Speaker
Description
Decades of research has demonstrated the necessity of using kinetic plasma models to accurately model the flux of heat and particles through the closed-field line region of tokamaks. In the much colder open-field-line region beyond the last closed flux surface (LCFS), fluid models are typically used to model the flux of heat and particles to the divertor. Recently, kinetic plasma models have been developed for the open-field-line region [1-4], using a variety of numerical methods with differing degrees of physics fidelity. These models will be important for assessing the importance of prompt losses of hot particles from the LCFS to the fluxes at the divertor plate, as well as determining whether there is a direct impact from the wall boundary on upstream physics within the LCFS.
In this work, we present a study of the role of the E×B drift in a drift kinetic model of the open-field-line region of a magnetic confinement device [5]. The model evolves drift-kinetic ions, and Boltzmann electrons, with the option to include a kinetic neutral species (please see the companion work [6]). The magnetic geometry is helical, with wall boundaries limiting the extent of the axial coordinate. We include a range of model collision operators, including Krook operators for ion-neutral collisions and a model pitch-angle scattering operator for ion-ion collisions. We include small spatial numerical viscosity as a proxy for finite-Larmor-radius physics. The numerical implementation is explicit in time using a strongly stability preserving Runge-Kutta algorithm, and a spectral-element discretisation for the spatial and velocity dimensions. The implementation is tested with manufactured solutions tests, demonstrating the potential for good performance.
With a source of particles injected into the centre of the domain and a plausible initial condition, the model allows us to study relaxation of the plasma and the formation of the steady-state sheath entrance at the wall boundary. In the absence of radial variation, we can verify that the steady-state solutions satisfy the kinetic Chodura condition [7]. We study the impact of a radially varying source and the consequent E x B drifts on the behaviour of the ion distribution function as it enters the sheath. In the absence of radial numerical dissipation, we find a wave-like instability that persists if fluctuations in the radial electric field are permitted to be non-zero. We carry out the linear stability analysis for the model drift kinetic system and we compare our findings with the instability observed in the numerical simulations.
References:
[1] C. S. Chang et al. Phys. Plasmas, 11:2649, (2004).
[2] E. L. Shi et al. J. Plasma Phys., 83:905830304, (2017).
[3] Q. Pan et al. Phys. Plasmas, 25:062303, (2018).
[4] M. Dorf et al. Phys. Plasmas, 23:56102, (2016).
[5] M. R. Hardman et al. Varenna-Lausanne Workshop P29 (2022)
[6] J. Omotani et al. European Fusion Theory Conference (2023)
[7] A. Geraldini et al. 2018. Plasma Phys. Control. Fusion 60, 125002.
