Relativistic critical density increase and relaxation and high-power pulse propagation

Abstract

High-power laser pulse propagation in an overdense plasma due to the relativistic critical density increase has been investigated in one dimension. In a first step the conditions for the existence of a relativistic critical density are delimited and supported by particle-in-cell simulations. Its accurate determination is made possible by the installation of a new numerical diagnostics. Guided by this we show that the critical density increase strongly depends on both laser polarization and plasma density profile. Further, we find a new relaxation time ranging from several to many laser cycles, which sets a limit for short laser pulse manipulation and tailoring. Paramountly, it is proved that in the power optics domain the pulse propagation velocity is inhibited by the relativistic energy density in the medium and by the efficient reflection, in contrast to the group velocity from standard dispersion optics.

Transparency of near-critical density plasmas under extreme laser intensities

“…The laser field then confronts plasma densities much higher than the initial ones. Hence the RT threshold for laser field is increased [5][6][7][8][9]. Other effects like electron temperature and pulse profile could also alter the threshold by some degree [10,11].…”

Ultra-intense laser pulse propagation in plasmas: from classic hole-boring to incomplete hole-boring with relativistic transparency

“…As the interaction of a relativistically intense laser pulse with matter is the basis of phenomena such as fast ignition of controlled fusion pellets [1][2][3][4] and laser particle acceleration [5,6], it has attracted increasing attention from both the laser and the plasma communities during the last few years [7]. At the intensity I 10 18 W cm −2 , laser pulse propagation into the overdense plasma is accomplished by relativistic transparency (RT) [8][9][10] or/and by holeboring (HB) [11][12][13][14][15][16][17][18][19]. Reinforcement of these two phenomena may happen by beam selffocusing [20,21] and filamentation [22].…”

“…RT is entirely based on the fast electron response, i.e. their energy conservation [9,10], but in the HB regime the dynamics occurs on the ion time scale and hence is governed by the conservation of ion momentum. The electrostatic field induced by the ponderomotive force becomes sufficiently strong to drive a collisionless shock into the plasma and to accelerate the ions to high velocity [13][14][15][16][17][18][19]23].…”

Collisionless electrostatic shock formation and ion acceleration in intense laser interactions with near critical density plasmas

“…7(b). In the case of n e = 5n c , the laser pulse at first can penetrate into the target via the relativistic transparency, but such a penetration is highly dissipative 31 . Therefore, the laser pulse has been greatly weakened in the target.…”

“…In the first stage (t ≤ 80T 0 ), the forward velocity of the laser front v laser decreases gradually from 0.27c to 0.05c. In To understand the distinct plasma dynamics in these two stages, we display the time evolution of longitudinal electric field E x in Fig Since the initial laser-plasma conditions satisfy a = 10 > n e /n c = 5, the relativistic transparency(RT) rather than the hole-boring(HB) dominates the laser propagation at the beginning of the first stage 31 . As a result, the initial forward velocity of the laser front v laser ≃ 0.27c is much higher than the hole-boring velocity v HB ≃ I/m i n i c 3 ≃ 0.07c.…”