DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 482: Test of resistive MHD linear stability threshold and saturated island structure against models | ||
| Name: | Lang L Lao ( |
Affiliation: | General Atomics |
| Research Area: | Integrated Modeling | Presentation time: | Not requested | Co-Author(s): | M. Chu, V. Izzo, T. Luce, R. Prater |
| Description: | Test resistive MHD classical linear stability threshold and saturated magnetic island structure against theoretical models by variation of local current density gradient and magnetic shear using ECCD, target density, and current ramp techniques. | ||
| Experimental Approach/Plan: | Produce low NBI L-mode plasmas and excite 3/2 and 2/1 classical tearing modes by varying the local current density gradient and magnetic shear at the 3/2 and 2/1 surfaces using ECCD, gas puff, and current ramp techniques. Fully document plasma equilibrium pressure and current profiles and fluctuation data for comparison against models and predictions from resistive MHD codes such as PEST3 and NIMROD. For ease of comparison against NIMROD, use a plasma with low magnetic Reynolds number. Start with a moderately high q95 plasma so that the q=2 surface is not too close to the edge. Apply co-ECCD to excite 3/2 and 2/1 modes. Reduce beam power if necessary to keep the islands saturated at constant amplitude. Then slowly rotating the island by varying the co-counter beam mix to move the island past the profile diagnostics (CER, BES, Thomson, MSE, etc.) It is expected that the saturated island size is largest when the co-ECCD sweeps across the singular surface and then drops precipitously across when the current drive layer moves outside. Repeat with current ramp technique and reduced target density. | ||
| Background: | Resistive MHD instabilities such as tearing modes play an important role in plasma confinement and stability. Although ideal MHD has become well established as a reliable predictive tool for tokamak stability limits, resistive MHD is less well understood. Resistive MHD models and codes such as PEST3 and NIMROD are available for evaluation of these instabilities, but very limited benchmarkings against experimental data have been performed. In a tokamak, tearing instabilities are driven primarily by the radial gradient of the equilibrium current density. The free energy driving theses instabilities arises from the non-uniform distribution of the current density that is concentrated near the neighborhood of a rational magnetic surface. This driving force can be characterized by the delta' parameter. As a first step toward development of a predictive resistive MHD tool, we propose to validate their linear stability threshold and saturated island structure by testing their dependence on the local current density gradient and magnetic shear against PEST3 and NIMROD predictions. | ||
| Resource Requirements: | 1 day experiment, 3 gyrotrons, 4 NBI sources (co+counter) | ||
| Diagnostic Requirements: | Thomson, CER, MSE, ECE, BES, SXR | ||
| Analysis Requirements: | Kinetic EFITs, PEST3, NIMROD, TORAY-GA | ||
| Other Requirements: | -- | ||
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