Measurement of a time dependent spatial beam profile of an RF-driven H- ion source.

The AC component of a beam current extracted from a negative hydrogen (H-) ion source was detected through a 0.1 mm wide, 66.5 mm long entrance slit to observe the spatial distribution. An internal antenna type multicusp source driven by a 2 MHz radio frequency (RF) power delivered beams to an electrostatic accelerator coupled to a pair of magnetic lenses. The local beam intensity measured by a Faraday cup after the entrance slit exhibited an oscillation showing two main frequency components: the RF power supply frequency and the frequency two times the driving RF. The frequency spectrum of the detected signal showed sharp peaks at 2 MHz, 4 MHz, and 6 MHz as well as at 3 MHz and 5 MHz. A 1 mm displacement of the Faraday cup slit position from the center of the beam axis increased the oscillation amplitude, corresponding to a larger amplitude of the AC component at the beam edge.

Evaluation of two-stage system for neutron measurement aiming at increase in count rate at Japan Atomic Energy Agency-Fusion Neutronics Source.

In order to increase the count rate capability of a neutron detection system as a whole, we propose a multi-stage neutron detection system. Experiments to test the effectiveness of this concept were carried out on Fusion Neutronics Source. Comparing four configurations of alignment, it was found that the influence of an anterior stage on a posterior stage was negligible for the pulse height distribution. The two-stage system using 25 mm thickness scintillator was about 1.65 times the count rate capability of a single detector system for d-D neutrons and was about 1.8 times the count rate capability for d-T neutrons. The results suggested that the concept of a multi-stage detection system will work in practice.

Influence of electric field penetration by a three-electrode beam extraction system on hydrogen negative ion source plasma.

We study influence of electric field penetration into H(-) ion source plasma with three-electrode beam extraction system. Clear change in the plasma potential due to the field penetration is observed in case of low gap voltage between a plasma electrode and an extraction electrode. Influence of lens voltage on the second electrode, which is normally utilized to focus the extracted beam, on ion source plasma is evaluated separating contributions of H(-) density in the plasma and extraction probability of H(-) ions from the plasma by two kinds of photodetachment techniques. In our operation condition, we found that the lens voltage is also useful to enhance the H(-) density in the plasma, though it negatively affects the extraction probability.

Angular distributions of surface produced H(-) ions for reflection and desorption processes.

A numerical simulation code, Atomic Collision in Amorphous Target, has been run to clarify the effects due to the incident angle of hydrogen flux onto surface collision cascade in the subsurface region of a Cs covered Mo plasma grid. The code has taken into account the threshold energy for negative hydrogen (H(-)) ions to leave the surface. This modification has caused the shift of energy distribution functions of H(-) from that of hydrogen atoms leaving the surface. The results have shown that large incident angle of hydrogen particle tilt the angular distribution of reflection component, while it caused a small effect onto the angular distribution of desorption component. The reflection coefficient has increased, while the desorption yield has decreased for increased angle of incidence measured from the surface normal.

Effects of roughness and temperature on low-energy hydrogen positive and negative ion reflection from silicon and carbon surfaces.

Angle-resolved energy distribution functions of positive and negative hydrogen ions produced from a rough-finished Si surface under 1 keV proton irradiation have been measured. The corresponding distribution from a crystalline surface and a carbon surface are also measured for comparison. Intensities of positive and negative ions from the rough-finished Si are substantially smaller than those from crystalline Si. The angular distributions of these species are broader for rough surface than the crystalline surface. No significant temperature dependence for positive and negative ion intensities is observed for all samples in the temperature range from 300 to 400 K.

Development of a He- and He0 beam source for alpha particle measurement in a burning plasma.

Proof of principle experiments of neutral helium beam production for alpha particle diagnostics was carried out on a test stand. Negative helium ions were produced in the Li charge exchange cell, in which stable and long time operation was possible. He(-) beam was accelerated to 157 keV. Finally, He(0) beam was successfully produced after the flight in the drift-tube through the auto-electron-detachment process from He(-) to He(0). A neutral beam detector using a pyroelectric device was also developed to measure He(0) beam intensity. The metastable component in the neutral helium beam was found to be less than 2%.

Extraction of a strongly focusing He+ beam from three-stage concave electrodes for alpha particle measurement system in ITER.

A strongly focusing He(+) ion beam source equipped with concave multi-aperture electrodes was developed for production of He(-) through a charge exchange cell. The beam was extracted at a voltage less than 20 kV from 301 apertures distributed in an area of 100 mm φ, and focused at 750 mm distance. The beam current and the beam size of 2 A and 20 mm in diameter, respectively, were achieved with an arc power less than 10 kW. The optimum perveance was obtained at 0.02 A∕kV(1.5) at the beam energy less than 20 keV which is suitable for the conversion to He(-) in an alkali vapor cell.

An alpha particle measurement system using an energetic neutral helium beam in ITER (invited).

An energetic helium neutral beam is involved in the beam neutralization measurement system of alpha particles confined in a DT fusion plasma. A full size strong-focusing He(+) ion source (2 A, the beam radius of 11.3 mm, the beam energy less than 20 keV). Present strong-focusing He(+) ion source shows an emittance diagram separated for each beamlet of multiple apertures without phase space mixing, despite the space charge of a beamlet is asymmetric and the beam flow is non-laminar. The emittance of beamlets in the peripheral region was larger than that of center. The heat load to the plasma electrode was studied to estimate the duty factor for the ITER application.

Fusion product diagnostics planned for Large Helical Device deuterium experiment.

Deuterium experiment on the Large Helical Device (LHD) is now being planned at the National Institute for Fusion Science. The fusion product diagnostics systems currently considered for installation on LHD are described in this paper. The systems will include a time-resolved neutron yield monitor based on neutron gas counters, a time-integrated neutron yield monitor based on activation techniques, a multicollimator scintillation detector array for diagnosing spatial distribution of neutron emission rate, 2.5 MeV neutron spectrometer, 14 MeV neutron counter, and prompt γ-ray diagnostics.

Detection of lost alpha particle by concealed lost ion probe.

Full orbit-following calculation is performed for the final orbit of the lost alpha particles, showing some orbits escaping from the last closed flux surface could be detected by a concealed lost ion probe (CLIP) installed under the shadow of the original first wall surface. While both passing and trapped orbits hit the same wall panel, detecting a trapped orbit by the CLIP is easier than detecting passing orbits. Whether the final orbit is detected or not is determined by the position of the reflection point. The CLIP successfully detects the trapped orbits, which are reflected before they hit to a first wall. Then the pitch angles of the orbits at the CLIP are close to and smaller than 90°. Optimization of the position of the CLIP in terms of broader detection window is investigated.

Development and irradiation test of lost alpha detection system for ITER.

We developed a lost alpha detection system to use in burning plasma experiments. The scintillators of Ag:ZnS and polycrystalline Ce:YAG were designed for a high-temperature environment, and the optical transmission line was designed to transmit from the scintillator to the port plug. The required optical components of lenses and mirrors were irradiated using the fission reactor with the initial result that there was no clear change after the irradiation with a neutron flux of 9.6×10(17) nm(-2) s(-1) for 48 h. We propose a diagnostic of alpha particle loss, so-called alpha particle induced gamma ray spectroscopy. The initial laboratory test has been carried out by the use of the Ce doped Lu(2)SiO(5) scintillator detector and an Am-Be source to detect the 4.44 MeV high energy gamma ray due to the (9)Be(α,nγ)(12)C reaction.

Strategy for the absolute neutron emission measurement on ITER.

Accuracy of 10% is demanded to the absolute fusion measurement on ITER. To achieve this accuracy, a functional combination of several types of neutron measurement subsystem, cross calibration among them, and in situ calibration are needed. Neutron transport calculation shows the suitable calibration source is a DT/DD neutron generator of source strength higher than 10(10) n/s (neutron/second) for DT and 10(8) n/s for DD. It will take eight weeks at the minimum with this source to calibrate flux monitors, profile monitors, and the activation system.

Fast neutron-gamma discrimination on neutron emission profile measurement on JT-60U.

A digital signal processing (DSP) system is applied to stilbene scintillation detectors of the multichannel neutron emission profile monitor in JT-60U. Automatic analysis of the neutron-γ pulse shape discrimination is a key issue to diminish the processing time in the DSP system, and it has been applied using the two-dimensional (2D) map. Linear discriminant function is used to determine the dividing line between neutron events and γ-ray events on a 2D map. In order to verify the validity of the dividing line determination, the pulse shape discrimination quality is evaluated. As a result, the γ-ray contamination in most of the beam heating phase was negligible compared with the statistical error with 10 ms time resolution.

Fine-structure characteristics in the emittance images of a strongly focusing He+ beam.

The phase space distribution of a strongly focused He(+) ion beam source equipped with concave multiaperture electrodes was measured using a pepper-pot plate and a Kapton foil. The substructure of 301 merging He beamlets was clearly observed on a footprint of pepper-pot hole at the beam waist, where the beam density was 500 mA/cm(2). The position and the width of each beamlet substructure show the effect of interference of beamlets with surrounding one.

Influence of H- velocity on H- extraction probability from a negative ion source.

We investigate influence of H(-) initial transport direction and kinetic energy on H(-) extraction probability with three-dimensional Monte Carlo calculation. As a result, lower energy H(-) ions are strongly trapped by the electrical potential structure, so that initial condition of H(-) transport direction is cancelled by alignment of the electric field; thus, it has lower influence for H(-) extraction probability. Besides, the potential hill induced by the beam extraction voltage more effectively enhances H(-) extraction probability for the lower energy H(-) ions. The correlation between the magnitude of the local plasma potential near the extraction region and the mean velocity of H(-) ions in the region should determine the H(-) extraction probability from the ion source.

Full orbit calculation for lost alpha particle measurement on ITER.

An orbit following calculation code with full gyromotion under the ITER magnetic field configuration has been developed to investigate escaping alpha particle orbits in ITER and to determine the geometrical arrangement for alpha particle detection. The code contained the full geometrical information of the first wall panels. It was carefully investigated whether an alpha particle escaping from the plasma through the last closed flux surface does not touch or intersect the first wall boundary before reaching the detection point. Candidates of blanket module modification have been studied to achieve effective measurement geometry for escaping alpha particle detection. The calculations showed that direct orbit loss and banana diffusion can be detected with a probe head recessed from the first wall surface.

Effects of filament geometry on the arc efficiency of a high-intensity He+ ion source.

A strongly focusing high-intensity He(+) ion source equipped with three concave electrodes has been designed and constructed as the beam source for a high-energy He(0) neutral beam probe system to diagnose fusion-produced alpha particles in thermonuclear fusion plasmas. The reduction of heat load onto the concave extraction electrodes is particularly important for a long pulse operation, as the heat load deforms the electrodes and thus the beam focal length. The effects on the arc efficiency (beam current/arc power) of the ion source due to the discharge filament structure (straight-type and L-shape-type filaments), size (filament diameters of 2 and 1.5 mm), number, and the locations have been studied. Choice of the appropriate filament structure improved the arc efficiency by 17%.

Energy straggling of low-energy ion beam in a charge exchange cell for negative ion production.

Energy straggling in a charge exchange cell, which is frequently used for negative ion production, was studied experimentally and compared with the results of theoretical evaluation. The change of the energy spectrum of a He(+) beam due to charge exchange processes in argon gas was measured in the energy range of 2-6 keV. Energy straggling by multiple collisions is expressed by the energy loss formula due to inelastic and elastic processes. The impact parameter is related to the elastic scattering angle, and the geometry of the charge exchange cell and other components of the beam transportation system determines the maximum acceptable scattering angle. The energy spread was evaluated taking the integral limit over the impact parameter into consideration. The theoretical results showed good agreement with those of actual measurement.

A beam transport system for an intense He(-) beam source.

We have been developing a test stand for fast He(0) beam production. One of the major issues is how to transport effectively the He(+) and He(-) beams from which the He(0) beam is produced. The beam should be focused in two transverse focal points, the center of the charge exchange cell and the electrostatic accelerator. We studied the beam transport system and effect of space charge neutralization in the test stand by experiments and calculation.

Diagnostics of a He(+) beam extracted from a compact magnetic bucket-type ion source.

Performance of a compact He(+) ion source was investigated before installing it to the experiment system developed for studying the efficiency of autodetachment neutralization from He(-) to He(0). Dependence of the extracted ion current upon the plasma parameters and that upon the extraction voltage indicated that the source performance was limited by space charge effect. The beam emittance was measured with a multislit system for various operation conditions. The experimentally determined emittance agreed well with the emittance obtained from ion trajectories calculated with input plasma parameters measured by a Langmuir probe. The beam current density of 60 mA/cm(2) and the emittance of less than 25pi mm mrad are expected to deliver enough current to the downstream of the He(-) neutralizer system to clarify factors affecting the final He(0) intensity.