Charles W. Roberson - Doctorate: Physics, 1971
I have often thought that one of the luckiest things that have ever happened to me was when the Chairman of the Physics Department at The University of Texas told me they had lost my application for a teaching assistantship. I had been accepted in the graduate school, but needed a job. Both my girlfriend and I were graduating college and planned to get married. I needed support to attend graduate school at UT. The department chairman, Professor Robertson, called me and told me my application had been lost and all the positions had been filled. However, he told me that he remembered my application to graduate school because my work for my masters’ thesis was similar to work done in his lab. He offered me a research assistantship starting in June. He suggested that before classes in the fall I should take the opportunity to see if there were any other areas of physics where I would like to work. Within a matter of weeks I was on my way to Austin with a U-Haul truck full of wedding gifts and Joan, my new bride.
Gentle is My Thesis Advisor
I did not stay with Professor Robertson. His research was indeed similar to the work I had done for my master’s thesis on the Ball-of-Fire Discharge at Cal State Los Angeles.
There was a new wave of plasma physics at UT. Around the hall from Professor Robertson’s lab I met Professor Gentle. Gentle was a new hire from MIT, well funded by the National Science Foundation and in need of students. I became his first student.
Professor Gentle was hired by The University of Texas to carry out fundamental plasmas experiments in the regime where collisionless process dominated over collisional processes. It was recognized that projected Fusion Reactors would operate in this range, and there was an opportunity to develop the experimental basis for the physics of these processes.
One of the choices Professor Gentle gave me for a thesis was the interaction of a warm beam with plasma. Plasma is an ionized gas. The most common example is our Sun.
An ideal beam is when all the particles in the beam have the same velocity as the beam. The extent to which a beam is warm or cold depends on the velocity spread of the beam particles and the application of the beam.
I had no idea why this was considered an important problem. In every other case, we try to make the beam cold, or ideal. I was happy to have a well-defined problem. The critical path for me was to determine how warm the beam had to be to satisfy the assumptions of quasilinear theory, which we were going to test experimentally. We had to figure out how to make the beam warm.
When I met Professor Gentle he was just starting to build his experiment. We were winding magnetic field coils, building vacuum chambers and testing electronic equipment. After I finished my course requirements, I built a test apparatus to explore ways to make warm beams to shoot into the collision-less apparatus we had built. In hindsight, it was a simple solution, but it depended on the fact that the experiment was carried out in a magnetic field and an averaging over the beam radius by the plasma waves.
While preparing to give my first talk on the results of this experiment at an American Physical Society (APS) meeting, I learned that a theorist at a laboratory in Siberia had theories similar to the Quasilinear Theory developed in the United States.
My first American Physical Society talk in 1970 was in Washington D. C. to a standing-room only crowd. My confidence level was high. I tried to inject some humor with the comment “We have built an experiment in Texas,” I told the audience, “to test the hypothesis that if a theory is developed in the United States and Siberia, and they get the same results, the theory is correct.” The paper was well received. My first paper was published in Physical Review Letters, perhaps the most sought after journal in physics.
Professor Gentle asked me to stay a few months after I finished my thesis to do some experiments related to a new theory of the nonlinear interaction of a cold beam with a collisionless plasma.
The transition from warm to cold beam was simple for us. I simply had to remove the magnetic shield from the electron gun that provided the energy spread. All else was the same.
The experiment worked immediately. All our years of building, tuning and improving the apparatus paid off. Every point from the theory paper we asked the machine to address, it provided answers on demand. We had a winner and we knew it. In retrospect, it was the most exciting experiment in my 40 years of research in plasma physics and I walked away from it to take my next career step.
I worked long hours every day for several weeks. On the last day I gave all the data to Professor Gentle who had worked with me, emptied my desk and went home to pack for the first job.
Two days after the experiment, I was in a U-Haul Truck on my way to New Haven. Joan was following in our car with our two year old son and her grandmother.
Our years in Austin were a transformation period. We had a baby and moved into the university owned Deep Eddy Apartments. At $35/Month for a two-bedroom apartment, this was the best deal in town. We learned to race one-design sailboats and became active in the Snipe Class. I had the opportunity to work with Professor Gentle at a critical time in his career and make significant contributions. I left to make my way to Yale for a postdoc and 40 more years of adventures in beam physics.
When we arrived at Yale, our apartment in Faculty-Staff housing was not ready. So they put us up in Connecticut Hall, the oldest building on campus. We were living in the old Campus, just across the street from the Town Green. It was an interesting introduction to New Haven.
The people living in Faculty-Staff apartments made Yale an adventure. There were bright people from all over the world. They also knew they were temporary. A few years at Yale and move on, that was the expectation. Most of the wives did not work outside the home. When a ground floor apartment could not be rented because of occasional flooding, the wives got permission to use the apartment as a pre-school. The women took turns volunteering to spend time with the kids, so they had free time. This “Yale pre-school” was a perk that was priceless.
Through the Newcomers club we got in the Gourmet dinner club, which was a feast and a chance for Joan to show her talents. Our Saturday night potluck dinners at the apartments were international food affairs.
We joined the Yale Faculty Yacht club. The University used 420s for intercollegiate racing. During the summer when classes were out, the club made these and the facilities on Long Island sound available to the faculty and staff. There were organized races on Sunday afternoon that we were very active in. We took advantage of the babysitting pool at the apartments to participate in the races.
The second summer at Yale I was awarded a Fellowship to attend a month long NSF-NATO sponsored summer school. The school is in the French Alps near Mount Blanc, the highest mountain in Europe. Joan, Stephen and I were off to our first European trip. It was 1972.
The summer school was in the mountains above Les Houches, a small town 7 km from Chamonix, where the first winter Olympics was held. The lectures were given in the morning and we had lunch together at the school. The afternoons were free for hiking in the Alps. There was an early evening discussion group. During one of the break out discussion, one of the Professors from MIT discovered my work had been done with Professor Gentle at The University of Texas. His “Those were beautiful experiments” comment sustained me for the controversy to come in the following weeks.
The lecturer from Ecole Polytechnique’s talk on Nonlinear Dynamics gave high praise to my quasilinear experiments. He claimed none of the previous experiments met the initial conditions required by the theory and none of the computer experiments had sufficient numbers of particles to be valid. Mine was the only experiment and there were no simulations.
The lecturer from Iowa had long been a critic of the fundamental assumptions of the Quasilinear Theory and issued a challenge. He said he would devote his evening discussion to criticisms of quasilinear theory and I could follow that with a discussion of the experiment. The Iowa Professor spent his time expressing his angst about what was wrong with the theory. My job was simpler. I knew what I knew and had talked about it enough times that I was comfortable discussing it. The sympathy of the school was with me. I was catapulted into the limelight. One year out of graduate school and my name was a household word in the French plasma community.
After Yale I went to the University of California at Irvine. It was good to be back in the Southern California sun and near family. I was involved in the startup of a new intense beam plasma laboratory. The beam energies were up to a million volts and currents up to 100kA. The experiments involved the generation of high power microwaves from beam plasmas interactions, collective ion acceleration and the formation of reverse field geometry with a rotating beam as a fusion confinement configuration. At the end of my stay at Irvine, I went to Europe and gave a paper on our rotating beam fusion experiment at the prestigious International Atomic Energy Agency meeting in Innsbruck, Austria. It was an ego trip. The Department of Energy combined our paper with similar results from the Naval Research Laboratory and the University of Maryland. When I gave the talk, the paper was translated into French, Spanish and Russian as I spoke. The translation alone was enough to make you think it was important work.
After 5 years at the University of California at Irvine, I went to work at the Naval Research Laboratory (NRL) in Washington, D.C. At NRL I worked on Free Electron Lasers and high current cyclic accelerator concepts until 1982. My next job was at the Office of Naval Research (ONR), a funding agency. I went to work at ONR to run the plasma program in the physics department. ONR was one of the first government funding agencies started after the Second World War to support university research. It was a role model for the National Science Foundation that was established a couple of years latter. When I started work at ONR the agency had a 35-year track record of supporting individual investigator research in fundamental science with a view towards Naval applications. There was always pressure from the top to create more focused programs. One of my investigators had demonstrated the ability to make the first thermal equilibrium plasma. It was a pure electron plasma in a Penning trap, which confines the particles radially by a magnetic field and axially by an electric field. This provided the stimulus for a five-year special focus program in Nonneutral Plasmas, which is still active 20 years later.
Nonneutral Plasma Physics
The development of microwave sources was one of the critical technologies during the Second World War and is widely used world wide in radar systems. Today most homes have microwave ovens based on that technology. Accelerators were developed primarily for nuclear physics research (i.e. cyclotrons) and later for high-energy physics (Synchrotrons). The discovery of electron synchrotron radiation at short wavelengths led to the development of incoherent light sources. These light sources have thousands of users in the physics, chemistry, and biology fields and electronics industry. All of these devices are beam related technologies. When beam currents become sufficiently high, plasma collective effects dominate over single particle effects in the beam. The beams behave like streaming plasmas.
The free electron laser is an accelerator based radiation source whose technology base is routed in microwave development. Demonstration of lasing in the Infrared in the late seventies has lead to a revival of this technology. The attractive features are the potential for coherent radiation sources that are accessible and tunable from microwave to X-rays and capable of high power operation.
The creation of the first thermal equilibrium plasma in a trap was a scientific break through. The thermal equilibrium states of solids, liquids and gasses had long been established and provided the foundation for the development of many industrial processes. Neutral Plasmas require an energy source to maintain them (steady state) or they recombine to form a gas. To form a thermal equilibrium, single component plasmas (electrons, ions, positrons, antiprotons) are required.
The first Nonneutral Plasma workshop was held in 1988 in collaboration with the National Academy of Sciences in Washington, D.C. The Academy was in the process of considering a Plasma Science Committee. Subsequent workshops focused on the opportunities provided by the creation of thermal equilibrium plasmas in traps. The outstanding results, reported in workshops since the Washington, meeting were at Irvine, CA (1992), Berkeley, CA (1994), Boulder, CO (1997), Princeton, NJ (1999), San Diego, CA (2001), Santa Fe, NM (2003), Aarhus, Denmark, (2006) and New York, New York (2008), have provided the basis for a new subfield in plasmas physics with unique properties such as confinement times of months, and pure ion crystals and antimatter plasma.
In the mid1980’s the Strategic Defense Initiative (Star Wars) adopted the free electron laser as a high power lasers for missile defense. The Office of the Secretary of Defense funded a program of five university medical centers to explore unique medical applications based on the tunability and wavelength accessibility of the Free Electron Lasers.
In the traditional Office of Naval Research, Scientific Officers were encouraged to remain active in Research. The policy was to provide travel funds and allow up to 20% of your time to do personal research. The first paper I published in this program was at the Orcas Island Free Electron Laser Conference. It was on Free Electron Laser Beam Quality (1983). The dispersion relation of the high gain free electron laser has the same form as the plasma two stream instability. That meant the physics of the beam plasma experiments at Texas should be applicable to the beam dynamics of the free electron laser.
The critical issue of scaling to short wavelengths and high powers was the quality of the electron beam. At one wavelength the electron beam could appear to be cold in the FEL interaction. At shorter wavelengths the beam could appear to be warm and the efficiency reduced.
In 1989 I published a Review Paper on Free Electron Lasers. During the 2008 meeting celebrating the 50th Anniversary of the Plasma Physics Division my review paper was listed as one of the 300 most highly cited papers of the past 50 years of the Plasma Division.
In the 90’s the light source community adopted the Free Electron Laser as a forth generation light source and set the goal of developing a 1 Angstrom Free Electron Laser. This FEL would require GeV, multi-kilo amp beams. To get the required beam currents, the beam had to be compressed during the acceleration process. A beam that could appear cold during the initial phase could appear warm after compression and efficiency reduced. This was predictable. A colleague and I developed a theory for this process and applied to an early proposal for an X-ray FEL from the Stanford Linear Accelerator. Our prediction was they would be operating in the warm beam regime after beam compression. We published this prediction in the proceedings of the Rome Free Electron Laser Conference (1997).
The Department of Energy funded the Stanford Linear Accelerator’s Linear Coherent Light Source (LCLS) proposal to build an X-ray FEL. The European community has funded a project to build an X-ray Free Electron Laser using superconducting accelerator technology at the DESY light source in Hamburg, Germany. The Japanese have launched an X-ray FEL experiment at a lab near Kyoto (Spring using room temperature RF technology. With their GeV energies and multi kilo amp beams they all require beam compression. The required wigglers, which are unique, are as long as a football field. These free electron lasers are in the billion dollar class devices.
Before going to Liverpool, England to attend the 31st International Free Electron Laser Conference in 2009, I checked on our latest calculations to see what the predictions for the most recent design parameters of the Stanford Linear Accelerator’s LCLS X-ray FEL were. The calculation clearly indicated they would be operating in the warm beam regime after beam compression.
The first talk at the FEL Conference was the FEL prize talk. The speaker was discussing the LCLS X-ray FEL growth rate curve. He compared the experimental results with the computer simulations and theory. The shape of the growth curve for the LCLS X-ray FEL was the same as the warm beam plasma experiments I had done in 1971 at The University of Texas. This was what I expected. Still, I was in awe. The beam physics that I had learned as a graduate student 40 years earlier was applicable to one of the latest billion-dollar class Free Electron Laser experiments. Physics is a predictable science.
The American Physical Society has acknowledged my research contribution by electing me to be a Fellow of the Society in 1995 with the citation “ In recognition of his seminal contributions to free electron beam quality, stellarator focusing of intense beams and outstanding beam plasma experiments.”
Texas Independence day has become a special event for me. The Austin Grill in Alexandria, Virginia offers a free lunch to anyone born in Texas or who graduated from a school in Texas. I go to lunch with my son Stephen, who was born in Austin, and my grandson Maxwell. We wear our shirts with the lone star on them. On Texas Independence day we are Texans.
Had the physics department not lost my application for a teaching assistantship and Professor Robertson not made me the generous offer to work in his lab for the summer without long term obligations, I am sure my life story would have been unimaginably different.