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Smooth Plasma Would Improve X-Ray Lasers

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Kathleen G. Tatterson

Smoothing the plasma from an x-ray laser amplifier could improve the device's efficiency by up to 50 percent, according to researchers at the Lawrence Livermore National Laboratory.

The research team, led by Robert Cauble and Luiz Da Silva, made the discovery while seeking the answer to a puzzle related to the theoretical and experimental output of x-ray lasers.

"The persistent problem is that our state-of-the-art (theoretical) calculations couldn't give an average gain that was consistent with the actual measurements," Cauble explained. "Predictions for gains were consistently about 50 percent too high. We would start with a local gain of 10 to 15 cm21 and, with losses due to refraction in propagation, estimate an average amplification of 7 or 8 cm21, but we could only measure average gains of 5 to 6 cm21."

To account for the discrepancy, the team set up a Mach-Zehnder interferometer using a neon-like yttrium laser source. "In essence, we developed the tool to measure itself," Cauble said. One Nova laser beam irradiated a 210-nm-thick yttrium target to create the probe x-ray laser beam, while another Nova beam drove the amplifier that the researchers tested (Figure 1). The system simultaneously measured the local gain and electron density with nearly 1-µm resolution.

The resulting electron density profile revealed that the plasma's lasing action was not uniform on a large scale (around 100 µm) and was highly inhomogeneous on a small scale (around 10 µm). This accounted for not only the reduced experimental gain compared with the theoretical (scientists had been calculating a uniform gain based on the assumption of smooth plasma), but also the observed lack of coherence in the x-ray laser.

The researchers will use this information to develop ways of controlling the inhomogeneity. Amplified x-ray lasers that are more coherent would result in improved performance in imaging systems, x-ray microscopy and holography of biological specimens.
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Published: November 1996
Glossary
mach-zehnder interferometer
A Mach-Zehnder interferometer is an optical device used to measure the phase difference between two collimated beams of light. It is named after the physicists Ludwig Mach and Ludwig Zehnder, who independently proposed the design in the early 20th century. The Mach-Zehnder interferometer consists of a beamsplitter, two mirrors, and two beam combiners. Here is a basic description of its components and operation: Beamsplitter: The incoming light beam is split into two beams by a...
Basic ScienceLawrence Livermore National LaboratoryLuiz Da SilvaMach-Zehnder interferometerMicroscopyResearch & TechnologyRobert CaubleTech Pulse

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