Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H₂/D₂ plasmas

Abstract

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...

Pulsed DC bias for the study of negative-ion production on surfaces of insulating materials in low pressure hydrogen plasmas

“…This is attributed to the enhanced sp2 phases etching at higher temperature. The decrease above 400-500°C is explained as in DC bias mode by the decrease of the material hydrogenation 9,23,24 . Concerning HOPG the results are very similar in pulsed and DC bias modes, except for the increase of the signal at room temperature in pulsed mode due to the higher material hydrogenation.…”

“…The density and electrical resistivity of HOPG were 2.265 g·cm −3 and 3.5×10 −5 Ω·cm, respectively. Diamond is another perspective material for high NI surface production rate 9 . It is well known for its ability to emit electrons at high temperature and even at low electric fields 21 .…”

“…Electron capture by incoming particles on dielectric material is harder due to the absence of electron in the conduction band. However, the transfer back of the captured electron from the negative-ion to the material is decreased or suppressed due to the presence of the gap 9 . This is resulting in an ionization efficiency that might be very high as demonstrated in beam experiments 10 .…”

“…This is resulting in an ionization efficiency that might be very high as demonstrated in beam experiments 10 . Despite the processes of electron capture and loss by incoming particles are different for dielectrics and low-work function materials due to different electronic structures 9 , it is expected that the former could also give high negative ion yield when placed in contact with hydrogen plasma. Within this framework we investigate negative-ion surface production on carbon materials putting a strong focus on diamond which presents electronic properties favorable for surface ionization 9 .…”

Focus on sources of negatively charged ions

“…Coating the molybdenum plasma electrode with caesium lowers the surface work function and favours production of negative hydrogen and deuterium ions. Cartry investigates alternatives to avoid usage of this very reactive alkali; he reports on innovative approaches aiming at replacing caesium by diamond coating [31].…”