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 Orléans cedex 02, France Phone: (33 2) ; Fax: (33 2) ; EJSM JGO/RPWI Team Meeting, Feb Mutual Impedance MEasurements, MIME as part of the EJSM JGO/RPWI Jean Gabriel TROTIGNON, Jean Louis Rauch and Fabrice Colin LPC 2 E, CNRS, Université d’Orléans, Orléans, France

EJSM JGO/RPWI Team Meeting, Feb Mutual Impedance MEasurements, MIME as part of the EJSM JGO/RPWI Téléphone: (33 2) Secrétariat: (33 2) Télécopie (Fax): (33 2) 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 Presentation Outline

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) 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.

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 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. 1. Self-impedance measurements (cont’d) Self- and mutual-impedance probes (cont’d)

2.1. double-probe antennae The mutual-impedance technique could be a combination of the double- probe 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. 2. Mutual-impedance measurements

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. Self- and mutual-impedance probes (cont’d)

Advantages of quadripole probes Electrodes perturb the plasma: the charge, usually < 0, acquired by the electrodes perturbs the plasma over distances of several λ D. 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) 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 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 λ D was of the order of 5 cm. 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. 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. R2 R1 T1-T2

Due to the S304 sensor size, the N e and T e might be determined so long as λ D was in the m range. The sphere size, 8-10 cm in diameter, was much < λ D. In most cases, the transmitter-receiver distance (about 20 m), was also much larger than 2 λ D, which is required for a reliable thermal plasma diagnosis. Representative quadripole impedance probes (cont’d) Impedance modulus (left) measured at 1475 km altitude by the ISOPROBE quadripole probe (right). Anti-resonances occur at gyroharmonics nF c, while peaks are observed at the plasma frequency F p, the upper-hybrid frequency F t, and maximum frequencies of the Bernstein’s modes F qn. ARCAD-3 ionospheric/ magnetospheric spacecraft

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, cm in length and 2 cm in diameter.

Owing to its principle, the MIP technique implies that the distance between receiving and transmitting electrodes is greater than about 2 λ D. As this distance is 40 cm, only plasmas with λ D < 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. Representative quadripole impedance probes (cont’d)

How to implement active plasma measurements onboard EJSM/JGO 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 Orléans cedex 02, France Phone: (33 2) ; Fax: (33 2) ; Different scenarii may be considered depending on the available Langmuir probes and electric-field antennae.

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

For a wire monopole antenna the free space capacitor is C o = 2π ε o L / ln (L / a), where a is the radius (m), L the length (m) of the monopole, and ε o = / 36π (F/m). For L = 6 m and a = 0.5 mm, C o 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 C o = f x (A). Self-impedance measurements (cont’d) 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.

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)

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 Orléans cedex 02, France Phone: (33 2) ; Fax: (33 2) ; 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.