Negative ion source operation with deuterium

Abstract

When the working gas of a negative ion source is changed from hydrogen to its isotope, deuterium, an ‘isotope effect’ is observed; namely, several plasma characteristics such as the electron energy distribution, the atomic fraction and the spectra of rovibrationally excited molecules change. The understanding of the effect becomes more important, as research and development aiming at ITER power level operation is being challenged with feeding deuterium to the ion sources. As a historical review of the effort to develop hydrogen/deuterium negative ion sources, several types of negative ion sources designed for the neutral beam plasma heating are described: double charge exchange sources, volume sources and surface-plasma sources. The early results with volume sources operated with and without cesium are introduced. The characteristics of the source charged with deuterium are compared to those of the source charged with hydrogen. The isotope effect did not appear pronounced as the negative ion density was measured in a small source but became more pronounced when the plasma source size was enlarged and the discharge power density was increased to higher values. Surface plasma sources were optimized for deuterium operation but could not achieve the same performance as a source operated with hydrogen at the same power and pressure. The lower velocity of negative deuterium ions leaving the low work function surface seemed to limit the production efficiency. Fundamental processes causing these differences in negative ion source operation are summarized. After explaining the current status of negative ion source research and development, the acquired knowledge is utilized to the development of large negative ion sources for nuclear fusion research and to the development of compact negative ion sources for neutron source applications.

TALIF measurements of hydrogen and deuterium surface loss probabilities on quartz in low pressure high density plasmas

“…Dissociative attachment of an electron on a vibrationally excited hydrogen or deuterium molecule is producing one negative ion and one atom. It is known to present strong isotope dependence, the cross sections for hydrogen being higher by orders of magnitude than those of deuterium [51,[54][55][56]. Based on cross sections of [49] (slightly lower than the original cross sections of Ref.…”

Direct measurements of electronic ground state ro-vibrationally excited D2 molecules produced on ECR plasma-facing materials by means of VUV-FT absorption spectroscopy

“…Negative ions could be employed to limit damages induced by charging effects inside the etched patterns 13 . A high density of negative ions obtained via DEA is also of interest in negative ion sources: for high intensity particle accelerators 14,15 , negative H − ions charge-exchange injection has allowed a noticeable increase of the emittance and of the beam intensity compared to proton injection 16,17 ; and in the neutral beam injectors (NBI) of a tokamak 18,19 , the acceleration of negative ions prior to their neutralization can transfer enough energy to neutrals to satisfy the Lawson criterion, the theoretical condition for the ignition of the fusion reaction 20 . Until now, high density beams of negative ions have been created on ceasiated surfaces: neutral atoms (D 0 ) are backscattered from the surface as negative ions (D − ).…”

“…The use of Cs also requires regular maintenance of the ion source on ITER injector, which will be quickly activated and such operations will have to be done via remote handling. Hence, alongside theoretical works 22,23 and experimental studies [24][25][26][27] (close to full scale for some of them) devoted to optimise and to secure the use of caesium in negative ion sources, theoretical and experimental studies 18,28 (at laboratory scale) on "Cs-free" ion source have been also undertaken. N-doped surfaces of diamond 29 or barium 30,31 can theoretically produce negative ion beams (Fig.…”

Latest experimental and theoretical advances in the production of negative ions in caesium-free plasmas

“…It should be noted that this section does not consider other important aspects of negative ion sources, such as the isotopic effect, the role of the wall and the effect of wall material, the role of the magnetic filter, which will be extensively considered in other parts of the manuscript including the reviews by Bacal and Wada [73][74][75].…”

“…Contrary to surface production sources, volume production sources operate with pure H 2 or D 2 , having the practical advantage of inherently caesium-free operation, which makes them an attractive alternative. While different processes might be responsible for the formation of negative ions [67,73,74,103], the volume production mechanism is an efficient path and mainly refers to the process of dissociative attachment (DA) of low-energy (cold) electrons to highly vibrationally excited molecules [19,66,73,103]:…”