1. Append 2
http://www.aero.org/publications/crosslink/spring2011/irnd.html
Emerging Technologies Volume 12, Number 1 (Spring 2011)
Preventing Radio-Frequency Breakdown in Satellite Components
Both military and commercial satellites rely on radio-frequency (RF) systems for communication
and navigation payloads. The RF power demand for these systems has continued to grow with
increasing user needs and higher available satellite power. Global Positioning System (GPS) III
and the Mobile User Objective System (MUOS) are just two examples of satellites with
unparalleled RF power requirements at multiple frequencies.
With increasing power levels comes increasing risk for RF breakdown within high-power
components. RF breakdown is an electrical discharge—such as a plasma or multipactor
discharge—that can degrade high-fidelity communication signals and cause physical damage to
susceptible components. These discharges can lead to complete loss of essential communication
or navigation signals and prevent proper satellite operation. As such, preventing RF breakdown
is essential.
In response to this growing risk, The Aerospace Corporation is leading new research into plasma
and multipactor breakdown. This program, led by Timothy Graves, Space Materials Laboratory,
is pursuing basic research into the underlying phenomenology while helping contractors develop
better hardware and testing requirements.
"Aerospace has a unique window into the real-world issues experienced by RF component
manufacturers. This allows us to tailor our research programs to solve problems of today and
tomorrow through a physics-based understanding of these concerns," said Graves. "Our goal is to
decrease risk through an improved understanding throughout the satellite process. From
component design, through extensive ground testing, to on-orbit operation, we depend on the
success of these RF components. Our research is providing new ways to improve success in each
of these areas."
Multipactor breakdown is one of the highest concerns for high-power RF component designers
today. Also referred to as multipaction, this discharge type can occur when electrons impact
material surfaces in resonance with the RF electric field. This resonance depends primarily on
three components: the RF voltage (how fast the electron is accelerated), the RF frequency (how
long before the electric field changes direction), and the geometry (how far the electron travels
before hitting a surface).
As electrons resonantly impact electrode surfaces, the electron density grows through secondary
electron emission. The secondary electron yield, defined as the number of emitted electrons per
incoming electron, is a fourth parameter for multipactor breakdown, such that the secondary
electron yield is greater than 1 to develop the discharge. When these conditions are met, the
formation of a large electron density can perturb the RF system and substantially increase the
risk of plasma breakdown and component damage.

2. Detecting multipaction in complex devices can be difficult, yet early detection in product
development is critical for satellite cost and schedule. In some cases, devices with undetected
multipaction in unit-level tests may experience catastrophic failures after integration into the
satellite system. To prevent this, Aerospace has characterized various breakdown signatures and
developed new diagnostics for improved detection sensitivity.
New software-based phase-nulling diagnostics for multipactor detection have been recently
developed at Aerospace using fast analog-to-digital processing to analyze the relationship
between forward and reflected power signals. With software, the system monitors for any
complex impedance change caused by multipactor or plasma formation. These software-based
systems have significant advantages over manually controlled analog devices, including higher
stability, improved interpretation, and greater sensitivity.
Multipaction depends on the material surface, which can vary strongly with contamination.
These discharges also dynamically change as multipacting electrons impact surfaces, desorb
contaminants, and/or form new surface thin films, a process known as multipactor conditioning.
Aerospace has performed extensive research into multipactor contamination and multipactor
conditioning on various materials, specifically characterizing a new multipactor mode referred to
as transient-mode multipactor discharge.
"The transient-mode multipactor discharge forms similarly to a conventional discharge, yet as
the electrons remove contaminants and change the secondary electron yield, the multipactor is
extinguished," Graves said. "This transient process can repeat indefinitely under continued
contamination until the device is damaged." Several Aerospace surface science studies are
investigating dynamic surface changes with multipactor exposure. Initial results have shown the
formation of thin films that can initially improve the voltage threshold for multipaction. Further
studies are planned with potential application to surface science and nanotechnology research
areas.
Graves credits the success of this program to the diverse scientific backgrounds available at
Aerospace. "Our multidisciplinary team includes researchers in RF engineering, plasma physics,
materials science, and systems engineering. With experts in each of these areas, we have made
strong and unique contributions toward mitigation of RF breakdown." The program's team
includes participants across many departments, including William Cox, Tom Curtiss, Rostislav
Spektor, and Jason Young, all of Electric Propulsion and Plasma Science; Gouri Radhakrishnan
and David Witkin, of Materials Science; Jerry Michaelson, of Communication Systems
Implementation; and Frank Villegas, of Antenna Systems.
The program has also had strong participation from students at UCLA, Loyola Marymount
University, Embry-Riddle Aeronautical University, Purdue University, and the University of
Maryland. Graves also cites an "excellent collaboration with many government contractors to
work together toward the common goal of better device performance and reliability."
"Our research will continue to adapt to meet our customer needs. We hope to pave the way for
improved computational prediction capability in complex structures, improved device testing
with enhanced diagnostics, and, lastly, improved understanding of breakdown phenomenology—
toward our main goal of ensuring space mission success."