The Effects of Surface Modification on Spacecraft Charging Parameters

Charging of materials by incident radiation is affected by both environmental and physical conditions. Modifying a material’s physical surface will change its reflection, transmission and absorption of the incident radiation which are integrally related to the accumulation of charge and energy deposition in the material. General arguments for incident and emitted photons, electrons and ions are considered. An optical analysis of the effects of surface modification on spacecraft charging parameters on prototypical polyimide Kapton HNTM and Cu samples is presented. Samples were roughened with abrasive compounds ranging from 0.5 to 10 μm in size, comparable to the range of incident wavelengths. They were also contaminated with thin layers of DC 704 diffusion pump oil. Using a UV/VIS/NIR light source and a diffraction grating spectrometer, measurements were performed on pristine and modified materials. The measured spectra confirmed that surface modifications induce expected changes in optical reflection, transmission, and absorption. The generally increased absorption observed results in increased photon energy deposited in the material, leading to increased charge emission through the photoelectric effect

The Effects of Surface Modification on Spacecraft Charging Parameters

Charging of materials by incident radiation is affected by both environmental and physical conditions. Modifying a material’s physical surface will change its reflection, transmission and absorption of the incident radiation which are integrally related to the accumulation of charge and energy deposition in the material. An optical analysis of the effect of surface modification on spacecraft charging parameters on prototypical Kapton HN and Cu samples is presented. Samples were roughened with abrasive compounds ranging from 0.5 to 10 μm in size, comparable to the range of incident wavelengths. They were also contaminated with thin layers of DC 704 diffusion pump oil. Using a UV/VIS/NIR light source and a diffraction grating spectrometer, measurements were performed on pristine and modified materials. The measured spectra confirmed that surface modification does induce changes in optical reflection, transmission, and absorption. The generally increased absorption observed results in increased photon energy deposited in the material, leading to increased charge emission through the photoelectric effect. Index Terms—About reflectivity, surface modification, spacecraft charging, photoyiel

Electron Induced Charging and Arcing of Multilayered Dielectric Materials

Measurements of the charge distribution in electron-bombarded, thin-film, multilayered dielectric samples showed that charging of multilayered materials evolves with time and is highly dependent on incident energy; this is driven by electron penetration depth, electron emission and material conductivity. Based on the net surface potential’s dependence on beam current, electron range, electron emission and conductivity, measurements of the surface potential, displacement current and beam energy allow the charge distribution to be inferred. To take these measurements, a thin-film disordered SiO2 structure with a conductive middle layer was charged using 200 eV and 5 keV electron beams with regular 15 s pulses at 1 nA/cm2 to 500 nA/cm2. Results show that there are two basic charging scenarios which are consistent with simple charging models; these are analyzed using independent determinations of the material’s electron range, yields, and conductivity. Large negative net surface potentials led to electrostatic breakdown and large visible arcs, which have been observed to lead to detrimental spacecraft charging effects

Low Temperature Cathodoluminescence in Disordered SiO2

Disordered SiO2 is commonly used for optical instrumentation and coatings. In space telescope applications, these materials can be exposed to low temperature (particularly for IR telescopes) and simultaneous electron fluxes from the space plasma environment. During recent charging tests of this dielectric material, a discernable glow was detected emanating from the surface of the SiO2, indicating that the incident electron beam induced a luminescent effect, termed cathodoluminescence. As the sample cooled from 300 K to 120 K, a change in the intensity and energy spectrum of the glow was observed between 250 nm and 1700 nm, demonstrating that the SiO2 cathodoluminescence is temperature dependent. Cathodoluminescence occurs when a high energy electron excites a valence band electron into the conduction band, then a transition takes place between the extended conduction states and the localized states below the mobility edge resulting from structural defects. This final electron transition is the origin of the emitted photon, hence the luminescence. As sample temperature and the thermal energy of the electrons vary, the trap state population, distribution of accessible trap states, and transitions between states also vary. A dynamic model of electrons in these localized trap states is proposed to explain the temperature dependent experimental cathodoluminescence spectra collected. Using our experimental results in conjunction with literature references, the specific structural defects in SiO2 responsible for distinct features in the cathodoluminescence spectra can be identified. From our experimental results, a simple qualitative model of disordered band theory has been developed to describe the states and electron dynamics in our SiO2 samples. Ultimately, such knowledge is important in the optimal design of space telescope optics

Modeling the Defect Density of States of Disordered SiO2 Through Cathodoluminescence

This study measures the electron-induced luminescence (cathodoluminescence) for various samples of fused silica. With a band gap of ~8.9 eV, visible and near-IR (NIR) luminescence occurs only if there are states (localized defect or trap states) within the forbidden band gap for electrons to occupy. A model is presented based on the electronic band structure and defect density of states—used to explain electron transport in highly disordered insulating materials—which has been extended to describe the relative cathodoluminescent intensity and spectral bands as a function of incident beam energy and current density, sample temperatures, and emitted photon wavelength. Tests were conducted on two types of disordered SiO2 samples, the first type containing two variations: (i) thin (~60 nm) coatings on reflective metal substrates, and (ii) ~80 μm thick bulk samples. Luminescence was measured using a visible range SLR CCD still camera, a VIS/NIR image-intensified video camera, a NIR video camera, and a UV/VIS spectrometer. Sample temperature was varied from ~295 K to 40 K. The results of these tests were fit with the proposed model using saturation dose rate and mean shallow trap energy as fitting parameters and are summarized below. First, each incident energy has a corresponding penetration depth, or range, which determines the fraction of energy absorbed in the material. In the thinner samples, the range exceeds the thickness of the sample; therefore, the intensity decreases with increasing energy. However, for the thicker samples, the range is less than the sample thickness and the intensity increases linearly with incident energy. Next, at low current densities, luminescent intensity is linearly proportional to incident current density through the dose rate. At very high current densities, saturation is observed. Finally, the overall luminescent intensity increased exponentially as T decreased, until reaching an optimum temperature, where it falls off to zero (as the model predicts). The spectra show four distinct bands of emitted photon wavelengths, corresponding to four distinct energy distributions of defect states within the band gap, each behaving differently with temperature. The response of each band to temperature is indicative of the extent to which it is filled

The Effects of Surface Roughness on Diffuse Optical Reflection and Photoyields on Spacecraft Materials

The goal of this project was to measure the change in the absorbance of spacecraft materials due to changes in the surface of the material. The absorbance was obtained by measuring reflectance and transmittance. We found that modifying the surface of a material did affect the material’s specular reflectance. However, the change may not have been entirely due to an increase in absorbance, but may also imply an increase in the diffuse reflection. To understand the affect on absorbance, diffuse reflectance and transmission will need to be measured. This will lead to a prediction of how surface modification affects the charging of spacecraft

Extending the Band Model of Disordered SiO2 Through Cathodoluminescence Studies

Optical coatings of disordered thin film SiO2/SiOx dielectric samples on reflective metal substrates exhibited electron-induced luminescence (cathodoluminescence) under electron beam irradiation in an ultrahigh vacuum chamber at the USU facilities,. These experiments provided measurements of the absolute radiance and emission spectra as functions of incident electron energy, flux and power over a range of sample temperatures (300 K to 40 K). Early results from these experiments have led to a preliminary model of the band structure of highly disordered trapped states within the band gap of SiO2. We now extend this model to further describe the excitation of electrons from the valence band to the conduction band and subsequent relaxion into trapped states. The model for cathodoluminescence is used to describe the experimental observations, providing a fundamental basis for understanding the dependence of cathodoluminescence on irradiation time, incident flux and energy, and sample thickness and temperature. *This work was supported by funds from NASA Goddard Space Flight Center and a NASA Space Technology Research Fellowship

Power and Charge Deposition in Multilayer Dielectrics from Monoenergetic Electron Bombardment

Power and charge deposition in multilayer dielectrics from electron bombardment is dependent upon the flux and electron range of the electron beam, where the range,--a lso known as the penetration depth—is dependent upon the incident beam energy. Using the Continuous Slow Down Approximation (CSDA), a composite analytical formula has been developed to relate the electron range to the dose rate and subsequently to the deposited power in each subsequent layer. Based on the constituent layer geometry and material , the deposited charge can also be inferred. To validate these models two separate experiments were conducted, one based on the net surface potential and the second on electron induced luminescence. The first experiment used a disordered SiO2 based multilayer dielectric with a conductive middle layer. The sample was charged using 15 s pulses from an electron beam with an energy range from 200 eV to 5 keV. The second experiment also used a disordered SiO2 based multilayer dielectric, but with energies from 5 keV to 25 keV. Results of these experiments showed the power and charge deposition’s dependence on electron beam flux and incident energy, which compare favorably with the model predictions *Work supported by the NASA Goddard Space Flight Center

Charging Effects of Multilayered Dielectric Spacecraft Materials: Surface Voltage, Discharge and Arcing

Charging of thin-film, multilayer dielectric materials subject to electron bombardment was found to evolve with time. The charging behavior was also highly dependent on the incident energy of the monoenergetic electron beams; this is driven by energy dependant processes including the electron penetration depth, electron emission, and material conductivity. The electron penetration depth is the average range to which incident electrons at a given incident energy penetrate into the material, thus defining the mean depth of an embedded charge layer. The secondary electron yield is the ratio of electrons emitted from the surface to the number of incident electrons; this ratio largely controls the magnitude of the surface charge layer. The material conductivity determines how easily charge layers can move with and out of the material, and hence the time evolution of the charge distribution. While range and yield are highly energy dependent, the conductivity depends more on beam power and is highly temperature dependent. Radiation induced conductivity becomes significant for high electron fluxes. Models based on the dependence of electron range, electron emission and conductivity on incident energy, flux and sample temperature allow the evolution of the internal charge distribution to be inferred from measurements of the net surface potential and displacement current. Measurements were made for thin film SiO2 multilayer structures with a conductive middle layer charged, using 200 eV and 5 keV electron beams with regular 15 s pulses at 1 nA/cm2 to 500 nA/cm2. Results show that there are two basic charging scenarios. The first scenario, for incident energies between the crossover energies, is characterized by small positive net surface potentials, limited by electron emission and re-attraction of emitted electrons to the positive surface. The second scenario, for incident energies above the second crossover energy, is characterized by larger negative net surface potentials, limited by the conductivity and the breakdown voltage at which the material can no longer sustain electric fields produced by the deposited charge layers. Large negative net surface potentials led to electrostatic breakdown and large visible arcs, which can to lead to detrimental spacecraft charging effects. These measurements are shown to be quantitatively consistent with the simple charging models described above, using previous results of independent electron range, yields and conductivity measurements