A new model of the stationary electrostatic plasma sheath in front of divertor plates is developed, which takes into account strong inelastic processes. Using particle-in-cell simulations and ...analytic estimates it is demonstrated, that the properties of the tokamak divertor plasma sheath can significantly deviate from the properties of the classical sheath model. The most significant deviations are the increased energy flux to the plates and non-monotonic potential and ion velocity profiles in the presheath. Two main reasons for these deviations are identified: strong inelastic collisionality of the plasma presheath and the presence of super-thermal plasma particles originating from the upstream scrape-off layer.
Implementation of self‐consistent model of plasma recombination into the BIT1 PIC code and the simulation of detached SOL plasma are described. Our simulations indicate that in a strongly recombining ...plasma edge the sheath properties do not change qualitatively. The most affected parameter is the sheath heat transmission coefficient, which can increase by order of magnitude.
We study tungsten prompt re-deposition processes at the divertor plates via kinetic modelling of the JET SOL for different divertor plasma parameters. Our simulations demonstrate that the electric ...field and not the Lorentz Force is the major contributor to the prompt re-deposition process. The fraction of tungsten ions escaping from the divertor plasma is defined by the number of tungsten atoms ionized outside the magnetic sheath and does not exceed 3% of the ions sputtered from the divertor surface. We derived the corresponding fit function for estimation of the re-deposition coefficient.
The time evolution of a one-dimensional, uni-polar ion sheath (an “ion matrix sheath”) is
investigated. The analytical solutions for the ion-fluid and Poisson’s equations are found
for an arbitrary ...time dependence of the wall-applied negative potential. In the case that
the wall potential is large and remains constant after its ramp-up application, the
explicit time dependencies of the sheath’s parameters during the initial stage of the
process are given. The characteristic rate of approaching the stationary state, satisfying
the Child–Langmuir law, is determined.
We present results of massively parallel kinetic simulations of the triple Langmuir probes at JET. These results indicate that the probes under certain conditions, e.g. during ELMs, can significantly ...under/over estimate the electron temperature.
In this work a fully kinetic model of the JET SOL with tungsten divertor plates has been developed. It includes the dynamics of main-ions (D+) and electrons, the neutrals (D, C, W) and the impurity ...particles (C+m, W+n). Our simulations show extremely low concentration of W impurity. We identify two reasons which are responsible for this effect: (1) for low temperature divertor plasma the energy of most of the main-ions and the impurities in a low-ionization state impinging the divertor plates is below the W-sputtering threshold energy; (2) with increasing temperature the W-sputtering increases, but the potential drop across the divertor plasma increases too, so that most of the W ions are reabsorbed at the divertors.
It has been shown recently by Kos et al. Phys. Plasmas 25, 043509 (2018) that the common plasma-sheath boundary is characterized by three well defined characteristic points, namely the plasma edge ...(PE), the sheath edge (SE) and the sonic point. Moreover, it has been shown that the sheath profiles, when properly normalized at the SE, as well as the potential drop in the plasma–sheath transition region (PST), (region between between PE and SE) in collision-free (CF) discharges are rather independent of discharge parameters, such as the plasma source profile, ion temperature and plasma density, providing that the sheath thickness is kept well bellow the plasma length. While these findings were obtained by theoretical means under idealized discharge conditions, the question arises whether and to which extent they are relevant under more complex physical scenarios. As a first step toward answering this question the CF discharge with warm ions is examined in this work via kinetic simulation method in which some of the model assumptions, such as independence of time and the Boltzmann distribution of electrons can hardly be ensured. Special attention is payed to effects of ion creation inside the sheath. It is found that only with considerably increased sheath thickness the sonic point always shifts from SE towards the wall. Whether the absolute value of ion directional velocity at the sonic point will increase or decrease depends on the ion temperature and the source strength inside the sheath. In addition preliminary comparison of results obtained under CF assumption with the representative ones obtained with strongly enhanced Coulomb collisions (CC), indicate the relevancy of hypothesis that the VDF of B&J can be considered as a universal one in future reliable kinetic modeling and solving the plasma boundary and sheath problem in both collisional and collision-free plasmas.
A simple method is described for optimization of particle-in-cell codes by improved memory management. This includes a faster calculation of Monte-Carlo collision operators. It is demonstrated that ...the CPU time can be reduced by a factor of 2 and more without reduction of the simulation accuracy.
Preliminary design of the COMPASS upgrade tokamak Vondracek, P.; Panek, R.; Hron, M. ...
Fusion engineering and design,
August 2021, 2021-08-00, 20210801, 2021-08-01, Letnik:
169, Številka:
C
Journal Article
Recenzirano
Odprti dostop
COMPASS Upgrade is a new medium size, high magnetic field tokamak (R = 0.9 m, Bt = 5 T, Ip = 2 MA) currently under design in the Czech Republic. It will provide unique capabilities for addressing ...some of the key challenges in plasma exhaust physics, advanced confinement modes and advanced plasma configurations as well as testing new plasma facing materials and liquid metal divertor concepts.
This paper contains an overview of the preliminary engineering design of the main systems of the COMPASS Upgrade tokamak (vacuum vessel, central solenoid and poloidal field coils, toroidal field coils, support structure, cryostat, cryogenic system, power supply system and machine monitoring and protection system). The description of foreseen auxiliary plasma heating systems and plasma diagnostics is also provided as well as a summary of expected plasma performance and available plasma configurations.
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