Negative ions in low-pressure plasma sources are created either in the plasma volume by dissociative attachment or, at the plasma surface interaction due to surface ionization of backscattered or sputtered particles. Negative-ions formed on surfaces are accelerated towards the plasma by the sheath. They can influence the plasma kinetics through collisions with plasma species, or are self-extracted from the plasma thanks to the energy acquired in the sheath. Self-extraction of negative-ions can affect processes like sputtering, where the negative-ions formed on the cathode bombard the layer being deposited. In applications such as negative-ion sources for accelerator or fusion devices, it is taken advantage of negative-ion surface production. A low work-function material (usually caesium-covered metals) is in contact with the plasma and greatly enhances negative-ion production because of the low energy required to extract an electron from the surface. However, caesium free negative-ion sources would be greatly valuable for fusion applications because of the strong maintenance constraints induced by caesium injection.
The negative ion electrostatic accelerator for the neutral beam injector of the International Thermonuclear Experimental Reactor (ITER) is designed to deliver a negative deuterium current of 40 A at 1 MeV. Inside the accelerator there are several types of interactions that may create secondary particles. The dominating process originates from the single and double stripping of the accelerated negative ion by collision with the residual molecular deuterium gas (' 29% losses). The resulting secondary particles (positive ions, neutrals, and electrons) are accelerated and deflected by the electric and magnetic fields inside the accelerator and may induce more secondaries after a likely impact with the accelerator grids. This chain of reactions is responsible for a non-negligible heat load on the grids and must be understood in detail. In this paper, we will provide a comprehensive summary of the physics involved in the process of secondary emission in a typical ITER-like negative ion electrostatic accelerator together with a precise description of the numerical method and approximations involved. As an example, the multiaperture-multigrid accelerator concept will be discussed.
This paper deals with a study of H − /D − negative ion surface production on diamond in low pressure H 2 /D 2 plasmas. A sample placed in the plasma is negatively biased with respect to plasma potential. Upon positive ion impacts on the sample, some negative ions are formed and detected according to their mass and energy by a mass spectrometer placed in front of the sample. The experimental methods developed to study negative ion surface production and obtain negative ion energy and angle distribution functions are first presented. Different diamond materials ranging from nanocrystalline to single crystal layers, either doped with boron or intrinsic, are then investigated and compared with graphite. The negative ion yields obtained are presented as a function of different experimental parameters such as the exposure time, the sample bias which determines the positive ion impact energy and the sample surface temperature. It is concluded from these experiments that the electronic properties of diamond materials, among them the negative electron affinity, seem to be favourable for negative-ion surface production. However, the negative ion yield decreases with the plasma induced defect density. fusion power-plant prototype producing electrical energy, targeting ∼1 GW of electrical power coupled to the grid [23,24]. In the ITER and DEMO devices, the heating of the plasma will mainly be produced by neutral beam injection (NBI). NBIs systems are key components in achieving high fusion energetic-performances. The ITER NBIs are required to inject 1 MeV beams of neutral deuterium atoms (D) into the tokamak, providing plasma heating and current drive. At such high velocities, much larger than classical electron orbit velocities of hydrogen atoms, the probability of electron capture from D + ions is too low, so that production of D relies on electron detachment from high-intensity D − beams. D − negative-ions are produced in a low-pressure plasma source and subsequently extracted and accelerated.The ITER negative ion source, currently under development at IPP Garching [7,25] in Germany, operates with a high-density, low-pressure inductively coupled plasma. Extracted D − current density of 200 A m −2 , over a large surface of 1.2 m 2 , with 5%-10% uniformity and low co-extracted electron-current (below one electron per negative ion), during long operation period (3600 s) is targeted. To reach such a high D − negative-ion current, the only up-to-date scientific solution is the use of caesium. Deuterium negative-ions are created at the extraction region by backscattering of positive ions or neutrals on the plasma grid. Deposition of caesium on the grid lowers the material work function and allows for high electron-capture efficiency by incident particles and thus, high negative ion yields. Studies conducted at IPP Garching show that the ITER negative-ion source can reach the required high current densities. However, drawbacks to the use of caesium have been identified. First, the caesium is continuously injected in the source a...
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