X-ray lasers driven by optical lasers

Recent topics in the optical-laser-driven x-ray lasers are reviewed. With the collisional excitation x-ray lasers, pumping laser energy has been reduced over 100 times by reducing the pumping pulse width from ns to ps. A high gain of 30-40 cm-1 has been achieved with the transient collisional excitation using ps pumping. A new scheme for a charge exchange x-ray laser using clusters such as C60 irradiated with ultra short laser pulses has been introduced. In the inner shell ionization x-ray lasers, creation of a population inversion utilizing the difference in the Coster-Kronig decay rates between the upper and lower laser levels has been proposed as a scheme to enable pumping with short bursts of electrons. High gain over long duration is expected with hollow atoms created under intense broad-band x-ray irradiation emitted from oscillating electrons under relativistic intensities (Larmor radiation). The x-ray laser program at the Advanced Photon Research Center at the new site in Kyoto is presented