Laser beat-wave accelerator and plasma noise

Abstract

Plasma noise effects on the beat-wave resonance condition and the subsequent reduction of the accelerating electric field for the laser beat-wave accelerator are investigated. Plasma noise or turbulence causes the accelerating electric field to saturate at a reduced level ε=$L_{\mathrm{eff}/\mathrm{L}}$ of the ideal coherent accelerating field, where $L_{\mathrm{eff}}$ is the effective growth length for the plasma wave and L is the ideal growth length. When the noise parameter σ=〈(δn${)}^2$〉/$n^2$ is above a threshold value, we find $L_{\mathrm{eff}\mathrm{\simeq }2\mathrm{c}/{\omega }_p^2{\tau }_c\mathrm{\sigma }}$, where ${\tau }_c$ is the correlation time of the noise spectrum along ω=${\mathrm{kv}}_p$. Slower noise is less effective in limiting the plasma wave growth. For strong noise $L_{\mathrm{eff}}$ becomes shortened to the correlation length in the noise $L_c$=$v_p$ ${\tau }_c$. In the case of slowly increasing mismatch in the beat-wave resonance condition, the effective growth length $L_{\mathrm{eff}}$ becomes the geometric mean of the density gradient scale length $L_n$ and the collisionless skin depth ($L_n$c/${\omega }_p$ ${)}^{1/2}$. .AE