Momentum flux measurements in a negative ion beam for fusion research

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

Negative ion beam applications in tandem accelerators are used for nuclear research, environmental studies, materials analysis, medical treatments, and ion implantation in semiconductor devices. Conventional methods for generating negative ions for tandem accelerators rely on metallic vapors (typically alkali) for charge exchange, which pose challenges like contamination, electrical shorting and breakdowns, and maintenance issues. To address these drawbacks, this work explores an alternative approach to produce negative ions using a non-metallic charge exchange process. It involves directing negative hydrogen ions into neutral gases within a specially designed charge exchange cell equipped with an electrostatic accelerator. The method is applied to various gas targets, including He, H2 and O2, to accelerate and measure resulting negative ions. This innovative approach aims to mitigate contamination concerns associated with metallic vapor double-charge exchange methods and explore novel avenues for negative ion production through charge transfer. Any newly formed negative ion beam current conversion ratios from the incident H− beam will be reported as progress in this research.

A source of epithermal neutrons based on a vacuum-insulated tandem accelerator and a lithium target was proposed and developed for the technique of boron neutron capture therapy. A stationary proton beam of 2 MeV with a current of up to 6 mA was produced by the accelerator. To optimize the injection of a beam of negative hydrogen ions into the accelerator, a wire scanner OWS-30 (D-Pace, Canada) was installed [1]. The main problem is that using the profilometer in the form in which it was provided does not allow us to determine the main beam parameters such as position and size. Also, the electron emission does not allow measurement of the total beam current and may lead to incorrect measurement of the beam profile. We have modernized the scanner by placing metal rings in front and behind the scanner with a negative potential to suppress the secondary emission of electrons from the scanner wires. We have developed software in which methods for calculating the position and size of the beam, methods for calculating the total current are implemented. Modernization of the scanner has made it possible to expand its capabilities. The suppression of the secondary electron emission made it possible to reconstruct the current profile of the ion beam and determine the value of the total current. The developed program allowed to display the coordinates of the beam, its dimensions and the total current. We are the first who proposed and implemented a new way of measurement of the beam emittance. A movable diaphragm was inserted in front of the wire scanner. Ion beams passing through the aperture of the diaphragm were measured by the wire scanner with а high level of detail when the diaphragm was moved along a radius. The use of a modernized scanner made it possible to detect the effect of space charge and the effect of the spherical aberrations of the focusing magnetic lenses on a beam of negative hydrogen ions. The use of the modernized scanner made it possible to optimize the injection of a beam of negative hydrogen ions into the accelerator, which led to an increase in the proton current and an improvement of the accelerator stability. The modernized scanner with an additional program for processing the results data and visualization has become a reliable device for beam diagnostics and for controlling beam entry into the accelerator.

Negative hydrogen ion beam can be compensated by the trapping of ions into the beam potential. When the beam propagates through a neutral gas, these ions arise due to gas ionization by the beam ions. However, the high neutral gas pressure may cause serious negative hydrogen ion beam loss, while low neutral gas pressure may lead to ion-ion instability and decompensation. To better understand the space charge compensation processes within a negative hydrogen beam, experimental study and numerical simulation were carried out at Peking University (PKU). The simulation code for negative hydrogen ion beam is improved from a 2D particle-in-cell-Monte Carlo collision code which has been successfully applied to H(+) beam compensated with Ar gas. Impacts among ions, electrons, and neutral gases in negative hydrogen beam compensation processes are carefully treated. The results of the beam simulations were compared with current and emittance measurements of an H(-) beam from a 2.45 GHz microwave driven H(-) ion source in PKU. Compensation gas was injected directly into the beam transport region to modify the space charge compensation degree. The experimental results were in good agreement with the simulation results.

A short review of the studies carried out at the Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences (BINP SB RAS) on the photon neutralization of the beams of negative ions is presented. The principal distinctive feature of the presented approach consists in the nonresonant accumulation of photons in a limited space. Their confinement is based on the adiabatic motion of photons in a system of concave mirrors, which is insensitive to the quality of the injected radiation. An analysis is carried out of the possibility of using the neutralizer based on such a nonresonant photon trap in large-scale installations such as ITER and TRT, and a future experiment is described on the photon neutralization using a beam of negative hydrogen ions with energy up to 130 keV and a current of about 10 mA.

A short review of the studies carried out at the Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences (BINP SB RAS) on the photon neutralization of the beams of negative ions is presented. The principal distinctive feature of the presented approach consists in the nonresonant accumulation of photons in a limited space. Their confinement is based on the adiabatic motion of photons in a system of concave mirrors, which is insensitive to the quality of the injected radiation. An analysis is carried out of the possibility of using the neutralizer based on such a nonresonant photon trap in large-scale installations such as ITER and TRT, and a future experiment is described on the photon neutralization using a beam of negative hydrogen ions with energy up to 130 keV and a current of about 10 mA.

Large and powerful negative hydrogen ion sources are required for the neutral beam injection (NBI) systems of future fusion devices. Simplicity and maintenance-free operation favors RF sources, which are developed intensively at the Max-Planck-Institut für Plasmaphysik (IPP) since many years. The negative hydrogen ions are generated by caesium-enhanced surface conversion of atoms and positive ions on the plasma grid surface. With a small scale prototype the required high ion current density and the low fraction of co-extracted electrons at low pressure as well as stable pulses up to 1 h could be demonstrated. The modular design allows extension to large source dimensions. This has led to the decision to choose RF sources for the NBI of the international fusion reactor, ITER. As an intermediate step towards the full size ITER source at IPP, the development will be continued with a half-size source on the new ELISE testbed. This will enable to gain experience for the first time with negative hydrogen ion beams from RF sources of these dimensions.

Negative ion beam extraction in volume mode on the RF negative ion source at ASIPP

Extraction of a negative hydrogen ion beam from a microwave ion source

Progress of neutral beam R&D for plasma heating and current drive at JAERI

The work is devoted to considering the possibility of improving the focusing properties of a space charge lens due to charge retention of positive ions by an electric field that varies both in time and along the axis of the system. Numerical calculations of motion trajectories of positive argon ion during interaction with an alternating electric field with oscillation frequencies in the range of 1...8 MHz and amplitude in the range of 1.5...6 kV were carried out. A significant increase in the residence time of positive argon ion in the overcompensated beam of negative hydrogen ions is shown, which indicates the possibility of significant influencing focusing charge in the lens for focusing beams of negative hydrogen ions.

A beam of negative hydrogen ions accelerated by a cyclotron has been passed through various gases and the resulting charge states determined by magnetic analysis. Reduction of the data yields values of the cross sections for removing one or both electrons from the negative ion and the cross section for removing the electron from the hydrogen atom. Measurements were made on hydrogen gas at 4.2, 7.4, 9.8, 14.6, and 17.9 MeV; and on helium, nitrogen, oxygen, and argon gases at 14.6 MeV.

ROBIN, the first step in the Indian R&D program on negative ion beams has reached an important milestone, with the production of negative ions in the surface conversion mode through Cesium (Cs) vapor injection into the source. In the present set-up, negative hydrogen ion beam extraction is effected through an extraction area of ∼73.38 cm2 (146 apertures of 8mm diameter). The three grid electrostatic accelerator system of ROBIN is fed by high voltage DC power supplies (Extraction Power Supply System: 11kV, 35A and Acceleration Power Supply System: 35kV, 15A). Though, a considerable reduction of co-extracted electron current is usually observed during surface mode operation, in order to increase the negative ion current, various other parameters such as plasma grid temperature, plasma grid bias, extraction to acceleration voltage ratio, impurity control and Cs recycling need to be optimized. In the present experiments, to control and to understand the impurity behavior, a Cryopump (14,000 l/s for Hydrogen) is installed along with a Residual Gas Analyzer (RGA). To characterize the source plasma, two sets of Langmuir probes are inserted through the diagnostic flange ports available at the extraction plane. To characterize the beam properties, thermal differential calorimeter, Doppler Shift Spectroscopy and electrical current measurements are implemented in ROBIN. In the present set up, all the negative ion beam extraction experiments have been performed by varying different experimental parameters e.g. RF power (30-70 kW), source operational pressure (0.3 – 0.6Pa), plasma grid bias voltage, extraction & acceleration voltage combination etc. The experiments in surface mode operation is resulted a reduction of co-extracted electron current having electron to ion ratio (e/i) ∼2 whereas the extracted negative ion current density was increased. However, further increase in negative ion current density is expected to be improved after a systematic optimization of the operational parameters and Cs conditioning of the source. It was also found out that a better performance of ROBIN is achieved in the pressure range: 0.5-0.6 Pa. In this paper, the preliminary results on parametric study of ROBIN operation and beam optimization in surface mode are discussed.

Beam tomography is a non-invasive diagnostic that allows us to reconstruct the beam emission profile by measuring the light emitted by the beam particles interacting with the background gas, along an elevated number of lines of sight, which is related to the beam density by assuming a uniform background gas. In the framework of the heating and current drive of future nuclear fusion reactors, negative ion beams of hydrogen and deuterium are required for neutral beam injectors (NBIs) due to their elevated neutralization efficiency at high energy (in the MeV range). Beside the beam energy, beam divergence and homogeneity are two critical aspects in the design of future NBIs. In this paper, the characterization of the negative ion beam of the negative ion source NIO1 (a small-sized radio-frequency driven negative ion source, with 130 mA of total extracted H- current and 60 kV of maximum acceleration) using the tomographic system composed of two visible cameras is presented. The Simultaneous Algebraic Reconstruction Technique (SART) is used as an inversion technique to reconstruct the 3 × 3 matrix of the extracted beamlets, and the beam divergence and homogeneity are studied. The results are compared with the measurements of the other diagnostics and correlated with the source physics. The suitability of visible cameras as a diagnostics system for the characterization of the NIO1 negative ion beam is a small-scale experimental demonstration of the possibility to reconstruct more complicated multi-beamlet profiles, resulting in a powerful diagnostic for large NBIs.

The negative ion beam optics is studied quantitatively from the viewpoints of the heat loads due to the beam halo in the accelerator and the emittance of the negative ion beam by using the 3D3V PIC model combined with a Monte Carlo calculation, in which negative ion beam from plasma meniscus formation to the beam acceleration is modeled. As for the heat loads of the A2G and the GRG, the simulation result almost agrees with the experimental result. The simulation result indicates that the secondary electrons from the extractor also contribute to the heat load of the A1G. Moreover, the normalized rms emittance of the negative ion beam after the exit of the GRG are estimated to be around 0.25π mm mrad, which are compatible with the typical values of the negative ion sources.