Optical Probing of CO2 Laser-Plasma Interactions at Near Critical Density
- Author(s): Gong, Chao
- Advisor(s): Joshi, Chandra
- Mori, Warren
- et al.
The interaction of a high-power laser beam with plasma has been explored extensively in the context of laser-driven fusion, plasma-based acceleration of ions and electrons and high energy-density physics. One of the fundamental processes common to all these studies is the penetration of intense light into a dense matter through the hole boring effect and self-induced transparency. Light with a given wavelength will be reflected once the electron density equals the critical electron plasma density . The radiation pressure exerted on the critical density layer is characterized by the ponderomotive force of a focused laser pulse which scales with a laser intensity, as Wμm2/cm2. At ~1017 Wμm2/cm2 and above, it becomes possible for the laser pulse not only to steepen the plasma profile but to push the overcritical plasma with creating a cavity or a hole in the target. The phenomenon of hole boring, whereby a laser pulse propagates through a reduced density cavity to reach and push the critical density layer, is of importance in fast-ignition fusion because it may allow the laser pulse to deliver its energy closer to the compressed fuel where it can be converted into fast electrons that are needed to ignite a small portion of the fuel. The layer of plasma pushed by the radiation pressure can reflect and accelerate ions via the so called Hole Boring Radiation Pressure Acceleration mechanism. Also the density pile- up in combination with the strong electron heating at the critical density layer can facilitate the formation of a collisionless shock. This shock wave acceleration can produce high energy ion beams with a narrow energy spread.
Numerous experiments have been carried out to study dynamics of laser plasma interaction indirectly using solid state targets that are opaque for 1μm laser. However, by using a longer wavelength CO2 laser, = 10.6μm, the critical plasma density is decreased by a factor of 100 ( ). Therefore overdense CO2 laser plasma is transparent for a fast optical probe which opens a unique opportunity to study the physics of laser hole boring in the temporal and spatial domain simultaneously. This thesis reports the first direct measurements of the laser hole boring velocity in an overdense plasma using a four-frame picosecond green pulse interferometry.
The plasma was created by tunnel ionization of a plume of He gas from a gas jet using a train of 3ps long CO2 laser pulses. For probing the plasma, a ~2ps frequency-doubled (532nm) Nd:Glass laser pulse is used in a four-frame interferometry scheme. Spatio-temporal dynamics of hole boring in CO2 laser-plasma interaction at 2×1018 Wμm2/cm2 are studied with a ~15µm spatial and better than 2ps time resolutions. Experimental measurements indicate the hole boring process is determined by a balance between the radiation pressure and thermal pressure . As a result of this competition, increases during the risetime of the laser pulse where but decreases with a larger slope rate on the falling edge. This is because the plasma electrons once heated cool slowly and diminishes the efficacy of even faster.