Long-Pulse Laser Fires Up at Linac Coherent Light Source

September 27, 2012

A powerful new laser has fired up at SLAC and is in its final steps toward full operation. It will be an integral component of an instrument designed to study extremely dense, hot matter at levels of detail never before possible.

The laser, which produces twin beams of green light, is part of the Matter in Extreme Conditions (MEC) instrument at the Linac Coherent Light Source (LCLS), the world’s most powerful X-ray laser facility. One of two conventional lasers there, it will be used to shock samples of material to produce extreme states of matter for examination with the X-ray laser.

Known as a long-pulse or nanosecond laser, it delivers pulses that are 2-200 nanoseconds, or billionths of a second, long. Pulses of this length generate long-duration shocks that can turn samples of material into warm, dense matter or high-energy-density plasmas for study. They also generate high pressures for the study of shock effects and chemical reactions, as examples.

The laser also delivers pulses with about 50 times more energy than those of any other conventional laser at SLAC. This is four orders of magnitude more total energy than the LCLS’s X-ray pulses deliver – although the X-ray pulses pack in more photons, deeply penetrating matter and enabling atomic-scale images. When used in conjunction with the long-pulse laser, the X-ray pulses can be used to analyze matter as it is shocked into exotic states. 

An array of large-aperture optics directs the nearly 2-inch-diameter, long-pulse laser beam along a winding, mazelike path on a 25-by-4-foot table, tuning and refining its characteristics. Due to the very high energy requirements, the beam is split and directed into two glass amplifier stages – each producing as much as 60 joules of near-infrared light, which is then frequency-doubled to achieve as much as 25 joules of green, visible light from each output. One joule is roughly equivalent to the energy it takes to raise an apple a few feet in the air. 

The two green laser beams will fire through a double window into the MEC chamber and strike samples simultaneously. The long-pulse laser is so powerful that it can fire just once every 10 minutes; this allows sufficient cooling between shots to prevent damage to its components. Research is in progress on ways to speed up this rate.

"It is an extremely high-energy laser. There's nothing approaching it here at SLAC," said Steve Edstrom, a laser engineer who worked closely with laser engineer Marc Welch to lead the design, installation and testing of the long-pulse laser system. He said a computer system records and saves "various recipes" for the characteristics of the laser’s pulses, which can be recalled based on what's needed in a particular experiment. Every individual pulse of the laser will be monitored and characterized to assist in analyzing experimental results. 

MEC is the last of six instruments installed at LCLS, and is still in a commissioning process. The crew of researchers assigned to MEC is working to bring all of the instrument's components online in preparation for a U.S. Department of Energy Critical Decision 4 (CD-4) review on Oct. 16. A positive outcome in that review would formally mark the project's completion and start of regular operations. Final commissioning is expected in early 2013.

The laser crew at SLAC worked to install the long-pulse laser and bring it online over a six-week period. "We anticipated it could take two months or more to bring the laser on line, but with careful planning and vital assistance from some of SLAC’s controls and infrastructure groups, we were able to accelerate the commissioning schedule," Edstrom said. 

Working in a confined space in the MEC hutch, around all of the other activities related to preparing the entire instrument for full operation, posed a challenge, he added: "It was tough to get time, especially given the fact that the laser can only be in operation while the hutch is in a mode that requires very restricted access."

Hae Ja Lee and Bob Nagler, MEC instrument scientists, said the high-power capabilities of the MEC optical lasers, coupled with the accuracy and quality of data from its specialized detectors, make the instrument unique.

Lee noted that there are still many scientific unknowns associated with the state of matter that MEC is designed to study, and its tools create a new window for exploration and understanding.

"Word is spreading" about the capabilities at MEC, Nagler said. "I think we're going to be flooded by proposals."

In addition to the long-pulse laser, another MEC laser features pulses that can be millions of times shorter – as short as 35 femtoseconds, or quadrillionths of a second, with a power of about 150 millijoules, or thousandths of a joule. This short-pulse laser is particularly useful for experiments involving super-thin samples, as well as in creating plasmas. It can be used separately or in tandem with the long-pulse and X-ray lasers.

Earlier this year, several user experiments were conducted using the short-pulse and X-ray lasers at MEC. The long-pulse laser will be ready for the next set of user experiments, which begin in October. Five user experiments are planned from October through December – more than for any other LCLS instrument during this stage of commissioning.