Measuring in the femtosecond range
The determination of the duration and of the time profile of electron bunches that are highly relativistic and only a few femtoseconds long (like those produced by laser-driven particle acceleration) is an important but also a challenging task.
A multi-national team from the JAI Oxford (Bartolini, Doucas, Hooker, Perry and Reichold), Orsay Paris (Delerue) and FACET SLAC (C. Clarke, herself an Oxford alumna) have developed a technique based on the measurement of the spectral distribution of the coherent radiation emitted when the electron bunch passes close to the surface of a metallic grating; this is also known as Smith-Purcell radiation. This was one of the first four experiments approved for installation at the new FACET facility and is a significant step in the developing close collaboration between the JAI and SLAC.
The installation and commissioning took place in the second half of August, in parallel with the commissioning of the accelerator. Preliminary results, obtained with a 19.65GeV beam, indicate that the FACET bunch was about 350fs (FWHM) long; these results were presented at the IPAC Conference in early September. The analysis of the data is continuing and a second run is expected in the spring of 2012, when the FACET bunch is expected to be even shorter, down to about 130fs (FWHM). The objective of these measurements is not only to explore the details of this technique in the fs regime but also to enable the team to specify a future device that would be able to determine the bunch profile in a compact and non-destructive fashion and in a single shot. That would be something worth having!