LBNL

The United States should build a powerful new X-ray laser that could make movies of electrons moving in materials and chemical reactions, a US government advisory panel recommended today.

In so doing, the Basic Energy Sciences Advisory Committee (BESAC), which advises the US Department of Energy’s Office of Science and met today in Bethesda, Maryland, offered no ringing endorsement of any of four proposals for future X-ray sources that had actually been presented to it. Instead, the committee set out a more ambitious vision, which panel members suggested could be realized if proponents of the various camps joined forces.

“You want to make a revolutionary machine that really stands out,” says William Barletta, an accelerator physicist at the Massachusetts Institute of Technology in Cambridge, who served on the BESAC subpanel that studied the issue. The desired machine, Barletta says, would be a ‘free electron laser’, a machine that uses magnets to wiggle an electron beam so that it emits X-rays that are coherent, or in phase with one another. The new laser’s specifications, the panel suggested, should include a fast X-ray pulse repetition rate and a large X-ray photon energy range.

The idea of a free electron laser with a high pulse repetition rate resonates strongly with a proposal from Lawrence Berkeley National Laboratory in California to build a free electron laser that uses an electron beam accelerated by superconducting magnets. That proposal, called the Next Generation Light Source (NGLS) (pictured) has already been approved in principle by the Energy Department but has received scrutiny from appropriators in the US Congress.

However, the panel’s preferred energy range is larger than what was proposed for the NGLS, instead falling within the range of another proposal, from Stanford Linear Accelerator Center, in Menlo Park, California, to upgrade the Linear Coherent Light Source (LCLS), a free electron laser that is already operating there.

Berkeley Lab director Paul Alivisatos  says that the NGLS has been through a number of descopings, aimed at bringing construction estimates below the level of US$1 billion, and that more ambitious proposals do exist to go up to near the panel’s desired level of 5 kiloelectronvolts, but at higher cost. “It’s possible to do that and we think it’s a straightforward extension of our proposal,” he says. Barletta says that the panel felt that both the NGLS and the proposed upgrade to the LCLS had demerits and strengths, and that the two labs needed to work together on a unification of their ideas.

The panel also heard from proponents of ‘ultimate storage rings’, which are similar to the X-ray synchrotrons that already exist at several US National labs but able to emit coherent X-rays, and from proponents of an Energy Recovery Linac, a linear accelerator that, like a synchrotron, would save energy by recycling its beam.

Barletta says that the key insight from studying the proposal for an ultimate storage ring is that the Energy Department should carefully review plans to upgrade existing synchrotrons to ensure that the money would not be better spent building a new ultimate storage ring instead. Several other countries are building synchrotrons more advanced than those in the United States today, including Sweden, Brazil and Japan.