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Structural controllability of complex systems proved [1] that a graph-theoretical technique can be used to identify the key processes through which full control of a system is achievable. This theory has been recently applied to the network of processes of a negative ion source [2,3] to test the sensitivity of the source behaviour to the driver processes. Since chemical reaction networks can be seen as graphs where reactants and products are considered as nodes of the network [4, 5](Aerts, et al., 2015), in this contribution the same technique has been applied to chemical reaction networks to identify the driver nodes of a reaction, that is, those species whose role in the reaction is particularly relevant. A network model has been generated from both a full [6] and a reduced [5] dissociation model, showing some differences between the two sets of the driver species. From a graph-theoretical point of view, these systems are thus not equivalent. To investigate this aspect, species that do not participate in both systems were removed from the network representation of the full system. The resulting network still shares more than 70% of the network structure with the network representation of the reduced model, showing that the network structure is resilient. Among the missing species, those whose removal induces differences between the driver node sets of the two models have been identified and their role discussed.

Negative ion sources are a key component of neutral beam Injection systems, which are used in fusion experiments to raise the plasma parameters close to start ignition. A novel concept for a negative ion source based on existing well tested Hall thrusters (HT) is presented. The thruster scheme is modified in order to maximize the hydrogen dissociation so as to produce an atom flux at an energy optimized to maximize the yield of negative ions when impinging on a low work function surface. The novel concept can in principle offer several advantages among which a limited amount of co-extracted electrons, a more uniform generation of negative ions and a lower rate of destruction of negative ions. In this contribution the numerical simulations aimed to identify the optimum geometry, magnetic field map and operational parameters of the source are presented discussing the effect of the HT working point on the dissociation rate. A preliminary design of the experimental equipment is also presented including the ion source and the vacuum chamber, focusing on the aspects related to the gas injection and the operational pressure and describing the main diagnostics.

We report on the characteristics of the beam-generated plasma in the multibeamlet case of a large hydrogen negative ion (H-) beam at NIFS. The plasma potential, and the energy of secondary particles in the drift region of an ion beam, offer an insight into the mechanisms that allow beam transport in low pressure gasses. The first measurements reported here were made by means of a four-gridded retarding field energy analyzer, combined with the measurement of the drain current at the beam dump, and with infrared beamlet monitoring technique. The retarding field analyzer measures the energy distributions of particles emitted radially from the beam-generated plasma [1]. The beam is dumped onto a graphite calorimeter, which is electrically insulated, so that it can be electrically biased and the collected current can be measured. The thermal image of the calorimeter is acquired during the beam pulse, thus offering a quantitative estimation of the single beamlet optics and multibeamlet focusing [2]. The influence of the neutral gas density is studied by puffing hydrogen gas in this drift region. Neither the ion nor electron saturation current are stable in low pressure regime, while above 10 mPa the characteristics can be obtained for a ~50 keV, 300 mA beam. The measured ion and electron energy distribution functions are strongly affected by the electric bias of the calorimeter, giving indications on the plasma potential profile in the diffusion region. In the compensation region, the beam potential is set by the calorimeter potential. The measured ion distribution functions suggest the possible presence of two populations with different temperatures. The collected data will be used to improve and validate numerical models of beam-plasma formation [3].

An innovative scheme of a neutral beam injector (NBI), based on the electrostatic acceleration and magneto-static deflection of negative ions, suitable for DEMO-like fusion reactor, is proposed and analyzed in term of feasibility and performance. The scheme is based on the deflection of a high energy 2 MV and high current (some tens of Amperes) negative ion beam by a large magnetic deflector placed between Beam Source (BS) and neutralizer. This scheme has the potential of solving two key issues concerning the operation of neutral beam injectors for a fusion reactor: the maximum acceleration voltage and the direct exposure of the Beam Source (BS) to the neutron flux from the fusion reactor. A beam deflection of 45 degrees has been applied as it is sufficient to screen the BS from the neutron flux therefore allowing the electrostatic accelerator to be insulated from the grounded vessel by compressed SF6. The latter is essential to obtain a stable injector performance at higher accelerating voltages. The scheme has been analyzed from the point of view of the beam optics in order to ensure the divergence is within acceptable values and it has been found that the scheme is quite effective as the deflector is able to guarantee, not only the steering effect, but also the necessary beam aiming.

The thermonuclear fusion experiment ITER, requires 33 MW of auxiliary heating power from two Neutral Beam Injectors (NBI), each of them providing 40 A of negative deuterium ions. The EU activities oriented to the realisation of the electrostatic accelerator comprise the construction in Padova of SPIDER, a facility devoted to the optimisation of the beam source. SPIDER parameters are: 100 keV acceleration, 40/60 A (deuterium/hydrogen) current. For the optimised SPIDER accelerator the present contribution provides a characterisation of secondary particles, which include electrons produced by impact of ions on grid surfaces, stripped from negative ions inside the accelerator, and produced by ionisation of the background gas, and the corresponding positive ions. Currents and heat deposited on the various grids and spatial distribution by secondaries will be described. It is found that most of the heat loads on the accelerator grids is due to electrons; moreover the features of secondaries exiting the accelerator and back-streaming towards the source will be presented. The results will be compared with old investigations concerning the NBI 1 MeV accelerator., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

Coherent structures emerging from the background turbulence have been detected by electrostatic measurements in the edge region of two Reversed Field Pinch experiments, RFX (Padua) and Extrap-T2R (Stockholm). Measurements have been performed by arrays of Langmuir probes which allowed simultaneous measurements of temperature, potential and density to be carried out. These structures have been interpreted as a dynamic balance of dipolar and monopolar vortices, whose relative population are found to depend on the local mean $E\times B$ flow shear. The contribution to the anomalous transport of these structures has been investigated and it has been found that the corresponding diffusion coefficient accounts up to 50% of the total diffusivity. The experimental findings indicate that the diffusion coefficient associated to the coherent structures depends on the relative population of the two types of vortices and is minimum when the two populations are equal. An interpretative model is proposed to explain this feature.

Magneto-Plasma-Dynamic (MPD) thrusters are currently under investigation as they constitute a possible, high power electric propulsion option for primary space mission, ranging from orbit raising to interplanetary mission of large spacecraft. They are essentially electromagnetic plasma accelerators, in which a high current discharge ionizes a gas, that is accelerated by the Lorentz force produced by the interaction between the current and a self induced (or applied) magnetic field. Presently, the performance of this class of thrusters is limited by the onset of critical regimes observed when the current rises beyond a threshold value. In this regime, large fluctuations in the arc voltage signals and damages to the anode are observed along with thruster performance degradation. Our experimental investigation carried out by several arrays of magnetic and electric probes and by two photomultiplier arrays of 16 channel each, has shown that the onset of these critical regimes is accompanied by the growth of large scale MHD instabilities. These instabilities have m/n = 1/1 azimuthal and axial periodicity and have features of a helical kink mode. We have found for the first time[1] that the threshold value for the current is well described by the Kruskal-Shafranov criterion currently applied in Thermonuclear Fusion research to interpret MHD stability of a screw pinch. The spatial structure of the kink has been reconstructed by the magnetic and electrostatic probes and preliminary results of the photomultiplier arrays for total radiation in the range 350-850nm confirms the helical structure of the kink. [1] M.Zuin et al., Phys. Rev. Lett. 92, 225003 (2004)

The development of neutral beam injectors (NBI) for tokamak like the ITER project and beyond requires high performance and huge negative ion sources (40 A of D- beam required); it was recently accepted that inductive plasma coupled (ICP) radiofrequency sources are the preferred option. It is therefore useful to have a moderate size source of modular design to test and verify both construction technologies and components and simulation codes; here the NIO1 design (60 kV, 9 beamlets of 15 mA H⁻ each) and construction status are described. Source is assembled from disk shaped modules, for rapid replacement; the beamlets are arranged in 3 times 3 square matrix so that 90 degree rotation of modules is possible and allows to cross or to align the magnetic filters used in the source. The 2 MHz rf coil and the rf window are a simply replaceable module. Extensive rf absorption and magnetic coil simulations were performed. Related beam simulation and fast emittance scanner development are described elsewhere., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan