Validation Study of gyrokinetic simulation (GYRO) near the edge in Alcator C-Mod ohmic discharges

Transcription

1 Validation Study of gyrokinetic simulation (GYRO) near the edge in Alcator C-Mod ohmic discharges C. Sung, A. E. White, N. T. Howard, D. Mikkelsen, C. Holland, J. Rice, M. Reinke, C. Gao, P. Ennever, M. Porkolab, R. Churchill, C. Theiler, J. Walk, J. Hughes, A. Hubbard, M. Greenwald and the Alcator C-Mod team 56th Annual Meeting of the APS Division of Plasma Physics Oct 27-Oct 31, 2014 New Orleans, LA Research supported by USDoE awards DE-SC , DE-FC02-99ER54512.

2 2 / 11 The ohmic confinement transition is an old mystery in transport research. Rice POP 2012 As density increases, ohmic confinement regime is changed from Linear Ohmic Confinement () regime to Saturated Ohmic Confinement () regime. Old hypothesis about / : Trapped Electron Mode (TEM) is dominant in the regime, and Ion Temperature Gradient (ITG) mode is dominant in the regime

3 3 / 11 The ohmic confinement transition is an old mystery in transport research. Rice POP 2012 Across the / transition, interesting phenomena have been observed and studied in C-Mod. Core rotation reversal [Rice NF 2011] Non-local transport [Gao NF 2014] Impurity transport [Rice PO ] Dilutioffect [Ennever PO ] Heat pulse analysis [Edlund PO ] Turbulence measurements and gyrokinetic analysis [Porkolab PPCF 2012, Sung NF 2013]

4 4 / 11 Reduction of T e /T e near the edge (r/a~0.85) across / transition T e /T e [%] in ohmic discharges r/a~ Only line-integrated were used to study the / transition in the past noise level We observed the changes in local T e /T e (k y ρ s <0.3) near the edge (r/a~0.85) across the / transition. /n crit

5 Gyrokinetic analysis using GYRO was performed for C-Mod ohmic discharges to interpret the reduction of T e /T e and explore / transition physics. 5 / 11 We first performed a GYRO validation for two C-Mod ohmic discharges (/) at r/a~0.85 by comparing the simulated heat fluxes and synthetic T e /T e with experiments. Then, the changes in turbulence across the / transition were investigated. Non-linear GYRO setup Local simulation Electrostatic Ion scale (k θ ρ s < ~1.7) Kinetic electrons with e-i collisions Rotation / ExB effects were included Boron (Z=5) was used as impurity with the estimated dilution ( : 18(±9)%, : 5(±3)%)

6 6 / 11 GYRO can reproduce experimental Q i and T e fluctuation level simultaneously at r/a~0.85 in C- Mod ohmic discharges Ion heat flux [MW/m 2 ] Q i from power balance Q i from GYRO Relative T e fluctuation level (%) 1.2 Measured T e fluct. Synthetic T e fluct With increase of a/l Ti within its uncertainty(~15%) from the local base run, GYRO reproduced experimental ion heat flux. In these Qi matched runs, GYRO can reproduce experimental T e /T e level within uncertainties.

7 / 11 Sensitivity analysis shows simulated Q i and synthetic T e /T e level vary too large to identify the reduction of T e /T e within uncertainties of inputs. Extreme (highest/lowest) values in the sensitivity analysis Ion heat flux [MW/m 2 ] Q i from power balance Q i from GYRO Relative T e fluctuation level (%) Measured T e fluct. Synthetic T e fluct Input parameters not used in the Q i matched runs were varied within the uncertainties in the sensitivity analysis. (a/l Te, a/l n, ν ei, n D / for, a/l Te, ν ei, n D / for )

8 Synthetic T e /T e spectral shape can be similar to the measured spectrum over the broad frequency range within the uncertainty of the measured E r values. 8 / 11 Comparison of T e /T e spectrum (E r ~9kV/m, E r,exp =3±12kV/m (CXRS*)) (Q i matched run) (low n D / (=0.92) from Q i matched run) Exp. T e /T e (1.0± 0.1 %) (0-170kHz) Syn. T e /T e (Er~9kV/m) (1.03 ± 0.09 %) (0-170kHz) Exp. T e /T e (0.7± 0.07 %) (0-170kHz) Syn. T e /T e (Er~9kV/m) (0.71 ± 0.04 %) (0-170kHz) *R. Churchill (BI )

9 GYRO under-predicts Q e, and cannot reproduce experimental Q e by adjusting input parameters within their uncertainties. Electron heat flux [MW/m 2 ] (from Qi matched runs) 9 / 11 Electron heat flux [MW/m 2 ] (from all runs in sensitivity scan) 0.15 Q e from power balance Q e from GYRO 0.15 Q e from power balance Q e from GYRO Although Q e is under-predicted, low-k non-linear runs can reproduce T e /T e (k θ ρ s <0.3) within its uncertainty. High k (ETG) turbulence can be a reason of the under-prediction of Q e. Linear runs found unstable high k turbulence, but its contribution to electron transport is unknown until multi-scale simulations are done [N. Howard TP ].

10 Toroidal mode number, n Toroidal mode number, n GYRO shows no changes in turbulence mode across the / transition. Power spectrum of simulated T e fluctuations per toroidal number, n 10 / (r/a~0.85) T e 300 (r/a~0.85) T e ω [c s /a] ω [c s /a] *Spectrum is obtained from the radially averaged fluctuations on the outer mid-plane. Hard to identify propagating direction at r/a~0.85 in both /, consistent with the linear simulations at r/a~0.85. At r/a=0.6, turbulence propagates in ion direction in both /. T e /T e level responds both ITG and TEM relevant changes Reduction of T e /T e does not necessarily support ITG/TEM transition across /.

11 11 / 11 More Realistic picture should be provided to understand the / transition. The GYRO validation study shows that non-linear GYRO can reproduce experimental Q i at r/a~0.85 in C-Mod ohmic discharges within the uncertainties of input parameters, but Q e is under-predicted. Synthetic T e /T e level and its spectral shape, which comes from ion scale turbulence (k θ ρ s <0.3), can be matched with the measurements within the uncertainties. From GYRO simulations, we learned that the simple linear mode transition from TEM to ITG is not a valid explanation for the / transition.