Transition from transparency to hole-boring in relativistic laser-solid interactions at the Texas Petawatt




Wagner, Craig Franklin

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This dissertation examines the motion of the electron critical density surface during interactions between a relativistically intense laser pulse and a solid density target. On short time scales the laser field increases the electron quiver velocity, increasing the effective electron mass which increases the plasma frequency at a given electron density. This leads to transmission of a given laser frequency deeper into the target, known as relativistic self induced transparency with the effective critical density surface propagating quickly into the target bulk. On longer time scales, the ponderomotive force from the laser leads to large light pressure which pushes a sheet of electrons into the target bulk. This leads to a strong electric potential between the electron sheet and ions that are left behind. These ions are accelerated by the potential in a process known as hole boring. Both processes are present during laser-solid interactions, but relative contributions to critical surface motion depend on the density profile of the target and the da0/dt of the laser pulse. We have conducted experiments which use the spectral shift in second and third harmonic light generated at the target surface to measure the velocity of the laser reflection point for plastic, copper, and gold targets. This data is time integrated, with the signal limited to the period of high laser intensity. The measured second harmonic spectra show a unique two peak structure which indicates the existence of two velocity regimes in the critical surface dynamics. Qualitatively, there is a small difference between the target types. In shots on gold, the signal in the high velocity peak is slightly greater than that in the low velocity peak, and this trend is reversed for plastic targets, with copper somewhere in the middle. There is more variation in the third harmonic spectrum as spectra from gold are generally high velocity dominant, copper is transitional and show both velocity peaks, and spectra from plastic targets have only the low velocity signal. This two velocity structure is explained by observing that there is a plasma density gradient in front of the solid density target on shot. This preplasma electron density profile is measured via interferometry up to 300 ps before peak intensity arrival on shot for gold and plastic targets. This data is used to evaluate contrast enhancement through adjustments to the pumping of the Texas Petawatt OPA stages, showing that a small adjustment leads to a significantly steeper plasma profile on shot. In addition, the plasma measurements taken are used to validate hydrodynamic simulations of the plasma density profile on shot. 1-D particle in cell simulations have been performed using the code EPOCH to evaluate the influence of preplasma on critical surface movement during the TPW interaction. Simulations with simplified preplasma profiles show two things. First, the cold critical density surface barely moves during the interaction. Second, for a preplasma with two scale lengths, one shallow and one steep, there is an initial phase of swift relativistic critical surface movement due to a relativistic self-induced transparency (RIT) front propagating into the plasma bulk. This is followed by a period of slower velocity hole boring. We also find that in simulations using the simulated preplasma profile, these features are also present. The simulation shows two velocity regimes which correspond to the velocity peaks seen experimentally in the time integrated spectra. Further simulations show that, for a given density profile and peak intensity, changing the pulse profile of incident laser can shift the transition between RIT and hole boring. A pulse with a longer rise time means that ions behind the laser front are less shielded by electrons bypassed in the RIT process. This means that hole boring plays a larger role in interaction with longer rise time pulses. Further explorations of the transition between RIT and hole boring at the Texas Petawatt Laser are proposed to investigate changes in front dynamics due to differing pulse rise time. Time resolved surface dynamic measurements are also proposed using an ultra-broadband GRENOUILLE which has been fielded, and is described in this thesis. Finally, initial studies on the feasibility of using laser wakefield electron accelerators to generate high fluence muon beams are described.



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