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	319: Impact of fast-ions on the RWM stability boundary
Name:	Francesca Turco turcof@fusion.gat.com	Affiliation:	Columbia University
Research Area:	General Physics	Presentation time:	Not requested
Co-Author(s):	J. Berkery, J. Bialek, J. Hanson, M. J. Lanctot, G. A. Navratil, M. Okabayashi, C. Paz-Soldan, S. Sabbagh, T. Strait, A. Turnbull	ITPA Joint Experiment :	No
Description:	The goal of this experiment is to study the physics of the RWM in the absence of fast-ions, in order to determine if the apparent stability of DIII-D discharges to the destabilization of the RWM is due to the presence of stabilizing fast particles. We propose to eliminate the presence of fast-ions completely, and asses the RWM stability under those conditions, by producing ECH-only plasmas (similar discharge obtained in the 2012 campaign, plus an extra 0.9 MW of ECH available in 2013) and then ramping the plasma current up quickly to produce an unstable current-driven RWM. The goal is to map the stability boundary in the complete absence of fast-ions, then progressively add NBI power (and hence adding fast-ions to the scenario) to determine whether the stability boundary moves, and ultimately if the RWM is stabilized in the presence of fast-ions, and by what amount and distribution of particles. When adding NBI power, care must be taken to use balanced co-counter injection to obtain the same level of rotation with and without beams. Even though the proposed scenario has intrinsically rather low betaN (~1.8), the physics of the destabilization of the RWM with and without fast-ions is universal and this experiment would provide crucial information on whether RWM stability poses a threat to the future machines that will not rely on fast-particle-producing heating systems. Moreover, if a way is found to run the scenario with qmin>1 for long enough, the experiment would provide the perfect (and only) platform to benchmark the stability codes (MARS-K, MISK, etc) that are needed to predict the MHD characteristics of future machines.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	a. Produce an ECH-only, diverted H-mode discharge (a starting point is 150840), with all the available ECH power (3.6 MW?). Insert a fast enough Ip ramp to destabilize a current-driven RWM. b. Obtain some data on the Ip ramp-rate necessary to destabilize the mode, and attempt to modify the scenario to obtain qmin>1 (early heating, co-ECCD off-axis, counter-ECCD on axis?). c. When the RWM is systematically reproducible, add balanced NBI power, as much as it can be used while maintaining a fixed betaN level (estimated 1-3 MW NBI), with on-axis or off-axis sources. Observe how (and if) the RWM stability boundary changes: is the original discharge stabilized? Is a faster/slower Ip ramp necessary to destabilize the mode? How does the stability change with on-axis vs off-axis fast-ion distributions?
Background:	Theoretical models suggest that the RWM stability is strongly affected by the presence of fast particles. In particular, is has been postulated that the RWM is stabilized in the DIII-D scenarios because of the fast ions produced by the NBI power, and machines that do not rely on NBI for heating and current drive may have issues with unstable RWMs when operating above the no-wall limit. Studies to assess the effects of fast-ions on the RWM stability have been preformed in DIII-D. However, while it was possible to modify the localization of the fast-ions and the fast-ion distribution function by means of the OA-NBI system in the previous campaign, this approach has proven difficult and the interpretation of the results hard to determine. This experiment proposes to tackle the problem in a different way, focusing on nailing down the physics in a scenario that allows for significant changes in the amount of fast-ions, and for a simple way to assess the RWM stability. The ECH-only shot would have no fast particles, and the discharges with different levels of NBI power and different injection angles will provide the direct knob to determine the effect of the amount and distribution of fast-ions. This test will allow us to create a map of the RWM stability under controlled conditions for fast-particles and rotation profiles.
Resource Requirements:	30 and 330 NBI sources, both 210 NBI sources. 6 gyrotrons at max power and max duration.
Diagnostic Requirements:	Magnetics, MSE and CER when NBI usage allows, Thomson scattering, ECE radiometer, density interferometer
Analysis Requirements:
Other Requirements: