Photons R Us

December 5, 2008

This one is especially for my condensed matter readers, of whom I know there are at least two …

SLAC is undergoing a transformation, from particle science to photon science. This includes both catching photons (astronomy) and shooting photons (light sources). I guess I am part of the trend on the catching-photons side. A few years back, someone like me would probably not have thought of Stanford for a sabbatical; but now it seems like a happening place, with Fermi flying, the LSST camera being built and the database under design, and AGN types to hang out with, like Roger Blandford and Greg Madjewski. On the shooting-photons side, there is of course a long synchrotron science tradition, with SSRL; but now the big new thing, the world beating monster, the core of the new SLAC, is LCLS, the Linac Coherent Light Source. Today, along with other SLAC employees, I got the chance the tour the beast.

This was both depressing and inspiring. Depressing because it starts in the historic Linear Accelerator, a stark reminder of how PEP II and BaBar are dead. The BaBar kit sits there still, waiting for a bright idea. But of course visiting LCLS was also inspiring, just like it always is when visiting a huge telescope or a monumental building; you marvel at the scale, the ambition, and the amazing detail attended to throughout the vast space. Like the LHC, like the VLT, like the cathedral at Chartres, the LCLS is a triumph of human co-operation.

Electrons are accelerated to 13 GeV in an underground tunnel; then they coast through a tiny pipe four football fields long while a sequence of devices align and squeeze the beam; then into another huge tunnel with temperature controlled to a tenth of a degree, where magnets wiggle the beam by a few microns; the accelerating electrons radiate; the electric field of the emitted light bunches the electrons; and lo there is lasing … two football fields later, the electrons are pulled down into The Dump and the light sails on, where samples and detectors await their arrival. Well, free electron lasers have been made before, but not at X-ray wavelengths, not with such intensity, and not in such short bursts. The pulse time is shorter than the chemical reaction time of most molecules, so a protein wacked by the X-rays doesn’t have time to change before the diffraction image is complete.

At least thats the plan … the first electron beam run is in a few days, but the undulator magnets don’t go in until March; first light, as it were, will be next summer. (Anybody want to tell me how this all compares to Diamond ? Back in the Yookay, do we have the next great thing, or the great white elephant ?)

A cynic might watch the transformation of SLAC as the B mesons fade away, and see an organisation trying to preserve itself even when its purpose has gone.  So whats wrong with that ? Its one of the world’s great labs. It has expertise and skills in engineering, project management, computing, and accelerator physics. It has a working culture, and a proud esprit de corps. How dumb would it be to waste all that ? Today’s tours had secretaries, lab technicians and scientists, and whether they were involved in LCLS or not, you could see their chests swelling as they thought “Hey, we did this. It was us.”

Hey, it could have been different. SLAC could have become a Science and Innovation Campus while Fermilab got all the new toys…