ALP Seminar - Measuring Electron Distribution Functions with Thomson Scattering

Optical Thomson scattering has become a workhorse diagnostic to measure conditions in laser-produced plasmas that has allowed the hydrodynamic uncertainties to be isolated from laser-plasma instability physics. To better study, the kinetic effects in these systems, an angularly resolved Thomson-scattering diagnostic was invented to measure electron distribution functions over five orders of magnitude [1]. This was achieved by simultaneously probing the plasma’s linear frequency response across a range of scales, from collective to non-collective, that were determined by the angle between the Thomson-scattering probe beam and the scattering photons. At each angle, the spectrum is most sensitive to a different part of the distribution function, which allowed the angularly resolved spectrum to define a unique electron distribution function over a large dynamic range. The initial experiments measured non-Maxwellian electron distribution functions at slower velocities driven by inverse bremsstrahlung heating by laser beams and Maxwellian distribution at high electron velocities. Current experiments are expanding this concept to measure the electron distribution function along a temperature gradient to determine the electron velocities that are responsible for carrying the heat down the temperature gradient and those that maintain charge neutrality through a return current propagating up the temperature gradient.

This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856 and the Office of Fusion Energy Sciences under Award No. DESC0016253.

REFERENCES

 [1] A. L. Milder, J. Katz, R. Boni, J. P. Palastro, M. Sherlock, W. Rozmus, and D. H Froula, “Measurements of non-Maxwellian electron distribution functions and their effect on laser heating,” Physical Review Letters 127(1), 015001 (2021); https://doi.org/10.1103/PhysRevLett.127.015001