle. Reversing the direction of the grad-Bt drift reduced this plasma-baffle interaction. The same kind of interaction was observed again during the summer�??s radiating divertor experiments. More recent UEDGE analysis indicates that drifts in the SOL and divertor are playing key roles.
Engineering implications of this are significant: If the electrons and ions are decoupled in the SOL outside the slot, then radiative divertor operation has little effect on heat flux outside the slot. Hence, the width of the slot must be widened when the grad-Bt drift direction is out of the divertor. If our supposition about drifts is correct, then a narrower slot can be used if the grad-Bt drift is into the divertor. Should the "slot" divertor concept be applied to future generation, high power tokamaks, our understanding of this result would be particularly important. | ||
| Resource Requirements: | Machine time: 0.2 day (forward Bt) + 0.2 day (reverse Bt), lower divertor cryo-pumps cold, minimum 6 beam sources. | ||
| Diagnostic Requirements: | Asdex gauges inside the lower outer baffle, IRTV monitoring the lower divertor, core Thomson scattering, all of the lower divertor fixed Langmuir probes, filterscopes, and CER. | ||
| Analysis Requirements: | UEDGE | ||
| Other Requirements: | -- | ||
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