|
Tabletop plasma-driven X-ray lasers hold the promise to overcome beamtime limitation of accelerator facilities. Such unlimited access is essential for training purposes, industrial processes as well as for going beyond the "proof-of-principle" research. Tabletop X-ray lasers are demonstrated by single-pass amplified spontaneous emission (ASE) across a plasma medium by means of transient collisional excitation. The latter is either a discharge or a laser-produced plasma obtained by target material irradiation. The homogeneity of the plasma medium is crucial to minimize refraction or self-absorption. Generation of ASE across a plasma column is dramatically improved under traveling-wave excitation (TWE). This means that a pump laser pulse irradiates a target with a certain angle of attack, instead of classical normal incidence. In the latter case hot spots and cold spots are formed. In the case of TWE a single sweep of plasma is formed in the same direction of pump pulse. The latter is optimized when the sweep propagation is close to the speed of light (i.e the ASE speed), which implies small-angle of attack. Yet, short-wavelength plasma lasing needs large-angle target irradiation, in order to increase the pump penetration into the denser plasma core. High electron density is needed for high collisional pumping rates. The apparent tradeoff large angle (for short wavelength) vs small angle (for efficient gain) was solved by pulse shaping, i.e. pulse-front back-tilt. In fact, the TWE speed depends on the pulse-front slope (envelope of propagation front), whereas the optical penetration depth depends on the wavefront slope (envelope of phase). Pulse-front tilt was accomplished by means of pulse compressor misalignment ("easing"). The latter is the final stage of a chirped-pulse amplification (CPA) system that recompresses the stretched pulse. In this study it was found effective to use the easing of the compressor, only if coupled with a high-magnification frontend imaging/focusing component, and that higher order terms are marginal. It is concluded that sweep speed matching should be accomplished with minimal compressor misalignment of a few degrees and maximal imaging magnification. A complete computational study by means of Fourier Optics was performed and validated experimentally.
|
