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Register a new userMany-body collision contributions to electron momentum damping rates in a plasma influenced by electron strong coupling
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Experimental studies of electron-ion collision rates in an ultracold neutral plasma (UNP) can be conducted through measuring the rate of electron plasma oscillation damping. For sufficiently cold and dense conditions where strong coupling influences are important, the measured damping rate was faster by 37% than theoretical expectations [W. Chen, C. Witte, and J. Roberts, Phys. Rev. E textbf{96}, 013203 (2017)]. We have conducted a series of numerical simulations to isolate the primary source of this difference. By analyzing the distribution of electron velocity changes due to collisions in a molecular dynamics simulation, examining the trajectory of electrons with high deflection angle in such simulations, and examining the oscillation damping rate while varying the ratio of two-body to three-body electron-ion collision rates, we have found that the difference is consistent with the effect due to many-body collisions leading to bound electrons. This has implications for other electron-ion collision related transport properties in addition to electron oscillation damping.
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Collisional effects can play an essential role in the dynamics of plasma waves by setting a minimum damping rate and by interfering with wave-particle resonances. Kinetic simulations of the effects of electron-ion pitch angle scattering on Electron Plasma Waves (EPWs) are presented here. In particular, the effects of such collisions on the frequency and damping of small-amplitude EPWs for a range of collision rates and wave phase velocities are computed and compared with theory. Both the Vlasov simulations and linear kinetic theory find the direct contribution of electron-ion collisions to wave damping is about a factor of two smaller than is obtained from linearized fluid theory. To our knowledge, this simple result has not been published before. Simulations have been carried out using a grid-based (Vlasov) approach, based on a high-order conservative finite difference method for discretizing the Fokker-Planck equation describing the evolution of the electron distribution function. Details of the implementation of the collision operator within this framework are presented. Such a grid-based approach, which is not subject to numerical noise, is of particular interest for the accurate measurements of the wave damping rates.
In this paper we have criticized the so-called Landau damping theory. We have analyzed solutions of the standard dispersion equations for longitudinal (electric) and transversal (electromagnetic and electron) waves in half-infinite slab of the uniform collisionless plasmas with non-Maxwellian and Maxwellian-like electron energy distribution functions. One considered the most typical cases of both the delta-function type distribution function (the plasma stream with monochromatic electrons) and distribution functions, different from Maxwellian ones as with a surplus as well as with a shortage in the Maxwellian distribution function tail. It is shown that there are present for the considered cases both collisionless damping and also non-damping electron waves even in the case of non-Maxwellian distribution function.
We propose a new method for self-injection of high-quality electron bunches in the plasma wakefield structure in the blowout regime utilizing a flying focus produced by a drive-beam with an energy-chirp. In a flying focus the speed of the density centroid of the drive bunch can be superluminal or subluminal by utilizing the chromatic dependence of the focusing optics. We first derive the focal velocity and the characteristic length of the focal spot in terms of the focal length and an energy chirp. We then demonstrate using multi-dimensional particle-in-cell simulations that a wake driven by a superluminally propagating flying focus of an electron beam can generate GeV-level electron bunches with ultra-low normalized slice emittance ($sim$30 nm rad), high current ($sim$ 17 kA), low slice energy-spread ($sim$0.1%) and therefore high normalized brightness ($>10^{19}$ A/rad$^2$/m$^2$) in a plasma of density $sim10^{19}$ cm$^{-3}$. The injection process is highly controllable and tunable by changing the focal velocity and shaping the drive beam current. Near-term experiments using the new FACET II beam could potentially produce beams with brightness exceeding $10^{20}$ A/rad$^2$/m$^2$.
Ultracold plasmas (UCP) provide a well-controlled system for studying multiple aspects in plasma physics that include collisions and strong coupling effects. By applying a short electric field pulse to a UCP, a plasma electron center-of-mass (CM) oscillation can be initiated. In accessible parameter ranges, the damping rate of this oscillation is determined by the electron-ion collision rate. We performed measurements of the oscillation damping rate with such parameters and compared the measured rates to both a molecular dynamic (MD) simulation that includes strong coupling effects and to Monte-Carlo collisional operator simulation designed to predict the damping rate including only weak coupling considerations. We found agreement between experimentally measured damping rate and the MD result. This agreement did require including the influence of a previously unreported UCP heating mechanism whereby the presence of a DC electric field during ionization increased the electron temperature, but estimations and simulations indicate that such a heating mechanism should be present for our parameters. The measured damping rate at our coldest electron temperature conditions was much faster than the weak coupling prediction obtained from the Monte-Carlo operator simulation, which indicates the presence of significant strong coupling influence. The density averaged electron strong coupling parameter $Gamma$ measured at our coldest electron temperature conditions was 0.35.
In the recent years, the hydrofluoroolefine HFO1234ze(E) (C3H2F4) has been subject to a growing interest for refrigerant applications. This novel gas is also considered a promising alternative in gaseous particle detectors and gaseous voltage insulation devices, thanks to its low global warming potential. In this contribution, measurements of electron transport coefficients and reaction rates in C3H2F4 are analyzed by an iterative procedure. A complete set of scattering cross sections for electrons in C3H2F4 has been obtained and validated by a systematic comparison of electron swarm parameters with experimental measurements in pure C3H2F4 as well as in gas mixtures of C3H2F4/CO2. Since the electron attachment of C3H2F4 is strongly influenced by the gas density, accurate calculations of the critical electric field strength in such mixtures are also reported in this paper.
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