DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 104: Excitation of the Geodesic Acoustic Mode via Radial Field Oscillation | ||
| Name: | George R. McKee ( |
Affiliation: | University of Wisconsin, Madison |
| Research Area: | Transport | Presentation time: | Requested | Co-Author(s): | G. McKee, A. Garofalo, C. Holland, G. Jackson, M. Shafer, D. Schlossberg |
| Description: | Attempt to perturb, excite and amplify the Geodesic Acoustic Mode, a coherent electrostatic zonal flow oscillation, using the high-frequency capability of the I-Coils. Measure the turbulence and GAM response to this radial field perturbation. | ||
| Experimental Approach/Plan: | Establish plasma conditions were the GAM has been observed: USN plasmas at moderate power (1-2 sources, co-injected, including 150L (steady) and 30L and 330L modulated out of phase). q-scaling experiments indicate that the GAM oscillation is stronger in higher q-discharges, so pick a higher q95 condition, e.g., Ip=1.0 MA, B_t=2.0 MA (119526). The I-Coil would be setup in an n=0, m=1 configuration (upper and lower coils 180 out of phase) and run near 15 kHz (SPAs operate at up to 40-100 kHz, so this should is feasible). The SPAs/I-Coils can operate at up to 250A (estimated by Gary Jackson) at 15 kHz. Radial field estimates can be made in the future.
Establish basic plasma condition and benchmark GAM parameters with radial scan of BES. Turn on I-Coil in above configuration at near 15 kHz, the known GAM frequency. Repeat this at several different frequencies in the expected GAM range (14-18 kHz). | ||
| Background: | The Geodesic Acoustic Mode (GAM), a class of high frequency zonal flows, has been observed at DIII-D in the outer regions of L-mode discharges. It has been measured via high-frequency poloidal velocity analysis of the turbulence obtained with BES. It is predicted to be radially localized, but is poloidally and azimuthally symmetric (m=0, n=0), consistent with the flow measurements obtained at the outboard midplane. Theoretically, it is predicted to have an m=1, n=0 pressure sideband as a result of the non-uniform ExB flow on a flux surface. The pressure buildup, nominally at the "top" and "bottom" of the plasma, relaxes via a radial drift current which gives rise to the very coherent GAM oscillation under some plasma conditions.
Typically, the GAM is observed near 15 kHz, peaking near r/a = 0.85-0.95, consistent with its predicted frequency of omega=c_s/R. The GAM is in principle capable of shearing turbulent eddies, and thus controlling and mitigating the saturated level of turbulence and resulting transport. It has also been shown to interact nonlinearly with the turbulence, driving a forward transfer of internal energy [C. Holland, PoP (2007)]. Estimates of GAM shearing rates suggest that its shearing rate is comparable to the turbulence decorrelation rate and thus might be playing a role in turbulence saturation. If it were possible to amplify the GAM, it might be feasible to control and perhaps further reduce turbulence and resulting transport. The question is whether there is a feasible method of tweaking/perturbing/amplifying the the GAM. The new high frequency I-Coil and audio amplifiers implemented at DIII-D potentially offers such a mechanism. The concept would be to generate an n=0, m=~1 radial magnetic field perturbation at or near the GAM frequency with the I-Coil. The I-Coils produce a radial magnetic field. This might interact with the GAM in one of two (or more?) ways: 1) by producing a radial field that amplifies the radial drift current and thus the pressure relaxation, and 2) by creating a small pressure perturbation through small but finite equilibrium shape modification that enhances the pressure sideband. Quantitative estimates of the radial magnetic field should be performed to assess the feasibility of this. | ||
| Resource Requirements: | Neutral beams: 150L, 330L, 30L, I-Coils, SPAs configured for n=0, m=1,10-20 kHz operation | ||
| Diagnostic Requirements: | BES, Doppler Reflectometer, CECE | ||
| Analysis Requirements: | TDE analysis of BES data for GAM studies | ||
| Other Requirements: | -- | ||
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