Ryan C. Hoffmann

• Principal Investigator at United States Air Force Research Laboratory

Spacecraft materials play a crucial role in safeguarding satellites from space’s harsh conditions. However, exposure to solar radiation and aggressive chemical species in the upper atmosphere alters these materials’ physical, chemical, and optical properties over time. Understanding material evolution during a mission enhances spacecraft reliabilit...

The interaction between the materials on the surface of a spacecraft and the space environment can lead to mechanical, chemical, and optical changes that affect their properties. It is crucial to comprehend how these materials respond to the different aspects of the space environment once they are in orbit and how these properties change over time....

Thermal control coatings (TCCs) represent a part of the spacecraft thermal control system, which provides a given temperature regime due to its thermo-radiation characteristics. The space-weather induced deterioration of the TCCs' optical properties causes a change in these characteristics. This work presents a comparative study of the optical prop...

View Video Presentation: https://doi.org/10.2514/6.2022-0797.vid The harsh space environment imposes stringent requirements upon spacecraft materials, especially those located on exterior surfaces of space objects in low Earth orbit (LEO). As humankind moves from space exploration to space commercialization, these polymers may have to last for 15–2...

GPS satellites undergo surface contamination on the solar array coverglasses from repeated arcing events. Using NASA Air Force Spacecraft Charging Analyzer Program (Nascap-2 K) spacecraft charging simulation software, a GPS Block IIF satellite model was constructed and analyzed in realistic Medium Earth Orbit environments. GPS Block IIF satellites...

The harsh radiation environment at geosynchronous Earth orbit (GEO) induces chemical changes in organic polymers, leading to alteration of optical, mechanical, and charge transport properties. This work focuses on the evaluation of laboratory-simulated GEO environmental effects on the properties of common spacecraft materials and on methods for rem...

Thermal control paints are widely used in spacecraft industry to protect the spacecraft/satellite surfaces from the deleterious effects of the space environment. Dynamic reflectivity of spacecraft materials must be taken into account for improved space situational awareness (SSA). Additionally, a thorough characterization of each spacecraft materia...

Global positioning system (GPS) satellites are strongly suspected to arc into the space environment. Arcing on spacecraft due to differential charging can be blamed for a host of operational anomalies resulting in the loss of billions of dollars and productivity. In addition to anomalous behavior on spacecraft, arcing can be accused of an otherwise...

A project to investigate space environment effects on specific materials with interest to remote sensing was initiated in 2016. The goal of the project is to better characterize changes in the optical properties of polymers found in multi-layered spacecraft insulation (MLI) induced by electron bombardment. Previous analysis shows that chemical bond...

Polyimide (PI, Kapton-H®) films are widely utilized in the spacecraft industry for their insulating properties, mechanical durability, light weight, and chemical resistance to radiation. Still PI materials remain exposed to a combination of high-energy electrons, protons, and ultraviolet (UV) photons, particles primarily responsible for radiation-i...

Low-density polyethylene (LDPE) is a nonpolar polymer commonly used as an insulating material. Although its applications in the spacecraft industry are limited, LDPE is widely utilized as a model system to study electron transport through disordered insulating materials under simulated space weather conditions. The charge transport models developed...

Polyimides, particularly poly-(pyromellitic dianhydride-4,40-oxydianiline) or PMDA-ODA (Kapton-H ® ), are ubiquitous in the aircraft and aerospace industries, flexible printed circuits, and many other high-performance applications. For many of these applications, the polymer's stability during and after radiation damage is a serious concern. The ef...

When electrical arcs occur in space, a plasma expands away from the arc-site, neutralizing adjacent surfaces (a current), and causing a current to be produced at the arc-site (source of neutralization current). The speed of this plasma expansion depends on the plasma species, which in turn depend on the ionizable materials near the initial electros...

Ground based measurements are critical to understanding the space environment-induced modifications of spacecraft materials and predictive spacecraft modeling. The interaction of high-energy electrons with spacecraft materials, such as polyimide (PI, Kapton-H®), is known to modify the material's chemical and consequently physical properties. Highly...

Most spacecraft arcing studies have been of the typical inverted gradient arcing because geomagnetic substorms tend to charge spacecraft very negative of dielectric surfaces. However, normal gradient arcing, where the spacecraft is positive of its dielectric surfaces, may be more damaging because it usually occurs at higher differential potentials...

The Mumbo space environment simulation chamber discussed here comprises a set of tools to calibrate a variety of low flux, low energy electron and ion detectors used in satellite-mounted particle sensors. The chamber features electron and ion beam sources, a Lyman-alpha ultraviolet lamp, a gimbal table sensor mounting system, cryogenic sample mount...

Interaction of high energy electrons with spacecraft materials, such as polyimide (PI, Kapton-H®), is known to cause their physical degradation. However, understanding of the chemical nature of this damage and the effect on the electrical and optical properties of PI is still limited. This lack of understanding limits predictive spacecraft models (...

Flashover propagation of plasma on a solar panel is considered from a physics standpoint. Although useful as a first attempt to describe the flashover phenomenon, the previous perimeter theory is shown to have many areas in need of improvement before it can be used as a guide for spacecraft solar array design. A new, more physical picture is develo...

Since electrons are the primary charged particles at the geosynchronous Earth orbit (GEO), understanding of their interactions with spacecraft materials, such as polyimide (PI, Kapton-H®), is important. Understanding of the chemical nature of electron damage and its effect on PI's electrical and optical properties is still limited. Thus, predictive...

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 modificati...

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 flight experiments, it was heretofore unknown that arcs into the space plasma could turn into arcs between adjacent solar array strings at high interstrin...

Interaction of high energy electrons with spacecraft materials, such as polyimide (PI, Kapton®), is known to cause their physical degradation. In this paper, we investigated chemical and optical changes in Kapton® during and after irradiation with high energy (90 keV) electrons. Several complimentary characterization techniques were utilized to qua...

A study was conducted to demonstrate electrostatic charging of static dissipative polyimide labels. The experimental chamber consists of a four-axis motion system. Materials were irradiated using high-energy monoenergetic electron radiation from the Kimball Physics EG8105-UD electron gun with a range of 1-100 keV. ATrek 20 kV electrostatic nonconta...

How dielectric materials collect and store electrical charge is critical to spacecraft construction and operational anomaly resolution. Electrical conduction, and thus spacecraft potential, is known to change as materials are damaged by the space environment. However, the exact extent and nature of the aging is unknown and this dynamic aging is a m...

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...

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 m...

The first preliminary results are reported from the U.S. Round-Robin Test on Plasma Expansion Speed. The tests were performed at the NASA Glenn Research Center on two coupons (six strings) of International Space Station (ISS) solar cell arrays, with a separate small array to obtain arcs (because it is so difficult to get ISS arrays to arc). ISS arr...

form only given. It is well known that dielectric surfaces such as those on solar arrays are susceptible to surface charging in the space environment. If the charging is sufficient, a spontaneous arc can occur, triggering an outward-propagating electrostatic discharge plasma. The propagation speed and the fraction of charge cleared by this so-calle...

High energy electrons incident on highly disordered insulating materials undergo quasielastic collisions that imparts both charge and energy to the material; this can excite multiple intrinsic electrons from valence band or low level trap states into the extended states of the conduction band. These excited electrons provide a significant conductio...

Results are reported from the second stage of the U.S. Round-Robin Test on Plasma Expansion Speed. The tests were performed at the NASA Glenn Research Center on two coupons (six strings) of ISS solar cell arrays, with two separate small coupons to obtain arcs (because it is difficult to get ISS arrays to arc). ISS arrays were used because they have...

One of the most important, yet most poorly known quantities in determining ESD currents for spacecraft arcing events is the plasma propagation speed. When an ESD event occurs in GEO, the return current waveform depends on how rapidly and completely spacecraft dielectric surfaces are discharged by the plasma created by the ESD event as it expands ou...

The Spacecraft Charging and Instrument Calibration Laboratory (SCICL) is now operational at Kirtland AFB, Albuquerque, NM. SCICL is a comprehensive one-stop shop for testing R&D spacecraft solar array coupons, novel materials, dielectric charging, and modeling of spacecraft effects from relevant materials. Electron guns of up to 100keV or LEO plasm...

Spacecraft charging codes require accurate models of electron yields as a function of accumulated charge to correctly predict the charge buildup on spacecraft. The accumulated charge creates equilibrium surface potentials on spacecraft resulting from interactions with the space plasma environment. There is, however, a complex relation between these...

As mankind reaches to explore extreme environments in space, the application of ceramics surface coatings is increasing. The 2005 mission concept for Solar Probe used a unique design to achieve the necessary thermal control for a very close approach to the solar corona, including the use of a highly refractory, electrically insulating ceramic coati...

Presented here are electron-induced electron yield measurements from high-resistivity, high-yield materials to support a model for the yield of uncharged insulators. These measurements are made using a low-fluence, pulsed electron beam and charge neutralization to minimize charge accumulation. They show charging induced changes in the total yield,...

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 electron-irradiated dielectric material...

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 LDP...

New instrumentation has been developed for in situ measurements of the electron beam-induced surface voltage of high resistivity spacecraft materials in an existing ultra-high vacuum electron emission analysis chamber. Design details, calibration and characterization measurements of the system are presented, showing sensitivity to a range of surfac...

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...

The electron emission and electron transport properties of highly insulating materials depend on both the fluence and flux of incident charged particle beams. These properties are affected by the deposition, accumulation and dissipation of both charge and energy in the material. We describe the effects of these processes on three distinct experimen...

Understanding the characteristics of electron beam bombardment that induce electrostatic discharge (ESD) of insulating materials is crucial to constructing an electrically stable spacecraft. A measurement system has been designed to determine the beam energy and charge flux densities at which spacecraft materials (bare and conductor-coated polymeri...

Understanding the photon-induced charging of spacecraft materials is necessary in modeling the overall charging of a spacecraft. Measuring the photoyields of insulators requires sophistication, since insulators' electrons must overcome a greater potential energy barrier, than electrons in a metal, to move within a solid. In order to determine the p...

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...

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 elect...

We report on initial measurements of Radiation Induced Conductivity (RIC) for twelve thin film polymer materials that are used in the cabling of the James Webb Space Telescope. Results will be used to model possible detrimental arching due to space craft charging effects. RIC occurs when incident ionizing radiation deposits energy in a material and...

When a sufficient electric field is placed across a dielectric material, electrical breakdown occurs. The field strength at which this occurs is referred to as the dielectric strength or electrostatic discharge (ESD) voltage. The dielectric strength of thin (25 mum to 250 mum) film polymer samples (low density polyethylene, Teflon, Kapton, Mylar, a...

When a material is irradiated with energetic particles electrons can be emitted from the material. For electron-induced emission, the number of electrons that leave a particular material depends on the incident energy of the electrons, among other things. There are two critical energies where the ratio of emitted electrons to incident electrons cro...

When incident energetic electrons interact with a material, they excite electrons within the material to escape energies. The electron emission is quantified as the ratio of emitted electrons to incident particle flux, termed electron yield. Measuring the electron yield of insulators is difficult due to dynamic surface charge accumulation which dir...

Measuring the electron emission energy spectrum from a material yields a similar curve regardless of the incident energy source, whether it is electrons, ions, photons (photoemission), or even thermal energy (thermionic emission). When measuring the spectrum for electron induced electron emission, there is a question of whether it is possible to di...

We report on new instrumentation to measure Radiation Induced Conductivity (RIC). RIC occurs when incident ionizing radiation deposits energy in a material and excites electrons into the conduction band of insulators. Conductivity is determined by measuring the current through the thin film samples in a parallel-plate geometry under a constant appl...

Accurate measurements of the electron emission properties of extreme insulators require highly controlled experimental techniques. Due to the poor electron mobility in insulators, charge can accumulate which will affect future incident and secondary electrons; subsequently, the electron yield will evolve. This evolution is the prime difficulty in m...