PRESENT STATUS ON THE DEVELOPMENT OF A BEAM PLASMA HEAVYION SOURCE (BPHIS-1) AND THE FUTURE PROGRAM
H. Kametani, M. Horie, K. Komatsu
H. Ijiri, and M. Matoba
Department of Energy Conversion Engineering, Graduate School ofEngineering Sciences, Kyushu University, Kasuga 816, Fukuoka, Japan
1. Introduction
Present status on the development ofabeam plasma heavy ion source (BPHIS-1)
and the program in future are described. Up to now, a Pierce-type electron gun
with a large concave LaB 6 cathode, a plasma chamber and an ion-extraction systemhave been completed. Beam currents of argon ions as functions of several parameters, the extraction voltage, the extraction geometry and gas pressure, havebeen measured.
The plasma-sheath distance and the shape of the ion-emission surface, i.e.,
the plasma surface, are obtained by analyzing the experimental results and also
by computer simulation.
BPHIS-1 will be changed to an ion source of an ECR-type, in which an electron beam is to be injected.
2. Experiments and theoretical analysis
The construction of BPHIS-1 is shown in Fig. 1. BPHIS-1 is composed of anelectron gun of a Pierce-type, a plasma chamber and an extraction system. Theelectron gun 1 ) possesses an LaB 6 cathodewith20 mm concave radius and 30 mm
diameter. The electron currents emitted from the LaB 6 cathode are higher than
200 rnA when the cathode is heated to its operating temperature above 1300°C.
Neutral gas is introduced from a gas inlet into the plasma chamber with an external solenoid. The primary electron beam accelerated to several keV bombards
the neutral gas so that aplasma is generated in the chamber. Stripping of electrons takes still more place in the plasma chamber due to a beam-plasma interaction. In this condition, ions are extracted from the plasma chamber.
Fig. 2 shows the results of experiments; the ordinate and the abscissa
designate the normalized ion currents and the extraction voltage, respectively.
The experiment shows that the normalized ion current varies in proportion as
the minus second power of the distance between the electron collector and theion extractor. Therefore, it is concluded that the relation of normalized ioncurrents and the distance between the electrodes obeys the current limited byspace-charge. The current I is given by312
2Ze 112 _V·l _4I=-£(--)
dsz
mi9°

where e 0 is the dielectric constant for vacuum, Z the effective charge state, mi
the ion mass, Vi the extraction voltage, and ds the sheath distance. Under theassumption that the effective charge state Z is constant and independent on extraction voltage, the experimental data leads results shown in Fig. 3.
In order to evaluate the position and the shape of the plasma surface atthe extraction system, and to obtain ion beam trajectories, computer simulation
has been performed by relaxation techniques2) on a matrix with a maximum size of1815 points in following procedure:(1) Geometry of the electron collector and the ion extractor, and voltages applied on each electrode are introduced. The voltages at points within the extraction system are computed by iteratively solving Laplace's equation expressedin difference forms.
(2) By solving the equation of the Lorentz force described in different forms,
electron trajectories are obtained.
(3) New potential, which is thought over the effect of space-charge on theelectrontrajectories, is determined by solving the Poisson's equation.
(4) Ions are emitted from the surface determined by potential of the spacecharge.
-243-Results of computer simulation are shown in Fig. 4 and Fig. 5.
3. Results
Fig. 3 indicates that the sheath distance increases with the increase ofapplied negative voltage. The lines in Fig. 3 denote the sheath distances which
are obtained by estimating results of computer simulation. What is the conclusion from comparing the experimental results with the simulated results is anagreement ofatendency between both results and no absolute agreement.
As the voltage applied on the electron extractor varies, a change of sheath
distance and that of the plasma surface are shown by the experiments and computer
simulation so that the conditions to get beam quality were obtained.
4. Future program
In next experiments, the extracted ions will be analyzed by a bending magnet.
It is hoped that we can study the change of charge state distribution with thedistance between the plasma surface and the ion extractor.
To obtain high charge states and strong beam intensity, the following plansare considered: the plasma chamber will be changed to magnetic mirror, and aplasma trapped between the magnetic mirrors will be heated by an induced microwave power.
References
1) T. Shintake, K. Ohba, M. Matoba, and A. Katase: Jpn. J. Appl. Phys. 20(1981) 341
2) J.E. Boers: IEEE Trans. Elect. Dev. ED-12 (1965) 425I.

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heavy ion source (BPHIS-1)

Fig. 2 Normalized ion
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Fig. 3 Sheath distanceversus the extraction
voltage