D. Boilson

Plasma diagnostic tools for optimizing negative hydrogen ion sources

The powerful diagnostic tool of optical emission spectroscopy is used to measure the plasma parameters in negative hydrogen ion sources based on the surface mechanism. Results for electron temperature, electron density, atomic-to-molecular hydrogen density ratio, and gas temperature are presented for two types of sources, a rf source and an arc source, which are currently under development for a neutral beam heating system of ITER. The amount of cesium in the plasma volume is obtained from cesium radiation: the Cs neutral density is five to ten orders of magnitude lower than the hydrogen density and the Cs ion density is two to three orders of magnitude lower than the electron density in front of the grid. It is shown that monitoring of cesium lines is very useful for monitoring the cesium balance in the source. From a line-ratio method negative ion densities are determined. In a well-conditioned source the negative ion density is of the same order of magnitude as the electron density and correlates with ...

Some lessons from long pulse operation of negative ion sources and accelerators

Operation of the neutral beam system of ITER for the entire ITER pulse is foreseen, with pulse lengths extending up to 1 h. Operation for such a pulse length is entirely new for neutral beam systems and the associated sub systems. This paper reviews some of the experience gained so far in the operation of the MANTIS test bed where long pulse operation of the ITER reference type of negative ion source is being studied. The three identified adverse effects of long pulse operation—reduced negative ion yield, reduced plasma grid temperature effect (see below) and increased caesium consumption—are still being studied, but some tentative explanations for each effect are given. In addition to the aforementioned effects, some operational difficulties associated with long pulses will be discussed, and some areas where caution will be needed in the future long pulse, high power operation of the ITER injectors are indicated.

Long pulse operation of the KAMABOKO III negative ion source

The target performance for the KAMABOKO ion source on the MANTIS test bed in Cadarache is to accelerate a beam of D− with a current density of 200 A/m2 and <1 extracted electron per accelerated D− ion, at an injected power ranging between 1 and 2 kW per liter of source volume, at a source pressure of 0.3 Pa. For ITER, a continuous neutral beam must be assured for pulse lengths of 500 s, but beams of up to 3600 s are also envisaged. During the last campaign, continuous beam pulses of duration up to 1000 s were demonstrated both in hydrogen and in deuterium. In this article, the source performance, the effect of the plasma grid temperature for long pulse operation, and the limits of the present experimental setup are described. Additionally, the effect of changing the strength of the magnetic filter in the ion source on the extracted ion and electron currents and the beam transmission is reported.

Voltage holding and dark currents in the Cadarache 1 MV ion beam facility

We report our experience in obtaining high voltages up to 1 MV required for the acceleration of D/sup -/ beams. We find that satisfactory voltage holding at this level can be obtained by adding gas up to a few 10/sup -4/ mb. Obtaining the required voltage at lower pressures is made cumbersome by the appearance of leakage (or dark) currents. Since the reduction of the dark current by conditioning is very time consuming the most practical remedy is the reduction of the cathode and anode surfaces of the system.

Characterization of the ITER model negative ion source during long pulse operation

It is foreseen to operate the neutral beam system of the International Thermonuclear Experimental Reactor (ITER) for pulse lengths extending up to 1h. The performance of the KAMABOKO III negative ion source, which is a model of the source designed for ITER, is being studied on the MANTIS test bed at Cadarache. This article reports the latest results from the characterization of the ion source, in particular electron energy distribution measurements and the comparison between positive ion and negative ion extraction from the source.

Latest results from the Cadarache 1 MV SINGAP experiment

Abstract The SINGle Aperture—SINgle GAP (SINGAP) acceleration concept has been developed as a simplified alternative to the Multi-Aperture, Multi-Grid (MAMuG) acceleration of the ITER Neutral Beam reference design. The objective of the present experiments is to demonstrate reliable multi-second acceleration of a D − beam to 1 MeV (∼100 mA, 200 A/m 2 ), relevant to the ITER neutral beam injection (NBI) requirements. During the previous studies it has been demonstrated that the SINGAP concept works and good quality 860 keV H − beams (43 mA, 1 s) and 630 keV D − beams (106 mA, 1 s) have been produced. In the present R&D programme another attempt has been made to reach the full beam energy and increase the extracted beam current density with a refurbished 1 MV epoxy insulator/bushing. Refurbishment had become necessary after the top two of the nine epoxy rings making up the insulator had been perforated and carbonised due to high voltage discharges. The best results achieved during the present campaign are 914 keV D − beams (58 mA, 1 s) with a D − current density of 30 A/m 2 and 600 keV D − beams (76 mA, 1 s) with a D − current density of 70 A/m 2 . These results are world records. The beam optics is largely as expected from the modelling with the Vector Fields OPERA 3D code.

Power Transmission From The ITER Model Negative Ion Source

In Cadarache development on negative ion sources is being carried out on the KAMABOKO III ion source on the MANTIS test bed. This is a model of the ion source designed for the neutral beam injectors of ITER. This ion source has been developed in collaboration with JAERI, Japan, who also designed and supplied the ion source. Its target performance is to accelerate a D− beam, with a current density of 200 A/m2 and <1 electron extracted per accelerated D− ion, at a source filling pressure of 0.3 Pa. For ITER a continuous ion beam must be assured for pulse lengths of 1000 s, but beams of up to 3,600 s are also envisaged. The ion source is attached to a 3 grid 30 keV accelerator (also supplied by JAERI) and the accelerated negative ion current is determined from the energy deposited on a calorimeter. During long pulse operation (⩽1000 s) it was found that the current density of both D− and H− beams, measured at the calorimeter was lower than expected and that a large discrepancy existed between the accelerated currents measured electrically and those transmitted to the calorimeter. The possibility that this discrepancy arose because the accelerated current included electrons (which would not be able to reach the calorimeter) was investigated and subsequently eliminated. Further studies have shown that the fraction of the electrical current reaching the calorimeter varies with the pulse length, which led to the suggestion that one or more of the accelerator grids were distorting due to the incident power during operation, leading to a progressive deterioration in the beam quality.. New extraction and acceleration grids have been designed and installed, which should have a better tolerance to thermal loads than those previously used. This paper describes the measurements of the power transmission and distribution using these grids.

Experimental Results with the New ITER‐like 1 MV SINGAP Accelerator

A new “ITER‐like” accelerator, which is a scaled down version of the ITER SINGAP (SINgle GAP, SINGle APerture) accelerator, has been built and installed on the Cadarache 1 MV test bed. The objective is to demonstrate reliable D− beam acceleration as close as possible to 1 MeV with a current density j− ≈ 200 A/m2 with the beam optics required for ITER, i.e. a beamlet divergence of ⩽7 mrad and beamlet steering within ±2 mrad of that specified. High voltage hold off tests have been performed and 940 kV has been held without breakdowns. The first beams up to 850 keV (D−, 15 A/m2) have been obtained after 4 weeks of experiments and the highest current density that has been obtained so far is 85 A/m2 (D−, 580 keV).

Effect of argon seeding on the negative ion yield of the Kamaboko III ion source

Abstract It has been previously reported that the addition of argon to a hydrogen plasma in an RF driven ion source can substantially increase (up to a factor 4) the extracted and accelerated negative ion (H−) current (W. Kraus et al., Development of large radio frequency negative-ion sources for nuclear fusion, Rev. Sci. Instrum. 73(2) (2002)). Realizing such an increase in the filamented arc discharge negative ion sources used for neutral beam injection systems would have significant benefits. Unfortunately the reported studies of argon addition to filamented sources have not shown a similar gain, but so far these have been carried out with arc powers and plasma densities far from those typical of the plasma in the negative ion sources used on neutral beam injectors (N. Nishiura et al., Cooling effect of hydrogen negative ions in argon gas mixture, Rev. Sci. Instrum. 73(2) (2002); N. Curran et al., The effect of the addition of noble gases on H- production in a dc filament discharge in hydrogen, Plasma Scources, Sci. Technol. 9 (2000)). The Kamaboko III ion source operates at the pressure and plasma density close to those anticipated in the ion source proposed for the ITER neutral beam injectors. Measurements have been made of the plasma density, electron temperature and the negative ion yield as a function of the argon seeding rate. The plasma parameters are determined with a fast spatially scanning Langmuir probe system. The effect on the H− yield is determined from the effect on the current extracted and accelerated from the source. Data will be presented for source filling pressures between 0.1 and 0.5 Pa of hydrogen, additions of argon from 0 to 30%, and a discharge power of 38 kW. All these data are collected in pure volume non-caesiated discharges. Some increase in the extracted H− yield is measured for small percentage additions of argon, (0–20%), but only at the highest H2 pressure used, 0.5 Pa.

First Simulations of the Cadarache SINGAP Experiments

The SINGAP (SINGle APerture ‐ SINgle GAP) acceleration concept is a simplified alternative to the multi‐aperture, multi‐grid acceleration of the ITER Neutral Beam reference design. Our project aims to demonstrate reliable multi‐second acceleration of a D− beam to 1 MeV (∼100 mA, 200 A/m2), relevant to the ITER Neutral Beam Injection requirements and to validate the predictions of the simulation codes used to design the SINGAP accelerator for ITER. The present campaign achieved (911 keV, 30A/m2) and (600 keV, 60 A/m2) D− beams. The highest space charge effects have been obtained with 400 keV, 50 A/m2 D−, which, although still a factor of 2.5 below the ITER value for space charge, is beginning to test the space charge aspects of the codes. Simulation results are compared with the experimental data for a variety of cases.