Laser probe design could improve plasma characterization

A proposed laser probe could offer researchers a quicker and simpler diagnostic for plasma. The new technique would use two long pulse lasers, one monochromatic and another with a high bandwidth, that could analyze the ion acoustic resonance of plasma in a single test.

“You’ll no longer have to do a bunch of experiments. You get results in one laser shot,” said Joshua Ludwig, co-author on the new study. “And then there’s a possibility of actually getting all these variables, like electron density, ion temperature, electron temperature and even a flow velocity.”

Currently, the main technique for plasma characterization relies on Thompson scattering, which is prone to high levels of noise due to internal scattering processes. But by using two lasers, one monochromatic and the other as a high bandwidth laser with a different polarization angle, the new technique reduces the noise associated with existing techniques. Additionally, the technique of using crossed beams with increased bandwidth allows measurements to be taken in a single shot, as opposed to the multiple shots required in the past, saving computational time.

Simulating the setup with wave interaction code, the proposed technique was shown to be able to recover a plasma’s complex index of refraction, which is directly related to the particles’ distribution function and can be used to recover plasma parameters.

The new design remains to be demonstrated, but if confirmed, it could be a viable replacement to upgrade some existing instruments. Specifically, such an instrument could be widely useful in inertial confinement fusion experiments, particularly for characterizing low-density plasma.

Source: “Single shot high bandwidth laser plasma probe,” by J. D. Ludwig, P. Michel, T. Chapman, M. A. Belyaev, and W. Rozmus, Physics of Plasmas (2019). The article can be accessed at https://doi.org/10.1063/1.5124401 .