22 October at lunchtime, the first single-bunch electron beam was delivered to the 1.5 GeV storage ring at MAX IV and put to use at the FinEstBeAMS beamline. These are still preliminary trials and the response from FinEstBeAMS will determine the path forward.
Normally the electrons in the storage rings come in so-called multi-bunch formation. You could think of this as several locomotives with many wagons travelling around the ring. In single-bunch mode, there is only one locomotive “on the track”. The abstract of Christian Strålman’s PhD thesis On the Challenges for Time-of-Flight Electron Spectroscopy at Storage Rings gives a good overview of the topic in Swedish.
The single-bunch mode will give the scientists access to a wider portfolio of measurement techniques in several research areas such as atmospheric chemistry, environmental science (in particular renewable energy sources), molecular reaction dynamics, cluster chemistry and physics, materials science, chemistry–chemical reactions at surfaces or in solution and photocatalysis.
What are the challenges in producing the single-bunch electron beam?
Francis Cullinan from the Accelerator Development Group explains: “In the single-bunch mode of operation, the scientists want short regular X-ray pulses that are more separated in time than they get with a multibunch beam. It is therefore very important that there is no impurity: stray electron bunches in the ring that will emit unwanted x-rays at the wrong time. These small bunches are very difficult to detect because they are of such low charge. We, therefore, need to develop very sensitive diagnostics to show that we have a pure single bunch. This development is still ongoing.”
“Injection of a single bunch is also more challenging”, continues Francis; “We currently have two electron sources at the beginning of the LINAC, a thermionic gun and a photogun. The thermionic gun typically produces long pulses of closely-spaced electron bunches that are synchronised with the lower-frequency ring RF (100 MHz) using an RF chopper. The photogun produces single electron bunches but is not normally synchronised with the ring RF. Neither gun is therefore set up by design to deliver a single bunch to one of the rings. Today, we have used the thermionic gun with special timing settings to get as close as we can to a single bunch. We then use the bunch-by-bunch feedback system in the storage ring to kick out the unwanted bunches that seep through. In parallel, our LINAC colleagues have been working on synchronising the photogun with the RF in the storage rings so that we can use the photogun for single-bunch injections in the future. This has been demonstrated already but requires more work so that the photogun can quickly switch between injecting into one of the rings and delivering to the SPF (short pulse facility).
So, how often will single-bunch be available for scientists in the future?
“Again,” says Francis, “everything is at a very preliminary stage at the moment. So far, three eight-hour shifts have been scheduled on three different days this autumn. The goal of these trials are to test whether we can produce single bunches that are pure enough and bright enough to be useful for time-resolved experiments. If these trials are successful, the longterm plan would be to schedule single weeks in the future for single-bunch delivery to specific beamlines that have the capability of performing time-resolved experiments.
Pseudo-single-bunch mode for a 100 MHz storage ring serving soft X-ray timing experiments
Author links open overlay panelT. Olsson, S.C. Leemann, G. Georgiev, G. Paraskaki
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Volume 894, 21 June 2018, Pages 145-156 https://doi.org/10.1016/j.nima.2018.03.067
Timing Modes for the MAX IV Storage Rings Teresia Olsson, 2018, PhD thesis