previous versions at the Internet Archive. Yahoo! is not affiliated with the authors of this page or responsible for its content.
PRINCETON PLASMA PHYSICS LABORATORY PRINCETON UNIVERSITY, PRINCETON, NEW JERSEY
PREPARED FOR THE U.S. DEPARTMENT OF ENERGY,
UNDER CONTRACT DE-AC02-76CH03073
PRINCETON PLASMA PHYSICS LABORATORY
PRINCETON UNIVERSITY, PRINCETON, NEW JERSEY
PPPL-3622
PPPL-3622
UC-70
High-frequency Probing Diagnostic
for Hall Current Plasma Thrusters
by
A.A. Litvak, Y. Raitses, and N.J. Fisch
October 2001 PPPL Reports Disclaimer
This report was prepared as an account of work sponsored by an
agency of the United States Government. Neither the United States
Government nor any agency thereof, nor any of their employees, makes any
warranty, express or implied, or assumes any legal liability or
responsibility for the accuracy, completeness, or usefulness of any
information, apparatus, product, or process disclosed, or represents that its
use would not infringe privately owned rights. Reference herein to any
specific commercial product, process, or service by trade name, trademark,
manufacturer, or otherwise, does not necessarily constitute or imply its
endorsement, recommendation, or favoring by the United States
Government or any agency thereof. The views and opinions of authors
expressed herein do not necessarily state or reflect those of the United
States Government or any agency thereof.
Availability
This report is posted on the U.S. Department of Energys Princeton
Plasma Physics Laboratory Publications and Reports web site in Fiscal
Year 2002. The home page for PPPL Reports and Publications is:
http://www.pppl.gov/pub_report/
DOE and DOE Contractors can obtain copies of this report from:
U.S. Department of Energy
Office of Scientific and Technical Information
DOE Technical Information Services (DTIS)
P.O. Box 62
Oak Ridge, TN 37831
Telephone: (865) 576-8401
Fax: (865) 576-5728
Email: reports@adonis.osti.gov
This report is available to the general public from:
National Technical Information Service
U.S. Department of Commerce
5285 Port Royal Road
Springfield, VA 22161
Telephone: 1-800-553-6847 or
(703) 605-6000
Fax: (703) 321-8547
Internet: http://www.ntis.gov/ordering.htm 1
High-frequency Probing Diagnostic for Hall Current Plasma
Thrusters
A.A. Litvak, Y. Raitses, and N.J. Fisch
Princeton Plasma Physics Laboratory,
Princeton University, Princeton, NJ 08543
Abstract
High-frequency oscillations (1-100 MHz) in Hall thrusters have apparently eluded
significant experimental scrutiny. A diagnostic setup, consisting of single Langmuir
probe, special shielded probe connector-positioner, and electronic impedance-matching
circuit, was successfully built and calibrated. Through simultaneous high-frequency
probing of the Hall thruster plasma at multiple locations, high-frequency plasma waves
have been identified and characterized for various thruster operating conditions. 2
I. INTRODUCTION
Hall thrusters are now considered as the preferred candidate for spacecraft propulsion in
certain near-Earth missions. One of the important issues that could stand in the way of
successful integration of the Hall thruster in spacecraft [1] is the presence of plasma
oscillations, which could interfere with RF communication, or the thruster operation
itself. Both theoretical and experimental studies of plasma oscillatory behavior have
been performed since the earliest Hall thruster investigations [2] and are still under way
[3].
In spite of widely recognized importance of the oscillations in the high-frequency band
for thruster operation, the insight on the physical properties of these modes is very
limited both theoretically and experimentally [4]. Lack of experimental data regarding
plasma instabilities with the frequencies of a few tens of MHz is apparently due to
technical difficulties one encounters in detecting and diagnosing these modes.
This paper is organized as follows. The technical problems encountered in diagnosing of
high-frequency phenomena in a Hall thruster plasma are discussed in Section II. Section
III describes the instrument setup which allows to detect and characterize high-frequency
oscillations inside a laboratory Hall thruster, while Section IV describes the calibration
and experimental procedures for high-frequency measurements. 3
II. HIGH FREQUENCY PROBE DIAGNOSTIC
Measurements of the plasma oscillations in this frequency range have become feasible
due to recent progress in the fabrication of miniaturized semiconductor devices. Use of
such devices allows placement of the signal conditioning electronics inside the vacuum
vessel in the proximity of the probe, needed to achieve acceptable signal-to-noise ratio.
Such measurements were recently successfully performed, for example, in the Magnetic
Reconnection eXperiment [5], however Hall thrusters present additional problems for the
use of probe diagnostics. For example, use of the double probe in Hall thrusters is
restricted by sputtering of the probe material, which can produce a short-circuit between
the probe tips. Also, double probe characteristics are very difficult to interpret in the
presence of a magnetic field and for a flow of ions with an unknown energy distribution.
For larger Hall thrusters, the use of the coil-type antenna for the detection of such
oscillations might be feasible, but, in thruster models with the overall channel diameter
less than 10 cm, localized measurements will require antenna sizes of ~ 1mm diameter.
Such an antenna would be very difficult to implement technically due to short lifetime in
harsh environment of Hall thruster plasma, and may not yield a sufficient level of
detected signal due to small pick-up area of the antenna.
The single Langmuir probe is one of the most commonly used plasma diagnostic tools,
but it too has very serious constraints while used to study Hall thruster plasma. Probes
inside the acceleration channel tend to disturb the discharge. It is also difficult to
maintain probe integrity inside high-temperature region [6]. Thus, the only accessible 4
fixed location of such probing is on the outer wall of the ceramic channel close to the
channel exit. At the same time, probe tip size must obey [7], r << e
, where r is the
probe radius and e
is the electron gyro-radius. For the typical laboratory Hall thruster
(1kW power range) with applied magnetic field of a 100-200 Gs and discharge voltage of
200-300 V the diameter of the probe tip should then not exceed 0.5 mm on the outer wall
of the acceleration channel.
The amount of current, collected by the surface of such a small probe, even for the
steady-state measurements is such, that the impedance of the probe-to-plasma interface is
of the order of 100 kOhm (the ratio of probe floating potential to the probe ion saturation
current). At the same time, the oscillations in the frequency range around 30MHz
correspond to relatively short wavelengths ~1m in free space and even shorter in the
coaxial cables. This means that all transmission of the signal from the probes to the
recording point (oscilloscope, spectrum analyzer etc.) should be performed by the way of
coaxial shielded transmission lines with matched impedance. This condition is very
difficult to meet using standard low-impedance cables. Therefore, a matching circuit
should be constructed and placed close to the probe to minimize the effect of impedance
mismatch between the probe and the cables.
During steady state thruster operation the probe tip is bombarded by energetic ions which
erodes the probe. Therefore the probe system should accommodate the easy replacement
of the probe tip and easy adjustment of its protrusion into the channel. 5
III. INSTRUMENTAL SETUP
To overcome the limitations and technical difficulties of operating high-frequency probes
in the harsh Hall thruster environment, the following probe diagnostic was successfully
developed and tested.
The probe is constructed of Tungsten wire 0.25mm dia, protruding into the discharge area
of the thruster from the outer ceramic wall of acceleration channel. On the outside of the
thruster, the probe wire is insulated by alumina tube 0.8mm dia. To prevent the probe
wire from the pickup of electromagnetic noise, outside the thruster a Molybdenum tube
shields the alumina, essentially providing a coaxial transmission channel for the signal
from the probe. On the other end the probe wire is coupled to a regular coaxial cable
(silicone-coated for vacuum compatibility) through a specially designed connector (Fig.
1). This connector by a single bolt on the back allows easy regulation of the length of the
probe protruding into the plasma. The connector is also easy to disassemble in-situ for
probe wire replacement. Both of these features are necessary to compensate for fast
erosion of the probe tip during thruster operation. At the same time the connector is
designed in such way that the whole transmission line stays coaxially shielded.
Oscillations in the plasma density can be related to the oscillations in the ion saturation
current of the probe. After the probe setup (without circuitry) was assembled, the I-V
characteristics (e.g. Fig. 2) of the probe were experimentally measured at vari