Smooth Plasma Would Improve X-Ray Lasers

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.