We performed a detailed characterization a plasmoid in the afterglow region of an Ar supersonic microwave cavity discharge. The supersonic flow was generated using a convergent-divergent nozzle upstream of the discharge region. A cylindrical cavity was used to sustain a discharge in the pressure range of 100-600 Pa. Optical emission spectroscopy was used to observe populations of excited and ionic species in the plasmoid region. Plasmoid formation in the supersonic flowing afterglow located downstream from the primary microwave cavity discharge was characterized by measuring the radial and axial distributions of Argon excited states and Argon ions. More experiments are being carried out on the plasmoid to understand the discharge parameters within the region, i.e. rotational temperature, vibrational temperature, electron density, and how the electrodynamic and aerodynamic effects combine to form this plasmoid

Drake, D. J.

Miller, S.

Nikolic, M.

Popovic, S.

Vuskovic, L.

NASA Technical Reports Server

29th ICPIG, July 12-17, 2009, Cancún, México
Characterization of a Plasmoid in the Afterglow of a 
Supersonic Flowing Microwave Discharge
D. J. Drake1, S. Miller1, M. Nikolić1, S. Popović1, and L. Vušković1
1Department of Physics, Old Dominion University, 4600 Elkhorn Ave., Norfolk, VA 23529
P
We performed a detailed characterization a plasmoid in the afterglow region of an Ar supersonic 
microwave cavity discharge. The supersonic flow was generated using a convergent-divergent 
nozzle upstream of the discharge region.  A cylindrical cavity was used to sustain a discharge in 
the pressure range of 100-600 Pa.  Optical emission spectroscopy was used to observe populations 
of excited and ionic species in the plasmoid region.
1. Introduction
We describe in this report a plasmoid-like structure 
appearing as a secondary, downstream phenomenon 
coupled to the microwave cavity discharge in the 
supersonic  f low in  argon.  The  p lasmoid  i s  
apparently sustained by a low-power surface wave 
which propagates along its surface and the surface of 
the containing quartz tube. Plasmoid formation is 
observed in pure argon in supersonic flow, while it 
is not sustained in the subsonic flow or in the 
mixture of argon with hydrogen (>1%). Some 
characteristics of the discharge suggest behavior
similar to other surface microwave discharges [1], 
but the full interpretation requires inclusion of the 
aerodynamic arguments. For instance, the Abel-
inverted line intensities of Argon ion lines have the 
characteristic contour profiles of the surface wave-
sustained discharges. The fact that the plasmoid 
structure is detached from the plasmoid and depends 
on the flow velocity suggests the importance of 
aerodynamic mechanism. 
2. Experimental Approach
The experimental set-up  shown in  F ig .  1  i s  a  
combination of supersonic flow tube and a
microwave cavity discharge.  Using an evacuated 
quartz tube at pressures of 1–3 Torr, supersonic flow 
was generated with a Mach 2 cylindrical convergent-
divergent (de Laval) nozzle upstream of the cavity.  
We used a commercial microwave generator, 
operating in the S-band at 2.45 GHz, to sustain a 
cylindrical cavity discharge at power density 
between 0.5 and 4 W/cm3.  Argon gas was fed into 
the stagnation chamber through a gas manifold.  The 
gas flow was established by using a roots blower in 
conjunction with two roughing pumps.  The capacity 
of the pumping system allowed a supersonic flow to 
be generated in a pressure range of 1 – 20 Torr.
FIG. 1. Scheme of the supersonic flowing experiment.
We used optical emission spectroscopy as our 
primary diagnostic tool to observe the spectra of the 
excited states.  Although there are uncertainties in 
this method, it is useful in these experiments since it 
is simple, nonintrusive, and in situ.  In Fig. 2, we 
show that a CCD camera was used in conjunction 
with a spectrometer for measurement of the gas and 
electron temperature [2].  
FIG. 2. Optical emission spectroscopy set-up.
The observed spectral frames were calibrated using a 
Spectra Physics Pen lamp and the intensity was 
calibrated using a Newport/Oriel absolute blackbody 
Topic number: 09
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29th ICPIG, July 12-17, 2009, Cancún, México
irradiance source.  Graphs of irradiance per count 
versus wavelength were calculated for the three 
spectrometer  grat ings .   We determined the  
population of particular excited state transitions by 
using these graphs.  
The populations of Ar excited states in a model free 
flow are shown in Fig. 3.  By comparing the 
population values with a Boltzmann plot (straight 
line), we observe that at higher energies, the 
population decreased.  This effect was more 
pronounced at the higher power density.
FIG. 3. Population of ArI states as a function of the 
energy level.
3. Analysis of a Plasmoid in the Afterglow Region
In the afterglow region, we observed the formation 
of a plasmoid, see Fig. 4.  
FIG. 4. Picture of the plasmoid in an Ar discharge at a 
pressure of 2.5 Torr and a power density of 1.45 W/cm3. 
We made measurements of the populations of the Ar 
excited states in this region at 1.0 cm increments 
from the microwave cavity.  In Fig. 5 we observe 
that the population of the Ar 4p[3/2] → 4s[3/2] o
transition at 706.72 nm changes by an order of 
magnitude across the plasmoid region.  In addition, 
to measurements along the axis of symmetry for the 
quartz tube, we measured the population at 0.3 cm 
increments vertically.  The collected data was used 
to interpolate the change in the population over the 
entire plasmoid region and we present these results 
in Fig. 6.  From the figure we see that the population 
of the excited states is greater at the head of the 
plasmoid.  However there is also a distinctive 
separation of the ionic states in the plasmoid, one at 
a height of 0.7 cm and the other at 2.4 cm.  This 
separation could be due to the fact that our plasmoid 
is more doughnut shaped.
FIG. 5. Population of the ArI 4p[3/2] → 4s[3/2] o state at 
706.72 nm along the axis of symmetry of the plasmoid as 
a function of the distance from the microwave cavity.
29th ICPIG, July 12-17, 2009, Cancún, México
FIG. 6. Population of the ArI 4p[3/2] → 4s[3/2] o state at 706.72 nm of the plasmoid as a function of the distance from 
the microwave cavity and the height in the quartz tube.
FIG. 7. Population of the ArII 4p4Po → 4s4P state at 480.602 nm as a function of the distance from the microwave 
cavity.
We also measured the populations of the Ar II 
4p4Po → 4s4P state at 480.602 nm, see Fig. 7.  As 
in the case of the lower energy transitions, we 
observe that population is greatest at the head of 
the plasmoid.  In addition, we also observed two 
distinct peaks in the population like in the case of 
the Ar excited state.
The evaluated values of the population in Figs. 5-
7 are based on the measured spectral line 
intensities, I(x), in the discharge.  However, 
measurements taken in the side-on direction were 
integrated over the entire depth of the discharge 
region.  Assuming cylindrical symmetry, then the 
radial emission coefficient, ε(r), is related to the 
measured intensities by the Abel transform,
0
0
( ) ( )
y
y
I x r dy

  . (1)
Inverting Eq. (1) , provides a way to determine 
ε(r) from the experimental values of I(x),
2 2
1 '( )( )
R
r
I xr dx
x r


 
 . (2)
Here I’(x) is the first derivative of I(x) with 
respect to x and R is the plasma radius.  
Measurements of the spectral lines provide only 
discrete values of I(x).  Thus solutions of Eq. (2) 
can only be approximated.  In addition, there is a 
discontinuity at x = r and ε(r) is dependent on 
I’(x) .   To proceed, we assumed 2N+1 discrete 
points for the intensity distribution I(xi), where xi
= i Δx and i = -N...0...N.  This distribution is used 
to find a N +1 distribution ε(rj), where rj = j Δr 
and j = 0...N.  In addition, we must assume that Δr 
= Δx, r0 =  x0 = 0, and rN = xN = R.  Direct 
discretization [3] of Eq. (2) gives a good 
approximation of the values of ε(rj) provided that 
I(xi)  h as been previously interpolated and 
smoothed,
1
1
2
2
( ) ( )1( )
2
N
i i
j
i j
i j
I x I xr
xx r





 
    
 . (3)
Applying Eq.  (3)  to  the  da ta  in  F ig .  7  we  
calculated the populations for the Ar II 4p4Po → 
4s4P state at 480.602 nm and present the results in 
Fig. 8.  Once can see from the figure that the 
discharge seems to be located at the surface of the 
quartz tube.
29th ICPIG, July 12-17, 2009, Cancún, México
FIG. 8. Population of the ArII 4p4Po → 4s4P state at 480.602 nm as a function of the distance from the microwave 
cavity.
In addition, we also observed the change in the 
plasmoid with pressure.  In Fig. 9, we present the 
normalized population for the 706.72 nm line at 5.1, 
2.4, and 1.5 Torr.  From the figure we observe that 
the center of the plasmoid, where the population was 
largest, appeared to be moving closer to the cavity as 
the pressure is increased.  This change in the 
location of the plasmoid was confirmed visually.
FIG. 9. Normalized population for the state at 706 nm at 
pressures of 5.1, 2.4, and 1.5 Torr.
The electron temperature distribution across the 
plasmoid region was obtained from the intensity 
ratio of an Ar atomic and an Ar ionic spectral line.  
For the discharge we used the Ar I 5p[½] → 4s’[½]o
transition at 470.232 nm and the Ar II 4p2Po  → 4s2P 
state at 476.487 nm.  Then by assuming chemical 
equilibrium between the Ar neutrals and Ar ions, we 
can apply the Saha-Boltzmann equation to determine 
the electron temperature (Te) [4],
3 22
exp
2 2
e i o n e x c
e B e B e
N g E EN h
N g m k T k T

 
       
   
. (4)
Here N is the population of each state, g is the 
statistical weight, h is Planck's constant, me is the 
mass of the electron, kB is the Boltzmann constant, 
Eion is the ionization energy of the electron in the 
5p[½]  state for the Ar I transition that is 1.2956 eV, 
and E+exc is the excitation energy of the  4p2Po state 
of the Ar II transition that is 19.867 eV. We present 
these results in Fig. 10.  From the figure we observe 
that the change in Te was small over the entire 
plasmoid region (Δ = ± 90 K), which would seem to 
indicate that this phenomenon is most likely due to 
aerodynamic effects.  
To confirm these results we moved the microwave 
cavity at 1.0 cm increments.  If the plasmoid is 
mostly due to aerodynamics, then the plasmoid 
would stay in the same place within the quartz tube.  
However, we found that the plasmoid moved with 
the microwave cavity, which indicated that there are 
electrodynamic effects causing this phenomenon as 
well as the aerodynamic effects.
29th ICPIG, July 12-17, 2009, Cancún, México
FIG. 10. Electron temperature as a function of the distance from the microwave cavity and the height in the quartz tube 
for an Ar discharge at a pressure of 2.5 Torr.
4. Conclusions
Plasmoid formation in the supersonic 
flowing afterglow located downstream from the 
pr i m a r y  m i c r o w a v e  c a v i t y  d i s c h a r g e  w a s  
characterized by measuring the radial and axial 
distributions of Argon excited states and Argon ions. 
More experiments are being carried out on the 
plasmoid to understand the discharge parameters 
within the region, i.e. rotational temperature, 
vibrational temperature, electron density, and how 
the electrodynamic and aerodynamic effects 
combine to form this plasmoid.
5. References
[1] C. M. Ferreira and M. Moissan, Physica Scripta, 
38, 382 (1988).
[2] D. J. Drake, S. Popović, and L. Vušković, J. 
Appl. Phys. 104, 063305 (2008).
[3] R. Álvarez, A. Rodero, and M. C. Qunitero, 
Spectrochimi. Acta B 57, 1665 (2002).
[4] A. Rodero, M. C. Garcia, M. C. Quintero, A. 
Sola, and A. Gamero, J. Phys. D: Appl. Phys. 
29, 681 (1996).


Characterization of a Plasmoid in the Afterglow of a Supersonic Flowing Microwave Discharge

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