The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Characteristics of plasma jet emitter
Takayuki INOUE, Ryuichi UENO, Takeji SAKAE, Takanori YAMAMOTO
Department of Nuclear Engineering, Kyushu University, Fukuoka 812, JAPAN

Abstract
In order to investigate characteristics of plasma jet emitter, a proton source including the emitter is developed. The
method to generate the pulsed plasma with reliability issuggested. The influences of the magnetic field, the pressure
and the timing of the pulse power supplies on the djschargeare studied.

GasPowersupply A1 Introduction
To diagnose the plasma for nuclear fusion, it is necessary
for neutral beams to have small emittance. The neutral
beams can be inserted into high magnetic field in Tokamak.
Further, the technology to produce intense beams is demanded for the purpose. The beams with small divergent arealso useful to develop a new type of ion source which needs
good quality of beams in magnetic field[l ]. These small
divergent beams are required to have current densities of
0.1-1.0 A/cm2 at a distance of several meters from the ion
source and total current of up to I 0 A.
For usual plasma ion sources, the beams are extracted byan electrical field from the plasma which is produced by adischarge. This type of extracting system makes a distortion
on the plasma emission. The distortion influences the quality of the extracting beams, which is a big problem for thebeam production. To solve the problem, and make intenseand small divergent beams, plasma jet emitter is developed[2]. In this work, the plasma jet emitter is newly developed to investigate the mechanism of the beam formation.
The characteristics of discharge and plasma expansion arediscussed from experimental data.

2 The system of the ion beam formation
The schematic drawing of the ion beam formation in thePlasma expansion space

Trigger signalFig. 2

The schematic drawing of the electrical circuit
for the arc discharge

plasma jet emitter is shown in Fig. I. The beam formatiQn
has some original characteristics. For example, the plasma
expansion is utilized without collision[2]. The plasma isexpanded by a pressure difference between the plasma generation space and the plasma expansion space. The transverse ion temperature to the beam's axis is decreased rapidly
by the plasma expansion because of the effect of plasma
adiabatic cooling. As one of other characteristics, the plasma jet emitter has the four accele-decele multi-slit lens system. The multi-slit lens system consists of molybdenous
wires which are fixed at intervals of 1.5 mm. To produce thebeams with small divergence, the part which has the parallel
velocities to the beam's axis is utilized by restriction insolid angle.

3 The experimental equipment for discharge and extraction

Multislit extracting electrodes
Fig. lThe schematic drawing of the ion beam formation

The schematic drawing of the electrical circuit for arc
discharge in the plasma jet emitter is shown in Fig. 2. In theexperiment, the discharge is ignited three times to decrease
the plasma impedance, which generates the pulsed plasma.
of a few hundreds amperes with reliability. Time intervals
for the three discharges can be controlled by using of two
triggers which are made in the timing circuit. The first discharge is ignited between the electrode A and the cathode,when the first trigger arrives at the pulse power supply A in
Fig. 2. Current of a few amperes flows between the cathode-154-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997l!

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0200

400

600

800

1000

Fig. 41200

The wave form of the arc current in the pulse
mode

Supplied voltage [V]Fig. 3

The relations between the arc current and thesupplied voltage

and the anode, when the second discharge is generated by
the stored charges in the condenser C 1• The second discharge is necessary to ignite a big discharge (the third). The gate(G) of the thyristor, which drives the stored charge in thecondenser c2 and c3 between the cathode and the anode, isopened for the third discharge, when the second trigger
arrives at pulse power supply B in Fig. 2. Thus, the plasmaof a few hundreds amperes is generated between the cathode and the anode.
For accele-decele lens system which makes proton beams
from the plasma, two pulse power supplies of several 10
amperes and 5 kV are utilized.

Under the conditions of the constant supplied voltage andpressure, in the pulse and the continuous mode, the arc cu~­
rents are influenced hardly by the change of the magnettc
field. Similarly, under the conditions of the constant supplied voltage and magnetic field, those are influenced
hardly by the change of the pressure.

5 conclusions
The method to ignite three discharges is useful to generate the pulsed plasma of a few hundreds amperes with ~eli­ability and periodically. In the pulse mode and the contmuous mode, the supplied voltage has a great influence on.thearc current. The magnetic field and the pressure have a httle
influence on it.

References

4 The experimental results and the discussion
To investigate the characteristics of the discharge, thepulsed arc discharge is ignited in static gas pressure. This iscalled the continuous mode in this paper. In the case of continuous mode, the discharges are ignited in only the plasma
generation space in Fig. 1 without flowing out from thespace. In the case of the. pulse mode, the plasma flows out
from the plasma generation space to the plasma expansion
space by the pressure difference between those spaces.
The relations between the arc current and the supplied
voltage are shown in Fig. 3. The plot (a) indicates a data forthe case of the continuous mode, and (b) indicates a data forthe case of the pulse mode. In the pulse mode, the wave
form of the arc current is shown in Fig. 4. As a value of thearc current, the peak of the wave form is measured. In theexperiment, the discharge times of the pulse and the continuous modes are about 300 J.1S and 100 J.1S, respectively. InFig. 3, the arc current of the pulse mode is lower than thatof the continuous mode, which is caused by the followingthing. The flowed out charges from the condensers are almost the same quantities in those modes, therefore, thosearc currents depend on those discharge times.

[l]

[2]

-155-'-A. N. Zelenski et al., "Pulsed optically pumped polarized H- ion source development" , Rev. Sci. Instrum.
67(3) (1996) 1359.
v. I. Davydenko, et al., "Reception of precise ·highintensity ionic and atomic beams" ,Dokl. Aka£1. NaukSSSR271 (1983) 1380.