Abstract

X-ray lasers have been available for almost ten years with the majority still being pumped by high energy (≈ 1 kJ) modest duration (≈ 500 psec) optical lasers. The recent availability of ultrashort pulse (≈ 100 fsec) optical lasers with modest energies (≈ 1 J), has renewed the interest in x-ray laser schemes that could be pumped by these relatively inexpensive, compact lasers. In one x-ray laser scheme, the ultrashort pulse, high intensity laser ionizes the target gas via the electric field of the laser with subsequent recombination and lasing between the first excited state and the ground state. In contrast with collisional ionization, this ionization mechanism may result in nearly complete emptying of a given ionization stage and has the potential of leaving relatively cold free electrons. There is potential for lasing down to wavelengths of order 100Å, e.g. Li-like Ne at 98Å, using lasers with energies of order 1 J.¹ Lasing at significantly shorter wavelengths, e.g. Li-like Al at 52Å or H-like B at 48Å, appears to be very difficult because of excessive heating associated with Raman instabilities.² There have been recent experimental results showing possible evidence of lasing in H-like Li at 135Å.^3,4 We discuss theoretical efforts on this approach to lasing and modeling efforts to understand the recent data. A second approach to x-ray lasing using ultrashort pulse lasers is based on innershell photoionization which has the potential of reaching short wavelengths, e.g., 15Å in Ne. The indirect ionization process of this scheme, where incoherent x rays emitted from a nearby plasma ionize the innershell electron, results in higher energy requirements. For example, lasing in Ne is calculated to require a pump laser with approximately 10 J in a 100 fsec pulse. The energy requirements are significantly reduced by considering longer wavelengths, e.g., 45Å in C, but a rapid rise time (≈ 50 fsec) for the x rays is still required to minimize collisional ionization. For both approaches the use of modest energy ultrashort pulse lasers allow for much higher repetition rates than available with conventional x-ray lasers pumped by high energy laser drivers. We discuss new applications appropriate for such high-repetition-rate x-ray lasers.

© 1994 Optical Society of America