New technologies for PETRA IV

 This process requires an injector – a complex assembly of pre-accelerators that first produces the particles and then brings them up to speed before they are injected into the 2.3-kilometre PETRA III electron storage ring. For its planned successor PETRA IV, an innovative laser plasma accelerator is being developed that will inject the electrons directly into the storage ring without the detour through a pre-accelerator chain. This would save space and energy. In a recently published conceptual design report (CDR), the research centre DESY describes what such an injector might look like.

“The publication of this study is an important milestone for us,” says Alberto Martinez de la Ossa, corresponding author of the study. “We show that it is in principle possible to use a plasma injector for a high-performance source like PETRA IV and we outline the challenges that still need to be overcome.” The realization of this study was made possible by a team of scientists from two distinct communities: plasma-based and radiofrequency-based accelerators. “Closely working together has been crucial to coming up with the most promising design for the plasma injector,” adds de la Ossa.

The plasma injector is based on laser plasma acceleration which is still a relatively young technology. Instead of using powerful radio-frequency waves to accelerate the electron bunches to high energies, as in a conventional system, a laser fires short, extremely intense pulses of light into a gas-filled tube. Here, the light pulses create strong electric fields which can literally catapult electrons away. This technology allows powerful accelerating fields to be produced in a tiny space, permitting the construction of very compact accelerators. DESY has been developing and refining this technology for several years.

With the conceptual design for a PETRA IV injector, the research centre is now outlining a potential first concrete application of this pioneering technique. Currently, an electron gun generates the particle bunches for the PETRA III injector and a 70-metre linear accelerator (linac) brings them up to speed. The electrons then enter the DESY II accelerator – a ring-shaped synchrotron with a circumference of 300 metres, which accelerates the particles to their final energy of 6 GeV before sending them on to the PETRA III ring. “Using a plasma injector, we would only need a fraction of the space,” explains de la Ossa.

“The ideal version would be a small building right next to the storage ring,” adds Andreas Maier, lead scientist for plasma acceleration at DESY. “The laser could be housed on the upper floor and the plasma accelerator on the lower floor.” By connecting it directly to the ring, it would be possible to dispense with the components that are currently required to transfer the electron bunches. This, together with the plasma acceleration, could save a lot of energy.

Various innovations are being developed so that a practical injector can be built. The quality of the electron bunches is crucial – after all, PETRA IV is expected to produce significantly narrower and more intense X-rays than the current ring. To do this, the future machine requires electrons whose energy distribution fluctuates by no more than one percent. “That was probably the most fundamental challenge for a plasma injector,” explains de la Ossa, “because laser plasma accelerators tend to have a relatively broad energy spectrum.” The scientists have already overcome this hurdle. They recently developed an energy compressor, in which the plasma stage is followed by a short conventional accelerator. Thanks to a clever arrangement, the energy distribution of the electron bunches is compressed to within the required range. The concept has already been successfully implemented in a demonstrator experiment, whose results were recently published in the journal Nature.

To enable full energy direct injection into the storage ring, electrons with an energy level of 6 GeV are required. The DESY team wants to resort to a special variant of plasma acceleration and develop it further: a plasma channel guided laser plasma accelerator. In this method, a weaker light pulse is fired into a gas ahead of the actual laser pulse. This ionises the gas, turning it into a plasma and creating a channel for the main laser pulse following immediately behind it. As a result, the latter remains sharply focused over dozens of centimetres, so that it can accelerate electrons over a longer distance – and thus to higher energies. Another requirement is that, in order to allow a large number of experiments to be conducted at PETRA IV, the ring must be replenished with new electrons every few hours – which must happen as quickly as possible. To achieve this, the laser driving the future plasma injector needs to be able to fire around 10 to 30 high-intensity light pulses per second, depending on the amount of electrons each pulse carries. And finally, the laser-based system must prove that it can operate at the same level of reliability as the current, proven radio-frequency technology.

To study the complex interaction between the different technologies, DESY is able to draw on state-of-the-art computer simulations. New software was developed specifically for these studies. “Modelling the entire chain precisely – from the plasma accelerator to the PETRA IV storage ring – is a complex task, but crucial for such sophisticated studies,” says Maxence Thévenet, team leader for theory and simulations at DESY’s Plasma Acceleration Group. “Working in an open-source environment ensures high standards and promotes collaboration,” adds Thévenet.

In publishing the CDR, the team has now identified a number of outstanding issues and the project has entered the process of achieving technology maturity. The next milestone will be a functional demonstrator by the end of 2026. For this purpose, plasma-accelerated electrons will be fed into the existing DESY II synchrotron and later also into the PETRA III ring. The aim is to test the core components – the plasma accelerator, the energy compressor and the injection optics – under real conditions. “If we succeed in taking this step, it will mark a turning point,” says Wim Leemans, Director of the Accelerator Division at DESY. “This would be the first application of plasma technology under practical conditions.”

The current baseline design involves a conventional chain, consisting of a linear pre-accelerator and synchrotron. “A plasma injector is fully included into our plans, but not yet mature enough to forego the well-established conventional injection option,” explains Maier. “Our aim is to gather sufficient experimental evidence, as quickly as possible, so that a decision can be reached on this issue in the next year or two,” Leemans adds. A hybrid system is also conceivable: in this scenario, PETRA IV would initially use a conventional injector system, while the team gains practical experience with a plasma injector installed in parallel. When the new technology proves to be robust, efficient and reliable, the facility could switch permanently to plasma acceleration.