Overdense plasma

In the case of nonrelativistic laser intensity, linear theory does not allow propagation in overdense plasmas, namely when to 1 < iop(. = e(An/rn,.) 2n,J 2. In the extreme case of ultra-relativistic laser intensity (ao 2> 1), the cutoff frequency for propagation drops from u pe down to wpe/(l Tag)1/4 [11], where ao = eA/mec is the dimensionless amplitude of the laser field. Then, in order for the propagation to occur at plasma density appreciably higher than the ordinary critical density, ao 2> 1 is needed. This is also the case of overdense thin plasma layers (as proved by simulation [12]) whose thickness exceeds the skin penetration depth of the e.m. wave. Theoretical background and basic... 

Fig. 8.1. Overdense plasma transmittivity vs. laser intensity on target for a focused Ti Sa laser pulse (30 fs, 800 nm) [4]...

The pulse shapes from the optimal control experiment provide strong support to the nanoplasma model. The optimal result can easily be interpreted in terms of a first pulse that creates an overdense plasma which subsequently undergoes expansion and a second pulse, which interacts with the plasma when the plasma frequency equals the laser frequency. 

Wilks SC, Kruer WL (1997) Absorption of ultrashort, ultra-intense laser light by solid and overdense plasmas. IEEE J Quant Electron 33 1954-1968, and references therein... 

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