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The magnetic field is a key feature that distinguishes magnetron sputtering from simple diode sputtering. It effectively increases the residence time of electrons close to the cathode surface and by that increases the energy efficiency of the discharge. This becomes apparent in high power impulse magnetron sputtering (HiPIMS) discharges, as small changes in the magnetic field can result in large variations in the discharge characteristics, notably the peak discharge current and/or the discharge voltage during a pulse. Here, we analyze the influence of the magnetic field on the electron density and temperature, how the discharge voltage is split between the cathode sheath and the ionization region, and the electron heating mechanism in a HiPIMS discharge. We relate the results to the energy efficiency of the discharge and discuss them in terms of the probability of target species ionization. The energy efficiency of the discharge is related to the fraction of pulse power absorbed by the electrons. Ohmic heating of electrons in the ionization region leads to higher energy efficiency than electron energization in the sheath. We find that the electron density and ionization probability of the sputtered species depend largely on the discharge current. The results suggest ways to adjust electron density and electron temperature using the discharge current and the magnetic field, respectively, and how they influence the ionization probability. [ABSTRACT FROM AUTHOR]

The future experimental fusion reactor International Thermonuclear Experimental Reactor will include two neutral beam injectors, which will contribute to heat the plasma to thermonuclear temperatures and to sustain the magnetic equilibrium by driving the toroidal current. Each injector will consist of a 40 A negative ion source, followed by a 1 MeV electrostatic accelerator and a gas neutralizer. The modelling of the transport of beam particles through the accelerator is a key ingredient in the design of the accelerator optics. It has been recently shown (Mochalskyy et al 2012 J. Appl. Phys. 111 113303) that the space distribution of the negative ion beam at the extraction plane of the ion source drastically differs from the ones used in previous studies of the accelerator optics. A numerical study of the transport of beam particles through the accelerator using these realistic geometries for the input beam is presented here. The simulations are performed with a new 3D particle-in-cell Monte Carlo collision code called ONAC (Orsay Negative ion Accelerator). The code is able to deal with accurate 3D magnetic field configurations and electrode geometries. Simulations show that the initial beam distribution drastically affects the beam transport and the power load on the grids. A significant fraction of beam particles collides with the accelerating grids, with a corresponding power loss of ~20% of the total power carried by the beam. The inclusion of beam loading effects (intrinsically taken into account in PIC codes) and of a fully 3D model is shown to be necessary to provide a correct description of the transport. [ABSTRACT FROM AUTHOR]