Ultrafast work at SPPS

This week’s item is inspired by my recent visit to Stanford, in anticipation of the world’s first x-ray free electron laser – LCLS, which will open in 2009. LCLS is a facility that will feature ultrashort (200 fs) x-ray pulses that are fully coherent in transverse directions, and will be using one third of a 3-km linear accelerator as the source of electrons that will be used for “lasing” in a total of about 100m of undulator. In comparison, current third generation hard x-ray sources like APS, ESRF or SPRING-8 use undulators that are only a couple of meters long, and pulses that are about 1,000 times longer – on the order of 100 ps.

In preparation to LCLS, scientists built a temporary facility called Sub-Picosecond Pulse Source (SPPS). It used SLAC’s LINAC electron beam outfitted with a relatively short undulator. The total flux was nowhere near projected LCLS values or even sources like APS – in fact (as reported here previously) when LCLS becomes operational it will produce more x-rays in the first 10 seconds than SPPS produced in its entire 3-year lifetime. But SPPS was a unique source of sub-ps x-rays, and produced some really interesting science. One such result was featured in Science earlier this year: a report by D. Fritz et al., includes almost 40 collaborators from 20 different institutions.

The basic idea of experiment is pump-probe – hitting Bi crystal with a ultrafast powerful laser and use x-rays to probe the lattice structure some short time later. Because both laser and x-ray pulses are sub-ps, one can in principle shoot a movie of structural lattice dynamics in response to the initial laser pulse, with sub-ps time resolution – something other x-ray sources currently cannot do. But in order to do this, one has to be able to vary the delay between laser and x-ray pulse – which was problematic at SPPS due to jitter, which results in random delay between the two pulses. However, using a clever time-tagging technique, the jitter was used to the experiment’s advantage – by figuring out the time delay between the two pulses that resulted from jitter for each pulse, scientists were able to use jitter to scan the time delay – even though the time of arrival is highly random, this randomness provides a way for varying the delay.

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