It was back in the early 1990s that DESY, together with partners from abroad, first started work on the development of a pioneering superconducting accelerator technology. The concept has proved so successful that it was chosen for the next major project in the world of particle physics, the International Linear Collider ILC. At the same time, it has also turned out to be ideal for operating a free-electron laser in the X-ray region. As one of the leading institutions in this field, DESY is playing a major role in the further development of the superconducting TESLA technology.

The TESLA project

It is now almost 20 years ago that work first began on developing the next major facility for particle physics: a linear accelerator capable of boosting electrons and their antiparticles, positrons, to energies ranging from 500 to 1000 billion electronvolts (gigaelectronvolts, GeV). Under the leadership of DESY, the international TESLA Collaboration put together a design concept based on the use of superconducting cavities made of the metal niobium. While this technology offers decisive advantages with regard to acceleration capability, it also represents a massive stride into unexplored technological territory.

It is the use of superconducting accelerator technology that marked the decisive difference between the TESLA and rival projects, which were to rely on conventional, normally conducting technology. In the course of the development work for TESLA, it emerged that this type of superconducting linear accelerator would also be ideal for operating a free-electron laser in the X-ray range. TESLA was therefore planned as a combined facility, comprising a linear collider for particle physics experiments and an integrated X-ray laser for photon science.

Pioneering work for TESLA

In order to test the technology for TESLA under realistic conditions, the TESLA Collaboration constructed a fully functional test accelerator at DESY – the 100-metre-long TESLA Test Facility TTF – which featured all of the components required for the operation of a superconducting linear accelerator. Thanks to the quality of the development groundwork, the TTF got off to a flying start in 1997. Indeed, the first eight of the superconducting niobium cavities immediately achieved an accelerating field strength (gradient) in excess of the 15 megavolts per metre (MV/m) originally planned. This was a world premiere and also marked a decisive advance on the early days of the project, when the maximum gradient achievable in beam operation was only 5 to 8 MV/m. Meanwhile, it has proven possible to more than double the initially recorded value of 15 MV/m. In this way, the TESLA Collaboration has been able to deliver conclusive proof of the technical feasibility of a superconducting electron-positron linear accelerator.

The TTF team then went on to demonstrate the feasibility of the second part of the TESLA project, the superconducting X-ray laser. The decisive proof arrived at the start of 2000, when the TTF – following its conversion to a free-electron laser – first produced laser pulses with a wavelength of less than 100 nanometres. After it had successfully fulfilled this mission, the TTF was extended and converted to FLASH, the world’s first ever free-electron laser in the soft X-ray range. Since 2005 it has been available to scientists from around the world for their experiments.

From TESLA to European XFEL and ILC

At the beginning of 2003 Germany’s Federal Ministry of Education and Research took a significant decision in favour of the X-ray laser part of the TESLA project. Whereas the X-ray laser was granted approval in principle as an independent European facility, a resolution regarding the linear collider was deferred pending global developments in the rest of the field. De facto, the two projects were thereby separated. Today the X-ray laser project is known as the European XFEL and is being realized in Hamburg and Schleswig-Holstein with substantial participation by DESY. The responsibility for the linear collider project has been transferred to the international level.

Following assessment of the various accelerator projects submitted, including TESLA, the international commission responsible decided in summer 2004 that the future linear collider – henceforth to be officially known as the International Linear Collider ILC – would be based on the TESLA technology developed by DESY and its partners. Since then, the development and planning work for the ILC has been proceeding at full speed, with a global collaboration now comprising more than 2000 scientists from over 25 countries currently working on the project. As experts in superconducting accelerator technology, DESY and its partners from abroad – now known as the TESLA Technology Collaboration – are playing a major role in these developments.