JAERI-Conf

95-021

PA06
PLASMA JET EMITTER:MECHANISM OF ION BEAM FORMATION WITH LOW EMITTANCE
Takeji SAKAE, Kenzo OGAWA and Takanori YAMAMOTO
Department of Nuclear Engineering, Kyushu University
Fukuoka 812, Japan
Abstract
Plasma emitter, which has been developed inNovosibirsk 1l for diagnostics in Tokamak type plasma, isinvestigated concep~ally in order to apply ithe source toa high intensity particle generation with long range
focusing. The effect of thermodynamic cooling in theexpansion of charged particles is examined as well as theeffect of the restriction of radius in phase diagram of
expanded collisionless plasma. The magnetic field is
calculated for designing the geometric parameter of theplasma jet emitter.
1. Introduction
A charged particle emitter has been developed forhigh brightness ion and atomic beam formation by
Davydenko et al. 1l. The emitter can supply beams withhigh brightness because of significant reduction of thetransverse ion temperature in the plasma. The normalized
brightness of 1keV proton beam arrived in 3xl08
A/cm 2rad2 by using a sophisticated extraction lens system.
The source should be useful for applications which needa dense and focused beam. As a mechanism of the ion
beam formation, the effect of thermodynamic cooling isexpected to make the beam emittance lower. The coolingshould be investigated to realize a new performance forlow emittance beam, as well as optimization of theplasma electrodes and nozzle.

2. Ion beam fonnation for low emittance
The schematic drawing of the electrodes of theplasma emitter is shown in Fig. 1 for a half part divided
by the section.

10°Q).
:::::J

10-1Cii>-oQ).!::!

10-2CiiE....
0Z· 10-3

10-4

5010

1520

Distance from nozzle (d)Fig. 3 Ideal thermodynamic cooling in a free jet.
The construction involves a cathode, an anode andfloating electrodes between the biased electrodes. The
discharge is excited just after the expansion of the gas
generated by a specially designed pulse valve which isconnected to the cathode. The beam formation has been
explained originally by the effect of the expansion ofcollisionless plasma1l. The mechanisms of ion beam
formation are drawn in Fig. 2 schematically. The
transverse effective temperature of the io.ns is reduced bya factor X2/do2 (>>1), where X is the distance from nozzle
to the extraction system, and do is the diameter of particle
quitting surface. In the figure, it is shown that thefluctuation of the transverse ion velocity dV,~ is mainly
depend on the solid angle of ion quitting surtace.

Fig. 1 Schematic drawing ofElectrodes of the plasma emitter.
XdVtJ0Plasma jetemitter

Quasi-continuum
flow region

Free particle flow region

Fig. 2 Ion beam formation by the plasma emitter.

-38-Extraction
electrodeJAERI-Conf 95-021

By considering quasi-continuum flow region at thenearest point to the aperture of the nozzle, thermodynamic cooling should come into effect. The ideal thermodynamic analysis of a neutral gas expansion gives changingof the temperature and the particle density as shown inFig. 32). This effect appears only for a dense region of theplasma Experiments using small PIG source plasma,show the optimum gas pressure of less than 10 mTorr for
intense beam extraction from the plasma On thiscondition, the mean free pass of the ion is over 1 em.
Therefore, the discharge should be examined at about 50
mTorr gas pressure in order to realize the quasi-continuum flow. The study of the plasma formation for highpressure is needed by experimental and computational
works for the effective cooling.
3. Calculation of the niagnetic: field
The magnetic field components H and R arecalculated by using integral expressions for the field ofthe plasma emitter. The expressions are included
complete elliptic integral. The values are evaluated bynumerical integration. Iron pieces are taken into accountby assuming infinite permeability and by representing themagnetization by Ampere's currents on the surface of
these pieces. The boundary problems are solved directly
by Gauss Jordanian elimination. The ratio of the valuesof magnetic field should be 8.0 for cathode region andanode region, according to the information from
Novosibirsk. By some modification of the shape of theiron yoke, the ratio can be achieved in the final shape ofthe yoke shown in Fig. 4. The calculated fields are shownin Fig.S for the axial strength Hz and the radial strength~-4. Conclusion
In order to achieve the adiabatic cooling in theexpanding plasma, dense discharge should be generated
initially in the plasma emitter. A discharge in highmagnetic field is one of the solutions for the requirement.
The mechanism of ion beam formation of plasma jetemitter should be confirmed by experimental method andsimulations.
Acknowledgement
We would like to thank V.I. Davydenko, C.D.P. Levy
and A.N. Zelenski for suggestive discussions at TRIUMF.
Reference1) V.I. Davydenko, G.I. Dimov and G.V. Raslyakov,
Sov. Phys. Dokl. 28(8), (1983) 685.
2) G. Scoles edited, Atomic and Molecular Beam
Methods, Vol.l, New York, Oxford, (1988) 22.

-39-Fig. 4 Shape of the yoke forthe plasma emitter.

10c:- 8·o0N

E 60<..... 4cuE
20~:I:

0-2-1 0 1 2 3 4 5 6 7 8 9 10z (em)Fig. 5 Magnetic field in the plasma emitter.