Alternative RF coupling configurations for H− ion sources

Abstract

Abstract. RF heated sources for negative hydrogen ions both for fusion and accelerators require very high RF powers in order to achieve the required H − current what poses high demands on the RF generators and the RF circuit. Therefore it is highly desirable to improve the RF efficiency of the sources. This could be achieved by applying different RF coupling concepts than the currently used inductive coupling via a helical antenna, namely Helicon coupling or coupling via a planar ICP antenna enhanced with ferrites. In order to investigate the feasibility of these concepts, two small laboratory experiments have been set up. The PlanICE experiment, where the enhanced inductive coupling is going to be investigated, is currently under assembly. At the CHARLIE experiment systematic measurements concerning Helicon coupling in hydrogen and deuterium are carried out. The investigations show that a prominent feature of Helicon discharges occurs: the so-called low-field peak. This is a local improvement of the coupling efficiency at a magnetic field strength of a few mT which results in an increased electron density and dissociation degree. The full Helicon mode has not been achieved yet due to the limited available RF power and magnetic field strength but it might be sufficient for the application of the coupling concept to ion sources to operate the discharge in the low-field-peak region.

Towards powerful negative ion beams at the test facility ELISE for the ITER and DEMO NBI systems

“…The standard helicon wave coupling usually requires magnetic field strengths up to 100 mT which might not be feasible to accommodate for in a DEMO source concept. In the present configuration, the low-field peak is observed at magnetic field strengths of 2 -4 mT for hydrogen and deuterium depending on the pressure [30,32].…”

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.…”

Application of a Langmuir probe AC technique for reliable access to the low energy range of electron energy distribution functions in low pressure plasmas

“…As shown in the schematic block diagram in Fig. 5, the AC system is implemented at a well characterized ICP experiment 38 consisting of a cylindrical stainless steel vacuum vessel with 15 cm in diameter and 10 cm in height. A water cooled planar RF antenna with seven windings is used, which is located in an evacuated chamber on top of the vessel and separated from the plasma chamber by a 3 mm thin dielectric quartz plate.…”