Anomalous inverse bremsstrahlung heating of laser-driven plasmas

9th International Conference on Inertial Fusion Sciences and Applications

Kundu, Mrityunjay^1

Institute for Plasma Research, Bhat, Gandhinagar, Gujarat

382 428, India^1

关键词: Amplified spontaneous emissions;    Coulomb logarithms;    Electron-ion collision;    Inverse bremsstrahlung heating;    Inverse bremsstrahlungs;    Laser intensities;    Laser plasma experiments;    Velocity-dependent;   

Others  :  https://iopscience.iop.org/article/10.1088/1742-6596/717/1/012068/pdf
DOI  :  10.1088/1742-6596/717/1/012068

来源: IOP

【 摘 要 】

Absorption of laser light in plasma via electron-ion collision (inverse bremsstrahlung) is known to decrease with the laser intensity as I0-3/2or with the electron temperature as Te-3/2where Coulomb logarithm ln Λ = 0.5ln(1 + k2min/k2max) in the expression of electron-ion collision frequency veiis assumed to be independent of ponderomotive velocity v0= E0/ω which is unjustified. Here k-1min= vth/max(ω, ωp), and k-1max= Z/v2thare maximum and minimum cut-off distances of the colliding electron from the ion, vth= √Teis its thermal velocity, ω, ωpare laser and plasma frequency. Earlier with a total velocity v = (v20+ v2th)1/2dependent ln Λ(v) it was reported that veiand corresponding fractional laser absorption (α) initially increases with increasing intensity, reaches a maximum value, and then fall according to the conventional I0-3/2scaling. This anomalous increase in veiand α may be objected due to an artifact introduced in ln Λ(v) through k-1min∝ v. Here we show similar anomalous increase of veiand α versus I0(in the low temperature and under-dense density regime) with quantum and classical kinetic models of vei without using ln Λ, but a proper choice of the total velocity dependent inverse cut-off length kmax-1∝ v2(in classical case) or kmax∝ v (in quantum case). For a given I014Wcm-2, veiversus Tealso exhibits so far unnoticed identical anomalous increase as veiversus Io, even if the conventional kmax∝ v2th, or kmax∝ vthis chosen. However, for higher Te> 15 eV, anomalous growth of vei and a disappear. The total velocity dependent kmaxin kinetic models, as proposed here, may explain anomalous increase of a with I0measured in some earlier laser-plasma experiments. This work may be important to understand collisional absorption in the under-dense pre-plasma region due to low intensity pre-pulses and amplified spontaneous emission (ASE) pedestal in the context of laser induced inertial confinement fusion.