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In the neutralizer of the ITER Neutral Beam Injector, a 1 MeV-D- beam passes through an structure filled with D2 gas, where negative ions are mainly converted to fast D0 atoms. Once that the beam is neutralized no further optical correction is possible, i.e., transport from the neutralizer to the confinement chamber is ballistic. Because of this, the transport through the neutralizer determines ultimately the geometrical properties of the neutral beams. The ionization of the buffer gas (D2) filling the neutralizer induced by the D beam creates a rarefied and low temperature plasma (ionization degree ≈10-3, electron temperature ≈20 eV). This plasma can screen the electrostatic well of the D beam and, consequently, affect the properties of the extracted beam and the energy transport to the neutralizer walls. On the other hand, the plasma will eventually escape from the neutralizer and move back in the accelerator chain, toward the accelerating grids and the source. We present particle-in-cell simulations of the beam propagation and plasma formation through the neutralizer. Particle-particle and particle-wall collisions are treated using a Monte Carlo approach. Simulations show that the secondary plasma effectively screens the beam space charge preventing beam radial expansion due to Coulomb repulsion between beam ions. First results suggest that the current of plasma ions (D2+) into the accelerator would be of the order of I(D-)/100, with I(D-) the negative ion current. [ABSTRACT FROM AUTHOR]

This article reports a new approach for probe manufacturing, which is the key component in scanning near-field optical microscope (SNOM). The wet-etching process, to create the tip at the apex of a tapered fiber, has been optimized. Typical tip features are short tapers, large cone angles (30°), and very small diameters (<50 nm). Next process steps are performed in an original arrangement of plasma device, based on a modified hollow cathode discharge. It is used for both, to remove the dust particles or the etching residues from the tip surface and to coat the tapered region with a metallic ultrathin and compact film. To complete the probe’s fabrication, the tips are opened by dry electrolytic erosion. These probes have been successfully tested for SNOM applications. [ABSTRACT FROM AUTHOR]

Decreasing the co-extracted electron current while simultaneously keeping negative ion (NI) current sufficiently high is a crucial issue on the development plasma source system for ITER Neutral Beam Injector. To support finding the best extraction conditions the 3D Particle-in-Cell Monte Carlo Collision electrostatic code ONIX (Orsay Negative Ion eXtraction) has been developed. Close collaboration with experiments and other numerical models allows performing realistic simulations with relevant input parameters: plasma properties, geometry of the extraction aperture, full 3D magnetic field map, etc. For the first time ONIX has been benchmarked with commercial positive ions tracing code KOBRA3D. A very good agreement in terms of the meniscus position and depth has been found. Simulation of NI extraction with different e/NI ratio in bulk plasma shows high relevance of the direct negative ion extraction from the surface produced NI in order to obtain extracted NI current as in the experimental results from BATMAN testbed. [ABSTRACT FROM AUTHOR]