Eiji Asano

Cavity Ring-Down System for Density Measurement of Negative Hydrogen Ion on Negative Ion Source

A Cavity Ring‐Down (CRD) system was applied to measure the density of negative hydrogen ion (H−) in vicinity of extraction surface in the H− source for the development of neutral beam injector on Large Helical Device (LHD). The density measurement with sampling time of 50 ms was carried out. The measured density with the CRD system is relatively good agreement with the density evaluated from extracted beam‐current with applying a similar relation of positive ion sources. In cesium seeded into ion‐source plasma, the linearity between an arc power of the discharge and the measured density with the CRD system was observed. Additionally, the measured density was proportional to the extracted beam current. These characteristics indicate the CRD system worked well for H− density measurement in the region of H− and extraction.

Production of high-current large-area H/sup -/ beams by a bucket-type ion source equipped with a magnetic filter

A negative-ion-based neutral beam injection (NBI) system is planned for plasma heating of the Large Helical Device (LHD). We have developed a negative ion source, which is 1/3 the scale of the source for the NBI. A magnetic filter held was generated by external permanent magnets to lower the electron temperature in a large-area bucket plasma source (35 cm/spl times/62 cm) for efficient H/sup -/ production. We investigated the magnetic field configuration and found a low electron temperature high density plasma ( 15 A: 1/3 current of LHD ion source). Based on the results, we are designing a negative ion source for the LHD.

Correction of Beam Distortion in Negative Hydrogen Ion Source with Multi‐Slot Grounded Grid

The new beam accelerator with multi‐slot grounded grid (MSGG) has been developed to increase the port‐through power into large helical device (LHD). Using the accelerator, the maximum power of 5.7 MW was achieved at the beam energy of 186 keV in the beam injection to LHD plasma last year. Although the port‐through power increased compared with conventional accelerators with multi‐hole grounded grid (MHGG), the accelerator with the MSGG includes a disadvantage of bi‐focal condition in parallel and perpendicular direction to the long side of the slots. When the beam width in one of those directions gets narrower, the width in another direction becomes wider. This disadvantage includes the loss of beam port‐through power and induces internal damages in neutral beam line. In order to reduce the disadvantage, an experiment has been done using a small‐scaled negative ion source with racetrack‐shaped apertures for the steering grid installed at beam upstream of the MSGG. By applying the racetrack apertures to th...

High‐Power Negative Ion Sources for Neutral Beam Injectors in Large Helical Device

Large‐scaled hydrogen negative‐ion sources, in which cesium is introduced in the source plasma, have been developed for neutral beam injectors in Large Helical Device, and their operational characteristics are reviewed. For high‐efficient negative ion production, configuration of the magnetic filter field and the cusp magnetic field was optimized, resulting in a high arc efficiency for the negative ion production of 0.23A/kW. With use of a multi‐slotted grounded grid, the gas pressure in the acceleration gap is lowered, leading to reduction of the heat load of the grounded grid. As a result, the voltage holding ability is much improved, and the rated energy of 180 keV is achieved in a short conditioning period of 4 days. The injection power is increased linearly to the 5/2 power of the beam energy and reached 5.7MW with an energy of 184keV, which exceeds the specified value of 180keV–5MW. Beam uniformity has been improved with an individual control of the local arc discharge by adjusting 12‐divided output...