Time dependent evolution of RF-generated non-thermal particle distributions in fusion plasmas

Abstract

We describe fully self-consistent time-dependent simulations of radio frequency (RF) generated ion distributions in the ion cyclotron range of frequencies and RF-generated electron distributions in the lower hybrid range of frequencies using combined Fokker-Planck and full wave electromagnetic field solvers. In each regime, the non-thermal particle distributions have been used in synthetic diagnostic codes to compare with diagnostic measurements from experiment, thus providing validation of the simulation capability. The computational intensive simulations require multiple full wave code runs that iterate with a Fokker-Planck code. We will discuss advanced algorithms that have been implemented to accelerate both the massively parallel full wave simulations as well as the iteration with the distribution code. A vector extrapolation method (Sidi A 2008 Comput. Math. Appl. 56) that permits Jacobian-free acceleration of the traditional fixed point iteration technique is used to reduce the number of iterations needed between the distribution and wave codes to converge to self-consistency. The computational burden of the parallel full wave codes has been reduced by using a more efficient two level parallel decomposition that improves the strong scaling of the codes and reduces the communication overhead.

Validation metrics for turbulent plasma transport

“…Verification and validation of the various codes and models used in these power balance calculations are challenging exercises in their own right, and it is essential to always bear in mind that the lack of direct measurements of local fluxes is a significant constraint on the validation of these tools. However, through combinations of extensive verification exercises, 95 global cross-checks (e.g., comparisons of predicted and measured neutron production rates to constrain fast ion densities injected by neutral beams [96][97][98] or hard X-ray emissions associated with interactions between fast electrons and lower hybrid, electron cyclotron, and ion cyclotron waves [99][100][101] ) and comparisons with both measured changes in equilibrium profiles [102][103][104][105][106] and core fluctuations, 107,108 quantitative confidence in these models to a fairly high level has been established for many operating conditions of interest. Nonetheless, power balance analyses are still subject to significant aleatory (i.e., statistical) and systematic uncertainties.…”

20 years of research on the Alcator C-Mod tokamak

“…This disagreement might be related to the finite banana width and gyro-orbit size of the ion tail or to non-diffusive effects of the RF on the distribution function. 315 Results from the ORBIT-RF 316 and DC 317 codes suggest that the waveparticle interactions modify the distribution function, causing it to evolve faster than expected from collisional processes alone. A computational approach was developed that uses the ICRF wave fields from the AORSA solver in DC.…”

“…Preliminary application of this technique has greatly improved the agreement between the measured and simulated formation times of the energetic tail in C-Mod minority heating experiments. 315 In the mode conversion regime, direct, localized heating of the electrons near the mode conversion layer is expected. This prediction was tested using modulated RF power and a break in slope analysis of the electron temperature profiles.…”

Modelling of the EAST lower-hybrid current drive experiment using GENRAY/CQL3D and TORLH/CQL3D

“…Planck code in order to capture important interference effects in the RF diffusion coefficient that have been shown to be important in weak damping regimes [Wright, 2014].…”