DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 298: Balance between FW edge absorption by far-field sheaths and central absorption | ||
| Name: | Robert I. Pinsker ( |
Affiliation: | General Atomics |
| Research Area: | Heating & Current Drive | Presentation time: | Not requested | Co-Author(s): | J.C. Hosea, F.W. Baity, E. Fredd, M. Porkolab |
| Description: | DIII-D experiments have shown that the central FW current drive efficiency can be accounted for quantitatively only if an edge loss on the order of a couple of percent per bounce is assumed. Experiments in ELMing H-modes showed that the edge loss increases significantly if the ELMs are so frequent that the FW cutoff moves near to the wall. These facts can be explained if the edge loss is predominantly due to rectified rf sheaths at the wall. If the edge losses are at all comparable to the central absorption, by changing only the edge loss, the fraction of power absorbed in the core should change, despite the core absorption per pass being held constant. In this proposed experiment, we measure the FWCD efficiency in L-mode, using the minimum amount of beam power necessary to obtain MSE data and using ECH to prepare the sawtooth-free portion of the discharge in which the current drive is measured. We vary the conditions in the plasma edge such that the dissipated power in sheaths should be reduced by increasing the separatrix/wall clearance in different poloidal locations, in an attempt to get a better idea where the important sheaths are located. When the outer gap is increased, the lower antenna loading will mean that the power per antenna will have to be lowered to be able to keep the total coupled FW power constant. Hence, the scan should begin with the maximum outer gap case in order to find the maximum reliably coupled FW power, and as the outer gap is reduced, it should be easy to keep the power constant. The prediction is that when the plasma/wall clearance in the important poloidal location is reduced, the core absorbed power will drop and hence the driven current will decrease. | ||
| Experimental Approach/Plan: | See description. | ||
| Background: | Experiments in NSTX on HHFW heating have arrived at a similar conclusion. In those experiments, by increasing the density at which the FW begins to propagate, the distance between that cutoff layer and the wall is increased, the edge losses decrease, and the core heating efficiency increases substantially. For NSTX, this increase in the cutoff density is achieved by raising the toroidal field. The complementary experiment in DIII-D, in which the toroidal field would be lowered and observe a decrease in central absorption, is difficult, because many things would change at the same time (proximity to the H-mode power threshold, electron thermal transport, ECH location). Hence in this proposal, we would vary the edge loss by changing the distance between the plasma edge and the wall, which should not affect the core absorption at all. | ||
| Resource Requirements: | 1 day experiment. 30L beam, 4 gyrotrons, all three FW systems. | ||
| Diagnostic Requirements: | MSE is the main diagnostic; all standard profile diagnostics. Edge reflectometry would be very useful to monitor edge density profiles directly. | ||
| Analysis Requirements: | Standard current drive analysis. | ||
| Other Requirements: | -- | ||
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