Investigation of Helicon discharges as RF coupling concept of negative hydrogen ion sources

Abstract

Abstract. The ITER reference source for H − and D − requires a high RF input power (up to 90 kW per driver). To reduce the demands on the RF circuit, it is highly desirable to reduce the power consumption while retaining the values of the relevant plasma parameters namely the positive ion density and the atomic hydrogen density. Helicon plasmas are a promising alternative RF coupling concept but they are typically generated in long thin discharge tubes using rare gases and an RF frequency of 13.56 MHz. Hence the applicability to the ITER reference source geometry, frequency and the utilization of hydrogen/deuterium has to be proved. In this paper the strategy of the approach for using Helicon discharges for ITER reference source parameters is introduced and the first promising measurements which were carried out at a small laboratory experiment are presented. With increasing RF power a mode transition to the Helicon regime was observed for argon and argon/hydrogen mixtures. In pure hydrogen/deuterium the mode transition could not yet be achieved as the available RF power is too low. In deuterium a special feature of Helicon discharges, the socalled low field peak, could be observed at a moderate B-field of 3 mT.

Experimental benchmark of the NINJA code for application to the Linac4 H−ion source plasma

“…The laboratory experiment CHARLIE (Concept studies for Helicon Assisted RF Low pressure Ion sourcEs) is actually intended to investigate the possibility of applying Helicon coupling for RF driven H − ion sources [17,18]. However, during the benchmark measurement campaign for the NINJA code, the experiment was operated as ICP using a helical coil made out of copper tube (6 mm diameter) with 5 windings.…”

Alternative RF coupling configurations for H− ion sources

“…The applicability of Helicon coupling to hydrogen or deuterium has not been investigated systematically yet 1 . In order to investigate and optimize Helicon coupling for H 2 and D 2 the laboratory experiment CHARLIE (Concept studies for Helicon Assisted RF Low pressure Ion sourcEs) has been set up [9]. Measurements between 0.3 and 10 Pa pressure can be carried out which covers the relevant range both for ion sources for fusion and accelerators.…”

Conceptual design of the beam source for the DEMO Neutral Beam Injectors

“…Helicon sources have found a wide range of applications, including semiconductor manufacturing and ion thruster systems for satellites. Recently, it has been proposed that helicon sources may be an interesting option for negative ions production for NBI systems for fusion [18]. In this context, helicon sources have the following advantages over traditional ICP generators: (1) reduced required RF power, leading to increase operational domain and lower recirculating power; (2) stable operation at low pressure (<1 Pa), reducing negative ion losses by electron stripping; (3) lower electron temperatures, resulting in higher efficiency of negative ion production; (4) high degree of molecular dissociation in hydrogen plasmas.…”