Conway, G.

Creely, A.

Freethy, S.

Görler, T.

Rodriguez-Fernandez, P.

White, A.

English

MPG.PuRe

45th EPS Conference on Plasma Physics, 2018, Prague, Czech Republic Multi-Machine, Multi-Discharge Validation of TGLF on Alcator C-Mod and ASDEX Upgrade  A.J. Creely1, G.D. Conway2, S.J. Freethy1,2, T. Görler2,  P. Rodriguez-Fernandez1, A.E. White1, and the ASDEX Upgrade Team  1Massachusetts Institute of Technology, Plasma Science and Fusion Center, Cambridge, USA 2 Max Planck Institute for Plasma Physics, Garching, Germany  A multi-machine, multi-discharge validation study of the turbulent transport code TGLF [1] has been performed with experimental data from 11 discharges on Alcator C-Mod and ASDEX Upgrade.  Results are used in order to determine when multi-scale simulations are necessary, and when ion-scale simulations are sufficient. If one would like to eventually use turbulent transport models to predict the performance of future fusion devices (such as ITER and SPARC), one must first validate the outputs of these models against current experimental results.  There are many levels of physics fidelity of turbulent transport models.  At the top (including the most physics) are multi-scale gyrokinetic simulations [2, 3], though these can take tens of millions of CPU hours for a single local simulation.  In many, but not all, cases, however, ion-scale gyrokinetic simulations (which only simulate down to roughly the ion gyro-radius) contain enough physics to sufficiently model the plasma behavior, and can run tens or hundreds of times faster.  Specifically, cases have been identified in which ion-scale simulations: match both heat fluxes and other constraints [4], match heat fluxes but not other constraints [3], and match neither heat fluxes nor other constraints [2].  In cases in these particular studies where ion-scale simulations disagree with experiment, the inclusion of multi-scale effects resolves the discrepancy.  When eventually predicting the performance of a future machine, it would be convenient to be able to run ion-scale simulations, but one would like to know when the significantly more expensive multi-scale simulations are necessary. Below nonlinear gyrokinetic simulations in the fidelity hierarchy are quasi-linear gyro-fluid models, such as TGLF [1].  These models do not contain the nonlinear physics that is in gyrokinetic codes, but instead use the results of gyrokinetic simulations to ‘tune’ an approximation of the nonlinear interactions of different turbulent modes.  The disadvantage of these models is that one likely does not trust their results outside of the parameter space where they have been tuned.  The major advantage is that they run much, much faster than gyrokinetic simulations (seconds or minutes on a few cores).  TGLF can also be run in ion- and multi-scale configurations. t onference on Plasma Physics P1.108345th EPS Conference on Plasma Physics, 2018, Prague, Czech Republic  This study uses TGLF in order to determine when multi-scale physics is important, in the hope of informing when one must run multi-scale gyrokinetic simulations in the future, and when ion-scale gyrokinetic simulations are sufficient.  In the course of this process, this study also validates TGLF on all of the discharges under consideration. While ion and electron heat fluxes may eventually be the most relevant predictions for the purpose of predicting temperature profiles in a future machine, a significant amount of recent work has shown that one must compare many experimental parameters to the outputs of simulation in order to avoid fortuitous agreement between the simulations and experiment (getting the right answer for the heat fluxes in one particular parameter space, but for the wrong physics reasons) [5].  For this reason, this validation study will compare heat fluxes (calculated with TRANSP), electron temperature fluctuations (measured with Correlation Electron Cyclotron Emission [6, 7, 8]), and perturbative thermal diffusivity (measured with partial sawtooth heat pulses [4, 9]). Unlike most past validation studies, which focus on one or maybe two discharges, this study will use 11 discharges, some with multiple radial locations (for a total of 17 cases), on both Alcator C-Mod and ASDEX Upgrade.  In order to determine what makes the multi-scale effects important in simulations, one must use many discharges and attempt to separate those in which multi-scale effects are important from those in which ion-scale simulations are sufficient, using some set of parameters.  Though several attempts have been made in the past to develop a ‘rule of thumb’ as to when multi-scale effects are important, a very small number of discharges makes any such analysis difficult.  The large number of discharges in this study, on the other hand, elucidates larger trends.  This new validation methodology is facilitated by the VITALS framework [10], which optimizes inputs (within experimental uncertainty) into TGLF in order to best match experimental constraints.  This study allows the input ion temperature gradient, electron temperature gradient, density gradient, and effective charge to vary within experimental uncertainty.  All together, this study employs 2 machines, 11 plasma discharges, 17 total plasma cases, 4 varied inputs, 4 validation constraints, and 2 TGLF settings.  The five Alcator C-Mod discharges are: 1120706008, 1120706017, 1120706018, 1120706019, and 1120706030.  The six ASDEX Upgrade discharge are: 33585, 34301, 34303, 34309, 34508, and 34623.  All 11 are L-mode plasmas. Alcator C-Mod discharges were all run with 5.4 T on-axis magnetic field and 0.8 MA plasma current. Heating power ranged from 1.2 to 4.5 MW of ICRH.  Simulations were performed at ρtor = 0.75 and local densities ranged from 6.0 to 10.0 x 1019 m-3.  ASDEX Upgrade discharges were run at 2.5 T t onference on Plasma Physics P1.108345th EPS Conference on Plasma Physics, 2018, Prague, Czech Republic on-axis magnetic field with currents from 0.6 to 1.0 MA.  Heating power ranged from 0.3 to 0.6 MW of ECH.  Simulations were performed at two radii in each discharge, an inner radius (ρtor = 0.3 - 0.5 depending on the simulation) and an outer radius (ρtor = 0.65 - 0.75).  Local densities ranged from 1.4 to 3.9 x 1019 m-3.  In all cases, TGLF was run with the SAT-1 saturation rule [1].  The ‘mutli-scale’ TGLF runs utilized wavenumbers up to kθ ρs ≈ 24.0, while the ‘ion-scale’ TGLF runs utilized wavenumber up to kθ ρs ≈1.0.  The results of this validation study show that the multi-scale TGLF model agrees with all available experimental validation constraints within experimental uncertainty for all 17 cases studied here.  The ion-scale TGLF model agrees in only 6 of the 17 cases.  Preliminary analysis reveals that the ratio of the peak linear growth rate at electron scales (γhigh-k) to the peak linear growth rate at ion scales (γlow-k), is correlated with the importance of multi-scale effects.  Higher values of (γhigh-k/γlow-k) generally lead to a larger difference between the ion- and multi-scale models.  Future work will include additional analysis of the relationship between linear growth rates and the importance of multi-scale effects, as well as investigations into the physical mechanisms behind these relationships.                  t onference on Plasma Physics P1.108345th EPS Conference on Plasma Physics, 2018, Prague, Czech Republic References:  [1] G.M. Staebler et al., Phys. Plasmas 23, 062518 (2016). [2] N.T. Howard et al., Phys. Plasmas 23, 056109 (2016). [3] N.T. Howard et al., Plasma Phys. Controlled Fusion 60, 014034 (2018). [4] A.J. Creely et al., Nucl. Fusion, Submitted. [5] C. Holland, Phys. Plasmas 23, 060901 (2016). [6] C. Sung et al., Rev. Sci. Instrum. 83, 10E311 (2012). [7] S.J. Freethy et al., Rev. Sci. Instrum. 87, 11E102 (2016). [8] A.J. Creely et al., Rev. Sci. Instrum. 89, 053503 (2018). [9] A.J. Creely et al., Nucl. Fusion 56, 036003 (2016). [10] P. Rodriguez-Fernandez et al., Fusion Sci. Technol. 74, 65 (2018).    This work is supported by the US DOE under Grants DE-SC0006419 and DE-FC02-99ER54512-CMOD. In addition, this research was supported by the US DoD and the Air Force Office of Scientific Research under the National Defense Science and Engineering Graduate (ND- SEG) Fellowship, 32 CFR 168a.  This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.    t onference on Plasma Physics P1.1083

Multi-Machine, Multi-Discharge Validation of TGLF on Alcator C-Mod and ASDEX Upgrade

http://hdl.handle.net/21.11116/0000-0008-77CA-F

Multi-Machine, Multi-Discharge Validation of TGLF on Alcator C-Mod and ASDEX Upgrade

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