William Herrmannsfeldt

Notice: We are in the process of migrating Oral History Interview metadata to this new version of our website.

During this migration, the following fields associated with interviews may be incomplete: Institutions, Additional Persons, and Subjects. Our Browse Subjects feature is also affected by this migration.

We encourage researchers to utilize the full-text search on this page to navigate our oral histories or to use our catalog to locate oral history interviews by keyword.

Please contact [email protected] with any feedback.

ORAL HISTORIES
Image of William Herrmannsfeldt

Credit: SLAC

Interviewed by
David Zierler
Interview date
Location
Video conference
Disclaimer text

This transcript may not be quoted, reproduced or redistributed in whole or in part by any means except with the written permission of the American Institute of Physics.

This transcript is based on a tape-recorded interview deposited at the Center for History of Physics of the American Institute of Physics. The AIP's interviews have generally been transcribed from tape, edited by the interviewer for clarity, and then further edited by the interviewee. If this interview is important to you, you should consult earlier versions of the transcript or listen to the original tape. For many interviews, the AIP retains substantial files with further information about the interviewee and the interview itself. Please contact us for information about accessing these materials.

Please bear in mind that: 1) This material is a transcript of the spoken word rather than a literary product; 2) An interview must be read with the awareness that different people's memories about an event will often differ, and that memories can change with time for many reasons including subsequent experiences, interactions with others, and one's feelings about an event. Disclaimer: This transcript was scanned from a typescript, introducing occasional spelling errors. The original typescript is available.

In footnotes or endnotes please cite AIP interviews like this:

Interview of William Herrmannsfeldt by David Zierler on May 14, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
www.aip.org/history-programs/niels-bohr-library/oral-histories/47179

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

Interview with William Herrmannsfeldt, Staff Physicist at SLAC. Herrmannsfeldt recounts his German heritage, his upbringing in Ohio, and his early interests in physics which he pursued as an undergraduate at Miami University. He discusses his graduate work on beta decay and nuclear physics at the University of Illinois, under the direction of James Allen, and he describes his postdoctoral appointment at Los Alamos where he made detectors for bomb tests. Herrmannsfeldt explains the connection between his work at Los Alamos on electron optics and his initial research at SLAC, and he describes his work on linear accelerators. He describes his tenure as Secretary of the Advanced Development Group and his role at the AEC to concentrate on accelerator physics for Fermilab. Herrmannsfeldt explains the decision to move ahead with the PEP project and his LINAC work at Berkeley. Herrmannsfeldt explains the relevance of this research to nuclear fusion, and he describes some of the technical challenges in building the superconducting RF system. At the end of the interview, Herrmannsfeldt conveys the sense of fun he felt in learning new technological systems, the inherent challenges of beam dynamics, and he reflects on how SLAC has changed since its inception. 

Transcript

Zierler:

This is David Zierler, oral historian for the American Institute of Physics. It is May 14th, 2021. I'm very happy to be here with Dr. William B. Herrmannsfeldt. Bill, it's great to see you. Thank you for joining me today.

Herrmannsfeldt:

It is my pleasure, David. This sounds like it might be a lot of fun.

Zierler:

I hope so. [laugh]

Herrmannsfeldt:

I have not thought about these things in years…

Zierler:

Bill, to start, will you please tell me your most current title and institutional affiliation?

Herrmannsfeldt:

I retired in 2005 from Stanford, so nothing since then.

Zierler:

And what was your title at SLAC?

Herrmannsfeldt:

Just staff member.

Zierler:

Staff member at SLAC. Bill, in what ways have you remained connected with SLAC over the years since retirement?

Herrmannsfeldt:

Not much. Less as time went on with the virus and so forth. That pretty much stopped everything. We did have a group that would come to a Chinese restaurant every month, but we cannot do that now.

Zierler:

Bill, let's take it back to the beginning. Tell me a little bit about your parents and where they were from.

Herrmannsfeldt:

Mother and father came from Hamburg, Germany, my father first and then my mother with the intention of getting married when she arrived, which they did in Chicago. I was born in Chicago. But they left then and went first to West Virginia and then to Cleveland, Ohio, so that was where I started school. Then to Barberton, Ohio, for the rest of my growing up years. My mother stayed in Barberton and lived to 102. She was a widow for about 38 years.

Zierler:

What year did your parents arrive in this country?

Herrmannsfeldt:

Around 1928, 1929. I was born in 1931.

Zierler:

Did your parents suffer during the Great Depression?

Herrmannsfeldt:

Yes. My father was a trained German engineer, and we are not Jewish, so the Germans were interested in him coming back home to Germany. I recall one discussion with my father in which he said that a major reason not to go back to Germany was that I was born in America, and he wanted me to stay American. My parents received their American citizenship as soon as they possibly could. That would have been a few years later, probably something like 1937.

Zierler:

What are some memories that you have of World War II as a boy?

Herrmannsfeldt:

I was a kid who had a long German name. That caused a little bit of trouble but nothing serious. My father was able to keep on working and what he was doing was important for the war effort. At one point, he attempted to move his career to Goodyear Aircraft Company. But they seemed to think that going to work for a defense company, (they were making airplanes in Akron at that time) might be a problem. They thought that he would be a little bit too German to do that. He stayed in Barberton for the rest of his career. He worked for the Babcock & Wilcox Company which made boilers primarily for power plants.

Zierler:

Bill, when did you start to get interested in science?

Herrmannsfeldt:

In high school certainly. I got a chemistry set and decided pretty early that I didn't really like chemistry. My chemistry experiments did not always work the way they were supposed to work. I liked physics better because if you put the parts together right with physics, things would run right. Chemistry sometimes you do not know why it didn't work.

Zierler:

Was it physics specifically that you wanted to focus on for undergraduate at Miami University?

Herrmannsfeldt:

Yes, that was specifically for physics. I went first to what is now Cleveland State University. It was called Fenn College in those days. And there I had a professor advisor who recognized things in my makeup that I did not necessarily understand or realize. He told me I should go to Miami. He had graduated from Miami himself and he wanted me to be with Ray Lee Edwards who was in those days a well-known undergraduate professor. Edwards kept track of how many PhDs he had fostered as their undergraduate advisor. So that opportunity was offered to me. Miami in those days like other state schools, was not too expensive. The Miami physics faculty picked one graduate school where they would recommend each undergrad student to go. They recommended that I apply to the University of Illinois.

Zierler:

What kind of physics did you want to do when you got to the University of Illinois? Did you want to focus more on theory or experiment?

Herrmannsfeldt:

No. I definitely am an experimentalist. Theory is totally different.

Zierler:

Who was your advisor at Illinois?

Herrmannsfeldt:

It was James S. Allen, who was the head of the cyclotron laboratory. I got involved with a project there that was defining the nature of beta decay, which was one of the leading topics of nuclear physics in those days.

Zierler:

What were some of the big theories that might've been relevant for your experimental work at Illinois?

Herrmannsfeldt:

There were two variations of the theory. One had been tested and published by a group at Columbia. I don’t recall their names. You are pushing me now, you realize that? [laugh] You have me going back almost 70 years. But anyway, we then repeated the experiment with the cyclotron in Illinois with a different isotopic beta decay process. We could not get the same answer that the Columbia group had gotten. And since we knew what their answer was, we could work on our approach to try to get closer to their answer. The harder we worked at confirming their answer the further away we got. When we published, they tried to check some of their approach but by that time their equipment didn't exist anymore. They did what they could do to study it. There were some issues with a gas that carried the radioactive isotope. This resulted in my going to the traditional APS meeting where you give your thesis talk at a New York APS meeting. And then the chief of the Columbia group followed my talk with essentially an apology talk that they had gotten the wrong answer. He had some explanation about gas flow that put a bias into their results. The determination of the nature of beta decay had been the project that Allen had pursued for quite a few years. I was at least a second or third PhD from that project. My thesis then was the one that was accepted as giving the right answer.

Zierler:

What was Allen like as a person? What was it like to work with him?

Herrmannsfeldt:

[laugh] He was a nice guy. Nothing hard driving about him. He had been involved in the Manhattan Project at Los Alamos. He then went to Chicago and finally Illinois. I then, with my family and after graduating with a PhD from Illinois, went to Los Alamos for four years. We stayed in touch with Allen who had some connections with the committees managing the lab.

Zierler:

Bill, in graduate school, what would you say some of the key findings of your thesis research were?

Herrmannsfeldt:

Saying that my project defined beta decay was such a finding. If you look at what other people with PhD theses did, it is hard to make a definitive statement like that.

Zierler:

What was the connection that got you out to Los Alamos? Did Allen know someone there? Did you apply for an open position?

Herrmannsfeldt:

No, not really. I visited a couple of other places. By that time, we had a baby. We were married during my second year of my graduate work. She had the baby shortly after I received my PhD and so we didn't consider going out of the country. Allen had recommended that I go to Scandinavia someplace. I think I had a little bit of a sense of okay, I had stayed out of the military because I was in school, but I felt that I owed the country a debt. And by working for a defense laboratory I had a sense that I was paying that debt. And we stayed there four years before then coming to Stanford.

Zierler:

What was your work at Los Alamos? What group were you a part of?

Herrmannsfeldt:

It was a weapons group. The connection with Allen was we were trying to make detectors for bomb tests that the Russians might be doing in outer space. The connection with the work at Illinois was that the detectors for bomb tests were similar in design to the detectors I made for the beta decay experiments. The fact was that the Russians had more sense than to try to design weapons using tests in outer space. The result was that we were involved in developing detector systems for radiation in outer space.

Zierler:

Did you enjoy your time at Los Alamos?

Herrmannsfeldt:

Yes, but four years is about enough to experience most of what there is to see and do. After four years you have seen it and now you want to do something else. Either that or you become part of the New Mexico community.

Zierler:

Bill, in what ways was your graduate research relevant for your work at Los Alamos, or was it really different physics?

Herrmannsfeldt:

It was different physics, but the technology was related because it could use the same kind of detectors and a lot of the instrumentation was relatively the same.

Zierler:

How did the opportunity at SLAC come up in 1962, right at the beginning? What was the connecting point there?

Herrmannsfeldt:

The connection was the electron optics design for the SLAC accelerator injector. I had that background when I was working at Los Alamos.

Zierler:

When you got there, what was already built and what was planning to be built in 1962?

Herrmannsfeldt:

I joined the group that was building the linear accelerator, but I joined the small group, two or three of us, for the injector. That is the first three feet of the accelerator, first three of the 10,000 feet of the two-mile machine.

Zierler:

Who were you working with during those early days? Who were your key partners?

Herrmannsfeldt:

I was hired by Roger Miller, and I mentioned him to you before. You said you weren't able to contact him.

Zierler:

Right.

Herrmannsfeldt:

We were just a small group as part of the accelerator department. It was essentially Roger and me and we had a couple of technicians.

Zierler:

What were some of the early technical challenges you encountered?

Herrmannsfeldt:

Well, the project was to make the injector, and to do this first we needed an electron gun. To design the gun we used a computer program based on some of the things that I had done at Los Alamos using some of the same software but advancing it with some programs that were available from the electronics department at Stanford—not SLAC but Stanford. And I started writing the computer program which is, to this day, still being used for computer software. Using software this old is pretty remarkable. But I got a request just yesterday from somebody in England who might like to use the program. We then developed what was really a business for selling this program.

The first issue that came up when I started selling it from SLAC was, could we do that legally? Could we sell a program that I had written for SLAC? And during that whole period, our son, the one who was born in Illinois, had grown up, had been around when I was doing that work, followed it a little bit. Then he himself went back to Illinois for his PhD work, and took the program that I had written in an off-standard language and converted it to what was in the more advanced language called C. He became the programmer for the program. He then gave me the program back again and I went to the lawyer at SLAC and asked, “can we sell this? And how can we reward the university and SLAC for it if we do sell it? Do we give them some money or something?” And the lawyer was not impressed with how promising this was going to be, but he said, "You can do anything you want with it because your son translated it to C from the Fortran that it was in. And he did that without government computers while he was in Illinois. Since he did that without government computers, and it is in a different language, it is totally yours to do anything you want with it." He did not think we were going to get any money out of it anyway, while, in fact, we sold many copies over the years, and there are still a few hundred thousand dollars in the accounts that will become Glen's. Over the years it was quite successful. The program is called EGUN.

Zierler:

Did you have interactions with Panofsky in the early years?

Herrmannsfeldt:

Yes.

Zierler:

What was his interest in what you were doing?

Herrmannsfeldt:

He encouraged it. He was a busy man in those days. Eventually as part of the construction project I built the laser alignment system for the LINAC. And one close encounter with Panofsky was taking him out to the far end of the machine two miles away from the lab and letting him see the alignment system operating. He thought that was pretty neat and it was fun being able to do that with him.

Zierler:

Bill, on the two-mile accelerator, can you tell me about the decision to make the accelerator structure a constant gradient as opposed to constant impedance?

Herrmannsfeldt:

Yes. This was not my department so I can pretty much only go by what was going on around me and history. Most linear accelerators that had been built with RF structures, of which there were several in the world, and then three at Stanford, had been built with all the cavities the same. When all the cavities are the same, then the cavity impedance is the same all the way through the whole machine. The way you choose the cavity design is you want to put in whatever RF power you have and get the highest possible voltage. If you add more power later, eventually you start getting discharge in the cavity. You look for a design that does not discharge if all the cavities are the same. These cavities are a few centimeters each in a structure that is a few meters long. In the case of the two-mile linear accelerator, then each section was three meters long. RF power enters in the input coupler and is reduced when it gets to the load end, the downstream end. There is less power there. It has been used up heating the structure. That means that if you keep on adding voltage, more power, eventually you will have some breakdown at the input end. To try to avoid that happening, you change the design of the cavities so that the impedance is not the same at each one. Each cavity is a little different than the one next to it. The result is that all the cavities have the same gradient but not the same impedance. That lets you run at a higher power level, all of which was pretty much irrelevant in the early days because we had klystrons that were not so powerful that we would break anything down anyway. But eventually, by looking ahead and anticipating that development, the accelerator sections were designed with constant gradients.

The best information about this would come from Gregory Loew. Of the people whom I would think you might still talk with; he would be the most obvious one about this question that you posed to me. So anyway, the point of the constant gradient is that you can run the whole thing at a higher power level since you are dealing more with the average rather than the peak. But it makes it harder to build. Every cavity is a little bit different, and they all must be tuned so they all resonate. In each three-meter-long section, every cavity must be touched up a little bit. As I told you in that little note, a more important reason is to avoid beam breakup. The instability that causes pulses of electrons to encounter transverse fields and causes the beam pulses to be shortened was known as beam breakup.

Zierler:

What did you work on next after the two-mile project? What was the next big one?

Herrmannsfeldt:

As far as the big ones were concerned, we were not quite ready for big ones, but we finished the first one. The machine was running pretty much in the early 1970s and I was given an opportunity to either go someplace else like Illinois for Fermilab, and I actually applied for a job there, or stay at SLAC and think about what other things SLAC could do. And Panofsky urged me to stay at SLAC. He gave me the title Secretary of the Advanced Development Group to think about what we might do next. And I did that for a little while, and then I was offered a chance to go to the AEC into the Office of High-Energy Physics.

Zierler:

Bill, what year was this? When did you get this offer to join the AEC?

Herrmannsfeldt:

It was 1974. We went to Maryland for a two-year appointment, a term appointment, and it was obvious that I could stay there. It was not so obvious that I wanted to. We kept our house here in Los Altos when we went to Maryland. We kept it and rented it. If we had not done that, we could not have come back because in the ten years between the time we came to SLAC and the time of going to Maryland, the housing prices had roughly doubled. In the two years that we were in Maryland they doubled again. In order to come back again, we would have had to buy a house that was not going be in Los Altos. By keeping the house, we could come back to Los Altos. The housing price business, as you are probably aware, gets to be a very important part of one's life and career decisions. Just yesterday I met a lady who had just moved onto our street, standing out chatting with some of the other ladies along the street when I took our little dog for a walk. She moved into a house that I remember from the early years. I could remember that all our neighbors got quite interested at one point when a house on the next street came on the market for $100,000. Everybody wanted to go see the $100,000 house, because $100,000 was a lot of money at that time.

Zierler:

[laugh] It was a novelty.

Herrmannsfeldt:

Unimaginable, unimaginable to us in those days. Housing prices go up with inflation.

Zierler:

Bill, what was the work at AEC? What was the job as it was laid out?

Herrmannsfeldt:

Accelerators. I owned all the accelerators in the atomic energy business. And the project of that era was Fermilab. I was required not to work on anything from Stanford. It was a conflict of interest. But Fermilab was the main issue in those days, so I went there quite a few times.

Zierler:

What were some of the big projects at Fermilab at that point?

Herrmannsfeldt:

Building the machine. Building the laboratory.

Zierler:

What were some of the technical challenges in building the machine?

Herrmannsfeldt:

That's something I'm not really ready to—I haven't thought about this in a long time. [laugh] They got into some trouble with the design and fabrication of the structures. Fermilab has a proton machine, a proton circular machine three miles around.

Zierler:

What was the trouble exactly?

Herrmannsfeldt:

I don't remember really anymore. There were fabrication questions with details of the accelerator structure that had to be modified.

Zierler:

Bill, how long did you stay in this role for?

Herrmannsfeldt:

Oh, just two years. My wife very much wanted to come back to California. We became well acquainted with William Wallenmeyer who was the head of that office there, the high-energy physics office. And every time we met Bill Wallenmeyer socially, my wife would remind him that we were going back to California after two years. And virtually when I walked around the office saying goodbye to everybody, he offered me a job to stay with the AEC. The AEC by that time was ERDA, the Energy Research and Development Administration, which then ultimately morphed into the Department of Energy.

So anyway, he would keep me there; and offered me a job. By then we were committed to coming back. It was not so clear what I was going to do when we got back and so I was interested in staying. But when he waited that long to do anything about it, I was committed to go back. The question was the one you just asked, “what are you going to do next?” I got involved with two things at that point. One was what to do with SLAC. SLAC then was becoming a colliding beam facility, so I got involved with that. The other thing was something that had come up while I was with the AEC which was inertial confinement fusion. It involves using accelerators for making fusion by crashing heavy ions together. That was going on at Berkeley, so then I started working with them doing beam physics. I roughly worked half time SLAC and half time the heavy ion fusion program at Berkeley.

Zierler:

What did you choose? How many options did you have at that point when you came back?

Herrmannsfeldt:

I was promoting to increase in the energy of the LINAC by circulating the beam around again. Then about the same time other people wanted to make a colliding beam facility. That turned out to be about the third iteration of SLAC then was PEP, positron-electron colliding beams. I was working on the alternative to that with increasing the energy of the LINAC, which we didn't do.

Zierler:

That was a policy decision or scientifically it was not feasible?

Herrmannsfeldt:

It was a choice, and the choice was to get the best physics, and so then the PEP project won that choice. You go back to your first question and we settled on the constant gradient design and that limited the energy of the machine to about 20 GeV. And then people wanted more. If you can increase the energy in the machine by a factor of two, you get 40 GeV. If you increase the gradient by just adding power, then you use more electricity just to make that much RF power. That means more transformers and bigger modulators and all of that, all of which is very expensive and doesn't get you very much in physics. In fact, the lab pretty well had done that because it's not very high tech to do that, but it did require changing all of the klystrons and the modulators. But to avoid making it terribly expensive, they reduced the pulse repetition rate. If you make the average power of the beam the same, then you can make the power you are taking from the power lines the same, but you take it with a lower-duty cycle. You change from 360 pulses per second to 60 per second, but you have higher energy electrons. That was then done that way to go further than I was proposing circulating the beam around again.

That was a project which we then did not proceed with. The choice was the PEP colliding beam. Ultimately there was the B factory, which took positrons and electrons together on the machine and collided them. The machine stayed looking the same for all those years but, in fact, it was operating in entirely different ways, entirely different modes as time went on from the original 20 GeV to being colliding beams.

Zierler:

What were some of the arguments in favor of increasing the energies?

Herrmannsfeldt:

You assume that you are going to get different physics. The physics questions, are you looking for other isotopes and so forth? In those days, the physics was looking at strange particles, so ultimately you were looking for the B factory and the B mesons. You don't know you're going to do that until you complete a couple of steps in the process. In the early days, you do not know you were going to do that.

Zierler:

What was your work for PEP? What did you do specifically?

Herrmannsfeldt:

I was not involved with PEP really. I was the alternative; running the beam through the LINAC twice. The alternative was that that gets you, only your total energy, and they wanted more, much more. You get much more when you collide beams.

Zierler:

What did you work on next, Bill?

Herrmannsfeldt:

Well, by that time—let's see. Where were we now?

Zierler:

The LINAC.

Herrmannsfeldt:

We’re back here and I'm, at that time, mostly involved with the heavy ion fusion program. I'm the only one at SLAC doing that. I had to go to Berkeley to do that.

Zierler:

Why is that? Why Berkeley? Why was this not at SLAC?

Herrmannsfeldt:

That's where the people were who were promoting the projects. And I was involved partly because I had been at DOE when the project was created, so I was going to heavy ion fusion meetings.

Zierler:

What were some of the major questions in heavy ion fusion at that point?

Herrmannsfeldt:

The same question that still is there. The question is, can you get fusion to work? Can you collide beams in a way that they actually fuse?

Zierler:

Were there any questions about applications of this technology?

Herrmannsfeldt:

The application is making energy, so it was—people were well aware in those days already of the issues like global warming and so forth, that you need to do something different for making energy. People were not thinking that we were going to have solar cells all over the place. We thought we had to do it with fusion. There is always a bias against fission reactors because of the bomb connection, I suppose. Fusion is all right, fission is not, politically. And fusion is always for the future. Fusion is 40 years away and it will always be 40 years away.

Zierler:

I've heard that. [laugh] From your vantagepoint, Bill, what did you recognize about this even then, about how difficult it would be to harness this technology?

Herrmannsfeldt:

You look at the way SLAC was built. You have an accelerator at Stanford that is a few hundred feet long and you feel like, OK, that's doing good work and now you know you can make one that's three miles long and it will work. That is a scaling up that you can do with that technology. Now, in fusion it is not so nice because you don't have something that is fairly easy to do and fairly cheap to do, relatively speaking, that is, that you can scale up to or from. That is the problem that all the fusion programs in the world still have, and they have been working for years on the tokamak in France, and there are several other tokamaks that come ahead of that one. The costs are going up to that amount and the time it takes is very great. I would not say prohibitive because they are, in fact, doing it, but that is only a step. And it does not prove that you can make the next step, which will be eventually much higher in cost. It is that scaling that is a big problem with fusion. Inertial fusion is a little bit easier but only a little bit. For the laser people, for example, who were doing inertial fusion, it is ridiculously expensive. They cannot make very much energy, but they could prove whether they can actually make the accelerator beam to make that process work. And they have not succeeded in that, so they have been pretty well stuck. That is where heavy ions came along. Well, we could have done that, but the scaling is prohibitive.

Zierler:

How long did you stay with the fusion project?

Herrmannsfeldt:

Oh, until I retired, and I am still associated with it through personal connections with people. I am not being paid for anything.

Zierler:

Did other people from SLAC join or you were always the solo representative from SLAC?

Herrmannsfeldt:

Pretty much just me, because it had to do with the way I had learned to make beams for the injector, for the LINAC, the coding and so forth for injecting beams, for the technology that I used there for the heavy ion program. But instead of accelerating electrons you are accelerating heavy ions. And heavy is, by definition, anything well above hydrogen or helium. It does not have to be lead or something, although it can be. You are having trouble, for one thing, getting an answer to your question that you posed right at the beginning, the one about constant gradient or constant impedance that defined the way the LINAC was built, and people knew about beam breakup, but only sort of. They didn't really believe in it very much. With the constant impedance, all cavities are the same so they all have the same modes that can be excited. There are transverse modes. Transverse modes bend the beam sideways. That is not good. You don't want them getting too excited, those transverse modes. If all the cavities have the same structure, then the transverse modes are going to be the same in every cavity of the string of two miles long. But nobody really thought too much about that.

I asked you if you looked at the SLAC Bible, and because of this meeting I dug it off my bookshelf and tried to see what was said in there. And the explanation in the SLAC Bible is that people understood beam breakup and they chose to do the constant gradient structure wisely to avoid the beam breakup. At the same time, it says, well, we did not really know about beam breakup. So that chapter in the book is self-contradictory if you know the history well enough to pick it up. And then, the question is, if you make the wrong choice what happens? We had a meeting that I was able to go to. There is something called the International Accelerator Conference which moves around from country to country, and at one point there was an earlier version of it in Europe, in Russia. My wife and I went to the meeting in Europe, in Armenia, and then following the meeting we went on a post-conference meeting in Kharkov in the Ukraine. And there the Russians had built an accelerator the same design as SLAC but constant impedance, so all cavities being the same, the same frequency, same dimensions, but only a few hundred meters long. I don't remember how many. Not nearly the length of ours. We went and had a talk with them, and they had beam breakup. Even though their machine was a tenth of the length of ours they could not get very much current through it. We proved that we were lucky. As I commented to you in the little note I sent to you, sometimes it's good to be good, but it's also good to be lucky.

Zierler:

[laugh]

Herrmannsfeldt:

Being good and lucky is especially good.

Zierler:

That's right.

Herrmannsfeldt:

And the Russians were—they weren't so bad, but they were not lucky.

Zierler:

Yeah.

Herrmannsfeldt:

And we worked harder at it but not for the same reasons. The important reason was the beam breakup. Now, we didn't solve it that way. It was not a terrible problem, but it definitely limited how much current we could get through. We changed the focusing system for the whole machine to suppress the beam breakup to keep the transverse modes from being too serious. That then was the way the machine was run. The rest of it—it still is today, but today they are not trying to just get more beam power. But they always have to work around that problem a little bit.

Zierler:

Bill, did you work on the fusion project exclusively until you retired, or did you do other things at SLAC in your later years?

Herrmannsfeldt:

I was always involved with whatever advanced beam accelerator work there might be, but what I was doing got a little bit farther away from what SLAC was doing. SLAC then started working with free electron lasers, so the machine itself turns into being a laser, and I was not involved in that directly. You know about free electron lasers?

Zierler:

I do, yes.

Herrmannsfeldt:

That's SLAC's primary mode now, x-ray free electron laser. An x-ray laser is pretty unique. Now the LINAC is being rebuilt. They have taken out my old alignment system. I built the laser alignment system. And they are now installing a lower energy but higher average power superconducting RF structure for a free electron laser.

Zierler:

Now, why did this laser alignment system need to be replaced? What could it not do?

Herrmannsfeldt:

Well, they needed part of the length of it for their superconducting RF structure. The pipes were bigger around and got in the way of the RF system. The alignment laser was in a 2-foot diameter vacuum pipe, 2-mile long, 2-foot diameter, and that was in the way. But the laser requirements are tougher than the beam accelerator requirements, so it wasn't that good an alignment system anymore. It needed to be more sensitive. There are ways of doing it with the beam itself, actually use the beam itself to be the alignment device. And anyway, the laser alignment system is gone now.

Zierler:

When did you decide it was time to retire? What were the considerations?

Herrmannsfeldt:

[laugh] Yeah. I was 74, so I was pretty well past the traditional 65, and the thing that I was primarily interested in was the fusion program which was not doing well. It was politically not connected very well so it was not getting much support out of the government. I was not particularly challenged by the work that I was doing.

Zierler:

What's the status of the fusion program today?

Herrmannsfeldt:

Government programs never quite go away—

Zierler:

[laugh]

Herrmannsfeldt:

—but they don't necessarily advance either, especially ones that are as expensive as laser fusion at Livermore. It is not really doing any fusion. I'm not following it very closely. Some of us did get together just before the pandemic business started, had a meeting here at Stanford and then for a lunch and chatted about it. But there are a few people that are still hanging in there at Berkeley, not so much anymore.

Zierler:

Bill, for the last part of our talk, I'd like to ask some overall questions about your career. The first one is, of all of the projects you were involved with building, building instrumentation, increasing knowledge in physics, what was the most fun? What was the most fun project you ever worked on?

Herrmannsfeldt:

The most fun was starting the little business selling the computer program for electron optics and electron gun design. The fun is in appreciating the technology and learning how to make these calculations in a way that are useful for a lot of people, a lot of different applications. It is developing the software for electron gun design, for electron optics. That is the part that you can recognize when you're making progress. You are doing it yourself. You are not doing it with thousands of other people or dozens of other people, but you are doing it yourself and you are doing what you are personally interested in doing. In my case, I involved my son, but otherwise there was really just the two of us. And I think that would have to be the favorite, because talk about the actual results of any one project and I almost hate to go back to my thesis work about the nature of the beta decay. That is the real physics issue. Everything else is some technology development, but getting a physics result itself is special. It's both of those two things, I would think, and they're not the same. They are not related.

Zierler:

Bill, of course, every project you have been involved in has been unique, but when thinking about technical challenges, what might be some common themes that connect all of these problems, issues that you always had to overcome no matter what the project was?

Herrmannsfeldt:

Beam dynamics mainly, causing particles to go where you need them to go, changing the beam quality and energy. It is accelerator design and accelerator usage and the most rewarding part of it is the particles you're accelerating. You are making them do what you need them to do, from heavy ions to electrons and everything in between.

Zierler:

What has been most intellectually satisfying in terms of all of your contributions to these physics projects? What's been most intellectually satisfying for you?

Herrmannsfeldt:

The gun design issue, electron optics for gun design, doing that for the heavy ion program in spite of the fact that the heavy ion program itself isn’t going anywhere today. In those days, we thought we could get it to go, and a few people still think so.

Zierler:

Bill, given that you were there, really right at the very beginning in 1962, in what ways do you see the founding mission of SLAC continue to animate and inform what SLAC is and does even today?

Herrmannsfeldt:

When we came, we did not know this area at all. I got a job, I moved the family into a motel, and we started house hunting. We found, in fact, this house that I am sitting in. Then we had to figure out a way to pay for it. I went to a bank and asked for a loan. And the banker asked me what I did, and I told him I was working for Stanford. The bank was actually on Stanford land at the Stanford shopping center. I told him I was working for the university at SLAC. And he asked me about how long we thought that job was going to last, and I told him, "Well, the director says he's quite sure that we're going be there for 10 years, and beyond that it's hard to tell."

Zierler:

[laugh]

Herrmannsfeldt:

The banker didn't like that answer. He wouldn't give me a loan. Ten years was not enough.

Zierler:

[laugh]

Herrmannsfeldt:

I went to a different funding place and I carefully avoided getting into that same discussion again.

Zierler:

[laugh]

Herrmannsfeldt:

I don't know how that banker proceeded with his career, but I would guess that he probably didn’t make it ten years. In any case, you think about it that way, it has been quite a thing to be here in this area and this house is now worth, like, a hundred times as much as we paid for it, if that makes any kind of sense.

Zierler:

Bill, last question, looking to the future. Given your long tenure in the field, what do you see as some of the most important unanswered questions for accelerators?

Herrmannsfeldt:

I don't know. I don't think I've got a good answer for that.

Zierler:

What kinds of new projects would you want to see done that don't currently exist?

Herrmannsfeldt:

The field has changed from particle beams, to free electron lasers, so SLAC is not an accelerator laboratory anymore, it's a laser program, it's a laser lab. You can use the same structure and make a laser, but these are x-rays. It is an x-ray laser, which is unique. You had—I'm trying to think—your sample program, what was his name? The man that you sent me the—

Zierler:

Oh, Claudio Pellegrini?

Herrmannsfeldt:

Pellegrini. Claudio is responsible for showing how you could make a laser out of SLAC, out of the linear accelerator. And that is getting into whole different kinds of physics than you can do with just crashing particles into each other. And that's got a long way to go yet. But it is not my field anymore. [laugh]

Zierler:

[laugh] Bill, I want to thank you so much for spending this time with me. It's been a lot of fun listening to your perspective and I'm so happy we were able to do this, so thank you so much.

Herrmannsfeldt:

OK.