John Sheffield ISSE U-Tennessee April 5, 2011. Adding Impurities to the Core Plasma to Alleviate Plasma Material Interaction Problems. John Sheffield ISSE U-Tennessee April 5, 2011.

Numerous studies of adding impurities at the plasma edge to increase radiation losses (plasma mantle) and alleviate plasma material interaction problems [e.g., Gibson 1978, Watkins 1981]. The ARIES-AT tokamak [Najmabadi et al] is a notable example of a power plant in which impurities were added to the core to reduce the conduction losses to the separatrix and spread the radiated power more benignly over the wall. A design study, such as ARIES-AT, may be based on assumptions that aggressive physics parameters will be obtained. But experiments designed to test PMI issues will have to be more conservative to give assurance that testing at interesting parameters can be done in the presence of a large fraction of power radiated by impurities. This means conservative values for HH and balance between inductive and non-inductive current drive.

Tokamaks that will test PMI issues will need to be able to add impurities “The key tests will involve trying various combinations of core and mantle radiation using a variety of impurities; in order to establish the optimum conditions for a DEMO. The tokamaks in question include non D-T, PMI-testing machines such as NHTX (Goldston et al) and Vulcan, and the D-T VNS and Pilot Plant. Generally, in designing these, efforts have been made to make them as small as possible to test variously, the nuclear technologies, PMI effects, and Qeng ~ 1 at the minimum useful wall loading. My talk is intended simply to emphasize that to do the kind of tests necessary for the PMI program one should make test facilities flexible enough to handle uncertainties in being able to achieve the necessary plasma conditions. This includes having a lot of auxiliary power to be able to handle high core radiation loss scenarios and Have P/R and P/PL-H in the range of interest to the DEMO. Both ARIES-AT and NHTX have P/PL-H ≥ 6.

Confinement and Power ITER H-mode scaling is tE = 0.1444 HH I0.93 B0.15 n0.41 M0.19 R1.97 e0.58 kX0.78Pc-0.69 (s) From the energy balance, the required tE = 4.737Ra2k<nT>/Pc Note that in the ITER-FEAT studies kX/k ≈ 1.1. Combining the two equations yields with M ~ 2, <nT> = 0.037 HH I0.93 B0.15 n0.41(R/a)0.39 a-1.03k0.78Pc0.31

Varying the radiated fraction Now Pc = P(1 – fR) where fR is the radiated fraction of the total power and <nT> = 0.037 HH I0.93 B0.15 n0.41(R/a)0.39 a-1.03k0.78P0.31(1 –fR)0.31 Alternatively, if the goal is to do testing at a fixed P/R, <nT> = 0.037 HH I0.93 B0.15 n0.41(R/a)0.70 a-0.72k0.78(P/R)0.31(1 –fR)0.31

NHTX study shows importance of achieving 100s of seconds pulse lengths. In a given experiment operating at fixed q and P/R, <nT> = Const. HH B1.08n0.41(1 – fR)0.31 An issue, in the absence of inductive current drive, is how to maintain the current, while varying n, T, and fR over the range of interest.

Options for sustaining the current in regions of interest Vary HH at constant B and ne as the impurity radiation fraction is varied from say 0.20 to 0.6. When fR from 0.2  0.6, HH goes from 1.00  1.24 or in a conservatively designed system from 0.81  1.00. Have enough inductive drive to compensate for variations in the bootstrap current and inductive current drive as the plasma parameters are varied at constant more or less HH.

Systems relying more on inductive current drive to achieve pulse lengths. Having some inductive drive offers more flexibility in varying the plasma parameters than non-inductive drive. In fact, having both is desirable because of the ability of non-inductive drive to influence the current profile. Another possibility is to have more current drive capability than needed for the easier to achieve conditions.

Conclusions The points above raise the question of whether it is best to use a low aspect ratio system; although it would be easier to achieve a high value of P/R at smaller R, as indicated in the NHTX study (R = 1.0 m, a = 0.55 m) it would be more difficult to provide Volt-seconds. Also, if the DEMO is ARIES-AT-like, it might be desirable to do the testing at higher R/a ~ 3+, for example, R= 1.5 m and a = 0.5 m, with P/R ~ 33 MW--less than DEMO but P/A comparable. In any case, it would be instructive to see n, T, Tp, P plots for a range of experiments satisfying acceptable P/R and P/A requirements.

Acknowledgements I appreciate receiving useful advice and information from Rob Goldston, Wayne Houlberg, Stan Kaye, and Dale Meade.