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	105: Optimization and exploration of the q95<2 scenario in DIII-D
Name:	Paolo Piovesan paolo.piovesan@igi.cnr.it	Affiliation:	Consorzio RFX
Research Area:	Stability & Disruption Avoidance	Presentation time:	Requested
Co-Author(s):	J. Bialek, A. Garofalo, G. Jackson, J. Hanson, M. J. Lanctot, E. Lazarus, L. Marrelli, P. Martin, G. A. Navratil, M. Okabayashi, C. Paz-Soldan, E. Strait, F. Turco, A. Turnbull, P. Zanca	ITPA Joint Experiment :	No
Description:	In the 2012 campaign, discharges with q95<2 have been realized in DIII-D in the context of the TJA, following an idea originally developed at RFX-mod. Careful optimization of the plasma start-up and shape and magnetic feedback control of MHD stability were crucial to the success of this proposal. Building on these results, we propose an experimental plan to fully explore and optimize this new scenario. The proposal would not only develop an interesting fusion scenario, but it would also give precious information on the physics of MHD stability control.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	The following options are listed in order of priority, with each step requiring the previous one to be completed. (a) Error field correction. Starting from shot 150593, further reduce the impact of error fields through the standard dynamic error field correction algorithm, i.e. apply feedforward coil currents based on what feedback ''suggests'' in previous shots and iterate for a few shots. This will allow to discriminate among error field correction and direct stabilization of the 2/1 RWM. (b) RWM FEEDBACK. If the 2/1 mode is still unstable, optimize fast feedback using the I-coils. To this end, we plan to use AC compensation and complex gains, varying the phase between the n=1 applied field and the n=1 plasma response. If these first two steps are successful, we plan to try and sustain the q95<2 phase as much as possible in L-mode. Fast feedback may be switched-off in short windows to test the 2/1 mode stability. (c) H-mode. Then we propose to try a transition to H-mode in the q95<2 phase by increasing the NBI power. The MHD stability of such a plasma is unknown and may require to re-optimize feedback control, and in particular DEFC and fast RWM feedback as in (b). (d) LOWER q95. As a last point, we propose to explore even lower q95 values by lowering the toroidal field since the beginning of the discharge. Even this step may require further optimization of magnetic feedback.
Background:	The 2012 TJA experiment on the q95<2 scenarios showed that overcoming the q95=2 limit is possible by magnetic feedback control of MHD stability. Given the limited experimental time, a full optimization of the feedback control was not possible. In fact, basically due to feedback limits the 2/1 RWM was not completely suppressed and eventually leaded to a disruption. Analysis of these experiments suggests two possible causes and solutions. First, a significant uncorrected error field is still present and tends to excite the 2/1 RWM at a finite amplitude. Dynamic error field correction should allow to further reduce it to lower level. If the mode is marginally stable, this may even be sufficient to avoid its growth, as observed with beta-driven RWMs. Second, modeling of this discharges with a code developed and tested on RFX-mod tokamak plasmas [P Zanca et al 2012 Plasma Phys. Control. Fusion 54 094004], and recently adapted to DIII-D, suggests that a small error in the 2/1 RWM phase used for feedback forces the mode into rotation. This is indeed observed in the experiment and causes the voltage saturation eventually responsible for the disruption. Such an error can be compensated using a complex gain, which can be determined in the experiment by varying the phase between the n=1 applied field and the n=1 plasma response. With this correction, the mode rotation should be avoided and the model predicts full suppression of the mode. A better estimation of the mode amplitude and phase would be obtained using the AC compensation technique [L Piron et al 2011 PPCF 53 084004] instead of the DC one, already developed and tested in DIII-D, but not used in the 2012 TJA experiment. The work described above is preliminary to any further optimization and exploration of the q95<2 scenario. Once these steps will be successfully completed, the experiment will continue by exploring H-mode and/or even lower q95 values, where new MHD stability windows may open.
Resource Requirements:	1,2 co-Ip NBI sources, audio amplifiers on I-coils, SPA supplies on C-coils.
Diagnostic Requirements:	Magnetics, MSE, CER, Thomson scattering, ECE radiometer, density interferometer.
Analysis Requirements:	--
Other Requirements:	--