Measurements of electron density and electric field strength have been made in an argon plasma contained in a resonant microwave cavity at 2.45 GHz. Spatial measurements of electron density, n sub e, are correlated with fluorescence observations of the discharge. Measurements of n sub e were made with Stark broadening and compared with n sub 3 calculated from measured plasma conductivity. Additional measurements of n sub 3 as a function of pressure and in mixtures of argon and oxygen are presented for pressures from 10 Torr to 1 atm. Measurements in flowing gases and in static systems are presented. In addition, limitations of these measurements are identified

Asmussen, J.

Kerber, R.

Peters, M.

Rogers, J.

Whitehair, S.

English

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SPATIAL ELECTRON DENSITY AND ELECTRIC FIELD STRENGTH
MEASUREMENTS IN MICROWAVE CAVITY EXPERIMENTS
by	 Sfp 1984
M. ?eters, J. Rogers ,` S. n'hitehair,	
RE(;E1VED
J. Asmussen,^"' and R. Kerber**
U^	 QU;, iii
	
College of Engineering	 42"_
Michigan State University
East Lansing, Michigan 48824
Abstract
Measurements of electron densit y and electric
field strength have been made in an argon plasma
contained in a resonant microwave cavity at 2.45
GHz. Spatial measurements of electron density, ne,
are correlated with fluorescence observations of
the discharge. Measurements of n e
 were made with
Stark broadening and compared with n e
 calculated
from measured plasma conductivity. Additional
measurements of n e as a function of pressure in
mixtures of argon and oxygen are presented for
pressures from 10 Torr to 1 atm. Measurements in
flowing gases and in static systems are presented.
In addition, limitations of these measurements are
identified.
Introduction
In at , earlier study Rogers l
 measured proper-
ties of microwave filamentary argon discharges
created inside microwave cavities. The general
properties, such as electron density, effective
collision frequency, *cc., of these discharges were
investigated as a function of cw microwave power
(10- 1 50W) at 2.45 GHz. argon gas pressure (40-1,200
Torr) and discharge geometry. Photographic and
electromagnetic probing measursments were combined
with conventional microwave circuit measurements to
+ield average discharge complex conductivity from
which discharge average electron densit y
 and
effective collision frequency could be calculated.
Brake at x1. 2 have measured rho electron density
ising the Stark broadening technique and compared
it :ith the work of Rogers l.3 for surface wave
plasmas. Following this work• the goal of the
pre,ent study has been to perform point measure-
ments of the electron density using Stark broaden-
ing and to correlate these measurements with the
observations of Rogers l in order to gain i^eight
into the coupling of electromagnetic energy to such
plasmas with an ultimate goal of coupling electro-
magnetic energy more effectively into solids and
liquids in microwave systems as well.
Experimental ,apparatus
The experimental cavity, shown in Figure 1,
consists of a 1 7 .8 cm i.d., brass cylinder, a
g liding short and adjustable excitation p robe to
provide tuning required to minimize reflected
power. The argon gas was contained inside a + mm
i.d. quirtz 'air that passed axialiv through the
cavit•..
EMIT Lincoln Laboratory, Lexington, MA 0 2103
*This work was supported by NASA Grant No. NAG3-305
**AIAA :member.
Figure 1. Cylindrical microwave cavity
(from ref. (11).
'he cavit y was oriented so thrt its axis was
vertical and was ignited with 2.45 GHz microwave
power in the TM912 mode for all the experiments. A
detailed description of the cavity and the expected
resulting electromagnetic field distributions are
given by Rogers , . A schematic diagram of the
cavit y and the microwave circuit are shown in
Figure 2. The input system consisted of a continu-
ousl y variable 0-500 W cw 2.45 GHz oscillator,
three port circulator, matched load, and direc-
tional couplers leading to two power meters which
measure incident power, P i , and reflected power,
P r . - hus, power coupled into the cavity is given
by P i -Pr = P a . Earlier mea q urements l have shown
that under the experimental conditions discussed
here, 97% or more of the power coupled into the
cavity is actually absorbed by the discharge.
Microcoaxial (0.08 in. c.d.) E field probes
were either inserted through small, tightl y fitting
holes drilled in the cavit y side walls as shown in
Figures 1 and 3 or were placed thrcugha long
narrow slot cut axiall y in the outer cavity
. linder. This -,-or -illcwed -ontinuous sampling of
+hr cavity electric field vs axial position.
The spectral and optical measurements were
made through a screened viewing port soldered into
the c y lindrical wall of the cavity. This viewing
port is different from the one shown in the cavity
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cylindrical microwave
	
.n Figur,- 1.	 It consisted of a 3cm by %cm screenea
flexible
	
Cavity
	
slat cut !xially in the outer 	 cavity cylinder.
coax Thus, during experimencai runs, the microwave
filamentary : 4 schArges could ba viewed axiall y , and
optical point measurements could be performed
through this window axially along and radially
through the discharge.
The light emitted from the plasma was gathered
with two 3.7 cm diameter lenses with focal lengths
of 25 cm. The first lens was placed 25 cm from the
discharge, and the second lens was placed 25 cm
from the entrance slit of the spectrometer leaving
approximately 50 c n between the lenses. At the
exit slit of the grating spectrometer (Spex, No.
1704, f9, 1 meter, Cserny Turn-9r) was a high
quantum efficiency photomultiplier tube, PHT (F-4I
9658R). The output of the PMT was read oa a
digital pico-ammeter (Kiethley 480) and a st.tp
chart recorder (Bristrol 71A-4PG). The spectro-
:
star, lenses and microwave cavity were aligned
with a H e - N. laser at the exit slit of the
spectrometer. The hydrogen Hg line at 4861 A was
used to determine the electron density. There was
enough residual Hi in the argon bottle (99.8x) to
nassure the linewidth. The full width at half
height (fwhh) was measured and the contribution of
the Doppler and instrument broadening to the line
wars used to "decunvolute" the measured linewidth
using tables of the Voigt profiles 4 . From earlier
studies, the translational temperature was assumed
to be 1000'K for determination of the Doppler
width s . The instrument broadening was determined
during each set of experiments by measuring the
width of the argon line at 4200 A which has
negligible Doppler and Stark broadening. The
electron density was determined from the expres-
sion
tight	 2 mm o.d.	 3/2
slip	 n	 C (n	 T) 5 1	 (1)
fit
	
microcoax	 •	 •	 •	 e
where the values of C(n a . Te) were taken from
Griem 6 . The electron temperature, T. was assumed
brass piece	 to be 10,000'K from previous studies 5 ; however, the
value of C(ne. Te) and the resulting value of ne
cavity wall are very insensitive to these assumptions made with
regard to T and T e . A more detailed description of
this apparatus, the calibration technique and the
2 mm exposed	 significance of these assumptions is given by
center conducto	 arake5.
microwave
source
incident
power meter
waveguide r
	
to coax	
T30 d8 attenuatortransition
	
circu-	 1
	
lator	
20 dB directicnal coupler
reflected
power meter
30 dB
attenuator
matched
20 dB directional	 load
coupler
Figure 2. Experimental microwave circuit (from
ref. (1)).
to power meter
power
microwa
power
probe
Figure 3. E-field probes and circuit (from
ref. (1J).
Results
Results of the measurement of the electron
density as a function of axial position are shown
in Figure 4 along with the plasma fluorescence as
recorded by the camera. The electron density is
constant vs length at a value of -10 14 /cm3 . The
electron density clearly dips at the node of the
field; however, the existing apparatus did not
permit sufficient spatial resolution to follow it
through the node. Experimental measurements with
the E-field probes shown in Figures 1 and 2
indicate that maintaining a discharge filament in
the cavit y reduces the T4012 loaned cavity Q from -
3,000-6,000 without a filament to 50-100 in the
presence of a filamentary discharge. Thus, for a
given cavity input power the square of the electric
field strength inside the cavity decreases by
50-100 times when the plasma filament is loading
the cavity.	 Slotted cavity experiments also
yielded similar results. However, they also
indicated that coupling from TMO12 mods to surface
filamentary modes is possible.
L
IG::
J
OF P^
Theradial	 distribution	 of	 the	 electron measurements	 and	 analysis	 is	 required to	 identify
densit y vas	 also measured with stark broadening and ind	 understand the coupling of T4o 1 2 mode power to
the	 value	 again
	 appeared	 to	 r-ma_n nearl y 	constant other
	 plasma cavity ^+odes.
in	 the	 r._ ama 	 regiof..	 However,	 it	 is	 strongly
'	 suspected	 that	 internal	 reflections	 produced	 this Using	 Stark	 bruadening the measured electron
i	 result;	 and,	 hence,	 we were	 not	 actuali y measuring density was	 found to be	 insensitive to the	 ibsorbed
the	 value of no as a function of radius but rather microwave	 power,	 Ar!Oo	 ratios	 from 5	 to m ,	 and
an	 average value.
	
In viewing the onset of signal spatial	 position	 within	 the	 plasma. the electron
as	 a	 function	 of	 position,	 the spatial resolution density	 +as a strong function of	 she gas	 pressure,
appeared
	
to	 be	 approximately	 1	 mn in the axial and the physical	 size of the plasma was dependent
direction. on most parameters studied.
Since	 the constant value of no was counter to
our intuition,
	 several other measurements were made Acknowledgements
to check sensitivity of the measurements to various
parameters.	 First,	 the value of electron density The	 authors	 acknowledge	 the	 support	 of
was measured as a function of pressure and compared NASA-Lewis Research Center, Grant No. 	 NAG 3-305. 
srwith the measurements of Rogers'. 	 This coparison,
shown	 in	 Figure	 5,	 indicates	 that	 the	 Stark References
broadenitg values	 are	 a	 factor of 2
	
to	 6 larger
than	 the "average" electron densities measured by (1). J.	 Rogers,	 "Properties	 Steady Stale,	 High
the conductivity method. 	 For these measurements,
ve
Argon Microwave Discharges," Ph.D.
the gas	 flow rate was
	 lcc/sec	 and the power was
s
Thesis,	 Michigan	 State	 University,	 East
Thesis,
tuned to the point of lowest reflected power in the
r
Lansing,	 Michigan	 (1982).
TNO12 mode.	 The	 tuned absorbed power, Pa ,	 ranged
from	 50-70 watts.	 A subsequent measurement of ne (2). M.	 Brake,	 M.	 Peters,	 J.	 Rogers,	 J.	 Asmussen,
showed that no was relativel y insensitive to power, and R. Kerber, "Electron Density Measurements
(see	 Figure	 6).	 This	 insensitivity	 was	 also of High Pressure Argon Surface Wave Plasmas
observed	 byRogera l .	 The	 optically	 observable IEEF	 International	 Conference	 on	 Plasma
variation	 in	 spatial	 dimensions
	 of	 the	 plasma Science,	 San	 Diego,	 CA (May	 23-25,	 1983).
fluorescence	 is	 the most noticeable parameter to
change during tuning. (3). J.	 Rogers	 and	 J.	 Aamussen, "Standing Waves
Since these measurements take several minutes, Along	 a Microwave	 Generated	 Surface	 Wave
the value of no	 was measured with Stark broadening Plasma,"	 IEEE	 Trans.	 on	 Plasma	 Science,
under constant conditions over a period of time. PS-10,	 11(1982).
For times	 less than 90 minutes, which is typically
twice	 the length of most experiments, there was no (4). J.	 Davies	 and J.	 Vaughan,	 "A New Tabv..lation
change	 in	 the	 observed	 value	 of	 n o .	 This	 is of the Voight Profile," J. 	
of Astrophys.	 137,
displayed	 graphically	 in	 Figure	 7.	 After	 90 1302	 (1963).
minutes,	 the power supply would often overheat and
(5). M.L.	 Brake,	 "A Theoretical and Experimental 	
jshut off. Investigation of the Chemical Kinetics of an TTT
Finally,	 measurements of no using the Stark Oxygen Microwave	 Discharge,"	 Ph.D.	 Thesis,
technique were made while	 the flow rate and the Michigan	 State	 University,	 East	 Lansing,
Ar/0i	 ratio were varied.	 As	 shown	 in	 Figures 8 Michigan	 (1983).
and	 9,	 these	 results	 indicate	 that	 over	 the
parametric	 range	 shown,	 n„ is	 insensitive to flow (6). Grim,	 Plasma Spectroscopy,	 McGraw Hia.'.NH.
ew Yorkk
rate and Ar,Oi	 ratio.	 For	 smaller ratios of Ar/02, Ne,	 1964.
the	 plasma	 could	 not	 be	 sustained	 Lnder	 these
conditions.
i
Conclusions
An experimental study has been performed to
examine the relative spatial distribution of
electron density and electromagnetic fields in a
resonant cavity. At a constant discharge pressure
the electron density and electric field strength
were observed to be nearly constant inside the
discharge region. The electron densities measured
with the Stark broadening method were 2-6 times
larger than those inferred from the plasma conduc-
tivity. This difference can be explained in part
by noting the difference between the two measure-
ment techniques. The electromagnetic method of
Rogers' measures th.. electron density averaged
over the entire plasma volume. The plasma volume,
determined from calibrated photography, was defined
as that volume that emitted intense visible light.
A more detailed knowledge of the actual plasm n
volume may resolve this difference in density
measurements.
	 The initial slotted cavity experi-
ments also indicated that further, more detailed,
_ • 1 .- ^ry►^11 'sue+
`CL
I
P • 200 torr
Pa
 • 55-100 watts
Q
ORIGINAL PAr- 79
OF POOR QUHLIII—,'
1015
I
b
101
0	 1	 2	 3	 4	 5	 6	 7	 8	 9
AXIAL DISTANCE (cc)
Figure 4. (a) Electron density vs. axial position as measured by Stark
broadening at 200 torr. (b) 1. sketch of plasma fluorescence vs.
length of the upper two parts of the plasma filament as it appears
along the cavity axis. The absorbed power was tuned to 55-100
watts during the Stark broadening measurements.
1015	 7
•	 1
•	 r:
1014
.^'
• PIOffO COn011C I1v11'
• Stark Lroaft Ing
Pa 
- 
40-10 watts
	 3'
1013 11.1 1
102
	103	 I
PRESSURE (torn
figure 5. A comparison between measured electron density
by the conductivity method Tand 2tark broadening.
The argon flow rate was 1 cm isec in a 4 mm i.d.
quartz tube with 40-70 watts absorbed power tuned
to minimum reflection.
4
4 a
OF POOR QL' %1 TV
1E
u
m 101
m
C
b
E
m
30 40 50 60 70 80 90
AISORB[D POWER (WATTS)
Figure 6. Effect of absorbed power on electron density.
The argon pressure was 150 torr with no flow.
101l,
U
t, tn14
A
Wn
0
a
v
tr
y
1013
0	 1	 2
TIME (HOURS)
Figure 7. Effect of discharge time on electron density.
The argon pressure was 150 Corr with no flow.
5
,^ r .I•^ s's' 'phi ^ -	 ^	 -
WN
C
W
W
W
r
6
a.^ r 11
ti
	
0^tCIN4L PP,*. -
 ' c
OF POC,R Q
l^
{i
0	 1	 2	 3	 4	 b	 o
FLOWMATE 1CW5/NCI
Figure 8. Effect of flow rate on electron density.
The argon pressure was 750 torr with
+5 watts abs:r , ed power.
1	 lu	 lvv
^' o.rtnriuria
{'are 4.	 _. :' _c:	 I, r.1tiJ on electron
density . The has mixture was held
at ?00 torr with no flow. The absorbed
power was 22-350 watts.


Spatial electron density and electric field strength measurements in microwave cavity experiments

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