Rosemary S. Walling

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...

Observation of soft x‐ray amplification in neonlike molybdenum

Thin molybdenum coated foils have been irradiated in line focus geometry with from 3 to 8×1014 W cm−2 of 0.53‐μm light at the Nova laser. The resulting exploding foil plasma has demonstrated x‐ray laser gain at four wavelengths (106.4, 131.0, 132.7, and 139.4 A), identified as 3s‐3p transitions in neonlike Mo. The J=0–1, a 3s–3p transition at 141.6 A has been identified, but does not show evidence of significant gain in disagreement with the theory.

The "nuclear car wash": a scanner to detect illicit special nuclear material in cargo containers

There is an urgent need to improve the reliability of screening cargo containers for illicit nuclear material that may be hidden there for terrorist purposes. A screening system is described for the detection of fissionable material hidden in maritime cargo containers. The system makes use of a low-intensity neutron beam for producing fission and the detection of the abundant high-energy /spl gamma/ rays emitted in the /spl beta/-decay of short-lived fission products and /spl beta/-delayed neutrons. The abundance of the delayed /spl gamma/ rays is almost an order of magnitude larger than that of the delayed neutrons normally used to detect fission, and they are emitted on about the same time scale as the delayed neutrons, i.e., /spl sim/1 min. The energy and temporal distributions of the delayed /spl gamma/ rays provide a unique signature of fission. Because of their high energy, these delayed /spl gamma/ rays penetrate low-Z cargoes much more readily than the delayed neutrons. Coupled with their higher abundance, the signal from the delayed /spl gamma/ rays escaping from the container is predicted to be as much as six decades more intense than the delayed neutron signal, depending upon the type and thickness of the intervening cargo. The /spl gamma/ rays are detected in a large array of scintillators located along the sides of the container as it is moved through them. Measurements have confirmed the signal strength in somewhat idealized experiments and have also identified one interference when 14.5-MeV neutrons from the D, T reaction are used for the interrogation. The interference can be removed easily by the appropriate choice of the neutron source.

Scaling of neonlike lasers using exploding foil targets

We present a set of calculations for laser-gain predictions in a neonlike collisional excitation scheme using laser-driven exploding foil targets. The calculation includes three steps: the ionization balance, the neonlike excited-state kinetics, and the hydrodynamics of the exploding foil target. The ionization-balance model solves steady-state rate equations, including excited states, using scaled hydrogenic atomic physics. The model for the neonlike excited-state kinetics is also steady state and includes the ground state and the 36 n = 3 excited states, with radiative and collisional transitions connecting these states. The plasma conditions in the exploding foil targets are calculated by using the similarity model of London and Rosen [ Phys. Fluids29, 3813 ( 1986)]. For selected elements in the range 20 < Z < 56, we predict the gain for the two most prominent 2p53p to 2p53s (J = 2–1) transitions seen in experiments, the plasma conditions necessary to maximize the gain, and the specifications for the laser driver and target required to reach those plasma conditions. Our predicted gains are larger than those measured in experiments, for reasons we discuss, but our calculations agree qualitatively with the observed trends; the gain peaks for elements around selenium and falls off for both lighter and heavier ions. Neglected effects, such as time-dependent kinetics and radiation trapping, are also discussed.

Spectroscopic investigations of hard x-ray emission from 120-ps laser-produced plasmas at intensities near 10(17) W cm(-2)

Spectroscopic investigations of the x-ray emission of plasmas heated by 120 ps, frequency doubled pulses from the JANUS Nd:glass laser are presented. High Z K-shell spectra emitted from slab targets heated to near 1017 W cm-2 intensity are investigated. High resolution ((lambda) /

Reduced conduction cooling in high energy-density plasmas using ultrashort pulse laser heated thin foil targets

DELTA(lambda) > 5000) x-ray spectra of multicharged ions of He-like Ti, Co, Ni, Cu, and also H-like Sc in the spectral range 1.5-3.0 angstrom are obtained in single laser shots using a spherically bent Mica crystal spectrograph with a 186 mm radius of curvature. The spectra have 1D spatial resolution of about 25 micrometers and indicate that the size of the emission zone of the resonance transitions is < 25 micrometers . Simultaneous x-ray images of the plasma from a charge-coupled device pinhole camera confirmed that the plasma x-ray emission is from a similar sized source. Survey spectra ((lambda) /(Delta) (lambda) equals 500 - 1000) taken with a flat LiF (200) crystal spectrometer with a charge-coupled device detector complement the high resolution data. 2D LASNEX modeling of the laser target conditions indicate that the high K-shell charge states are produced in the hot dense region of the plasma with electron temperature > keV and density approximately 1022 cm-3. These experiments demonstrate that with modest laser energy, plasmas heated by high-intensity 120 ps lasers provide a very bright source of hard approximately 8 keV x-ray emission.

Ionization balance and gain calculations for neon-like selenium x-ray laser plasmas

Abstract We have conducted experiments to determine the effects of target thickness on the time history and spectra from short pulse laser produced plasmas. A reduced contribution to conduction cooling was achieved by using thin targets. The experiment was done using aluminum foils of thickness ranging from 250 A to 1250 A. The foils were heated with a 400 nm, 150 fs (FWHM), 250 mJ laser focused to a spot size of 2 μm, resulting in a peak intensity of 5.3 × 1019 W/cm2. The ls2-ls2p transition in He-like aluminum was temporally resolved using a 900 fs x-ray streak camera. An analytic model is derived to describe the time rate of change of the ls2-ls2p transition in He-like aluminum.

Characterization of germanium stripe x-ray lasers

Abstract We have developed compact collisional-radiative models to describe the ionization balance and excitation mechanisms in neon-like selenium x-ray laser plasmas. These models can be used for calculations of the ionization dynamics, detailed emission spectra, and gain coefficients. Careful attention has been paid to indirect processes such as dielectronic recombination, excitation-autoionization, and resonant excitation. We discuss the importance of different atomic processes and model approximations in the ionization balance and gain calculations. These results will be compared to experimental measurements and to previous calculations.

Ray and wave optics modeling of laboratory x-ray lasers

One method of improving the transverse spatial coherence of x-ray lasers (XRLs) is by adaptive spatial filtering of XRL apertures using geometric shaping in the form of bowtie or wedge XRLs. However, we must maintain the desired geometric shapes in exploding foil or slab configurations during the lasing period. As a first step toward understanding lasing in such geometries, we study the behavior of simple stripe XRLs. Past experience with stripe XRLs deposited on thick plastic substrates resulted in significantly weaker laser intensities as compared to line-focused slab XRLs. Possible reasons for this intensity reduction of stripe XRLs could include mixing at the laser boundary, and changes in plasma, kinetics, and hydrodynamic properties that affect laser gains and propagation. We present experimental and theoretical characterizations of germanium line-focused slab and stripe XRLs. Key experimental parameters we are studying include images of emission profiles of the laser blowoff, angular divergences, XRL output intensities, and ionization balances as we vary XRL designs. We compare the experimental results with 2-D laser deposition and hydrodynamics simulations using LASNEX, and study the changes in ionization balances and level populations from postprocessing LASNEX results.

Characterization of short pulse laser-produced plasmas

Much progress has been made recently in characterizing the emission from neon- like Yttrium exploding foil x-ray lasers. Concomitant with that effort, we have carried out detailed modeling to enhance our understanding of the experiments and improve their design. Our modeling includes target hydrodynamics, calculation of gain, and both ray and wave optics propagation. We will describe our modeling of Yt x-ray lasers, including first simulations using a two transverse dimensional gain calculation. Our calculations indicate that the time-integrated signal is very sensitive to the time history of the gain, because of the rapid sweep of the beam in angle measured with respect to the plane of the foil.