  This dissertation explores the interaction between high-intensity lasers and
plasmas to accelerate electrons and produce radiation via experimental and
computational efforts. The laser pulses used in this dissertation have
ultrashort duration (< 100 fs), near-infrared to mid-infrared wavelength (0.8
$\mu$m, 2 $\mu$m, or 3.9 $\mu$m), millijoules of energy, and high repetition
rates (480 Hz or 20 Hz). The plasma sources applied are from solid-density
targets (overdense) or gaseous targets (underdense). With the
high-repetition-rate capability, statistical methods are employed to optimize
certain aspect of the experiments and to interpret the physics.
