J. Plasma Fusion Res. SERIES, Vol.5 (2002) 302-305

Hydrogen Plasma Production in the Lower-Hybrid
Frequency Range
KIKUCHI Hideki, SAKAWA Youichi and SHOJI Tatsuo
Department of Energy Engineering and Science, Nagoya University, Nagoya 464-8603, Japan(Received: 17 December 2001 | Accepted: 5 July 2002)

Abstract
Hydrogen plasmas are produced by using the radio frequency (r1; discharge. Plasma density ro
depends strongly on the strength of the external magnetic field Bs and has a peak when Bs and thefrequency satisfy the lower-hybrid resonance condition. Various types of antennas are used, and thehelical antenna gives the maximum no of 5 x 1012 cm-3 at 3.5 kW and 200 G.

Keywords:
hydrogen plasma, lower hybrid resonance, antenna configuration, loop antenna, helical antenna

1. Introduction

because they can produce high-density plasmas at
relatively low gas pressure of a few mTorr [1-3].

The maximum magnetic field strength (86) on thechamber axis is 1400 G within 5 7o uniformity in theplasma region. Hydrogen gas is.used in the pressure Ps= 9.1 mTorr, and the flow rate is 20 sccm. An rf (powerP,r < 4.5 kW and frequency..if = 13.56 - 28.4 MHz) ispulse modulated (pulse length - 3 ms and repetition
time = 67 ms) and applied to antennas wound on thePyrex tube. Radially-movable Langmuir probes areplaced at z = 3 and 40 cm. The plasma density n o isHowever, when hydrogen gas is used, plasma density ismeasured from the ion saturation current of the probe atCompact high-density hydrogen beam sources areuseful for measuring local ion and electron temperaturesand densities in nuclear fusion research. Plasma
production by using wave electric field in the frequency
range of a"i K. o) << @"" is known as a helicon-wavedischarge. Helicon-wave plasma sources are attractive

low compared with rare-gas plasmas [4]. Our purpose isto develop a compact high-density hydrogen-plasma
source by using radio frequency (rf) discharges. In thisz4,nprcbe

Langmuir probe

study, plasma production near the lower-hybrid
resonance condition is investigated. Various types of(z= 40 cm)(z=3cm)o*mantennas are used and optimization of antenna structure

is conducted.
End2. Experimental Setup
The experiment is conducted in a linear device
shown in Fig. 1. A vacuum chamber is made of stainlessPyrex tube
( Rldiu = 25cm )m mmVacuum pumps

steal (inner diameter is 36.5 cm and length is 150 cm).

f = 13.56 - 28.5 MHz

A Pyrex tube with inner diameter of 5 cm and length ofFig. 1 Schematic view of the experiment setup.

90 cm is connected to one end of the vacuum chamber.
C orre spondin g aut ho r' s e -mail : h- kikuc hi @ e e s. na g oya - u. acjp302@2002by The Japan Society of Plasma
Science and Nuclear Fusion ResearchKikuchi H. et al., Hydrogen Plasma Production in the Lower-Hybrid Frequency Rangez = 3 cm. Here. z = 0 is the position of the antenna edge.

increases, occur at 500 W and 1.5 kW when 8o = 0 and

The electron temperature I" is measured by the probe at

200 G, respectively. We define the discharge modes
before and after the density jump as the low-density(LD) mode and the high-density (HD) mode,respectively. When 86 = 0 G, in the LD mode theplasma production is done by the electric field due toz = 40 cm, which accomplishes an rf compensation [5].

3. Antenna Configurations
Figure 2 shows three types of antennas that areused to exanrine efficiency of hydrogen plasma
production. All antennas are made of a copper strip. One

the antenna voltage V6 (E discharge), on the other hand,in the HD mode plasma is produced by the electric field
due to the antenna current I,i (H discharge) [7]. When Bs

is the helix type [Fig. 2(a)], in which the rf current flows
with a pitch angle 0 with respect to 86. The copper strip= 200 G, even though the threshold P6 is larger than thatfor 86 = 0 G, the density jump corresponds to thetransition from E to H discharge. In the LD mode, no forthe one-turn loop antenna is smaller than that for thetwo-turn loop antenna because of the smaller V6 due toits lower inductance. When the one-tum loop antenna isused, the density jump or the transition from E to H
discharge does not occur. This is because the induction
field for the one-turn loop antenna is half of that for theis wound along the Pyrex tube and its length on thesurface of the tube is kept constant with 11 cm. WhenI= 0, the antenna is identical to Nagoya type III antenna[6], and induces the rf electric field parallel to Bs, E., invacuum. As 0 increases, both E, and the azimuthalcomponent of the electric field Es are induced. When 0= 90o, the antenna is nearly a half-turn loop antenna.
The antenna induces mainly E6, and E in vacuum
vanishes. In Fig. 2(b) a two-turn loop antenna is shown.
L and W are defined as the distance between twoantenna loops and width of antenna strip, respectively.
Figure 2(c) shows a helical type antenna, in which thelength along Bs and the helical pitch is 20 cm and23",
respectively. Ihis antenna induces both Ee afid Ez

two-turn one.

10"
(acomponents.

th 10"

4. Results
The variation of zo as a function of P.1 for Bo = 0:

and 200 G with the two-tum loop antenna (L = 0 cm andro'o

W = 1 cm) and for Bo = 150 G with the one-turn loop

antenna (W = | cm) is shown in Fig. 3(a). Density
jumps, which mean discontinuous increase in zo as Pr1

10'0.1I
RF power P" ( kW )l.s to "(b)

(.)

(b)

e l1ot2hoq
II+IE 510ttt

0200 400 600 E00 1m0
External magnetic field 86 (Gauss)

Fig. 3 (a) Dependence of no on Pn with and without 4
using the two-turn (L = 0 cm and W = 1 cm) and
one-turn loop antennas (W= 1 cm). (b) Plasma
density no versus 8o using the two-turn loop antenna (L = 0 cm and W= 1 cm) at the HD mode(P" = 3 kW)'

Fig. 2 Antenna configurations. (a) Helix antenna, (b) twoturn loop antenna and (c) helical antenna.
303

Kikuchi H. et al., Hydrogen Plasma Production in the Lower-Hybrid Frequency Range

Figure 3(b) shows the dependence of no on Be forthe two-turn loop antenna at the HD mode (P.1 = 3 kW).

90' of the helix antennas at P6 = 3 kW is shown in Fig.

where crl"" and ar"1 are the electron and ion cyclotron frequencies, respectively, and tr)'" is the electron plasma5(a). When 0 = 27", np peaks strongly near the lower
hybrid resonance and the discharge is in the HD mode.
While when 0 = 0o, the density peak around the lower
hybrid resonance is not observed. When 0 = 90", thedischarge is in the LD mode because of low induction
field.
Figure 5(b) shows no at Bo- 200 G, i.e., near the
lower-hybrid resonance condition, as a function of 0.
The plasma density np peaks at 0 = 45". When 0 > 70',no values are small and the discharges are in the LD

frequency. At the high-density limit of 4, >> d", >mode.

o)esaei, ay11is given by

The dependence of no on -86 at the HD mode (Po =3.5 kW) with the helical antenna, which has the both E,

np peaks around 200 G, where the lower-hybrid
resonance condition (D= a)ru is satisfied. Here, atls isdefined by the following equation,o)2y11= a)""a)"ii'i''*'*3'' I ,(l)\ a;"+ @t" J(2)@rt'_so - \t@;o;.

and Es [see Fig. 2(b)], is shown in Fig. 6. The

Nearly two times larger n, at 86 - 200 G than that at 86= 0 G is caused by the contribution of the high-efficient
plasma production at the lower hybrid resonance in addition to H discharge.
The dependence of no on Bs is measured as afunction of/using the two-tum loop antenna (Z = l0 cm
and I4l = 3 cm) at the HD mode P.1 = I kW). When/isvaried, np peaks at different values of,86 that satisfy thelower-hybrid resonance condition, and reaches no = 6 xl0rr cm-3. Figure 4 shows ,86 at the peak of n, Bp,versus 1f. The calculated B0 that satisfies a = o)ys_11e,
Brs-uo, is also shown. The measured Bo shows similar/
dependence to Bpg_11p. The difference between Bo and8111-sp can be explained by considering the contributionof the plasma density shown in eq. (1).
The plasma density nD versus 86 for 0 = 0", 2'1" andmaximum density of 5 x 1012 cm-3 is achieved at 200 G.
Furthermore, nr> 1.6 x 1012 cm-3 is obtained in a wide
86 range of 86 = 100 - 900 G.

7to"
(.1L

610"

I510"

?: 410"'d6 3to"pzro"
0iI10"
00200 400 600 800 1000 1200 1400
External Magnetic Field 86 ( Gauss )110"(b?atBo
LH-HD,E 400
F300Jzoo810 ".

|_Bota610 "a410 "
q210 "

cs 1000100051015202530

0 ( Degree )RF frequency f (MHz)Fig. 5 (a) Plasma density np versus Bofor 0- Qo,27" and90" of the helix antennas at P; = 3 kW. (b) Plasma
density no around the lower-hybrid resonancecondition 14 = 200 G) as a function of 0 at Pn = 3Fig. 4 HF frequency fdependence of the measured Bo at
the HD mode (Pn = 1 kW) using the two-turn loop
antenna (L =10 cm and W = 3 cm) and the catcu-lated Bls-11e.

kw.
304

Kikuchi H. et al., Hydrogen Plasma Production in the Lower-Hybrid Frequency Range{(ienhancement of zo occurs as shown in Fig. 5(a). If theloop antennas having E6 component are used, theproduction of the space charge is possible due to theradial motion of electrons caused by E6x Bo drift. The
resultant radial electrostatic fields couple to the slow
waves near the lower hybrid resonance as shown inFigs. 3(b) and 4.
When the helical antenna is used, the maximum noof 5 x l0r2 cm-3 is achieved at Bs = 200 G, and no > 1 .6x l0r2 cm-3 is obtained for 86 - 100 - 900 G. According

d4
x1h,,

:1o

E0200 400 600 800

01m0

to the results shown in Figs. 5 and 6, the antennasf)xternal Magnetic Field Bg (Gauss)having both Ee and E, such as the 27" antenna and thehelical antenna, are effective to produce high-density

Fig.6 Plasma density ne versus 8o with the helical an-hydrogen plasmas. Especially, the helical antenna issuitable to excite the helicon waves with azimuthalmode m = +1 [3]. Therefore, it is likely that the helicon
waves start to contribute to the plasma production in thisdensity region in addition to that of the slow waves.
In order to clarify the contribution of waves on thetenna at P.r = 3.5 kW.

5. Discussion
The dispersion relation of electromagnetic waves ina uniform plasrna is expressed as [8]plasma production, we are planning to measure the wave

Snl-r InTr(S + P) - (RL + PS)l*r-r P(n7- R)(n\r - L) = o,magnetic field using a magnetic probe.
(3)

6. Gonclusion

where n1 = ckLla, nrt = ckrrla, c is the speed of light,
and k1 (k11) is the wave number perpendicular (parallel)
to .B0, respectively. R, L, P, S and D arc the dielectric
tensor elementr; [8]. When a"i< a < oce, two branches
exist: one is the slow waves and the other is the fast
waves (helicon waves) [4]. The slow and fast waves ex-

Hydrogen-plasma production using the rf discharge

in the frequency range of oJ"i < a4 @"" is investigated.
Plasma density zo peaks when Bs satisfies the iowerhybrid resonance condition. The dependence of no on
the pitch angle (0) is measured using the helix typeantennas. Erand E6 ilre necessary for efficient hydrogenplasma production. The maximum values of no obtained
at 3.5 kW are 5 x 1012 cm-3 for the helical antenna, 1.7
x 1012 cm-3 for the two-turn loop antenna and 7 x 10rr
cm-3 for the helix antenna.

ist in the low-density and high-density regions, respectively. When 86 ( Bls-11p, the slow and fast waves
merge each other at N? = 45. On the other hand, when
Bo > Btr-"o the slow wavgs satisfy the lower hybrid
resonance at S = 0. In the density region below l0r2
cm-3, it is difficult to excite the helicon waves since theaxial wavelenl;th becomes too long compared with theReferences[]

plasma length. Therefore, enhancement of r?p at thelower hybrid resonance in this density region might beR.W. Boswell, Plasma Phys. Control. Fusion 26,

rr47 (r98q.

[2] F.F. Chen, Plasma Phys. Control. Fusion 33, 364(leel).

caused by the slow waves.

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Near the lower hybrid resonance, the dispersion
relation of the slow waves is identical to that of the cold
electrostatic \vaves, and the slow waves propagate
almost perpendicular to 86 in the radial direction [4].
Therefore, as approaching to the resonance the radial
component of the electric field E, becomes large. In
order to make rhe antenna field couple effectively to thewaves near tht: resonance, antennas producing large E,
should be useC.. Therefore, it seems that O = 0" antenna
which induces large E in vacuum is not appropriate to6,47s9 (1999).

[5] I.D. Sudit and F.F. Chen, Plasma Sources Sci.
Technol. 3,162 (1994).

[6] T. Watari et al.,Phys. Fluids 21, 2016 (1918).
t7l A. Degeling, N. Mikhelson, R. Boswell and N.
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[8] T.H. Stix, The Theory of Plasma Waves (McGrawHill. New York. 1962).

excite the wave electric field near the resonance and no305