22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Performance of low-power nitrogen and helium arcjets at various
back-pressures
X. Meng, W. Pan, H. Huang and C. Wu
State Key Laboratory of High Temperature Gas Dynamics, Institute of Mechanics, Chinese Academy of Sciences,
100190 Beijing, P.R. China
Abstract: Experiments on a low power arcjet thruster with natural radiation-cooled nozzle
have been carried out, and pure nitrogen and helium have been used as the propellant. The
specific impulse, thrust efficiency and arc voltage have been measured and analysed.
Results show that pressure in the test chamber is an important factor that influences the
performance of the arcjet thrusters even though flow in the nozzle is considered supersonic.
Keywords: low-power arcjet, specific impulse, thrust efficiency, voltage, pressure
1. Introduction
Arc-heated jet (arcjet) thruster is a kind of simple and
effective electric propulsion system for space applications
such as North-South station-keeping of geosynchronous
satellites and orbit transfer applications, and 2-kW class
hydrazine arcjet thrusters have been successfully applied
to many satellites since the 1990s [1]. In this kind of
thruster, gaseous propellant is heated by a DC arc
discharge to a temperature over 104 K, and subsequently,
converted into a supersonic plasma jet through a
convergent-divergent nozzle, issuing into the ambient
atmosphere, where the pressure is low, producing a thrust
force. The precise measurement of the thrust is a
precondition to obtain the thruster performance. In
general, the thrust can be measured by direct method that
directly mounting the thrusters on a thrust stand and
taking the measurement with a force measuring system, or
by indirect method mostly measuring the impulse of the
arcjet [2].
In principle, arcjet is capable of operating on a variety
of propellants, to meet space mission requirements. Many
research papers have reported on the lifetime of the arcjet,
anode erosion, plume parameter measurement and
performance of the arcjet thrusters with different
propellants such as hydrazine, ammonia, hydrogen,
N 2 /H 2 mixtures and so on [3-5]. Helium has the highest
ionization energy among all the atoms, but a smaller
atomic mass. and therefore helium arcjet can deposit more
input electric power per unit mass into the propellant. As
a monatomic gas, it has no internal vibrational and
rotational energy modes. Arcjet thrusters with helium
propellant have a higher efficiency of conversion from the
input electric energy by arc heating of the working gas to
the exhaust gas kinetic energy. The experimental study of
helium arcjet has attracted wider attention in recent
years[6].
In this work, pure nitrogen and helium have been used
as the propellant. In space application, the environmental
pressure is near vacuum, but laboratory tests are made in
chambers at low pressure. The pressure in the test
P-I-13-8
chamber in these experiments has been changed from 1
Pa to 10 Pa to observe its effect on the performance and
the voltage characteristics of the arcjet thruster.
2. Experimental
Experiments have been performed in the Aerospace
Plasma Dynamics Facility [7], which consists of a
vacuum chamber of 2m diameter and 4m length, equipped
with vacuum pump systems, gas and power supplies,
thrust measurement stand, experimental data collecting
and traversing systems and so on. Two sets of vacuum
pump systems have been used to keep the chamber
pressure at ~10 Pa and ~1 Pa, i.e., a Roots blower
combined with a mechanical pump for higher operational
volume flow rates at 10 ~ 20 Pa chamber pressure, and
with an additional diffusion pump and molecular pumps
for lower volume flow rates at ~ 1 Pa.
The thruster is mounted on a three-dimentsional
moveable table driven by stepping motors, which can
move along the jet axial and radial directions, and provide
the distance changes between the arcjet thruster and the
flat plate of the thrust stand. Impulse method has been
applied to measure the thrust in this work [2]. A heatresistant metal plate of 200 mm in diameter is used to
receive the impact of the jet plume, which is set
perpendicular to the jet plume axis. The gas flow rate, arc
current and voltage, pressures, etc. are measured by
transducers. According to the measured thrust, gas flow
rate, arc current and voltage, the specific impulse and
thrust efficiency can be obtained, which are two important
parameters to characterize the performance of the thruster.
Fig. 1. Schematic diagram of the arcjet.
1
~1Pa:
~10Pa:
N2
N2
7A
8A
8A
10A
9A
10A
He
11A
9A
12A
10A
11A He
10A
12A
12A
14A
14A
15A
80
Efficienc (%)
Due to the high dissociation energy and ionization
potential and thermal conductivity of nitrogen and helium,
it is not so easy to maintain a stable arc and diffused arc
root on the anode surface. Therefore, a natural radiationcooled high-temperature anode/nozzle has been used. The
dimensions of the nozzle are: throat diameter 0.6 mm,
constrictor length 0.5mm, half angle of the divergent
section 15o, and area expansion ratio of exit to throat 220.
60
He, 1Pa
40
He, 10Pa
20
N2, 1Pa
3. Results and discussions
N2, 10Pa
0
0
2
4
6
8
10
12
Gas flow rate (L/min)
Fig. 4. Variations of thrust efficiency against volumetric
flow rate.
(a) Pure He, 8.3 L/min, 10 A, 480 W, 10 Pa
(b) Pure He, 4.1 L/min, 12 A, 440 W, 1 Pa
Fig. 2. Photographs showing the appearance of the helium
plasma plumes.
Fig. 2 shows photographs of the plasma plumes with
helium propellant at vacuum chamber of 10 Pa (Fig. 2a)
and 1 Pa (Fig. 2b). It is obvious that the plasma plume
expanded to a much broader area at chamber pressure of 1
Pa than that injected into the atmosphere of 10 Pa, which
indicates a much more sufficient expanding process of the
helium plume in the chamber pressure of 1Pa.
1000
~10Pa ： ~1Pa:
N2
N2
7A
8A
8A
10A
9A
10A
He
11A
9A
12A
10A
11A He
10A
12A
12A
14A
14A
15A
Specific impulse (s)
800
He, 1Pa
600
400
He, 10Pa
N2, 1Pa
Fig. 3 and Fig. 4 plot the variations of specific
impulse and thrust efficiency against volume gas flow rate
respectively. It is seen that the specific impulse increases
with the increase of volumetric flow rate. At similar
volume gas flow rate, arc current and chamber pressure
conditions, thruster with helium propellant can produce
higher specific impulse than that with nitrogen propellant.
Much higher specific impulse can be obtained with the
plasma plume issuing into the lower chamber pressure
atmosphere of 1Pa, and the maximum specific impulse is
close to 700s in this work. Thrust efficiency has the
similar trend of variation, especially for helium propellant.
From Fig. 3 and Fig. 4, one can see that the vacuum
chamber pressure has a definite effect on specific impulse
and thrust efficiency, which is not consistent with the
ordinary concepts of supersonic nozzle flow. In general, if
the area expansion ratio of the nozzle is over 100, then the
main flow would definitely be in the high supersonic
region, and the expansion would be quite complete,
converting most enthalpy of the gas into directional exit
velocity, i.e., sonic condition is reached at the throat, the
vacuum chamber pressure cannot influence the gas
expansion, and the specific impulse should be the same
value whether the chamber pressure is 10 Pa or 1 Pa. But
in the present study, the plasma flow is quite complicated,
having non-uniformity in all parameters, especially for
helium, which has a low mass flow rate and high viscosity
and thus low Reynolds number. Low-speed boundary
layer occupies a significant portion of the flow in such
case. This is why vacuum chamber pressure can affect the
flow and expansion process[7].
200
N2, 10Pa
0
0
2
4
6
8
10
12
Gas flow rate (L/min)
Fig. 3. Variations of specific impulse against volumetric
flow rate.
2
P-I-13-8
4. Conclusions
~10Pa ：
N2
He
9A
8A
10A
10A
11A
12A
14A
15A
100
Voltage (V)
80
N2, 1Pa
60
N2, 10Pa
40
~1Pa:
N2
7A
8A
9A
10A
11A
12A
He
10A
12A
14A
Experiment results show that the vacuum chamber
pressure can be an important factor that influences the
measured performance of the arcjet thruster. Arcjet
thruster with helium propellant has much higher specific
impulse than that with nitrogen propellant, and the
maximum specific impulse is close to 700s in this work.
He, 10Pa
He, 1Pa
Acknowledgements
This work is supported by the National Natural Science
Foundation of China (Nos. 11475239 and 11175226).
20
0
0
2
4
6
8
10
12
Gas flow rate (L/min)
Fig. 5. Variations of arc voltage against volumetric flow
rate.
Fig. 5 shows the variations of arc voltage against
volumetric flow rate. It is seen that the arc voltage with
nitrogen propellant is about 2.5 times as high as that with
helium propellant at the similar arc current and gas flow
rate, this could be caused by the difference in arc column
constriction in the throat passage of the arcjet with
different propellants. Higher arc voltage indicates higher
input power, and can produce higher thrust. However, the
molecular weight of nitrogen is 7 times larger than the
atomic weight of helium, which results in the much higher
specific impulse with helium propellant. This shows the
advantage of using low molecular weight gases as
propellants. Fig. 5 also shows lower voltage at higher
current. Effect of vacuum chamber pressure on the arc
voltage is not the same in the two propellants, different
chamber pressures cause different voltages in nitrogen,
but not so evidently in helium. This behaviour could be
due to the different ways the arc column is affected by the
vacuum chamber pressure in the two propellants
P-I-13-8
5. References
[1] R. L. Sackheim, Journal of Propulsion and Power, 22,
1310(2006).
[2] C. K. Wu, H. X. Wang, X. Meng, X. Chen, W. X. Pan,
Acta Mechanica Sinica, 27, 152(2011).
[3] W. X. Pan, X. Meng, H. J. Huang, C. K. Wu, Plasma
Sources Sciences and Technology, 20, 065006(2011).
[4]D. Zube, P. Lichon, D. Cohen, D. Linctin, J. Bailey, N.
Chilleli, AIAA 99-2292,1999.
[5] H. B. Tang, X. A. Zhang, Y. Liu, H. X. Wang, C. B.
Shi, Journal of Propulsion and Power, 27, 218(2011).
[6] A. Rybakov, M. Auweter-Kurtz, H. Kurtz, M. Riehle,
AIAA 2002-3659, 2002.
[7]C. K. Wu, W. X. Pan, X. Meng, H. X. Wang,
Mechanisms for non-ideal flow in low-power arc-heated
supersonic nozzles, Acta Mechanica Sinica, accepted.
3