DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2013 EXPERIMENTAL CAMPAIGN Review | Direct submission with log-in | Request submission without log-in
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Title	32: High Beta Hybrids and Pressure Profile Broadening
Name:	Clinton (Craig) Petty petty@fusion.gat.com	Affiliation:	General Atomics
Research Area:	Steady State Heating and Current Drive	Presentation time:	Not requested
Co-Author(s):		ITPA Joint Experiment :	No
Description:	Use 5 MW of off-axis beam injection to broaden the total pressure profile compared to on-axis injection. Most of this will be due to a change in the fast ion pressure profile, but some broadening of the thermal pressure profile may also occur depending upon how stiff the transport dependence is. Determine whether the broader pressure profile allows a high beta_N to be obtained in steady-state hybrid plasmas, with the goal being beta_N=4. Will also calculate whether the ideal wall limit changes significantly with the broader pressure profile for these low q_min discharges. The off-axis beam will probably not effect the current drive profile much since the off-axis NBCD efficiency remains high, and the poloidal magnetic flux pumping inherent in hybrids tends to keep the total current profile constant regardless of the driven current profile. For steady-state considerations, the co-ECCD should be deposited inside the q=1.5 surface for this experiment. However, we could broaden the scope of this experiment by re-directing some of the ECH power (probably 4 gyrotrons) to deposit at the q=2 surface to see if we can suppress the 2/1 mode. This would be deem a success if the beta limit comes from a RWM rather than a 2/1 mode (the latter is the current situation).	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	(1) Begin with a 1 MA, high-beta hybrid case with 4 co-/on-axis beams that would serve as a fiducial. Lower BT until a hard beta limit is found (likely from a 2/1 mode); we expect to at least reach beta_N of 3.4-3.5 given previous results. (2) Repeat the fiducial case, but add the 150 beamline tilting fully downwards. Scan the NBI power for the other co-beams to vary beta_N. Determine the limit for the 2/1 mode, the goal being beta_N=4. (3) If time permits, compare the stability limit for cases where all the co-ECCD is deposited inside the q=1.5 surface, and where a minimum of 4 gyrotrons are aimed at the q=2 surface.
Background:	High beta hybrids have been operated stably (to the 2/1 mode) up to at least beta_N=3.5 at high density, which is well above the ideal no-wall limit. At lower densities and with central co-ECCD, high beta hybrid plasmas have been created with beta_N=3.4 and nearly zero loop voltage (10 mV). TRANSP calculations show that these plasmas should be very close to fully noninductive. The ideal wall stability limit is calculated to be above beta_N=4 by DCON. During one half-day experiment, however, a lower stability limit of beta_N<3.2 was found. It was found that these discharges had a systematically more peaked pressure profile than the previous cases, which can explain the lower beta limit. The near term goal of high beta hybrid research is to obtain beta_N=4 with zero loop voltage for as long as the beams will run. Based upon experimental experience that broader pressure profiles yield higher stability limits, some broadening of the pressure profile is desirable. This can be achieved using the off-axis beam. Since the NBCD efficiency remains high even for off-axis injection (especially with positive B_T), we do not have to give up on the steady-state goal to do this experiment.
Resource Requirements:	NBI: Tilted 150 beamline is critical. All 6 co-beams are needed. ECH: 6 gyrotrons required.
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