Surface Production Of Negative Ions Research Articles

Langmuir-probe measurement through the plasma grid aperture of hydrogen negative ion source

The extraction physics of negative-ion sources is still unknown because of the complication of the plasma-beam boundary (meniscus), which includes electronegative plasma, the magnetic field, and a surface production of negative ions. This paper shows results from Langmuir probe measurements inside of the extraction aperture (EA) and the criteria for locating the meniscus. The criteria are defined by the appearance and disappearance of Maxwellian feature of plasma. The result gives direct information on the location and shape of the plasma-beam boundary.

Computationally studying H- extraction and beamlet formation: the impact of the plasma model

The ITER NBI requires H-/D- beamlets with a low divergence, to maximize the transmitted power through the beamline. Ion extraction and beamlet formation are typically studied with gun-type codes such as IBSimu, which do not treat the plasma explicitly. These codes neglect many physical processes, such as the surface production of negative ions, particle drifts in the plasma as a result of magnetic fields, and sheath formation at surfaces. 3D PIC simulations show that the Debye sheath, which forms between the plasma and the plasma grid, has an impact on the optics of negative ions extracted from the plasma. In this paper, a repelling potential around the plasma grid is implemented ad-hoc in the IBSimu plasma model. The impact of the sheath is investigated for the BUG-MLE grid system for negative ions that come from the plasma. An electron repelling sheath increases the H- extraction probability since fewer particles are lost on the plasma grid, however the extracted current density at the edge of the aperture is reduced. The sheath increases the perpendicular energy distribution of the extracted H-, because particles are reflected towards the extraction region. In the underperveant regime, the sheath decreases the divergence, since it prevents H- from being extracted at the edge of the aperture, and these particles are highly divergent in the underperveant regime. The divergence optimum is shifted towards lower extracted current density with the sheath. The sheath leads to a higher minimum divergence due to the increased perpendicular energy distribution in the extraction region and the changed current density distribution.

Study of the charged particle flow near the plasma grid in negative ion source

Flows of the charged particles in hydrogen (H) and deuterium (D) plasmas were measured with the single-tip directional Langmuir probe. The flow patterns of positive and negative ions have been constructed from the data by moving the probe near the plasma grid. The positive ion flow from driver to extraction regions showed the different velocity equivalent to the mass difference between H and D. The negative ion flow indicated the surface production of negative ions on the plasma grid, and changed the direction corresponding to the magnitude of the extraction electric field.

Towards the optimization of the Cs evaporation configuration for long pulse operation of negative ion sources

Negative ion sources for neutral beam injection rely on surface production of negative ions on a caesiated low work-function surface (plasma grid). To maintain the low work function in long pulses (one hour) and the desired source performance (extracted H-/D- ions and limited co-extracted electrons), Cs needs to be constantly delivered onto the plasma grid. The CsFlow3D code was applied to the RF driven negative ion source ELISE to simulate the evaporation and the plasma-assisted redistribution of Cs. The Cs flux stability is investigated for consecutive one hour plasma pulses and compared with experiments. The effect of different positions of the Cs oven for the case of ELISE equipped with two racetrack shaped RF drivers was investigated: a considerable increase of the Cs flux occurs when evaporating where the plasma is in contact with surfaces, as confirmed also by the back-tracking of the Cs particles reaching the PG. Additionally, an alternative evaporation method close to the plasma grid was designed with the help of CsFlow3D and tested in BATMAN Upgrade, with the aim of increasing the Cs flux onto the PG and achieve a better control of caesiation in long pulses.

Surface production of negative ions from pulse-biased nitrogen doped diamond within a low-pressure deuterium plasma

The production of negative ions is of significant interest for applications including mass spectrometry, materials surface processing, and neutral beam injection for magnetic confined fusion. Neutral beam injection sources maximise negative ion production through the use of surface production processes and low work function metals, which introduce complex engineering. Investigating materials and techniques to avoid the use of low work function metals is of interest to broaden the application of negative ion sources and simplify future devices. In this study, we use pulsed sample biasing to investigate the surface production of negative ions from nitrogen doped diamond. The use of a pulsed bias allows for the study of insulating samples in a preserved surface state at temperatures between 150 ∘C and 700 ∘C in a 2 Pa, 130 W, (ne ∼ 109 cm−3, Te ∼ 0.6 eV) inductively coupled deuterium plasma. The negative ion yield during the application of a pulsed negative bias is measured using a mass spectrometer and found to be approximately 20% higher for nitrogen doped diamond compared to non-doped diamond. It is also shown that the pulsed sample bias has a lower peak negative ion yield compared to a continuous sample bias, which suggests that the formation of an optimum ratio of defects on its surface can be favourable for negative ion production.

Enhancing surface production of negative ions using nitrogen doped diamond in a deuterium plasma

The production of negative ions is of significant interest for applications including mass spectrometry, particle acceleration, material surface processing, and neutral beam injection for magnetic confinement fusion. Methods to improve the efficiency of the surface production of negative ions, without the use of low work function metals, are of interest for mitigating the complex engineering challenges these materials introduce. In this study we investigate the production of negative ions by doping diamond with nitrogen. Negatively biased (−20 V or −130 V), nitrogen doped micro-crystalline diamond films are introduced to a low pressure deuterium plasma (helicon source operated in capacitive mode, 2 Pa, 26 W) and negative ion energy distribution functions are measured via mass spectrometry with respect to the surface temperature (30 °C to 750 °C) and dopant concentration. The results suggest that nitrogen doping has little influence on the yield when the sample is biased at −130 V, but when a relatively small bias voltage of −20 V is applied the yield is increased by a factor of 2 above that of un-doped diamond when its temperature reaches 550 °C. The doping of diamond with nitrogen is a new method for controlling the surface production of negative ions, which continues to be of significant interest for a wide variety of practical applications.

Pulsed DC bias for the study of negative-ion production on surfaces of insulating materials in low pressure hydrogen plasmas

In this work, negative-ion production on the surface of a sample negatively DC biased in a hydrogen plasma is studied. The negative ions created under positive ion bombardment are accelerated toward the plasma, self-extracted, and detected according to their energy and mass, by a mass spectrometer placed in front of the sample. The use of a pulsed bias allows applying a quasi-DC bias on insulating material during a short period of time and offers the possibility to extend the measurement method to nonconductive samples. The pulsed-bias tests were performed first with highly oriented pyrolytic graphite (HOPG), a conductive material, to demonstrate the feasibility of the method. By changing the pulsed-bias frequency, it was possible to obtain HOPG material with different hydrogen surface coverages and hence different surface states leading to an increase of negative-ion production by up to 30%–50% as compared to the continuous bias case. To establish a protocol for insulating materials, charge accumulation on the surface during the bias pulse and influence of the bias duration and frequency were explored using microcrystalline diamond (MCD) thin layers. By using a pulse short enough (10 μs) at 1 kHz frequency, it has been possible to measure negative-ions on the MCD sample at a quasi-constant surface bias of 130 V, with only 1 V variation during the measurement. Negative-ion surface production on MCD has been studied in pulsed mode with surface temperature from room temperature to 800 °C. It is shown that pulsing the bias and increasing the temperature allows limiting defect creation on MCD which is favorable for negative-ion production. Consequently, at 400 °C, the yield on MCD in pulsed mode is one order of magnitude higher than the yield on HOPG in continuous mode at room temperature.

Single-crystal and polycrystalline diamond erosion studies in Pilot-PSI

Diamond is a promising candidate for enhancing the negative-ion surface production in the ion sources for neutral injection in fusion reactors; hence evaluation of its reactivity towards hydrogen plasma is of high importance. Single crystal and polycrystalline diamond samples were exposed in Pilot-PSI with the D+ flux of (4‒7)·1024 m−2s−1 and the impact energy of 7–9 eV per deuteron at different surface temperatures; under such conditions physical sputtering is negligible, however chemical sputtering is important. Net chemical sputtering yield Y = 9.7·10−3 at/ion at 800 °C was precisely measured ex-situ using a protective platinum mask (5 × 10 × 2 μm) deposited beforehand on a single crystal followed by the post-mortem analysis using Transmission Electron Microscopy (TEM). The structural properties of the exposed diamond surface were analyzed by Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS). Gross chemical sputtering yields were determined in-situ by means of optical emission spectroscopy of the molecular CH A-X band for several surface temperatures. A bell-shaped dependence of the erosion yield versus temperature between 400 °C and 1200 °C was observed, with a maximum yield of ∼1.5·10−2 at/ion attained at 900 °C. The yields obtained for diamond are relatively high (0.5–1.5)·10−2 at/ion, comparable with those of graphite. XPS analysis shows amorphization of diamond surface within 1 nm depth, in a good agreement with molecular dynamics (MD) simulation. MD was also applied to study the hydrogen impact energy threshold for erosion of [100] diamond surface at different temperatures.

Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas

This paper deals with a study of H−/D− negative ion surface production on diamond in low pressure H2/D2 plasmas. A sample placed in the plasma is negatively biased with respect to plasma potential. Upon positive ion impacts on the sample, some negative ions are formed and detected according to their mass and energy by a mass spectrometer placed in front of the sample. The experimental methods developed to study negative ion surface production and obtain negative ion energy and angle distribution functions are first presented. Different diamond materials ranging from nanocrystalline to single crystal layers, either doped with boron or intrinsic, are then investigated and compared with graphite. The negative ion yields obtained are presented as a function of different experimental parameters such as the exposure time, the sample bias which determines the positive ion impact energy and the sample surface temperature. It is concluded from these experiments that the electronic properties of diamond materials, among them the negative electron affinity, seem to be favourable for negative-ion surface production. However, the negative ion yield decreases with the plasma induced defect density.

Development of negative ion extractor in the high-power and long-pulse negative ion source for fusion application

High power and long-pulse negative ion extractor, which is composed of the plasma grid (PG) and the extraction grid (EXG), is newly developed toward the neutral beam injector for heating and current drive of future fusion machines such as ITER, JT-60 Super Advanced and DEMO reactor. The PG is designed to enhance surface production of negative ions efficiently by applying the chamfered aperture. The efficiency of the negative ion production for the discharge power increased by a factor of 1.3 against that of the conventional PG. The EXG is also designed with the thermal analysis to upgrade the cooling capability for the long pulse operation of >1000 s required in ITER. Though the magnetic field for electron suppression is reduced to 0.75 of that in the conventional EXG due to this upgrade, it was experimentally confirmed that the extracted electron current can be suppressed to the allowable level for the long pulse operation. These results show that newly developed extractor has the high potential for the long pulse extraction of the negative ions.

Role of positive ions on the surface production of negative ions in a fusion plasma reactor type negative ion source—Insights from a three dimensional particle-in-cell Monte Carlo collisions model

Results from a 3D self-consistent Particle-In-Cell Monte Carlo Collisions (PIC MCC) model of a high power fusion-type negative ion source are presented for the first time. The model is used to calculate the plasma characteristics of the ITER prototype BATMAN ion source developed in Garching. Special emphasis is put on the production of negative ions on the plasma grid surface. The question of the relative roles of the impact of neutral hydrogen atoms and positive ions on the cesiated grid surface has attracted much attention recently and the 3D PIC MCC model is used to address this question. The results show that the production of negative ions by positive ion impact on the plasma grid is small with respect to the production by atomic hydrogen or deuterium bombardment (less than 10%).

Electric Potential in Surface Produced Negative Ion Source with Magnetic Field Increasing Toward a Wall

Electric potential near a wall for plasma with the surface produced negative ions with magnetic field increasing toward a wall is investigated analytically. The potential profile is derived analytically by using a plasma-sheath equation, where negative ions produced on the plasma grid (PG) surface are considered in addition to positive ions and electrons. The potential profile depends on the amount and the temperature of the surface produced negative ions and the profile of the magnetic field. The negative potential peak is formed in the sheath region near the PG surface for the case of strong surface production of negative ions or low temperature negative ions. As the increase rate of the magnetic field near the wall becomes large, the negative potential peak becomes small.

Negative-ion surface production in hydrogen plasmas: modeling of negative-ion energy distribution functions and comparison with experiments

A negatively biased graphite sample (highly oriented pyrolitic graphite) was placed in a H2 low-pressure plasma. The negative ions were produced on the graphite surface upon positive-ion bombardment. Surface-produced H− negative-ion distribution functions (NIDFs) were measured by means of an energy-resolved mass spectrometer. The shapes of the recorded NIDFs depend not only on the surface production mechanisms but also on the negative-ion trajectories in the plasma and their collection probability by the mass spectrometer. In order to gain an insight into the surface production mechanisms, NIDFs were computed using Stopping and Range of Ions in Matter simulations and calculations of the ion transmission function between the sample and the mass spectrometer detector. The calculations were based on 3D modeling of the sheath potential in the extraction region and 3D modeling of ion transport inside the mass spectrometer. The excellent agreement between experiments and computations led to a better understanding of the experimental NIDFs. The method developed in this work to study H-surface production on graphite can be generalized to any other negative ions and/or surface material.

Effect of fast positive ions incident on caesiated plasma grid of negative ion source

This paper describes the effect on negative ion formation on a caesiated surface of the backscattering of positive ions approaching it with energy of a few tens of eV. For a positive ion energy of 45 eV, the surface produced negative ion current density due to these fast positive ions is 12 times larger than that due to thermal atoms, thus dominating the negative ion surface production instead of the thermal atoms, as considered until now.

Negative ion surface production through sputtering in hydrogen plasma

A graphite sample is negatively biased in a hydrogen plasma. H− ions created on the surface are accelerated by the sheath toward a mass spectrometer where they are detected according to their energy. The ion distribution function shows two contributions: a main peak followed by an energetic tail. It was previously proven backscattering of positive ions as negative ions is responsible for the high energy tail. In this paper we demonstrate 75% of negative ion surface production is coming from sputtering of adsorbed hydrogen atoms as negative ions. The positive ion energy threshold for this mechanism is around 20 eV and its maximum around 70 eV.

Negative ion production in cesium seeded high electron temperature plasmas

In experiments on uniformity improvement in a large negative ion source, steep gradients have been observed in the profiles of electron temperature and H(-) ion beam intensity. It has been observed that the gradient in the H(-) ion beam intensity is altered by seeding cesium, though the electron temperature distribution is not affected by Cs. Thus in the Cs seeded condition, the H(-) ion beam intensity is enhanced in local area illuminated by high electron temperature plasmas. A brief analysis suggests possible advantages of high electron temperature plasmas for the negative ion surface production, by enhancement of dissociation to yield proton or atoms as parent particles of the negative ions.

Negative Ion Production in High Electron Temperature Plasmas

In our recent experiment, it was found that intense negative ion beams could be extracted from high-density and high-electron temperature plasmas under a Cesium seeded condition. This indicates that such plasmas contribute to the surface production of negative ions, and that the negative ion production exceeds the destruction process via electron detachment. Thus, it has been suggested that highly ionized and dissociated plasmas are suitable for the surface production of negative ions. This paper reports the results of specific experiments to confirm the negative ion production in plasmas with high-electron temperature. In these experiments, high electron temperature plasmas were produced by (1) placing filaments (acting as cathodes) near the extraction area, and (2) reducing the magnetic filter strength, both to allow fast electrons access to the beam extraction area. The results support the production of high-density H- ions even in plasmas with high-electron temperature. Also included is a discussion on the surface production mechanism of negative ions under a Cs-seeded condition.

1 MeV, ampere class accelerator R&D for ITER

The present objective of the 1 MeV electrostatic accelerator R&D for the ITER NB system is the acceleration of ampere class negative ion beams at the required current density (200 A m−2) up to the beam energy of 1 MeV. To produce such high current density H− ions, the KAMABOKO negative ion source was operated under high power arc discharge (≤40 kW) and with Caesium seeding for enhancement of the negative ion surface production. The H− ion beams of 146 A m−2 (total ion current: 0.2 A) have been obtained stably at the beam energy of 836 keV (pulse length: ≥0.2 s). The pulse length was limited due to the power density of the beams, which is more than twice as high as those of the existing negative ion based NB systems (JT-60U and LHD). A preliminary measurement of beam optics showed a beamlet divergence of ≈5 mrad even in the beams of such high power density. The Bremsstrahlung generation as a consequence of electron acceleration was estimated. A discussion then follows for photoelectron production and possible breakdowns in insulators for the acceleration of higher current negative ions.

NBI:核融合炉に向けた物理•工学の進展 5.核融合炉用負イオン源•加速器の開発の現状

This article reviews R&D progress of a MeV accelerator and a large negative ion source at the Japan Atomic Energy Research Institute (JAERI). The JAERI MeV accelerator at present aims at acceleration of high current density (200 A/m2) H- ion beams up to MeV range energy. According to the progress of vacuum insulation technology, the MeV accelerator sustained 1 MV for more than two hours stably, followed by beam acceleration experiments of high current density. So far, H- ions beams of 146 A/m2, 836 keV have been obtained, which is the world's first demonstration of “ITER class” high power density beams. Recent investigation of negative ion uniformity in a large negative ion source revealed fast electron leakage through a magnetic filter, followed by destruction of negative ions in the local extraction region during pure volume (without Cs) operation. On the other hand, the leaked fast electrons enhance surface production of negative ions locally under Cs seeded operation. This implies possible high current density and uniform negative ion production by a high-density plasma in the magnetic configuration like positive ion sources, to be coupled with RF driven plasma production for an NB system of a fusion DEMO reactor.

Analysis of plasma dynamics of a negative ion source based on probe measurements

Measurements and analysis of the plasma flow in an ion source made for negative ion extraction are reported in this article. The plasma flow has been measured using a Mach probe having two orthogonal probe heads. The plasma flow along the axis is driven by the electron pressure gradient, dragging along the ions via a measured ambipolar electric field against the collisional drag on the background gas. The force on the ions created by the electric field is mainly balanced by the collisional drag force. The collision between the ions and the background gas creates a pressure gradient along the flow direction. The one-dimensional plasma dynamic analysis supports the consistency of the experimental observations. The presence of a transverse magnetic filter reduces the plasma flow velocity, which could affect the negative ion production on the cesiated grid surface. A simple analysis shows that a strong plasma flow could enhance the surface production of negative ions.