Search

A compact ECR ion source was used to investigate the effect that the shape of the plasma electrode (PE) has on the surface production of negative hydrogen (H−) ions. A C12A7 electride PE produced ten times more H-ion current compared with a molybdenum (Mo) PE of the identical design. Four times more H- ion current was obtained when the ion source plasma faces the 45-degree tapered hole opened on the surface of the electride PE as compared to the current obtained from a plasma facing the flat side of the PE. This enhancement in H- ion current may indicate larger surface production of H− ions by the enlarged surface area. A Mo PE also showed a similar effect but with less pronounced H- ion current enhancement. A beam trajectory study showed that the beam divergence was larger when the source plasma faced against the tapered side of the PE. [ABSTRACT FROM AUTHOR]

Key issues to fabricate a beam extraction system of a H- source with a plasma electrode (PE) made from C12A7 electride which has low work function were studied. Preliminary study of the H- extraction using a compact ECR ion source reveals, that high H- current can be extracted from a source without PE bias of high voltage. A bucket type with ECR source is being designed. Beam simulation for the extraction through the designed electrodes has been carried out using the OPERA code. It predicts that the optimum perveance is 0.6 mA/kV1.5, optimum acceleration to extraction voltage ratio Vacc/Vext is 4, with the minimum beam divergence of 9 mrad, which are enough to demonstrate the capability to produce a high H- current density. [ABSTRACT FROM AUTHOR]

A thermal beam emittance measurement method is developed specifically to measure high power density beams for the ITER neutral beam injection system (NBI)’s. A one-dimensional carbon-fiber composite (CFC) target is utilized to withstand the high current densities of the negative hydrogen beam making the beam emittance measurement possible. Schemes for the emittance measurement were discussed since beam instabilities are present in various stages of the beam phase. The allowable time for the measurement will be less than 2 seconds and it’s necessary to monitor the CFC temperature to not exceed 2000°C. The proposed measurement scheme to minimize beam instabilities is through the constant operation of the arc discharge and pulsed beam extraction. Mechanical structures are also being designed for rapid measurements to select different phases of the beam and for implementing the measurement system to long pulse beam operation. [ABSTRACT FROM AUTHOR]

The next generation of compact tandem-type DD or DT neutron generators requires a robust electron stripper with high charge exchange efficiency. In this study, stripping foils of various types were tested, and the H- to H+ conversion efficiency, endurance to the heat load, and durability were investigated in terms of suitability in the tandem-type neutron generator. In the experiments, a H- beam was accelerated to about 180 keV, passes through a stripping foil, and produces a mixed beam of H-, H0, and H+. These ions were separated by an electric field, and detected by a movable Faraday cup to determine the conversion efficiency. The experimental results using thin foils of diamond-like carbon, gold, and carbon nano-tubes revealed issues on the robustness. As a new concept, a H- beam was injected onto a metal surface with an oblique angle, and reflected H+ ions are detected. It was found that the conversion efficiency, H+ fraction in the reflected particles, depends on the surface condition, with the maximum value of about 90%. [ABSTRACT FROM AUTHOR]

The study of VUV emission from a hydrogen plasma produced in a filament discharge in a magnetic multicusp device showed that the use of tantalum and tungsten filaments leads to significant differences in the spectra. The effect of the filament material is interpreted in terms of the fresh film of this material, deposited on the wall. The synthetic spectrum convoluted with our apparatus function for the conditions of this experiment (gas temperature 500 K, electron energy 100 eV) agrees roughly well with the spectrum obtained with tungsten covered walls, but not with the spectrum obtained with tantalum covered walls. We show that in the case of tungsten covered walls the E-V singlet excitation is indeed a two-step Franck-Condon transition, going through either B or C state from an initial H[sub 2] molecule with v'=0, added to a Franck-Condon transition to highly excited states cascading to the B or C states. The excitation process to high v' states in the case of tantalum covered walls is a three step process, in which the first step is the formation by recombinative desorption on the wall of a vibrationally excited molecule with v'=1 or 2, which serves as the initial molecule in the subsequent E-V excitation through the B state. The results indicate a larger recombination coefficient of atoms on the tantalum covered wall. [ABSTRACT FROM AUTHOR]

Negative hydrogen (H-) ion density near the plasma electrode (PE) depends upon the materials which are absorbed the PE surface. Tungsten and tantalum were compared as adsorbates on the PE by means of evaporation from the filaments made of these elements in a hydrogen discharge plasma. The difference of the filament material against the plasma electrode bias voltage showed different characteristics of the extraction current and that of the H- ion current. For the tantalum filament operation, the photodetachment signal showed a dependence upon the PE bias similar to the one for the extraction current, while the tungsten filament operation indicated a plateau near the anode potential. In this paper, the possible reasons for observing the difference between W and Ta filament operations are discussed. [ABSTRACT FROM AUTHOR]

For an unique experimental approach to study low-energy electron transport in a plasma, a "sub" filament was introduced in a volume-produced, DC filament-driven hydrogen negative ion (H-) source as an additional electron source. The sub-filament engages in the special task which introduces additional electrons into the plasma with controlling injection energies. Transports of the additional electrons are observed by the difference of the electron density profiles with and without the electron introduction. The experimental result here indicates that the electron transport is strongly restricted by the filter magnetic field. On the other hand, analysis with Particle-In-Cell simulation seems to imply that the restriction by the magnetic field can be changed by the controlling electron energy in the sub-filament experiments for the research of the electron transport. [ABSTRACT FROM AUTHOR]

Exposure to Cs added hydrogen discharge makes surface of plasma grid of a negative hydrogen ion source covered with Cs and hydrogen. A Monte-Carlo particle simulation code ACAT was run to evaluate the effects due to adsorbed Cs and H atoms upon the angular and energy distributions of H atoms leaving the surface. Accumulation of H atoms on the surface reduces particle reflection coefficients and the mean energy of backscattered H atoms. Angular distributions of H atoms reflected from the hydrogen covered surface tend to be under-cosine at lower energies. Desorption of adsorbed H atoms is more efficient for hydrogen positive ions than for Cs positive ions at lower incident energy. At higher energy more than 100 eV, Cs ions desorb adsorbed H atoms more efficiently than hydrogen ions. [ABSTRACT FROM AUTHOR]

A test stand for the proof of principle experiments of He0 beam production has been constructed aiming at development of an alpha particle diagnostic system in next generation fusion reactors. The characteristics of a He- beam produced in Lithium vapor has been studied. With an increase in the vapor pressure in the cell, mitigation of space charge in a He+ beam and bunching of a He- beam which travels part of the way with the He+ beam have been observed. [ABSTRACT FROM AUTHOR]

A probing He0 beam for confined alpha particle measurement using a double charge exchange process is now under development. A proof of principle experiment for ground-state He0 beam production will be performed on a test stand. Several methods are developed to measure the metastable fraction of a He0 beam. A full-size strong-focusing He+ source has been constructed and sufficient beam current was achieved with a beam size tolerable to be used on ITER. [ABSTRACT FROM AUTHOR]

The ICRF heating in toroidal plasmas is studied using two global simulation codes: a full wave field solver TASK/WM and a drift kinetic equation solver GNET. The codes are applied to both tokamaks and helical systems. The full wave code TASK/WM evaluates the realistic wave electric field, in which the effect of the self-consistent non-Maxwellian velocity distribution on the wave propagation is taken into account. GNET solves a linearized drift kinetic equation (5D phase-space) for energetic ions including complicated behavior of trapped particles in helical systemsCharacteristics of energetic ion distributions in the phase space are investigatedSelf-consistent analysis including the effect of energetic ion distribution on the fast wave propagation is also reported for tokamak plasmas. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The confinement of high-energy particles produced by ICRF heating was studied in LHD experimentally setting magnetic axis at 3.6m and 3.75m in two methods. It was concluded that the confinement of high-energy particles with Rax=3.6 m was better than that with Rax=3.75m. Based on this result the long pulse experiments were conducted in the magnetic configuration of Rax=3.6m. The plasma was sustained for 150sec. The total input power reached 71MJ though the pulse length was limited by the collapse of plasma due to density increase. The second harmonic experiment was done also with Rax=3.6m. The stored energy increased and the particles with the high-energy were observed. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

The correlation between the work function of the plasma electrode of a H[sup -] ion source and H[sup -] density was investigated in H[sub 2] plasma, which contains the vapor of an alkali metal (cesium or rubidium). For the work function measurement, the photoelectric current was measured with laser irradiation to the plasma electrode, while the negative ion signal was measured by the laser photodetachment technique simultaneously with an electrostatic probe, As the work function of the plasma electrode was reduced due to alkali metal adsoption, the ratio of H[sup -] density to electron density, n[sub -]/n[sub e], increased gradually. The n[sub -]/n[sub e] ratio became the maximum value when the work function reached the minimum value. The measured work function was well correlated with the n[sub -]/n[sub e] ratio. The dependence of plasma parameters on the discharge current is compared between pure H[sub 2] and H[sub 2] + (Rb/Cs) mixture discharge. [ABSTRACT FROM AUTHOR]

The effect of the cesium on hydrogen negative ions is discussed using the vacuum ultraviolet emissions (VUV) from vibrational states of hydrogen molecules. The VUV spectrum from 90 to 165 nm is related to the H- production from the dissociative attachment of electrons to hydrogen molecules. The VUV spectra in hydrogen plasmas are measured in the extraction region of a negative ion source for neutral beam injector. Under the pure hydrogen discharge, the cesium vapor is introduced into the ion source to enhance the hydrogen negative ion density. When the same arc power of ∼99 kW is applied in both with and without cesium admixture cases, the ratio of the observed VUV spectrum without to with cesium is not changed clearly. Therefore the production process of H- related to the wall/electrode surface would be enhanced rather than the volume production process. [ABSTRACT FROM AUTHOR]

We compared the effect of a tantalum (Ta) fresh coating with that of a Ta thin sheet covering the plasma electrode of a negative hydrogen ion source on the ratio of extracted negative hydrogen (H−) ion current to extracted electron current. Fresh Ta evaporation from a thin hot Ta filament showed about a 22 % increase of extracted H− ion current compared to the operation with a tungsten (W) hot filament. On the other hand, a thin Ta foil covering the plasma electrode decreased the extracted electron current without a large influence on H- ion current. Meanwhile, when a titanium (Ti) foil covered the plasma electrode, both the extracted H− ion current and electron current decreased substantially. With a plasma electrode bias higher than the plasma potential, the ratio of H- ion current to the electron current for the Ti covered plasma electrode exceeded 80%. [ABSTRACT FROM AUTHOR]

In the last decade, a series of high power neutral beam injectors for plasma heating and diagnostics in magnetic fusion experiments has been developed at the Budker Institute of Nuclear Physics. In the injectors, the plasma emitter is produced by inductively coupled radio frequency discharge. With these emitters of different size, positive ion hydrogen and deuterium beams with energy up to 60 keV and power up to 1.6 MW can be provided. In the last few years, the negative ion sources with the RF drivers have been developed for the injector with higher energies. This paper presents an overview of the design, parameters and performance of the developed plasma drivers. [ABSTRACT FROM AUTHOR]

The NIO1 (Negative Ion Optimization 1) source is developing a systematic test programme of magnetic configurations, including permanent magnet filter up to 15 mT dipole strength, tunable current filter up to 3 mT strength (parallel or perpendicular to the previous), and ion deflection compensation system in the extraction gaps. Results for both perpendicular and parallel configurations are presented. The radiofrequency (rf) system takes full advantage of the programmable frequency amplifier capability, to start the plasma and later to optimize coupling with plasma. Operation experience with borosilicate rf window and improved power limit are reported, as well breakdown voltages Vbr are discussed, together with improvements obtained with the installation of a cryogenic pump. The interrelation between bias voltage, bias plate current and magnetic configuration is outlined; cesium evaporators are in advanced development. Major diagnostic systems (including cameras and emission spectrometers) have been integrated with the acquisition system along with the data measured by several power supplies (including the high voltage supplies, the rf generator and bias supplies, the pressure measurements and the beam currents): among the several diagnostic used, the role of simple current measurement and some optical observation are noted. [ABSTRACT FROM AUTHOR]

The intensity of negative hydrogen (H-) ion current and that of electron current extracted from a negative ion source show different characteristics against the change in plasma electrode bias depending upon the material covering the plasma electrode surface. The knowledge of these characteristics is of importance for a proper design of an efficient H- ion source. This paper discusses this subject based upon two kind of experiments i) experiments with plasma electrodes covered with tungsten or tantalum evaporated from filaments made of these metals ii) experiments with caesium covered plasma electrode. The tantalum and caesium covered plasma electrodes lead to an enhancement of the extracted H- ion current by a factor of 2 compared to the tungsten coverage on the plasma electrode. The electron current is also affected by the material covering the plasma electrode. The reasons for observing these characteristics are also elucidated. [ABSTRACT FROM AUTHOR]

The key parameter for the performance of negative hydrogen ion sources based on surface conversion is the work function of the converter surface. In order to enhance the negative ion yield caesium is introduced into the source which lowers the converter work function upon adsorption. However, the Cs layer is subject to background pressures of 10-7-10-6 mbar and to H2 or D2 low-temperature plasmas which can have a major impact on the resultant work function. In order to determine the work function of caesiated surfaces under ion source relevant conditions and to identify influencing parameters, systematic investigations are performed at the laboratory experiment ACCesS. It is confirmed, that the work function of a sample surface in a vacuum of 10-6 mbar is decreased upon the evaporation and adsorption of caesium. However, the adsorbed layer is determined by the formation of Cs compounds with residual gases from the background pressure leading to an increased work function by about 0.6 eV compared to pure Cs layers. Elevated surface temperatures are beneficial for cleaning and against degradation in absence of Cs evaporation, but no further enhancement of the work function can be achieved by temperatures up to 260 °C if a Cs flux onto the surface is already present. Thus, the minimally achievable work function of a stainless steel surface in ion source relevant vacuum conditions is 2.75 eV. The influence of the substrate material and plasma surface interaction will be investigated next. [ABSTRACT FROM AUTHOR]

We have demonstrated an optical parametric chirped pulse amplification (OPCPA)/Yb:YAG ceramic thin disk hybrid laser system having hundred mJ level pulse energy sub-picosecond pulse duration with high temporal contrast. At an input chirped-pulse energy of 3.8 mJ from an OPCPA preamplifer an output energy of 130 mJ has been generated from multipass laser diode-pumped Yb:YAG ceramic thin disk amplifier. A recompressed pulse duration of 450 fs with a contrast level of less than 7.2×10-9 at -150 ps before the main pulse has been obtained. The contrast level is the highest value achieved in Yb:YAG chirped pulse amplification (CPA) laser system with hundred mJ level. [ABSTRACT FROM AUTHOR]

A systematic study of the extraction probability of surface-produced H- ions from the ion source has been done with H- and neutral Monte-Carlo transport modeling. Without the effects of the flow velocity of background H+ ions and neutrals, the extraction probability (E.P.) depends strongly on the strength of the magnetic filter field BMF for the low gas pressure condition. On the other hand, the E.P. does not depend so much on BMF for the high gas pressure condition. Neutral transport modeling shows that H2 neutral possibly has a relatively high velocity (∼1 km/s). The effect on the E.P., however, is not significant even for the high gas pressure regime. [ABSTRACT FROM AUTHOR]

We have been developing a negative hydrogen ion (H- ion) source for a spatial beam profile monitor of a high intensity positive ion beam as a new diagnostic tool. In case of a high intensity continuous-wave (CW) deuteron (D+) beam for the International Fusion Materials Irradiation Facility (IFMIF), it is difficult to measure the beam qualities in the severe high radiation environment during about one-year cyclic operation period. Conventional techniques are next to unusable for diagnostics in the operation period of about eleven months and for maintenance in the one-month shutdown period. Therefore, we have proposed an active beam probe system by using a negative ion beam and started an experimental study for the proof-of-principle (PoP) of the new spatial beam profile monitoring tool. In this paper, we present the status of development of the H- ion source as a probe beam source for the PoP experiment. [ABSTRACT FROM AUTHOR]

The roles of fusion products in diagnosing plasmas are quite diversified and rich in scientific content. From the early days to the new frontier of burning plasmas, the study of fusion products has allowed a deeper understanding of the physics found in these plasmas, spanning all configurations, in both magnetic and inertial confinement. In many early experiments, the measurement of plasma performance was based almost entirely on neutron yield measurements. Since then, the capability to measure fusion products has grown tremendously, and many new measuring techniques have been successfully tested and used in experiments. The first role is based on single-particle diagnosis, which can yield important information on performance, confinement, source and current profiles, plasma fluctuations, effects of ripples, MHD mode structure, etc. The second role, which has expanded tremendously in the last few years, includes the diagnosis of collective effects, which can yield additional information on instabilities, heating, and many others. This scientific capability is enhanced by the large number of fusion products (neutrons, charged particles, gammas) generated by the primary or secondary fusion processes such as the commonly studied, D+D, D+T and D+3He reactions. This paper will review the applications of fusion product measurements used in the scientific study of plasma confinement and stability, and will describe the prospects and opportunities offered in burning plasma experiments such as ITER. [ABSTRACT FROM AUTHOR]

We studied the negative ion current extracted from the plasma created by seven elementary ECR sources, operating at 2.45 GHz, placed in the magnetic multipole chamber “Camembert III”. We varied the pressure from 1 to 4 mTorr, with a maximum power of 1 kW and studied the plasma created in this system by measuring the various plasma parameters, including the density and temperature of the negative hydrogen ions. We found that the electron temperature is optimal for negative hydrogen ion production at 9.5 cm from the ECR sources. The tantalum-covered wall surface pollution reduces the extracted negative ion current and enhances the electron current. Tantalum evaporation has a positive effect. The use of a grid and of a collar in front of the plasma electrode did not lead to any enhancement of the extracted negative ion current. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]