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The sheath structure around a rocket payload charged up to 460 V negative relative to the ambient ionospheric plasma is investigated experimentally and by computer simulations. Our experimental results come from the CHARGE 2 sounding rocket experiment in which the payload was split into two separate sections (mother and daughter) connected with a conducting, insulated tether. In one of the experimental modes, the voltage between the payloads was increased linearly from 0 to 460 V in 2.5 s. In this case the tethered mother/daughter functioned as a double probe, the negative probe (mother) reaching large negative potentials, while the positive probe (daughter) stayed close to the ambient plasma potential. A floating probe array was mounted on the mother with probes located 25, 50, 75, and 100 cm from the rocket surface. The internal impedance of the array was smaller than the probe/plasma impedance, which influenced the potential measurements. However, the measurements contain signatures, which we interpret as resulting from the outward expansion of the ion sheath with increasing negative mother potential. This conclusion is substantiated by NASCAP/LEO computer simulations of space charge limited flow. At high potentials, the observed ion current flowing to the mother increased more strongly with bias potential than found from the simulations. It is suggested that the enhancement of the current is generated by secondary electrons emitted by the ions bombarding the payload skin. The effects of the motion of the mother (540–580 m/s) and of the ambient magnetic field have been assessed by the code. It was estimated that the ion current to the mother was increased by 20% relative to a stationary payload, while the incorporation of a magnetic field had no practical influence on the simulation results.
The sheath structure around a rocket payload charged up to 460 V negative relative to the ambient ionospheric plasma is investigated experimentally and by computer simulations. Our experimental results come from the CHARGE 2 sounding rocket experiment in which the payload was split into two separate sections (mother and daughter) connected with a conducting, insulated tether. In one of the experimental modes, the voltage between the payloads was increased linearly from 0 to 460 V in 2.5 s. In this case the tethered mother/daughter functioned as a double probe, the negative probe (mother) reaching large negative potentials, while the positive probe (daughter) stayed close to the ambient plasma potential. A floating probe array was mounted on the mother with probes located 25, 50, 75, and 100 cm from the rocket surface. The internal impedance of the array was smaller than the probe/plasma impedance, which influenced the potential measurements. However, the measurements contain signatures, which we interpret as resulting from the outward expansion of the ion sheath with increasing negative mother potential. This conclusion is substantiated by NASCAP/LEO computer simulations of space charge limited flow. At high potentials, the observed ion current flowing to the mother increased more strongly with bias potential than found from the simulations. It is suggested that the enhancement of the current is generated by secondary electrons emitted by the ions bombarding the payload skin. The effects of the motion of the mother (540–580 m/s) and of the ambient magnetic field have been assessed by the code. It was estimated that the ion current to the mother was increased by 20% relative to a stationary payload, while the incorporation of a magnetic field had no practical influence on the simulation results.
A general analysis of the electrodynamic interactions between a space station with two exposed charged platforms and the ionospheric plasma is presented. We show that this problem can be separated into a far-field problem, concerned with the electromagnetic interference surrounding the entire space station, and a near-field problem, concentrated on the interactions in the vicinity of the biased platforms. Computer particle simulations as well as approximate analysis were carried out in the near field of the charged platform. Results of the plasma flowfield, the presheath and sheath structure, and current collection characteristics are obtained. The near-field solution is used to construct the perturbation current source in the far-field problem, which is solved by application of plasma fluid theory. It is found that the space station will generate a radiation field composed of the Alfven waves forming a "wing" structure. Based on our analysis, a global description of the space station's electrodynamic environment is obtained. Nomenclaturespeed of light, Alfven speed, and ion sound speed d sh = sheath thickness E = electric field E A = Alfven wave electric field e , = electron charge / = current /, j = current density K = Boltzmann constant k -wave vector M• -ion flow Mach number m e , m t = electron and ion mass «o»«/, n e -ambient plasma, ion and electron density «o» n f = plasma and ion density at the sheath boundary P sh ,P p resh = sheath and presheath power near ,^rad = near-field and far-field power R ce , RCJ = electron and ion Lamor radius /? near , j? far = near-field zone and far-field zone dimension T e -electron temperature u = velocity Vo>Vti>v te -orbital, ion thermal and electron thermal velocity Xi,x 2 ,X3 = coordinates in the rest frame of the plasma x 1 9 xi 9 x£ = moving coordinates with origin at the center box of station x'y' = moving coordinates with origin at the center of the station projected on the plane 5,4 x, z = moving coordinates for a (x{ 9 x^) plane cutting through the platform Z/,Z /7 = radiation impedance in band I and band II Z rad = total radiation impedance Presented as Paper 91-0114 at a. = angle of attack F = ion flux \ d = Debye length Xmfp = mean free patĥ wave = wavelength X X ,X|| = wavelength component in x and z direction $ = electric potential $ w >$sh -potential at the plate surface and at the sheath boundary 0 0 > OA -Mach angle and Alfen angle Upe>Upi -electron and ion plasma frequency -lower and upper hybrid frequency = electron and ion gyro frequency
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