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Title
A compact permanent-magnet helicon thruster
Permalink
https://escholarship.org/uc/item/98x5j6vw
Journal
IEEE Transactions on Plasma Science, 43(1)
ISSN
0093-3813
Author
Chen, FF
Publication Date
2015
DOI
10.1109/TPS.2014.2361476
Peer reviewed
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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 43, NO. 1, JANUARY 2015 195
A Compact Permanent-Magnet Helicon Thruster
Francis F. Chen
Abstract—A small helicon source using a permanent magnet
has been tested for possible application as a spacecraft thruster.
Ion energy distributions measured with a retarding-field ion
analyzer show that ions are ejected with energies of ∼5 KT
e
,
in agreement with theory. The specific impulse can be increased
by applying a positive bias to the endplate of the discharge.
Index Terms— Ambipolar thruster, compact thruster, helicon,
helicon thruster, permanent-magnet helicons, spacecraft thruster,
thrusters
I. DISCHARGE CONFIGURATION
N
ORMAL spacecraft thrusters eject a fast ion beam, which
has to be neutralized by electrons from an auxiliary
source to prevent the spacecraft from charging up negatively.
This is not necessary in ambipolar thrusters, which eject
neutral plasma. Helicon discharges require less power to
generate a given plasma density than other ambipolar sources,
but they require a dc magnetic field B. This obstacle has been
overcome by the use [1], [2] of vertically polarized annular
magnets located away from the discharge, as shown in Fig. 1.
As seen in Fig. 2, the B-field below the magnet reaches a stag-
nation point not far from the magnet; the discharge is located
below this, where the field is quite uniform and nearly vertical.
In previous experiments, specially designed neodymium
(NdFeB) magnets were used, but that work showed that
B-fields greater than ∼60 G (6 mT) yielded negligible
improvement in plasma density [3]. As a result, a smaller
system using a commercially available magnet was designed
and tested. The magnet shown in Fig. 1 is of 2-in ID, 4-in
OD, and 0.5-in thickness. The discharge tube is of 2-in ID
and 2-in height, topped by a grounded aluminum plate (to
reflect the back wave). The discharge runs in argon from
0.5 to 60 mTorr with 50 to 2000 W of 27.12-MHz RF. The
single loop antenna is located near the exit to minimize
plasma loss to the walls. Maximum density inside the tube is
≈ 5× 10
12
cm
−3
.Fig.3shows|B
z
| versus. z and the location
of the discharge for |B|≈60 G.
II. M
EASUREMENTS
As plasma exits the source, the electron density decreases,
following the diverging field lines. Since the electrons are
Maxwellian, the plasma potential also decreases. Thus, there
is an electric field that accelerates the ions along B.Usinga
retarding-field ion analyzer (RFIA) made by Impedans, Ltd. of
Ireland, we have measured the ion energy distribution function
Manuscript received February 11, 2014; revised March 7, 2014; accepted
September 16, 2014. Date of publication October 1, 2014; date of current
version January 6, 2015.
The author is with the University of California at Los Angeles, Los Angeles,
CA 90024 USA (e-mail: ffchen@ee.ucla.edu).
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPS.2014.2361476
Fig. 1. Argon helicon discharge with a permanent magnet.
Fig. 2. Small squares: B-lines around a large annular magnet. Large squares:
two possible positions of the discharge tube. The B-field can be varied by
moving the magnet vertically relative to the discharge.
at various positions below the source. A sample power scan,
at 5 mTorr, is shown in Fig. 4. It is seen that the ion energy
peaks at ∼12–14 eV. A normal sheath drop at the wall of an
argon discharge is ∼5 KT
e
,or≈10 eV for KT
e
≈ 2eV[3].
Thus, the ion acceleration has the approximate expected mag-
nitude. The apparent occurrence of ions at negative voltages
is due to the RF filtering circuit and collisions with neutrals
before reaching the sensor. Fig. 5 shows that higher ion
velocities can be obtained at lower pressures.
0093-3813 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
196 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 43, NO. 1, JANUARY 2015
Fig. 3. B-field in the region below the stagnation point (at three
radial positions), and the location of the discharge tube when the antenna
is at 60 G.
Fig. 4. RFIDs versus voltage on the collector plate relative to ground at
5 mTorr and various RF powers.
Fig. 5. RFIDs versus voltage relative to ground at 0.5 mTorr.
Fig. 6 shows how the RFIA is mounted in the experimental
chamber. The RFIA is a disk 4-in in diameter and 1/4-in thick,
encased in oxidized aluminum. Since it is an RF conductor, it
affects the discharge in its uppermost positions by becoming
a second endplate for the helicon waves. More importantly,
it blocks the position where the double-layer studied in [4]
would normally occur (Fig. 7). Nonetheless, though a sudden
potential drop cannot be seen, the ion acceleration is still there
and can be seen downstream.
Fig. 6. Schematic of the experimental chamber showing locations of three
probe ports and the sliding mount for the RFIA sensor.
Fig. 7. Location where a double-layer would occur in the absence of the
RFIA sensor. The location is not a single point because B is not uniform in
the discharge.
The retarding-field ion distributions (RFIDs) at Port 2,
16.9-cm below the source, are shown in Fig. 8 for various
RF powers. It is seen that the ion flux increases with power,
but the energy peak does not move much. These results show
a much smaller ion acceleration than was reported in [5] at
higher B-fields and lower pressures.
III. A
PPLICATION TO THRUSTERS
At our normal operating pressure of 15 mTorr, Fig. 8 shows
that the peak ion energy is only ∼10 eV; but at pressures in
thrusters, the ion energy is much larger, as seen in Fig. 5.
The question is whether this energy can be increased even
CHEN: COMPACT PERMANENT-MAGNET HELICON THRUSTER 197
Fig. 8. Downstream ion distributions at Port 2 versus RF power at 15 mTorr.
Two curves at 700 W show reproducibility at beginning and end of run.
Fig. 9. Ion distributions at Port 2 with at 1000 and 400 W with top plate
voltages of 0 V (black curves), −24 V (red curves), and +24 V (blue curves).
further by biasing the top plate of the discharge relative to
the spacecraft ground. We have tested this at 15 mTorr, and
Fig. 9 shows that it is indeed possible. Because of the severe
RF environment, an electronic power supply can be used only
with filtering by large electrolytic capacitors. Instead, we used
two 12-V lead-acid batteries in series to supply ±24 V to the
top plate. The peak ion energy is indeed altered, but the shift
is less than the applied voltage, being only 15 and −7V,
respectively. Nonetheless, the thrust can, in principle, be
increased arbitrarily by applying a dc voltage, as is done
in existing ion and Hall thrusters. Compared with these, a
helicon thruster can provide a much denser ion beam with
automatic electron neutralization.
Thrusters are characterized by their specific impulse I
sp
,
among other criteria
I
sp
≡
v
ex
g
where v
ex
is the exhaust velocity of the ions, and g is the
gravitational constant 9.8 m/s
2
. Fig. 5 shows that the ion
distribution peaks above 30 eV at low pressures. This energy
corresponds to an I
sp
of ∼1200. To obtain an I
sp
of over 3000,
one needs to increase the ion energy to 200 eV or so, which
can easily be done with a positive top plate bias. The current
drawn by a biased top plate is only at the milliamp level;
apparently, the sheath there changes so that a large electron
current is not necessary to provide a helicon equilibrium. This
preliminary experiment suggests the possibility of developing
a permanent-magnet helicon thruster.
A
CKNOWLEDGMENT
The author would like to thank D. Gahan from Impedans,
Ltd., Dublin, Ireland, for the loan of the hardware and
software of their SEMion system for automatic measurement
and plotting of ion distribution functions.
R
EFERENCES
[1] F. F. Chen and H. Torreblanca, “Large-area helicon plasma source with
permanent magnets,” Plasma Phys. Controlled Fusion, vol. 49, no. 5A,
pp. A81–A93, May 2007.
[2] F. F. Chen, “Helicon plasma source with permanent magnets,”
U.S. Patent 8 179050, May 15, 2012.
[3] F. F. Chen, “Performance of a permanent-magnet helicon source at
27and13MHz,”Phys. Plasmas, vol. 19, no. 9, p. 093509, Sep. 2012.
[4] C. Charles, “A review of recent laboratory double layer experiments,”
Plasma Sour. Sci. Technol., vol. 16, no. 4, pp. R1–R25, Nov. 2007.
[5] M. Wiebold, Y.-T. Sung, and J. E. Scharer, “Ion acceleration
in a helicon source due to the self-bias effect,” Phys. Plasmas, vol. 19,
no. 5, p. 053503, May 2012.
Authors’ photographs and biographies not available at the time of publication.