Ritwik Sain Final Defense

The initial state of a plasma driven by high-intensity laser fields is central to

the operation of diverse laser–plasma applications, including laser wakefield acceleration,

ion acceleration, coherent radiation sources, and plasma waveguide formation.

The physical processes that establish this initial configuration from the target constitute

a “preparatory phase” of the laser–plasma interaction that varies by application,

and span several orders of magnitude in time, including sub-picosecond field ionization

and heating, picosecond collisionless expansion of the laser-heated plasmas,

and nanosecond radial hydrodynamic evolution of laser-produced plasma columns.

Because the performance of the applications depends directly on this initial plasma

configuration, establishing predictive modeling frameworks for the preparatory phase

is essential for design, optimization, and reproducibility of the applications.

This thesis develops three physics-matched reduced modeling frameworks for

preparatory phase processes at the sub-picosecond, picosecond, and nanosecond timescales

respectively. The Laser-Plasma Heating and Ionization (LAPHI) framework provides

a sub-cycle resolved treatment of field ionization and the two heating channels active

in the non-relativistic laser regime, above-threshold ionization (ATI) and inverse

bremsstrahlung (IB) absorption. The self-similar solutions of heated plasma

expansion models the dynamics of a collisionless, electrostatic two-fluid system with

time-dependent heating as an input parameter, extending previous treatments to

the non-polytropic regime with electron-ion charge separation. The Plasma-neutral

Hydrodynamic Radial Evolution (PHYRE) solver is a two-fluid finite-volume treatment

for the collisionally coupled plasma–neutral system, that simulates the longer

timescale radial evolution of laser-produced plasma columns.

Applied to their respective preparatory phase problems, the three frameworks

yield structural insights. LAPHI identifies a novel IB-dominated sub-picosecond laser

produced channel formation regime. The unified model of heated plasma expansion

identifies five qualitatively distinct expansion regimes across the parameter space

of heating rate and plasma scale length, including an expanding hot electron cloud

regime and an ablation-like expansion regime hosting a novel mechanism for narrow-spread

ion features. PHYRE reveals a distinct solution structure governing the plasma

channel profiles, with unique two-fluid features that single-fluid models cannot access.

Collectively, these reduced frameworks occupy a critical role within the laser–plasma

modeling ecosystem, bridging the gap toward sequential multiscale simulation workflows

and enabling the parametric exploration of the preparatory phase of laser–plasma applications.