Direct Characterization of the Electronic Structure of Shocked and Heated Materials

摘要

Detailed knowledge of how materials respond to strong shocks or other extreme conditions on rapid timescales (such as laser heating) are required to support LLNL missions of national security and stockpile stewardship. This project started in FY01 to develop and demonstrate a new pump-probe characterization capability for investigating ultrafast changes in the chemical and electronic structure of materials under extreme conditions with picosecond time resolution. The LLNL COMET (Compact Multipulse Terawatt) is a compact 15 TW laser facility operating at 1054 nm wavelength, and utilizes the technique of chirped pulse amplification to produce two high power beams at a rate of 1 shot every 4 minutes. A short pulse length varied from 500 fs to 25 ps and a long 600 ps (FWHM) pulse is focused in a high intensity line focus with a traveling wave geometry to generate an intense Ni-like Pd ion 4d-4p x-ray laser (XRL) line at 14.7 nm (84.5 eV). Total energy in the two beams is of order 3-7 J to produce lasing where the peak-to-peak delay between the laser pulses is found to be optimal at 700 ps with the short pulse arriving after the long pulse. Typical COMET x-ray laser characteristics are summarized. High photon flux/shot, high monochromaticity, and picosecond pulse duration when combined with small source area and beam divergence properties of the 14.7 nm linegive ultra-high peak brightness (approx) 10(sup 24)-10(sup 25) ph. mm(sup -2) mrad(sup -2) s(sup -1) (0.1% BW)(sup -1). Overall, the 14.7 nm peak brightness is 5-6 orders of magnitude higher than 3rd generation synchrotron undulator sources. However, third generation synchrotron undulator sources still have higher average brightness of 0.5-6 x 10(sup 18) ph.mm(sup -2) mrad(sup -2) s(sup -1) (0.1% BW)(sup -1) at 50-10 nm, respectively. The technique of electron time-of-flight (e-ToF) spectroscopy requires a monochromatic, ps pulsed source for efficient x-ray laser induced photoelectron spectroscopy (PES).