A. Hatayama

Effects of a weak transverse magnetic field on negative ion transport in negative ion sources

The effects of a weak transverse magnetic field on negative ion (H−) extraction in a negative ion source have been studied by two-dimensional electrostatic particle simulation. A particle-in-cell model is used which simulates motion of charged particles in their self-consistent electric field. The extraction of H− ions is significantly improved by the weak transverse magnetic field in simulation as well as in experiments. Since electrons are deflected by the weak magnetic field, more H− ions arrive instead of electrons in the region close to the plasma grid in order to ensure plasma neutrality. The presence of the weak magnetic field produces important modifications in positive ion flow and, as a result, in the structure of the spatial potential which collects H− ions.

Improvement of beam uniformity by magnetic filter optimization in a Cs-seeded large negative-ion source

The influence of magnetic filter configuration on the beam uniformity was examined to improve beam uniformity in a large Cs-seeded negative-ion source. By reducing the filter strength of the transverse magnetic field used in a typical negative-ion source, the beam uniformity was largely improved with the improvement of the plasma uniformity while the beam intensity was kept to be nearly constant. However, the coextracted electron current greatly increased. To suppress the coextracted electron current, a tent-shaped magnetic filter was applied together with modifications in the cusp magnets to form a typical multicusp positive-ion source arrangement. The uniformity in longitudinal beam profile was improved with the deviation of local beam intensity within 16% that was nearly equal to the deviation obtained at 50Gcm of the transverse filter strength. In the meantime, the coextracted electron current was kept to be the same as the H− ion current. The present result suggests that the uniformity of H− ion-beam...

The origin of beam non-uniformity in a large Cs-seeded negative ion source

The origin of longitudinal beam non-uniformity, which is one of the key issues in large caesium (Cs)-seeded negative ion sources for fusion application, was experimentally investigated in the JAERI 10 A negative ion source. A sufficient caesium was seeded in the negative ion source to enhance the negative ion production. The beam intensity with Cs was typically four times higher than that without Cs, and was relatively high in the upper region where the electron temperature and plasma density were relatively high. This distribution was significantly different from that without Cs. From the correlation between the beam intensity and the plasma parameters, it was found that the beam non-uniformity was due to the localization of the plasma.The front edge of the filaments was bent towards the plasma grid to suppress the plasma localization. This resulted in the significant improvement of the beam uniformity. The root-mean-square deviation of the beam intensity from the averaged value decreased to a half of that before the modification while the beam intensity integrated along the longitudinal direction was kept constant. This result indicates that the suppression of the plasma localization by the filament modification is effective for improving the beam uniformity in the Cs-seeded negative ion source.

Status and operation of the Linac4 ion source prototypes.

CERNs Linac4 45 kV H(-) ion sources prototypes are installed at a dedicated ion source test stand and in the Linac4 tunnel. The operation of the pulsed hydrogen injection, RF sustained plasma, and pulsed high voltages are described. The first experimental results of two prototypes relying on 2 MHz RF-plasma heating are presented. The plasma is ignited via capacitive coupling, and sustained by inductive coupling. The light emitted from the plasma is collected by viewports pointing to the plasma chamber wall in the middle of the RF solenoid and to the plasma chamber axis. Preliminary measurements of optical emission spectroscopy and photometry of the plasma have been performed. The design of a cesiated ion source is presented. The volume source has produced a 45 keV H(-) beam of 16-22 mA which has successfully been used for the commissioning of the Low Energy Beam Transport (LEBT), Radio Frequency Quadrupole (RFQ) accelerator, and chopper of Linac4.

Monte Carlo simulation of negative ion production in the negative hydrogen ion source

Two Monte Carlo simulation codes: (a) neutral transport code and (b) negative ion (H−) transport code, have been developed to understand transport phenomena in negative ion sources. In the neutral transport code, Boltzmann equations for hydrogen molecules (H2) and atoms (H) are solved. Three dimensional (3D) spatial distributions of H2, H, and H− production are obtained for a tandem negative ion source. The volume production of H− is limited to the area around the gas inlet in the first chamber and near the plasma grid in the second chamber. On the other hand, distribution of H− surface production is shown to be almost uniform over all the plasma grid surface. In the negative ion code, H− trajectories are calculated by numerically solving the 3D equation of motion for H− ions. The effects of the magnetic filter on the extraction probability of surface produced H− ions are mainly studied. The dependence of the extraction probability on the field strength is small.

Linac4 H⁻ ion sources.

CERNs 160 MeV H(-) linear accelerator (Linac4) is a key constituent of the injector chain upgrade of the Large Hadron Collider that is being installed and commissioned. A cesiated surface ion source prototype is being tested and has delivered a beam intensity of 45 mA within an emittance of 0.3 π ⋅ mm ⋅ mrad. The optimum ratio of the co-extracted electron- to ion-current is below 1 and the best production efficiency, defined as the ratio of the beam current to the 2 MHz RF-power transmitted to the plasma, reached 1.1 mA/kW. The H(-) source prototype and the first tests of the new ion source optics, electron-dump, and front end developed to minimize the beam emittance are presented. A temperature regulated magnetron H(-) source developed by the Brookhaven National Laboratory was built at CERN. The first tests of the magnetron operated at 0.8 Hz repetition rate are described.

Meniscus and beam halo formation in a tandem-type negative ion source with surface production

A meniscus of plasma-beam boundary in H− ion sources largely affects the extracted H− ion beam optics. Although it is hypothesized that the shape of the meniscus is one of the main reasons for the beam halo observed in experiments, a physical mechanism of the beam halo formation is not yet fully understood. In this letter, it is first shown by the 2D particle in cell simulation that the H− ions extracted from the periphery of the meniscus cause a beam halo since the surface produced H− ions penetrate into the bulk plasma, and, thus, the resultant meniscus has a relatively large curvature.

Numerical analysis of the production profile of H0 atoms and subsequent H− ions in large negative ion sources

The production and transport processes of H0 atoms are numerically simulated using a three-dimensional Monte Carlo transport code. The code is applied to the large JAEA 10ampere negative ion source under the Cs-seeded condition to obtain a spatial distribution of surface-produced H− ions. In this analysis, the amount of H0 atoms produced through dissociation processes of H2 molecules is calculated from the electron temperature and density obtained by Langmuir probe measurements. The high-energy tail of electrons, which greatly affects H0 atom production, is taken into account by fitting a single-probe characteristic as a two-temperature Maxwellian distribution. In the H0 atom transport process, the energy relaxation of the H0 atoms, which affects the surface H− ion production rate, is taken into account. The result indicates that the surface H− ion production is enhanced near the high-electron-temperature region where H0 atom production is localized.

Development of multidimensional Monte Carlo simulation code for H− ion and neutral transport in H− ion sources

Two multidimensional Monte Carlo simulation codes—(a) neutral (H2,H) transport code and (b) negative ion (H−) transport code—have been developed. This article focuses on the recent simulation results by the neutral transport code for the H− production in a large, hybrid negative ion source, “Camembert III.” Two-dimensional spatial profiles of vibrationally excited molecules H2(v) and H− production are obtained for a given background plasma profile. Both H2(v) and H− ions are mainly produced near the filaments in the driver region. However, the H− source density has double peak in its spatial structure, while the density profile of H2(v) is characterized by the “mushroom” structure with a single peak. These results indicate a large potential of the neutral transport code, not only for the understanding of underlying physics, but also for designing ion sources, including complicating effects of geometry, spatial and velocity distribution of particles, and atomic and wall processes.

Monte Carlo simulation of negative ion transport in the negative ion source (Camembert III)

Transport process of negative hydrogen ions (H−) in a large hybrid multicusp H− source, “Camembert III,” has been analyzed by a three-dimensional Monte Carlo simulation code. The realistic geometry and multicusp magnetic-field configuration are taken into account. Various important destruction processes of H− and Coulomb collision with background plasma are also included in the model. Both the volume- and surface-produced H− ion trajectories are followed. For volume-produced H− ions, most of the H− ions can reach the wall in the low-pressure case (1 mTorr), while in the high-pressure case (3 mTorr) most of the H− ions are destructed by volume loss reactions before reaching the wall. This shows that the wall loss is significant at low pressure as in the experiments. For surface-produced H− ions, the influence of its birthplace on the H− current is studied. Negative ions created on the sidewall hardly can reach the center of the source due to trapping by the multicusp magnetic field. As a result, H− ions cr...