Channeling experiments on OMEGA EP

Thesis (Ph. D.)--University of Rochester. Department of Mechanical Engineering, 2015.The creation of stable, straight channels in dense plasma with high intensity laser beams is of interest to a number of applications in laser-produced plasma science. This includes the generation of MeV to GeV electron beams in laser wakefield accelerators (LWFA), coherent and brilliant X-ray generation from electron betatron radiation, and fast ignition of inertial confinement fusion targets (FI ICF). This thesis studies the creation of channels through dense preformed plasma on the OMEGA EP laser system. A new optical probing method was developed to image the channeling process. The experiments show that the channel bores faster than the ponderomotive hole boring velocity of Wilks et al. predicted by a balance of light pressure at the channel front. In the wake of the channeling pulse, a shock-driven parabolic density profile exists in the radial direction, allowing for the guiding of a subsequent high intensity pulse to an embedded steep gradient. A structure of this type is beneficial to the applications mentioned above. The results of this study can be extended to an integrated FI experiment on OMEGA, planned for the upcoming year. Current laser systems are able to create high energy density (HED) systems where the radiation-pressure in the laser field totally overwhelms the thermal pressure of the material. In this radiation-pressure dominated regime, the radiation-pressure, 2I/c, where I is the intensity of the radiation field and c the speed of light, is orders of magnitude greater than the thermal pressure, nkBT, where n is the material density and kBT is the specific energy of the system. This radiation-pressure driven flow modifies the plasma density in the region of the laser. This work presents measurements of a channel or density cavitation bored into an inhomogeneous HED plasma. A high-energy chirped-pulse amplification (CPA) laser beam (> I ∼ 1018 W/cm2) was used to drive a radiation-pressure driven blast-wave into the plasma. A 10-ps, 10-mJ, 3.6 μm resolution optical probing system was developed to image the interaction with unparalleled resolution with high spatial and temporal resolution. In a second experiment, a second, co-propagating high-intensity pulse was guided through the blast-wave prepared channel established by the first pulse. The pulse is transmitted efficiently and maintains a spatial profile close to the vacuum mode. This is most likely due to the intense heating experienced by the plasma during the channeling beam. The experimental data show a self-similar cylindrical blast-wave developing from the channeling laser. The forward velocity of the channel is measured and is quantitatively consistent with a radiation-pressure driven flow model that predicts the propagation velocity. The residual plasma in the channel is heated to multi-MeV temperatures (11,000,000,000 K) and the long time expansion (∼ 500 ps) is consistent with 2-D radiation-hydrodynamics simulations. This work shows the creation of radiation-pressure driven flows in plasma. The creation of these conditions may be scaled to a regime where the dimensionless ratio of radiation pressure to thermal pressure, 2I/nTc is >> 1. Conditions in this regime are found in physical conditions ranging from stellar interiors and radiation generated winds, to the formations of “photon bubbles” in very hot stars and accretion disks and in high-energy astrophysical environments