Justin J. Likar

First Preliminary Results From U.S. Round-Robin Tests

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 arrays were used because they have no exposed interconnects to act as bare current collectors and confuse the experimental results. The preconstructed ISS strings were laid out approximately parallel to the plasma expansion velocity to allow for the best test of the simple plasma expansion front current waveform model. Several Langmuir probes were arranged above and to the sides of the sample to allow for measurement of the plasma propagation speed. In the initial set of tests, primary arcs and the consequent current waveforms were measured in a Low Earth Orbit-type plasma. In a second set of tests, two electron guns with diffusers were used to provide an approximately uniform Geosynchronous Earth Orbit-type environment, and primary arcs and current waveforms were obtained. The objective of this and other round-robin tests is to characterize primary arc waveforms in terms of speed and degree of discharge of arc plasmas produced by primary arcs, and their dependences on environment, capacitance per unit area, arc voltage, temperature, and so on. The final goal is to allow engineering estimates of arc current peaks and half-widths to allow confident design and construction of space solar arrays, and to allow mitigation techniques to be evaluated. This is the first in an extended series of tests to be performed at six different U.S. facilities, and with participation from ten different U.S. organizations.

Interaction of Charged Spacecraft with Electric Propulsion Plume: On Orbit Data and Ground Test Results

On-orbit observations and ground tests demonstrate interaction between charged spacecraft and electrothermal thruster-generated plasma. On-orbit measurements and test results are presented for plasma diagnostics and solar array performance during long-term exposure of flight solar panel. The long-term performance of a flight 70 V 2mtimes4 m GEO solar array exposed to a 2 kW arcjet plasma environment was studied

The Timescale of Surface-Charging Events

The timescale for creating high potentials on shadowed spacecraft surfaces depends on the conductivity of the surface in question, whether the neighboring surfaces are tied to the spacecraft frame or not, and on the space environment input. It is understood from laboratory and spaceflight measurements that the likelihood of a large surface potential and the timescale over which it might occur depends on these variables, yet the complex interplay between them makes the hazard difficult to assess even in controlled experiments. In this paper, we approach the specific question of the timescale of surface charging using several data sets in several orbit regimes: geostationary orbit (GEO), highly elliptical orbit (HEO), and low-Earth orbit (LEO). The measurements that we will show involve different approaches to the question of surface charging and subsequent electrostatic discharge (ESD) (GEO: surface charge monitors; HEO: direct plasma measurements; LEO: anomalies due to surface charging). However, the main strengths of the data are derived from their long duration covering multiple years and their occasional overlap in time. Here, we report on the timescale of surface-charging events at different locations in the magnetosphere across ~11 years of geomagnetic activity. The results will be relevant for assessments of space system impacts due to surface discharges and ESD, simulations of surface charging, and laboratory testing of flight system designs.

On-Orbit Error Rates of RHBD SRAMs: Comparison of Calculation Techniques and Space Environmental Models With Observed Performance

SEU data for more than 250 equivalent on-orbit SRAM device years is compared with upset rate calculations using various environmental models and contributions of both direct ionization and nuclear interactions.

Shielding Effectiveness and Closeout Methods for Composite Spacecraft Structural Panels

Increasingly, the latest spacecraft designs utilize partially conducting composite materials such as graphite epoxy or carbon-loaded Kevlar panels. These materials provide significant weight and mechanical advantages compared to conventional metallic structures, but their radio-frequency (RF) shielding properties are not well understood. In order to maintain a specified level of electromagnetic shielding on the spacecraft, it is necessary to use these composites and other materials to construct complex electromagnetic interference closeouts where harnesses, propulsion lines, RF cables, and waveguides penetrate the spacecraft Faraday cage. Shielding effectiveness measurements of these composite materials with standard seams and penetrations were performed. Closeout methodology was based on best commercial practices and included materials such as metallic mesh screen, double-sided metal-clad dielectric, conductive thermal blankets, and metallic foil tape with conductive adhesive. The test was conducted using standard methods inside a MIL-STD-461E anechoic chamber. Results demonstrated that the spacecraft composite structure with closeouts typical for commercial spacecraft can provide greater than 40 dB of shielding effectiveness from 200 MHz to 10 GHz and greater than 30 dB of shielding effectiveness from 10 to 40 GHz.

Spacecraft Charging Monitoring at GEO: Natural and Electric Propulsion Environment Measurements

Surface charging sensors are used on spacecraft to characterize the potential difference between a sample material surface relative to spacecraft common ground. A review has been performed of data collected over a period representative of a typical spacecraft lifetime from more than 10 such sensors aboard 6 Lockheed Martin built satellites operated by SES and INTELSAT which employ the use of electric propulsion systems at geosynchronous altitudes. Data are used to study the effects on surface charging caused by the electric propulsion plasma / GEO plasma correlative environment. Data are also used to review the performance of the sensors, characterize worst-case charging conditions experienced by the vehicles, and evaluate the frequency of severe charging events over the mission lifetimes.

Charging of Composites in Space Environment: Ground Test Data

Results of an extensive ground test routine, during which the charging characteristics of numerous composites commonly used in spacecraft design, are presented for plasmas representative of spacecraft exterior and shielded locations at GEO.

Total Ionizing Dose and Dose Rate Effects in Candidate Spacecraft Electronic Devices

Total dose tests of common devices reveal unexpected dose rate sensitivity. Devices from the same vendor procured to SMD versus military specifications exhibit substantially different dose rate effects. Behavior and critical parameters are compared and discussed.

Influence of Solar Cell Shape, Interconnect Shape, and Coverglass Coatings on Solar Array Arcing Parameters

Arc inception voltage studies have been performed on several common solar cell types. Results yielded quantitatively significant findings indicating specific designs are susceptible to arc inception at bias voltages readily achievable at near Earth orbits.

On-Orbit SEU Rates of UC1864 PWM: Comparison of Ground Based Rate Calculations and Observed Performance

More than 10 years of on-orbit UC1864 SEU data are compared with typical upset rate calculation methodologies and GCR environments. Results suggest that the most common calculation methods overestimate upset rates by at least 2×. Mitigation techniques are also verified via direct in orbit observations.