Performance of multi-aperture grid extraction systems for an ITER-relevant RF-driven negative hydrogen ion source

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 ion source of dimension 1.9 × 0.9 m 2 by an acceleration system consisting of seven grids with 1280 apertures each. Currently, apertures with a diameter of 14 mm in the first grid are foreseen.In 2007, the IPP RF source was chosen as the ITER reference source due to its reduced maintenance compared with arc-driven sources and the successful development at the BATMAN test facility of being equipped with the small IPP prototype RF source (∼ 1 of the area of the ITER NBI source). These results, however, were obtained with an extraction system with 8 mm diameter apertures. This paper reports on the comparison of the source performance at BATMAN of an ITER-relevant extraction system equipped with chamfered apertures with a 14 mm diameter and 8 mm diameter aperture extraction system. The most important result is that there is almost no difference in the achieved current density-being consistent with ion trajectory calculations-and the amount of co-extracted electrons. Furthermore, some aspects of the beam optics of both extraction systems are discussed.

PIC modeling of negative ion sources for fusion

“…(i) preserve the same production area A prod  530 cm 2 (black region in figure 2(b)) of the so-called Large Extraction Grid (LAG) used in BATMAN [27]; this means that the total current of negative ion produced on the surface -I H s , is equal to the experiment and therefore, a correct current on PG = --…”

“…Nevertheless, due to 2.5D representation, this choice leads to a larger extraction area (red region in figure 2(b)) by a factor 4 (A extr =256 cm 2 against 63 cm 2 of LAG [27]). For this reason, in order to compare the model with experiment, we have decided to present results in term of extracted current density instead of total extracted current.…”

Upgrade of the BATMAN test facility for H− source development

“…Hence, all further experiments have been performed with the dipole magnetic field. The plasma grid (PG) was changed several times, the major campaigns were carried out and ITER relevant parameters were achieved with a 2 mm thick plasma grid and a 2 mm thick cover plate on top with chamfered holes, edge heated up to 200°C with a heating wire clamped onto it [10]. The PG can be biased against the source body and an additional bias plate is installed in front of the plasma grid which is on source potential.…”

“…Furthermore the LAG was designed for a perveance optimum, e.g. a good beamlet divergence, at 5 kV extraction voltage (U ex ) while it was shown meanwhile that 9 -10 kV are needed to achieve the ITER requirements on the extracted negative ion current [10]. At this U ex and the low ratio between U acc and U ex the beam becomes highly divergent (>3.5°) and is partially hitting the EG [10].…”

Status of the ELISE test facility

“…The negative ion beam formation at the plasma boundary is space-charge driven (for details see Ref. [30]) and can be described by the Child-Langmuir law. The main parameter driving the beam…”