Chaos control theory could provide synchrotrons with opportunities for new regimes of operation with improved stability and intensity. That’s the finding of a French team of scientists from the University of Lille and the SOLEIL synchrotron facility (see image) near Paris. Clement Evain and co-workers report that the chaos control theory developed by Edward Ott, Celso Grebogi and James Yorke at the University of Maryland in the 1990s can be used to stabilize inherently unstable terahertz synchrotron radiation (Nat. Phys. https://doi.org/10.1038/s41567-019-0488-6; 2019).

Credit: Synchrotron SOLEIL - CAVOK Production - Laurent PERSIN

Synchrotrons are large-scale light sources that harness the radiation emitted by relativistic electron bunches circulating around a storage ring. Their high brightness, short pulse duration and tunable nature (photon emission energy can span from terahertz frequencies to hard X-rays) make them a very useful source of electromagnetic radiation for studies in biology, materials science and physics.

It has been known for many years that unstable microstructure patterns of electron charge density can spontaneously form within the storage ring and result in transient bursts of terahertz emission that are many orders of magnitude more powerful than usual. However, the irregularity and unstable nature of these microstructure-induced emissions have prevented their practical use.

The innovation of the French team is that they have shown that a feedback scheme inspired by chaos control can be used to stabilize these microstructures and their terahertz emission, allowing the system to operate in an otherwise unstable solution. The team used a fast bolometer (1 μs response time) to measure fluctuations in the power of the coherent terahertz radiation emitted in SOLEIL. A suitable control signal to modulate the amplitude of the radio-frequency wave injected into one of SOLEIL’s acceleration cavities is then calculated and used to modify the electron bunch length, and mitigate the bursting behaviour. The result is that within just a few milliseconds the terahertz power is stabilized. A modification in the strength of the radio-frequency signal of less than 0.3% is found to be sufficient to reduce fluctuations in the terahertz output by more than 40 dB.