Multiaperture Grids Research Articles

Characteristics of the extracted negative-ion beam in a cesium-free negative-ion source using TPDsheet-U

The progress toward realizing a high-performance cesium (Cs)-free negative-ion source based on volume production in a magnetized sheet plasma device (TPDsheet-U) is reported. In the experiments, H− ions were successfully extracted from sheet plasma using single-/multi-aperture grids. The experimental results of the single-aperture grid show that the negative hydrogen ion current density (J C(H−)) performance of Cs-free H− ion beams in TPDsheet-U is about one-fourth that of Cs-containing H− ion sources in the ITER negative-ion neutral-beam injector (NNBI). (i) The J C(H−) was ∼7.5 mA cm−2 at an extraction voltage of 10 kV, a discharge current I d of 90 A, and a gas pressure of 0.3 Pa. (ii) The electron current J EG(e) with this soft magnetic filter (SMF) was only ∼10% of the electron current without the SMF; The J EG(e)/J C(H−) ratio was reduced below 0.4 at y < 18 mm for a single-aperture grid with the SMF at I d = 50 A and external magnetic field strength of 38 mT.

Design and first results of a retractable 1D-CFC beam target for BATMAN upgrade

BATMAN Upgrade (BUG) is a test facility for the production of negative ion beams from an RF driven plasma ion source devoted to supporting the development of the ITER Neutral Beam Injection (NBI). BUG consists of the IPP RF prototype source and a multi-aperture grid system corresponding roughly to a beamlet group of the ITER NBI grid system, i.e. an arrangement of 14 (vertical) ×5 (horizontal) Ø14 mm apertures, meaning the ion beam is comprised of 70 individual beamlets. The beams can be extracted with up to 10 kV and accelerated up to a total voltage of 45 kV for up to 10 s (steady state operation is underway). One of the goals of BUG is a detailed study of the ion beam optics. So far, the permanently installed beam diagnostic consisted of several lines of sight of Beam Emission Spectroscopy (BES) at two different positions and thermocouple measurements on the beam dump calorimeter. Resolving the divergence and deflection of single beamlets is technically exceptionally difficult with the existing diagnostic tools, therefore, a beam target made out of a one-directional carbon fiber composite material (1D-CFC) with dimensions 376×142×20 mm³ has been installed at a distance of 851 mm from the exit of the grid system. The thermal footprints of the beamlets are observed from the back side via infrared imaging with a high spatial resolution (<0.7 mm/pixel) that allows characterization of the beamlets at low divergence. The target can be rotated out of the beam to make it compatible with long pulses and the forthcoming steady state operation. The safety analysis of the target operation for different operational scenarios is presented together with an energy-based interlock. Finally, the accuracy of estimating the shape of the heat flux on the front of the target from the temperature profile at the back is assessed, based on FEM simulations of a single beamlet.

CRISP: A compact RF ion source prototype for emittance scanner testing.

A movable Allison type emittance scanner is being developed to characterize the phase-space distribution of the beamlets of spectral phase interferometry for direct electric-field reconstruction, the prototype RF negative ion source of the ITER heating neutral beam injector. To test the electronics and verify the capability of the device to resolve nearby beamlets, a compact RF ion source prototype has been set up, capable of accelerating 1 mA of helium ions up to a voltage of 2 kV. A commercial 100 W RF generator creates a plasma inside a Pyrex tube, with a density between 1015 and 1016 m-3 and an electron temperature up to 15 eV. Three multi-aperture grids in accel-decel configuration extract and accelerate the ions, which are measured with a Faraday cup. We present in this paper the characterization of the ion source and its first operation, showing that it is suitable for the commissioning of the Allison scanner.

Manufacturing technology development for an ‘angled’ accelerator grid segment for DNB Beam Source

The accelerator for the Diagnostics Neutral Beam (DNB) beam source is composed of a multi-aperture grid system with three water cooled grids made from Oxygen free Copper. To achieve the focusing requirements at the distance of >20m, the grid segments are designed with two stage angles (0.222° and 0.665°) from the centerline in the horizontal direction. The configuration of this kind of ‘angled segment’ includes the water cooling channels milled in the angular form, subsequently closed by copper electrodeposition, providing the angles on front and back surface and then drilling of apertures on the angular plane. The long beam path and low energy beam demands the tight tolerances on each of these mechanical features and therefore demands the high degree of manufacturing controls on each of the processes.To unveil the challenges those could appear during the production of such grid, a 1:1 prototype of the most complex type of grid has been manufactured. This paper shall present the technical data generated out of manufacturing of this prototype, summarizing the recommendations for real grid production on: optimization of the sequence of manufacturing, effect of each of the operations, post-manufacturing handling and identifying the measurement techniques. The experience gathered here provides a recipe for the best manufacturing practices for the accelerators of NB system for ITER and upcoming devices.

Optical transparency radial distribution of ion thruster

The optical system is one of the main components of an ion thruster, which consists of electrically biased multi-aperture grids. The grid design is critical to the ion thruster operation since its transparency has an important influence on the thruster efficiency and thrust. To further optimize the optical system performance and evaluate effectively the efficiency of ion thruster, the optical transparency radial distribution of ion thruster is analyzed and discussed in experiment and simulation. The process of beam extraction is simulated by the particleincell-Monte Carlo collision (PIC-MCC) method, and the movement of the ions is investigated by the PIC method while the collisions of particles are handled by the MCC method. Then the interdependency among the transparency of screen grid, the accelerator grid, optics system and the number of ion extracted is analyzed. Taking into account the distribution of ion density at the exit of discharge chamber, the radial distribution of the screen grid transparency, accelerator grid transparency and optical system transparency are acquired. An experiment is performed to verify the simulation based derivation, indicating the good agreement between experimental and simulation results. The results show that the radial distribution of screen grid transparency increases gradually along the radial direction and has a good central axial symmetry, and its minimum value is located in the center of the thruster while the maximum value is near the margin region of screen gird. The radial distribution of accelerator grid transparency is opposite to that of the screen grid transparency, which decreases along the radial direction, and its maximum value is located at the axis of the thruster. The radial distribution of optical system transparency is the same as that of the screen grid transparency. And its minimum value is in the center of optics system, which indicates that the effect of accelerator grid transparency on the optical system transparency is little. In addition, the study also finds that the total optical transparency of ion thruster decreases slowly as the beam current increases. This work will provide a lot of support for the optimal design of ion thruster optics system.

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.

Beamlet deflection due to beamlet-beamlet interaction in a large-area multiaperture negative ion source for JT-60U

The JT-60U negative ion source has been designed to produce high current beams of 22 A through grids of 1080 apertures (five segments with nine rows of 24 apertures). One of the key issues is to steer such a high current beam through the multiaperture grids in order to focus the overall beam envelope because the beamlet-beamlet interaction may deflect the outer beamlets outward due to unbalanced space charge repulsion. To clarify the beam deflection in the JT-60U negative ion source, the beamlet trajectory in a multiaperture ion source was calculated by a three-dimensional simulation code. The measured angles of the outmost beamlets were in agreement with the calculated results where space charge of the beamlets was taken into account. It is noticed that the deflection of the outermost beamlet due to the beamlet-beamlet interaction is saturated at 5.2 mrad outward for beamlets more than ten.

Present status of the negative ion based NBI system for long pulse operation on JT-60U

The 500 keV negative-ion based neutral beam injector for JT-60U started operations in 1996. The availability of the negative ion based neutral beam injection system has been improved gradually by modifying the ion source and optimizing its operation parameters. Recently, the extension of the pulse duration up to 30 s has been intended to study quasi-steady state plasma on JT-60U. The most serious issue is to reduce the heat load on the grids for long pulse operation. Two modifications have been proposed to reduce the heat load. One is to suppress the spread of beamlet-bundle which may be caused by beamlet–beamlet interaction in the multi-aperture grid due to the space charge force. Indeed, the investigation of the beam deflection, which was measured by the infrared camera on the target plate set 3.5 m away from the grid, indicates that the spread of beamlet-bundle is in proportion to the current density. Field-shaping plates were attached on the extraction grid to modify the local electric field. The plate thickness was optimized to steer the beamlet deflection. The other is to reduce the stripping loss, where the electron of the negative ion beam is stripped and accelerated in the accelerator and then collides with the grids. The ion source was modified to reduce the pressure in the accelerator column to suppress the beam-ion stripping loss. To date, long pulse injection of 19 s of 1.5–1.6 MW at a high energy beam of 360 keV, 9–10 A for D− has been obtained by one ion source with these modifications.

Development of a high brightness negative hydrogen ion source

A high brightness negative hydrogen ion source has been developed for a high intensity proton linear accelerator. The ion source is volume production type. Negative ions are generated in a magnetically filtered multicusp plasma generator. The negative ion production is enhanced by seeding a small amount of cesium into the plasma generator. A negative hydrogen ion beam of 36 mA was extracted from a single aperture at an acceleration voltage of 50 kV. To obtain a higher ion beam current, the focusing of beamlets extracted from the multiaperture grid has been demonstrated with an aperture displacement technique.

New high-power fast atom beam source

This article describes a new high-power fast atom beam (FAB) source incorporating an electromagnet. The electromagnet increases the total path length of electrons entering the source, enabling this source to produce higher plasma densities. Beam current densities up to 1 mA/cm2, which is about ten times that of a conventional FAB source, are obtained by using an electromagnet and a multiaperture grid. The beam current density can also be controlled by adjusting the flux density of the electromagnet and the gas pressure in the source. The beam neutralization coefficient is more than 90% almost independent of the flux density, discharge current, and source configuration. This coefficient does, however, strongly depend on the gas pressure of the FAB source, rapidly increasing as gas pressure increases. Most of the fast atoms have a kinetic energy about equal to the discharge voltage. Ions generated in the glow discharge are accelerated, and collisions with gas molecules near the cathode convert these ions into fast atoms without loss of kinetic energy.

Demonstration and evaluation of carbon-carbon ion optics

The principal life-limiting component of ion thrusters is the ion optics set used to electrostatically accelerate ionized propellant from the thruster's discharge chamber. An ion optics set consists of two or three thin, closely spaced, multiaperture grids, usually made of molybdenum. Grid lifetime is limited by sputter erosion from ion impingement. Data in the literature show that carbon is about five times more resistant to sputter erosion than molybdenum. Carbon in the form of graphite is used for laboratory ion optics, but graphite is too fragile for spacecraft use. Use of carbon-carbon composites may enable fabrication of rugged, thermomechanically stable optics with greater lifetime than molybdenum. This article describes a laboratory demonstration of a flat, twogrid, 10-cm-diam carbon-carbon ion optics set, with an evaluation of selected performance characteristics relative to a molybdenum optics set of roughly similar geometry. The carbon-carbon optics delivered stable operation at a cold grid gap of 0.2 mm, an average current density of 2.2 mA/cm, and an inferred voltage gradient of 6400 V/mm. Differences observed in the maximum perveance condition, the electron backstreaming limit, and the defocusing limit between the two optics sets were consistent with the known geometrical differences.

High-power fast-atom beam source and its application to dry etching

We have developed a new high-power fast-atom beam (FAB) source by modifying a McIlraith-type ion source. The beam current density emitted from a source with an electromagnet and multiaperture grid is about ten times higher (up to 1 mA/cm2) than that emitted from a conventional source. Silicon dioxide can therefore be etched at 60 nm/min when CF4 gas is used, or about six times faster than by using a conventional source with the same discharge current. The beam neutralization coefficient of the new FAB source is nearly 100%. We have used this FAB source to etch Si, SiO2, GaAs, and other compound materials. We could easily etch an anisotropic profile (nearly vertical with an aspect ratio of more than 10 for a 0.6 μm L&amp;S pattern), etch at a high rate (0.3–0.5 μm/min), and etch with high selectivity (GaAs:resist =30:1). We also fabricated a SiO2 fine pattern in 0.6 μm L&amp;S by using CF4 reactive-fast-atom beam etching. The uniformity of the etch depth is about ±5% over a 4-in. wafer. Surface radiation damage (total oxide charge) caused by FAB bombardment of the SiO2/Si structure is very close to that of reference (not etched) samples. This damage is significantly less than that caused by ion beam bombardment under almost the same radiation conditions.

H- Source Deveidpment at ANL

The H- source development program at ANL1-7 has produced the first operational source for direct H- injection into a proton synchrotron and is now directed toward the development of a 30 Hz H- source for booster injection into a synchroton. On October 1, 1976, direct injection of 50 MeV H- ions into the ANL Zero Gradient Synchrotron (ZGS) and stripping of the H- to obtain protons for acceleration to full energy became the normal mode of operation. With >6 mA of 50 MeV H- beam injected and ~1012 protons per mA accelerated to full energy during each cycle, new ZGS intensity records have been established.8 In the operational tandem-acceleration H source, a single-aperture source grid is followed by a multislot parallel-ribbon extraction grid. Full and fractional energy H- ions derived from H+, H+2 and H+3 are produced by electron attachment in a target gas consisting of the hydrogen outflow from the plasma source. Power dissipation in the extractor grid, and lifetime considerations, limit the repetition rate to ~1 Hz. A source which uses two or more aligned multiaperture grids to minimize grid dissipation is being developed for 30 Hz injection into the rapid-cycling booster-injector9,10 for the ZGS. A modification of this source will be combined with a recirculating supersonic sodium vapor jet charge exchange cell to obtain a greater H- yield. A sodium cell has been built and is being tested.11,12