Measuring Unruh radiation from accelerated electrons
Abstract:
Detecting thermal Unruh radiation from accelerated electrons has presented a formidable challenge due not only to technical difficulties but also for lack of conceptual clarity about what is actually seen by a laboratory observer. We give a summary of the current interpretations along with a simpler heuristic description that draws on the analogy between the Unruh effect and radiation from a two-level atomic system. We propose an experiment to test whether there is emission of thermal photons from an accelerated electron.
Multimessenger measurements of the static structure of shock-compressed liquid silicon at 100 GPa
Abstract:
Multimessenger measurements of the static structure of shock-compressed liquid silicon at 100 GPa
Abstract:
The ionic structure of high-pressure, high-temperature fluids is a challenging theoretical problem with applications to planetary interiors and fusion capsules. Here we report a multimessenger platform using velocimetry and in situ angularly and spectrally resolved x-ray scattering to measure the thermodynamic conditions and ion structure factor of materials at extreme pressures. We document the pressure, density, and temperature of shocked silicon near 100GPa with uncertainties of 6%, 2%, and 20%, respectively. The measurements are sufficient to distinguish between and rule out some ion screening models.
Exploring relaxation dynamics in warm dense plasmas by tailoring non-thermal electron distributions with a free electron laser
Kinetic stability of Chapman–Enskog plasmas
Abstract:
In this paper, we investigate the kinetic stability of classical, collisional plasma – that is, plasma in which the mean-free-path λ of constituent particles is short compared with the length scale L over which fields and bulk motions in the plasma vary macroscopically, and the collision time is short compared with the evolution time. Fluid equations are typically used to describe such plasmas, since their distribution functions are close to being Maxwellian. The small deviations from the Maxwellian distribution are calculated via the Chapman–Enskog (CE) expansion in λ/L≪1, and determine macroscopic momentum and heat fluxes in the plasma. Such a calculation is only valid if the underlying CE distribution function is stable at collisionless length scales and/or time scales. We find that at sufficiently high plasma β, the CE distribution function can be subject to numerous microinstabilities across a wide range of scales. For a particular form of the CE distribution function arising in strongly magnetised plasma (viz. plasma in which the Larmor periods of particles are much smaller than collision times), we provide a detailed analytic characterisation of all significant microinstabilities, including peak growth rates and their associated wavenumbers. Of specific note is the discovery of several new microinstabilities, including one at sub-electron-Larmor scales (the ‘whisper instability’) whose growth rate in certain parameter regimes is large compared with other instabilities. Our approach enables us to construct the kinetic stability maps of classical, two-species collisional plasma in terms of λ, the electron inertial scale de and the plasma β. This work is of general consequence in emphasising the fact that high-β collisional plasmas can be kinetically unstable; for strongly magnetised CE plasmas, the condition for instability is β≳L/λ. In this situation, the determination of transport coefficients via the standard CE approach is not valid.