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.

The discharge properties of a weakly magnetized inductively coupled hydrogen discharge (operating pressure 1 Pa) are evaluated by using optical emission spectroscopy. The behaviour of the electron density n e , temperature T e and the density ratio of atomic to molecular hydrogen n H /n H2 with varying magnetic field strength (up to 12 mT) is investigated. The results obtained from the OES measurements performed with a line of sight directed along the central axis of the cylindrical discharge vessel are compared to the case when the ICP antenna is replaced by a Nagoya-type-III Helicon antenna. In the ICP case, the electron temperature and density at the axis of the cylindrical discharge vessel decrease with increasing magnetic field due to the hindered radial electron diffusion. This results in a gradual transition from a homogeneous radial emission profile to a hollow profile with minimal emission in the discharge centre. Concerning the density ratio of atomic to molecular hydrogen, one obtains very high values of up to 0.32 at low B field and a decreasing behaviour with higher magnetic fields. For the Helicon case, the obtained values of n e and T e are virtually unaffected by the external magnetic field. Furthermore, a hollow radial emission profile is observed already at low B field strengths. In the Helicon setup one obtains an increasing trend for n H /n H2 with a maximum of about 0.2 at 12 mT.

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Large RF driven negative hydrogen ion sources are being developed at IPP Garching for the future NBI system of ITER. The overall power efficiency of these sources is low, because for the RF power supply self-excited generators are utilized and the plasma is generated in small cylindrical sources ("drivers") and expands into the source main volume. At IPP experiments to reduce the primary power and the RF power required for the plasma production are performed in two ways: The oscillator generator of the prototype source has been replaced by a transistorized RF transmitter and two alternative driver concepts, a spiral coil, in which the field is concentrated by ferrites, which omits the losses by plasma expansion and a helicon source are being tested.

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