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	262: Test Pedestal Structure Model Where Paleoclassical Transport Is Dominant
Name:	James Callen jdcallen@wisc.edu	Affiliation:	University of Wisconsin
Research Area:	Pedestal Structure	Presentation time:	Not requested
Co-Author(s):	J. Canik, T. Osborne, S. Smith	ITPA Joint Experiment :	No
Description:	The main objective of this ROF proposal is to test the pedestal structure model (see Background section below) predictions in regimes where paleoclassical plasma transport processes are predicted to be dominant -- mainly high n_e and low T_e. During the 2012 DIII-D campaign, studies of the slow evolution of the pedestal just above the L-H transition power produced some potentially relevant discharges for these studies. However, the parameter ranges of these pedestals were limited mainly by the emphasis on regimes where the density was low and they mainly did not use NBI heating -- so plunging probes could be inserted into the edge plasma. Further studies of the paleoclassical-based pedestal structure model are needed to: 1) determine if paleoclassical processes do indeed provide the minimum transport level in the pedestal; 2) explore the maximum T_e and n_e gradients that can be obtained in H-mode pedestals; 3) provide a base transport model for the higher collisionality pedestals in DIII-D, AUG, NSTX and MAST before RMPs are applied to such pedestals; and 4) further explore the properties of H-mode pedestals at power levels just above the L-H threshold which are likely to be the most important situations in ITER.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	The basic approach would be similar to the types of discharges in the 2012 campaign (e.g., shot 148692) in which the ECH power was initially just above the L-H transition power and then reduced slightly to keep the plasma just barely in the H-mode. The difference from those shots would be to do them at higher collisionality via higher n_e and hence lower T_e, and perhaps increased Z_eff. Also, it would be desirable to do them at the highest possible B_t and I_p where the pedestal beta is smallest and KBMs are least likely. Finally, long nearly steady ELM-free periods would be desirable to facilitate good Thomson pedestal profile measurements, with good pumping to limit the pedestal density buildup.
Background:	A pedestal structure model based on paleoclassical processes providing the minimum level of plasma transport in H-mode pedestals has been developed and published recently: 1) J.D. Callen, J.M. Canik and S.P. Smith, "Pedestal Structure Model," Phys. Rev. Lett. 108, 245003 (2012). It had been shown earlier that this pedestal structure model provided reasonably good agreement with the chi_e and n_e profiles in NSTX H-mode pedestals both with and without Lithium coated plasma facing components: 2) J.M. Canik et al., "Edge transport and turbulence reduction with lithium coated plasma facing components in the National Spherical Torus Experiment," Phys. Plasmas 18, 056118 (2011). Extensive, relatively successful comparisons (factor ~< 2 for pedestal T_e gradient and n_e) have been made of this pedestal structure model with 158 pedestals from the DIII-D 2010 and 2011 pedestal databases: 3) S.P. Smith et al., "Comparisons of paleoclassical based pedestal model predictions of electron quantities to measured DIII-D H-mode profiles," Nucl. Fusion 52, 114016 (2012). The last reference indicated that paleoclassical transport processes may be dominant at low beta and high collisionality, while other processes (e.g., KBMs) are likely dominant at high beta and low collisionality. However, more dedicated and extensive studies with higher collisionality pedestals are needed to firm up this conclusion.
Resource Requirements:	Mainly low NBI power, high B_t, I_p low recycling, well pumped discharges held just barely in the H-mode regime for long periods without ELMs.
Diagnostic Requirements:	Mainly good Thomson n_e and T_e profiles. But also good CER data so the Z_eff profile in the pedestal can be determined.
Analysis Requirements:	Good kinetic EFITs, ONETWO and SOLPS transport analyses.
Other Requirements:	--