Drifting plasma collection by a positive biased tether wire in LEO-like plasma conditions: current measurement and plasma diagnostic

BETs is a three-year project financed by the Space Program of the European Commission, aimed at developing an efficient deorbit system that could be carried on board any future satellite launched into Low Earth Orbit (LEO). The operational system involves a conductive tape-tether left bare to establish anodic contact with the ambient plasma as a giant Langmuir probe. As a part of this project, we are carrying out both numerical and experimental approaches to estimate the collected current by the positive part of the tether. This paper deals with experimental measurements performed in the IONospheric Atmosphere Simulator (JONAS) plasma chamber of the Onera-Space Environment Department. The JONAS facility is a 9- m3 vacuum chamber equipped with a plasma source providing drifting plasma simulating LEO conditions in terms of density and temperature. A thin metallic cylinder, simulating the tether, is set inside the chamber and polarized up to 1000 V. The Earth's magnetic field is neutralized inside the chamber. In a first time, tether collected current versus tether polarization is measured for different plasma source energies and densities. In complement, several types of Langmuir probes are used at the same location to allow the extraction of both ion densities and electron parameters by computer modeling (classical Langmuir probe characteristics are not accurate enough in the present situation). These two measurements permit estimation of the discrepancies between the theoretical collection laws, orbital motion limited law in particular, and the experimental data in LEO-like conditions without magnetic fields. In a second time, the spatial variations and the time evolutions of the plasma properties around the tether are investigated. Spherical and emissive Langmuir probes are also used for a more extensive characterization of the plasma in space and time dependent analysis. Results show the ion depletion because of the wake effect and the accumulation of- ions upstream of the tether. In some regimes (at large positive potential), oscillations are observed on the tether collected current and on Langmuir probe collected current in specific sites.

Electrodynamic tether for scientific mission in low Jovian orbit

An electrodynamic bare tether is shown to allow carrying out scientific observations very close to Jupiter, for exploration of its surface and subsurface, and ionospheric and atmospheric in-situ measurements. Starting at a circular equatorial orbit of radius about 1.3/1.4 times the Jovian radius, continuous propellantless Lorentz drag on a thin-tape tether in the 1-5 km length range would make a spacecraft many times as heavy as the tape slowly spiral in, over a period of many months, while generating power at a load plugged in the tether circuit for powering instruments in science data acquisition and transmission. Lying under the Jovian radiation belts, the tape would avoid the most severe problem facing tethers in Jupiter, which are capable of producing both power and propulsion but, operating slowly, could otherwise accumulate too high a radiation dose . The tether would be made to spin in its orbit to keep taut; how to balance the Lorentz torque is discussed. Constraints on heating and bowing are also discussed, comparing conditions for prograde versus retrograde orbits. The system adapts well to the moderate changes in plasma density and motional electric field through the limited radial range in their steep gradients near Jupiter.

A proposed bare-tether experiment on board a sounding rocket

A mission on board a sounding rocket to carry out two bare-tether experiments is proposed: a test of orbital-motion-limited (OML) collection and the proof-of-flight of a technique to determine the (neutral) density vertical profile in the critical E-layer. Since full bias from the motional field will be small (~ 20V), corresponding to a tape 1 km long and V rocket <8 km/s, a power source with a range of supply voltages of few kV would be used. First, the negative terminal of the supply would be connected to the tape, and the positive terminal to a round, conductive boom of length 10 - 20 m; electrons collected by the boom cross the supply into the tape, where they leak out at the rate of ion impact plus secondary emission. Determination of the density profile from measurements of auroral emissions observed from the rocket, as secondaries racing down the magnetic field reach an E-layer footprint, are discussed. Next the positive terminal of the voltage supply is connected to the tape, and the negative terminal to a Hollow Cathode (HC); electrons now collected by the tape cross the supply, and are ejected at the HC. The opposite connections, with current collection operated by tape and boom, and operating on electrons and ions, and through partial switching in the supply, allow testing OML collection in almost all respects it depends on.

Floating Bare Tether as Upper Atmosphere Probe

Use of a (bare) conductive tape electrically floating in LEO as an effective e-beam source that produces artificial auroras, and is free of problems that have marred standard beams, is considered. Ambient ions impacting the tape with KeV energies over most of its length liberate secondary electrons, which race down the magnetic field and excite neutrals in the E-layer, resulting in auroral emissions. The tether would operate at night-time with both a power supply and a plasma contactor off; power and contactor would be on at daytime for reboost. The optimal tape thickness yielding a minimum mass for an autonomous system is determined; the alternative use of an electric thruster for day reboost, depending on mission duration, is discussed. Measurements of emission brightness from the spacecraft could allow determination of the (neutral) density vertical profile in the critical E-layer; the flux and energy in the beam, varying along the tether, allow imaging line-of-sight integrated emissions that mix effects with altitude-dependent neutral density and lead to a brightness peak in the beam footprint at the E-layer. Difficulties in tomographic inversion, to determine the density profile, result from beam broadening, due to elastic collisions, which flattens the peak, and to the highly nonlinear functional dependency of line-of-sight brightness. Some dynamical issues are discussed.

Conductive-tether design for de-orbiting from given altitude and inclination

A bare tether with thin-tape cross section is both i) the most effective electrodinamic tether for given length and mass, and ii) capable of effective design for an arbitrary mission through its three disparate dimensions. It handily beats the fully insulated tether that exchanges current at both ends, a result resting in advantages of 2D current collection as against 3D collection; it has much greater perimeter than the round bare tether and much lower fatal debris-impact rate, leading to greatly faster de-orbiting and greatly higher probability of survival; and it only allows multi-line tethers reaching a few hundred lines to stand competitive. In selecting the disparate values of length L, width w, and thickness h for a de-orbit mission, performance involves three criteria: a) tether-tospacecraft mass ratio must be small; b) probability of survival against the debris environment must be high; and c) de-orbiting must be fast to reduce manoeuvres for avoiding catastrophic collisions with big active/passive satellites around. Beyond determining tether mass through the product Lwh, main dimension parameters affecting performance are L/h2li characterizing ohmic effects, and w determining electron collection. An algorithm for optimal selection of tape dimensions is elaborated.

Survivability analysis of tape-tether against multiple impact with tiny debris

We show that for a tether at 800 km altitude, which is 5 km long, 2 cm wide and 0.05 mm thick, the risk of substantial damage during a 3 month period due to multiple impacts with debris or micrometeoroids is low, of about 1.4%. By substantial damage we mean that if the tape is divided in 2 cm2 cm squares, then in some square the damaged area by bombardment with debris or micrometeoroids exceeds 11% of the area of the square. Furthermore, we show that the danger posed by the micrometeoroids is negligible compared to the risk posed by the debris.

Space demostration of bare electrodynamic tape-tether technology on the sounding rocket S520-25

A spaceflight validation of bare electro dynamic tape tether technology was conducted. A S520-25 sounding rocket was launched successfully at 05:00am on 31 August 2010 and successfully deployed 132.6m of tape tether over 120 seconds in a ballistic flight. The electrodynamic performance of the bare tape tether employed as an atmospheric probe was measured. Flight results are introduced through the present progressive report of the demonstration and the results of flight experiment are examined as the premier report of the international cooperation between Japan, Europe, USA and Australia. Future plans for maturing space tether technology, which will play an important role for future space activities, are also discussed.

Effect of magnetic field on current colection on bare tether in LEO

An eiectrodynamic Tether is a long thin conductive string deployed from a spacecraft. A part of the ED tether near one end, which is rendered positive by the Electromotive force (EMF)along the tether, collects electrons from the ambient plasma. In the frame of reference moving with theter, ions flow toward the tether, get deflected near the tether by its high positive potential and create a wake. Due to the asymmetry of plasma distribution and the weak but significant Geomagnetic field, the conventional probe theory becomes almost inapplicable. Computational work for the prediction of current collection is thus necessiated.. In this paper, we analyze effects of magnetic field on velocity distribution funtion at a point that is far from the tether, and discuss a new way to treat electrons at computational boundary. Three cases with different magnetic field are simulated and compiled so as to provide a part of the pre-flight prediction of the space experiment by NASA ProSEDS, which is planned September 2002.

PIC computation of electron current collection to a moving bare tether in the mesothermal condition

In tethered satellite technology, it is important to estimate how many electrons a spacecraft can collect from its ambient plasma by a bare electrodynamic tether. The analysis is however very difficult because of the small but significant Geo-magnetic field and the spacecraft’s relative motion to both ions and electrons. The object of our work is the development of a numerical method, for this purpose. Particle-In-Cell (PIC) method, for the calculation of electron current to a positive bare tether moving at orbital velocity in the ionosphere, i.e. in a flowing magnetized plasma under Maxwellian collisionless conditions. In a PIC code, a number of particles are distributed in phase space and the computational domain has a grid on which Poisson equation is solved for field quantities. The code uses the quasi-neutrality condition to solve for the local potential at points in the plasma which coincide with the computational outside boundary. The quasi-neutrality condition imposes ne - ni on the boundary. The Poisson equation is solved in such a way that the presheath region can be captured in the computation. Results show that the collected current is higher than the Orbital Motion Limit (OML) theory. The OML current is the upper limit of current collection under steady collisionless unmagnetized conditions. In this work, we focus on the flowing effects of plasma as a possible cause of the current enhancement. A deficit electron density due to the flowing effects has been worked and removed by introducing adiabatic electron trapping into our model.

Towards a new generation of solar cells: silicon supersaturated with titanium or vanadium

We have analyzed the spectral sub-bandgap photoresponse of silicon (Si) samples implanted with vanadium (V) and titanium (Ti) at different doses and subsequently processed by pulsed-laser melting.

Intermediate band solar cells. The transition metal supersaturated Silicon approach

Within the framework of the third solar cell generation some new ideas to enlarge the spectral response of the solar cells toward the infrared have been proposed. Among them the inclusion of an Intermediate Band (IB) seems to be very promising. This paper will deal with one of the ways to generate the IB namely the deep level center approach. We will discuss not only its existence but also the carriers lifetime recovery which is necessary to obtain the expected increase of the solar cell efficiency.

Impact of Impurities on the Mechanical Strength of Multicrystalline Silicon

The usage of more inexpensive silicon feedstock for crystallizing mc-Si blocks promises cost reduction for the photovoltaic market. For example, less expensive substrates of upgraded metallurgical silicon (UMG-Si) are used as a mechanical support for the epitaxial solar cell. This feedstock has higher content of impurities which influences cell performance and mechanical strength of the wafers. Thus, it is of importance to know these effects in order to know which impurities should be preferentially removed or prevented during the crystallization process. Metals like aluminum (Al) can decrease the mechanical strength due to micro-cracking of the silicon matrix and introduction of high values of thermal residual stress. Additionally, silicon oxide (SiOx) lowers the mechanical strength of mc-Si due to thermal residual stresses and stress intensification when an external load is applied in the surrounding of the particle. Silicon carbide (SiC) introduces thermal residual stresses and intensifies slightly the stress in the surrounding of the particle but can have a toughening effect on the silicon matrix. Finally, silicon nitride (Si3N4) does not influence significantly the mechanical strength of mc- Si and can have a toughening effect on the silicon matrix.

Jovian Capture of a Spacecraft with a Self-Balanced Electrodynamic Bare Tether

This paper proposes and analyzes the use of a nonrotating tethered system for a direct capture in Jovian orbit using the electrodynamic force generated along the cable. A detailed dynamical model is developed showing a strong gravitational and electrodynamic coupling between the center of mass and the attitude motions. This paper shows the feasibility of a direct capture in Jovian orbit of a rigid tethered system preventing the tether from rotating. Additional mechanical–thermal requirements are explored, and preliminary operational limits are defined to complete the maneuver. In particular, to ensure that the system remains nonrotating, a nominal attitude profile for a self-balanced electrodynamic tether is proposed, as well as a simple feedback control.