The DESY-Fermilab Connection

by Kurt Riesselmann

July 3rd, 1983: Ferdinand Willeke, physicist from the German high-energy physics laboratory DESY (Deutsches Elektronen-Synchrotron), witnessed the Tevatron setting a new world energy record. "I happened to be on shift when acceleration up to 512 GeV was accomplished," says Willeke. "I was in the team of Frank Turkot and Hans Jöstlein, and I was declared `accelerator physics expert' of this team. It was great fun and very, very interesting. I still like to remember these days."

And so does Fermilab.

"Fermilab was quite a beneficiary of this collaboration," summarizes former Fermilab director John Peoples. "Siegfried Wolff and Hartwig Kaiser, physicists from DESY, contributed a lot to the development of the Tevatron magnets, the first use of superconducting magnets for building a high-energy accelerator."

The collaboration also benefited DESY. Wolff and Kaiser, returning to Hamburg, used their knowledge gained at Fermilab for the design of the superconducting magnets needed for the new HERA accelerator, Germanyís prime high-energy collider. As a result the "HERA magnet" with cold iron yoke was developed, providing an increase in magnetic field strength of about 20 percent compared to the original Tevatron design.

R&D for a Superconducting Linac

The story of successful DESY-Fermilab collaborations entered a new chapter in the early 1990s when former DESY director Bj–rn Wiik proposed an R&D program for a future superconducting linear accelerator. Helen and Don Edwards, husband and wife, both veterans of the Tevatron magnet program, were immediately interested.

Helen Edwards, in 1992 a member of the machine advisory committee at DESY, became the first project manager for the TESLA test facility (TTF) in Hamburg. She and Don have spent many months at DESY Hamburg ever since.

"He did have a vision," Helen says of Wiik, who died in a tragic accident in 1999. "That's why I am interested in TTF."

Many Fermilab engineers contributed to the success of TTF. Mark Champion (couplers), Tom Peterson (cryogenic design) and Dan Wolff (modulators) are the people who were most closely involved in the collaboration. Champion and Peterson both worked more than a year in Hamburg. Peterson also is credited with the development of the vertical test stand used to check the quality of the superconducting cavities at TTF.

Albrecht Wagner, director of DESY, acknowledges that "the tremendous progress made in super-conducting radiofrequency (RF) acceleration in TESLA would not have been possible without the strong engagement of Fermilab."

A highlight of the DESY-Fermilab collaboration on TTF was the construction of the RF gun, a device used to create a high-quality electron beam. It was designed and built by Fermilab engineers and scientists. In 1998 it was installed at the TTF facility of DESY Hamburg.

"The RF gun is a very sophisticated device," comments Reinhard Brinkmann, project manager of TTF. "We operate it day and night, seven days a week. It is extremely reliable, constantly proving Fermilab's competence."

In 1994 Peoples, at that time director of Fermilab, also wanted the lab to become involved in photo-injector physics: "I wanted to have something here at Fermilab that was part of TTF, some work that was sort of pushing the envelope. I thought that accelerator technology was very valuable for Fermilab to know. Then Helen pro-posed the idea of the photoinjector."

The construction of the Fermilab photoinjector, in design similar to its TTF counterpart, was again a truly international collaboration, with DESY providing a nine-cell TESLA cavity and INFN Milan contributing a cathode preparation facility. Further input came from several U.S. universities, including the University of Rochester, and HEP institutes around the world.

Beam dynamics studies at Fermilab's photoinjector are relevant for a future low-emittance linear collider. The results are important both for the Next Linear Collider (NLC) and TeV-Energy Superconducting Linear Accelerator (TESLA) designs. They also show promise for application to high-energy electron cooling at the TeV energy scale. If successful, this technique could be used to maintain the quality of the beams at the Tevatron for long periods of time.

In early May, Reinhard Brinkmann and Klaus Flöttmann came to Fermilab to participate in measurements, triggered by the promising experimental results that the Fermilab photoinjector group had just obtained. Together with Yaroslav Derbenev, Brinkmann and Flöttmann discovered a new and completely surprising concept for improving the electron beam in an accelerator. They predicted that, without using expensive damping rings, it would be possible to convert a round electron beam to a flat beam with better quality. Experimenters at the Fermilab photoinjector were the first to observe the effect, a major breakthrough.

If physicists achieve the electron beam specifications of a linear collider without having to build a damping ring, "this would save about 100 million dollars, independent of the final accelerator technology [NLC or TESLA] that is eventually used," Brinkmann says. He would like to see the DESY-Fermilab co-operation intensified, especially since there are plenty of research topics, like beam focusing, that need investigation no matter what kind of machine will be built in the future.

Co-operation beyond accelerator technology

The DESY-Fermilab cooperation is not limited to beam and accelerator technology. Matthias Kasemann, physicist and former DESY employee working in the ZEUS detector group, came to Fermilab in 1998. He became head of the Fermilab Computing Division, a group of about 250 computer scientists, electrical engineers and physicists. Kasemann emphasizes that ěthe interdisciplinary setup of Fermilab's computing division is very positive,î an aspect that eventually led him to accepting Fermilab's job offer.

Kasemann has contributed to the DESY-Fermilab connection beyond his personal story. He has initiated an ongoing DESY-Fermilab co-operation on the development of mass storage. Both DESY and Fermilab use similar components, including commercial tape robots for data recording. To store data more efficiently, DESY extended its software to steer the tape robots. Fermilab, which faces unprecedented amounts of data when Tevatron Run II starts in 2001, joined DESY's efforts. In addition to the work done at Fermilab's Feynman Computing Center, Charles Waldmann, computing engineer in Fermilab's Computing Division, spent three months at DESY Hamburg to work on subsystems of tape recording software that will be used jointly at Fermilab and in the DESY system.

DESY alumnus Holtkamp receives Fermilab Employee Recognition Award

Norbert Holtkamp was looking for a new physics project when Helen Edwards, then project manager at DESY's TTF, suggested he should think about joining Fermilab. Holtkamp, intrigued by the idea of living in the United States, saw the great opportunity of helping to define Fermilab's future accelerator agenda. In 1998, twelve months after Edwards' proposal, he moved to the United States.

His wife and two sons originally were a little skeptical about leaving Hamburg. Things changed when he told his sons about living in a house with a garden big enough to keep a dog, something that did not seem possible when living in Hamburg. Eventually his family accepted, and a dog became the fifth member of the Holtkamp family soon after their arrival.

Holtkamp quickly managed to establish himself as an expert on accelerator technology for future machines. Together with David Finley he coordinated the technical study for a future neutrino factory, a machine that would use a muon storage ring to produce the most intense neutrino beam ever produced on earth. Their report, complemented by the report of Steve Geer and Heidi Schellman (Northwestern University) on the physics study, has received recognition by high-energy specialists around the world. Holtkamp, Finley and Geer were honored with a Fermilab Employee Recognition Award on May 8.

Wake Fields and X-rays

Fermilab's photoinjector is presently used for many other experiments related to beam physics. James Rosenzweig, professor at UCLA, and his graduate students are carrying out experiments on beam acceleration using plasma wake fields. Having worked in the field for more than ten years, Rosenzweig is finally harvesting the results of his persistence. Using the high-quality electron beam of the photoinjector, Rosenzweig's group is able to achieve acceleration gradients of 500 MeV per meter ń but presently they are only able to build a plasma device a few millimeters in length.

Fermilab physicist Richard Carrigan is using the photoinjector to create x-ray radiation that is potentially hundreds of times more powerful than that obtained from sources currently used at hospitals. Guiding the high-quality electron beam through a crystal, the intense x-ray radiation is produced by a mechanism called channeling radiation. Carrigan's collaborators at TH Darmstadt, Germany, have probed the possibility that hospitals could build x-ray facilities for heart studies and mammogram screening that could provide much better contrast, without having to rely on very large synchrotron light sources.

Globally expanding connections

At the International Conference on Future Accelerators meeting in October of 1999, Wagner initiated a discussion of what a worldwide collaboration for a future accelerator might look like. Laboratories around the world would not only be involved in the construction but also the operation of such an accelerator. The successful collaborations for particle detectors such as CDF point in the direction one needs to go, he said.

Fermilab director Mike Witherell confirms this point of view: "If there will be only enough funding for one TeV-scale collider, it is important that it will be built as a broadly based global project tied to laboratories around the world. No matter where a linear collider is built, it needs to be a part of the core program for laboratories elsewhere." He credits Wagner with taking a leading role in that effort.

At a time of reduced budgets for high-energy physics, a single laboratory cannot explore all aspects relevant to a future accelerator. Laboratories need to expand their collaboration efforts. Regarding Fermilab's role, Witherell sees these priorities: "Linear colliders are an issue of intense interest for the worldwide high-energy physics community. It will come to a decision point in the near future, and Fermilab needs to be involved in that decision."

Collaborations like those that have been so effective for DESY and Fermilab in the past will be essential for the future.

Connecting with DESY Zeuthen

DESY Zeuthen had few connections with Fermilab until last year, when DESY decided to build a photoinjector, the third of its kind, at its Zeuthen location near Berlin. Since then engineers and physicists from Zeuthen have visited Fermilab to learn about its photoinjector and related R&D activities.

Another connection between DESY Zeuthen and Fermilab was established on May 1. Kurt Riesselmann, formerly working as public relations officer at Zeuthen, joined Fermilab's Office of Public Affairs. He already had strong connections with the Midwest: He received his Ph.D. in physics from the University of Wisconsin-Madison, and his wife, Lisa, is from Milwaukee.