EXC/P2-02. Experiments and Simulations of ITER-like Plasmas in Alcator C-Mod

Transcription

1 EXC/P2-02 Experiments and Simulations of ITER-like Plasmas in Alcator C-Mod J. R. Wilson 1, C. E. Kessel 1, S. Wolfe 2, I. Hutchinson 2, P. Bonoli 2, C. Fiore 2, A. Hubbard 2, J. Hughes 2, Y. Lin 2, Y. Ma 2, D. Mikkelsen 1, M. Reinke 2, S. Scott 1, A.C.C. Sips 3, S. Wukitch 2 and the C-Mod Team 1) Princeton Plasma Physics Laboratory, 2) Plasma Science and Fusion Center MIT, 3) JET EFDA

2 ITER-like Plasma Development on Alcator C-Mod Rampup discharges: Target several current diffusion times for rampup time Establish large bore plasmas early and early divert time (at 0.15 of rampup time) Ohmic, ICRF, and LH heating and current drive Examine volt-second requirements, li(t) evolution, and their controllability or limitations Density variations Rampdown discharges: OH/PF coil responses to H to L-mode transition Ip rampdown rates, li(t) evolution and vertical stability Density behavior in rampdown H-mode sustainment and pedestal behavior in rampdown

3 ITER Simulations of Ip Rampup Show that H/CD Sources Can Control li and V-s rampup flattop rampup flattop Lower Hybrid Electron Cyclotron Ion Cyclotron Ohmic Center of OH coil

4 Alcator C-Mod Targeting ITER Parameters C-Mod is a high field compact tokamak, the ITER-like discharges parameters: Ip = MA B T = 5.4 T R = 0.67 m a = 0.22 m κ= q 95 = 3.0 Rampup: n/n Gr ~ (ITER ) β N ~ (ITER ) H 98 ~ (ITER ) Flattop: (in progress) n/n Gr ~ 0.5 (ITER 0.85) β N ~ 1.0 (ITER 1.7) H 98 ~ (ITER 1.0) TSC model of C-Mod

5 C-Mod Rampup Discharges: Ohmic and ICRF Heated li values are very similar in rampup, in spite of ICRF heating ITER simulations showed lower li with ICRF OH coil shows lower current which implies lower V-s required, these are preserved to the end of flattop

6 Te Profiles show a stiff outer profile, only the core responds to ICRF heating Profile times ICRF deposits near the plasma core Radiated powers are different among the discharges This does not affect the outer ½ of the plasma temperature profile, it is the same as the ohmic case Te in the outer half of the plasma most strongly affects the current profile J ~ Te 3/2 li ~ B p 2 dv

7 ICRF deposition is within ρ~ 0.3, P elec > P ion varying from 67:33 to 60:40 TRANSP Narrow region for power deposition Radiated power tends to increase with ICRF power H-minority heating deposits almost all power on H This subsequently heats thermal electron and ion populations Te profile responds to this deposition

8 Simulations show the V-s savings and the lack of a current profile effect from ICRF TSC simulations show the resistive V-s savings with ICRF heating over an Ohmic L-mode discharge At the L to H the current profile broadens, and at the H to L it peaks, the V-s associated with this are not preserved to the end of flattop Early time eq. reconstructions are being examined o o o o o o The li values are very similar in the simulations as well, indicating the poloidal flux diffusion and a diffusive transport approach are consistent with the experiment

9 Density variation in rampup for Ohmic discharges shows nearly uniform Te(ρ) response V-s saving for lowest density Delay in sawtooth onset Density and temperature at end of ramp

10 Initial results with LH injection in current rampup show it saves V-s LH, low density OH, low density LH, higher density OH, higher density OH1 coil has reduced di/dt promptly after LH injection begins Low density shows LH saves significant V-s in spite of only ~ 400 kw injected power Higher density shows weakening LH effects that should be compensated by higher powers X-rays signals are seen at all densities

11 OH coil current rise at H to L transition (beginning of rampdown) can be mitigated with prolonged H-mode Rise in OH coil current at H to L transition could cause ITER to reach a coil limit Black earliest H to L Blue later H to L Green is latest H to L The OH coil current rise is diminished as the H-mode is prolonged Faster Ip rampdown (Red) can allow an earlier H to L transition

12 EDA H-modes in the rampdown show progressive decrease in stored energy with decreasing Ip Black Ohmic L-mode EDA-H-modes with ICRF heating show strong reduction of Wth with Ip Density shows decrease with Ip that largely keeps n/n Gr roughly fixed Only one discharge with Ohmic heating only, not clear if this trend is preserved is auxiliary heating required?

13 Rampdown plasmas show some generic behavior 1) li(t) ~ Ip(t) 2) For given Ip(t), there is an L-mode li(t) and an H- mode li(t) 3) Delay in li(t) from H-mode most significant in first ½ of rampdown, then diminishes Delay in li(t) from H-mode

14 Predictive Simulations of C-Mod experiments with TSC and TRANSP Data taken from experiment: OH/EF coil currents Plasma current Toroidal field n(0) from Thomson n(ρ) from Thomson (L-mode & H- mode) Z eff P rad (ρ) P ICRF L-H and H-L transition times Interpretation data (TRANSP): P e,icrf (ρ) P i, ICRF (ρ) Can use derived χ e, i (ρ) TSC is time-dependent axisymmetric transport evolution code Solves 2D MHD-Maxwell s eqns on (R,Z) grid 1D flux surface averaged transport for energy, particles, current density Models for transport coefficients PF coils, structures, and feedback systems

15 Coppi-Tang energy transport model and density profiles used in simulations Coppi-Tang model has a profile assumption governed by a broadness parameter, and a scaling factor that are used to obtain agreement with Te(ρ), Te(0), li, and sawtooth onset Density profiles are relatively stiff in C-Mod, so an fixed L-mode and fixed H-mode profile shape is used through the discharge simulation, scaled to match the n(0) from Thomson data

16 Treating impurities explicitly was not possible, enforce the experimental radiation profiles and Z eff Bolometer arrays provide the profile of total radiated power from the plasma, combined with the Z eff measurement allow the impurity effects and proper electron power balance for the simulations TSC simulation

17 Simulations have several difficulties reproducing experiments Transitions are not well modeled, such as L-H and H-L, simulations are too abrupt Density profile variations should be expanded, hollow profiles give n L > n(0) Smoothing in time occurs with timeresolution of input data, can conflict with actual features, such ICRF or density transients

18 Kinetic EFITs are required in the early phase of rampup and in H-mode to obtain reasonable current profiles Early in rampup q(0) > 1 and kinetic constraints enforce pressure contribution In H-modes the pedestal must be enforced to place j BS properly near the plasma edge o o o o o o

19 Comparison of semi-empirical energy transport models on C-Mod discharges: Coppi-Tang, Bohm-gyroBohm, CDBM Ohmic ICRF heated TSC/Coppi-Tang expt Original Coppi-Tang model settings give too peaked Te profiles, and overestimate Te(0), leading to higher li The Te near the plasma edge is critical to getting li evolution correct

20 Ohmic rampup comparison Expt, BgB, CDBM, CT Te(R), ev

21 Expt, BgB, CDBM, CT ICRF rampup comparison Te(R)

22 C-Mod has several activities addressing the physics and operational basis for the baseline ITER inductive scenario C-Mod has provided ITER-like rampup experiments to identify V-s savings and current profile evolution under Ohmic, ICRF and LH heating Established large bore and early divert ITER-like rampup discharges V-s savings in rampup are clearly achieved with ICRF and LH Current profile effects on V-s evolution clearly identified Current profile modification with ICRF appears weak, simulations confirm the experimental results, still unclear why this is the case C-Mod has provided ITER-like rampdown experiments to identify plasma evolution and OH coil response OH coil over current identified at H to L transition EDA H-mode sustained into rampdown can mitigate this over-current Shown that density decreases with Ip, keeping n/n Gr roughly fixed Shown that pedestal T and n both decrease as Ip drops

23 Pursuing ITER-like flattop phase at lower B T / Ip configurations Simulations with free-boundary TSC and TRANSP transport evolution codes provides comparisons with ITER projections and challenges modeling capabilities to reproduce experimental discharges OV/3-2 Marmar Overview of Recent Results from Alcator C-Mod Including Applications to ITER Scenarios EX/1-3 Whyte I-Mode: An H-mode energy confinement regime with L-mode particle confinement on Alcator C-Mod EX/3-3 Rice Progress towards a physics based phenomenology of intrinsic rotation in H-mode and I-mode EXC/P3-06 Hughes Power requirements for superior H-mode confinement on Alcator C-Mod: Experiments in support of ITER EXD/P3-37 Wukitch ICRF Impurity Behavior with Boron Coated Molybdenum Tiles in Alcator C- Mod FTP/P6-14 Shiraiwa Design and Commissioning of Novel LHCD launcher on Alcator C-Mod EXW/P7-28 Wallace Reduction of lower hybrid current drive efficiency at high density in Alcator C-Mod