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Abstract:

Two issues are discussed. First, a new sheath model that takes into account charge-saturation is implemented in HPHall. Second, the transition between the quasineutral solution and the sheaths at the lateral walls is found to be treated deficiently in the original code. The use of finer meshes yields better solutions but do not solve the problem completely. Hall thrusters; particle-in-cell codes; sheaths.

Abstract:

An axial model for a two-stage discharge with an electron-emissive electrode is further examined. Scaling laws are derived and help to understand two-stage physics. Efficiency gains are obtained when the second-stage is placed in the upstream part of the acceleration region and the first and two stage voltages are comparable. A parametric study to determine the best position and voltage of the intermediate electrode is carried out. © 2003 by The Authors.

Abstract:

In electrostatic simulations of MAST plasma at electron-gyroradius scales, using the local flux-tube gyrokinetic code GS2 with adiabatic ions, we find that the long-time saturated electron heat flux (the level most relevant to energy transport) decreases as the electron collisionality decreases. At early simulation times, the heat flux "quasi-saturates" without any strong dependence on collisionality, and with the turbulence dominated by streamer-like radially elongated structures. However, the zonal fluctuation component continues to grow slowly until much later times, eventually leading to a new saturated state dominated by zonal modes and with the heat flux proportional to the collision rate, in approximate agreement with the experimentally observed collisionality scaling of the energy confinement in MAST. We outline an explanation of this effect based on a model of ETG turbulence dominated by zonal-nonzonal interactions and on an analytically derived scaling of the zonal-mode damping rate with the electron-ion collisionality. Improved energy confinement with decreasing collisionality is favourable towards the performance of future, hotter devices.

Abstract:

The magnetic presheath is a boundary layer occurring when magnetized plasma is in contact with a wall and the angle $\alpha$ between the wall and the magnetic field $\vec{B}$ is oblique. Here, we consider the fusion-relevant case of a shallow-angle, $\alpha \ll 1$, electron-repelling sheath, with the electron density given by a Boltzmann distribution, valid for $\alpha / \sqrt{\tau+1} \gg \sqrt{m_{\text{e}}/m_{\text{i}}}$, where $m_{\text{e}}$ is the electron mass, $m_{\text{i}}$ is the ion mass, $\tau = T_{\text{i}}/ZT_{\text{e}}$, $T_{\text{e}}$ is the electron temperature, $T_{\text{i}}$ is the ion temperature, and $Z$ is the ionic charge state. The thickness of the magnetic presheath is of the order of a few ion sound Larmor radii $\rho_{\text{s}} = \sqrt{m_{\text{i}} \left(ZT_{\text{e}} + T_{\text{i}} \right) } / ZeB$, where $e$ is the proton charge and $B = |\vec{B}|$ is the magnitude of the magnetic field. We study the dependence on $\tau $ of the electrostatic potential and ion distribution function in the magnetic presheath by using a set of prescribed ion distribution functions at the magnetic presheath entrance, parameterized by $\tau$. The kinetic model is shown to be asymptotically equivalent to Chodura's fluid model at small ion temperature, $\tau \ll 1$, for $|\ln \alpha| > 3|\ln \tau | \gg 1$. In this limit, despite the fact that fluid equations give a reasonable approximation to the potential, ion gyro-orbits acquire a spatial extent that occupies a large portion of the magnetic presheath. At large ion temperature, $\tau \gg 1$, relevant because $T_{\text{i}}$ is measured to be a few times larger than $T_{\text{e}}$ near divertor targets of fusion devices, ions reach the Debye sheath entrance (and subsequently the wall) at a shallow angle whose size is given by $\sqrt{\alpha}$ or $1/\sqrt{\tau}$, depending on which is largest.