Beam and installation improvements of the NIO1 ion source.

The NIO1 (Negative Ion Optimization phase 1) source can provide continuous beam operation, which is convenient for systematic parameter and equipment studies. Even in the pure volume production regime, the source yield was found to depend on conditioning procedures. Magnetic configuration tests continued adding magnets to the existing setup; the filter field component B has been progressively extended to span the -12 to 5 mT range, and as a trend, source performances improved with |B|.

First experiments with the negative ion source NIO1.

Neutral Beam Injectors (NBIs), which need to be strongly optimized in the perspective of DEMO reactor, request a thorough understanding of the negative ion source used and of the multi-beamlet optics. A relatively compact radio frequency (rf) ion source, named NIO1 (Negative Ion Optimization 1), with 9 beam apertures for a total H(-) current of 130 mA, 60 kV acceleration voltage, was installed at Consorzio RFX, including a high voltage deck and an X-ray shield, to provide a test bench for source optimizations for activities in support to the ITER NBI test facility. NIO1 status and plasma experiments both with air and with hydrogen as filling gas are described.

Particle model of full-size ITER-relevant negative ion source.

This work represents the first attempt to model the full-size ITER-relevant negative ion source including the expansion, extraction, and part of the acceleration regions keeping the mesh size fine enough to resolve every single aperture. The model consists of a 2.5D particle-in-cell Monte Carlo collision representation of the plane perpendicular to the filter field lines.

The characterization and optimization of NIO1 ion source extraction aperture using a 3D particle-in-cell code.

The geometry of a single aperture in the extraction grid plays a relevant role for the optimization of negative ion transport and extraction probability in a hybrid negative ion source. For this reason, a three-dimensional particle-in-cell/Monte Carlo collision model of the extraction region around the single aperture including part of the source and part of the acceleration (up to the extraction grid (EG) middle) regions has been developed for the new aperture design prepared for negative ion optimization 1 source. Results have shown that the dimension of the flat and chamfered parts and the slope of the latter in front of the source region maximize the product of production rate and extraction probability (allowing the best EG field penetration) of surface-produced negative ions.

Negative ion extraction by particle model.

More self-consistent injection boundary conditions from the source region have been used in the extraction region model to examine the negative ion formation and transport. Bulk kinetic, plasma-surface, and gas-surface processes have been all included. This work represents a first example of coupling between different models, and it shows the important role of positive ion conversion on plasma grid for the extracted negative ion current.