Negative Ion Beam Research Articles

Breakdown Oscillations of the ISIS H− Penning Ion-Source Hydrogen–Cesium Discharge

The ISIS Penning ion source is a pulsed dc discharge surface plasma source delivering a 55-mA beam of negative hydrogen ions (H−) in 250- ${ mu }\text{s}$ pulses to the ISIS accelerator for neutron and muon physics at 50-Hz pulse repetition rate. The H− ions are extracted from the hydrogen–cesium discharge sustained by a current-regulated pulsed power supply delivering a current of 50–60 A. The discharge exhibits an ignition transient that requires the pulselength to be prolonged to 700 ${ mu }\text{s}$ to extract a clean H− beam pulse. The current and voltage fluctuations during the breakdown oscillation enhance the sputtering damage of the ion-source cathodes, thus contributing to the lifetime limitation of the device. It has been hypothesized that the breakdown oscillations are due to cesium dynamics causing the surface work function and electron emission from the cathode surfaces to fluctuate periodically. Experiments with the ISIS H− Penning ion source, reported here, reveal that the breakdown oscillations are primarily driven by the discharge power supply and its auxiliary circuits, whereas cesium dynamics have only a minor effect on the ignition transient. The conclusion is derived from careful characterization of the discharge current–voltage characteristics, optical emission measurements combined with a 0-D model, and comparison of different discharge geometries. Engineering solutions to mitigate the breakdown oscillations are presented and the potential effect of the applied modifications on the molybdenum cathode erosion by cesium sputtering is estimated.

Formation of large negative deuterium ion beams at ELISE.

Negative ion sources for neutral beam injection (NBI) in fusion experiments are based on the surface production of H- or D- on cesiated low work function surfaces. In the recent years, it was demonstrated at the large RF driven ion source of the ELISE (Extraction from a Large Ion Source Experiment) test facility that the requirements for the ITER NBI systems can be fulfilled by hydrogen. This is a big step toward the first operational period of ITER, planned for up to 2035. However, for the following operational period, neutral beam systems working in deuterium are needed. Operation of negative hydrogen ion sources in deuterium is significantly more demanding than in hydrogen: the amount of coextracted electrons is much higher and their increase during pulses is much more pronounced, limiting the achievable performance. This paper presents the results of investigations aimed to improve the insight into the physics related to this isotope effect. Due to the higher atomic mass of deuterium, cesium is removed much more effectively from reservoirs at the walls, resulting in a depletion of these reservoirs and a strongly increased cesium density in the plasma. Additionally, a correlation between the fluxes of charged particles toward the inner ion source surfaces and the coextracted electrons is identified.

Physics and engineering design of 400 keV H- accelerator for negative ion based neutral beam injection system in China.

A research project of the China Fusion Engineering Test Reactor (CFETR) Negative ion-based Neutral Beam Injection (NNBI) prototype has been started in China. The objectives of the CFETR NNBI prototype are to produce a negative hydrogen ion beam of >20 A up to 400 keV for 3600 s and to attain a neutralization efficiency of >50%. In order to identify and optimize the design of the negative ion accelerator, a self-consistent model has been developed to consider all key physics and engineering issues (electric and magnetic fields, background gas flow, beam optics, beam-gas interaction, secondary particle trajectories, power deposition on grids, heat removal design, and mounting pattern). This paper presents the primary results by applying the self-consistent model to the current design of the 400 keV H- accelerator of the CFETR NNBI prototype.

Study of continuous wave 33 keV H- beam transport through the low energy beam transport section.

The transport of a continuous wave 33 keV negative ion beam through the low energy beam transport section, designed for beam injection into the tandem accelerator, was studied. The continuous wave H- beam, produced by the Penning surface-plasma source with hollow cathodes and cesium addition, was separated and focused by the 90° bending magnet and then transported through the 0.8 m long transport tube, equipped with beam electrical and optical diagnostics. The beam current was measured by a water-cooled Faraday cup at the transport tube exit. Beam sizes and profiles were measured by CCD cameras looking into the beam from the back and sides. The additions of hydrogen, argon, and xenon to the transport tube and to the bending magnet chamber in the pressure range of 10-6 to 10-4 Torr were tested. The influence of gas addition on the beam space charge compensation and beam transport was studied. A mechanism for the H- beam focusing due to space charge overcompensation and beam compression by the positive charge, produced with positive ion accumulation in the beam, is discussed.

Extending the pulse length of the ISIS H− Penning ion source

The ISIS Penning ion source can routinely produce a 55 mA beam of negative hydrogen ions in 250 µs pulses at 50 Hz repetition rate. Extending the beam pulse length to 2 ms requires eliminating the 15%–30% droop of the current observed in long pulse operation and benefits from the suppression of discharge breakdown oscillations otherwise forcing to prolong the pulse length to 2.2 ms or longer. The droop can be compensated by ramping the discharge current during the pulse, whereas the discharge oscillations are suppressed by modifying the ancillary circuit of the pulsed arc power supply.

The particle tracking code BBCNI for large negative ion beams and their diagnostics

Heating and current drive in the next generation tokamak ITER requires the use of large and powerful neutral beams, generated by a precursor ion beam from an ion source around 1 m × 2 m in cross-section. To avoid energy losses and component damage, strict requirements are placed on the divergence and uniformity of this ion beam, which is comprised of many individual beamlets. Understanding, controlling, and predicting the behaviour of these large ion beams requires knowledge of these individual beamlets and their interactions with one another. This is hindered by available experimental diagnostics on these large beams typically only having access to volume averaged information. A forward simulation of beam diagnostics would allow the connection of experimental results with otherwise unobtainable individual beamlet properties. The particle tracking and ray tracing code Bavarian Beam Code for Negative Ions was developed for this reason, and takes into account the interaction of individual component beamlets with whole-beam diagnostics to produce synthetic data that can be compared with experimental results. In this work a significantly reworked and upgraded version of the code is presented and example results are given and analysed for the ITER relevant test facility BATMAN Upgrade. It is shown how the simulation can recreate experimental results, and that one must consider the whole beam in order to do so. The impact of beamlet mixing on beam emission spectroscopy results is shown, as is the importance of long range magnetic fields on the beam transport. The capabilities and limitations of the code are discussed with a view toward application to ITER size ion sources.

Physical design of the neutralizer for CFETR negative ion based neutral beam injection prototype

A research project of the CFETR Negative ion based Neutral Beam Injection (NNBI) prototype has been started in China. The objectives of the CFETR NNBI protype are to produce a negative hydrogen ion beam of >20 A to 200–400 kV for 3600 s and to attain a neutralization efficiency of >50%. Thus, a neutral beam power of >2 MW is foreseen onto the calorimeter (i.e., beam dump). A gas neutralizer is applied inside the beamline of CFETR NNBI prototype, to achieve the neutralization of negative ion beam through the charge-exchange collisions with the gas target. The most critical issues for the gas neutralizer are to balance the contradictive requirements of high neutralization efficiency, low gas inflow, and low heat load. During the physical design of the neutralizer for CFETR NNBI prototype, two proposals have been considered and studied in detail. One has single beam channel and other one has two narrow channels separated by a panel. The narrow channel design is benefic to decrease gas conductance, and thus reduce the required gas inflow to attain the maximal neutralization efficiency. However, the inserted panel will restrict the extraction area of the beam source, where the aperture columns corresponding to the panel should be masked to avoid the direct beam interception. But the leading edge of the inserted panel will still suffer a high thermal load due to the stray particles from the beam source. A 3D model of the whole beamline of CFETR NNBI prototype has been developed to study the gas flow and the beam transport in the beamline. Based on this model, these two neutralizer designs have been evaluated in detail, in terms of the relation between neutralization efficiency and gas inflow, and the power load on the neutralizer components.

Magnetic field configurational study on a helicon-based plasma source for future neutral beam systems

A helicon-based plasma source is under development and exploration at CEA-IRFM to produce an efficient and dense magnetized plasma column. The final objective of this development is the extraction and acceleration of a blade-like negative ion beam in view of future neutral beam injector systems for fusion reactors. The extraction of a negative ion beam from a magnetized plasma column requires a specific topology of the magnetic confinement, which significantly impacts the plasma parameters and wave propagation along the column. The magnetic confinement under investigation is based on water-cooled internal coils implemented under vacuum along the source (column) axis; these coils are supplied with a high DC current (∼1000 A), providing the axial magnetic field (B// ∼ 10 mT). Different diagnostics developed in academic laboratories, such as 3D B-dot probes, optical emission spectroscopy and Langmuir probes, have been used for plasma characterization. The paper reports the experimental results obtained under different operating conditions of this particular magnetic confinement.

Offline 2, ISOLDE’s target, laser and beams development facility

Offline 2 is an entirely new research and development mass separator laboratory for the ISOLDE facility located on the Meyrin site of CERN. It closely resembles beam production (for non-radioactive beams) of the on-line (radioactive) systems at ISOLDE with machine versatility and flexibility at the core of its design. The beam optics and down stream beam preparation can deliver bunched beams or continuous beams of negative or positive ions with kinetic energies up to 60 keV. The mass resolving power of the separator operating with a typical ISOLDE ion source is R∼500. All of this is housed within a highly accessible 125 m2 work area with multiple beam optics instruments to determine the beam quality, quantify modifications, develop new technologies and create alternate beam tunes, with the purpose of improving the beams and targets used during on-line operations.

Negative Ion Beam Production in an Ion Source with Chamfered Extraction Opening

Negative Ion Beam Production in an Ion Source with Chamfered Extraction Opening

Achievement of ITER-relevant accelerated negative hydrogen ion current densities over 1000 s at the ELISE test facility

The neutral beam heating system for the future international fusion experiment ITER will be based on radiofrequency driven ion sources delivering a large (≈1 × 2 m) and homogeneous negative hydrogen or deuterium ion beam of severals tens of amperes for up to 1 h. Such beams have never been produced before and a dedicated R&D process has been ongoing for more than two decades. An important intermediate step is the size scaling test facility ELISE (Extraction from a Large Ion Source Experiment) with its half-ITER size ion source. Recently, ELISE has fulfilled its first main aim, demonstrating hydrogen ITER-relevant accelerated negative ion current densities over 1000 s, at the required filling pressure of 0.3 Pa, with an electron–ion ratio below one and a beam homogeneity better than 90%. The measures identified as essential for achieving such pulses are the introduction of external permanent magnets and internal potential rods as well as a dedicated caesium conditioning technique.

Progress of the CFETR design

The Chinese Fusion Engineering Testing Reactor (CFETR), complementing the ITER facility, is aiming to demonstrate fusion energy production up to 200 MW initially and to eventually reach DEMO relevant power level 1 GW, to manifest a high duty factor of 0.3–0.5, and to pursue tritium self-sufficiency with tritium breeding ratio (TBR) >1. The key challenge to meet the missions of the CFETR is to run the machine in steady state (or long pulse) and high duty factor. By using a multi-dimensional code suite with physics-based models, self-consistent steady-state and hybrid mode scenarios for CFETR have been developed under a high magnetic field up to 6.5 T. The negative-ion neutral beam injection together with high frequency electron cyclotron wave and lower hybrid wave (and/or fast wave) are proposed to be used to drive the current. Subsequently the engineering design of CFETR including the magnet system, vacuum system, tritium breeding blanket, divertor, remote handling and maintenance system will be introduced. Some research and development (R&D) activities are also introduced in this paper.

36Cl in a new light: AMS measurements assisted by ion-laser interaction

The experimental setup at the Vienna Environmental Research Accelerator (VERA) was extended with an ion beam cooler and a 18 W laser to perform Ion-Laser InterAction Mass Spectrometry (ILIAMS). Gas collisions of the decelerated negative ion beam with He buffer gas inside a radio-frequency quadrupole slow down the ions, which facilitates element selective photodetachment to remove isobaric interferences. Here we present the first successful implementation of a suppression of the isobar of 36Cl via laser-photodetachment into an AMS measurement. Whereas conventional AMS measurements require separation of the isobar 36S at high beam energy, ILIAMS takes advantage of the higher electron affinities of Cl− (3.6 eV) compared to S− (2.1 eV). Using 532 nm photons (Eph = 2.3 eV) the isobar 36S is suppressed by 10 orders of magnitude. A high transmission of the Cl− beam through the ion cooler (up to 80%) and a quantitative suppression of the isobaric interference allows for competitive measurements at low accelerator voltages and with lower charge states. This results in high yields for 36Cl transport to the detector. The ILIAMS system is stable over several days of beamtime and blank levels below 36Cl/Cl = 10−15 are reached. The accomplishment of a 36Cl AMS measurement at a final beam energy as low as 5.4 MeV with full separation of isobars and m/q interferences shows the potential of the new isobar suppression method to widen the capabilities of smaller and middle-sized AMS facilities.

Characteristics of cesium-free negative hydrogen/deuterium ion source by sheet plasma

The use of negative ion sources without Cs seeding is desired in neutral beam injectors of fusion experiments. We have designed a magnetized-sheet plasma that produces negative ions by volume production without Cs seeding. The developed system was experimentally tested on steady-state hydrogen and deuterium plasmas. The negative hydrogen ion beam was extracted by a two-grid extraction system located at the periphery of the sheet plasma. The negative hydrogen/deuterium ions were successfully extracted from the sheet plasma. The current densities of the ion beams increased with increasing discharge current. At an extraction voltage of 7 kV and a discharge current of 80 A, the approximate current densities of the negative hydrogen and deuterium ion beams were 3.8 and 1.2 mA/cm2, respectively.

Negative ion beam source as a complex system: identification of main processes and key interdependence

Complex network theory offers new tools to tackle controllability of systems such as processes affecting generation, extraction and acceleration of negative ions in ion beam sources. A model has therefore been developed and tested for the negative ion beam source NIO1. Preliminary results show good agreement between model predictions and experimental behaviour of the source. A subset of driver processes which in principle allow the system to be controlled has been identified. One of the driver processes related to the meniscus formation has been found to play a key role in the global controllability of the system, so that a detailed discussion of the physics behind its formation and uniformity has been provided.

SPIDER in the roadmap of the ITER neutral beams

To reach fusion conditions and control plasma configuration in ITER, a suitable combination of additional heating and current drive systems is necessary. Among them, two Neutral Beam Injectors (NBI) will provide 33 MW hydrogen/deuterium particles electrostatically accelerated to 1 MeV; efficient gas-cell neutralisation at such beam energy requires negative ions, obtained by caesium-catalysed surface conversion of atoms inside the ion source. As ITER NBI requirements have never been simultaneously attained, a Neutral Beam Test Facility (NBTF) was set up at Consorzio RFX (Italy), including two experiments. MITICA is the full-scale NBI prototype with 1 MeV particle energy. SPIDER, with 100 keV particle energy, aims at testing and optimising the full-scale ion source: extracted beam uniformity, negative ion current density (for one hour) and beam optics (beam divergence <7 mrad; beam aiming direction within 2 mrad). This paper outlines the worldwide effort towards the ITER NBI realisation: the main results of the ELISE facility (IPP-Garching, Germany), equipped with a half-size source, are described along with the status of MITICA; specific issues are investigated by small specific facilities and by joint experiments at QST and NIFS (Japan). The SPIDER experiment, just come into operation, will profit from strong modelling activities, to simulate and interpret experimental scenarios, and from advanced diagnostic instruments, providing thorough plasma and beam characterisation. Finally, the results of the first experiments in SPIDER are presented, aimed at a preliminary source plasma characterisation by plasma light detectors and plasma spectroscopy.

Thermo-mechanical analysis of unidirectional carbon-carbon composite for thermal imaging diagnostic of a particle beam

Unidirectional carbon fiber-carbon matrix (CFC) composite tiles can be used to diagnose the main features of a particle beam such as its power, divergence and uniformity. The diagnostic calorimeter STRIKE will be used with such an aim for the negative hydrogen ion beam produced by the ITER ion source prototype SPIDER, which is in operation at Consorzio RFX (Padova, Italy) since 2018.In the present work, a transient non-linear parametric finite element model developed in ANSYS and already validated on the basis of data collected in other test facilities, is used to predict the operational limits of STRIKE, i.e. at which performance (in terms of beam power, divergence and pulse duration) SPIDER may operate when the beam is dumped onto STRIKE. Powerful long lasting pulses in fact, may induce cracks in the tiles, affecting the heat transfer and thus limiting their diagnostic capability. The effects of different configurations for the STRIKE setup on the thermo-mechanical stresses are also discussed.

Neural network based prediction of heat flux profiles on STRIKE

The instrumented calorimeter STRIKE (Short-Time Retractable Instrumented Kalorimeter Experiment) has been designed with the main purpose of characterizing the SPIDER (Source for Production of Ion of Deuterium Extracted from Radio Frequency plasma) negative ion beam in terms of beam uniformity and divergence during short pulse operations. STRIKE is made of 16 1D Carbon Fiber Composite (CFC) tiles, intercepting the whole beam and observed on the rear side by infrared (IR) cameras. The front observation presents some drawbacks due to optically emitting layer caused by the excited gas between the beam source and the calorimeter, and the material sublimated from the calorimeter surfaces due to the heating itself. This paper proposes a Neural Network-based approach to solve the inverse non-linear problem of determining the energy flux profile impinging on the calorimeter, considering the 2D temperature pattern measured on the rear side of the tiles. Most of the conventional methods used to evaluate the inverse heat flux are unbearably time consuming; since the objective is having a tool for heat flux evaluation for STRIKE real time operation, the need to have a ready-to-go instrument to understand the beam condition becomes stringent. For this reason, in this paper, a Multi-Layer Perceptron has been used to solve the problem. Once properly trained, the neural networks provide a fast evaluation of the impinging flux. Furthermore, there is no need to optimize any parameter since this operation is already included in the self-adjustment of the network weights during the training. The achieved results show the reliability of the proposed method both with stationary and non-stationary heat fluxes.

Поверхностно-плазменный метод получения пучков отрицательных ионов

Поверхностно-плазменный метод получения пучков отрицательных ионов

Characteristics of Ion–Ion Thruster Generating Continuously Positive and Negative Ion Beams

The neutralizers in ion thruster systems do not generate thrust force. Hence, the power consumption of a neutralizer limits the thrust efficiency of the ion thruster system. Therefore, an ion thruster system is proposed that uses a negative ion source that generates thrust force as well as neutralizes the positive ion beam. In this study, in order to verify the feasibility of this ion thruster system by ground experiments, a negative fullerene ion source was developed and three experiments were conducted. First, in order to demonstrate that the satellite can accelerate ions without becoming charged, the positive and negative ion beams were accelerated in the floating condition using a feedback circuit. Second, in order to verify whether the formation of the virtual anode could be suppressed, the space potential and the beam profile in the downstream region of the ion beam were measured. Third, in order to demonstrate that the thruster system generates thrust force, the thrust with a beam target was measured. From the aforementioned results, it was concluded that the experimental verification of the feasibility of the ion thruster with combined positive and negative ions without a neutralizer was successful.