An overview of the recent operations and the main results of cesium injection in the Source for the Production of Ions of Deuterium Extracted from Rf plasma (SPIDER) negative ion source are described in this contribution. In experiments without cesium injection, all SPIDER plants were tested to verify the basic expectations on the operational parameters (e.g., electron cooling effectiveness of magnetic filter field) and to determine its operational region. For beam properties, it was shown that the current density varies across the beam in the vertical direction. In preliminary cesium experiments, the expected increase of negative ion current and simultaneous decrease of co-extracted electrons were found, along with the influence of the control parameters (polarization of the plasma electrodes, magnetic filter field) on the SPIDER beam uniformity in the horizontal and vertical directions. It was shown that non-Gaussian tails can be identified in the angular distribution on the plane perpendicular to the beam propagation direction. Stray particles, nonhomogeneous beam and large divergence might result in unexpected heat and particle loads over ITER neutral beam injector (NBI) accelerator grids; it is the goal of SPIDER to assess and possibly to identify suitable methods for controlling these beam features. A major shutdown, planned for late 2021, to solve the issues identified during the operation and to carry out scheduled modifications, is outlined. Such improvements are expected to allow SPIDER to pursue the ITER requirements in terms of negative ion current, electron-to-ion ratio, and beam duration.

The negative-ion based neutral beam injector for heating and current drive of the ITER plasma (ITER HNB) is under development, at present focusing on the optimization of the full-scale plasma source in the SPIDER test stand. The production of H − or D − ions in the ion source is based on the low work function surfaces obtained by caesium evaporation. This paper describes the caesium conditioning procedure and the corresponding beam performances during the first operation of SPIDER with caesium. Technical solutions to overcome present limitations of the test stand are described. The influence of source parameters on the caesium effectiveness was investigated in short beam pulse operation; with total radio-frequency (RF) power of 400 kW and filling pressure below 0.4 Pa, and a limited number of extraction apertures, a negative ion current density of about 200 A m −2 was extracted in hydrogen, with beam energy lower than 60 keV. Beam optics and beam uniformity were assessed thanks to the acceleration of isolated ion beamlets. A possible procedure to accelerate a uniform beam was demonstrated at low RF power. The results obtained in this first investigation provided key indications on the operation of one of the largest existing sources of accelerated negative hydrogen-like ions.

The SPIDER experiment is operating at the ITER Neutral Beam Test Facility PRIMA since June 2018 to study the operation of a full ITER-size negative ion source. In order to increase the negative ion extracted current and reduce the co-extracted electron current, caesium is evaporated inside the source. In fact, the adsorption of Cs onto the plasma grid, i.e. the first grid of the extraction system, lowers the surface work function, enhancing the probability of surface conversion of impinging hydrogen particles into negative ions. At the SPIDER test facility, Cs will be evaporated by heating a liquid Cs reservoir in three Cs ovens located at the back-plate of the ion source. The ovens must assure a controllable and reproducible evaporation. A SPIDER Cs oven prototype was procured and tested in the CAesium Test Stand (CATS) at PRIMA site to validate the design. Following the experimental tests, few improvements were introduced in the final design of the three Cs ovens for the SPIDER source. Then, the ovens were procured in 2019 and commissioned before their installation in SPIDER in 2020. This paper reports the functional tests of the SPIDER Cs ovens and of their embedded diagnostics. These tests were carried out at CATS, with the aim of characterizing the three ovens, in order to validate them for their use in SPIDER in the experimental campaign with caesium evaporation planned for the end of 2020. The ovens were successfully tested and characterized in terms of evaporation rate for different oven temperatures. Moreover, the three ovens show a good reproducibility and reliability.

Negative ion sources of neutral beam injection (NBI) systems for future fusion devices like ITER (“The Way” in Latin) rely on the surface conversion of hydrogen (or deuterium) atoms and positive ions to negative ions in an inductively coupled plasma (ICP). The efficiency of this process depends on the work function of the converter surface. By introducing caesium into the ion source the work function decreases, enhancing the negative ion yield. In order to study the isotope effect on the negative ion density at different work functions, fundamental investigations are performed in a planar ICP laboratory experiment where the work function and the negative ion density in front of a sample can be simultaneously and absolutely determined. For work functions above 2.7 eV, the main contribution to the negative hydrogen ion density is solely due to volume formation, which can be modeled via the rate balance model YACORA H−, while below 2.7 eV the surface conversion become significant and the negative ion density increases. For a work function of 2.1 eV (bulk Cs), the H− density increases by at least a factor of 2.8 with respect to a non-caesiated surface. With a deuterium plasma, the D− density measured at 2.1 eV is a factor of 2.5 higher with respect to a non-caesiated surface, reaching densities of surface produced negative ions comparable to the hydrogen case.

To reach fusion conditions and control the plasma configuration in ITER, the next step in tokamak fusion research, two neutral beam injectors (NBIs) will supply 16.5 MW each, by neutralizing accelerated negative hydrogen or deuterium ions. The requirements of ITER NBIs (40A/1 MeV D-ions for <1 h, 46A/870 keV H-ions for <1000 s) have never been simultaneously attained. So in the Neutral Beam Test Facility (NBTF, Consorzio RFX, Italy) the operation of the full-scale ITER NBI prototype (MITICA) will be tested and optimised up to full performances, focussing on accelerator (including voltage holding), beam optics, neutralisation, residual ion removal. The NBTF includes also the full-scale prototype of the ITER NBI source with 100 keV particle energy (SPIDER), for early investigation of: negative ion production and extraction, source uniformity, negative ion current density and beam optics. This paper will describe the main results of the first two years of SPIDER operation, devoted to characterizing plasma and beam parameters, including investigation of RF-plasma coupling efficiency and magnetic filter field effectiveness in reducing co-extracted electrons. SPIDER is progressing towards the first caesium injection, which aims at increasing the negative ion density. A major shutdown, planned for 2021, to solve the issues identified during the operation and to carry out programmed modifications, will be outlined. The installation of each MITICA power supply and auxiliary system is completed; in-vessel mechanical components are under procurement by Fusion for Energy (F4E). Integration, commissioning and test of the power supplies, procured by F4E and QST, as the Japanese Domestic Agency (JADA), will be presented. In particular, 1.0MV insulating tests were carried out step-by-step and successfully completed. In 2020 integrated tests of the power supplies on the accelerator dummy load started, including the assessment of their resilience to accelerator grid breakdowns using a short-circuit device located in vacuum. The aggressive programme, to validate the NBI design at NBTF and to meet ITER schedule (requiring NBIs in operation in 2032), will be outlined. Unfortunately, in 2020 the coronavirus disease infection affected the NBTF activities. A solution to proceed with integrated power tests despite the coronavirus is presented.

The influence of low-pressure and low-temperature hydrogen plasma on a pure molybdenum sample implanted with caesium is investigated by means of sputtering-XPS. A portion of the sample was exposed to the plasma, while the rest of the sample was covered by stainless steel and thus shielded from direct plasma flux. The results show that the part of the sample exposed to the plasma exhibits both on and beneath the surface an increased amount of H2O and MoO2 with respect to the covered sample part which can be explained by a chemical reduction of MoO3 oxides by the hydrogen plasma species. The XPS Cs lines show oxidation peaks. A sputtering depth of roughly 13 nm is necessary to recover an identical spectrum, indicating that the plasma changes the chemical composition of the implanted samples not only on the surface but also inside the material.

Beam extraction at the large ELISE test facility is currently possible only pulsed, with short extraction phases, so-called extraction blips, of up to 10 s each ≈150 s. Over the past years, a good insight into the physics of this operational mode has been gained for both hydrogen and deuterium operation. The uniformity of the co-extracted electrons was identified as a key issue and it was possible to achieve 1000 s plasma pulses in hydrogen with repetitive extraction blips and an extracted current density of over 90 % of the ITER target value by improving the co-extracted electron symmetry. In deuterium roughly 67 % of the ITER target for the extracted current density has been achieved for long pulses. During such pulses an overall increase of the co-extracted electron current is typically observed between one blip and the next one, even though the electron current is observed to actually decrease during each blip. These opposing effects are explained by different caesium dynamics during the source plasma phase when compared to the beam phase and they were one motivation behind the currently ongoing upgrade of ELISE to a CW extraction system. This update consists of two main hardware changes: i) installation of a new CW high voltage power supply and ii) installation of a CW beam calorimeter. Being able to achieve beam pulses of up to 1 hour will allow knowledge to be gained on the physics of caesium redistribution and conditioning over the long timescales needed for ITER operation.

The requirements of ITER neutral beam injectors (1 MeV, 40 A negative deuterium ion current for 1 h) have never been simultaneously attained; therefore, a dedicated Neutral Beam Test Facility (NBTF) was set up at Consorzio RFX (Padova, Italy). The NBTF includes two experiments: SPIDER (Source for the Production of Ions of Deuterium Extracted from Rf plasma), the full-scale prototype of the source of ITER injectors, with a 100 keV accelerator, to investigate and optimize the properties of the ion source; and MITICA, the full-scale prototype of the entire injector, devoted to the issues related to the accelerator, including voltage holding at low gas pressure. The present paper gives an account of the status of the procurements, of the timeline, and of the voltage holding tests and experiments for MITICA. As for SPIDER, the first year of operation is described, regarding the solution of some issues connected with the radiofrequency power, the source operation, and the characterization of the first negative ion beam.

Negative hydrogen ion sources for NBI systems at fusion devices rely on the surface conversion of hydrogen atoms and positive ions to negative hydrogen ions. In these sources the surface work function is decreased by adsorption of caesium (work function of 2.1 eV), enhancing consequently the negative ion yield. However, the performance of the ion source decreases during plasma pulses up to one hour, suggesting a deterioration of the work function. Fundamental investigations are performed in a laboratory experiment in order to study the impact of the plasma on the work function of a freshly caesiated stainless steel surface. A work function of 2.1 eV is achieved in the first 10 s of plasma, while further plasma exposure leads to the removal of Cs from the surface and to the change of the work function: a value of around 1.8–1.9 eV is measured after 10–15 min of plasma exposure and then the work function increases, approaching the work function of the substrate (≥4.2 eV) after 5 h. The Cs removal must be counteracted by continuous Cs evaporation, and investigations performed varying the Cs flux towards the surface have shown that a Cs flux of at least 1.5 × 10 16 m −2 s −1 is required to maintain a work function of 2.1 eV during long plasma exposure at the laboratory experiment.

The cesium ovens for the prototype source of the ITER neutral beam injectors are currently tested in the CAesium Test Stand (CATS) facility, with a background pressure of 10-6 mbar. Different diagnostics are here installed: two Langmuir-Taylor detectors allow us to determine the Cs vapour evaporation rate from the oven and the Cs density at different positions in the vacuum chamber; and laser absorption spectroscopy is used to measure the density integrated over a line of sight and a quartz crystal microbalance to detect the cesium mass deposited in time over a surface. In this paper, we present a model to describe the dynamic equilibrium in the evaporation chamber of CATS with the first oven tested in order to gain information about the Cs sticking coefficient at the walls. The model hence includes sticking and energy accommodation of the Cs atoms to the walls, calculates the flux density at the surfaces, and provides the Cs atom density at any location in the volume. By this model, we simulate the Cs evaporation and the equilibrium density, comparing the modeled results with the experimental data. As a result, a sticking coefficient of 2% is obtained.

Europium is a ductile rare-earth element and has the lowest function among lanthanides (2.5 eV). Due to its low work function reachable without an activation procedure, Eu is a potential electron emitter. For its application to negative hydrogen ion sources for fusion, the work function of an Eu sample exposed to a low pressure low temperature hydrogen plasma is investigated here. First, a work function of 2.6±0.1 eV is achieved in the present vacuum conditions (∼10−6 mbar) by heating the sample up to 350°C. A short hydrogen plasma pulse of 1 min allows us to achieve the same low work function; however, a longer plasma exposure leads to an increase of the work function, which must be counteracted by heating the sample up to at least 400°C to maintain the low work function during plasma. Furthermore, the exposure to hydrogen plasma affects the reactivity of Eu to oxidation when it is exposed to air, showing evidence of a faster oxidation in the areas previously in contact with the plasma. In conclusion, due to the strong reaction to hydrogen plasma and the rapid oxidation in air, Eu is not suitable for application in negative ion sources for fusion.

The SPIDER H-/D- ion source is currently in operation in the Neutral Beam Test facility (NBTF) at Consorzio RFX (Padova, Italy) to prove the possibility of generating up to 40 A of negative ions, with a maximum extracted current density of 350 A/m2 (H)/285 A/m2 (D) and a fraction of co-extracted electrons not greater than 0.5 (H)/1 (D). These performances are required for the realization of the ITER Neutral Beam Injector (NBI), which should deliver 16.7 MW to the plasma by means of negative ions accelerated up to 1 MeV and neutralized before being injected into the ITER tokamak. In order to obtain such high extracted current densities and low co-extracted electron fractions it is necessary to lower the work function of the surface of the acceleration system grid facing the source; this will be accomplished by coating the surfaces with Cs, routinely evaporated by three ovens. The functionality of the ovens has been tested at the CAesium oven Test Stand (CATS), hosted at NBTF. The test stand is equipped with several diagnostics, among which a Laser Absorption Spectroscopy (LAS) diagnostic. Using a tunable laser diode, the LAS diagnostic gets the high resolution absorption spectrum of the Cs 852 nm D2 line along a line of sight to measure Cs density at ground state. The paper describes the test stand and the LAS diagnostic, together with the characterization of the ovens to be installed in SPIDER. The paper will also study the systematic density underestimation effect caused by Cs ground state depopulation, as a function of laser intensity and of Cs density, in the perspective of correcting the density evaluation.

Negative ion sources for fusion rely on the formation of negative hydrogen (or deuterium) ions by conversion of atomic hydrogen and positive hydrogen ions at a low work function caesiated surface. Cs is thus evaporated into the source to decrease the surface work function, which may change due to the removal and redistribution of Cs during plasma phases. To maintain a temporarily stable low work function during 1 h plasma, continuous evaporation of caesium is required, and this is performed by temperature controlled Cs ovens. The Cs ovens for ELISE (IPP Garching) and SPIDER (Consorzio RFX) are based on the evaporation of liquid Cs from a reservoir located at one end of the oven, which is controlled by the reservoir temperature. The ampoule Cs oven of ELISE is in operation since 2015, allowing for controllable and stable evaporation. The SPIDER oven is based on the ELISE oven although it required significant changes due to the vacuum environment and the oven location (at the back-plate instead of the sidewalls), leading to a different design of the oven and the nozzle. First investigations on the SPIDER oven in a dedicated test stand show that Cs evaporation is controllable, stable, and reproducible.

Based on the requirements for heating and current drive of a DEMO (tokamak) device a neutral beam injection (NBI) system faces several challenges, from which the achievement of high wall-plug efficiency is the driving parameter for the beam line design. For boosting the wall-plug efficiency of the ITER NBI system from about 0.25 to target values around 0.4 (pulsed DEMO) or 0.5-0.6 (steady state DEMO), the standard gas neutralizer concept has to be modified or replaced by the innovative laser neutralizer concept for which the modular concept of the ion source allows for adaption. The technology developments of ITER's high power RF-driven ion source are continued by optimization of the RF-driver geometry and the resulting overlap of the driver plasmas in a large expansion chamber. Insights in the caesium evaporation and distribution in the ion source could allow for a reduction of the caesium consumption. On the other hand, the caesium dynamics influences the temporal source performance and the beam uniformity such that investigations on alternatives to caesium became an R&D task. Finally, the status of proof-of-principle experiments on the laser neutralization technique and its challenges for up-scaling from a lab scale experiment to a beam line are discussed.

Materials with low work function (WF), such as the well-known caesiated surfaces, are most efficient in producing H− ions. In view of the applicability in future negative ion sources, alternative materials to Cs evaporation are studied in a dedicated experiment. Lanthanated (WL10, WL2, MoLa, LaB6) and bariated materials (TDC) as well as molybdenum implanted with Cs are investigated regarding their achievable WF at ion source relevant conditions, in particular for temperatures below 500 ◦C. In contrast to their usual application as electron emitters at temperatures above 1000 ◦C, the work functions of the investigated materials under these conditions do not decrease below 3.6 eV (measured global minimum, achieved with LaB6). For the lanthanated materials, the obtained WF values are stable under plasma exposure times of several hours. However, for all the materials the work function is subject to degradation in absence of heating or plasma exposure. Compared to Cs evaporation, with WF values measured down to 2.1 eV, none of the materials tested so far can be regarded as an actual alternative.

Negative hydrogen ion sources rely on the surface conversion of neutral atomic hydrogen and positive hydrogen ions to H−. The efficiency of this process depends on the actual work function of the converter surface. By introducing caesium into the source the work function decreases, enhancing the negative ion yield. In order to study the impact of the work function on the H− surface production at similar conditions to the ones in ion sources for fusion devices like ITER and DEMO, fundamental investigations are performed in a flexible laboratory experiment. The work function of the converter surface can be absolutely measured by photoelectric effect, while a newly installed cavity ring-down spectroscopy system (CRDS) measures the H− density. The CRDS is firstly tested and characterized by investigations on H− volume production. Caesiation of a stainless steel sample is then performed in vacuum and the plasma effect on the Cs layer is investigated also for long plasma-on times. A minimum work function of (1.9±0.1) eV is reached after some minutes of plasma treatment, resulting in a reduction by a value of 0.8 eV compared to vacuum measurements. The H− density above the surface is (2.1±0.5)×1015 m−3. With further plasma exposure of the caesiated surface, the work function increases up to 3.75 eV, due to the impinging plasma particles which gradually remove the Cs layer. As a result, the H− density decreases by a factor of at least 2.Negative hydrogen ion sources rely on the surface conversion of neutral atomic hydrogen and positive hydrogen ions to H−. The efficiency of this process depends on the actual work function of the converter surface. By introducing caesium into the source the work function decreases, enhancing the negative ion yield. In order to study the impact of the work function on the H− surface production at similar conditions to the ones in ion sources for fusion devices like ITER and DEMO, fundamental investigations are performed in a flexible laboratory experiment. The work function of the converter surface can be absolutely measured by photoelectric effect, while a newly installed cavity ring-down spectroscopy system (CRDS) measures the H− density. The CRDS is firstly tested and characterized by investigations on H− volume production. Caesiation of a stainless steel sample is then performed in vacuum and the plasma effect on the Cs layer is investigated also for long plasma-on times. A minimum work function of (1....

The ITER project requires additional heating provided by two injectors of neutral beams resulting from the neutralisation of accelerated negative ions. To study and optimise negative ion production, the SPIDER test facility (particle energy 100keV; beam current 50A) is under construction in Padova, with the aim of testing beam characteristics and to verify the source proper operation. The SPIDER beam will be characterised by the instrumented calorimeter STRIKE, whose main components are one-directional carbon fibre carbon composite tiles. Some prototype tiles have been employed in 2012 as a small-scale version (mini-STRIKE) of the entire system to investigate the features of the beam from BATMAN at IPP-Garching. As the BATMAN beamlets are superposed at the measurement position, about 1m from the grounded grid, an actively cooled copper mask is located in front of the tiles; holes in the mask create an artificial beamlet structure. Recently the mini-STRIKE has been updated, taking into account the results obtained in the first campaign. In particular the spatial resolution of the system has been improved by increasing the number of the copper mask holes. Moreover a custom measurement system has been realized for the thermocouple signals and employed in BATMAN in view of its use in SPIDER. The present contribution gives a description of the new design of the system as well as of the thermocouple measurements system and its field test. A new series of measurements has been carried out in BATMAN. The BATMAN beam characterisation in different experimental conditions is presented.