Gordon Bowden

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ORAL HISTORIES
Gordon Bowden

Photo courtesy of Gordon Bowden

Interviewed by
David Zierler
Interview date
Location
Video conference
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In footnotes or endnotes please cite AIP interviews like this:

Interview of Gordon Bowden by David Zierler on May 5, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
www.aip.org/history-programs/niels-bohr-library/oral-histories/46980

For multiple citations, "AIP" is the preferred abbreviation for the location.

Interview with Gordon Bowden, Staff Engineer for over fifty years at SLAC. Bowden recounts his childhood in California, Oregon and New Jersey as his father moved around for work, and he explains his interest in physics and his undergraduate experience at the University of Virginia. Bowden discusses his subsequent work at a small electronics company doing research on X-ray diffraction and building equipment for the Space Radiation Effects Laboratory. He describes his subsequent work at Boeing on fluidic development, and he explains his motivations to focus on National Laboratories for his next opportunity. Bowden discusses his initial work in the hydrogen bubble chamber operations group at SLAC and he describes Dick Taylor’s leadership of Group A. He surveys his contributions to DELCO and PEP 1 and explains why SLC was so significant and how the klystron group fit within the overall Lab structure. Bowden explains how LCLS was an outgrowth of SLC and how SLAC became involved in the LSST camera collaboration. He discusses the function of the Technology Innovation Directorate, and he describes his recent work on X-ray cancer therapy. At the end of the interview, Bowden reflects on how SLAC has shifted from its early reliance on visualization in its detectors, which has changed completely as a result of computer analysis. 

Transcript

Zierler:

Okay. This is David Zierler, oral historian for the American Institute of Physics. It is May 5, 2021. I am delighted to be here with Gordon Bowden. Gordon, it's great to see you. Thank you for joining me today.

Bowden:

Thank you. I'm surprised to be here, but I'm going to enjoy it.

Zierler:

[laughs] Gordon, to start, please tell me your current or most recent title and institutional affiliation.

Bowden:

Oh boy. When I was hired at SLAC, I was classified as an Engineering Physicist. I'm not actually sure exactly what the classification is now. I think it's just Staff Engineer or something like that. In recent years they've changed all of these classifications.

Zierler:

How long have you been with SLAC?

Bowden:

I came in May of 1968, and I'm still there until the second of June.

Zierler:

Oh wow.

Bowden:

And then after that, I'll be working part time probably.

Zierler:

A true SLAC lifer.

Bowden:

So that's something like, what, 52, 53 years, something like that.

Zierler:

Yeah. That makes you a SLAC lifer.

Bowden:

Pretty much. Yeah, I didn't have anything to do with the construction of the accelerator. I was hired sort of... well, to fill out the initial staff. It was the second year of the full-time operation, so I knew all of the people that built the accelerator. But I'm not a member of that select group. I don't think there are any of those people still around.

Zierler:

Gordon, from your perspective, how well or not has SLAC managed during the pandemic and the mandates of remote work?

Bowden:

Oh, I think it's been done fine. I go into work at this point once a week, just to keep in touch with what's going on in the shops, but I think it's worked out well. I mean, the next question is, how are we going to get back to normal, if you call it normal? I suspect it'll be a new normal.

Zierler:

Yeah.

Bowden:

But they're in the process of trying to figure out how to relax some of the restrictions now.

Zierler:

Are most people vaccinated at this point?

Bowden:

Yeah, I think so. Pretty much everyone that I interact with. I don't know of anybody that I interact with that hasn't already gotten all the vaccinations taken care of.

Zierler:

Yeah, yeah.

Bowden:

California is ahead of the curve, I think, even for the United States.

Zierler:

It is. It is. Well Gordon, let's take it all the way back to the beginning. Let's start first with your parents. Tell me a little bit about them and where they're from.

Bowden:

Say again?

Zierler:

Tell me a little bit about your parents and where they're from.

Bowden:

Oh boy. My father, he, let me see... how to place him. He worked for AT&T in the glory days when it was “maw” Bell. We moved around the country a lot because of that. His education, he was a sociologist, but when I was born in '42, he was teaching naval supply officers at the Harvard Business School in Cambridge. His father was what in the early days was called an educator. He was at one point, the first head of public education for South Dakota, when it was converted from a territory around 1900 and ended up being the first president of what is now the State Teachers College in New Mexico (Silver City NM). He finally completed his career as the head of the anthropology department at USC in LA.

My father sort of followed his footsteps. He ended up at 195 Broadway in Manhattan working for a vice president of AT&T called Greenleaf who had this dream of being able to teach leadership. Not management, but leadership. My father ended up being his liaison with the academic community. He set up a number of summer schools to identify people in AT&T that were going to end up in high management positions and he did some leadership training. Anyway, so that was his job. My mother came from a farm family in Pennsylvania. She was the first of five kids who went to college. She went to Penn State. That's where they married.

Zierler:

Where did you grow up mostly, since you moved around? Where would you say you spent most of your childhood?

Bowden:

Oh, on the coasts. I lived for about six years in LA right after the war when my father was starting out with AT&T. We moved up to Portland and lived there for a year, and then we went back to New Jersey, where my mother and father had spent some time during the war years. I got most of my K-12 education in a little town called Ramsey, New Jersey, which was a suburb of New York City. People rode the “weary” Erie railroad into Manhattan.

Zierler:

When would you say you started to get interested in science? Was it early on?

Bowden:

In SLAC?

Zierler:

No, in science. In science generally.

Bowden:

Oh, in science? Well, I was interested in it sort of right from the very beginning. I'm more of a technologist, but science was critical to that. So, all through, you know, later grade school years and in high school, those were subjects that were of interest to me.

Zierler:

What kind of colleges did you apply to coming out of high school?

Bowden:

Well, my father of course wanted me to go to Harvard. Because that's where he went. I did not get into any of the Ivy League schools, and I ended up at the University of Virginia.

Zierler:

And you majored in physics there?

Bowden:

I majored in physics, that's right.

Zierler:

Why physics?

Bowden:

That's an interesting question. When I was five years old, I thought I was going to be a farmer like my grandfather. I spent the last year of the war (WWII) on the farm when I was between three and four. So, I was going to be a farmer. Then after that, during grade school I thought I would become an architect, because I'm a visual person and there's a certain amount of art interest in my background. Entering high school, I thought I was going to become a naval architect. And by the time I was ready for college, I wanted to be an aeronautical engineer. Since none of these technical backgrounds had anything to do with anybody in the family, my father said, "Well, you don't want to get into those things right away. You need a more general education. So why don't you just pick physics and you'll be able to choose later?"

Zierler:

Now, because of your other interests, was it more on the experimentation side rather than theory that was most interesting for you?

Bowden:

Yeah, yeah, no, I'm very much a practical person. Although, the connection between practice and theory has always been one of the exciting aspects of my work. I've always enjoyed seeing the two come together and fit together.

Zierler:

Yeah. What did you want to do after undergraduate? What was available to you?

Bowden:

Oh, that was a difficult time. My family was in trouble, it took about ten years for it to disassemble, my mother ended up in the hospital. She was manic depressive. There were a lot of problems, and so the transition from college education to a career was a very awkward time. I failed a course in quantum mechanics in my last semester in the fourth year. So, I did not graduate with my class. I graduated a semester later. That was a time when the divorce was finally being completed.

So, it was just a time that I really had trouble deciding how to move forward as an adult. The transition from a child, to a goal-oriented adult was very difficult for me. It was a pivotal year. It was 1964, and I realized that I was not going to graduate school, at least not immediately, and I had to decide what the future would be. For a while I seriously considered going into the military. I was going to join the Navy, and I went to Norfolk and took the test for aviation officer's candidate school. I was going to go to Pensacola. I qualified in every respect, except for being seven pounds underweight. I went home for the summer to gain the weight, I knew I was going to have to have one more semester to finish my degree, and I was able to get a job in Northern New Jersey. It was a little bit like Silicon Valley, or what Silicon Valley used to be. There were lots of small shops, machine shops and high tech for the time.

I ended up getting a job in a little company called Electronics and Alloys which I found not by any advertisement, but by simply sitting down and looking through the telephone book. People had telephone books in those days. Not in the yellow pages, but in the white pages just looking for names of companies that might have something to do with the background that I was interested in. Anyway, there was a physicist at Columbia, Bakish was his name I believe, who had this small company Electronics and Alloys in Engelwood, New Jersey that made X-ray diffraction instruments. So, I just drove around and walked into their door one morning at the beginning of the summer, and they said, “Who sent you? How did you know we needed somebody?” So, I was hired immediately, and I spent that summer working for an Italian instrument maker. It was a very valuable education.

Zierler:

Gordon, did you have any experience in X-ray diffraction from your college days?

Bowden:

No, I knew what it was, but I had not worked in any labs where X-ray diffraction was used. But I ended up earning enough money to go back to school, and when I got back, I was picked up by Dr. Klaus Ziock in the physics department and given a research assistant's job, that's how I finished my degree, and it just kept on going. After that, I went on full time. Over the summer I realized that I was really not cut out for aviation officer's candidate school and so that got canceled. That's where I sort of turned the corner. It was a fork in the road. It’s only in recent years that I have realized how critical this fork was. At the extremes, I would have either washed out, or ended up off the coast of North Vietnam.

Anyway, I worked for a couple of years in the physics department. I realized I would either have to go back to graduate school or get a job in some larger organization, because the physics department did not have a long-term job for me. I built the equipment for some nuclear physics experiments that were done at a place called SREL. Space Radiation Effects Laboratory. It was outside Norfolk, Virginia. It was a part of NASA, and it was sort of an industrial copy of a 600 MeV synchrocyclotron from CERN that was used by NASA to do radiation effects testing on equipment that was going to go into space, go through the Van Allen Belts, and suffer radiation degradation. But there was a certain amount of machine time devoted to physics, and University of Virginia was one of three... let's see, William & Mary, University of Virginia, and probably VPI. I can't remember the details. But these institutions were given a certain amount of machine time in payback for running the lab. I built equipment, spark chambers and scintillator arrays for muon experiments at that place. It's now the Jefferson Lab. It's been completely refurbished and built as a new laboratory.

Zierler:

Gordon, did you feel at a certain distance that you were contributing to the space race of the 1960s?

Bowden:

No, no, well, our contract that supported the research was through NASA. Doctor Ziock’s dream was to do satellite-based astrophysics, basically. What's become a really big deal. But he was sort of ahead of his time, and the contracts were never let... most of that time was all devoted to getting to the moon. But he was on the list of receivers for all publications that came out of NASA, and I still have some of these manuals and books on heat transfer that were associated with the engineering for the Apollo missions.

Zierler:

So, you were a UVA employee at this point?

Bowden:

Yes, that's right.

Zierler:

And then how did the opportunity at Boeing come about?

Bowden:

Well, if you pick up all the Scientific American issues from that era, you'll see that all of the advertisers were the major industrial military complex companies that were looking for manpower. All the ads were for jobs. So, it was not hard. During my college years, I found usually a couple weeks of vacation at the end of the summer between my summer job and going back to school, I drove across the country with friends and we car camped and ultimately spent a lot of time in the Pacific Northwest. I was just fascinated by the opportunities for mountaineering in the Northwest, I immediately thought, well, Boeing was the place to go. It wasn't hard to get a job.

Zierler:

What was the initial project that you joined when you got to Boeing?

Bowden:

I was hired for R&D into a small group of, well there were four or five people who were tasked with the investigation of the fluidic technology that had come out of missile development. This was a pneumatic or gas-driven analog to electronics. It was used in radiation-hard ICBMs (Inter Continental Ballistic Missiles). The company was interested in the general technology and whether it had any application to civilian aircraft. I worked in the Airplane Division. So, we had a number of projects. One was a complete cabin heating and cooling system for aircraft based on fluidic controls. There were no moving parts. It was all just pneumatic circuitry. In another example we built a fluidic switch for the sink water drain in the 727 lavatory. We built a no-moving-part switch that diverted water to a holding tank when the plane was on the tarmac and vented it overboard through a drain mast when the plane was airborne depending on cabin-to-outside pressure differential. We never succeeded with the 727 project engineering people in getting it incorporated. It was a little bit of a radical thing. I think they used some sort of solenoid valve.

Zierler:

Was Boeing an exciting place to be at this point?

Bowden:

It was... I have to say it was almost a necessary career experience as far as I was concerned. I learned a lot, but not very much technically. But I learned about organizations there. I learned that I did not fit into a 50,000-man engineering company. I bought stock in the company, and I got the company stockholder's brochure the year that I went to work there which was 1967. Boeing hired 10% of all the engineering and science graduates in the United States that year. It took only six months for me to realize that I wasn't going to be able to spend my life there. It was just too big a place. I'd gone from the extreme of just a small group of four or five people at Virginia working on a small accelerator-based experiment to a gigantic corporation where most of the people I interacted with were simply phone numbers. There were 3 or 4 company satellite plants around the Seattle area and people were just working together through phone lines. I didn't really fit into that kind of an organization. So, by December of that year, I started sending out resumes to Los Alamos, to Livermore, and to SLAC.

Zierler:

It was national laboratories you were specifically focused on?

Bowden:

Yes.

Bowden:

Yeah, those were the three places that I was attracted to for first choices, and it was obvious from the applications that SLAC was the most desirable; fit my orientation the best. The application clearly asked all the right questions. They wanted to know all of your background. There was a section at the back of the application, four or five pages long, just lists of extraneous hobbies and knowledge base outside of a formal degree that you had to check off. Languages, history, crafts, all kinds of things. That indicated that they really wanted to place you where you would be able to make a contribution.

Zierler:

What was the first group that you joined when you got to SLAC?

Bowden:

Well, I was hired into the hydrogen bubble chamber operations group. My original dream when I made the application… I had just finished reading an issue of Scientific American where there was an article on the streamer chamber, which was a new visual track instrument that was in competition with bubble chambers and cloud chambers and all the other visual particle track detectors. It was a Russian invention, but a large streamer chamber had been built at SLAC. I never got to run it. Well, ultimately, I was offered the operation of that instrument, but it turned out that the hydrogen bubble chamber had sort of supplanted it.

I can remember in 1965 I believe, there was an issue of Time magazine (each issue had a page devoted to science in those days.) that described the fire that burned at the Cambridge Electron Accelerator when the hydrogen bubble chamber there ruptured and ignited. That made me think, boy, that’s one particular technology I really don’t think I should get into. But it was only two years later that I was being offered a job to run those things. Of course, I took it. At SLAC, they had built a one meter bubble chamber with the engineering people at SLAC, but in addition to that they also took what had been the largest bubble chamber built by Alvarez at Berkeley, the 72 inch, and made an arrangement with Berkeley to transfer that bubble chamber from the Bevatron at Berkeley down to SLAC with the agreement that when it was put into operation at SLAC, the experimental groups at Berkeley would get a number of years of operational time and data. That move involved not just taking the bubble chamber but taking the entire technical group that operated it and transplanting them from Berkeley down to SLAC. That was the group that I was hired into. I was a SLAC employee, but I really worked with people that had all come from Berkeley.

Zierler:

What were some of the major research questions surrounding initially the hydrogen bubble chamber?

Bowden:

Well, it was originally dedicated to photoelectric interactions with protons in hydrogen and neutrons in deuterium. There were a number of secondary beams that came from the end station at SLAC that were built just for the bubble chamber. There were gamma beams, pion beams of course and there was also a kaon beam, and ultimately a muon beam. The hydrogen bubble chambers at SLAC were more or less the last of the standard bubble chambers before the final giant ones were built at CERN and Fermilab.

The experiments that I worked on during that period were a transitional kind of experiment that they called hybrid counter-controlled experiments where the bubble chamber was used in conjunction with electronic counters: spark chambers, scintillation counters, and Cherenkov counters that were used to analyze the radiation coming out of the back of the chamber from the collisions inside. The technique used the three thousandths of a second delay between the formation of bubbles and the time that it took for them to grow to visible photographable size. A minicomputer used this 3-millisecond interval to decide whether to photograph the interaction. So, the bubble chamber sat there and pulsed as fast as it could, and the flash tubes that actually exposed the film would be flashed three milliseconds after the beam went through, so that the number of pictures that were taken was greatly, greatly reduced. At this stage in experimental particle physics, most of the things that you could see in a bubble chamber had already been seen, and most of the experiments were experiments involving large statistics on rare interactions. The number of photographs you might have to take if you had no way to filter through and select the ones that contained the events you were interested in was just amazing. Massive numbers. Hundreds of thousands of pictures were taken. But with this hybrid counter-controlled system you could reduce the number that you had to analyze greatly.

Zierler:

When did you first meet Dick Taylor?

Bowden:

Dick Taylor. Of course, Experimental Group A was doing the deep inelastic scattering of electrons off protons in Station A in 1970. After all that was done there were a lot of people who wanted to actually see what the strange particle production was from the electron interactions and so this muon beam was built to run into the bubble chamber with a counter control system behind it to pick out scattering events. It was Dick Taylor’s Exp Grp A that came and did this bubble chamber experiment, BC42 that took about a year. There were four or five runs to gather all the film. That's when I started interacting with Group A, and Dick was the head of that group. We had to make a lot of modifications to the bubble chamber. It was in continual evolution during that time. The number of expansions the bubble chamber could make in a given time was continually being increased.

Eventually, the bubble chamber got up to a point where it would run at a rate of ten expansions per second. We had to make modifications to the chamber that had to be approved. I can remember going up to Dick's office and presenting how we were going to modify the chamber so that it could run faster. At the end of that experiment, I started doing work for their group on the side. Eventually, it got to the point where the number modifications and the development of the bubble chamber had more or less plateaued, and you could tell that the physical process associated with the formation of the bubbles was going to limit the rate at which the chamber could run and there was very little we could do that would increase the pulse rate. I was looking for work outside of the general routine operation of the chamber. I was able to work part time with Dick's group, and eventually I ended up making a transfer out of the bubble chamber group to do that work.

Zierler:

Tell me about the value of the Pestov spark counters for Group A.

Bowden:

Oh well, yeah, Yuri Pestov had invented a type of very, very fast spark-type counter that could measure sub-nanosecond, picosecond type events. It was a timing instrument that depended on a semiconducting glass to quench the avalanche. I can remember the glass. It was black and was made with vanadium. I ended up making some of this glass in the lab using a home coffee grinder to grind up some of the salts and using a small electric oven that a hobbyist would use for glass enameling to melt and pour the glass. Pestov himself came to the United States and worked with people in Group A for about a year during that time. It never really took off. There were other competing detectors. I mean, phototubes. The speed with which phototubes could operate was following closely. It wasn't a big advantage and Pestov’s spark counter became kind of a niche that never really spread widely.

Zierler:

How did you get involved in Charlie Prescott's e122 experiment?

Bowden:

Well, that was in the works at the time I joined group A. I ended up building some of the polarization measuring instruments that were used to check the polarization of the beam for that experiment. I designed a Mott scattering tank and a Wien filter to rotate the polarization of the beam coming from the photocathode from an axial to a transverse polarization. So that whole experience, that was one of the few major experiments that I was able to watch directly as it was ongoing.

Zierler:

What are honeycomb beam pipes?

Bowden:

When the storage ring business got started, of course, the collisions all occur between counter-rotating bunches of particles inside the storage ring vacuum pipe at the interaction point. The detector has to be built around this interaction point, and the fragments from these interactions have to pass out through the walls of the storage ring beam pipe to be measured. Traditionally, it developed that those sections of the storage ring beam pipe were all made from beryllium, which is a difficult material to work with and to get a really transparent wall it has to be quite thin. It's on the verge of implosion. It's very delicate, and you can't have any dents or anything, or it won't stand the air pressure.

So, looking around for a cheaper and safer way to do this, we hit upon the honeycomb construction, which is a standard kind of construction used in aircraft. Hexcel made much of the honeycomb for Boeing and other companies, and they also made their own skis. They had a recreational downhill ski that was made of honeycomb construction. We hired a consultant to come over from Hexcel in the East Bay and give us a day's primer on how to fabricate honeycomb structures, and we built some pipes that were just as transparent to radiation as a beryllium pipe but could be fabricated with standard epoxy and honeycomb structure. The pipes had aluminum foil walls and a honeycomb core. They looked a lot thicker than a beam pipe that was made from beryllium. They were like a centimeter thick. But the foil aluminum is just as thin and transparent to radiation as a beryllium pipe would be.

Zierler:

How well-developed was PEP 1 by the time you got involved?

Bowden:

I ended up building the wire chambers for one of the PEP I storage ring detectors, I think there were six initial experiments. There were six places around the PEP storage ring where positrons and electrons collided. I think there were six bunches that orbited in each direction, so there were six places where you had interactions. Group A was connected with people from Caltech, Barry Barish. Together we made one of the proposals that was accepted. It was called DELCO, which was based on some physics that came out of SPEAR. DELCO stood for Direct Electron... I forget what the acronym was. But I made the wire chambers for that experiment at PEP 1. At that point, that was...1980, people involved in that collaboration were connected to work in Switzerland at SIN, which is now the Paul Scherrer Institute.

Zierler:

How did that opportunity at SIN come available to you? Were you looking for a sabbatical?

Bowden:

Well, there was a physicist by the name of Rainer Pithan who had been a school friend, I think, of one of the people that was leading this experiment at SIN. Rainer thought it would be a good idea if I got some experience overseas in another culture. He was the bridge that connected me with the people at SIN. They needed somebody to help with the design and construction of the wire chambers for the SINDRUM experiment that they were doing. It was a search for forbidden decay of a muon into 2 positrons and an electron. Two pluses and one negative as I remember, and it was one of these experiments where you were either looking for a violation of the forbidden decay or trying to push the certainty that it was forbidden down to a lower order of magnitude. I ended up spending a year in Switzerland working on that. A little more than a year, actually.

Zierler:

Now this was a leave of absence? You were always planning on returning to SLAC?

Bowden:

Yes. That's right. They didn't promise they'd take me back, but they did.

Zierler:

[laughs] I wonder what your feelings were when Dick Taylor won the Nobel Prize in 1990?

Bowden:

It was not a tremendous surprise. It took a little while. I mean, the work had been finished for some years, and it was work from the late 60s and the early 70s that got the prize there. It wasn't as immediate as Burt Richter's prize for the J/psi. But yeah, no it was in the works, it was going to happen.

Zierler:

Did you have opportunity at all to be included in the celebration?

Bowden:

No, but my neighbor, Bill Atwood, went. I had very little to do with it. When I came into the group, most of that work was done. My connection was through the bubble chamber muon experiments that Dick’s group did after the main deep inelastic scattering data was already taken in end station A. It was sort of anti-climactic. Nothing surprising came out of the muon experiment. People joked, you know, after a year's work, if you let the bubble chamber run with 100 muon tracks every pulse and added up all the expansions the chamber had done during that year even at 10 pulses per second you probably got the equivalent of just one direct electron beam pulse in end station A. In most bubble chamber experiments there's usually just, you know, four or five tracks in a pulse, because visually, it's hard to untangle a large number of particle tracks. For BC42 they upped the beam intensity to about 100 tracks per pulse. It was a large sample, but compared to the data associated with the end station spectrometer experiment, it was a very small fraction. What they were looking for in the bubble chamber was strange particle production, which is the sort of thing you can’t see in a spectrometer. You wouldn't know anything about it. But it would be easy to pick out from a bubble chamber experiment.

Zierler:

What was your first work when you returned to SLAC from Zürich?

Bowden:

Well, they wanted me to stay, actually. I could have stayed another year or more, who knows how it would have all turned out. But the SLC, SLAC's linear collider, was an idea of Burt Richter's and he was of course in direct competition with the LEP experiment at CERN to produce Z-zeros. Using the SLAC linac to collide electrons and positrons head-on at 50 GEV was an entirely new approach; it was the first example of a new kind of collider that would be based on linear accelerators. It was in the news. You'd read about it in Nature and Science that this race was on. I didn't want to miss it. So, I managed to get back and get a job working on the collider. The job I was given, was associated with alignment of magnets. The beams were going to be a few microns in diameter and whether micron-sized beams could ever be brought into collision was not certain at that time. Magnet alignment and making mechanisms that would successfully remotely align the magnets in a way that could be adjusted slowly to accommodate temperature changes and ground motion; that had not been done before, and that was the part that I ended up working on. Here's a lamination from the magnet for the SLC. There was about I’d say probably oh half a mile of magnets like this that were built into a tunnel that unlike most accelerator beam tunnels, was like a roller coaster. It went up and down and right and left. It wasn't in a plane.

The beam had to go through this little hole right in the middle here, where the three poles of the magnet come together. The idea was that we were going to use copper refrigerator tubing to pipe the beam into the collision point. That would be no easy task, and in fact it actually took the better part of five or six years to get the SLC to actually operate as originally designed. There were betatron oscillations in the beam coupled between vertical and horizontal that had to be decoupled. Developing the beam guiding system that set all of the magnets in the proper orientation and with the proper excitation to do that was a major, major problem. The magnets have trim coils, but because they were combined function, they had both a dipole field to bend the beam in the tunnel, but they also had a quadrupole field to focus it and these two fields were locked together in the design of the magnet, so the only way to completely control the beam was not only to adjust the current in the magnet, but to actually move the magnets around by tiny fractions of a millimeter. The magnet movers, as they were called for doing that, were what I worked on. The part that I actually spent most of my time on was the final focus part of the system where the beams actually collided. The last set of magnets before the collision point, they actually reached into the detector where the collisions were occurring, and they had to move by microns in order to get the beams to collide. There were three of us that worked on that part of it. I don't have any pictures. I can send you some pictures of that. But there were a number of innovations there. I ended up building a set of beam position monitors that could measure both the incident beam and the exiting beam from the collision point in the same instrument. These were monitors that had cabling coming out of both ends, so it saw the beam coming in and saw the beam going out, in the same detector. That was critical to measuring whether the two trajectories were coincident or not. It was called a directional coupler from the electrical standpoint. You got a signal coming out one end from the incoming beam, and then a signal coming out from the other end from the outgoing beam, and when one beam was steered across the other you could use these signals in the form of a spectrometer and measure the collision of one beam off the other. It was the standard deflection curve that Philip Bambade from Orsay developed.

It showed that as you swept one beam across the other, you'd see the deflection peak in one direction, and then suddenly drop through zero and peak in the other direction as one beam crossed the other. By identifying the location of this crossing point between a positive and negative deflection, you could get the beams to collide head-on, and that was how they did it. It's never been done since. It would be the way a normal, large-scale linear collider with two big linacs running against each other would be adjusted.

Zierler:

Now did you officially join the klystron group at some point? Or you just did work for them?

Bowden:

No, yeah, no I worked in the klystron group for about two years after SLC went into operation. I got to work on klystron-- they weren't klystrons actually. The type of RF amplifier that was of interest at that time was called a CFA. A cross-field amplifier is what they called them. It had a very very low input impedance for which there was no standard SLAC modulator. It required large amounts of current at high voltages compared to the standard klystron. Nobody knew anything about the kind of modulator that was going to be needed for the CFA. There was a group at Livermore that had used, had built induction linacs that needed large currents and used a magnetic pulse compression modulator. I spent two years more or less reading their papers and copying their design and building a comparable modulator at SLAC that would power this CFA. It was a remarkable machine. It had a pulse of about 80 nanoseconds and could deliver about five gigawatts of power. In other words, more than what a large nuclear power plant would deliver. But only for, oh, 80 nanoseconds. 80 x 10-9 seconds. It's a very, very short time. It delivered 400 joules of energy each pulse-- it could melt a few coins. It was an interesting device and I enjoyed very much seeing the mechanical and the electromagnetic design of the thing fit the calculations very well. It worked as predicted. But the CFA as a project, the cross-field amplifier, was canceled, and so the purpose for working on this thing just evaporated. We kept the modulator around for many, many years thinking that at some point it would be useful as a kicker magnet modulator in the accelerator, but it never came to be used for anything else after that.

Zierler:

Gordon, how did you get involved in the Bfactory, in the BaBar detector project?

Bowden:

Ah, how did I get? Well actually it was a connection between me and people in Group A. One of the physicist in Group A, Hobey de Staebler, he ended up becoming part of the Bfactory collaboration, and this whole complex of the detector and the accelerator machine structure at the interaction point had been one of his focal points. And we-- after the part that I had done for the SLC at the collision point there, it was natural that he would come after me when he started to work on this, and so he and a few other people were responsible for building the interaction region for the Bfactory. There was a lot of concern about how to actually incorporate the magnets for the Bfactory into the detector. Those magnets would have to be permanent magnets because they operated inside the detector field. There was no iron, you couldn't use any ferromagnetic material, and how to mount them inside the detector and align them was a serious problem. I ended up with-- Hobey was very concerned about the background level inside the detector which could blind the detector from seeing the interactions if there was too much radiation, so the masking, the tungsten masking that collimated the beams and shielded the detector was a major concern of his, and how to put all the magnets, the masks, and everything else inside the detector and get them aligned was a major problem just to assemble. How you bolted stuff together inside of a 4 pi detector that completely enclosed the interaction region.

I convinced Hobey that we should build all this stuff in a-- at the time, I called it a cartridge. It ended up being a long tube that we assembled in the lab with the magnets and the beam pipe and the actual silicon vertex detector of the main detector, all housed in a separate tube that was then inserted through the center of the physics detector. It took Hobey’s authority to convince the collaboration that this was going to work, because there were many people that did not want to see any physical walls between where the interactions were occurring and the rest of the detector. This system required that there be an actual thin aluminum walled cylinder that passed right through the detector. The radiation that came out from inside the barrel as we called it, or the cartridge to the rest of the detector had to go through this scattering as it went out. And god, there was an argument that raged in the collaboration for about a year as to how serious this material would be to seeing the full collision event. It turned out it was not going to cause any serious problem, and that debate was resolved. We had no problems with alignment or anything else during that time.

Zierler:

What was the Next Linear collider? What was next about it?

Bowden:

Well, the SLC was a prototype that was invented at SLAC making use of the two-mile linac for both the electrons and the positrons. Normally, you would have two separate accelerators firing at each other. One with a positron beam and one with an electron beam. But they found a way to make one accelerator function for both beams. The way they did it was to store positrons. The two-mile long linac had a pulse sequence where it fired one pulse into a target to make positrons up at the injector. These were captured in a damping ring, a small underground storage ring that allowed the beam to circulate and damp and the admittance of the beam to become very small. Then it was injected into the linac and accelerated through the two miles up to 50 TEV followed by, I don't know, 10 meters or so by a second pulse of electrons. These two pulses were sent to the end of the linac where there were some DC magnets that directed the electrons into one arc and the positrons into another arc, and these two arcs swept around in the underground tunnels that I talked about that went around like a roller coaster. These arcs brought the two beams into collision at the interaction point. That was the way it operated. But the next linear collider would never have this complicated set of arcs to transmit two beams from the same accelerator into collision. So that's where the name "next" linear collider... people have been working on various versions of the Next Linear Collider for, gosh, it's now 20 or 30 years. It's never gotten to the point of an actual accelerator start. I think people at (CERN) are still looking at it.

Zierler:

What was your involvement with the LCLS?

Bowden:

Ah, that came out of work on the movers that were used with SLC. The problem with LCLS was that the magnet structure, the undulator structure that converted electron energy into X-ray radiation, was extremely sensitive to alignment. The beam pipe in the first LCLS was a six-millimeter soda straw sized tube and the aperture in the undulator was extremely small. The alignment was so sensitive that it was subject to all sorts of seismic and thermal distortion. It had to be adjustable. The LCLS runs with what's called beam-based alignment. The initial alignment that surveyors do when the undulators are installed in the tunnel is just good enough to get the beam to go through the undulators and come out the other end. But it's not good enough to generate an efficient undulator X-ray beam.

So, after the beam is running, there is a series of monitors, the beam position monitors, along the undulator string that are used to track the beam's trajectory. From the observation of how the beam is transmitted, the magnet system is slowly dialed into proper alignment. There are some secondary alignment systems in LCLS. There's a wire monitor and a water level system that also help. But this beam-based alignment is the most critical part of it, and in order to do that, you had to have some way of shifting the position of various undulators in the string to tolerances of like a few microns or so. The undulators, they're a couple of meters long. They're heavy; on their raft they weigh about a ton. So, you need some sort of a mechanism that can make small movements on the order of at most a millimeter, to tolerances of like a micron. That technology was in hand after the initial SLC. When LCLS started, it was obvious that that's what they would use. We at SLAC did not build the undulator string for the first LCLS, but people at Argonne did, and they took the magnet mover from the SLC and implemented it in the LCLS, and that system has been trouble-free to this day.

Zierler:

Gordon, I'm curious with your work on the LSST camera, if you recognized more broadly at the time that SLAC was moving into astrophysics?

Bowden:

Oh, everybody did. I mean SLAC realized that it was at the end of its life as far as a site for a large linear collider like the NLC would have been, very early in the game. The synchrotron light source was obviously the direction that SLAC was going to go so SPEAR went through a number of upgrades and then of course the LCLS itself. But all this time, those people who were not part of the machine development, not part of the accelerator physics, and not part of the various X-ray sciences, that means the particle physicists themselves, they realized that their direction was increasingly going to go into astrophysics. The reason SLAC has anything to do with LSST is that the data rates, especially from the LSST camera are so large that they're very similar to the data rates that are coming out of large collider physics detectors. The physics groups at SLAC that were associated with some of the CERN experiments at LHC and other labs, realized that handling the data from the images at LSST would produce was something that they had expertise in, but of course, the astrophysics associated with it was the main interest. So that was the reason that SLAC had anything to do with it.

Zierler:

Did working on the LSST and the fact that this represented a shift for SLAC, did that change your day-to-day very much?

Bowden:

Not day-to-day, but it was a whole other set of technologies that I had interest in. Because of my background in cryogenics, it was naturally assumed that I would have some role with the refrigeration of the camera. The camera for LSST is, you know, a cryogenic camera. It runs at about 140, 150 degrees Kelvin. Refrigeration is a difficult problem for a ground-based telescope that's moving around in the sky with the camera, you know, 25 meters up in the top of the telescope. So that cryogenic part of it was an aspect that I contributed to. I also did a little work on the shutter. For LSST, you'd think there'd be some sort of advanced electronic-type shutter, but no, it has a mechanical shutter because of the amount of light that would be absorbed by any kind of polarization type shutter. You're going to lose half the light, and they didn't want to do that. They had to have a mechanical shutter to close off the camera while the data was read out. The dynamics for that shutter is important. It has a large aperture. The field of view is about 60 centimeters in diameter, so the shutter is a very large object, and it has to move very precisely to guarantee that all parts of the field are equally illuminated.

I worked on the dynamics of the shutter's motion and we developed a set of instruments to monitor the motion that didn't add any light. It was a magnetic system which used a bunch of, or a comb of, permanent magnets that was carried by the blades past a row of magnetic proximity sensors along the side of the track. That's another part that I worked on. But the refrigeration system was the most significant. Originally, solid state cameras for telescopes were refrigerated from a dewar of liquid nitrogen, but these were much smaller cameras. The dewar would be filled early in the evening before the sun went down and the camera would sit there overnight running on the boil-off from the nitrogen dewar. But when they got to LSST, the camera was so large that we computed it would take a couple of oil drums of the liquid nitrogen to refrigerate the thing, and it was just impractical.

The other aspect of it was how to get any cold cryogenic fluids up from some sort of ground-based source, up through the moving mechanism of the telescope to the camera. We avoided all of that by building a system that involved room-temperature hoses that carried the refrigerant up at room temperature and built the actual cryogenic part of the refrigerator into the back of the camera. So, there's no cold cryogenic equipment anywhere except in the camera itself. It's driven from the ground through hoses. The actual refrigeration technology itself is a field of cryogenics called mixed refrigerants. The refrigeration system that it runs on today is built with large versions of your home refrigerator compressor. The compressors that are used for LSST are made by Danfoss in Denmark, and they're basically the same as what you'll find in your home refrigerator. The only major difference is that instead of the normal chlorofluorocarbon refrigerants used in home refrigerators at minus 20 degrees centigrade, which they're phasing out now, the camera refrigerator uses a special mixture of cryogenic gases like argon plus a number of other refrigerants that are similar to the ones that are in your home refrigerator. This mixture will allow that kind of refrigerator to operate at not just minus 20, but minus 140 degrees centigrade. You can get close to LN temperatures. You can actually produce liquid nitrogen with this kind of a system. It took a long time to prove that you could operate a refrigerator up at 25 meters above the compressor system, transmit all of the lubricating oil associated with these reciprocating refrigerator compressors through the cryogenic system and have it all come back and run under equilibrium 24 hours a day for years. It's yet to be proven on the telescope, but we've had about four or five years of operational experience during the assembly of the camera, so I think it's probably going to work fine.

Zierler:

Gordon, what is the Technology Innovation Directorate, and how did you contribute to that?

Bowden:

Oh boy. (laughs) The TID is a place where they've gathered together a bunch of people at the lab who've had quite various jobs that they didn't really know what to do with but didn’t want to lose. They weren't really assigned to one of the major projects of the lab. Some years ago, the effort was made to diversify, well they called it "work for others." The idea was that the technology base that had accumulated at SLAC would be useful to outside projects that weren't part of SLAC's main mission. It was encouraged that people would find applications for technology that was developed at SLAC and partner with outside people to develop it. People with these very oddball backgrounds like myself, we were all just immediately transferred into TID for that reason. I mean, the LSST was of course a main, major SLAC objective, and I worked on LSST, but I came from the TID group after I had worked in the accelerator division for a number of years while we were trying to promote the Next Linear Collider which was a lot of development work and a lot of collaboration between SLAC and KEK in Japan on the development of x-band klystrons and accelerator structures for the NLC. That work has not yet led to a new accelerator facility. I came out of that group, and was transferred into TID when it was formed, but quickly ended up in LSST, where I spent close to 10 years, actually.

Zierler:

Gordon, just to bring our conversation right up to the present, what have you been working on in recent years at SLAC?

Bowden:

Well, recently I've been building accelerators mainly for X-ray cancer therapy and certain imaging applications that are associated with national security like monitoring shipping crates for nuclear proliferation problems. So, it's mainly RF accelerator fabrication for X-ray sources. There's a significant collaboration with the medical school down on campus for electron linac-driven X-ray sources for cancer therapy.

Zierler:

Gordon, now that we've worked right up to the present, I'd like to ask a few retrospective questions about your career, and then we'll end looking to the future. So, the first is, in what ways have technological advances really punctuated the work that you've done at SLAC over the years? Either in materials, either in computers, instrumentation? What stands out in your memory as some of the really big things that move the needle forward?

Bowden:

Well, the main transition was of course the transition from the visual track detectors that we were still operating when I came to the lab. There's a book that you probably are aware of called Image and Logic by Peter Galison. It describes this early period in particle physics when many of the detectors were visual. I mean, the first ones of course were cloud chambers and then there were emulsion stacks, and then there were bubble chambers and then there were visual spark chambers. That's the technology that I came into the lab working on. That's all history now. The electronic revolution has changed all of that. The main effect, I can draw the line... when was that? The year that I went to Switzerland, the lab still had a main frame computer and people had terminals in their office or out in the hall, but when I came back a year and a half later, everybody had a PC. That was a major revolution. The particle detectors of the era when I left was in the midst of complete conversion to total electronic applications. I mean, the wire chambers were my only crossover into that. The wire chambers were electronic equivalent to a visual track chamber.

Zierler:

A really broad question. Given that you arrived at SLAC very close to its beginning and you're still there today, in what ways has SLAC remained the same, and in what ways has it changed?

Bowden:

Well, the reason I'm still there is that I found the right place to work in the sense of the size of the laboratory. It wasn't this small physics department, and it wasn't a giant corporation like Boeing. It was just right. It was in the middle. The number of people that are direct SLAC employees today is not much different than what it was when I came here. It's something like 1500. And so, there was enough scope. There was a critical mass here. I had many, many jobs, and I didn't have to leave the lab to go from one to the next. The laboratory's management was enlightened enough to allow it to happen. That's not true everywhere. I mean, I very recently I read a study of major corporations where there was a woman hired to find out why they couldn't keep people, and what they found was that there were many people that left the corporation that were very promising people, but they left because they could not make horizontal transfers in the organization. That's not been a serious problem in my case. There were a couple of times when the letter from the director had to be written to release me from one job into another, but it was possible. And that's why I'm still here. [laughs]

Zierler:

Gordon, last question looking to the future. Given your powers of extrapolation since you've been there for so long, what's your confidence that SLAC will continue to be a source of innovation, and even adventure in science?

Bowden:

Well, it's physically there. And it's not going away. The organization is vastly different than it was when I came to the lab. It's gotten much more complex and just the management overhead associated with the laboratory is much larger today than it was when I came. When I came, the people at the top of the organization were actually participating in the science. They were in the control rooms. They were doing the experiments themselves. Panofsky, he would walk into the bubble chamber control room at 10 o'clock on Friday night and ask, you know, what was happening. Dick Taylor was often in the main control room for the accelerator demanding more beam when they were running the deep inelastic experiment. I could hear his voice, on the PA when I was down in the control room at the bubble chamber. He was right in the middle of it. That's no longer possible. The lab is in the process of diversifying into many, many directions. Initially, Pief wanted to keep the lab as a sort of single-function particle physics laboratory. There were many early efforts to diversify the lab. Some wanted to follow Berkeley into many new fields. Pief resisted that initially. It has happened now. It just happened because of necessity. But today, the laboratory is, heading off in many, many different directions. The photon science, I mean, LCLS and all that's come since then has brought a lot of new people into the laboratory that come from other fields. Biology and solid state physics and so forth. And these fields have nothing to do with the original purpose for the laboratory. It's all a new world now.

Zierler:

Well Gordon, it's been great getting your perspective on these things, and I'm so glad we were able to do this. And I'm so glad that the SLAC archivists encouraged us on including your perspective. So, I'm really happy we were able to do this.

Bowden:

All right.