Transport of negative hydrogen and deuterium ions in RF-driven ion sources

Abstract

Abstract. Negative hydrogen ion sources are major components of neutral beam injection systems for plasma heating in future large-scale fusion experiments like ITER. In order to fulfill the requirements of the ITER neutral beam injection, a high-performance, large-area RF-driven ion source for negative ions is being developed at the MPI für Plasmaphysik. Negative hydrogen ions are mainly generated on a converter surface by impinging neutral particles and positive ions under the influence of magnetic fields and the plasma sheath potential. The 3D transport code TrajAn has been applied in order to obtain the total and spatially resolved extraction probabilities for H − and D − ions under identical plasma parameters and the realistic magnetic field topology of the ion source. A comparison of the isotopes shows a lower total extraction probability in case of deuterium ions, caused by different transport effect. The transport calculation shows that distortions of the spatial distributions of ion birth and extraction by the magnetic electron suppression field are present for both negative hydrogen and deuterium ions.

Effect of fast positive ions incident on caesiated plasma grid of negative ion source

“…According to Gutser et al, 11 23% of the produced negative ions can be extracted when the starting energy is 1 eV (with a magnetic filter field of 7 mT), but only 15% when the starting energy is 5 eV. We applied these different extraction factors to the two groups of negative ions.…”

Moderately converging ion and electron flows in two-dimensional diodes

“…For example, for a fairly large beam θ c = 0.2 rad and α 0 = π /3, we have that 1 = 0.0579932 and current density is 6.5% less at beam center = 0 than it is at beam edge. For some H − sources it is speculated that current density is larger at beam edge, due to surface effects, 26,27 so that Eq. (25) is a closer approximation than a constant current is.…”

“…(27) exploits an analogy with classical mechanics perturbation schemes; we introduce the auxiliary "position" q(s) = a A (s)p(s)/a A0 with a A0 = a A (0) and let "s" play the role of time; one initial condition q(0) = p(0) = 0 will be sufficient, because s = 0 is a singular point also of Eq. (27).…”

Exploration of operational scenarios of the SPIDER accelerator

“…The most interesting case consists in a strong asymmetry of the extracted current density depending on the spatial distribution oh H-particles whose transport inside the source is affected by the magnets inserted in the EG to deflect the co-extracted electrons on the H-transport inside the source. This effect was predicted in [10] using 3D Monte Carlo method calculating the spatially resolved extraction probability for negative ions. In order to investigate the effect of such asymmetry on the beam optics a 3D electromagnetic analysis of the accelerator was carried out, by using macro-particles arranged so as to exhibit a current distribution reflecting the results of [10].…”

“…In order to investigate the effect of such asymmetry on the beam optics a 3D electromagnetic analysis of the accelerator was carried out, by using macro-particles arranged so as to exhibit a current distribution reflecting the results of [10]. This was obtained by creating a dedicated emitter whose macro-particles carry an amount of current 1 which depends on the position on the [x,y] plane, according to the following formula modulating a simple Gaussian of width Gw into the e direction: (2) where 10 the is ray current assuming a homogeneous distribution among rays (10 is the total extracted current divided by the number of macro-particles in the simulations) and Pais the minimum of the extraction probability distribution, here set to a value of 0.6 to match the results of [10], as reported in figure 4.…”