Development Of Negative Ion Sources Research Articles

Influence of cusp magnetic field configuration on plasma parameters of radio frequency driven negative ion source for CRAFT NNBI

Radio Frequency (RF) driven negative ion source is the preferred ion source for negative ion based Neutral Beam Injection system (NNBI). The plasma generator, which consists of RF driver(s) and an expansion chamber, is one of the key parts for NNBI. The distribution of cusp magnetic field which is generated by permanent magnets installed at the lateral wall of the expansion chamber is a common issue, due to the great influence on electron density, electron temperature and spatial uniformity of plasma density in the extraction area. Two kinds of configuration of cusp magnetic field (checkerboard type and symmetric type) and three types of arrangement of permanent magnets (with three, four and five rows of magnets) have been carried out with the approach of finite element and the experimental performance. On the RF negative ion source testbed, a moveable Langmuir probe has been adopted to investigate the characteristic of electron density and temperature near the extraction area. Since the symmetric type with three rows of permanent magnets generates the distribution of cusp magnetic field that contribute to extract the plasma from RF driver and improve the probability of particle collision easily, the higher electron density and the lower electron temperature was obtained via simulation. The experimental conclusion is that the symmetric type with three rows of permanent magnets leads to a higher electron density and lower electron temperature, showing a great agreement with simulation results. The results are helpful to optimize the structure and performance of high power ion source, and provide support for the development of large area RF negative ion sources with multi-driver.

Surface production of negative deuterium ions from plasma-exposed boron doped diamond and graphite: work function measurements using photoemission yield spectroscopy

Negative-ion sources are of considerable interest for applications such as materials processing and neutral beam injection for magnetic confinement fusion. The efficient production of negative ions in these sources often relies on surface production. Work function measurements are critical to enable a detailed understanding of the mechanisms that underpin this. In this study we used a combination of photoemission yield spectroscopy and the Fowler method to determine the work functions of boron doped diamond (BDD) and highly oriented pyrolytic graphite (HOPG) directly after exposure to a low-pressure inductively coupled deuterium plasma (150 W, 2 Pa). A magnetised retarding field energy analyser is used to measure the negative ion current from the samples. During plasma exposure, samples are biased at −130 V or −60 V and their temperature is varied between 50 ∘C and 750 ∘C. The results show that the increasing work function of the plasma exposed HOPG occurs over the same sample temperature range as the decreasing negative-ion current. In contrast, the work function of BDD does not show a clear relationship with negative-ion current, suggesting that different mechanisms influence the negative-ion production of metal-like HOPG and dielectric-like BDD. The necessity for an additional fitting parameter for the Fowler fits to BDD suggests that its electronic properties are changing under plasma exposure, unlike HOPG. For both materials, the maximum photocurrent measured from the samples displays a strong similarity with negative-ion current, suggesting they are driven by a common mechanism. The in-situ measurement of the work function using non-invasive techniques is of interest for the development of negative ion sources.

The effect of O impurity particle adsorption on the Cs/Mo (110) surface

In this work, the O impurity particle adsorption on the Cs/Mo(110) surface, to determine the influence of O on Cs stability on Mo(110) surface and also the work function of Cs/O/Mo(110) surface, has been investigated by performing density functional theory (DFT). There are four independent adsorption sites on Cs/Mo(110) surface for O atom. No matter which adsorption site O was adsorbed, the surface adsorption energy of Cs increased. O atoms attract more charges due to the difference in electronegativity. Although this weakens the interaction between the Cs and Mo atoms, the O atom does connect them like a bridge, resulting in enhanced adsorption of Cs atom. In our model, the surface work function of Cs/Mo(110) is calculated as 1.835 eV. In some cases, the adsorption of O can dramatically reduce the surface work function to 1.633 eV. Conversely, O may also increase it, but this increase is not pronounced. This result suggests that the appropriate addition of O atoms may be beneficial to the development of high-efficiency negative ion sources.

The Neural Correlates of Life Satisfaction: A Systematic Review of Magnetic Resonance Imaging

In this work, the O impurity particle adsorption on the Cs/Mo(110) surface, to determine the influence of O on Cs stability on Mo(110) surface and also the work function of Cs/O/Mo(110) surface, has been investigated by performing density functional theory (DFT). There are four independent adsorption sites on Cs/Mo(110) surface for O atom. No matter which adsorption site O was adsorbed, the surface adsorption energy of Cs increased. O atoms attract more charges due to the difference in electronegativity. Although this weakens the interaction between the Cs and Mo atoms, the O atom does connect them like a bridge, resulting in enhanced adsorption of Cs atom. In our model, the surface work function of Cs/Mo(110) is calculated as 1.835 eV. In some cases, the adsorption of O can dramatically reduce the surface work function to 1.633 eV. Conversely, O may also increase it, but this increase is not pronounced. This result suggests that the appropriate addition of O atoms may be beneficial to the development of high-efficiency negative ion sources.

Development of a Utility Negative Ion Test Equipment With RF Source at ASIPP

An RF-driven negative-ion source development program has been started at ASIPP-Hefei with a small test equipment, Hefei utility negative ion test equipment with RF source (HUNTER). It is devoted to some crucial issues of the RF-driven negative-ion source, such as the improvement of RF efficiency, the enhancement of the extracted negative-ion current density, and the promotion of the beam qualities. In the initial stage, the HUNTER negative-ion source conservatively adopted a similar structure of the ITER prototype source developed at IPP, Garching. It consists of single or double RF driver on the back side of a plasma expansion chamber, and a negative ion accelerator which is able to extract 120 beamlets up to 60 kV for 10 s. In the first phase operation of the HUNTER-ion source, a low-pressure (0.3 Pa) plasma has been successfully achieved by coupling 46-kW RF input power and different plasma parameters have been measured by using Langmuir probes and optical emission spectroscopy. Besides the negative-ion source, the HUNTER comprises the vacuum vessel, the special cooling system, the power supply system, and the source plasma and beam diagnostic system are also ready for the next phase operation of hydrogen negative ion beam extraction.

Indigenous Manufacturing Realization of TWIN Source and Its Auxiliary

Indian negative-ion source development program has gained momentum with planned integration of Indian Test Facility (INTF) for International Thermonuclear Experimental Reactor (ITER)—Diagnostic Neutral Beam (DNB) characterization at the Institute for Plasma Research (IPR). Eight RF driver-based negative-ion source, being developed for DNB will be tested and operated in INTF. The Two driver-based Indigenously built Negative ion source (TWIN) source provides a bridge between the operational single driver-based negative-ion source test facility, ROBIN in IPR, and an ITER-type multidriver-based ion source. The source is designed to be operated in continuous mode with 180 kW, 1 MHz, 5-s on/600-s off duty cycle and also in 5-Hz modulation mode with 3-s on/20-s off duty cycle for three such cycles. The complete design of TWIN source and its test facility, from conceptual to detailed engineering, has been carried out in IPR. The manufacturing design has been optimized to match the capability of Indian manufacturers, without compromising on the specifications. Some examples of optimization are: 1) an improvised design of the Faraday shields where electrodeposition has been replaced by vacuum brazing; 2) a simplified design of the side walls of the plasma source, where jointing process is simplified, without the application of Electron Beam Welding; and 3) introduction of a fiber reinforced polymer-based integrated electrical and vacuum isolation scheme that replaces the application of a large ceramic. Finite-element analysis (FEA) based on heat load and structural load calculation ensure the functionality and structural integrity of each component of the source. Due to nonnuclear environment in TWIN source experimental area, vacuum brazing is an acceptable manufacturing process. The contract for manufacturing of the ion source has been awarded to an Indian manufacturing company for the first indigenous production of a large-size fusion grade ion source. TWIN source is designed in such a way that it can be operated both in air mode (RF driver coil antennas are exposed to air) as well as vacuum mode. (Whole source is immersed inside vacuum.) The TWIN source shall be manufactured as per ASME guidelines for pressure vessel. Experiments on the TWIN source are foreseen in the last quarter of 2015, as all the auxiliary systems such as 180-kW, RF generator system, vacuum vessel with pumping station, cooling water system, data acquisition and control system, and other power supply systems are already installed in the lab premises. This paper discusses the FEA-based engineering design, simplified manufacturing design, manufacturing experience with highlighting quality control, and the system integration activities undertaken for the TWIN source test facility.

Negative ion source development for a photoneutralization based neutral beam system for future fusion reactors

In parallel to the developments dedicated to the ITER neutral beam (NB) system, CEA-IRFM with laboratories in France and Switzerland are studying the feasibility of a new generation of NB system able to provide heating and current drive for the future DEMOnstration fusion reactor. For the steady-state scenario, the NB system will have to provide a high NB power level with a high wall-plug efficiency (η ∼ 60%). Neutralization of the energetic negative ions by photodetachment (so called photoneutralization), if feasible, appears to be the ideal solution to meet these performances, in the sense that it could offer a high beam neutralization rate (>80%) and a wall-plug efficiency higher than 60%. The main challenge of this new injector concept is the achievement of a very high power photon flux which could be provided by 3 MW Fabry–Perot optical cavities implanted along the 1 MeV D− beam in the neutralizer stage. The beamline topology is tall and narrow to provide laminar ion beam sheets, which will be entirely illuminated by the intra-cavity photon beams propagating along the vertical axis. The paper describes the present R&D (experiments and modelling) addressing the development of a new ion source concept (Cybele source) which is based on a magnetized plasma column. Parametric studies of the source are performed using Langmuir probes in order to characterize and compare the plasma parameters in the source column with different plasma generators, such as filamented cathodes, radio-frequency driver and a helicon antenna specifically developed at SPC-EPFL satisfying the requirements for the Cybele (axial magnetic field of 10 mT, source operating pressure: 0.3 Pa in hydrogen or deuterium). The paper compares the performances of the three plasma generators. It is shown that the helicon plasma generator is a very promising candidate to provide an intense and uniform negative ion beam sheet.

Comparison of ONIX simulation results with experimental data from the BATMAN testbed for the study of negative ion extraction

The development of negative ion (NI) sources for the ITER neutral beam injector is strongly accompanied by modelling activities. The ONIX (Orsay Negative Ion eXtraction) code simulates the formation and extraction of negative hydrogen ions and co-extracted electrons produced in caesiated sources. In this paper the 3D geometry of the BATMAN extraction system, and the source characteristics such as the extraction and bias potential, and the 3D magnetic field were integrated in the model. Calculations were performed using plasma parameters experimentally obtained on BATMAN. The comparison of the ONIX calculated extracted NI density with the experimental results suggests that predictive calculations of the extraction of NIs are possible. The results show that for an ideal status of the Cs conditioning the extracted hydrogen NI current density could reach ~30 mA cm−2 at 10 kV and ~20 mA cm−2 at 5 kV extraction potential, with an electron/NI current density ratio of about 1, as measured in the experiments under the same plasma and source conditions. The dependency of the extracted NI current on the NI density in the bulk plasma region from both the modeling and the experiment was investigated. The separate distributions composing the NI beam originating from the plasma bulk region and the PG surface are presented for different NI plasma volume densities and NI emission rates from the plasma grid (PG) wall, respectively. The extracted current from the NIs produced at the Cs covered PG surface, initially moving towards the bulk plasma and then being bent towards the extraction surfaces, is lower compared to the extracted NI current from directly extracted surface produced ions.

Beam formation in CERNs cesiated surfaces and volume H− ion sources

At CERN, a high performance negative ion (NI) source is required for the 160 MeV H− linear accelerator named Linac4. The source should deliver 80 mA H− ion beams within an emittance of 0.25 mm·mrad. For this purpose two ion sources were developed: IS01 is based on the NI volume production and IS02 provides additional NI by surface production via H interaction on a cesiated Molybdenum plasma electrode. The development of negative ion sources for Linac4 is accompanied by modelling activities. ONIX code has been modified and adapted to investigate the transport of NI and electrons in the extraction region of the CERN negative ion sources. The simulated results from modeling of IS01 and IS02 extraction regions, which were obtained in 2012 during source commissioning, are presented and benchmarked with experimental measurements obtained after 2013. The formation of the plasma meniscus and the screening of the extraction field by the source plasma are discussed. The NI production is compared between two types of sources, the first one based on volume production only and the second one encompassing NI cesiated surface production. For the IS02 source, different states of conditioning were simulated by changing the NI emission flux from the plasma electrode and Cs+ density in the bulk plasma region. The numerical results show that in low work function regime, with high NI surface emission rate of 3000 A m−2 and Cs-density of nCs+ = 3.8 × 1016 m−3, the total extracted NI current could reach ~80 mA. At the less favorable Cs-coverage, when the surface NI emission rate becomes significantly lower, namely 300 A m−2 with nCs+ = 3.3 × 1015 m−3, the total extracted NI current only reaches ~20 mA. A good agreement between simulation and experimental results is observed in terms of extracted NI current for both extraction systems, including the case of reversed extraction potential that corresponds to positive (H+) ion extraction.

Towards a realistic 3D simulation of the extraction region in ITER NBI relevant ion source

The development of negative ion (NI) sources for ITER is strongly accompanied by modelling activities. The ONIX code addresses the physics of formation and extraction of negative hydrogen ions at caesiated sources as well as the amount of co-extracted electrons. In order to be closer to the experimental conditions the code has been improved. It includes now the bias potential applied to first grid (plasma grid) of the extraction system, and the presence of Cs+ ions in the plasma. The simulation results show that such aspects play an important role for the formation of an ion–ion plasma in the boundary region by reducing the depth of the negative potential well in vicinity to the plasma grid that limits the extraction of the NIs produced at the Cs covered plasma grid surface. The influence of the initial temperature of the surface produced NI and its emission rate on the NI density in the bulk plasma that in turn affects the beam formation region was analysed. The formation of the plasma meniscus, the boundary between the plasma and the beam, was investigated for the extraction potentials of 5 and 10 kV. At the smaller extraction potential the meniscus moves closer to the plasma grid but as in the case of 10 kV the deepest meniscus bend point is still outside of the aperture. Finally, a plasma containing the same amount of NI and electrons , representing good source conditioning, was simulated. It is shown that at such conditions the extracted NI current can reach values of ∼32 mA cm−2 using ITER-relevant extraction potential of 10 kV and ∼19 mA cm−2 at 5 kV. These results are in good agreement with experimental measurements performed at the small scale ITER prototype source at the test facility BATMAN.

Improvement of charged particles transport across a transverse magnetic filter field by electrostatic trapping of magnetized electrons

A study on the transport of charged particles across a magnetic filter field has been carried out in a double plasma device (DPD) and presented in this manuscript. The DPD is virtually divided into two parts viz. source and target regions by a transverse magnetic field (TMF) which is constructed by inserting strontium ferrite magnets into two stainless steel rectangular tubes. Plasma electrons are magnetized but ions are unmagnetized inside the TMF region. Negative voltages are applied to the TMF tubes in order to reduce the loss of electrons towards them. Plasma is produced in the source region by filament discharge method and allowed to flow towards the target region through this negatively biased TMF. It is observed that in the target region, plasma density can be increased and electron temperature decreased with the help of negatively biased TMF. This observation is beneficial for negative ion source development. Plasma diffusion across the negatively biased TMF follows Bohm or anomalous diffusion process when negative bias voltage is very less. At higher negative bias, diffusion coefficient starts deviating from the Bohm diffusion value, associated with enhanced plasma flow in the target region.

Two RF Driver-Based Negative Ion Source Experiment

Multidriver-based radio-frequency (RF) ion sources have emerged as the most promising option for producing the plasma in large ion sources required for fusion applications. In India, under domestic long-term fusion program, a negative ion source development program has been initiated. A 100-kW inductively coupled single RF driver-based negative ion source (ROBIN) has been installed and it is operational at the Institute for Plasma Research. The next step is to move to a multidriver source and address to several issues related to configuration and operation. A two-driver-based twin source has been configured for this purpose. The experimental plan is divided into two phases: 1) plasma operation phase and beam extraction and 2) acceleration phase. A vacuum chamber has been specially designed to accommodate these phases of operations. The data acquisition and control system (DACS) of the experimental system is designed based on International Thermonuclear Experimental Reactor-COntrol Data Access and Communication (ITER-CODAC) guidelines. Probe and spectroscopy diagnostics constitute the principal diagnostics. This paper presents the experimental system with emphasis on the aspects of mechanical, electrical, and DACS integration.

Kinetic modeling of particle dynamics in H− negative ion sources (invited)

Progress in the kinetic modeling of particle dynamics in H(-) negative ion source plasmas and their comparisons with experiments are reviewed, and discussed with some new results. Main focus is placed on the following two topics, which are important for the research and development of large negative ion sources and high power H(-) ion beams: (i) Effects of non-equilibrium features of EEDF (electron energy distribution function) on H(-) production, and (ii) extraction physics of H(-) ions and beam optics.

Influence of plasma loss area on transport of charged particles through a transverse magnetic field

Plasma transport in a double plasma device from the source region to the target region through a physical window comprising of electrically grounded magnet channels (filled with permanent magnet bars) for transverse magnetic field (TMF) and a pair of stainless steel (SS) plates is studied and presented in this manuscript. The study has relevance in negative ion source research and development where both TMF created by magnet channels and bias plate are used. The experiment is performed in two stages. In the first stage, a TMF is introduced between the two regions along with the SS plates, and corresponding plasma parameter data in the two regions are recorded by changing the distance between the TMF channels. In the second stage, the TMF is withdrawn from the system, and corresponding data are taken by changing the separation between the SS plates. The experimental results are then compared with a theoretical model. In the presence of TMF, where electrons are magnetized and ions are un-magnetized, it is observed that plasma transport perpendicular to the TMF is dominated by the ambipolar diffusion of ions. In the absence of TMF, plasma is un-magnetized, and plasma transport through the SS window aperture is almost independent of open area of the SS window.

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.

Negative ion source development for fusion application (invited)

Giant negative ion sources, producing high-current of several tens amps with high energy of several hundreds keV to 1 MeV, are required for a neutral beam injector (NBI) in a fusion device. The giant negative ion sources are cesium-seeded plasma sources, in which the negative ions are produced on the cesium-covered surface. Their characteristic features are discussed with the views of large-volume plasma production, large-area beam acceleration, and high-voltage dc holding. The international thermonuclear experimental reactor NBI employs a 1 MeV-40 A of deuterium negative ion source, and intensive development programs for the rf-driven source plasma production and the multistage electrostatic acceleration are in progress, including the long pulse operation for 3600 s. Present status of the development, as well as the achievements of the giant negative ion sources in the working injectors, is also summarized.

Spectroscopy—a powerful diagnostic tool in source development

The development of negative hydrogen ion sources for neutral beam systems is closely linked with an optimization of negative ion formation in hydrogen plasmas, which requires knowledge of the plasma parameters. Emission spectroscopy is introduced as a non-invasive and in situ diagnostic tool for line of sight averaged plasma parameters. Diagnostic lines and simplified analysis methods for a variety of plasma parameters, such as electron density and electron temperature, gas temperature, atomic and molecular hydrogen density, caesium densities (atoms and ions), and negative ion densities are identified and prepared for direct application. Emphasis is laid on results obtained in RF generated negative ion sources. Correlations of plasma parameters with extracted negative ion current densities are discussed. Stripping losses in the extraction system are quantified by using beam emission spectroscopy.

Two approaches for H− ion production with 2.45 GHz ion sources

Over the last few years, the accelerator community requested the development of improved negative hydrogen ion sources. For spallation sources, like SNS or ESS, pulsed high intensity H− ion beams of a few tens of milliamperes, with a duty cycle close to 10%, are required. New facilities like CERN also ask for high performance negative ion beams. Since CEA undertook an electron cyclotron resonance (ECR)-based ion source programme, a European network devoted to high performance negative ion source development has been created.In this group, several laboratories developing 2.45 GHz ECR sources follow different approaches to increase the extracted ion current. At Saclay, with a solenoidal magnetic structure based on coils, close to 3 mA H− ion beam is now extracted in pulsed mode (2 ms/100 ms). A metallic biased grid separates the plasma bulk from the H− ion production zone and significantly improves the H− extracted current. At Ecole Polytechnique, the source Camembert III operates in continuous wave and pulsed modes. Photodetachment measurements in continuous wave mode show considerable H− ion density (109–1010 cm−3), whether the primary electrons are provided by filaments or small ECR modules inserted into the plasma chamber. Similarities and differences between the ECR-driven and the filamented source are discussed. Representative experimental results from the sources of the two laboratories as well as future plans are reported here.

Positive and negative ion sources for magnetic fusion

The positive or negative ion sources which form the primary components of neutral beam injection systems used in controlled nuclear fusion using magnetic confinement have to meet simultaneously several demanding requirements. This paper describes the underlying physics of modern positive ion sources, which provide the required high proton fraction (>90%) and high current density (/spl ap/2 kA/m/sup 2/) at a low source pressure (0.4 Pa) with a high electrical efficiency and uniformity across the accelerator grids. The development of negative ion sources, which are required if high energy neutral beams are to be produced, is described, and the present understanding of the physics of negative ion production in sources is explained. The paper reports that negative ion sources have achieved many of the parameters required of sources for the neutral beam injectors of future fusion devices and reactors, >200 A/m/sup 2/ of D/sup -/ at low source pressure, <0.3 Pa, with a low co-extracted electron content. The development needed to meet all the requirements of future systems is briefly discussed.

Ion sources for fusion plasma diagnostics

Various probing beams have been developed and used for plasma diagnostics in thermonuclear fusion research for magnetic confinement. The required beam current, beam size, energy, beam characteristics such as divergence, energy spread, and emittance, etc., as well as ion species are different depending on the purpose of diagnostics, target plasma size and parameters. Hydrogen beams are used for charge-exchange recombination spectroscopy (CHERS) and motional stark effect (MSE) spectroscopy. Suitable beam energy for CHERS ranges in the area of 50 keV/amu, and a small beam divergence is required for MSE measurements. Heating beams are often used as diagnostic beams by sharing. Neutral lithium probing beams have been used as effective tools for measurement of density and fluctuations. A heavy ion beam probe (HIBP), made from alkali metal ions, is injected directly without neutralization to measure plasma potential and density, as well as their fluctuations. Recent progress in magnetic fusion experiments has resulted in advancement of the development of negative ion sources, such as He/sup -/ and Li/sup -/ sources for alpha-particle measurement, because of their advantages with regard to efficient neutralization in the high-energy region, and an Au/sup -/ source for HIBP with the application of tandem acceleration.