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Title	288: Amplifying the Geodesic Acoustic Mode via Resonant Radial Field Amplification: New Methods
Name:	George McKee mckee@fusion.gat.com	Affiliation:	University of Wisconsin
Research Area:	Turbulence & Transport	Presentation time:	Not requested
Co-Author(s):	K. Hallatschek, A. Garofalo, J. Hanson, G. Jackson, Z. Yan	ITPA Joint Experiment :	No
Description:	NOTE: This is a follow-up and continuation of an 2011 TJA experiment Amplify the naturally-occuring Geodesic Acoustic Mode, to control and suppress turbulence and associated transport near the plasma edge region while maintaining a non-ELMing L-mode condition. The goal is to achieve enhanced energy confinement via the resulting turbulence suppression. The experiment would exploit the high-frequency radial B-field capability of the DIII-D I-Coils, and measure the resulting turbulence and GAM response to this resonant radial field perturbation at the GAM frequency.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	Establish plasma conditions were the GAM has been clearly observed and has a relatively large "natural" amplitude: Upper-Single-Null L-mode plasmas at moderate power (2 sources, co-injected). Establish a moderate q95 condition, e.g., Ip=1.0 MA, B_t=2.0 MA, q95~6.5 (144872). The I-Coil will be configured in an n=0, m=0 configuration (upper and lower coils in phase) and run near 15 kHz using the high-frequency Audio Amplifiers connected to the I-Coils. Also try at lower field/current (1.0 T, 0.5 MA) to enhance the relative amplitude of applied field to plasma fields: increase B_r/B_T. Establish basic plasma conditions and benchmark GAM parameters with the 2D 8x8 BES array and toroidally-displaced DBS systems. Turn on I-Coil in above configuration at ~15 kHz, near the known GAM frequency range. Scan frequency in the expected GAM range (14-18 kHz). The radial field produced by the I-coil at these frequencies is relatively low: it is predicted to be of the order Br < 1 Gauss at this high frequency, based on measurements by G. Jackson. The relatively low field results from image currents in the wall at this frequency. It will need to be experimentally assessed whether this field is adequate to interact with and perturb or resonantly amplify the GAM.
Background:	The Geodesic Acoustic Mode (GAM), an electrostatic, coherent, radially-sheared zonal-flow oscillation, has been observed in DIII-D in the outer radial region of L-mode discharges. High-frequency poloidal velocity analysis of BES turbulence measurements have provided a detailed characterization of the GAM structure, which is also observed with the Doppler Backscattering diagnostic. The electrostatic potential and corresponding radial electric field is radially localized with well-defined k_radial, but is poloidally and azimuthally symmetric (m=0, n=0). 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, which has been observed in some experiments (AUG, HL-2A). The pressure oscillation, peaking at the "top" and "bottom" of the plasma, relaxes via a radial drift current which gives rise to the very coherent GAM oscillation under the right plasma conditions. Typically, the GAM is observed near 15 kHz, consistent with its predicted frequency of omega=c_s/R, and peaks spatially near r/a = 0.85-0.98. The GAM can shear turbulence, and thus reduce the saturated level of turbulence and resulting transport. It interacts nonlinearly with the turbulence, driving a forward transfer of internal energy to higher frequency/wavenumber [C. Holland, PoP (2007)]. Shearing rate estimates from the poloidal flow shear of the GAM, obtained from the time-varying radial gradient of poloidal velocity, suggests that its shearing rate is comparable to the turbulence decorrelation rate and thus should play a role in turbulence saturation and decorrelation. If it were possible to amplify the GAM, it might be feasible to control and reduce turbulence and resulting transport, thus improving energy confinement. The high frequency I-Coils and audio amplifiers implemented on DIII-D provide a possible mechanism to amplify the GAM. The concept is to generate a radial magnetic field perturbation at the GAM frequency with the I-Coils. It has been proposed (S. Cowley, Imperial College) that this field may interact with and amplify the GAM by creating a small pressure perturbation through equilibrium shape modulations, thus enhancing the pressure sideband by resonantly "squeezing" the flux surface at the GAM frequency. It is also possible that the radial field will interact with or amplify the radial drift current that creates the periodic pressure relaxation. An initial attempt of this experiment was performed in 2011 as the Torkil Jensen Award [McKee-May, 2011]. While the GAM was found and a radial field applied at the resonant frequency, no clear enhancement of the GAM was observed due to the resonant field. This may have been because the field amplitude was too low, or didn't have the best spectral mode structure. K. Hallatschek has since then performed simulations which suggest that the concept is viable, given adequate field amplitude, and suggests application of a different mode structure: even parity.
Resource Requirements:	I-coils configured in n=0, even parity configuration, connected to Audio Amps operating at high frequency (14-20 kHz). 2 NBI, USN plasma
Diagnostic Requirements:	BES (8x8 array configuration), DBS-5, DBS-8, CECE, Reciprocating probe with Reynolds Stress head
Analysis Requirements:
Other Requirements: