CHARGING AND ACCELERATION OF MICROPARTICLES
N.Kamiya,T.Sato,K.Yoshida,Y.Haruyama•,N.Imanishi,an d F.Fukuzawa
Department of Nuclear Engineering, Kyoto University, Kyoto Japan*Physics Laboratory, Kyoto Prefectural College, Kyoto Japan

P(Matm)Abstract

100

Nickel microparticles of the order of lJ.L m inradius were highly charged by contacting with finecarbon fibers l{ept at high voltage and accelerated
electrostatically up to 2MV. Preliminary experiment on
hypervelocity
thesewithformation
microcrater
microparticles has been made.
Introduction
The eA~riments to change the crystal structure ofsolid materials by using shock wave compression havebeen made for about 30 years. The special features ofthe shock wave compression are as follows:(1) The pressure due to shock wave compression is
extraordinarily high in comparison with that due tostatic compression.
(2) Time duration of shock compression is so short as10-"-10-5 sec.
compression isshock(3) Temperature-rise during
relatively low.
Due to the feature of (2) only a little modification of
initial crystal structure takes place by compression,and instead of the phase transition with random flow ofindividual atoms does not occur.
byIn usual, shock wave have been produced
hyperveloci tyaa solid target withbombarding
projectile mass. The bombarded target is broken inpieces which must. be gathered afterwards. If theprojectile is very small mass, only the surface of thetarget can be modified without destroying the bulk
Such modification of material surface isn~terial.
impossible with the static hyperpressure processes. The
bombardment with hypervelocity microparticles has two
excellent features; production of extraordinary highmodification.
pressure and possibility of surface
Therefore it is eA~cted to become the new techniquefor creating new materials. In Fig.l are shown shock
wave pressure and temperature-rise as a function ofcollision velocity. These n?lations are obtained formany combinations of colliding materials by solving theRankin-Hugoniot. equation with experimentally estimated
J:Xll'Hineters, and are found t.o be in the hatched belt
region in this figure.
aremicroparticles
trial to accelerateThe
w1expectedly few, and no work has been made about phasetransition with shock wave high pressure even after thetowayof acceleration. The convenient
success
accelerate microparticles is to charge up them and toaccelerate with some electrical accelerator. The mostimportant problem in this acceleration is to charge up
the particles as high as possible. Although several ion(macronsources for highly charged microparticles
source) were developed, they were somewhat complicatedstaticordinarywithusefor
unsuitableandaccelerators. We have developed simple macron sources
Cockcroft-Walton
aaccelerated macrons withandaJJ<~e 1 era tor

and a Van cle Gra.t:t f f~lacronac~r!e] era tor.

sources

Parallel plate macron source
For charging micropa.rticles, we must contact themwith high voltage electrodes or spray electrons onthem. We first studied a parallel plate type macronsource. This type of macron source is now used as afeeder for supplying microparticles to a macron source.

Sound Velocity
Air

101
0.1

fn

L____c!___Meteorites____J001
100.

__l_____ _ __ . __ _ _10v (km/s)
Fig.l Variation of shock pressure and~<i th collision velocity.

temperature

Two parallel plate electrodes are placed horizontally,and microparticles (pol,lder) of some metal are put on
the lower electrode.
h~en de voltage is applied between two electrodes,
microparticles on the lower electrode are charged andrepelled up against gravitation to the upper electrode,
where they reverse t,heir charge and are accelerated
downward. In such a way, Irnicrons move up and down
between two plates, and cause electric current.
The macron used in the present experiment issizeThenickel powder,availablecommerciallydistribution is the log-normal distribution for theparticle radius as shown in Fig.2.
With this powder, the current due to macron
oscillation between two electrodes was measured as afunction of applied voltage. The measured V-I relationis shown in Fig.3. This relation can be explained byconsidering the size distribution and charging processof macrons. In general, the charge q(r) on a sphericalmacron with radius r, loaded by contact with a sphereof radius R having surface electric fieldER, is givenby(1)q(r)=4•n eor2Er,(2)Er= ~(r/R)E.,
where ~ (r/R) is a function of r/R, and , ~ = n 2/6 forr<<R. In the case of a non-spherical m8cron, we canroughly estimate Er by using a form correlation factor
G, which is 1 for a sphere, and radius r of a spherewith same volume as the macron:(3)
Er=G.~ (r/R)E..
For the parallel plate electrodes with distance d andapplied voltage V, ER is V/d. The maximum size of therepelled macron is determined by balancing the Coulomb
repulsive force and gravitational force, and is afunction of applied voltage. The sum of carrying-57-Current (1-JA)

fLN/ r101

10°.......,' ....
....../.~. .-:~_.,..~.....
/Ni~''!-'/,

Nifi

w-1,:',i" ,'';/,10°/,,II.,E--rw-z:::1.

tD

011~

I"-·/. 1/. I
10-1: 7L()(")

II I,.-::) I I

It {'~G = 0.1

11/I-/1·1~0.2

w-3:f~g;·

10-z

11•

0.7

100.1

10w-30r ( 1-Jm) 100

Fig.2 Size Distribution of nickel powder
sphere of radius r.

as

654
3
2
1
Applied Field ( kV/cm)

Fig.3 V-I relation of amacron source.

thecharges of macrons of which size is smaller than themaxi_mum size gives total current. The curves in Fig.3
are the calculated ones, and in best agreement withexperimental data if we take G=O. 3.
The macrons are extracted through a hole at thecenter of the lower plate (Fig.4(a)) and focused by aunipotential lens. Each charge of macrons can bemeasured with a faraday cup connected to a preamp-ampsystem. In Fig.5 is shown an example of the charge
distribution (p.p.). The experimental results can be
reproduced fairly well by the calculation considering
the size distribution and the charge formulae Eqs,(l)and (3).

(a)parallelplate

Parallel Plate type~Vr~~~Vb~(b)

Blade typeI(c)Blade typeII(d)Fiber type-

Fiber type macron source
For an electric acceleration of macrons, it isdesirable to make a macron charge as high as possible.
On the other hand, there is a limiting maximum value ofa macron charge, corresponding to an initiation ofsurface discharge which takes_ place at the electricfield of about 2xl010V/m. In oUr present experiment,
macrons from the parallel plate type source have only
1/10• of this maximum value.
Use of sharp point or edge for contact charging
electrode gives very large value of E. in Eq. (3),resulting high charge on macrons. A razor blade Hithedge radius 4tt m was used for this purpose (Fig.4(b)).
By collision with the edge of a high voltage bladeedge, a macron charge increased to about 1/10• of thelimiting maximum value. Such collisions, hoHever, took
place only with small probability.
Next, we bundled many blades as shoHn in Fig.4(c).
extracted
weremacrons
high charge
first,At
plentifully. After operating a few minutes, however,clearances between adjacent edges were stuffed withlonger
and high charge macrons were nomacrons
produced.

-58-Fig.4 Various TypPS of macron source.

nickelFiber : 7 1Jm in diameter106

Vc=5.5kV
DP=6mm
DF=Bmm105

P. P.

.?:'(CclcvlaUon)

Fiber(ecJculaUonl"iiic 104(lJcClJ

10 3d2>a; 100:.1>.

101•

P.P. IExo.l
Fiber IE•o.i.

10°1b3106

107

Charge Q (q/e)
fromaFig.5 Charge distributions of macrons
parallel plate type (p.p.) and fiber type(Fiber) sources.

After many trials, we found that the fiber tY?Bmacron source is the most favorable. As shown l.nFig.4(d), an intermediate electrode which has a central
hole spanned with many fine carbon fibers (71-J m indiameter)is placed between twoparallel · -plateelectrodes. Macrons oscillate between the intennediate
electrode and two parallel plate electrodes. When amacron collides with a carbon fiber, it gets highcharge and is extracted through the hole of the lower
electrode. The collision probability of a macron with afiber is nruch larger than the case of blade type. InFig.5, the charge distribution of macrons from thefiber type source is shown and compared with that fromthe parallel plate type source. Solid curve in thisfigure is the charge distribution calculated by taking
into account the gravitational effect to the macron
trajectory between electrodes.

0ScmtoAce. TubeFig. 6 The fiber type macron source used on aeh,etron Van de Graaff accelerator additionof mechanicalandelectrical particle
f'""'dc,rs enabled Long time operation.

Acceleration and crater formation
The macrons are accelerated with the Van de Graaff
accelerator which is enclosed in a high pressure
insulating gas tank. We have replaced the electron gun
on the electron Van de Graaff accelerator with anegative macron source. The arrangement isshown
schematically in Fig.6. A mechanical macron feeder wasdeveloped to enable long time operation in such anenclosed type accelerator. The particle reservoir cancontain
40gof nickel
powder.
Byvibratingintennittently the supplying tube, proper quantity ofpowder is fed, through a TEl'! mesh, into the electricfeeder. With this mechanical feeder, we can operate themacron source for about 10 hours.
A macron beam extracted from this source isconverged with a unipotential lens and accelerated up
to 2MV to bombard targets. Fig.7 is a SEM photograph of
microcraters formed on the bombarded copper target.
~!any such microcraters are observed ru1d analyzed.
References
l.F.F\llmzawa,K.Yoshida,
H.Fujita,
M.TomitaandK.Higashi, Rep.Heeting J.J.A.P. (1983),(1985),(1986)
2.F.FuhuzaHa,
K.Yoshida,
H.Fujita,
M.Tomita andK.Higashi, Rep. Symp. Shock Wave (1985)
3.F.Fulmzawa ruld K.Yoshida, Hoshasen J{agaku 45 23(1988)
4.F.Fukuzawa, N.Imrulishi and K.Yoshida, Rep. Symp. NeH
Material (1989)
5.Y.C.Ji&}g 1 T.Satoh, N.Kamiya, K.Yoshida, N.Imrutishi
and F.F\llmzawa, Rep.Meeting J.J.A.P. (1989)

producedon Cu plateFig.7 Microparticles
bombarded by 2~W-accelerated nickel
SEM
angle 60°.

-59-