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The characterization of the plasma generated by a Hall Effect thruster operating in Nitrogen and Hydrogen is presented. Measurements of the spatial profiles of the electron temperature and density of the plume have been obtained by scanning the region outside the exit plane of the thruster through a Langmuir probe. The measurements are aimed to determine the best position of a caesiated target in order to maximize the surface production of negative Hydrogen ions. The thruster has been designed as a tunable plasma source for a novel concept of a negative ion source, which could in principle benefit from some positive features of the Hall Effect thruster, like the possibility of tuning the ion energy and the good uniformity of particle density at the channel exit, thanks to the axisymmetric magnetic field. [ABSTRACT FROM AUTHOR]

The NIO1 (Negative Ion Optimization 1) source is developing a systematic test programme of magnetic configurations, including permanent magnet filter up to 15 mT dipole strength, tunable current filter up to 3 mT strength (parallel or perpendicular to the previous), and ion deflection compensation system in the extraction gaps. Results for both perpendicular and parallel configurations are presented. The radiofrequency (rf) system takes full advantage of the programmable frequency amplifier capability, to start the plasma and later to optimize coupling with plasma. Operation experience with borosilicate rf window and improved power limit are reported, as well breakdown voltages Vbr are discussed, together with improvements obtained with the installation of a cryogenic pump. The interrelation between bias voltage, bias plate current and magnetic configuration is outlined; cesium evaporators are in advanced development. Major diagnostic systems (including cameras and emission spectrometers) have been integrated with the acquisition system along with the data measured by several power supplies (including the high voltage supplies, the rf generator and bias supplies, the pressure measurements and the beam currents): among the several diagnostic used, the role of simple current measurement and some optical observation are noted. [ABSTRACT FROM AUTHOR]

In June 2018 the SPIDER device, which is the full-size prototype of the negative ion source and extractor for the ITER Heating Neutral Beam has entered the first operation phase at the Neutral Beam Test Facility (NBTF) in Padova, Italy. This paper describes the present status of the device, the experimental plans and the results obtained during the first experimental campaign of the radio-frequency driven plasma source. [ABSTRACT FROM AUTHOR]

We report on the characteristics of the beam-generated plasma in the multibeamlet case of a hydrogen negative ion 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. Finally, the influence of the neutral gas density is studied by puffing hydrogen gas in this drift region. The measured ion and electron energy distribution functions are strongly affected by the electric bias of the calorimeter. For a 47kV, 0.3A beam, an electron temperature of 3eV at the plasma edge was found. The energy distribution of the ions for a pressure of about 0.03Pa is presented, from which the plasma potential can be inferred. [ABSTRACT FROM AUTHOR]

The measurement of the plasma potential and the energy spectrum of secondary particles in the drift region of a negative ion beam offers an insight into beam-induced plasma formation and beam transport in low pressure gasses. Plasma formation in negative-ion beam systems, and the characteristics of such a plasma are of interest especially for space charge compensation, plasma formation in neutralizers, and the development of improved schemes of beam-induced plasma neutralisers for future fusion devices. All these aspects have direct implications in the ITER Heating Neutral Beam and the operation of the prototypes, SPIDER and MITICA, and also have important role in the conceptual studies for NBI systems of DEMO, while at present experimental data are lacking. In this paper we present the design and development of an ion energy analyzer to measure the beam plasma formation and space charge compensation in negative ion beams. The diagnostic is a retarding field energy analyzer (RFEA), and will measure the transverse energy spectra of plasma molecular ions. The calculations that supported the design are reported, and a method to interpret the measurements in negative ion beam systems is also proposed. Finally, the experimental results of the first test in a magnetron plasma are presented. [ABSTRACT FROM AUTHOR]

The deployment of neutral beam injectors in future fusion plants is beset by the particularly poor efficiency of the neutralization process. Beam-generated plasma neutralizers were proposed as a passive and intrinsically safe scheme of efficient plasma neutralizers. The concept is based on the natural ionization of the gas target by the beam, and on a suitable confinement of the secondary plasma. The technological challenge of such a concept is the magnetic confinement of the secondary plasma: a proof-of-principle for the concept is needed. The possibility to test of such a system in the small negative ion beam system NIO1 is discussed in this paper. The constraints given by the facility are first discussed. A model of beam-gas interaction is developed to provide the charge-state of beam particles along the neutralizer, and to provide the source terms of plasma generation. By using a cylindrical model of plasma diffusion in magnetic fields, the ionization degree of the target is estimated. In the absence of magnetic fields the diffusion model is validated against experimental measurements of the space-charge compensation plasma in the drift region of NIO1. Finally, the feasibility study for a beam-generated plasma neutralizer in NIO is presented. The neutralizer length, required gas target thickness, and a very simple magnetic setup were considered, taking into account the integration in NIO1. For the basic design a low ionization degree (1%) is obtained, however a promising plasma density up to hundred times the beam density was calculated. The proposed test in NIO1 can be the starting point for studying advanced schemes of magnetic confinement aiming at ionization degrees in the order of 10%. [ABSTRACT FROM AUTHOR]

An innovative magnetic configuration called ADCM (Asymmetric Deflection Compensation Magnets) has been recently proposed for the cancellation of the "criss-cross" deflection of ion trajectories in multi-beamlet negative ion accelerators. Such undesired deflection occurs due to the transverse magnetic field, which is necessary for the suppression of co-extracted electrons. In view of the application of this concept to the ITER Heating Neutral Beam Injector, the new configuration has been experimentally tested for the first time in 2016, in the framework of a research collaboration between Consorzio RFX and QST. [ABSTRACT FROM AUTHOR]

In order to understand the physics mechanism of a negative ion extraction in negative ion sources, an emission surface of the negative ions around an aperture at a plasma grid, so-called a meniscus, has been analyzed by an inverse calculation of the negative ion trajectory in a two dimensional beam analysis code. In this method, the meniscus is defined as the final position of the negative ion trajectories which are inversely calculated from the measured beam profile to the plasma grid. In a case of the volume-produced negative ions, the calculated meniscus by the inverse calculation was similar to that obtained in conventional beam simulation codes for positive ion extractions such as BEAMORBT and SLACCAD. The negative ion current density was uniform along the meniscus. This indicates that the negative ions produced in the plasma are transported to the plasma grid uniformly as considered in the transportation of the positive ions. However, in a surface production case of negative ions, where the negative ions are generated near the plasma grid with lower work function by seeding cesium, the current density in the peripheral region of the meniscus close to the plasma grid surface was estimated to be 2 times larger than the center region, which suggested that the extraction process of the surfaceproduced negative ions was much different with that for the positive ions. Because this non-uniform profile of the current density made the meniscus shape strongly concave, the beam extracted from the peripheral region could have a large divergence angle, which might be one of origins of so-called beam halo. This is the first results of the determination of the meniscus based on the experiment, which is useful to improve the prediction of the meniscus shape and heat loads based on the beam trajectories including beam halo. [ABSTRACT FROM AUTHOR]

The ion source NIO1 (Negative Ion Optimization 1) was developed and installed as a reduced-size model of multiaperture sources used in neutral beam injectors. NIO1 beam optics is optimized for a 135 mA H- current (subdivided in 9 beamlets) at a Vs = 60 kV extraction voltage, with an electron-to-ion current ratio Rj up to 2. Depending on gas pressure used, NIO1 was up to now operated with Vs < 25 kV for beam extraction and Vs = 60 kV for insulation tests. The distinction between capacitively coupled plasma (E-mode, consistent with a low electron density plasma ne) and inductively coupled plasma (H-mode, requiring larger ne) was clearly related to several experimental signatures, and was confirmed for several gases, when applied radiofrequency power exceeds a given threshold Pt (with hysteresis). For hydrogen Pt was reduced below 1 kW, with a clean rf window and molybdenum liners on other walls; for oxygen Pt ≤ 400 W. Beams of H- and O- were separately extracted; since no caesium is yet introduced into the source, the expected ion currents are lower than 5 mA; this requires a lower acceleration voltage Vs (to keep the same perveance). NIO1 caesium oven was separately tested and Cs dispensers are in development. Increasing the current in the magnetic filter circuit, modifying its shape, and increasing the bias voltage were helpful to reduce Rj (still very large up to now, about 150 for oxygen, and 40 for hydrogen), in qualitative agreement with theoretical and numerical models. A second bias voltage was tested for hydrogen. Beam footprints and a spectral emission sample are shown. [ABSTRACT FROM AUTHOR]

Negative ion sources are a key component of neutral beam injection systems, which are used in fusion experiments to raise the plasma parameters 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, for instance 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, 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. [ABSTRACT FROM AUTHOR]

The ITER tokamak requires two injectors of neutral beams resulting from the neutralisation of accelerated negative hydrogen/deuterium ions. To optimise the source operation, a specific test facility ("PRIMA") was established in Padova, comprising the experiment SPIDER (full-size negative ion source with 40A beam, 100keV particle energy). The design of SPIDER accelerator was based on the most advanced numerical codes available for the investigation of the expected beam properties. Several diagnostic systems will characterise source and beam during the experiments. In view of the operation, this paper describes the preparation of the experiments by means of numerical simulations of the accelerator. The simulations provide also estimates of the current and power deposited onto the accelerator grids and will be the basis for further work aimed at computing the expected measurements by the various beam diagnostics. [ABSTRACT FROM AUTHOR]

After installation of Mo liners in the source NIO1 (Negative Ion Optimization phase 1), hydrogen plasmas in a continuous regime operation (much longer than one hour) are routinely maintained, with more than 1 kW rf power and 0.5 Pa pressure, allowing a systematic investigation of pure H- volume effect, which requests a much lower acceleration voltage Vs ≅ 12 kV than future Cs operations at Vs ≅ 60 kV . A new extraction grid EG was installed (replacing some eroded insulators) and preliminary beam images are compared to old EG ones, discussing effects of different deflection field strength and need of intermediate values. Large improvements in beam diagnostics and the effect of installation of a cryogenic pump are also reported. [ABSTRACT FROM AUTHOR]

In the framework of the accompanying activity for the development of the two neutral beam injectors for the ITER fusion experiment, an instrumented beam calorimeter is being designed at Consorzio RFX, to be used in the SPIDER test facility (particle energy 100keV; beam current 50A), with the aim of testing beam characteristics and to verify the source proper operation. The main components of the instrumented calorimeter are one-directional carbonfibre-carbon composite tiles. Some prototype tiles have been used as a small-scale version of the entire calorimeter in the test stand of the neutral beam injectors of the LHD experiment, with the aim of characterising the beam features in various operating conditions. The extraction system of the NIFS test stand source was modified, by applying a mask to the first gridded electrode, in order to isolate only a subset of the beamlets, arranged in two 3x5 matrices, resembling the beamlet groups of the ITER beam sources. The present contribution gives a description of the design of the diagnostic system, including the numerical simulations of the expected thermal pattern. Moreover the dedicated thermocouple measurement system is presented. The beamlet monitor was successfully used for a full experimental campaign, during which the main parameters of the source, mainly the arc power and the grid voltages, were varied. This contribution describes the methods of fitting and data analysis applied to the infrared images of the camera to recover the beamlet optics characteristics, in order to quantify the response of the system to different operational conditions. Some results concerning the beamlet features are presented as a function of the source parameters. [ABSTRACT FROM AUTHOR]

Enhancement of negative ion sources for production of large ion beams is a very active research field nowadays, driven from demand of plasma heating in nuclear fusion devices and accelerator applications. As a versatile test bench, the ion source NIO1 (Negative Ion Optimization 1) is being commissioned by Consorzio RFX and INFN. The nominal beam current of 135 mA at -60 kV is divided into 9 beamlets, with multiaperture extraction electrodes. The plasma is sustained by a 2 MHz radiofrequency power supply, with a standard matching box. A High Voltage Deck (HVD) placed inside the lead shielding surrounding NIO1 contains the radiofrequency generator, the gas control, electronics and power supplies for the ion source. An autonomous closed circuit water cooling system was installed for the whole system, with a branch towards the HVD, using carefully optimized helical tubing. Insulation transformer is installed in a nearby box. Tests of several magnetic configurations can be performed. Status of experiments, measured spectra and plasma luminosity are described. Upgrades of magnetic filter, beam calorimeter and extraction grid and related theoretical issues are reviewed. [ABSTRACT FROM AUTHOR]

The NIO1 project consisting of a 60 kV ion source (9 beamlets of 15 mA each of H-) is jointly developed by Consorzio RFX and INFN-LNL, with the purpose of providing a test ion source, capable of working in continuous mode and in condition similar to larger ion sources for Neutral Beam Injectors. The modular design allows for quick replacement and upgrading of parts. While the main body of the ion source construction is progressing at industry, some parts were separately developed at participating institution, as described in the following. A water free Carbon Fiber Composite (CFC) calorimeter is considered, together with more traditional water cooled calorimeters. A small rf plasma generator was installed at INFN-LNL and several rf matching boxes and a Cesium heater controller prototype were tested. Plasma generator (at ground) is followed by a puller and a positively biased Faraday cup, so that beam current can be measured. Plasma density estimated with a 4 wire Langmuir probe is consistent with plasma rf simulation, even if electron distribution deviation from Maxwellian seems large; new electronics with extended DC voltage sweep and a second Langmuir probe circuit are being tested. Finally preparation of the NIO1 site has begun at RFX and installation of source is expected to start in the end of 2012. [ABSTRACT FROM AUTHOR]

The ITER project requires additional heating by two neutral beam injectors, each accelerating to 1MV a 40A beam of negative deuterons, delivering to the plasma about 17MW up to one hour. As these requirements have never been experimentally met, it was decided to build a test facility, PRIMA (Padova Research on ITER Megavolt Accelerator), in Italy, including a full-size negative ion source, SPIDER, and a prototype of the whole ITER injector, MITICA, aiming to develop the heating injectors to be installed in ITER. The Japan and the India Domestic Agencies participate in the PRIMA enterprise; European laboratories, such as KIT-Karlsruhe, IPP-Garching, CCFE-Culham, CEA-Cadarache and others are also cooperating. In the paper the main requirements are discussed and the design of the main components and systems are described. [ABSTRACT FROM AUTHOR]

At the National Institute for Fusion Science (NIFS) large-scaled negative ion sources have been widely used for the Neutral Beam Injectors (NBIs) mounted on the Large Helical Device (LHD), which is the world-largest superconducting helical system. These injectors have achieved outstanding performances in terms of beam energy, negative-ion current and optics, and represent a reference for the development of heating and current drive NBIs for ITER. In the framework of the support activities for the ITER NBIs, the PRIMA test facility, which includes a RF-drive ion source with 100 keV accelerator (SPIDER) and a complete 1 MeV Neutral Beam system (MITICA) is under construction at Consorzio RFX in Padova. An experimental validation of the codes has been undertaken in order to prove the accuracy of the simulations and the soundness of the SPIDER and MITICA design. To this purpose, the whole set of codes have been applied to the LHD NBIs in a joint activity between Consorzio RFX and NIFS, with the goal of comparing and benchmarking the codes with the experimental data. A description of these modeling activities and a discussion of the main results obtained are reported in this paper. [ABSTRACT FROM AUTHOR]

In the context of the development of the 1 MV neutral beam injector for the ITER tokamak [1], the study on beam formation and acceleration has considerable importance. This effort includes the ion source and accelerator SPIDER (Source for Production of Ions of Deuterium Extracted from an Rf plasma) ion source, planned to be built in Padova, and designed to extract and accelerate a 355 A/m2 current of H- (or 285 A/m2 D-) up to 100 kV [2][3]. Exhaustive simulations were already carried out during the accelerator optimization leading to the present design [4]. However, as it is expected that the accelerator shall operate also in case of pre-programmed or undesired off-normal conditions, the investigation of a large set of off-normal scenarios is necessary. These analyses will also be useful for the evaluation of the real performances of the machine, and should help in interpreting experimental results, or in identifying dangerous operating conditions. The present contribution offers an overview of the results obtained during the investigation of these off-normal conditions, by means of different modeling tools and codes. The results, showed a good flexibility of the device in different operating conditions. Where the consequences of the abnormalities appeared to be problematic further analysis were addressed. [ABSTRACT FROM AUTHOR]

Consorzio RFX in Padova is currently using a comprehensive set of numerical and analytical codes, for the physics and engineering design of the SPIDER (Source for Production of Ion of Deuterium Extracted from RF plasma) and MITICA (Megavolt ITER Injector Concept Advancement) experiments, planned to be built at Consorzio RFX. This paper presents a set of studies on different possible geometries for the MITICA accelerator, with the objective to compare different design concepts and choose the most suitable one (or ones) to be further developed and possibly adopted in the experiment. Different design solutions have been discussed and compared, taking into account their advantages and drawbacks by both the physics and engineering points of view. [ABSTRACT FROM AUTHOR]

The NIO1 (Negative Ion Optimization phase 1) project consists of a multiaperture negative ion source mounted on a 60 kV accelerating column; up to 9 beamlets (15 mA H- each one) arranged in 3×3 matrix with 14 mm spacing can be extracted. The moderate size and the modular concept make some relative rotation of the source magnetic filter and the electrodes possible, so that the effect of crossed and aligned field can be easily compared. Other goals of source experimental program are emittance (and beam profile) measurement at several distances (for simulation code validation), testing of diagnostic components and of radiofrequency coupling. A full set of construction drawing was completed; also the Fast Emittance Scanner (FES) and its vacuum chambers were built (four mounting positions are reserved to FES). Some low power rf matching boxes were developed for a test plasma, approximately half the source size. A cesium oven compatible with NIO1 is being also developed; by using some industrial standard 100 W heaters (with a proper driver) a careful control of temperature is planned. [ABSTRACT FROM AUTHOR]

In the design of extraction systems for negative ion sources several fundamental questions still deserve further investigation, as the distribution of particles near the extraction sheath, the optimal magnetic structure and the space charge compensation length after acceleration. Large (and undesired) deflection differences may develop between beamlets of a multiaperture source, so that equalization of the magnetic field effect is necessary. To guarantee an uniform strength of filter field at extraction and in the acceleration, several configuration of arrays of permanent magnets were studied and fast simulation tools were developed. As an example of optimized magnetic configuration and as a possible experimental tool, the design of NIO1 (Negative Ion Optimization try 1) is here discussed. This project consists of a 3×3 matrix of 8 mm extraction holes, aimed at a total H- current about 130 mA with an extraction voltage Vs = -60 kV. A modular design is used, so several parts (the extraction grid, the acceleration grid, the filter assembly, the source multipoles) can be rotated by 90 degrees for versatility. Space charge compensation was included into a two dimensional self consistent code for negative beams, here used for NIO1 simulation. [ABSTRACT FROM AUTHOR]

A Test Facility is planned to be built in Padova to assemble and test the Neutral Beam Injector for ITER. In the same Test Facility the Ion Source will be tested in a dedicated facility planned to operate in parallel to the main 1 MV facility. Purpose of the full size Ion Source is to optimize the Ion Source performance by maximizing the extracted negative ion current density and its spatial uniformity and by minimizing the ratio of co-extracted electrons. In this contribution the design of the extractor and accelerator grids for a 100 kV, 60 A system is presented. The trajectories of the negative ions, calculated with the SLACCAD code [1], have been benchmarked by a new 2D code (BYPO [2]) which solves in a self consistent way the electric fields in presence of electric charge and magnetic fields. The energy flux intercepted by the grids is estimated by using the Montecarlo code EAMCC [3] and the grids designed according to the constraints set by the permanent magnets and by the cooling channels. The interaction of backstreaming ions due to the ionization process with the grids and the Ion Source backplate is investigated and its impact on the project and performance discussed. [ABSTRACT FROM AUTHOR]

In the Reversed Field Pinch experiment RFX a highly sheared ExB flow is observed in the edge region, with shear value close to the value required for turbulence suppression or reduction. Recent observations have shown that almost 50% of particle flux due to turbulence is due to intermittent events which tend to cluster during magnetic relaxation phase. These events have been associated in RFX to vortex like structures whose rotation direction depends on the local value of the mean ExB flow shear. [ABSTRACT FROM AUTHOR]

The mean values of electron temperature and density in the plume of a gas-fed, externally applied field, Magneto-Plasma Dynamic (MPD) thruster, have been measured with a balanced triple probe. The spatial distribution and the temporal behaviour of floating potential fluctuations have been also investigated using multielectrode probes. The addition of an external magnetic field results in an increase of the fluctuation levels of electron temperature, density and floating potential. Mean values in the anode region at the thruster outlet have been related to the Hall parameter to understand the Hall current's contribution to the thrust. Spectral analysis of the floating potential fluctuations shows the importance of the external magnetic field in the dynamic of plasma instabilities. [ABSTRACT FROM AUTHOR]