DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 480: Physics and Power Threshold of Low Rotation L- to H-mode Transitions | ||
| Name: | Lothar Schmitz ( |
Affiliation: | University of California, Los Angeles |
| Research Area: | Transport | Presentation time: | Not requested | Co-Author(s): | G. Wang, E. Doyle, T.L. Rhodes, G. McKee, A White, W.A. Peebles, J. DeBoo |
| Description: | This experiment investigates the L-to H-mode transition physics and power threshold in low rotation NBI and ECRH-generated H-modes. In particular, the experiment addresses the ubiquity and significance of the 2007 observation that the power threshold with NBI heating is lower at low plasma rotation, and approximately equal for both directions of grad-B. Radial flow profiles and the effects of equilibrium (shear) flow and Zonal flows on plasma turbulence will be investigated with four-channel Doppler Reflectometry and BES across the L-to H-mode transition. A second experimental goal is to quantify density, and Bt dependence of the L-H transition power threshold at low rotation with the goal to extrapolate to ITER. | ||
| Experimental Approach/Plan: | Utilize same plasma shapes as for 2007 NBI rotation studies with early NBI and late ECRH (only) heating to induce L-H transitions and determine threshold power for both scenarios. Beam torque will be scanned for both directions of grad-B, focusing on the low torque range. A subset of scans will be repeated for two other plasma density values, and for 1-2 different values of Bt (time permitting). Diagnostic beam blips for CER/MSE will be used during the ECRH phase. The average and time dependent poloidal flow will be measured simultaneously with low-k and medium k density fluctuations spectra by Doppler reflectometry. (2cm-1< k pol < 5 cm-1), combined with BES (k pol < 3 cm-1). Doppler reflectometry can also measure the instantaneous correlation of the flow amplitude with the density fluctuation level. Four x-mode reflectometry channels (50-70 Ghz) can obtain a radial flow profile for pedestal densities n ped < 2.5x 10^13 cm^-3. | ||
| Background: | : The discovery that the L-H transition power threshold is lower at low rotation is one of the most significant results from the 2006/7 DIII-D run period. However, previously it was claimed that the power threshold for NBI and RF driven plasmas was similar. As ECRH driven plasmas have low rotation, are these two results contradictory? Given the gap in time since the previous ECRH driven L-H transition studies, the only way to address this is via new comparison experiments. There are also implications for QH-mode operation and projections to ITER in resolving this issue.
The 2007 BES results show significant differences in edge flow profiles and fluctuation characteristics as the torque is varied. Four-channel Doppler Reflectometry will be used for simultaneous measurements of the rotation profile, intermediate-k fluctuations, and Zonal Flows including GAMs (Geodesic Acoustic Modes). Zonal Flows are thought to regulate local turbulence levels. We propose simultaneous low and intermediate-k fluctuation measurements, along with poloidal flow measurements (<v pol> and v~ pol) in ELM-free H-mode in the L-mode edge and the H-mode transport barrier (pedestal) | ||
| Resource Requirements: | 7 beam sources, 5-6 gyrotrons. | ||
| Diagnostic Requirements: | BES, FIR low/intermediate k, CECE, profile reflectometry | ||
| Analysis Requirements: | -- | ||
| Other Requirements: | -- | ||
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