Hypervelocity dust impact craters on photovoltaic devices imaged by ion beam induced charge

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Abstract

Hypervelocity dust has a speed of greater than 5 km/s and is a significant problem for equipment deployed in space such as satellites because of impacts that damage vulnerable components. Photovoltaic (PV) arrays are especially vulnerable because of their large surface area and the performance can be degraded owing to the disruption of the structure of the junction in the cells making up the array. Satellite PV arrays returned to Earth after service in orbit reveal a large number of craters larger than 5 μm in diameter arising from hypervelocity dust impacts. Extensive prior work has been done on the analysis of the morphology of craters in PV cells to understand the origin of the micrometeoroid that caused the crater and to study the corresponding mechanical damage to the structure of the cell. Generally, about half the craters arise from natural micrometeoroids, about one third from artificial Al-rich debris, probably from solid rocket exhausts, and the remainder from miscellaneous sources both known and unknown. However to date there has not been a microscopic study of the degradation of the electrical characteristics of PV cells exposed to hypervelocity dust impacts. Here we present an ion beam induced charge (IBIC) pilot study by a 2 MeV He microbeam of craters induced on a Hamamatsu PIN diode exposed to artificial hypervelocity Al dust from a dust accelerator. Numerous 5–30 μm diameter craters were identified and the charge collection efficiency of the crater and surrounds mapped with IBIC with bias voltages between 0 and 20 V. At highest bias, it was found the efficiency of the crater had been degraded by about 20% compared to the surrounding material. The speed distribution achieved in the Al dust accelerator was peaked at about 4 km/s compared to 11–68 km/s for dust encountered in low Earth orbit. We are able to extrapolate the charge collection efficiency degradation rate of unbiased cells in space based on our current measurements and the differences in the structure of the targets.

Introduction

The impact of ubiquitous space debris encountered by spacecraft in low Earth orbit results in numerous impact craters and holes in solar arrays from micrometres to several millimeters in size [1], [2]. In the case of the Hubble Space Telescope (HST), extensive studies have been performed on solar arrays returned to Earth after service in low Earth orbit. The cells in these arrays are based on silicon and comprise a 150 μm thick cover glass bonded with a 40 μm thick adhesive layer to a 250 μm thick silicon photovoltaic (PV) layer and a ∼250 μm backing layer of complex structure [2]. Thin tracks of Al spaced 1.25 mm apart form the front contacts to the PV layer with the back contacts forming part of the backing layer. Damage is observed from debris impacts with both the front and back surfaces and a number of impacts penetrate completely through the front or back layers to reach the PV layer, or blast a hole completely through the whole multilayer structure. From a combination of theory and experiment [2], the cumulative flux of impact craters of diameter d (i.e. the flux of the specified diameter or larger) ranges from around 4000 impacts/m2/year for d = 1 μm, ∼1000 impacts/m2/year for d = 10 μm, 130 impacts/m2/year for d = 100 μm, 3 impacts/m2/year for d = 1 mm and less than 1 impact/m2/100 year for d greater than 5 mm. Here d is defined to be the diameter of the conchoidal spallation which is roughly correlated with the diameter of the damage to the cover glass visible in an optical microscope. Degradation of the solar array can occur because of mechanical damage to the structure of the cell including electrical contacts or from occultation of the PV layer from spallation of the cover glass. The depth of a crater is typically 4 or 5 times less than d [3], however cracks from the crater may extend into the PV layer from both front and back impacts even if the crater does not reach the PV layer itself.

Analysis of the HST projectile residue alloyed with molten glass in a thin surface layer within the melt pit in the centre of the crater by analytical electron microscopy has identified the likely origins of the projectiles [3], [4]. Important elements are C, Cl, S, P, Ca, Mg, Cr, Ni, Fe and Al. The smaller craters (d < 50 μm) that have an identifiable trace elemental signature consist of about 30% from micrometeoroids and 70% from solid rocket combustion products (Al rich). Larger craters are mainly from micrometeoroids. About 25% of all craters have unknown origin and would probably benefit from more sensitive trace element analysis by proton induced X-ray emission in a nuclear microprobe.

Further studies have led to the development of detailed theoretical models for the degradation of the panel performance as a function of the crater diameter, projectile diameter and speed [5]. This study shows that the overall degradation of a panel in Low Earth Orbit (LEO) will be between 0.2 and 0.25% per year, but that the degradation is highly heterogeneous with 5 to 10% power loss possible for 10–20 large impacts. In adverse environments, such as those experienced by the Vega comet probe, more than 50% power loss occurred as a result of passage through the comet dust cloud [5]. In addition to the permanent mechanical disruption to the cell, transients associated with electrostatic effects can also be observed [6] and precautions have to be taken to ensure that accumulated charge can be dissipated to avoid exceeding the electrical stress limits of spacecraft structures.

Our model system in the present paper examines craters in the Class III category of impacts where the crater penetrates into the PV layer using the European Space Agency classification scheme [7].

Section snippets

Experiment

Measuring the degradation at the micrometre scale of the actual PV systems deployed in space is difficult because of the large capacitance of the cells which causes instrumentation difficulties associated with the controlled injection and measurement of charge. We have therefore elected to use a model system to examine the first order effects associated with projectile impact that is easier to manage but provides insights into the expected phenomena. Some previous studies with model systems

Conclusion

We have observed both the physical crater created by the dust impacts from SEM images and the “electrical” crater induced by similar impacts from IBIC images. If the degradation observed in the IBIC images is assumed to be representative of the craters in PV systems deployed in LEO then from the cumulative flux of craters reported by Moussi et al. [2] it is possible to calculate the approximate rate at which the degraded areas of a panel accumulate with time. If it is assumed that the craters

Acknowledgements

The authors acknowledge the assistance of Roland Szymanski at the University of Melbourne for assistance in performing the IBIC measurements and of Daniel Spemann for useful discussions. This work was supported by the Education Infrastructure Fund of the Australian Government.

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