ITER-like Discharges in C-Mod and Simulations

1. ITER-like Discharges in C-Mod and Simulations C. Kessel1, S.Wolfe2, A.C.C. Sips3 1PPPL, 2PSFC MIT, 3Max Planck - Garching

2. Discharges for Examination • ITER Rampup discharges (1080201) • Ohmic (1080201010) • ICRF heated, 80 MHz H minority (1080201031) • Ip = 1.35 MA in 500 ms • BT = 5.4 T • n(0) ≈ 2.5x1020 /m3 for ohmic, n(0) ≈ 2.0x1020 /m3 for ICRF heated • T(0) ≈ 2.5 keV for ohmic, T(0) ≈ 4.0 keV • ITER Rampdown discharges (1080409) • H & L-mode, slower rampdown (1080409016) • H-mode most of the way, faster rampdown (1080409021) • Elongation reduction, diverted plasma • ICRF heating in both cases, part of rampdown

3. C-Mod Rampup Expts. Ohmic ICRF-heated

4. C-Mod Rampdown Expts

5. TSC Simulation The experimental data used in the simulation directly are the coil currents. peak density (from profile data), injected ICRF power to the plasma, and plasma current. Since the input data to TSC is only given at discrete time points (24 points total) there is some loss of smoothness.The very ratchety data is smoothed (for example the ICRF power). Data approximated in the simulation is the density profile, using 2 fixed profiles, an L-mode and an H-mode.Experimental emissivity profile data used in simulation due to complexity of modeling impurities. Zeff loosely fit to measured data properly account for ohmic heating.The data targeted for matching/comparison is the peak electron temperature, electron temperature profile, li(1), R, a, elongation, sawtooth onset time, etc.Feedback systems are used in the simulation to keep the plasma current and plasma position approximately correct.The energy transport model used is the Coppi-Tang model [ ], based on profile consistency. The model was developed for L-mode plasmas and ohmic heating [ ], but has been found generally satisfactory for L-modes with auxiliary heating. H-mode temperature profiles are generated by locally depressing the thermal diffusivity in the pedestal region, producing a temperature pedestal. The height of the pedestal is determined by how low the thermal diffusivity is prescribed to be.

6. C-Mod Rampup, Ohmic TSC Expt Expt

7. C-Mod Rampup, Ohmic L-mode and H-mode profiles used in all simulations

8. C-Mod Rampup, Ohmic TSC/TSC, broader CT Broader CT improves Te & li match

9. C-Mod Rampup, ICRF heated Expt TSC Expt

10. C-Mod Rampup, ICRF heated

11. C-Mod Rampup, ICRF heated TSC/TSC, broader CT

12. C-Mod Rampdown, Slow TSC Expt Expt

13. C-Mod Rampdown, midplane coupled, but not strike point coupled t = 1.1 s t = 1.3 s t = 1.4 s J-phi

14. C-Mod Rampdown, Slow

15. C-Mod Rampdown, Slow Using finite Tsep TSC

16. C-Mod Rampdown, Fast TSC Expt Expt

17. C-Mod Rampdown, Fast

18. C-Mod Rampdown, Fast TSC Using finite Tsep

19. C-Mod Rampdown, Fast

20. Results • Coppi-Tang model performs better with a broader Te profile assumption, better match to li and sawtooth onset • The GLF model has problems in the rampup creating high Te near the plasma edge, which is difficult to suppress, and Te(0) tends to be underestimated (however, the H-mode works well with a prescribed pedestal) • What is the proper edge condition for GLF in L-mode plasmas? • Rampdown Te profiles are matched reasonably well with the broader Coppi-Tang • Using finite separatrix Te was tried in the rampdown simulations, but it is not clear from Thomson data that it should be finite • There is complex behavior in the rampdown, with H-L-H transitions when the power is reduced, will study this further