Extraction of negative charges from an ion source: Transition from an electron repelling to an electron attracting plasma close to the extraction surface

Abstract

Large-scale sources for negative hydrogen ions, capable of delivering an extracted ion current of several ten amperes, are a key component of the neutral beam injection system of the upcoming ITER fusion device. Since the created heat load of the inevitably co-extracted electrons after magnetic separation from the extracted beam limits their tolerable amount, special care must be taken for the reduction of coextracted electrons -in particular in deuterium operation, where the larger amount of co-extracted electrons often limits the source performance. By biasing the plasma grid (PG, first grid of the extraction system) positively with respect to the source body the plasma sheath in front of the PG can be changed from an electron repelling towards an electron attracting sheath. In this way, the flux of charged particles onto the PG can be varied, thus changing the bias current and inverse to it the amount of co-extracted electrons. The PG bias affects also the flux of surface-produced H − towards the plasma volume as well as the plasma symmetry in front of the plasma grid, strongly influenced by an ⃗ E × ⃗ B drift. The influence of varying PG sheath potential profile on the plasma drift, the negative hydrogen ion density and the source performance at the prototype H − source is presented, comparing hydrogen and deuterium operation. The transition in the PG sheath profile takes place in both isotopes, with a minimum of co-extracted electrons formed in case of the electron attracting PG sheath. The co-extracted electron density in deuterium operation is higher than in hydrogen operation, which is accompanied by an increased plasma density in deuterium.

Deuterium results at the negative ion source test facility ELISE

“…The plasma density in deuterium is in general higher at the same parameters. In particular close to the plasma grid a 60 % higher density has been measured in the BATMAN source by probes 12 . This is seen as the reason for the higher currents of co-extracted electrons compared to hydrogen operation observed in all experiments.…”

PIC modeling of negative ion sources for fusion

“…The bias plate and PG (in green and black, respectively, in figure 2) bias are set to a potential f f = = 0 BP PG (usually, to reduce the co-extraced electron current, PG is set to plasma potential condition at about f = 23 V PG [6]), while the EG (in red in figure 2) potential is f = 1 kV EG . The extraction voltage has been scaled assuming a generalized Child-Langmuir-like extraction law (s is the sheath thickness).…”

“…In experiments [5], ratios of coextracted electron current to extracted negative ion current around or below 1 can be reached, strongly depending on the status of the Caesium conditioning. Typically, in deuterium much higher values of this ratio are measured than in hydrogen [5,6]. In addition, a strong inhomogeneity (top-bottom asymmetry in the plane perpendicular to the filter filed lines) is present along the plasma grid [4], as effect of electron magnetic drifts.…”

Efficiency of Cs-free materials for negative ion production in H2 and D2 plasmas

“…In caesiated high power negative ion sources, however, surface conversion is the dominant H − formation mechanism. And indeed, in this case, an isotopic influence can be observed, as for example shown at the BATMAN test facility [28]: the typically increased dissociation degree in deuterium plasmas directly converts to an increased negative ion density.…”

“…Ion sources for fusion applications have to be driven in both H 2 and D 2 and the respective performance varies depending on the isotope [28]. Thus, for the most effective Cs-free converter material -MoLa-investigations comparing hydrogen and deuterium are performed.…”