Negative-Ion-Based Neutral Beam Injector for the LHD

Introduction

  In the LHD project, 15MW neutral injection is planned. Generally speaking, in the helical systems, it is neccessary to inject the beam tangentially to avoid the helical ripple loss of absorbed high energy ions. It is also important for the beam to heat the core of the plasma, i.e., the deposition profile should be peaked because no 'profile consistensy' has been observed in the helical systems. These two constraints make the beam energy high: 180keV for hydrogen, and we need negative ions to construct the system. The specifications of our beam injection system is as follows;
Table 1 Specifications of LHD NBI

Total injection power

15MW (port through )

Beam energy

180keV (H) / 360keV (D)*

Pulse length

10 sec.

Number of beam lines

2 (balanced injection)

Number of ion sources

4 (total)

*) planned in future

A cross-sectional view of a beam line and 1/3 section of LHD vacuum vessel is shown below.


Fig. 1 Horizontal cross-sectional view of a beam line and LHD vacuum vessel

　Balanced injection is required to study net-current free plasmas in the LHD. Although the LHD is a steady state machine, high power heating is only possible by pulse operation due to the lack of electric capacity from a commercial line. We use a flying wheel motor generator for the operation.

Negative-Ion-Based Neutral Beam Line

　The beam line is designed as dealing with a large current negative ion beam. Each beam line has two negative ion source that delivers 40A negative hydrogen ion beam with the beam divergence angle of 10mrad. The system has a large vacuum chamber with a cryosorption pump at the exit of the ion source, which is utilized as a gas expansion chamber to reduce the gas pressure in the beam accelerator. This is important for negative ion sources, because the negative ions are neutralized by electron stripping via collision with neutral particles during acceleration. Neutralizer becomes long to keep the gas line density for optimum neutralization, where we define 'optimum' so that the fraction of residual negative ions to positive ions is equal. Then the neutralization efficiency is about 58%. Residual ions are seperated from neutrals by a bending magnet to beam dumps. Beam dumps are made by swirl tube array, and the maximum heat load is assumed 16MW/m 2.



Fig.2 A schematic vertical view of a beam line

Negative Ion Source

Negative ion source is designed based on our R&D results of high current negative hydrogen ion sources, which has been carried out at the negative ion beam test stand in the plasma heating laboratory. Specifications of the ion source is as follows;

Table 2 Specifications of negative ion source and the results of R&D at NIFS
LHD Ion Source R&D 1/3 scale ion source

Type of ion source

Magnetic multipole with a strong external magnetic filter

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Beam Species

Hydrogen

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Beam energy

180 keV

125 keV

Beam current

40 A

16.2 A

Beam extracting area

25 cmW x 120 cmH

25 cmW x 50cmH

Operating gas pressure

0.4 Pa

0.5 Pa

Current density

> 35 mA/cm2

54 mA/cm2 @1.2A, 0.9Pa
45 mA/cm2 @16A, 0.9Pa
31 mA/cm2 @16.2A, 0.5Pa

Divergence angle

10 mrad.

5 mrad. (single hole)
9 mrad. (multi holes)

Electron fraction

< 75% of H- in the extracting region
< 12% of H- in the accelerating region

45% of H- in the extracting region
17% of H- in the accelerating region

Accelerator

Electro-static, single-stage acceleration

Compared single-stage with double-stage

Beam focusing

Geometric focusing of five divided grids
&
Beam steering by aperture displacement in each grid

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A schematic diagram of a negative ion source is shown in the figure below.

Fig.3 Schematic view of negative ion source (long direction)

　Construction Schedule

The design of the first injection system was started in 1997. The system is know under fabrication. The design of the second injector has also been started in 1996. The construction willbe completed in the end of 1997, and the commissioning will start 1n 1998. We expect the first injection in the middle of 1998.

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LHD > LHD information > LHD Technical Information > NBI