DESY : Albrecht Wagner se souvient…

Albrecht Wagner, ancien directeur de DESY, évoque ici ses 35 ans de relations avec ce laboratoire. Il y a travaillé pour la première fois en tant que physicien des particules, aux anneaux de stockage électron–positon (installation DORIS). Puis il est passé à l’expérience JADE auprès du collisionneur PETRA. Ensuite, après une dizaine d’années passées à l’expérience OPAL au CERN, il est revenu à DESY en 1991, juste au moment où le collisionneur HERA était sur le point de démarrer. Au cours de son expérience de directeur, il a vu le laboratoire prendre un rôle de premier plan dans la technologie des accélérateurs supraconducteurs, qu’il s’agisse de lasers à électrons libres à rayons X, ou d’un projet de futur collisionneur linéaire international.

DESY came to my attention for the first time in 1963 through a poster advertising its new summer student programme. Although I did not go to Hamburg that summer, this triggered my awareness of the laboratory. It took 11 more years before I finally went there, as a member of a group from Heidelberg, to work on the electron–positron storage rings, DORIS and then PETRA. It was 1974, the year of the discovery of the J/Ψ and it was in the midst of the related "November revolution" that DORIS started to provide its first collisions. The contributions that this machine was able to make in the understanding of the properties of the bound states of the charm and anti-charm quarks, as well as in the mass measurement of the τ-lepton, created a very stimulating atmosphere – which became the springboard for the next DESY project, the 2.3 km-circumference storage ring, PETRA.

PETRA, originally proposed as a proton–electron collider, was quickly converted into a positron–electron collider. Approved in 1976, it was built in the record time of two years and eight months, while staying 20% under the original budget. With the PETRA experiments being realized in international co-operation, DESY for the first time became a truly international laboratory and laid the foundation for its future development. The main drivers at DESY at that time were Herwig Schopper, Gustav-Adolf Voss, Erich Lohrmann and the many scientists, engineers and technicians from DESY, Germany and the partners abroad. For DESY, this international flavour was new and stimulating. The scientific programme for PETRA was broad, but interestingly enough did not contain what was to become the machine’s major highlight – the discovery of the gluon.

It was while working on JADE, one of the four experiments at PETRA, that I lived through the worst moment of my professional career, when early in 1979 the beams were lost in the middle of the detector, breaking many wires of the "jet" chamber on which I was working. But I also experienced extremely exciting, hard-working and very rewarding moments while trying to establish the true nature of the 3-jet events that proved to be the gluon’s signature. The scientific success of PETRA, and with it JADE, was paradoxically the reason for me to leave DESY in around 1980 – to work on the next electron–positron collider, the 27 km LEP at CERN. There I joined the OPAL experiment, the big brother of JADE.

I was called back to Hamburg, the university and DESY just as the hadron–electron storage ring, HERA, was getting ready to operate in 1991. HERA was built by three great personalities: Volker Soergel, Bjørn Wiik and (again) Gustav-Adolf Voss. This time not only the experiments but also the accelerator had been built through international collaboration, in a very successful way that became known as the "HERA model". Although I had moved from working on an experiment to science management, I kept close contact with the experiments and the physics at HERA. When HERA operations came to a close in 2007, we could look back on an impressive and unique harvest of scientific results, from the structure of the proton to the properties of the fundamental forces. Only one wish had not come true, the discovery of the unforeseen.

New technology

Around 1990, work on linear colliders started around the globe inspired by the continuing success of electron–positron colliders. It had become clear that circular machines would no longer be feasible and that a new concept with many challenges had to be tackled. By the mid-1990s DESY decided to concentrate on superconducting accelerator technology and the TESLA collaboration was formed with many international partners. Combining the world know-how in this area, the collaboration made major progress in raising acceleration gradients and also solved many other problems. To put the technology to the test under realistic conditions, the collaboration built the TESLA Test Facility (TTF) at DESY, which demonstrated the feasibility of the technology and its reliable operation.

At a major meeting in 2001, the collaboration presented a proposal for a 500 GeV linear collider with an integrated X-ray laser (XFEL), to be realized as an international project at DESY. Two years later, the German government decided to approve the XFEL, together with the conversion of PETRA into a synchrotron light source, and to fund continuing R&D for a linear collider. At the same time the TTF was turned into FLASH, a soft X-ray laser facility for science and a test-bed for future linear-collider work. In the same year the International Committee for Future Accelerators unanimously decided that the technology for the linear collider, now called the International Linear Collider, should be based on superconductivity. Together with its partners from the TESLA Collaboration, DESY thus continues to be one of the main players in the R&D work for the next major project of particle physics.

I have focused mainly on the particle physics aspect of DESY. At the same time, however, the lab has been a pioneer in the generation and use of synchrotron radiation. First experiments started in 1964 and the Hamburg Synchrotron Radiation Laboratory (HASYLAB) was founded in 1977 around DORIS – still the work horse, serving more than 2000 scientists a year. Today, with the new light sources PETRA III and FLASH, and as host for the European XFEL, DESY is building and operating a remarkable suite of new tools for photon science.

As a former director of DESY, I am delighted that the laboratory, despite its age, has remained young, flexible, ambitious and successful on a world scale. I hope for DESY, my former colleagues, and all of the guest scientists, that the same can be said in another 50 years.