Inner-shell photoionized x-ray lasers

Abstract

The inner-shell photoionized x-ray lasing scheme is an attractive method for achieving x-ray lasing at short wavelengths, via population inversion following inner-shell photoionization (ISPI). This scheme promises both a short wavelength and a short pulse source of coherent x rays with high average power. In this dissertation a very complete study of the ISPI x-ray laser scheme is done concerning target structure, filter design and lasant medium. An investigation of the rapid rise time of x-ray emission from targets heated by an ultra-short pulse high-intensity optical laser was conducted for use as the x-ray source for ISPI x-ray lasing. Lasing by this approach in C at a wavelength of 45 Å requires a short pulse (about 50 fsec) driving optical laser with an energy of 1-5 J and traveling wave optics with an accuracy of ~ 15 μm. The optical laser is incident on a high-Z target creating a high-density plasma which emits a broadband spectrum of x rays. This x-ray source is passed through a filter to eliminate the low-energy x rays. The remaining high-energy x rays preferentially photoionize inner-shell electrons resulting in a population inversion. Inner-shell photoionized x-ray lasing relies on the large energy of a K-α transition in the initially neutral lasant. The photo energy required to pump this scheme is only slightly greater than the photon energy of the lasing transition yielding a lasing scheme with high quantum efficiency. However, the overall efficiency is reduced due to low x-ray conversion efficiency and the large probability of Auger decay yielding an overall efficiency of ~ 10-7 resulting in an output energy of μJ's. They calculate that a driving laser with a pulse duration of 40 fs, a 10μm x 1 cm line focus, and an energy of 1 J gives an effective gain length product (gl) of 10 in C at 45 Å. At saturation (gl ~ 18) they expect an output of ~ 0.1 μJ per pulse. The short duration of x-ray lasing (< 100 fs) combined with a 10-Hz repetition rate (Pavg = 1μW) makes this source of coherent x rays ideal for pump-probe experiments to study fast dynamical processes in chemistry and material science.