Optical fiber approach diagnoses fast electrons in laser experiments

As lasers continue to improve, they have now become able to focus up to 10 quintillion watts of energy or more onto a square centimeter of a target. Intense beams often yield high-energy fast electrons, a primary product of laser-plasma experiments that provide a glimpse into laser absorption processes. Measuring fast electrons is key to understanding how laser-plasma interactions produce high-energy protons, X-rays and terahertz radiation. New work with optical fibers has provided one way of diagnosing electrons.

Liu et al. demonstrated a novel way to reliably diagnose fast electrons that escape when intense laser pulses irradiate solid targets at high repetition rates. The authors used loops of optical fibers to catch the electrons, which allowed them to measure both the number and the angular distribution of escaping electrons by analyzing the Cherenkov radiation in the fiber loops.

An increasing number of experiments employ ultra-intense lasers pulsed at frequencies of 1 hertz or faster, creating a demand for new diagnostic systems that can keep up with this repetition rate.

Experiments revealed that data from the fibers agreed well with measurements using image plate stacks, a standard approach for such experiments. The fiber loops held up well under high repetition and were insensitive to X-rays and ion beams, offering advantages over other common fast electron diagnostic techniques.

The results also indicate that the incidence angle of the electron beam onto optical fibers plays a key role in the energy selection. Next, the group plans to develop a fast electron diagnostic based on optical fibers that can make real-time and energy-resolved measurements of fast electrons’ number and angular distribution.

Source: “Cherenkov radiation-based optical fibre diagnostics of fast electrons generated in intense laser-plasma interactions,” by H. Liu, G.-Q. Liao, Y.-H. Zhang, B.-J. Zhu, Z. Zhang, Y.-T. Li, G. G. Scott, D. R. Rusby, C. Armstrong, E. Zemaityte, D. C. Carroll, S. Astbury, P. Bradford, N. C. Woolsey, P. McKenna, and D. Neely, Review of Scientific Instruments (2018). The article can be accessed at https://doi.org/10.1063/1.5024872 .