The negative ion sources currently used for Fusion applications produce tens of Amperes of Hydrogen or Deuterium ion current from a low-temperature (few eV) plasma. Being the negative ion current typically limited below 200 - 300 A/m2 at the extraction apertures, these sources can be quite large and their input power ranges from some tens to hundreds of kW. Two source types are currently used: the Japanese "kamaboko" sources, and the radio-frequency (RF) plasma-driver sources, mainly developed in EU. The former exploits an arc discharge produced by hot filaments biased w.r.t. the containing chamber, whereas the latter uses cylindrical coils operating at about 1-2 MHz. The generated plasma expands towards a cesiated Plasma Grid (PG), where some of the impinging particles are converted into negative ions and then accelerated through the grid apertures. In both types a power of about 2-4 MW/m2 is necessary for achieving the required negative ion current. The main reasons for this are: (1) the low-temperature plasma is confined just by the cusp-shaped magnetic field produced by permanent magnets located outside the plasma chamber; (2) a large fraction of the electric power of the RF coils is dissipated by eddy currents induced on the surrounding metallic structures. In principle, a current-free plasma (such as the one considered in these sources) could be confined using a purely poloidal magnetic field configuration. In fact, a dipole configuration allows to achieve an efficient plasma confinement, without requiring a net plasma current. This concept has already been applied in the LDX device [Garnier et al., Nucl. Fusion 49 (2009) 055023] and a similar configuration has been proposed for the Polomac device [F. Elio, Fusion Engineering and Design 89 (2014)]. In the paper we propose a novel Ion source, based on the Polomac configuration, estimate the particle trajectories so as to assess the equilibrium and low-temperature plasma confinement capabilities of such device.

Caesium-covered molybdenum surfaces are considered for negative ion production [1] [2]. Surface chemical-physics can help to understand the surface process influencing and determining device performances. Molecular Dynamics simulations based on ab initio calculations in the framework of DFT( Density Functional Theory) have been used to simulate the interactions occurring at the gas-surface interface [ [3] [4] [5]. In recent years, we have employed this computational tool to study the surface processes of interest in negative ion sources. The simulated processes occur on a short space/time scale but influence the complete process. Instead, the determined collisional data can be useful in the kinetic modelling of negative ion production [6] [7]. A few years ago, we started by simulating the process of the negative hydrogen ion formation [8] on a caesiated surface model [9]. Thus, we investigated and understood which formation mechanism (surface or volume) is the leading one [10]. The same investigation has been then conducted for deuterium [11]. Lately, given the non-optimal vacuum conditions in which these sources operate, we have started characterizing the interaction of oxygen (atomic and molecular) with the same surface model [12] [13]. In this contribution, after a quick overview of the previously obtained results, we will focus on those obtained for atomic and molecular oxygen interaction with the considered caesium-covered molybdenum surface. A detailed analysis of the occurring surface processes will be provided by foreseeing a possible way to attenuate the influence of the presence of oxygen on the negative ions production.

In view of the long term operation of neutral beam injectors (NBI) used for stellaratorand tokamak heating and current drive, the negative ion source must be carefully optimized,especially because not only plasma but also surface wall conditions are involved in H- or D-production. The NIO1 (negative ion optimization phase 1) installation was developed and isoperated since 2014 (in close collaboration between Consorzio RFX and INFN), as a convenientbenchmark to study innovative solutions and to address physical questions. The comparativelysmaller size of NIO1 (a relatively compact and modular 9 beamlet H- source) is of evidentadvantage for detailed modeling. The apparent plasma impedance is well in agreement withnegligible rf reflection at plasma on, and reflections with plasma off (about 25 %) are well withinlimits tolerable by rf amplifier; transition between E and H modes of rf coupling can be controlledby increasing rf power or by decreasing gas pressure. The filter field intensity, Bx, has beenextended to span the [-12, 5] mT range, and as a trend, source performances improve with |Bx|.Status and results of a first NIO1 cesium oven are reported. Installation of the Cavity Ring DownSpectrometer (CRDS) and recent years progress of beam diagnostics and of the quality of thevolume-produced H- beam are briefly discussed, together with the status of the NIO1 full power"end calorimeter/beam imaging attenuator", to be placed at the end flange with auxiliary flangesfor infrared observation. Conceptual tests of energy recovery system, perhaps using this fullpower calorimeter are also summarized.

In the non-isothermal gas flow through the electrodes of electrostatic accelerators, it is found that the accommodation of the gas temperature to the temperature of the electrode walls modify the density profile. The local gas density is, on turn, causing stripping losses and stray parti cle production. This document reports the activities carried out during 2015, dedicated to the numerical investigation of the gas-wall interaction at nanoscale, by molecular dynamics modelling of the particle- wall interaction. This report contains two annexes: a bachelor degree thesis (first annex), and a paper presented at the ICIS conference 201 5 (second annex).The calculated momentum and energy accommodation coefficients for the H 2 -Cu and D 2 -Cu systems are presented.