J. Plasma Fusion Res. SERIES, Vol.4 (2001) 313*316

Effects of lon-Beam Injection on prasma potential
HONZAWA Tadao
Department of Electric and Electronic Engineering,
Utsunomiya University, (Jts unomiya 3 2 I -g5 g S, Japan(Received: 5 December 2000 / Accepted: lg August 2001)

Abstract
It is experimentally shown that injection of a stationary ion beam always changes the backgroundplasma potential in a beam-plasma system. In this experiment, a technique of ion energy analysis
isadopted to determine the plasma potential in the system. Results indicate that the plasma potentialchanges sensitively depending on the beam-to-plasma density ratio, even if other parameters are notallowed to va"ry.

Keywords:
plasma potential, ion-beam, beam injection, energy analyzer

1. Introduction

chamber, a stationary ion beam was injected into thetarget plasma through a negatively biased (= -90 V)
separation grid. Thus, an ion-beam-plasma system was

How an ion beam affects the background plasma
potential when it is injected into a plasma is a quite

fundamental and very important problem in basic

formed in the target plasma region. In this case, thebeam energy was controllable by changing the voltage
V3. The one-dimensionality of the ion beam was ensured
by the plane grid with a large area.
In order to get information on the change of the
background plasma potential in the presence of an
injected ion beam, we measured the energy distributionof the plasma and beam ions by use of a gridded energy
analyzer [4], movable along the axis (r-axis) of thechamber and noted the shifts of the peak positions of theplasma and beam ions in the ion distribution. Typicalplasma physics. To our surprise, however, it remains anopen question. In most experiments in which the ion
beam injection is adopted, the change ofthe backgroundplasma potential, induced by ion beam injection, hasbeen neglected so far. However, the neglect of theplasma potential change is not justified in most cases, as
shown below. In particular, the plasma potential change

hinders the clarification of phenomena, caused by ion
beams with low energies below 2 eV. In this paper, wewish to report experimental results on the changes of thebackground plasma potentials induced by injection of astationary ion beam.

examples of a characteristic curve of the energy
analyzer and the energy distribution of the plasma andbeam ions are shown in Fig. l. The peak voltages V,

2. Experimental Methods

and V6 in the distribution are defined as theExperiments were carried out using a double
plasma device [1-3]. In the device, two argon plasmas
were produced by dc discharges. Plasma parameters

corresponding background plasma and beam potentials,

respectively. In addition, the ion currents I, and 16 atvoltages Vp - LV, and V6 - AV. (where A% = l0 V) aredefined as the total background plasma and beam ion
currents in the system, respectively. Using these total

were such as densities N, : (l-5) x 108 cm-3, electron
temperature T" = 2-3 eV and ion temperature Z, - 0.I
eV. Applying a dc voltage V6 to the driver plasma*Corresponding author's

e-mail: thonzawa@ ml g.alpha-net.ne.jp
313@2001 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchHonzawa T., Effects of Ion-Beam Injection on Plasma Potentialanalyzer's characteristic
=c;

rovg

F_zt!
EE: 1.5(J= 2.0E--2.5

F(Jr3.0:3.5JJ=4Olrl

vp

V60v

:0.5
:1.0

o:1.5
-- 5.0Fig. 1 Characteristic curve of ion energy analyzer andion energy distribution. Here, the corresponding+potentials V, and Vo are defined. The total ion
currents loand lo are also defined.

I:5.5

:6.0
:6.5
:7.0
z 7.5

-- so-12 -lo -8+

ion currents, the beam-to-plasma density ratio isdetermined such that N6lNo= I6lIr.

-6 -1 -2 0

2 I

COLLECTOR VOLTAGE vc (V )Fig. 2 Typical examples of the analyzer's characteristic
curves changing with the change of V".

The above method, determining the plasma
potential Vo from an ion energy distribution, isparticularly useful to know the change of thecorresponding background plasma potential in thecoexistence of both the plasma and beam ions. This isbecause under the effect of an ion beam the probe
technique, being usually used to measure the plasma

Typical ion energy distributions, measured at variousvalues of Vs, arc shown in Fig. 2. From these observed
distributions we can obtain the relations of Vo and V6
with V3, as shown in closed circles and in crosses in Fig.
3, respectively. For comparison, the relation of Vo with
Vu in the absence ofthe ion beam is also plotted in open
circles in Fig. 3. Results in Fig. 3 clearly indicate thatthe beam injection induces a shift of Vo, whose value ispotential, is expected to present only an average value ofthe background plasma and beam potentials and not togive an accurate datum on the plasma potential. From
this reason, most data of the plasma potentials wereobtained from the ion energy distributions measured byuse of a gridded energy analyzer in this experiment.
Here, it is noted that in the case of no beam effect theplasma potentials Vo, determined from the ion energychangeable depending on the values of yB'

3.2 Spatial Change of Plasma Potential
In the target plasma region, the intensity of an
injected ion beam was observed to decrease withincreasing x because of interparticle collisions, In

distribution, were experimentally confirmed to beapproximately proportional to the actual plasma
potentials V", observed by the probe measurement,
though the absolute values of V, were lower by about
1.7 V than the ones of V". This means that Vo = V, - 1.7

addition, the background plasma density was greatly
reduced in the sheath region close to the grid (xS 2 cm),V.

because the grid was biased at a highly negative
potential such as -90 V. Thus, the beam-to-plasma
density ratio N6lN, changes with distance -x. In themethod described above, we could observe both thespatial changes of the potentials Vo and V6 at various -x.
Typical examples of the spatial variations of yp with andwithout an injected ion beam are shown in closed andopen circles in Fig. 4, respectively.

3. Experimental Results
3.1 Effect of Voltage Vs on Plasma Potential
ChangeBy applying a DC voltage Vs between the twoplasma chambers, an ion beam was injected into thetarget plasma from the driver plasma. Then, energy
distributions of the background plasma and beam ionswere measured by use of a gridded energy anaLyzer.
314

Honzawa T., Effects of lon-Beam Iniection on plasma potential

3.3 Relation of Plasma Potential with Beam-toplasma Density Ratio

I3

In this experiment, we examined the dependence ofthe plasma potential Vo on the beam-to-plasma density

JFztr,ratio N6lNo by controlling N, (observed at a fixed;r),when N6 was invariable. The dependence is shown inFig. 5. Here, the beam-to-plasma ion current ratio 16/lo,
as defined in Fig. I, is almost equal to the density ratio

uo, t5-r(lff-zdl.**<.

A-3J(L(,-4z6z
O-ckToNhxl[:-gil'.T"Nt/Np.

4. Discussions and Conclusion\From the electron fluid equation of motion in aplasma, assuming the electron mass /?? -+ 0, the,,Boltzmann relation is derived aso-tn
lrt

i..--"r"""'-""'o olo o o o o o o o o o o o
kTs :3.9.Y
Vp (witho.rt beam )Gc-6I(JN, = No exp(eV/kT"),(1)

wherc 7", V, N" and N6 are the electron temperature, theplasma potential, the electron densities at V and at V = 0-8-8 -7 -6 -5 -1 -3 _2 _l
VoLTAGE V8 (v)V, respectively. e and k are the electron charge and theBoltzmann constant, respectively. In this paper, we try
to apply eq. (1) to the body of a stationary plasma,
assuming that it is not completely neutral. In otherwords, the relation is applied to the state of a sheathoFig. 3 Relations of the potentials V, and Vo with Vr. Here,l/o* means the plasma potential with ion-beaminjection.

built between the stationary plasma and the surrounding
wall. In such a case, we can say that Vis regarded as theplasma potential seen from the wall. Under thisvf =vp* -!Ie.1n1r.fft
kT" 14.0 eVJ(e )Fzo(Llrlts

J-zt!

o Vp (withoui beam)

aVx
trxtIX Xt I I'X

Fatt\lg: fixed , lp : variablexX

Fo(L_JkT" = 1'.9"Yo-ct(n=Jozo(Lvlozozo 7lt)
d.

(roL)
-l r.0o.
anlrJ -23t567

E.'IG,o(J-AXIAL DISTANCE X (cm)Fig. 4 Spatial variations of the plasma potential V" withand without the beam effect.

vX =h*$,"t''ffi1

900.2 0A

0.6 0.8

1.0

ION CURRENT RATIO l6/lp
Fig. 5 Relations of the potentials Vo and V, with ion

current ratio lollo.
315

Honzawa T., Effects of Ion-Beam Injection on Plasma Potential

nature of the dependence on Vs can be explained by eq.
(4), although kT"= 5.9 eV, adopted here, is considerably

assumption, we have a relation asNo = No

exP(eVolkT"),higher than the observed value kT" 5 3 eV. Result inFig.4 is explained as follows. Using the vallue of I6lIo=N6lNo observed at each I, AVis calculated as a functionof x, as shown in Fig. 4. Comparison of these results
indicate that they are in good agreement with each other
if kT" = 3 eV. Next, as seen from Fig. 5, the plasma(2)

for the background plasma with a density No and apotential Vo, regarding the electron temperature uniform
throughout the plasma. If an ion beam with a density N,
is injected into the plasma, then the plasma will have adifferent potential Vo* as follows. In this case, we havepotential Vo is found to change sensitively depending onthe beam-to-plasma density ratio N6lNo even if V3 isa relation as

N, + Nu = No exP(eVP*lkT")'

fixed. The dependence of Vo on N6lN, has never beenreported by anybody so far. Here, we try to explain thisdependence with the help of eq. (4). In this case the(3)better fit requires thatkT"

Here, plasma electrons with the same temperature areassumed to neutralize both the plasma and beam ions.
So, we can find from eqs. (2) and (3) that the plasma- 4 eV. In addition, the result

suggests that another unknown cause is more effective
for the change of Vo at small N6lNo.

potential is raised byReferences

tll R.J. Taylor, K.R. MacKenzie and H. Ikezi, Rev'Sci. Instrum. 43,1675 (1972).
t2l R. Limpaecher and K.R. MacKenzie, Rev. Sci.
Instrum. 44, 726 (1973).
[3] T. Honzawa, Ch. Hollenstein, J. Vaclavik and Y.
Kawai, Phys. Fluids 23,981 (1980).
T.
[4] Honzawa, T. Sekizawa, Y. Miyauchi and T.
Nagasawa, Jpn. J. Appl. Phys.32, 5148 (1993).

LV = Vp* - Vo= (kT"le) tn (1 + N6lNo) @)by the ion beam injection.
Based on this idea, we can explain almost all theexperimental results obtained in this experiment. InFigs. 3,4 and 5 theoretical curves are added in solid anddotted lines. Comparing experimental and theoreticalresults in Fig. 3, the solid line, where kT" = 5.0 eV, isfound to better fit the observed plots. The qualitative

3t6