DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 141: Enhanced ohmic confinemnt using edge RMPs | ||
| Name: | Todd E. Evans ( |
Affiliation: | General Atomics |
| Research Area: | Hydrogen Discharges | Presentation time: | Not requested | Co-Author(s): | tbd |
| Description: | The goal of this experiment is to obtain enhanced ohmic confinemnt discharges using I- and C-coil edge RMPs in order to study the basic physics of edge transport barriers. A stochastic transport barrier is attractive because its properties can be controlled using the current and mode spectrum of the external coils. This provides a tool that can be used to test transport barrier theories and to study the properties of plasmas (especially the physics of the pedestal formation, edge flows, Er profiles and changes in turbulence levels) with enhanced confinement over a range of parameters such as q_a, fueling rates, ohmic heating power levels and plasma shapes. | ||
| Experimental Approach/Plan: | Circular HFS wall limited ohimic plasmas will be used to minimize the level of cross field transport. Circualr plasmas are needed to minimize the area of contact with the HFS wall and the local recycling flux from the wall. The discharge will be initiated with the minimum amount of gas fueling needed to get stable operating conditions and the gas feed will be turned off as soon as possible after the plasma current has fully penetrated. The density will be allowed to deacy at the natural rate determined by the HFS recycling and the confinement. Once a stable density decay rate has been established various combinations of I- and C-coil currents and mode spectra will be used to induce the edge stochasic transport barrier (ESTB). After obtaining reproducible ESTBs q_a will be scanned by changing Bt while keeping Ip and the ohmic input power fixed. Then at fixed q_a we will scan the ohmic heating power by varying Ip/bt. If time permits we will increase the elongation of the plasma and the contact area of the plasma with the HFS wall. This will increase the wall fueling and reduce the rate of decay in the density after the intial gasa fueling is turned off. We will use hydrogen discharges for the first experiment of this type since this may help improve our understanding the physics of edge transport barrier formation in the hydrogen phase of ITER. | ||
| Background: | Signatures of an ESTB formation were first seen during ergodic magneic limiter (EML) experiments in TEXT (1985). Hints of improvements in the confinement were also seen in the helical island divertor experiment (HIDEX) in JIPP T-IIU (1987) and strong enhancement in the particle confinement were seen in circular HFS limited ergodic divertor (ED) experiments in Tore Supra with both ohmic (1988) and lower hybrid heated plasmas (1989). More recently TEXTOR (2006) has reproduced the Tore Supra results using NBI heating in circular HFS limited plasmas with the dynamic ergodic divertor (DED). Based on the accumulated experience gained in these earlier experiments we should be able to produce an ESTB in DIII-D using the C- and/or I-coils (assuming we have a reasonably good edge spectrum and sufficient coil currents which we believe is the case). Since DIII-D has significantly better edge diagnostics, pedestal profile and equilibrium reconstruction tools than these previous experiemnts we have a unique opportunity to develop a basic understanding of the edge transport, pedestal and barrier formation physics and to compare the properties of the ESTB to those of edge transport barriers in H-modes. This may result in a better understanding of the physics that controls the L-H power threshold. | ||
| Resource Requirements: | I- and C-coil. | ||
| Diagnostic Requirements: | The full set of edge turbulence and transport diagnostics (except BES and CER altough we may want to introduce short beam blips to obtain theis data if comaptable with the ESTB). | ||
| Analysis Requirements: | -- | ||
| Other Requirements: | -- | ||
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