Experimental study on the characteristics of low power microwave
plasma jet within local vacuum environment
Juan Yang*
Yang Qiao Xu
Bing Zhu
Gen Wang Mao
Liang Ming Zhu
College of Astronautics, Northwestern Polytechnic University, Xi’an 710072, China
A microwave plasma jet based on a coaxial cavity can be generated in atmosphere and vacuum environment.
It is shown that with argon gas and a power range of 53～60W, cavity efficiency ranges from 54 to 68%. The
electron density distribution and the microwave return loss of the confined plasma jet adjacent to a metal object
and their dependency on argon mass flow rate and power have been studied by applying emission/Langmuir probe
and spatial reflected wave diagnostic equipments in a low scatter vacuum environment. The results show that the
electron density ranges from 8.8×1014 to 7.53×1016 /m3, the electron density on the centerline of the jet decreases
exponentially from the nozzle exit plane, but its distribution off the centerline is in up heaved curve. Increasing
mass flow rate at constant power and increasing power at constant mass flow rate increase electron density mildly.
From typical measurements of microwave return loss, it is noted that the plasma jet attenuates microwaves in 6 to
8GHz range.
*yangjuan@nwpu.edu.cn
Ⅰ. Introduction
Plasma can be generated through electron-impact and photon ionization. In the electron-impact process,
energy for sustaining the plasma can be provided via hot arcs and radio frequency waves of all frequencies. Of all
the possibilities, microwave ionization for sustaining the plasma has the advantage of low power, cathodeless
operation than the hot arc. The microwave generated plasma has been applied in deep space detection1-2 and is
under study for future applications in space propulsion3-4. Another possible application is radar cloaking in which
an object with high scattering coefficient is covered with a plasma cloud which absorbs incident microwave
radiations. Plasma concealment is more effective and flexible5-8 compared with body design and wave absorbing
coating methods.
According to the theory of microwave propagation, effective wave absorption by non-collision plasma is
governed by the relationship between the microwave and plasma frequency. The condition of microwave
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2 2
propagation and absorption in a plasma is electron density n p  n cr , where n cr  4π m e ε 0f /e is the cutoff
density. When n p  n cr , the wave can not penetrate the plasma and is reflected. For collision plasma, the
relationship among wave attenuation, wave and plasma frequency, and particle collisional frequency is
complicated. The plasma characteristics such as electron density and particle collisional frequency greatly affect
wave attenuation9-10. Therefore, to be an attractive wave absorber, the plasma must have an appropriate electron
density for a given background pressure and given wave parameters.
For application study of radar cloaking, a large number of references cover numerical simulations11-15 and a
few experimental research16 related to the interaction between microwaves and plasmas. Numerical simulations
assume the plasma model as uniform or non-uniform slabs or balls with special electron density distributions.
However, actual plasma can not be represented by these regular shapes. It is a complicated task to simulate wave
propagations in plasma when it has a non-regular shape.
Based on the aforementioned background, it is necessary to experimentally study the characterization of the
actual plasma jet. As shown in Figure 1a), our experimental model consists of a low power microwave plasma
generator operating at 2.45GHz and less than 60W, and a quartz container and metal plate. The experimental
model is positioned in a large vacuum chamber to create a vacuum background. A plasma jet from an exit hole of
the generator cavity is sprayed into the container. Its electron density distribution is characterized by
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emission/Langmuir probe. The metal plate and the end of the cavity are strong scatter objects, and the plasma jet
enveloping them can be considered a microwave absorbent coating. When the experiment model is put into a low
scatter vacuum chamber, the microwave return loss in the near field of a measuring antenna can be obtained using
the spatial reflected wave measurement method. Experimental results and analysis explain the effects of an actual
plasma jet on microwave absorption in the near field of the antenna.
a)
b)
Fig.1 Experimental system (dimensions in mm)a) Integrated experimental set upb) Cavity structure and field distribution
Ⅱ．Experimental Setup
A．Microwave plasma generator and experimental model
As shown in Figure 1a), a low power microwave plasma generator is composed of three parts: solid state
microwave source, a cavity and a gas supply subsystem. The solid state microwave source changes the electric
power into microwave energy at 2.45GHz and less than 60W, which is transferred to the cavity through a coaxial
circulator with three ports. The reflected microwave energy from the cavity can be transmitted to the attenuated
detector for examination.
Figure 1b) shows the cavity, in the form of coaxial structure with a concentrated capacitance. It is composed
of an inner and outer conductor and a coupling probe. Radial and axial electric intensity Er and Ez, tangential
magnetic intensity HΦ exist in the cavity. Near the end tip of the inner conductor, Er and Ez become stronger,
creating a favorable condition for gas breakdown. Also, the gap between the end tip of inner conductor and the
inner side of outer conductor forms a concentrated capacitance, which results in the cavity length being less than a
quarter wave length. When gas is injected, it is ionized in the strong electric field zone and the resultant plasma jet
is accelerated through the nozzle.
In Figure 1b), the inside surface diameter of the outer conductor and outside surface diameter of the inner
conductor are chosen to be 40 and 10 mm, respectively, based on the consideration that the structure must
suppress the appearance of an inhomogeneous wave mode and support maximum power. Another compromise is
that the cavity must have high quality factor. The length of the cavity is chosen to be 31mm, which is less than the
quarter wave length at 2.45GHz. The nozzle throat diameter is chosen as 0.6mm in order to sustain appropriate
gas pressure in the cavity and appropriate mass flow rate.
Including the microwave plasma generator, the experimental model is built up as shown in Figure 1a). It is
composed of the cavity, metal plate and quartz container with 260mm in diameter and 300mm in length. A pipe is
joined on the metal plate, which is used to vacuum the container when this model is placed in a large vacuum
chamber.
B．Emission/Langmuir probe
The Langmuir probe is an effective tool for diagnosing the distribution of plasma electron density. The
theory of the Langmuir probe is based on the assumption that the probe radius is small compared to the collision
mean free paths but large compared to the Debye length, which means when charged particles impact the probe
biased from negative to positive voltage, they must be completely absorbed by the probe. It is important to define
the charged particle collecting area of the probe surface in order to calculate the electron and ion collection current.
2
Also, obtaining the electron saturation current is important for calculating the electron density according to the
I-V characteristics of probe. But when the probe is immersed in the plasma jet there will appear a wake region, in
which the charged particles may not be absorbed normally. Sometimes, the thickness of this plasma shell increases
slowly with an increase in the probe potential, which causes the electron collecting area and current to increase
slowly. This circumstance makes it difficult to accurately define the charged particle collecting area and electron
saturation current. Therefore, another method is needed to specify the point at which the electron saturation
current appears on the probe’s I-V curve. The emission probe is an effective tool for doing this.
The emission probe is a tungsten filament heated by current. When the current is too low to induce the
filament to emit electrons, the I-V characteristic of the probe is same as that of the Langmuir probe. But when
current is increased to a high level, the filament emits electrons. As shown in Figure 2, in the ion saturation region,
the filament potential is lower than that of the plasma, and the emission electrons can come into space without
resistance to generate greater emission electron or ion current than Langmuir probe. At the electron saturation
region, the filament potential is higher than that of the plasma, which leads the surrounding electrons to be
absorbed by the filament but the emission electrons are turned back. In this region, although the emission probe is
heated by current, the I-V curve is same as Langmuir probe. When the filament potential counterbalances with
that of plasma, the emission electrons will begin to come into the surrounding area and the I-V curve drops
suddenly. In this situation, plasma spatial potential can be
defined accurately and electron density can be calculated.
According to the similarity and difference between I-V
characteristics of emission and Langmuir probes, the emission
probe can be applied to diagnose not only the plasma spatial
potential but also electron density.
The diagnostic arrangement is illustrated in Figure 1a).
Since the experimental setup is fixed in a large vacuum
Fig.2 I-V characteristics measured by emission probe
chamber, the probe penetrating through the metal plate is
mounted on a stand that allows it to be shifted in the axial
direction of the cavity.
C．Low scatter vacuum experimental environment and microwave measuring equipment
The integrated experimental system is also shown in Figure 1a), in which the vacuum chamber of 1.2m in
diameter and 3m in length is lined with a rubber wave absorber for a low scatter vacuum environment. The
chamber is pumped by two sets of Lobe-type vacuum pumps. They maintain a background pressure of less than
10Pa throughout the experiments. Testing has shown that the chamber wall lined with the rubber wave absorber
can attenuate microwaves by 10dB from 4 to 10GHz. The experimental model is mounted in this environment to
investigate the effect of the plasma jet on shielding the metal plate from microwaves.
One pair of transmitting and receiving antennas is mounted 520mm from the metal plate. The distance
between the centerlines of the two antennae is 170mm. The antenna exit plane faces the metal plate, such that the
transmitting antenna emits a test wave supplied by the network analyzer toward the metal plate and cavity,
through the quartz container and the plasma jet. The receiving antenna receives the reflected microwave passing
through the reverse path and transmits it back to the network analyzer. The measured parameter is the wave return
loss Lr  10lg  Pr Po  , where P0 is the microwave power induced by the network analyzer and Pr is the
reflected power.
When the transmitting antenna propagates the test wave toward the metal plate and cavity, the plate and
cavity will reflect wave toward all sides. Neglecting the quartz effect on the microwave, the received microwave
power Pr by the receiving antenna is composed of Pr1 and Pr2. Pr1 is directly from the metal plate and cavity, Pr2 is
from the rubber wave absorber lining on the chamber wall. Pr2 is less than Pr1 because the absorber will attenuate
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the incident microwave power by about 90 percent (10dB return loss). Before generating plasma, Lr is first
calibrated to zero, which means that Pr is at a reference level. After plasma is generated, the plasma may also
'
reflect microwaves in all directions. Assuming the total power received by the receive antenna is Pr , which is also
'
'
'
'
composed of Pr1 and Pr2 , Pr1 comes directly from the metal plate and plasma jet. Pr2 is from the absorber.
Pr1' contains the main part of Pr' . Therefore the return loss
L' r of the test wave propagating into the plasma jet
surrounding the metal plate and cavity can be obtained
and gives the shield effect of the plasma jet.
One pair of horn antennas, operating frequency
ranges from 6 to 8GHz, is used in the measurement. One
Fig.3 Antenna structure
of their structures is shown in Figure 3, where the E and
H plane is defined as that parallel and perpendicular to the electric intensity. The dimensions of their exit planes
are chosen as 200mm×200mm for matching the cross section of the plasma.
Ⅲ．Experimental results and analysis
A. Microwave plasma jet generation
In order to reduce the reflected microwave power from the cavity to a minimum level, the cavity of plasma
generator must be regulated on network analyzer and the microwave source with low output power respectively to
match the impedances of cavity and its input. Then plasma can be formed within the cavity with high efficiency
and exits through the nozzle, at microwave output power up to 40W.
Experimental results:
(1) Microwave power for sustaining plasma can be reduced after the gas discharge.
After the plasma is started at 40W, it can be steadily sustained at 30W. Before an equilibrium state is reached,
electrons diffuse rapidly to the boundary and electrons determine the plasma diffusion coefficient.
After the ionized gas reaches equilibrium, plasma shell is formed and induces additional ion diffusion, which
establishes the plasma ambipolar diffusion coefficient as defined by fast electron and low speed ion movements.
Electrons in the equilibrium plasma will be confined by the static bipolar electric field. This makes the electron
diffusion coefficient less than that in a free moving environment and thus decreases the microwave power
required to maintain equilibrium plasma.
(2) Pressure required to sustain large volume plasma jet.
Experiments shows that 0.5～2atm absolute pressure within cavity can produce a large volume plasma jet.
When the cavity absolute pressure is greater than 2atm, the volume of cold gas passing through the discharge zone
is too large for the available microwave energy. This decreases the ionization percentage. On the other hand,
when the cavity absolute pressure is less than 0.5atm, although gas can absorb enough microwave energy and,
thus, there is a high degree of ionization, lower pressure produces lower plasma exit speed and a smaller jet
volume.
(3) Strong cavity coupling will reduce the reflected power from the cavity and stabilize operation.
The loaded quality factor QL is a performance parameter of the plasma generator cavity. It is defined as
1 QL =1 Q0 +1 Qe , where Q0 is the intrinsic quality factor of the cavity which is defined only by its structure,
Qe is the plasma load quality factor. Defining Pd as the power loss on the cavity wall and Pe as the power absorbed
by plasma within the cavity, results in a coupling coefficient β= Pe Pd = Q0 Qe , and QL = Q0 1+  17. At
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given cavity structure, QL is defined by β, which is affected by the coupling probe shape and the gap between
the probe and the inner conductor. Resonant experiments show that a high coupling coefficient corresponds to a
small gap between probe and inner conductor, and a low loaded quality factor QL reduces reflected microwave
power from the cavity and increases the operating frequency band, which makes the generator operation more
stable.
Defining
Fig.4 Cavity coupling efficiency vs. mass flow rate
eff  Pout  Pr  Pout as the cavity coupling
efficiency, where Pr is the reflected power from the cavity and Pout
is the output power from microwave source, the efficiency curves
as a function of the variation in argon mass flow rate at
atmosphere and vacuum conditions are given in Figure 4. The
curves show that the cavity efficiency ranges from 54 to 68% at
microwave output power levels of 53 to 60W. At a fixed mass flow
rate, cavity efficiency increases as output power increases. At
fixed output power and mass flow rate, the cavity efficiency in
vacuum is greater than that in atmosphere.
B．Electron density
Three bend probes are installed in the experimental model as shown in Figure 1a), in which one filament is
on the centerline, and the other two are 50mm and 80mm from the centerline. Although the probes can be moved
continuously in the axially direction, diagnostics are taken starting at 20mm from the nozzle exit plane and in
25mm intervals. The probe insertion depth into the experiment model is limited to 170mm because of the
interference between the probe stand and the experiment model. At each location, three points of power and five
points of argon mass flow rate are applied: 46W, 52W and 59W, 21 mg/s, 42 mg/s, 63 mg/s, 84 mg/s and 105mg/s.
(1) Spatial distribution at constant power and dependency on mass flow rate.
Figure 5 shows electron density profiles in the axial directions with various mass flow rates at microwave
output power of 59W. L and r are the distances from the nozzle exit plane and the centerline of the plasma jet,
respectively. The diagnosed density ranges from 9.6×1014 to 1016/m3.
It can be seen from the profiles showed in Figure 5a) that axial electron density along the centerline
decreases exponentially from the nozzle exit plane at every mass flow rate. The dependence is consistence with
that presented in Reference 18. The difference is due to the effect of the quartz container on the plasma jet. The
same variation of mass flow rate induces less discrepancy in electron density in a confined plasma jet, but
noticeable variation in a free plasma jet. Figures 5b) and c) show that the plasma in the middle part of jet is denser
and diffuses farther than the two ends, which was anticipated since it is known that the nozzle plume has the same
characterization. Electron density is more sensitive to the mass flow rate when the position is far from the
centerline and close to the nozzle exit plane.
a)
b)
c)
Fig.5 Axial electron density distribution with various mass flow rates at 59W output power a) r=0, b) r=50, c) r=80mm
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(2) Spatial distribution at constant mass flow rate and its dependency on output power.
Figure 6 shows electron density profiles in the axial directions at various microwave output powers and mass
flow rate of 63mg/s. Electron densities range from 8.8×1014 to 7.52×1016/m3. All the profiles show that increasing
microwave output power increases the electron density in the plasma jet. This is consistent with Figure 4 in the
plasma generation experiment. When the microwave power is transmitted into the cavity filled with about 1atm
gas, the collision frequency of the particles in the induced plasma is very high which means power is absorbed
without the limitation of the electron cutoff density. This results in an increasing percentage of ionization and
electron density with increasing microwave power. The profiles show that at far distances from the centerline and
nozzle exit plane, electron density is more sensitive to microwave power.
a)
b)
c)
Fig.6 Axial electron density distribution with various power levels at 63mg/s mass flow rate a) r=0, b) r=50, c) r=80mm
C．Microwave return loss
The antennas are mounted such that the E plane is parallel to the ground level, which is called horizontal
polarization. Two points of power and three points of argon mass flow rates are applied: 46W, and 59W; 21 mg/s,
63 mg/s, and 105mg/s. Calibration of the experimental model is accomplished by pumping the chamber to 0.1 Pa,
then adjusting the mass flow rate to 105 mg/s which causes the background pressure to increase to about 5 Pa.
Figure 7 shows the calibration plot with antennas in horizontal direction. The plot shows deviation amplitude of
0.135dB, which can not be zeroed out after many calibrations. It is
caused by the vibration of the experimental system.
The measurements of the horizontal polarized wave return
losses in the plasma jet are given in Figure 8. Different symbols
present the different experimental data taken at different power, and
different lines present the associated curve fits. Experimental data
shows an obvious frequency oscillation, which can be explained by
the vacuum chamber and plasma jet being regarded as a big
resonant cavity with a disturbance object in it. At a given frequency
point, if the plasma state is in resonance with the chamber, more
detection power will be absorbed by the plasma and the chamber.
Fig.7 Calibration profile of experimental model
We call this abnormal absorption. The return loss measured at a non
resonant frequency can actually give the wave attenuation in the plasma jet. Otherwise, curve fits can correct the
data at the resonant frequency. Profiles in Figure 8a) show wave return loss at mass flow rate of 21mg/s is nearly
zero because of the small volume of generated plasma. Figure 8b) gives the wave return loss at a mass flow rate of
63mg/s and microwave power of 59W and 46W. The profiles show the wave return loss decreases gradually from
5dB to 0. Figure 8c) gives the wave return loss at a mass flow rate of 105mg/s and microwave power of 59W and
46W. The profiles have the same variation as Figure 8b), but Figure 8c) shows from 6 to 8GHz the maximum of
the wave return loss reaches 7dB.
Reference 19 reports that the plasma plume of an arcjet operating at back ground pressure of 53Pa has the
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same electron density magnitude as this microwave plasma jet. The arcjet can only attenuate microwaves from 1
to 4GHz. The attenuation decreases with increasing frequency. The reason is the increasing wave frequency
induces low wave spatial damping rate. The variation trend reflected by Figure 8 is same as Reference 19, but
their magnitude is different, which is caused by the different experiment conditions.
a)
b)
c)
Fig.8 Wave return loss a) at 21mg/s flow rate, b) at 63mg/s flow rate, c) at 105mg/s flow rate
Ⅳ. Conclusions
A solid state microwave source at 2.45GHz frequency and a coaxial cavity comprises a compact, low power
microwave plasma jet generator. It produces a stable plasma jet in atmosphere and vacuum at a microwave power
of less than 60W.
The present work is unique in two respects. First, an emission probe is applied to diagnose the plasma jet
confined by a quartz container and enveloping a metal plate. This probe characterizes the shape of the plasma jet,
the electron density distribution and its dependence on mass flow rate and microwave power. At microwave
powers of 46W, 52W and 59W, and mass flow rates of 21mg/s to 105mg/s, the diagnosed electron density ranges
from 8.8×1014 to 7.53×1016/m3. The electron density on the centerline of jet decreases exponentially beyond the
nozzle exit plane. Its distribution off the centerline is in the shape of up heaved curve.
Second, using spatial reflected wave measurement diagnostic equipments, the attenuation of the horizontal
polarized wave by the confined plasma jet enveloping a metal plate is measured in a low scatter vacuum
environment. The data shows that the plasma jet can attenuate the incident microwave.
For construction of a low scatter environment, we can choose a different type of wave absorber, a soft rubber
with small bubbles and plain rubber. Although the soft rubber can attenuate microwaves more than 30dB and
provide a very low scatter environment, its volume is too large for the vacuum chamber, and out gassing from its
small bubbles overwhelms the vacuum system. Therefore, we chose plain rubber which is small in volume and has
a solid interior. It satisfies the spatial and vacuum pressure demands, but with a wave attenuation of only 10dB
from 1 to 10GHz, which may induces uncertainty in experiments. Therefore to get an exact understanding of the
interaction between microwaves and the plasma jet, further investigations are required utilizing an improved
vacuum environment and equipment.
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