Modification of atomic physics rates due to nonlocal electron parallel heat transport in divertor plasmas (original) (raw)
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Plasma Physics and Controlled Fusion, 2018
KInetic code for Plasma Periphery (KIPP) was used to assess the importance of kinetic effects of parallel electron transport in the SOL and divertor of JET high radiative H-mode inter-ELM plasma conditions with the ITER-like wall and strong nitrogen (N2) injection. Plasma parameter profiles along magnetic field from one of the EDGE2D-EIRENE simulation cases were used as an input for KIPP runs. Profiles were maintained by particle and power sources. KIPP generated electron distribution functions, fe, parallel power fluxes, electron-ion thermoforces, Debye sheath potential drops and electron sheath transmission factors at divertor targets. For heat fluxes in the main SOL, KIPP results showed deviations from classical (e.g. Braginskii) fluxes by factors typically ~ 1.5, sometimes up to 2, with the flux limiting for more upstream positions and flux enhancement near entrances to the divertor. In the divertor, at the same time, for radial positions closer to the separatrix, very large heat flux enhancement factors, up to 10 or even higher, indicative of a strong non-local heat transport, were found at the outer target, with heat power flux density exhibiting bump-on-tail features at high energies. Under such extreme conditions, however, contributions of conductive power fluxes to total power fluxes were strongly reduced, with convective power fluxes becoming comparable, or sometimes exceeding, conductive power fluxes. Electron-ion thermoforce, on the other hand, which is known to be determined mostly by thermal and sub-thermal electrons, was found to be in a good agreement with Braginskii formulas, including the Zeff dependence. Overall, KIPP results indicate, at least for plasma conditions used in this modelling, a sizable, but not dominant effect of kinetics on parallel electron transport.
2017
For a radiative collisional model, population densities of atomic levels are determined by a system of equations containing the various atomic process rates. The electron impact ionization is an important atomic process in the collisional radiative model as well as for the study of ionization balance. In many types of plasmas it has been observed that some electrons (hot) are governed by a non-Maxwellian energy distribution. The illustration of the effect of a non-Maxwellian distribution is provided for neutral helium emission lines and effective ionization rate coefficients. The ionization rates are generated from cross sections obtained by the Flexible Atomic Code (FAC), weighted by this distribution. We present, in this work, the effects of hot electrons on the ionization rates of Beryllium by using a non-Maxwellian distribution of hot electrons for different fractions. We study the influence of electron energy distribution functions on the calculation of ionization rate for neut...
Advanced simulation of electron heat transport in fusion plasmas
Journal of Physics: Conference Series, 2009
Electron transport in burning plasmas is more important since fusion products first heat electrons. First-principles simulations of electron turbulence are much more challenging due to the multi-scale dynamics of the electron turbulence, and have been made possible by close collaborations between plasma physicists and computational scientists. The GTC simulations of collisionless trapped electron mode (CTEM) turbulence show that the electron heat transport exhibits a gradual transition from Bohm to gyroBohm scaling when the device size is increased. The deviation from the gyroBohm scaling can be induced by large turbulence eddies, turbulence spreading, and non-diffusive transport processes. Analysis of radial correlation function shows that CTEM turbulence eddies are predominantly microscopic but with a significant tail in the mesoscale. A comprehensive analysis of kinetic and fluid time scales shows that zonal flow shearing is the dominant decorrelation mechanism. The mesoscale eddies result from a dynamical process of linear streamers breaking by zonal flows and merging of microscopic eddies. The radial profile of the electron heat conductivity only follows the profile of fluctuation intensity on a global scale, whereas the ion transport tracks more sensitively the local fluctuation intensity. This suggests the existence of a nondiffusive component in the electron heat flux, which arises from the ballistic radial E X B drift of trapped electrons due to a combination of the presence of mesoscale eddies and the weak de-tuning of the toroidal precessional resonance that drives the CTEM instability. On the other hand, the ion radial excursion is not affected by the mesoscale eddies due to a parallel decorrelation, which is not operational for the trapped electrons because of a bounce averaging process associated with the electron fast motion along magnetic field lines. The presence of the nondiffusive component raises question on the applicability of the usual quasilinear theory for the CTEM electron transport. This is in contrast to the good agreement between the quasilinear transport theory and simulation results of the electron heat transport in electron temperature gradient (ETG) turbulence, which is regulated by a wave-particle decorrelation. Therefore, the transport in the CTEM turbulence is a fluid-like eddy mixing process even though the linear CTEM instability is driven by a kinetic resonance. In contrast, a kinetic process dominates the transport in the ETG turbulence, which is characterized by macroscopic streamers.
Application of the collisional-radiative, atomic-molecular model to the recombining divertor plasma
Physics Letters A, 1996
Recombination of hydrogen plasma in the divertor volume can be considered as a simple explanation of divertor plasma detachment phenomena experimentally observed on many tokamaks. As will be shown in this report, the presence of rotationally and vibrationally excited molecules significantly enhance the conventional three-body recombination of plasma and expand the plasma temperature and density range responsible for plasma recombination to much higher temperatures and/or lower densities. The effective rate coefficient for plasma recombination due to chemical reactions with molecules, calculated on the basis of generalized collisional-radiative model (CRAMD code), was found be about 10-10 cm 3 /s in the wide range of plasma temperatures T, = 1-3 eV and densities
Physics of Plasmas, 2017
Three models for nonlocal electron thermal transport are here compared against Vlasov-Fokker-Planck (VFP) codes to assess their accuracy in situations relevant to both inertial fusion hohlraums and tokamak scrape-off layers. The models tested are (i) a moment-based approach using an eigenvector integral closure (EIC) originally developed by Ji, Held, and Sovinec [Phys. Plasmas 16, 022312 (2009)]; (ii) the non-Fourier Landau-fluid (NFLF) model of Dimits, Joseph, and Umansky [Phys. Plasmas 21, 055907 (2014)]; and (iii) Schurtz, Nicola€ ı, and Busquet's [Phys. Plasmas 7, 4238 (2000)] multigroup diffusion model (SNB). We find that while the EIC and NFLF models accurately predict the damping rate of a small-amplitude temperature perturbation (within 10% at moderate collisionalities), they overestimate the peak heat flow by as much as 35% and do not predict preheat in the more relevant case where there is a large temperature difference. The SNB model, however, agrees better with VFP results for the latter problem if care is taken with the definition of the mean free path. Additionally, we present for the first time a comparison of the SNB model against a VFP code for a hohlraum-relevant problem with inhomogeneous ionisation and show that the model overestimates the heat flow in the helium gas-fill by a factor of $2 despite predicting the peak heat flux to within 16%. V
Plasma Physics and Controlled Fusion, 2015
The first-derivative probe technique was applied to derive data for plasma parameters from the IV Langmuir probe characteristics measured in the plasma boundary region in the COMPASS tokamak and in the TJ-II stellarator. It is shown that in the COMPASS tokamak in the vicinity of the Last Closed Flux Surface (LCFS) the Electron Energy Distribution Function (EEDF) is bi-Maxwellian with the low-temperature electron fraction predominating over the higher temperature one, whereas in the far scrape off layer (SOL) the EEDF is Maxwellian. In the TJ-II stellarator during NBI heated plasma the EEDF in the confined plasma and close to the LCFS is bi-Maxwellian while in the far SOL the EEDF is Maxwellian. In contrast, during the ECR heating phase of the discharge both in the confined plasma and in the SOL the EEDF is bi-Maxwellian. The mechanism for the appearance of a bi-Maxwellian EEDF in the vicinity of the LCFS is discussed. The comparison of the results from probe measurements with ASTRA package and EIRENE code calculations suggests that the main reason of the appearance of a bi-Maxwellian EEDF in the vicinity of the LCFS is the ionization of the neutral atoms. Results for the electron temperatures and densities obtained by the first-derivative probe technique in the COMPASS tokamak and in the TJ-II stellarator were used to evaluate the radial distribution of the parallel power flux density. It is shown that in the vicinity of the LCFS where the EEDF is bi-Maxwellian, the radial distribution of the parallel power flux density is double exponential. It is pointed that in calculations of the parallel power flux density at the LCFS the energy losses from ionization mechanisms have to be taken into account.
Dense plasma effects on atomic data and line emission of He I for divertor plasma conditions
Journal of Nuclear Materials, 2007
Effects of collisions between autoionizing levels of helium on its dielectronic recombination rate coefficients are investigated for ionizing plasma conditions encountered in magnetic fusion devices. It is shown that the so-called density effects including collisions and level depression seriously affect dielectronic recombination rates and can alter the atomic/ionic fraction of helium which enters in several diagnostics employing population densities. On the other side, Stark broadening of the high members of the diffuse series of neutral helium is also examined for recombining plasma conditions relevant to divertor regions. It is shown that similarly to the Balmer lines of hydrogen isotopes, high-n helium lines can be used for electron temperature and density determination.