Negative Hydrogen Ion Source Research Articles

Optimizing the laser absorption technique for quantification of caesium densities in negative hydrogen ion sources

The performance of negative hydrogen ion sources, which rely on the formation of negative hydrogen ions on a surface with low work function, depends strongly on the caesium dynamics in the source. A quantitative measurement of the amount of caesium in the source during plasma-on and plasma-off (vacuum phase) is highly desirable. The laser absorption technique is optimized for the diagnostics of neutral caesium densities close to the extraction surface on which the negative hydrogen ions are generated. The setup is simplified as much as possible utilizing also an automatic data evaluation for online measurements at high power rf sources. The setup is tested and calibrated in a small scale laboratory experiment. The system and the analysis of the D2 caesium line at 852.1 nm are described in detail, including effects of line saturation and density depletion. The system is sensitive in the density range 1013–1017 m−3 (path length of about 15 cm), allowing also for a temporal resolution of 40 ms. First very promising results from the negative hydrogen ion source are presented, such as the increase in the caesium density due to the caesium evaporation and time traces before, during, and after the discharge indicating a strong caesium redistribution.

Caesium influence on plasma parameters and source performance during conditioning of the prototype ITER neutral beam injector negative ion source

Systematic Langmuir probe measurements were performed during the caesium (Cs) conditioning of the IPP prototype negative hydrogen ion source. Conditioning consists of slowly injecting caesium into the source in order to increase the extracted negative ion current and minimize the co-extracted electrons. This process usually takes several days. Measurements were taken near the plasma grid (PG) where the majority of the extracted negative ions are created. A clear transition between a poorly conditioned (volume dominated H− production) and a well conditioned (surface dominated H− production) source was seen. By following the plasma parameter variations, an overall decrease in the plasma potential, the floating potential and also the electron saturation current was observed. However, the ion saturation current remained constant. Consequently, near the PG where negative ions are generated and extracted, a transition occurred in the space charge balance (quasineutrality): the space charge is balanced by electrons when the source is poorly conditioned and by negative ions when the source is well conditioned.

Performance of multi-aperture grid extraction systems for an ITER-relevant RF-driven negative hydrogen ion source

The ITER neutral beam system requires a negative hydrogen ion beam of 48 A with an energy of 0.87 MeV, and a negative deuterium beam of 40 A with an energy of 1 MeV. The beam is extracted from a large ion source of dimension 1.9 × 0.9 m2 by an acceleration system consisting of seven grids with 1280 apertures each. Currently, apertures with a diameter of 14 mm in the first grid are foreseen.In 2007, the IPP RF source was chosen as the ITER reference source due to its reduced maintenance compared with arc-driven sources and the successful development at the BATMAN test facility of being equipped with the small IPP prototype RF source ( of the area of the ITER NBI source). These results, however, were obtained with an extraction system with 8 mm diameter apertures.This paper reports on the comparison of the source performance at BATMAN of an ITER-relevant extraction system equipped with chamfered apertures with a 14 mm diameter and 8 mm diameter aperture extraction system. The most important result is that there is almost no difference in the achieved current density—being consistent with ion trajectory calculations—and the amount of co-extracted electrons. Furthermore, some aspects of the beam optics of both extraction systems are discussed.

Conceptual design and circuit analyses for the power supplies of the NIO1 experiment

The NIO1 (Negative Ion Optimization phase 1) experiment, whose construction has started in 2010 at Consorzio RFX in Padova in collaboration with INFN-LNL, is a radio frequency Hydrogen negative ion source designed to produce a beam current of 130 mA with 60 keV particle energy. The aim of the experiment is to provide a highly flexible system to test material properties of source components and to benchmark simulation codes for beam optics and plasma behavior. The paper focuses on a conceptual design for NIO1 power supply system on the basis of given operational and performance requirements. The requested ratings fall well within those of products available on the market and the design includes standard commercial solutions for the individual power supplies. The ion source operates at −60 kV with respect to ground, leading to a number of design issues at system level and related to mutual connection between power supplies and load, layout and grounding. These aspects, along with the crucial point of ensuring protection to the arc prone beam source, are reviewed and discussed. The study leads to a scheme integrating high voltage power supplies, high current power supplies and a possible solution for passive protection components at the power supply output.

Development of negative hydrogen ion sources for fusion: Experiments and modelling

Large and powerful negative hydrogen ion sources have to be developed for the neutral beam injection system of the international fusion experiment ITER which is currently under construction. In order to fulfil the ITER requirements — high negative ion current densities and low co-extracted electron currents at low pressure operation (0.3Pa) — caesium is seeded into the discharges which lowers the work function of the converter surface. The paper addresses the development program at the three test facilities of the Max-Planck-Institut für Plasmaphysik (IPP) in Garching. Emphasis is given on a comparison of deuterium with hydrogen operation as well as on the complex caesium chemistry and the plasma surface interaction which are at present the most critical issues for optimising the source performance. An insight into the plasma chemistry and the processes relevant for source optimisation is provided by the well diagnosed plasma accompanied by modelling which is strongly coupled to the physics relevant for the experiments.

ITER DNB ion source movement mechanism

The 100 kV negative hydrogen ion source based Diagnostic Neutral Beam (DNB) injector, a part of Indian (IN) Procurement Package for ITER. DNB is expected to deliver 18–20 A of hydrogen neutral beam to the ITER plasma through a narrow blanket aperture of size 0.40( H) × 0.45( V) m 2, at a distance ∼20.67 m from the ion source position. Due to long transport length, a small misalignment of the ion source will cause significant transmission loss of the beam and produce asymmetric heat load on the beamline components and on the duct assembly. Mechanical misalignment and deflection due to tokamak stray magnetic field are also envisaged and therefore ion source positional adjustment is needed during DNB operation. This can be addressed by making provisions for the desired vertical and horizontal movements in the ion source support structure. Two independent translations for horizontal angular and linear misalignment adjustment are achieved by means of “master–slave” configuration arrangement in the support structure. The force analysis of the movement mechanism of the ion source support structure is carried out analytically by statics and generates inputs for an engineering design of such movement mechanism and discussed in the manuscript.

Numerical analysis of electronegative plasma in the extraction region of negative hydrogen ion sources

This numerical study focuses on the physical mechanisms involved in the extraction of volume-produced H− ions from a steady state laboratory negative hydrogen ion source with one opening in the plasma electrode (PE) on which a dc-bias voltage is applied. A weak magnetic field is applied in the source plasma transversely to the extracted beam. The goal is to highlight the combined effects of the weak magnetic field and the PE bias voltage (upon the extraction process of H− ions and electrons). To do so, we focus on the behavior of electrons and volume-produced negative ions within a two-dimensional model using the particle-in-cell method. No collision processes are taken into account, except for electron diffusion across the magnetic field using a simple random-walk model at each time step of the simulation. The results show first that applying the magnetic field (without PE bias) enhances H− ion extraction, while it drastically decreases the extracted electron current. Secondly, the extracted H− ion current has a maximum when the PE bias is equal to the plasma potential, while the extracted electron current is significantly reduced by applying the PE bias. The underlying mechanism leading to the above results is the gradual opening by the PE bias of the equipotential lines towards the parts of the extraction region facing the PE. The shape of these lines is due originally to the electron trapping by the magnetic field.

负氢离子源中多峰磁场形态数值模拟

负氢离子源中多峰磁场形态数值模拟

Negative ion density measurements in an inductively driven hydrogen discharge

The study presents experiments on the determination of the electronegativity and its axial variation in an inductively driven hydrogen discharge, performed by using the laser photodetachment technique. The results showing high electronegativity with nonmonotonic axial variations and a maximum next to the rf power deposition region provide an experimental indication that a design of an efficient source of negative hydrogen ions based on a single-chamber discharge might be possible.

Transport of negative hydrogen and deuterium ions in RF-driven ion sources

Negative hydrogen ion sources are major components of neutral beam injection systems for plasma heating in future large-scale fusion experiments such as ITER. In order to fulfill the requirements of the ITER neutral beam injection, a high-performance, large-area RF-driven ion source for negative ions is being developed at the MPI für Plasmaphysik. Negative hydrogen ions are mainly generated on a converter surface by impinging neutral particles and positive ions under the influence of magnetic fields and the plasma sheath potential. The 3D transport code TrajAn has been applied in order to obtain the total and spatially resolved extraction probabilities for H− and D− ions under identical plasma parameters and the realistic magnetic field topology of the ion source. A comparison of the isotopes shows a lower total extraction probability in the case of deuterium ions, caused by a different transport effect. The transport calculation shows that distortions of the spatial distributions of ion birth and extraction by the magnetic electron suppression field are present for both negative hydrogen and deuterium ions.

3D modeling of the electron energy distribution function in negative hydrogen ion sources

For optimization and accurate prediction of the amount of H-ion production in negative ion sources, analysis of electron energy distribution function (EEDF) is necessary. We are developing a numerical code which analyzes EEDF in the tandem-type arc-discharge source. It is a three-dimensional Monte Carlo simulation code with realistic geometry and magnetic configuration. Coulomb collision between electrons is treated with the "binary collision" model and collisions with hydrogen species are treated with the "null-collision" method. We applied this code to the analysis of the JAEA 10 A negative ion source. The numerical result shows that the obtained EEDF is in good agreement with experimental results.

Enhanced surface production in H− ion sources by introducing a negatively biased secondary electrode

A transformer coupled plasma negative hydrogen ion source with an external rf antenna has been developed at SNU, which is capable of continuous operation with long lifetime. A positively biased plasma electrode (PE) has been successfully used for the optimization of H(-) extraction. With molybdenum-coated stainless steel PE, the enhancement of H(-) production at the electrode surface was observed at the bias voltage lower than the plasma potential. However, the low bias voltage is unfavorable to H(-) beam extraction since the negative ions are repelled. A second electrode is inserted in front of the PE to enhance H(-) production at the electrode surface without impeding beam extraction. By biasing the secondary electrode (SE) more negatively, H(-) production is clearly enhanced although the SE itself reduces H(-) beam currents because of suppressed electron transport in front of the PE. In this configuration enhancement of surface productions is most pronounced in tantalum electrode among various electrode materials.

Fundamental experiments on evaporation of cesium in ion sources

Basic experiments are carried out to study the cesium evaporation and desorption from surfaces at different temperatures in an environment, which is very close to the conditions of negative hydrogen ion sources for fusion applications: in a vacuum base pressure of 10(-5) mbar and in a hydrogen plasma in the Pa-range. Several diagnostic techniques such as emission and absorption spectroscopy, a surface ionization detector, and a quartz-microbalance have been utilized to determine the cesium densities, evaporation and desorption rates. The work function of a cesiated surface measured by the photoelectric effect degrades with increasing plasma-off time. Impurities and cesium compounds are detected by a residual mass analyzer.

Optical Measurement of Cesium Behavior in a Large H− Ion Source for a Neutral Beam Injector

Optical emission in a negative hydrogen ion source for the Large Helical Device Neutral Beam Injector (LHD-NBI) has been measured to investigate the behavior of Cs. Two optical sight lines exist parallel to the plasma grid, in the discharge area and in the magnetic filter area near the plasma grid. In the discharge area, the spectrum intensity from Cs+ ions is considerably increased during 20 s of the beam extraction. This indicates a considerable increase in the Cs+ density inside the plasma due to the impact of back-streaming H+ ions. A strong neutral Cs spectrum is observed in the magnetic filter area, where the electron density is lower than in the discharge area. The rate of increase of neutral Cs is much enhanced after t = 30 s, probably because the Cs adsorbed on the cooled region inside the arc chamber evaporates because its temperature increases during the long pulse discharge.

Physical performance analysis and progress of the development of the negative ion RF source for the ITER NBI system

For heating and current drive the neutral beam injection (NBI) system for ITER requires a 1 MeV deuterium beam for up to 1 h pulse length. In order to inject the required 17 MW the large area source (1.9 m × 0.9 m) has to deliver 40 A of negative ion current at the specified source pressure of 0.3 Pa. In 2007, the IPP RF driven negative hydrogen ion source was chosen by the ITER board as the new reference source for the ITER NBI system due to, in principle, its maintenance free operation and the progress in the RF source development. The performance analysis of the IPP RF sources is strongly supported by an extensive diagnostic program and modelling of the source and beam extraction. The control of the plasma chemistry and the processes in the plasma region near the extraction system are the most critical topics for source optimization both for long pulse operation as well as for the source homogeneity. The long pulse stability has been demonstrated at the test facility MANITU which is now operating routinely at stable pulses of up to 10 min with parameters near the ITER requirements. A quite uniform plasma illumination of a large area source (0.8 m × 0.8 m) has been demonstrated at the ion source test facility RADI. The new test facility ELISE presently planned at IPP is being designed for long pulse plasma operation and short pulse, but large-scale extraction from a half-size ITER source which is an important intermediate step towards ITER NBI.

PIC code for the plasma sheath in large caesiated RF sources for negative hydrogen ions

Powerful negative hydrogen ion sources are required for heating and current drive at ITER. The physics of the production and extraction of high negative ion currents is much more complex than that for positive ions. One of the most relevant parameters is the shape of the plasma sheath, which determines the velocity of surface produced negative ions and thus the probability of the ions to reach the extraction system. In order to investigate the influence of hydrogen atoms, positive and negative hydrogen ions and positive caesium ions on the plasma sheath, a 1d3v particle in cell code (PIC) code for the plasma close to the extraction system has been developed. For typical plasma parameters of such ion sources, surface conversion of impinging atoms is the main negative ion production channel, while conversion of positive ions plays a minor role. Due to the formation of a potential minimum close to the surface, the emission of negative ions into the plasma is space charge limited. As a consequence, the flux of negative ions can be increased only by increasing the density of positive hydrogen ions. At identical plasma parameters, an isotope effect is determined by the mass of the particles only, resulting in lower fluxes of negative deuterium ions compared with hydrogen. A small amount of positive Cs does not change the plasma sheath and the H− flux significantly.

Particle-in-cell with Monte Carlo collision modeling of the electron and negative hydrogen ion transport across a localized transverse magnetic field

The control of the electron temperature and charged particle transport in negative hydrogen ion sources has a crucial role for the performance of the system. It is usually achieved by the use of a magnetic filter—localized transverse magnetic field, which reduces the electron temperature and enhances the negative ion yield. There are several works in literature on modeling of the magnetic filter effects based on fluid and kinetic modeling, which, however, suggest rather different mechanisms responsible for the electron cooling and particle transport through the filter. Here a kinetic modeling of the problem based on the particle-in-cell with Monte Carlo collisions method is presented. The charged particle transport across a magnetic filter is studied in hydrogen plasmas with and without including volume production of negative ions, in a one-dimensional Cartesian geometry. The simulation shows a classical (collisional) electron diffusion across the magnetic filter with reduction in the electron temperature but no selective effect in electron energy is observed (Coulomb collisions are not considered). When a bias voltage is applied, the plasma is split into an upstream electropositive and a downstream electronegative regions. Different configurations with respect to bias voltage and magnetic field strength are examined and discussed. Although the bias voltage allows negative ion extraction, the results show that volume production of negative ions in the downstream region is not really enhanced by the magnetic filter.

Plasma grid design for optimized filter field configuration for the NBI test facility ELISE

Maintenance-free RF sources for negative hydrogen ions with moderate extraction areas (100–200 cm 2) have been successfully developed in the last years at IPP Garching in the test facilities BATMAN and MANITU. A facility with larger extraction area (1000 cm 2), ELISE, is being designed with a “half-size” ITER-like extraction system, pulsed ion acceleration up to 60 kV for 10 s and plasma generation up to 1 h. Due to the large size of the source, the magnetic filter field (FF) cannot be produced solely by permanent magnets. Therefore, an additional magnetic field produced by current flowing through the plasma grid (PG current) is required. The filter field homogeneity and the interaction with the electron suppression magnetic field have been studied in detail by finite element method (FEM) during the ELISE design phase. Significant improvements regarding the field homogeneity have been introduced compared to the ITER reference design. Also, for the same PG current a 50% higher field in front of the grid has been achieved by optimizing the plasma grid geometry. Hollow spaces have been introduced in the plasma grid for a more homogeneous PG current distribution. The introduction of hollow spaces also allows the insertion of permanent magnets in the plasma grid.

Laser photodetachment on a high power, low pressure rf-driven negative hydrogen ion source

Powerful, low pressure negative hydrogen ion sources are a basic component of future neutral beam heating systems for fusion devices. The required high ion currents (>40 A) are obtained via the surface production process, which requires negative ion densities in the range of in the plasma region close to the extraction system. For spatially resolved diagnostics of the negative hydrogen ion densities, the laser photodetachment method has been applied to a high power, low pressure, rf-driven ion source (150 kW, 0.3 Pa) for the first time. The diagnostic setup and the data evaluation had to cope with the rf field (1 MHz), the high source potential during extraction (−25 kV) and the presence of magnetic fields (<10 mT). Horizontal profiles of negative ion densities and electron densities along 15 cm with a typical step length of 1 cm and a probe tip of 5 mm length show a broad maximum in the centre of the extraction region. The variation of a bias voltage applied to the plasma grid with respect to the source body yields a correlation between the detachment signals for the negative ion density and the electron density with the extracted ion and electron currents, respectively. The density ratio of negative hydrogen ions to electrons is in the range of , demonstrating that the negative ions are the dominant negatively charged species in these types of ion sources. Absolute negative ion densities are in good agreement with line-of-sight integrated results of cavity ring-down spectroscopy and optical emission spectroscopy.

Design of the “half-size” ITER neutral beam source for the test facility ELISE

In 2007 the radio frequency driven negative hydrogen ion source developed at IPP in Garching was chosen by the ITER board as the new reference source for the ITER neutral beam system. In order to support the design and the commissioning and operating phases of the ITER test facilities ISTF and NBTF in Padua, IPP is presently constructing a new test facility ELISE (Extraction from a Large Ion Source Experiment). ELISE will be operated with the so-called “half-size ITER source” which is an intermediate step between the present small IPP RF sources (1/8 ITER size) and the full size ITER source. The source will have approximately the width but only half the height of the ITER source. The modular concept with 4 drivers will allow an easy extrapolation to the full ITER size with 8 drivers. Pulsed beam extraction and acceleration up to 60 kV (corresponding to pre-acceleration voltage of SINGAP) is foreseen. The aim of the design of the ELISE source and extraction system was to be as close as possible to the ITER design; it has however some modifications allowing a better diagnostic access as well as more flexibility for exploring open questions. Therefore one major difference compared to the source of ITER, NBTF or ISTF is the possible operation in air. Specific requirements for RF sources as found on IPP test facilities BATMAN and MANITU are implemented [A. Stäbler, et al., Development of a RF-driven ion source for the ITER NBI system, SOFT Conference 2008, Fusion Engineering and Design, 84 (2009) 265–268].