Ryan Hoffmann

Low-Fluence Electron Yields of Highly Insulating Materials

Electron-induced electron yields of high-resistivity high-yield materials - ceramic polycrystalline aluminum oxide and polymer polyimide (Kapton HN) - were made by using a low-fluence pulsed incident electron beam and charge neutralization electron source to minimize charge accumulation. Large changes in the energy-dependent total yield curves and yield decay curves were observed, even for incident electron fluences of < 3 fC/mm2. The evolution of the electron yield as charge accumulates in the material is modeled in terms of electron recapture based on an extended Chung-Everhart model of the electron emission spectrum. This model is used to explain the anomalies measured in highly insulating high-yield materials and to provide a method for determining the limiting yield spectra of uncharged dielectrics. The relevance of these results to spacecraft charging is also discussed.

On the Computation of Secondary Electron Emission Models

Secondary electron emission is a critical contributor to the charge particle current balance in spacecraft charging. Spacecraft charging simulation codes use a parameterized expression for the secondary electron (SE) yield delta(E_o) as a function of the incident electron energy E_o. Simple three-step physics models of the electron penetration, transport, and emission from a solid are typically expressed in terms of the incident electron penetration depth at normal incidence R(E_o) and the mean free path of the SE lambda. In this paper, the authors recall classical models for the range R(E_o): a power law expression of the form b₁E_o ⁿ¹ and a more general empirical double power law R(E_o)=b₁E_o ⁿ¹+b₂E _o ⁿ². In most models, the yield is the result of an integral along the path length of incident electrons. An improved fourth-order numerical method to compute this integral is presented and compared to the standard second-order method. A critical step in accurately characterizing a particular spacecraft material is the determination of the model parameters in terms of the measured electron yield data. The fitting procedures and range models are applied to several measured data sets to compare their effectiveness in modeling the function delta(E_o) over the full range of energy of incident particles

Temperature Dependence of Radiation Induced Conductivity in Insulators

This study measures Radiation Induced Conductivity (RIC) of Low Density Polyethylene (LDPE) over temperatures ranging from ∼110 K to ∼350 K. RIC occurs when incident ionizing radiation deposits energy and excites electrons into the conduction band of insulators. Conductivity was measured when a voltage was applied across vacuum‐baked, thin film LDPE polymer samples in a parallel plate geometry. RIC was calculated as the difference in sample conductivity under no incident radiation and under an incident ∼4 MeV electron beam at low incident fluxes of 10−4–10−1 Gr/sec. The steady‐state RIC was found to agree well with the standard power law relation, σRIC = kRIC⋅DΔ between conductivity, σ and adsorbed dose rate, D. Both the proportionality constant, kRIC, and the power, δ, were found to be temperature dependant above ∼250 K, with behavior consistent with photoconductivity models developed for localized trap states in disordered semiconductors. Below ∼250 K, kRIC and Δ exhibited little change. The observed ...

Triggering Threshold Spacecraft Charging with Changes in Electron Emission from Materials

[Abstract] Modest changes in spacecraft charging conditions can lead to abrupt changes in the spacecraft equilibrium, from small positive potentials to large negative potentials relative to the space plasma; this phenomenon is referred to as threshold charging. It is well known that temporal changes of the space plasma environment (electron plasma temperature or density) can cause threshold charging. Threshold charging can also result from by temporal changes in the juxtaposition of the spacecraft to the environment, including spacecraft orbit, orientation, and geometry. This study focuses on the effects of possible changes in electron emission properties of representative spacecraft materials. It is found that for electron-induced emission, the possible threshold scenarios are very rich, since this type of electron emission can cause either positive or negative charging. Alternately, modification of photonor ion-induced electron emission is found to induce threshold charging only in certain favorable cases. Changes of emission properties discussed include modifications due to: contamination, degradation and roughening of surfaces and layered materials; biasing and charge accumulation; bandstructure occupation and density of states caused by heat, optical or particle radiation; optical reflectivity and absorptivity; and inaccuracies and errors in measurements and parameterization of materials properties. An established method is used here to quantitatively gauge the relative extent to which these various changes in electron emission alter a spacecraft’s charging behavior and possibly lead to threshold charging. The absolute charging behavior of a hypothetical flat, twodimensional satellite panel of a single material (either polycrystalline conductor Au or the polymeric polyimide KaptonTM H) is modeled as it undergoes modification and concomitant changes in spacecraft charging in three representative geosynchronous orbit environments, from full sunlight to full shade (eclipse) are considered.

AFRL Round-Robin Test Results on Plasma Propagation Velocity

The speed plasma propagates across a charged solar panel after a primary arc is one of the most important, yet poorly known, quantities in determining Electrostatic Discharge (ESD) currents for spacecraft arcing events. A review of the literature over the last two decades reveals that measured propagation velocity varies by as much as an order of magnitude. To overcome this deficiency, a round-robin set of tests was initiated with partners from industry, academia, NASA and the U.S. Air Force. This paper will provide the most recent results from the Air Force Research Laboratory testing conducted at the Spacecraft Charging and Instrument Calibration Laboratory.

Preliminary Measurements of ESD Propagation on a Round Robin Coupon

The propagation dynamics of spacecraft electrostatic discharge (ESD) flashover plasmas have been a topic of increasing interest in the past few years. To investigate ESD propagation and possible contributions from facilities, methodology, and analysis, we performed inverted gradient ESD tests of an International Space Station solar array coupon as part of a U.S.-wide Round-Robin test campaign. Geosynchronous earth orbit charging environments were simulated and current transients were simultaneously measured on ten solar array strings. In addition, images of ESD events were collected by low- and high-speed cameras, and an array of eight Langmuir probes positioned above the coupon was monitored. Preliminary results will be discussed, with special attention given to the effect of different analysis techniques on derived propagation velocity. Comparisons will be made with the results from other labs, and implications for spacecraft ESD events will be discussed.

1997–2002 Solar Array String Failures Revisited

In 1997, two commercial geostationary satellites experienced a new phenomenon: sustained solar array arcing. Although arcing on solar arrays in space had been expected from ground tests and space f...

Chemical and Electrical Dynamics of Polyimide Film Damaged by Electron Radiation

The processes of electrical charge accumulation and dissipation in dielectric materials are critical to spacecraft construction and operational anomaly resolution. Electrical conductivity, and therefore surface potential, of radiation-damaged materials undergoes unpredicted changes while on orbit. The space environment causes fundamental modifications in the chemical structure of spacecraft materials by breaking intermolecular bonds and creating free radicals that act as space charge traps. Over time, free radicals react with each other and the material recovers. The rates of free radical formation and loss determine the dynamics of the conductivity of spacecraft materials. Lack of knowledge about dynamic aging is a major impediment to accurate modeling of spacecraft behavior over its mission life. This paper presents an investigation of the chemical and physical properties of polyimide (PI) films during and after irradiation with high-energy (90 keV) electrons. The constant voltage method was utilized to monitor material conductivity during the recovery process. To quantify the concentration of free radicals within the irradiated material, the electron paramagnetic resonance technique was used. Changes in the infrared (IR) absorption profile of irradiated materials during the recovery process were identified using the directional-hemispherical reflectance technique coupled with the Fourier transform IR spectroscopy. This physical/chemical collaboration allowed correlation of chemical changes in PI with the dynamic nature of spacecraft material aging.

Optical and electrical properties of pseudomorphic glass

A flexible space solar cell coverglass replacement called Pseudomorphic Glass (PMG) has been under investigation in hopes of providing a robust, high transmissivity replacement for conventional coverglass. PMG is composed of ceria doped borosilicate or fused silica beads incorporated in a variety of polymer matrices. The glass beads provide the primary radiation protection and the polymer matrix provides the mechanical integrity. Ideally, PMG will be flexible, conductive, stable in the UV wavelengths, and emissive in the infrared wavelengths for thermal control. The infrared emittance of the beads and matrix that make up PMG was examined, demonstrating high emissivity of the adhesives in the infrared region. This may allow a PMG coated array to operate at lower temperatures than traditional coverglass. Conductivity of PMG was measured using investigated with multiple techniques. PMG demonstrated little improved conductivity over coverglass, prompting further exploration of incorporation of conductivity into PMG.

Relevance of Ground-based Electron-Induced Electrostatic Discharge Measurements to Space Plasma Environments

Electron-induced electrostatic discharge (ESD) can lead to severe spacecraft anomalies. It is crucial to the success of space missions that the likelihood of ESD occurrence is understood and mitigated. To aid in predicting ESD occurrence, a model for electric fields above and below the charge layer inside an electronirradiated dielectric material was developed. An instrumentation system was also designed to induce and detect ESD events. Because ESD events with a wide range of maximum current values can occur over a range of time intervals, multiple simultaneous detection methods were employed as charge was accumulated on a sample surface; these included monitoring of sample current and optical emissions from the sample surface. Data from ESD experimentation for James Webb Space Telescope (JWST) materials was used to verify that the instrumentation system was effective in inducing and observing ESD. Two types of discharge events were observed during JWST testing: a sudden-onset, decaying current accompanied by luminescence in the optical data, and an arc or flash in optical data. JWST test results were applied to the electric field models developed to determine the threshold electric field for luminescence onset. The models were also applied to the JWST materials in five different space plasma environments to determine the accumulated electric field as a function of time, and to thereby predict the likelihood of sample luminescence in each location.