The 2011 (ISC) Global Information

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The 2011 (ISC)2 Global Information Security Workforce Study A Frost & Sullivan Market Survey Sponsored by and Prepared by Robert Ayoub, CISSP Global Program Director, Information Security
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ALTERNATIVE HALL THRUSTER PROPELLANTS
KRYPTON AND BISMUTH:
SIMULATED PERFORMANCE AND CHARACTERIZATION
A DISSERTATION
SUBMITTED TO THE DEPARTMENT OF MECHANICAL
ENGINEERING
AND THE COMMITTEE ON GRADUATE STUDIES
OF STANFORD UNIVERSITY
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
David Brandon Scharfe
August 2009c° Copyright by David Brandon Scharfe 2010
All Rights Reserved
iiI certify that I have read this dissertation and that, in my opinion, it
is fully adequate in scope and quality as a dissertation for the degree
of Doctor of Philosophy.
(Mark A. Cappelli) Principal Adviser
I certify that I have read this dissertation and that, in my opinion, it
is fully adequate in scope and quality as a dissertation for the degree
of Doctor of Philosophy.
(Ronald K. Hanson)
I certify that I have read this dissertation and that, in my opinion, it
is fully adequate in scope and quality as a dissertation for the degree
of Doctor of Philosophy.
(William A. Hargus, Jr.)
Approved for the University Committee on Graduate Studies.
iiiivPreface
Though xenon has traditionally been used as the propellant for most Hall thrusters
and ion engines, its rarity and high cost create the need for alternatives for certain
missions. Bismuth and krypton are suggested as possible replacements for xenon
to meet mission requirements. Bismuth, a metal, is 1000 times less expensive per
mass than xenon, and is readily available as a byproduct of the reflning process of
other metals. Krypton, as a noble gas, is a much simpler replacement for xenon than
bismuth; krypton is ten times more abundant in the atmosphere than xenon, is 1/10
the cost per volume, and is essentially produced in ten times the quantity as xenon
during the fractional distillation of air.
Computer-based Hall thruster simulations presented in this dissertation indicate
that a thruster optimized for bismuth propellant should have a shorter channel than
one optimized for xenon; with a shortened channel, bismuth ofiers distinct perfor-
mance advantages over xenon, providing higher thrust, ionization fraction, and ef-
flciency. However, being a metal, bismuth would add additional complexity to the
designofaplasmathrustersystem. Krypton,anoblegas,isamoredirectreplacement
for xenon in a thruster system; simulations indicate that with the same mass ow,
the performance penalty incurred for utilizing krypton instead of xenon propellant is
likely to be rather small.
To assist in future experimental measurements of these alternative propellants,
analytical background work has been done to design optical diagnostics for measure-
ment of particle velocities and densities. For all discussed electronic transitions, the
hyperflne splitting has been calculated when data in the literature allows; though
the splitting of krypton transitions is comparably narrow, an understanding of the
vsplit proflles for bismuth will be crucial to accurately interpret future experimental
measurements. Atomicresonanceabsorption spectroscopyissuggestedfor measuring
propellant number densities by probing the ground states of neutral and ionized bis-
muth and krypton. Laser-induced uorescence (LIF) and emission spectroscopy are
suggested for measuring ion velocities. Based on data output from the Hall thruster
simulation, the likely recorded signals from such optical measurements are modeled.
Forabsorptionspectroscopy,resonancetransitionsforBiIat307nm,BiIIat143nm,
KrIat123.6nm,andKrIIat91.7nmaresuggestedandanalyzed. Whilethebismuth
neutral line poses no distinct di–culties, the other resonance transitions lie within
thevacuumultravioletrangeandwouldposesigniflcantexperimentalchallenges. For
velocimetry of ions via LIF or emission, the Bi II transition at 681 nm has been
selected and analyzed. For Kr II, several transitions connected to the metastable
¡1120,209.87 cm energy level have been identifled: 630.5 nm, 642.2 nm, 687.1 nm,
and 729.2 nm. Due to the availability of hyperflne splitting data, the 729 nm tran-
sition has been modeled in terms of LIF and emission spectroscopy, but it is noted
that the splitting of krypton transitions is suitably narrow that any of the suggested
lines could be used for analysis quite readily.
Finally, noting the particular di–culties associated with the analysis of a metal
vapor, a series of apparatuses used to analyze a bismuth plasma are presented. The
design, operation, and measurements from a microwave discharge, a heat-pipe appa-
ratus, and a linear E£B (Hall) discharge are detailed. The microwave discharge has
been identifled as a very weak source of bismuth plasma. From the heat-pipe appa-
ratus, the suggested Bi I absorption analysis has been verifled and the breakdown of
bismuth vapor was characterized. Low resolution emission measurements of the sug-
gestedtransitionsof ionizedbismuthhavebeen recorded. Thelinear E£B discharge
was used to characterize the bismuth plasma via various probe-based measurements.
Measurements recorded include emissive probe measurements to determine the pro-
fllesofsheathpotential,plasmapotential,andelectrontemperaturealongthethruster
axis. The combined use of a retarding potential analyzer, a Faraday probe, and an
emissive probe were used to compute the ion velocity distribution, the ion current
density, and the plasma density at a single point along the thruster axis.
viOptical measurements on various atomic and ionic bismuth transitions were also
recorded via the linear discharge. It is noted that while LIF may pose signiflcant
di–culties for analysis of a bismuth thruster, high-resolution emission measurements
may provide a suitable path to velocimetry analysis, while simultaneously increasing
experimental exibility in terms of transition selection.
Further, the details of the design of a coaxial Hall thruster, suitable for direct use
withkryptonandmodiflableforusewithbismuth, arealsopresentedforfuturework.
Additional suggestions are made for transition selection and measurement techniques
in the future analysis of bismuth Hall thrusters.
ThisworkrepresentsaflrstefiorttosimulatebismuthandkryptonasHallthruster
propellants using an advanced 2-D hybrid particle-in-cell (PIC) method. It also rep-
resents the flrst use of a Hall thruster simulation in a qualitatively predictive way:
to simulate a potential future under various conditions and suggest basic
optimizations to the geometry and operational settings. The diagnostic design work
in this dissertation is the flrst of its kind performed for bismuth and krypton, and
the flrst published efiort to use the outputs of a Hall thruster simulation to aid in di-
agnostic development. Finally, the experimental work performed represents a unique
compendiumofmeasurements,otherwiseunavailableintheliterature,tocharacterize
the nature of, and di–culties associated with, a bismuth plasma. This experimental
work is expected to aid in the decision-making process for experiments on this metal
vapor.
viiAcknowledgements
First and foremost, I owe an unspeakably large debt of gratitude to my wife, Michelle
([138], heh). We’ve known of each other since junior high, began dating late in high
school, managedto sufier through an oft-driven14 mile separation during undergrad,
and flnally inextricably tied ourselves to each other as we arranged to work under
the same adviser as we began grad school at Stanford. I will always owe her my
thanks for things too numerous to exhaustively list. She has always brought me joy.
Throughout these past years, she has supported me through several overly tortuous
classes (as I lent her my intuition and practicality for the classes that didn’t flt well
in her equation-oriented brain). She has stood by me in the lab through long hours
of frequently fruitless experiments, and she got me started on the less frustrating
simulation work. Perhaps most importantly of all, she has put up with my negativity
throughout the process, maintaining her love for me and helping me to see the dim
light at the end. Without Michelle, I do not know where I would be today...
Next, I must of course thank my family, and most especially my parents. I owe
them all my thanks for whatever combination of nature and nurture were required to
get me to where I am today, and wherever I go in the future. My entire family, from
my siblings Nathan and Angela, up through my grandparents, and extending out to
my aunts, uncles and cousins, has always believed in my potential, and has guided,
encouraged, and supported me along the way.
In a more direct sense, I owe great thanks to several people who shaped my early
developmentinthefleld. Dr. KeithGoodfellow’scoursesatUSCpiquedmyinterested
in space propulsion and especially in EP. Working in the USC labs of Professors
Ketsdever and Muntz, and at Dr. Lee Johnson’s lab at JPL put me further down
viiithe path toward my eventual graduate endeavors. Obviously, the culmination of this
path was my dissertation-related work under my adviser, Prof. Mark Cappelli.
Mark and the entire Cappelli group were instrumental in my completion of this
work. Most especially, I want to thank Scott, without whom the whole lab would
have fallen apart; his knowledge and experience is the linchpin that keeps everything
together and running. Nicolas played a similar role with his breadth of knowledge
of all things Hall thruster. Flavio deserves a special thanks for flnding us an o–ce
so Michelle and I didn’t have to stay in the loft, for sharing that o–ce with us,
and for providing us a place to crash during our numerous trips up to Stanford to
complete this whole process. San, the honorary member of the Cappelli group, also
gets a special thanks for his thesis-editing wedding gift. To Andrew, I owe a special
thanks for lending me his knowledge so that I could shortcut the design process for
my probe-based measurements. I also thank Tsuyo for working with me on several of
the heat pipets, and for being so awesomely Japanese. Every member of
the Cappelli group, of course, has also provided much appreciated support through
their friendship, intellectual discourse, and shared sufiering.
On the note of friendship, I must also thank all of the robots from USC. They
made undergrad more fun, and provided me with occasional stress relief during my
tenure at Stanford.
I would also like to thank my entire dissertation defense and reading committee,
including my readers, Professors Cappelli and Hanson, and Dr. Hargus. I’d also like
to thank my non-reader, Prof. Edwards, and my chair, Prof. Salleo. An additional
debt of gratitude is owed to C. William Larson at AFRL, who spent time with me,
helping me to flnd several references that I would not otherwise have come by, and
that were necessary for me to tie the whole dissertation together.
A debt of gratitude is also owed to the various administrators working behind the
scenesintheThermosciencesDivisionandthelargerMechanicalEngineeringDepart-
ment. IthankPerry,Christine,Indrani,andCitaforallthattheydo,andspeciflcally
for the help they ofiered me to ensure that my work progressed and ultimately cul-
minated in this dissertation.
Of course, without proper flnancing, continuing this far with my schooling would
ixhave been a great burden to bear. I must thank USC for their generous Trustee
Scholarship,theDepartmentofDefenseandASEEfortheNDSEGfellowship,andthe
NationalSciencefoundationfortheirfellowshipsupportviatheGRFP.Forextending
the flnancial support beyond my time at Stanford and funding my existence while
I flnished my dissertation, I owe my thanks to all the people instrumental in hiring
Michelle and myself at the Air Force Research Lab. I especially owe my thanks to
Prof. Andrew Ketsdever, Dr. Marcus Young, and Anthony Pancotti, with whom I
have worked closely for the past year. I also thank Dr. David Campbell, of ERC,
who o–cially hired me and allowed me to begin my career with the supportive ERC
family.
Finally, with my tongue in my cheek, I also thank the people responsible for
gizmodo.com and arstechnica.com for providing me with the sort of technical nerd-
stufi I like to read to relieve stress while simultaneously increasing the breadth of my
knowledge in flelds I have no business knowing about.
x