Magnetic filter field dependence of the performance of the RF driven IPP prototype source for negative hydrogen ions

Abstract

The ITER neutral beam system requires a negative hydrogen ion beam of 48 A with an energy of 0.87 MeV and a negative deuterium beam of 40 A with an energy of 1 MeV. The beam is extracted from a large RF driven ion source with the dimension of 1.9 × 0.9 m 2 . An important role for the transport of the negative hydrogen ions to the extractor and the suppression of the co-extracted electrons is the magnetic filter field in front of the extractor. For the large ITER source the filter field will be generated by a current of up to 4 kA flowing through the first grid of the extractor. The extrapolation of the results obtained with the small IPP RF prototype source, where the filter field has a different 3D structure as it is generated by permanent magnets, is not straightforward. Furthermore, the filter field is by far not optimized due to the technical constraints of the RF source. Therefore, a frame that surrounds the ion sources and hosts permanent magnets was constructed for a fast and flexible change of the filter field. First results in hydrogen show that a minimum field of 3 mT in front of the extractor is needed for a sufficiently large number of extracted negative hydrogen ions, whereas sufficient co-extracted electron suppression is achieved by a source integrated magnetic field of more than 1.0 mTm.

Modelling the ion source for ITER NBI: from the generation of negative hydrogen ions to their extraction

“…The design of the RF driven IPP negative hydrogen ion sources is based on the tandemconcept: in the cylindrical driver(s) a hot and dense (T e ≥ 10 eV, n e ≥ 10 18 m −3 ) plasma is generated by means of inductive coupling (f RF = 1 MHz, typical RF power: 70 to 100 kW per driver) and then cooled [13] (T e ≈ 1 eV at n e ≤ 4 · 10 17 m −3 ) in the expansion region by a transverse magnetic filter field [3]. Figure 2 shows a schematic diagram of the IPP prototype source.…”

“…mainly oxygen atoms but also hydrogen from the plasma as well as hydrogen-oxygen compounds) into the caesium layers at the surfaces [20]. Due to a short survival length of the negative ions (in the cm range in the hot driver plasma, up to a few tens of cm in the cold plasma in close proximity to the extraction system [3]), the most relevant converter surface is the surface of the plasma grid (PG), the first grid of a multi-grid, multi-aperture extraction system (consisting in the IPP test facilities of the PG, the extraction grid and the grounded grid, while for the ITER NBI system seven grids in total are foreseen [21]). Prior to their extraction the trajectories of negative ions produced at the PG are bent back towards the extraction apertures by the influence of the magnetic filter field, the field of the electron deflection magnets embedded in the extraction grid and by charge exchange collisions [22].…”

“…Up to now, optimization of these factors with respect to the co-extracted electrons have been performed only empirically. For example, in [3] results of a dedicated measurement campaign at BATMAN are presented indicating that a determining parameter for the electron transport is the integral value of the magnetic filter field ∫ B x dz between the driver and the PG. A theoretical code that describes in a self consistent manner the transport of charged particles in the boundary layer is highly desirable.…”

“…The heating beams (P = 16.5 MW per beamline, two beamlines are presently foreseen) will deliver a beam energy of up to 1 MeV [1,2] and hence the system has to be based on the production, extraction and acceleration of negative hydrogen or deuterium ions. Up to now maximum current densities in the range of a few 100 A/m 2 have been obtained in negative hydrogen ion sources [3]. Thus, in order to obtain the foreseen high accelerated negative ion currents (48 A of H − accelerated to 870 keV or, in deuterium operation, 40 A of D − accelerated to 1 MeV) the ITER source has to be rather large: the present design foresees an extraction area of 0.2 m 2 , consisting of 1280 extraction apertures for a total source area of 1.9×0.9 m 2 .…”

A collisional radiative model for caesium and its application to an RF source for negative hydrogen ions

“…The plasma then expands into the expansion region where it is cooled (from T e 10 eV to 1 eV at n e 10 17 m -3 ) by means of a magnetic filter field (of some mT). The filter field is generated by rods of permanent magnets embedded into an external frame [22]. The dominant component of the filter field points into the horizontal direction.…”