Large Negative Ion Source Research Articles

Development of a 500 keV, 22 A D− ion source for the neutral beam injector for JT-60U

The first results of the performance test of the large negative ion source for a JT-60U negative-ion-based neutral beam injector (N-NBI) are presented. The ion source consists of a cesium seeded multicusp plasma generator, where negative ions are produced via volume and surface processes, a 110 cm×45 cm multiaperture extractor, and a three-stage electrostatic accelerator. After negative ion production and voltage holding tests in test stands, the ion source was installed in the N-NBI system and the full power test began. Up to now, the ion source has produced 400 keV, 5.9 A (2.4 MW) D− ion beams, the world highest D− current and beam power, with a pulse duration of 0.1 s.

Investigation of a large volume negative hydrogen ion source

The electron and negative ion densities and temperatures are reported for a large volume hybrid multicusp negative ion source. Based on the scaling laws an analysis is made of the plasma formation and loss processes. It is shown that the positive ions are predominantly lost to the walls, although the observed scaling law is n+∝I0.57d. However, the total plasma loss scales linearly with the discharge current, in agreement with the theoretical model. The negative ion formation and loss is also discussed. It is shown that at low pressure (1 mTorr) the negative ion wall loss becomes a significant part of the total loss. The dependence of n−/ne versus the electron temperature is reported. When the negative ion wall loss is negligible, all the data on n−/ne versus the electron temperatures fit a single curve.

Improvement of the large current negative hydrogen ion source for neutral injection in the large helical device

Abstract A large negative hydrogen ion source with a rod-type magnetic filter was investigated for the construction of a neutral beam injection (NBI) system for the large helical device (LHD). A 1 3 - scale device was constructed and investigated. Three main phenomena are observed on seeding a small amount of cesium into the ion source. These are enhancement of the hydrogen negative ion (H−) current, a reduction in the electron beam extracted with the H− beam, and a decrease in the operating gas pressure. The enhancement factor of the H− current is 3–4, and a total H− current of 8.5 A is obtained from 20 × 40 extraction apertures 9 mm in diameter. This current corresponds to a current density of 17 mA cm−2. A multi-wall structure forming a lattice, called an egg-box cell, is installed between the magnetic filter rod and the plasma grid. The electron current with this cell is half as low as the electron current without the cell, but the H− current is hardly affected. This phenomenon is observed when cesium is seeded in the source chamber. The electron temperature and its density increase by a factor of 2 on strengthening the cusp field. Experiments to obtain a higher H− current density were carried out using the improved are plasma. A hydrogen negative ion current of 54 mA cm−2 was extracted from a restricted aperture of 36 extraction holes. The H− current was extracted from 20 × 40 full apertures of the ion source with strong cusp field. A total H− current of 10.4 A was obtained from these apertures. The optimum filling pressure is less than 1.0 Pa.

Development of high-power heating for large helical device

Abstract Three high-power heating mechanisms, i.e. electron cyclotron resonance heating, ion cyclotron resonance heating and neutral beam injection heating will be available to achieve operation of the Large Helical Device in the (i) high nτT mode, (ii) high T i mode and (iii) high β mode. The requirements for each heating method are (i) 10 MW of ECH at a frequency of 84 GHz with a pulse length of 10 s, (ii) 3 MW of ICRF heating in the frequency range 25–100 MHz in CW operation and (iii) 20 MW of NBI at a hydrogen injection energy of 125 keV and 10 s operation. The components of each heating mechanism which need development individually exist, i.e. (i) a high-power gyrotron with an output power of 1 MW in CW operation and a high-efficiency transmission line for ECH, (ii) a high-power co-axial transmission line with cooling and an automatic matching circuit for ICRF heating and (iii) a large negative ion source and some components of the beam line for NBI. The heating scenario and the present status of the development are described for each heating system.

Negative ion production in a large semicylindrical ion source

A large semicylindrical negative ion source, whose dimensions are 34 cm in diameter and 104 cm in length, has been developed. By optimizing a chamber depth and a filter strength, a H− current of 650 mA (7.7 mA/cm2) was obtained at an arc condition of 1.3 Pa, 70 V, and 1200 A. In order to increase the H− current and to reduce an operating pressure, a small amount of Cs was injected. As a result, the H− current was increased from 550 mA (6.5 mA/cm2) to 850 mA (10 mA/cm2) under an arc power of 70 V×800 A. The most significant feature of the cesium effect was the reduction of the operating pressure. With maintaining the sufficient current of 670 mA (8 mA/cm2), the operating pressure could be lowered to 0.03 Pa. At this pressure, the highest gas efficiency of 20% was achieved.

A 14 cm×36 cm volume negative ion source producing multi-ampere H− ion beams

A large volume negative ion source, which has a newly devised magnetic filter called a PG filter, was designed and tested. The PG filter produces a uniform magnetic filter field over a large extraction area of 14×36 cm2 by flowing a high current through the plasma grid itself. By optimizing the filter strength, we succeeded to produce 3.4-A 75-keV negative hydrogen ion beams for 50 ms from 253 apertures of 11.3 mm diam with an average current H− density of 13 mA/cm2.

Characteristics of a sheet plasma and its application to a large extraction area negative ion source

A sheet plasma with a thickness as thin as several millimeters was produced by the combination of a pair of dipole magnetic fields and a mirror magnetic field. The negative hydrogen ion produced in the peripheral region of the sheet plasma was measured with a movable Faraday cup. The maximum negative ion current was observed when the extraction electrode of the Faraday cup was biased slightly negative with respect to the plasma potential. The optimum position for the negative ion extraction appeared to be 2 to 3 cm from the center of the sheet plasma. A large concentration of high-energy electrons was found inside of the sheet plasma. The measured negative ion current showed a dependence on the density of high-energy electrons in the sheet plasma.

Self-extraction negative ion source

A large multicusp negative ion source has been constructed to generate H− or D− ions for high-energy neutral beam systems. With the presence of cesium, the source has produced a steady-state H− ion beam current greater than 1 A with an impurity level less than 1%. The vertical and horizontal profiles of the self-extracted beam have been measured with a gridded Faraday cup. The gas efficiency of the source and the electron component in the accelerated beam have been measured for two different pumping conditions.