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Title	249: Steady-state high beta with NCS and qmin>2
Name:	Andrea Garofalo garofalo@fusion.gat.com	Affiliation:	General Atomics
Research Area:	Steady State Heating and Current Drive	Presentation time:	Not requested
Co-Author(s):	J. Kinsey, C. Holland	ITPA Joint Experiment :	No
Description:	This is a re-entering of 2009 ROF proposal#376, which was a re-entering of 2005 ROF proposal #1118. Utilize new off-axis current drive capabilities and new lower divertor to extend duration and maximum betan of 2005 discharge 122959, which achieved betan=4 with qmin>2 and surface voltage=0 during plasma current flattop. This discharge used a Bt ramp-down technique for off-axis current drive. With tilted NBI and more gyrotron power this year, we may be able to replace the Bt ramp-down with steady-state compatible ECCD. The Bt ramp scenario also provides a unique scenario for several high priority FNSF studies: - study dependence of global energy confinement on negative shear of core q-profile, since Bt ramp discharges are unique in achieving very good confinement quality at high and very high qmin values. - divertor heat flux mitigation studies, since Bt ramp discharges are unique in maintaining very good confinement quality despite strong gas puffing leading to high upstream density value. - study of dependence of MHD stability limit and confinement quality on radius of qmin, since Bt ramp down is the most powerful tool available for current drive at rho~0.7 - producing optimal target for potential helicon off-axis current drive, since Bt ramp discharges are unique in the achievement of high electron beta at large minor radius	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	Restore discharge 122959. Test effect of new lower divertor on the ability to control the density. Investigate effect of varying the density on the evolution of the q-profile and the pressure profile (lower density is favorable for better ECCD efficiency). Use ECCD for co-Ip current drive at radius larger than rho(qmin). Use tilted beamline for off-axis current drive. Some counter-NBI may be used for counter-current drive at small radius, as long as lower rotation does not reduce confinement severely. Initially, use Bt ramp down as in 122959. Work toward reduced or no Bt ramp rate. Initially, keep betaN from exceeding 4 by using NBI feedback control. Work toward higher betan with improved noninductive current profile alignment.
Background:	Analysis of discharge 122959 shows that the high beta phase is terminated by a kink mode destabilized by qmin dropping below 2 because of noninductive current overdrive near rho of qmin. With the new lower divertor we should be able to keep the density low, slow down the qmin evolution and improve the ECCD efficiency. With higher available ECCD power and FW power we should be able to improve the noninductive current alignment, avoid the rapid qmin drop observed in 122959, and extend the duration at betaN=4 and qmin>2. The availabily of counter-Ip NBI provides an additional tool to oppose the noninductive current overdrive inside rho of qmin, and improve the noninductive current alignment.
Resource Requirements:	Normal Ip, reversed Bt for off-axis NBI CD. All gyrotron power
Diagnostic Requirements:	--
Analysis Requirements:	--
Other Requirements:	--