Earlier this year the German Federal Ministry of Education and Research reached a policy decision on the TESLA project that determines the way ahead for DESY. Albrecht Wagner, chairman of the DESY Directorate, assesses the impact of the decision for the laboratory in Hamburg.
In February 2003 Edelgard Bulmahn, the German federal minister of education and research, decided to support several large-scale facilities for basic scientific research. These included the 4 km long X-ray free-electron laser (XFEL), which was originally conceived as part of the project proposed by the international TESLA collaboration for a 33 km electron-positron linear collider to be built near DESY, Hamburg, together with an integrated X-ray laser laboratory. At the same time, the German government decided not to proceed nationally with the linear collider part of the TESLA project and not to propose a German site for such a machine at this moment, but to wait for international developments. These decisions will have important implications for DESY in the coming years.
The German government is thus proposing Hamburg as the site for a European XFEL facility, and is prepared to carry half of the investment costs of €673 million. A decision on construction should be possible within two years, and would be followed by a construction period of about six years. Since the announcement in February, the German government has entered into bilateral discussions with other European governments. The first goal is to set up two European working groups, one on scientific and technical issues, and one on organizational and administrative matters. In parallel, the European Strategy Forum on Research Infrastructure (ESFRI) recently organized a workshop at DESY, on 30-31 October, on the technological challenges of X-ray lasers. So far discussions have led to the conclusion that only one major facility for research with hard X-ray radiation should be developed in Europe. The XFEL is the only project proposal in Europe in this field.
In addition, the ministry foresees €120 million for the conversion of the PETRA storage ring – which currently serves as a pre-accelerator for HERA – into a high-performance third-generation synchrotron radiation source. This upgrade is scheduled to start in 2007, after the conclusion of the HERA physics programme, and is intended to strengthen further the research with synchrotron radiation.
Regarding the linear collider part of TESLA, the German government decided that DESY will continue work on the project as part of the international research and development effort. At the EPS conference on High Energy Physics held in Aachen in July, Hermann Schunck, director-general of the Federal Ministry of Education and Research, said: “We have to wait for international developments. But we will continue our efforts so that we can participate in a global linear collider project. Let me underline this – my government is the first one to have announced that it is in principle committed to participating in this project.”
Testing acceleration structures
For the past 10 years the TESLA collaboration has made decisive progress with the superconducting accelerator technology that forms the basis of the TESLA linear collider and the XFEL. A test accelerator of 250 MeV – the TESLA Test Facility (TTF) – has been built and has operated at DESY since 1997. The international partners in the project provided about 35% of the investment and personnel funding. At the TTF the collaboration successfully tested the superconducting acceleration structures and made groundbreaking progress on the SASE (Self-Amplified Spontaneous Emission) principle for a free-electron laser at short wavelengths around 100 nm. The first experiments with this new type of laser provided an impressive demonstration of the high scientific potential of free-electron lasers in the UV and X-ray region. The TTF is currently being extended to reach an energy of 1 GeV, that is, a length of 260 m. Starting in 2004, it will be available as a user facility for experiments with soft X-ray laser radiation above 6 nm wavelength. As such it will allow researchers to gain important experience in experimentation with free-electron lasers in the X-ray region, and it will provide valuable operating experience for the linear collider.
In co-operation with European partners, DESY is actively preparing for the construction of the 20 GeV superconducting linear accelerator for the XFEL laboratory, and is focusing on issues related to the industrialization, mass production, quality assurance and reliability of all the linear accelerator components. A first step in the concrete planning of the XFEL will be the commissioning of the free-electron laser for soft X-ray radiation at the expanded TTF. Since the XFEL is to be realized as a European project, discussions are being held with scientists and politicians in countries that are interested in participating in the effort. In these discussions a number of issues must be examined and clarified, such as the exact operational parameters of the laser and the organizational models for the laser laboratory. The inclusion of international partners from a very early stage in the planning and development of the superconducting accelerator within the TESLA project has proved very helpful in this respect.
At the same time, the TESLA collaboration continues to pursue the high-gradient programme to demonstrate the accelerating field of 35 MV/m that is required to reach 800 GeV for a 33 km TESLA collider. Substantial progress has been made in this area. In a test at low RF power, four nine-cell cavities have shown the required performance of 35 MV/m after electro-polishing. Two of these cavities were then fully assembled with all their ancillaries and have reached gradients above 35 MV/m in long-term testing under typical collider operating conditions (at Q > 5 x 109 with an RF loading as required for linear collider operation), but without beam. Each of these cavities corresponds to one-eighth of a TESLA cryo-module. This represents a significant step towards the milestones set by the International Linear Collider Technical Review Committee for “Phase II” of TESLA. A full test of one module – eight cavities – at 35 MV/m with beam will, however, take more time due to constraints on the resources available at DESY.
Towards a linear collider
The next major step towards a global collider project concerns the choice of technology. The International Linear Collider Steering Committee is currently setting up an advisory group (“wise persons”), which will be charged with performing an analysis of the status of the two competing technologies (“warm” and “cold”) and with making a technology recommendation before the end of 2004.
If the chosen linear collider technology is “cold”, a major synergy will exist between the work on the XFEL and the linear collider. In this case the contribution of DESY and the partners in the TESLA collaboration will most likely be in the area of the main accelerator of the collider. A recent analysis of the work needed to be done on the basic accelerator unit for the XFEL, the cryo-module, has shown that more than 90% of the issues to be tackled are the same for the XFEL and the linear collider. The synergy is therefore achieved by the fact that the work done now for the XFEL will be largely of direct use for the linear collider. In addition, the R&D funds now spent on the XFEL will not need to be spent again for a collider built using the cold technology.
If the chosen technology is “warm”, a major reassessment of the contributions will be necessary. In this case DESY will probably participate in other subsystems and its contribution will probably be less pronounced than for a “cold” machine due to the commitment to the XFEL.
DESY will continue to participate in the linear collider working groups of ICFA and ECFA and once the technology choice has been made, the laboratory will be a partner in a European team within an international linear collider design team. DESY will also play a major role in the design, construction and future operation of the collider detector(s).
The international efforts for the coming years aim at reaching an agreement, in principle, to start the construction of a linear collider in time for commissioning in 2014/15, in accordance with the recommendations of ACFA, ECFA, HEPAP and the OECD Global Science Forum. Taking into account a construction time of seven to eight years, this requires a decision to go ahead to be made in 2007. Such a schedule also requires the first funds to become available in 2007, although the major investment spending for the linear collider will typically begin three years after the project starts, i.e. around 2010, as has been the case for other major accelerators.
The future for DESY
The strength of DESY is the result of an in-house synergy in three key areas: accelerator development, particle physics and research with synchrotron radiation. Particle physics has been the driving force behind accelerator development, and this also applies to the TESLA project. The decisions of the research ministry have secured DESY’s long-term future as one of the world’s leading centres for research at accelerators.
In particle physics DESY’s research and its contributions to both the linear collider itself and the detector, will ensure that the laboratory remains a major contributor to the realization of the project, regardless of whether or not the facility is built in Germany.
