Transcript and mutual-impedance probes

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE
Laboratoire de Physique et Chimie de l'Environnement et de l’Espace
3A, avenue de la Recherche Scientifique
F-45071 Orléans cedex 02, France
Mutual Impedance MEasurements, MIME
as part of the EJSM JGO/RPWI
Jean Gabriel TROTIGNON, Jean Louis Rauch and Fabrice Colin
LPC2E, CNRS, Université d’Orléans, Orléans, France
EJSM JGO/RPWI Team Meeting, 18-19 Feb. 2010
Phone: (33 2) 38 25 52 63; Fax: (33 2) 38 63 12 34; E-mail: [email protected]
EJSM JGO/RPWI Team Meeting, 18-19 Feb. 2010
Mutual Impedance MEasurements, MIME as part of the EJSM JGO/RPWI
Presentation Outline
How does a classical impedance probe work?
Self- and mutual-impedance probes
Advantages of quadripole probes
Representative quadripole impedance probes
How to implement active plasma measurements
onboard EJSM/JGO
Conclusion
Téléphone: (33 2) 38 25 52 63 Secrétariat: (33 2) 38 25 52 64
Télécopie (Fax): (33 2) 38 63 12 34
E-mail: [email protected]
How does a classical impedance probe work?
A classical impedance probe consists in the probe itself and the electronics
that measure the impedance:
 The probe comprises transmitting/receiving electrodes immersed in
the plasma.
 The electronics measure the dynamic impedance between the
electrodes at several fixed frequencies over a range that includes the
electron plasma frequency, from which the total plasma density is directly
derived.
As the impedance depends on the parameters of the ambient plasma, such
as the electron density and temperature, impedance probes are powerful tools
for plasma diagnoses.
Self- and mutual-impedance probes
1. Self-impedance measurements
1.1. Double-probe configuration
Sinusoidal currents, in opposite phase, are injected in the two spherical
probes through a resistor/capacitor (10MΩ, 10pF) network, so that the
current may be considered as constant whatever the plasma conditions.
The potential difference that appears between the two probes, on open
circuit, is measured:
 either in a differential mode (only one amplifier and one acquisition
channel are required),
 or with reference to the S/C structure (with two amplifiers and two
acquisition channels).
1.2. Single-probe configuration
If only one probe is used, the a.c. current is injected in it and the voltage is
measured between the probe and the S/C body.
Self- and mutual-impedance probes (cont’d)
1. Self-impedance measurements (cont’d)
1.3. Double-wire configuration
Measurements similar to those obtained with the double-probe configuration
may be done with a double-wire configuration. Sinusoidal voltages, in
opposite phase, are applied to the shields of the two sets of boom harness,
in parallel with a resistor (10 kΩ) that link each shield to the S/C ground.
The current that flows through each harness (or only one, assuming that a
current of same magnitude flows through the other for symmetry reason) is
then measured with a current probe. One acquisition channel per available
current probe is necessary.
1.4. Single-wire configuration
In the single wire configuration, the voltage is only applied to the boom
harness equipped with the current probe. Again, only one acquisition channel
is required.
Self- and mutual-impedance probes (cont’d)
2. Mutual-impedance measurements
2.1. double-probe antennae
The mutual-impedance technique could be a combination of the doubleprobe and double-wire modes describes above, assuming that the
electric antenna is a double-probe antenna and that the shields of the two
sets of boom harness may be used as transmitting devices.
Two sinusoidal voltages, in opposite phases, are then applied to the two
boom harness shields and the difference in voltage between the two
spherical probes are measured (one acquisition channel is enough).
In addition, for calibration purpose, the current circulating on the harness
may be measured by one or two current probes.
Self- and mutual-impedance probes (cont’d)
2. Mutual-impedance measurements (cont’d)
2.2. Quadripole probe
A more elegant and efficient way should be to use four electrodes, two for
transmitting and the other two for receiving.
The transmitting electrodes are excited from a signal generator, while the
receiving electrodes are connected to a voltmeter with a very high input
impedance.
Providing that the internal impedance of the current source is very large
compared to the self-impedance of the transmitting electrodes, the current
may be considered as known and constant.
The mutual impedance of the two pairs of electrodes is then the ratio of the
received voltage to the transmitted current, Z = V / I.
Advantages of quadripole probes
Electrodes perturb the plasma: the charge, usually < 0, acquired by the
electrodes perturbs the plasma over distances of several λD.
The ion sheath thus created insulates the electrodes partially and the
impedance differs from the one that should be measured in the absence of
perturbation.
Another difficulty comes from the antenna sensitivity to quasi-static
disturbances originating in the spacecraft body and structures.
Monopoles are thus rarely used and symmetrical dipole preferred because
differential measurements nominally reduce these disturbances.
A way to minimize these perturbations is to implement a quadripole
probe, composed of 2 transmitting & 2 receiving electrodes, whose
measurements are relatively insensitive to the ion sheath effects.
Advantages of quadripole probes (cont’d)
Transmitter
I
Plasma
V
Receiver
Schematic illustration showing how the mutual impedance
Z = V/I of a quadripole probe is determined.
Ion sheaths influence self-impedance at transmitting electrodes, but have very
little influence on the way in which the current spreads out through the
unperturbed plasma, and their influence decreases as the distance to the
transmitting electrodes increases.
Whatever perturbations may affect the self-impedance of transmitting
electrodes, the source will force the nominal current out of these electrodes
and through the plasma between them (Storey, Aubry and Meyer, 1969).
Representative quadripole impedance probes
The CISASPE ionospheric rocket was launched from Kourou in 1971. The
quadripole probe consists in 4 booms bearing spheres, 3 cm in diameter.
Once deployed the spheres are at the corners of a plane square, 17 cm in
length. The encountered λD was of the order of 5 cm.
fT upper-hybrid frequency
nfb Gyrofrequencies
CISASPE rocket
Kourou
16 Dec. 1971
Quadripole impedance probe on the ionospheric rocket CISASPE & modulus of
the mutual impedance normalized w.r.t. its value in vacuum, at 227 km altitude.
Representative quadripole impedance probes (cont’d)
The 2 receiving spheres of GEOS-1 mutual impedance are separated by a
distance of 42 m, while the two transmitting spheres are along a line parallel to
receiving antenna, 3.3 m away from S/C and make a 1.4 m long dipole.
R2
T1-T2
R1
Locations of some of GEOS-1 plasma and wave instruments. S304 mutual impedance is
composed of 2 transmitting 10 cm diameter wire mesh spheres, located at tips of 3.3 m axial
booms & separated by a distance of 1.4 m, and 2 receiving 8 cm diameter spherical spheres,
mounted on 42 m tip to tip radial booms.
Representative quadripole impedance probes (cont’d)
Due to the S304 sensor size, the Ne and Te might be determined so long as λD
was in the 0.5-6 m range. The sphere size, 8-10 cm in diameter, was much < λD.
In most cases, the transmitter-receiver distance (about 20 m), was also
much larger than 2 λD, which is required for a reliable thermal plasma
diagnosis.
ARCAD-3
ionospheric/
magnetospheric
spacecraft
Impedance modulus (left) measured at 1475 km altitude by the ISOPROBE quadripole
probe (right). Anti-resonances occur at gyroharmonics nFc, while peaks are observed
at the plasma frequency Fp, the upper-hybrid frequency Ft, and maximum frequencies
of the Bernstein’s modes Fqn.
Representative quadripole impedance probes (cont’d)
The Mutual Impedance Probe, MIP, that is onboard ROSETTA consists of:
 an electronics board (470 g, 247 mm x 147 mm, 1.9 W peak) for
experiment managing, input/output data handling, and signal processing in
the 7 kHz-3.5 MHz range;
 a lightweight sensor unit (370 g). The electrode array is linear and
includes 1 receiving dipole and 2 transmitting monopoles supported by a
Carbon Fibre-Reinforced Plastic cylindrical bar, 103.5 cm in length and 2 cm
in diameter.
Representative quadripole impedance probes (cont’d)
Owing to its principle, the MIP technique implies that the distance between
receiving and transmitting electrodes is greater than about 2 λD. As this distance
is 40 cm, only plasmas with λD < 20 cm should be properly measured with
MIP alone (Short Debye Length mode).
To overshoot this limit, the Long Debye Length mode has been designed. In
this mode the LAP 2 spherical Langmuir probe that is located at about 4 m
from MIP is used as a transmitter.
CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE
Laboratoire de Physique et Chimie de l'Environnement et de l’Espace
3A, avenue de la Recherche Scientifique
F-45071 Orléans cedex 02, France
How to implement active plasma
measurements onboard EJSM/JGO
Different scenarii may be considered depending on the available
Langmuir probes and electric-field antennae.
Phone: (33 2) 38 25 52 63; Fax: (33 2) 38 63 12 34; E-mail: [email protected]
How to implement active plasma measurements onboard EJSM/JGO
Self-impedance measurements
In case the double-wire antenna may
be used, the self-impedance technique
could be implemented.
Self-impedance measurements here
require the current injected into the
plasma to be measured:
 directly with one or two simple
current probes,
 or alternately the antenna
capacitance and conductance to be
compared with variable capacitance
and conductance, the latter being
much more demanding in terms of
S/C resources.
How to implement active plasma measurements onboard EJSM/JGO (cont’d)
Self-impedance measurements (cont’d)
For a wire monopole antenna the free space capacitor is Co = 2π εo L / ln (L / a),
where a is the radius (m), L the length (m) of the monopole, and εo = 10-9 / 36π
(F/m). For L = 6 m and a = 0.5 mm, Co is 35.5 pF.
A voltage of a few 100 mV is injected on the opposite booms. The current in
the plasma is measured during the transmission. The current probe can be
placed around the coaxial cable that feeds the antenna. The grounding of the
antenna shield and the current probe should be placed carefully to avoid the effect
of stray capacitance.
The current probe consists of a toroidal magnetic
core on which are wound a main coil and a
secondary coil. The secondary coil provides a flat
gain over the frequency range of interest.
For a transmitted voltage of 200 mV, the current to
measure is I = 2π f V Co = f x 4.5 10-11 (A).
How to implement active plasma measurements onboard EJSM/JGO (cont’d)
Mutual-impedance measurements
Mutual-impedance measurements may be implemented in different ways
depending on the availability of 1 to 4 Langmuir probes and possibly two
monopoles of the HF wire antennae.
At least 1 Langmuir probe is required as a transmitting monopole, and 2 other
Langmuir probes or two monopoles of the HF wire antennae are necessary to
receive signals. A perfect mutual-impedance device would be composed of
4 Langmuir probes, two of them being used as transmitting electrodes and
the two others as a receiving dipole. They could be advantageously mounted
at the corners of one of the S/C panels.
Not to scale.
How to implement active plasma measurements onboard EJSM/JGO (cont’d)
Mutual-impedance measurements (cont’d)
Now, if these Langmuir probes are
designed in order to be used either as
transmitting electrodes or receiving
electrodes, both symmetrical (bottom) &
asymmetrical (top) configurations can be
considered.
For symmetrical quadripole the
impedance vanishes in an isotropic
medium, while it is finite in case of
asymmetrical configuration.
Nevertheless, in presence of a B-field,
the plasma becomes anisotropic and the
impedance will vary with the orientation
of the probe w.r.t. B, the plasma
diagnosis might therefore be improved.
How to implement active plasma measurements onboard EJSM/JGO (cont’d)
Mutual-impedance measurements (cont’d)
Classical
quadripole
impedance probe
block diagram.
How to implement active plasma measurements onboard EJSM/JGO (cont’d)
Mutual-impedance measurements (cont’d)
Two Langmuir
Probes (or at least
one) may also be used
as transmitting
electrodes, and two of
the HF monopoles as
a receiving dipole.
Not to scale.
How to implement active plasma measurements onboard EJSM/JGO (cont’d)
Mutual-impedance measurements (cont’d)
Two frequently asked questions:
Why is it preferable to use point sources and point measurements
instead of rod or wire ones?
It is to minimize the sheath effects and to have a unique line joining each
transmitting electrode to each receiving electrode that can be compared
with the B-field direction, if any.
What’s about the distance between electrodes?
The distance between electrodes must be larger than two times the Debye
length (plasma measurements) and much shorter than the wavelength of
any electromagnetic wave (quasi-electrostatic behaviour).
CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE
Laboratoire de Physique et Chimie de l'Environnement et de l’Espace
3A, avenue de la Recherche Scientifique
F-45071 Orléans cedex 02, France
Mutual Impedance MEasurements, MIME
as part of the EJSM JGO/RPWI
J. G. Trotignon, J. L. Rauch and Fabrice Colin
Conclusion
Several configurations, using Langmuir probes and/or wire electrodes,
would allow self/mutual antenna impedances to be measured and hence
plasma parameters, such as electron density and temperature, to be
determined.
MIME may therefore contribute to the study of the Jupiter’ system and help
out with in-flight sensor calibrations.
Phone: (33 2) 38 25 52 63; Fax: (33 2) 38 63 12 34; E-mail: [email protected]