The word “precision” doesn’t do justice to the feat achieved by one of DESY’s accelerator groups: it has managed to adjust the acceleration in a TESLA-type superconducting resonator, or cavity, to within 190 attoseconds – an attosecond is a billionth of a billionth of a second! The demonstration, which has just been reported in the journal Physical Review Accelerators and Beams, is a milestone in the control of free-electron lasers (FELs) and could give a boost especially to ultrafast pump-probe experiments at FELs.

For free-electron lasers like the European XFEL to deliver extremely short and reproducible X-ray laser pulses, the electron bunches that generate these pulses must be of outstanding quality and homogeneity. The radio-frequency fields used to accelerate the particles in the accelerator modules need to be extremely stable. Feedback systems measure the acceleration in “real time” in order to track and adjust them. A team from DESY’s Machine Beam Control (MSK) group has now taken the accuracy of these systems to a new level, making them many times more accurate than before.

The MSK team used a method known as carrier suppression, in which the fundamental frequency of the signal – in this case, the very powerful accelerating frequency of 1.3 gigahertz (GHz) – is suppressed before the measurement. This allows the (much weaker) fluctuations in this signal to be measured with greater accuracy. “You could compare it to observing the sun. If I hold up a disc to block out the sun itself, I can see its corona, or halo, in much greater detail than if I am blinded by the sun’s main rays,” says Frank Ludwig, first author of the study. In their experiment, the group actually suppressed the central signal in two stages.

The technique has been around for a very long time and has been continuously refined and applied by the group over the years. It is now so sensitive that all the components of the control unit have to be completely immobilised. “The slightest flutter, vibration or wobble of a connecting cable would destroy our measurement,” says Ludwig. Even thermal factors, such as the heat of your body when you enter the laboratory, show up immediately in the measured signals.

Following previous laboratory tests (see DESY News of 2 September 2020, “Record: DESY team measures electronic noise more precisely than ever before”), Ludwig and his colleagues Matthias Hoffmann, Uros Mavric and Heinrich Pryschelski have now succeeded in operating the system under real-life conditions for the first time. At DESY’s Cryomodule Test Bench (CMTB), they controlled a 1.3 GHz cavity with a resolution of 189 attoseconds (0.000 000 000 000 000 000 189 s). “With this, we are launching the next generation of RF control systems with attosecond resolutions, improving on current systems by a factor of more than 10,” says Holger Schlarb, head of the MSK group and last author of the publication. “In our free-electron lasers, this is particularly important at the beginning of the accelerator, where we shape and compress the particle bunch by selectively shifting the accelerating wave.”

“Our system provides the temporal resolution necessary for future experiments to be carried out with a precision in the single-digit femtosecond and even attosecond ranges,” says Ludwig. This is particularly interesting for pump-probe experiments, in which a system is first excited by a “pump” laser and then “probed” by a second beam. For comparison, the temporal stability of 190 femtoseconds corresponds to a spatial precision of less than 60 nanometres, equivalent to about 160 atomic layers of copper.

Wim Leemans, DESY Director of the Accelerator Division congratulates the team: “DESY is the place where the combination of huge instruments and highest precision leads to new innovations: This is a tremendous success which can bring X-ray lasers into the attosecond regime for even more reliable and precise investigations of the nano world.” However, applying this process not only marks a step forward in the operation of particle accelerators, it also promises a range of other applications, including higher data densities in telecommunications. Schlarb is convinced that “the technique of carrier suppression will extend the capabilities of conventional RF detectors and also lead to an enormous technological boost in future measurement technologies, and the selection and production of radio-frequency components, with broad industrial applications.”

Reference:
F. Ludwig et al., “RF controls based on carrier suppression detection with attosecond resolution”, Phys. Rev. Accel. Beams 28, 072803 (2025)
https://doi.org/10.1103/jhc3-dtzw