S. Maxon

Short wavelength x-ray laser research at the Lawrence Livermore National Laboratory*

Laboratory x‐ray lasers are currently being studied by researchers worldwide. This paper reviews some of the recent work carried out at Lawrence Livermore National Laboratory. Laser action has been demonstrated at wavelengths as short as 35.6 A while saturation of the small signal gain has been observed with longer wavelength schemes. Some of the most successful schemes to date have been collisionally pumped x‐ray lasers that use the thermal electron distribution within a laser‐produced plasma to excite electrons from closed shells in neon‐ and nickel‐like ions to metastable levels in the next shell. Attempts to quantify and improve the longitudinal and transverse coherence of collisionally pumped x‐ray lasers are motivated by the desire to produce sources for specific applications. Toward this goal there is a large effort underway to enhance the power output of the Ni‐like Ta x‐ray laser at 44.83 A as a source for x‐ray imaging of live cells. Improving the efficiency of x‐ray lasers in order to produce s...

X-ray laser research at the Lawrence Livermore National Laboratory Nova laser facility

We describe our optical-laser-pumped x-ray laser program. Our long-term goal is to develop and utilize a fully coherent, gigowatt-power-level sub-44-A laser. To this end we have been studying the characteristics of the exploding-foil amplifier coupled with various inversion schemes: Ne-like and Ni-like collisional excitation as well as H-like three-body recombination. Most of our experimental results to date are for the Ne-like schemes; we have observed ~15 laser transitions in Se, Y, and Mo having wavelengths from 26.3 to 10.6 nm. Output power to at least 1 MW has been observed for the Se J = 2 to 1 lines at 20.6 and 20.9 A along with geometrical divergence patterns for the beam. We have also observed time-dependent beam refraction from these amplifiers and have been able to demonstrate double-pass amplification by using a multilayer mirror operated at normal incidence. Future plans for improving beam coherence and producing lasing at wavelengths shorter than 44 A are discussed.

Two-dimensional magnetohydrodynamic calculations of the plasma focus

Two‐dimensional magnetohydrodynamic calculations on plasma focus experiments in the Mather geometry are described. The properties of the two‐dimensional numerical solutions are discussed. Detailed results are given for a specific problem which compares favorably with experiment. The numerical solution for the plasma focus previously given by Potter has been rerun with substantial disagreement. Arguments are given to show that the Potter code is incorrect.

A gas puff soft x‐ray laser target design

An x‐ray laser target to be driven by a Xe gas puff implosion powered by the Proto II machine at Sandia is proposed. The laser material is Sn pumped to the Ni‐like ionization state. The main laser transition (4d‐4p) is calculated to be near 110.5 eV.

Soft-x-ray amplification at 50.3 Å in nickellike ytterbium

Nickellike ions of ytterbium (Yb42+) have been produced in long exploding foil plasmas. The plasmas were formed by short-pulse laser irradiation of thin foils of ytterbium with line-focused high-intensity 0.53-μm light. Soft-x-ray line emissions at 50.26 and 56.09 A are identified as 4d–4p, J = 0−1 transitions in the nickellike ionization stage of ytterbium. The energy emitted in the line at 50.26 A along the axis of the plasma is observed to increase nonlinearly with plasma length, consistent with a small-signal gain of 1.2 ± 0.4 cm−1.

Cylindrical solitons in a warm, multi‐ion plasma

The soliton problem for ion acoustic waves propagating in a warm plasma consisting of a mixture of two ion species is solved for cylindrically symmetric ingoing waves. The small amplitude approximation is made and a modified Korteweg–de Vries equation is obtained. The early time solution show that the solitons for the two ion species plasma are narrower in spatial width and propagate faster than their counterpart in the single ion plasma. Also, a given initial disturbance will break up into more solitons in the multi‐ion, warm plasma than in the cold single‐ion plasma.

Radiation spectra from an imploding argon gas puff

The one‐dimensional magnetohydrodynamic equations coupled with the time‐dependent populations of the average ion model are numerically solved for an argon gas puff in a Z‐pinch configuration. Time‐dependent emission spectra are obtained and compared with experiment. There is agreement for the radiation emitted by the hot gas adjacent to the shock front and apparent disagreement for the cool, dense snowplowed gas adjacent to the outer boundary.

Estimated gains for a Ni‐like exploding foil target

It is shown that an exploding Eu foil target irradiated with 7.5×1013 W/cm2 should reach plasma parameters which look optimistic for soft x‐ray lasing. Among the lines of interest are two 4p‐4d transitions near 100 A and two near 80 A with gains estimated at 4–7 cm−1.

Photon trapping models for x-ray lasers

Optimum methods for calculating the effects of photon trapping are discussed. An efficient line-transfer algorithm that can calculate trapping when there are overlapping and interacting lines is described. Escape probability formulas are shown to be appropriate for calculating photon trapping for isolated lines and for the highest-energy line in a group of lines in many situations. Major computational savings are achieved by using cylindrical escape probabilities for recombination x-ray laser schemes. For collisional x-ray laser schemes it is shown that the calculation of line transfer in planar geometry is sufficiently fast that one only obtains substantial savings by exploiting the coarser spatial zoning that is possible when using escape probabilities in regions of steep velocity gradients. The use of escape probabilities is shown to be particularly well suited for single-zone parameter studies.

Cavity issues for Ni-like Ta x-ray laser

Lasing has been demonstrated at 44.83 angstroms in Ni-like Ta with a gain of order 3 cm-1 over a length of 3 cm. This wavelength is suitable for holography of biological specimens provided that sufficient energy is available. The duration of the x-ray laser pulse (FWHM of order 0.35 ns) allows the possibility of a second pass through the lasing medium to increase the output energy. The dependences of the output energy on the reflectivity of the multilayer mirror, the separation of the mirror from the lasing medium, and the duration of lasing are explored in this paper. The x-ray laser plasma is a strong source of x rays (energies up to 2 keV) that can damage the multilayer mirror. We examine the heating of the multilayer mirror as a function of the separation from the lasing medium. The effect of the mirror shape (radius of curvature and whether the surface is flat, concave, or convex) on the output energy and degree of coherence of the x-ray laser is given.