Negative Ion Beam Acceleration Research Articles

Development of air-cooled plasma grid system for long-pulse negative ion beam acceleration with ITER-relevant perveance

Long pulse beam experiment has been carried out with a multi-aperture and five-stage accelerator with the same gap length, the same aperture interval, and the same aperture diameter as ITER accelerator in order to demonstrate ITER-relevant beam of 230 Am−2 and 870 keV. An actively cooled plasma grid (PG) with compressed air has been newly developed to maintain the PG temperature within suitable range for surface production of hydrogen negative ion (H–) and to realize the stable H– production. It is confirmed that the air-cooled PG has a capability to control the PG temperature within target range for 300 s with feedback control of the air flow rate. The stable beam acceleration with ITER relevant perveance has been successfully achieved for 300 s at 275 keV with PG cooling, 100 s at 500 keV, and 10 s at 750 keV without PG cooling.

Towards self-consistent modelling of negative ion beam acceleration

Neutral beams are a valuable option to provide heat and current drive to a magnetically confined plasma. In addition they can be used as a diagnostics for the positive ion temperature and, in case of nuclear fusion reactions, for the amount of helium ash. In the case of ITER, stringent requirements are set on the acceptable beam divergence and aiming in order to propagate such beams up to the tokamak itself. Besides the well-known dependence of the beamlet optics on the beam energy and perveance, other important features affecting the optics are the ion spatial and velocity distribution at the plasma boundary facing the extractor, the so-called plasma meniscus. In a caesiated negative ion source, this distribution is influenced both by the birth energy of the negative ions and by the ion transport in the plasma itself. In this work we use a test particle code to investigate the negative ion transport up to the plasma meniscus and we use a standard ray-tracing code to trace the test particles that reached the meniscus inside the electrostatic accelerator. The coupling of the two codes is iterated up to numerical convergence. By this procedure, the angular distribution of the accelerated particles is found to be much more similar to the experimental ones than the one obtained by simple ray-tracing simulations.

Experimental validation of the dual positive and negative ion beam acceleration in the plasma propulsion with electronegative gases thruster

The PEGASES (Plasma Propulsion with Electronegative Gases) thruster is a gridded ion thruster, where both positive and negative ions are accelerated to generate thrust. In this way, additional downstream neutralization by electrons is redundant. To achieve this, the thruster accelerates alternately positive and negative ions from an ion-ion plasma where the electron density is three orders of magnitude lower than the ion densities. This paper presents a first experimental study of the alternate acceleration in PEGASES, where SF6 is used as the working gas. Various electrostatic probes are used to investigate the source plasma potential and the energy, composition, and current of the extracted beams. We show here that the plasma potential control in such system is key parameter defining success of ion extraction and is sensitive to both parasitic electron current paths in the source region and deposition of sulphur containing dielectric films on the grids. In addition, large oscillations in the ion-ion plasma potential are found in the negative ion extraction phase. The oscillation occurs when the primary plasma approaches the grounded parts of the main core via sub-millimetres technological inputs. By controlling and suppressing the various undesired effects, we achieve perfect ion-ion plasma potential control with stable oscillation-free operation in the range of the available acceleration voltages (±350 V). The measured positive and negative ion currents in the beam are about 10 mA for each component at RF power of 100 W and non-optimized extraction system. Two different energy analyzers with and without magnetic electron suppression system are used to measure and compare the negative and positive ion and electron fluxes formed by the thruster. It is found that at alternate ion-ion extraction the positive and negative ion energy peaks are similar in areas and symmetrical in position with +/− ion energy corresponding to the amplitude of the applied acceleration voltage.

Long-pulse beam acceleration of MeV-class H− ion beams for ITER NB accelerator

In order to realize neutral beam systems in International Thermonuclear Experimental Reactor whose target is to produce a 1 MeV, 200 A/m(2) during 3600 s D(-) ion beam, the electrostatic five-stages negative ion accelerator so-called "MeV accelerator" has been developed at Japan Atomic Energy Agency. To extend pulse length, heat load of the acceleration grids was reduced by controlling the ion beam trajectory. Namely, the beam deflection due to the residual magnetic field of filter magnet was suppressed with the newly developed extractor with a 0.5 mm off-set aperture displacement. The new extractor improved the deflection angle from 6 mrad to 1 mrad, resulting in the reduction of direct interception of negative ions from 23% to 15% of the total acceleration power, respectively. As a result, the pulse length of 130 A/m(2), 881 keV H(-) ion beam has been successfully extended from a previous value of 0.4 s to 8.7 s. This is the first long pulse negative ion beam acceleration over 100 MW/m(2).

Analysis of secondary particle behavior in multiaperture, multigrid accelerator for the ITER neutral beam injector

Heat load on acceleration grids by secondary particles such as electrons, neutrals, and positive ions, is a key issue for long pulse acceleration of negative ion beams. Complicated behaviors of the secondary particles in multiaperture, multigrid (MAMuG) accelerator have been analyzed using electrostatic accelerator Monte Carlo code. The analytical result is compared to experimental one obtained in a long pulse operation of a MeV accelerator, of which second acceleration grid (A2G) was removed for simplification of structure. The analytical results show that relatively high heat load on the third acceleration grid (A3G) since stripped electrons were deposited mainly on A3G. This heat load on the A3G can be suppressed by installing the A2G. Thus, capability of MAMuG accelerator is demonstrated for suppression of heat load due to secondary particles by the intermediate grids.

Acceleration of ampere class H− ion beam by MeV accelerator

The H(-) ion accelerator R&D to realize the international thermonuclear experimental reactor neutral beam is ongoing at Japan Atomic Energy Agency (JAEA). The required performance for the prototype MeV accelerator developed at JAEA is 1 MeV, 500 mA (current density of 200 A/m(2)) H(-) ion beam at the beamlet divergence angle of less than 7 mrad. Up to 2005, 836 keV, 146 A/m(2) H(-) ion beam was successfully accelerated as the highest record of the current density at MeV class energy beams. In the present work, high current negative ion beam acceleration test was performed by increasing the beam extraction apertures from 3 x 3 (9 apertures) to 3 x 5 (15 apertures). By fixing the air leak at the source chamber due to backstream ions as well as the improvement of voltage holding capability by a new fiber reinforced plastic insulator ring, the performance of the MeV accelerator was improved. So far, H(-) ion beam of 320 mA was successfully accelerated up to 796 keV with the beam divergence angle of 5.5 mrad. The accelerated drain current including the electron reaches close to the power supply limit for the MeV test facility. The heat flux by the backstream ion during the above beam acceleration was estimated to be 360 W/cm(2). The Cs leakage to the accelerator during the test campaign (Cs total input of 5.0 g) was 0.26 mg (7.0 microg/cm(2)). This is considered to be the allowable level from the viewpoint of voltage holding.

Hydrogen negative ion beam acceleration in a multiaperture five-stage electrostatic accelerator

To develop a high power negative ion source/accelerator for 1 MeV class neutral beam injector, hydrogen negative ion beam acceleration has been studied using a five-stage, multiaperture electrostatic accelerator. After conditioning each accelerator stage, the negative ions are accelerated to 1 MeV successfully with a drain current of 25 mA for 1 s. Cs was introduced into the ion source to produce higher current density. The highenst acceleration current density of 15 mA/cm2 was successfully accelerated up to an energy of 700 keV for 1 s, keeping the optimum perveance. The total acceleration current of 200 mA was extracted from nine central apertures 14 mm diameter each. A preliminary measurement of the heat load in the accelerator showed that the direct interception of the beam for the first grid and the third grid was negligibly small. The highest heat load was 4.5% of the input power at the second grid.

Recent progress of high-power negative ion beam development for fusion plasma heating

A negative-ion-based neutral beam injector (N-NBI) has been constructed for JT-60U. The N-NBI is designed to inject 500 keV, 10 MW neutral beams using two ion sources, each producing a 500 keV, 22 A D − ion beam. In the preliminary experiment using one ion source, a D − ion beam of 13.5 A has been successfully accelerated with an energy of 400 keV (5.4 MW) for 0.12 s at an operating pressure of 0.22 Pa. This is the highest D − beam current and power in the world. Co-extracted electron current was effectively suppressed to the ratio of Ie/ I D − < 1. The highest energy beam of 460 keV, 2.4 A, 0.44 s has also been obtained. To realize 1 MeV class NBI system for ITER (International Thermonuclear Experimental Reactor), demonstration of ampere class negative ion beam acceleration up to 1 MeV is an important mile stone. To achieve the mile stone, a prototype accelerator and a 1 MV, 1 A test facility called MeV Test Facility (MTF) were constructed. Up to now, an H − ion beam was accelerated up to the energy of 805 keV with an acceleration drain current of 150 mA for 1 s in a five stage electrostatic multi-aperture accelerator.

Recent progress on high power negative ion sources at JAERI

Abstract R & D's on high power negative ion source at JAERI are reviewed. A method of cesium injection into volume source has drastically progressed in development of the high current source. The H− ion current was enhanced up to 10 A only by seeding cesium of a few hundreds mg into a volume production type magnetically filtered multicusp source. Reductions of the optimum gas pressure and the electron current were also observed in the operation of cesium seeded sources. These effects are helpful in stable ac-celeration of the ion beams. High energy negative ion beam acceleration was demonstrated in the cesium seeded sources. So far, we have succeeded to accelerate 0.13 A H− ion beam up to 350 keV for 1 s with good beam optics of the divergence ωl/e ∼ 5 mrad. Two methods of the beam focussing were attempted; i.e. aperture displacement and curved grids extraction. The beam focussing by aperture displacement was found to be useful for the high power beam steering, while the beam merge experiment were being p...

Design of an electrostatic magnetic quadrupole accelerator

A new type of electrostatic acceleration system, electrostatic magnetic quadrupole (ESMQ) acceleration system, is proposed for efficient acceleration of negative ion beams. In this system, permanent magnets are buried in the acceleration electrodes so as to produce a quadrupole magnetic field in the electrode aperture region. Envelope simulation indicates that the quadrupole field can deflect electrons stripped from the negative ions. Beam envelope simulations for deuterium ions and electrons have been carried out using the beam envelope code trace. Electrons are largely divergent and most appear likely to hit downstream electrodes. Furthermore, maximum beam divergence of the deuterium ions is reduced to the focusing effect of the quadrupole magnetic field.

Negative ion beam acceleration and transport

The optimum conditions for focusing a negative ion beam are discussed. The distribution of the current between the Faraday cup and a large ring collector, surrounding this cup, has been investigated experimentally and theoretically, at constant extraction voltage (1.9 kV), with the acceleration voltage being varied in a wide range. The effects of beam space charge, negative ion temperature, and stripping are analyzed.