Experiments on the multiampere negative ion source in National Institute for Fusion Science

Abstract

The multiampere negative hydrogen ion source has been developed in National Institute for Fusion Science (NIFS). The ion source is a volume-production type multicusp one with an extraction area of 25×25 cm2. It is found that high density negative hydrogen ions of more than 1 × 1012 cm−3 are produced in the center of the arc chamber with a double magnetic filter configuration. A supply of a small amount of cesium vapor into the arc chamber has greatly enhanced the H− ion current and reduced the operation pressure. The H− ion current of 3.3 A has been extracted from a Cs-seeded plasma at the pressure of 0.9 Pa.

Negative hydrogen ion production mechanisms

“…The grid temperature control allowed obtaining H À ion beam current as large as 3.3 A from a Cs seeded ion source with 25 cm  25 cm extraction area. 245 Meanwhile, Cs seeded operation was confirmed effective for operation of a large negative ion source (34 cm diameter and 104 cm long). 246 Unlike the case of surface production source with a converter, leakage of Cs to the downstream accelerator was not large for Cs seeded volume sources, and 800 keV, 0.32 A H À ion beam was successfully extracted from a multi aperture arc discharge source.…”

Modeling of secondary emission processes in the negative ion based electrostatic accelerator of the International Thermonuclear Experimental Reactor

“…Production of a high ion current implies the use of cesium inside the ion source in order to enhance the surface production on the PG grid, which occurs because cesium lowers the work function of the PG surface [6]; then neutrals, and to some extent positive ions from the source plasma, may trap electrons from the PG valence band during impact and be reflected back into the plasma as negative ions. This reaction can be highly efficient and allows the production of a large numbers of negative ions [35][36][37][38][39].…”

Continuous pulse advances in the negative ion source NIO1

“…[5,8] This is only possible by evaporating Cs into the source; this lowers the works function of the PG surface, with an exponential increase of the negative ion production rate by surface reactions [9]. The work function depends on the purity and the thickness of the Cs layer deposited on the surface, which in turn depend not only on the evaporation rate but also on the Cs redistribution mechanisms caused by the hydrogen plasma and on the surface temperature [9][10][11][12][13][14][15]. The management of Cs is then a critical task for any negative ion source aimed to continuous operation.…”

“…In the perspective of improving the beam current, it was proven in other negative ion sources that increasing the PG temperature improves the conditioning of the PG surfaces; temperature values from 80 ÷ 100 • C up to 200 ÷ 250 • C were reported as optimal. [12][13][14]18] In NIO1, given the present technical constraints of the cooling circuit and of vacuum components, it was possible to raise the PG temperature to about 85 • C, while keeping the source body at about 30 • C. Section 2 shows the results of the Cs conditioning of the NIO1 source in continuous plasma and beam extraction conditions, and the interplay between the magnetic filter field and the Cs-plasma conditioning. Section 3 instead presents the effects of raising the PG temperature in NIO1.…”