The device is carefully lifted by the indoor crane. The weight displayed on the crane’s external screen shoots up and down, eventually settling down at about 11.5 tonnes. The weight of this so-called insertion device is mainly due to its heavy steel frame. The magnets installed inside this generate attractive forces of several tonnes. The device must be able to withstand this enormous field strength. In particle accelerators, the periodic arrangement of the magnets is used to deflect electrons, thereby generating synchrotron radiation – a special type of X-rays.
Pioneering work at PSI
At PSI, the insertion device was used for a very special purpose, however. Following a lecture by an American colleague on the generation of ultra-short X-ray pulses, the two PSI physicists Gerhard Ingold and Thomas Schmidt realised that the conditions at the Swiss Light Source SLS were ideal for such a technique. The technology is called femtoslicing, and it can be used to observe extremely fast processes, such as chemical reactions.
"Immediately after the lecture was over, we did our first calculations. A few days later, the calculations turned into a project and three years later, under the leadership of Gerhard Ingold, we were finally able to produce hard X-rays in the femtosecond range for the first time in the world – a pulse of high-energy X-rays lasting 0.000 000 000 000 1 second," as the head of the Insertion Device Group at PSI, Thomas Schmidt, recalls. Their approach was based on using the powerful magnetic field produced by this device as a modulator, so as to achieve resonance between the electrons and an external infrared laser, thereby transferring the pulse length of the latter to the X-rays. Since only a small fraction of the electrons are used in this process, namely those that overlap with the laser pulse, the technique is referred to as “slicing”.
The project resulted in numerous publications. Experiments were carried out on a range of different samples and new types of detectors were developed in order to process the extremely fast units of information. These findings were ultimately crucial to the development of free electron lasers, which are driven by linear accelerators and thus indirectly also the Swiss X-ray free-electron laser SwissFEL, where the first pilot experiments were carried out in 2017. However, this also heralded the end of the Femtoslicing Facility and thus of the insertion device in question. "SwissFEL allows us to generate much brighter and even shorter pulses of this kind of radiation than with the original facility. With this, extremely fast processes can be imaged at even higher resolutions," says Thomas Schmidt.
Since then, the insertion device has been sitting in the hall of the SLS, unused.
Important manufacturer based in Siberia
Insertion devices are high-precision instruments, and demand for them is limited. Because of this, only a few manufacturers in the world are prepared to take on the complex task of building these devices in the first place. The world’s most important supplier of superconducting wigglers (a special type of insertion device) is based in Russia. However, due to the war and the global sanctions against Russia, many countries have also stopped importing these rare devices.
"Suddenly, there was a demand for retired wigglers in our European network for synchrotron radiation sources (LEAPS - League of European Accelerator-based Photon Sources)," explains Thomas Schmidt. "First, SOLARIS, the National Synchrotron Radiation Centre in Poland, inquired about a device that was no longer needed – we immediately agreed and sent them the plans. Unfortunately, our device was not compatible with their facility." But just a short while later, the Australian Nuclear Science and Technology Organisation got in touch, also asking for a wiggler for their synchrotron in Melbourne. Again plans were sent – and this time everything fitted.
From Villigen to Melbourne
Swaying slowly to and fro, the insertion device moves across the ground suspended from its crane. The wooden pallet is ready and the unit, weighing many tonnes, descends precisely, settling gently into place. With practised movements, the two employees of the company cargopack tägi AG start packing it up. First the insertion device is treated with an anti-corrosion spray to prevent the salt water from causing it to rust during the long sea voyage. Then it is wrapped in plastic before being enclosed in sturdy wooden walls and finally disappearing altogether beneath the lid. A forklift truck specially approved for loads of this kind brings the crate outside and loads it into a truck for the journey to Rotterdam. There it is placed inside a shipping container and the journey across the high seas to the other end of the world can begin.
Identifying what is needed and how to help each other: this is what distinguishes international networks like LEAPS, underscoring once again how important they are for research. A new task awaits the newly packed insertion device, which was used intensively for research at PSI until 2017; in Australia it will serve the purpose for which it was originally designed – generating synchrotron radiation. Until then, we wish it a safe journey.