The effects of ionization potential depression on the spectra emitted by hot dense aluminium plasmas

Recent experiments at the Linac Coherent Light Source (LCLS) X-ray Free-Electron-Laser (FEL) have demonstrated that the standard model used for simulating ionization potential depression (IPD) in a plasma (the Stewart–Pyatt (SP) model, J.C. Stewart and K.D. Pyatt Jr., Astrophysical Journal 144 (1966) 1203) considerably underestimates the degree of IPD in a solid density aluminium plasma at temperatures up to 200 eV. In contrast, good agreement with the experimental data was found by use of a modified Ecker–Kröll (mEK) model (G. Ecker and W. Kröll, Physics of Fluids 6 (1963) 62–69). We present here detailed simulations, using the FLYCHK code, of the predicted spectra from hot dense, hydrogenic and helium-like aluminium plasmas ranging in densities from 0.1 to 4 times solid density, and at temperatures up to 1000 eV. Importantly, we find that the greater IPDs predicted by the mEK model result in the loss of the n = 3 states for the hydrogenic ions for all densities above ≈0.8 times solid density, and for the helium-like ions above ≈0.65 solid density. Therefore, we posit that if the mEK model holds at these higher temperatures, the temperature of solid density highly-charged aluminium plasmas cannot be determined by using spectral features associated with the n = 3 principal quantum number, and propose a re-evaluation of previous experimental data where high densities have been inferred from the spectra, and the SP model has been used.     

Isochoric heating of reduced mass targets by ultra-intense laser produced relativistic electrons

We present measurements of the chlorine K-alpha emission from reduced mass targets, irradiated with ultra-high intensity laser pulses. Chlorinated plastic targets with diameters down to 50 μm and mass of a few 10^?8 g were irradiated with up to 7 J of laser energy focused to intensities of several 10¹⁹ W/cm². The conversion of laser energy to K-alpha radiation is measured, and high-resolution spectra that allow observation of line shifts are observed, indicating isochoric heating of the target up to 18 eV. A zero-dimensional 2-temperature equilibration model, combined with electron impact K-shell ionization and post processed spectra from collisional radiative calculations reproduces the observed K-alpha yields and line shifts, and shows the importance of target expansion due to the hot electron pressure.     

X-ray absorption spectroscopy for wire-array Z-pinches at the non-radiative stage

Absorption spectroscopy was applied to wire-array Z-pinches on the 1 MA pulsed-power Zebra generator at the Nevada Terawatt Facility (NTF). The 50 TW Leopard laser was coupled with the Zebra generator for X-ray backlighting of wire arrays at the ablation stage. Broadband X-ray emission from a laser-produced Sm plasma was used to backlight Al star wire arrays in the range of 7–9 Å. Two time-integrated X-ray conical spectrometers recorded reference and absorption spectra. The spectrometers were shielded from the bright Z-pinch X-ray burst by collimators. The comparison of plasma-transmitted spectra with reference spectra indicates absorption lines in the range of 8.1–8.4 Å. Analysis of Al K-shell absorption spectra with detailed atomic kinetics models shows a distribution of electron temperature in the range of 10–30 eV that was fitted with an effective two-temperature model. Temperature and density distributions in wire-array plasma were simulated with a three-dimension magneto-hydrodynamic code. Post-processing of this code’s output yields synthetic transmission spectrum which is in general agreement with the data.     

Fluctuations and the ionization potential in dense plasmas

A semi-analytic model is developed to estimate continuum lowering in dense plasmas including fluctuations. The model is applied to aluminum and compared with recent experiments at the Linac Coherent Light Source [O. Ciricosta et al., Phys. Rev. Lett. 109 (2012) 065002] that reported the ionization potential depression of K-shell electrons in solid density aluminum at temperatures up to 180 eV. The analysis suggests fluctuations, which are neglected in most continuum lowering models but are essential to describe energy absorption by a system, are sufficiently large to impact the interpretation of the experimental results.     

Measurement and simulations of hollow atom X-ray spectra of solid-density relativistic plasma created by high-contrast PW optical laser pulses

K-shell spectra of solid Al excited by petawatt picosecond laser pulses have been investigated at the Vulcan PW facility. Laser pulses of ultrahigh contrast with an energy of 160 J on the target allow studies of interactions between the laser field and solid state matter at 10²⁰ W/cm². Intense X-ray emission of KK hollow atoms (atoms without n = 1 electrons) from thin aluminum foils is observed from optical laser plasma for the first time. Specifically for 1.5 μm thin foil targets the hollow atom yield dominates the resonance line emission. It is suggested that the hollow atoms are predominantly excited by the impact of X-ray photons generated by radiation friction to fast electron currents in solid-density plasma due to Thomson scattering and bremsstrahlung in the transverse plasma fields. Numerical simulations of Al hollow atom spectra using the ATOMIC code confirm that the impact of keV photons dominates the atom ionization. Our estimates demonstrate that solid-density plasma generated by relativistic optical laser pulses provide the source of a polychromatic keV range X-ray field of 10¹⁸ W/cm² intensity, and allows the study of excited matter in the radiation-dominated regime. High-resolution X-ray spectroscopy of hollow atom radiation is found to be a powerful tool to study the properties of high-energy density plasma created by intense X-ray radiation.     

Probing of laser-irradiated solid targets using coherent extreme ultra-violet radiation

The diagnostic potential of extreme ultraviolet (EUV) coherent probing within a laser produced plasma is investigated. A fluid code is used to model the interaction of a 35 fs, 2 × 10¹⁴ Wcm^?2 800 nm laser pulse with an 800 nm thick aluminium target. A post processor is used to calculate the refractive index and transmission to 45 eV radiation of the target. The effects of EUV radial phase variations at the rear of the target on the intensity distribution at a detector 1.5 m from the target are studied. An irradiated aluminium target is found to have little effect on the transmission of 45 eV radiation, however, there are significant phase retardation differences of the probing beam in the radial direction. These phase variations affect the subsequent propagation of the radiation, suggesting that a simple diagnostic that measures the far-field footprint of the coherent EUV radiation passing through an irradiated target is sensitive to radial variations of the target heating. Sample calculated footprint variations associated with a drop in laser absorption to an irradiance of 10¹⁴ Wcm^?2 at a radius from the focal centre of 50 μm are shown.     

Hard X-ray spectroscopy of inner-shell K transitions generated by MeV electron propagation from intense picosecond laser focal spots

The propagation of energetic electrons from the focal spots of intense picosecond laser pulses was studied using targets consisting of planar foils and fine metal wires. High-resolution K-shell spectra of elements with atomic numbers in the range 46–74 (Pd to W) and with energies from 21 keV to 69 keV were recorded by a Cauchois-type spectrometer using a curved transmission crystal. The K-shell spectra resulted from the collisional ionization of 1 s electrons by energetic electrons that were generated in the laser focal spot and propagated into the planar foil region beyond the focal spot or into the metal wires adjacent to an irradiated wire. The lateral spread of the energetic electrons from the focal spot was determined from the source broadening of the K spectral lines and from the relative intensities of the K spectra from an irradiated wire and neighboring wires of different metals. The propagation distances up to 1 mm in a variety of materials indicated electron energies up to 1 MeV were generated in the laser focal spot. Inhibited propagation in an electrically insulating material was observed that results from a weak return current and incomplete space charge neutralization.     

High-resolution (∼0.05%) red shift of a ∼ 60 keV Kβ line upon ionization

A recent measurement [1] demonstrates that iridium's Kα₂-line, centered at ?63286.96 eV for a cold atom, increases ?+10 eV in energy when it is emitted by a modestly (～17×) ionized plasma. This measurement, enabled by a near-coincident lutetium K-edge filter, agrees well with atomic physics computations. Not understood at the time was a similar measurement with a thulium filter at the ?59370 eV energy of ytterbium's Kβ₁ line, which indicated that its photon energy decreases with ionization. The computation reported here shows that the ionization energy shift for Yb's Kβ lines is indeed negative and agrees qualitatively with the measurements. For the K-lines the ionization energy shift may be most interesting in atomic physics, while for the L-lines the ionization energy shift is a promising plasma diagnostic [2].     

Tungsten L transition line shapes and energy shifts resulting from ionization in warm dense matter

Spectra of the W L transitions in the energy range 8–12 keV from warm dense plasmas generated by the Naval Research Laboratory's Gamble II pulsed power machine were recorded by a newly developed high-resolution transmission-crystal X-ray spectrometer with ±2 eV accuracy. The discharges have up to 2 MV voltage, 0.5 MA current, and produce up to 2.4 MJ/cm^?3 energy density. The plasma-filled rod pinch (PFRP) diode produces a plasma with N_e ≈ 10²² cm^?3 and T_e ≈ 50 eV during the time of maximum X-ray emission. By analyzing the line shapes, it was determined that the Lβ₂ inner-shell transition from the 4d_5/2 level was shifted to higher energy by up to 23 eV relative to nearby Lβ transitions from n = 3 levels. In addition, the Lβ₂ transition was significantly broader and asymmetric compared to the n = 3 transitions. The energy shift of the Lβ₂ transition results from the ionization of electrons outside the 4d shell that perturbs the transition energies in the ions to higher values. The increased line width and asymmetry result from unresolved transitions from a range of ionization states up to +28. The ionization distribution was determined by comparison of the measured energy shifts and widths to calculated transition energies in W ions, and the ionization was correlated with Gamble discharge parameters such as the anode type and the high voltage delay time. This work demonstrates a new hard X-ray spectroscopic diagnostic technique for the direct measurement of the ionization distribution in warm dense plasmas of the heavy elements W through U that is independent of the other plasma parameters and does not require interpretation by hydrodynamic, atomic kinetics, and radiative simulation codes.     

Two-dimensional radiation MHD modeling assessment of designs for argon gas puff distributions for future experiments on the refurbished Z machine

Argon Z-pinch experiments are to be performed on the refurbished Z machine (which we will refer to as ZR here in order to distinguish between pre-refurbishment Z) at Sandia National Laboratories with a new 8 cm diameter double-annulus gas puff nozzle constructed by Alameda Applied Sciences Corporation (AASC). The gas exits the nozzle from an outer and inner annulus and a central jet. The amount of gas present in each region can be varied. Here a two-dimensional radiation MHD (2DRMHD) model, MACH2-TCRE, with tabular collisional radiative equilibrium atomic kinetics is used to theoretically investigate stability and K-shell emission properties of several measured (interferometry) initial gas distributions emanating from this new nozzle. Of particular interest is to facilitate that the distributions employed in future experiments have stability and K-shell emission properties that are at least as good as the Titan nozzle generated distribution that was successfully fielded in earlier experiments on the Z machine before it underwent refurbishment. The model incorporates a self-consistent calculation for non-local thermodynamic equilibrium kinetics and ray-trace based radiation transport. This level of detail is necessary in order to model opacity effects, non-local radiation effects, and the high temperature state of K-shell emitting Z-pinch loads. Comparisons of radiation properties and stability of measured AASC gas profiles are made with that of the distribution used in the pre-refurbished Z experiments. Based on these comparisons, an optimal K-shell emission producing initial gas distribution is determined from among the AASC nozzle measured distributions and predictions are made for K-shell yields attainable from future ZR experiments.     

Intense ultrashort laser–Xe cluster interaction

The last several years have witnessed a surge of activity involving the interaction of clusters with intense ultrashort pulse lasers. The interest in laser–cluster interaction has not been only of academic interest, but also because of the wide variety of potential applications. Clusters can be used as a compact source of X-rays, incoherent as well as coherent, and of fast ions capable of driving a fusion reaction in deuterium plasmas. In one set of xenon cluster experiments, in particular, amplification of ～2.8 Å X-rays has been observed [28]. X-ray amplification in cluster media is a phenomenon of critical importance and may lead to applications such as EUV lithography, EUV and X-ray microscopy, X-ray tomography, and variety of applications in biology and material sciences. However, while amplification of ～2.8 Å X-rays has been documented in experiments, the mechanism for producing it remains to be fully understood. In this talk, a xenon model of laser–cluster interaction dynamics is presented to shed light on the processes responsible for amplification. The focus of this research is on the feasibility of creating population inversions and gain in some of the inner-shell hole state transitions within the M-shell of highly ionized xenon. The model couples a molecular dynamics (MD) treatment of the explosively-driven, non-Maxwellian cluster expansion to a comprehensive multiphoton-radiative ionization dynamic (ID) model including single- and double-hole state production within the Co- and Fe-like ionization stages of xenon. The hole-state dynamics is self-consistently coupled to a detailed valence-state collisional-radiative dynamics of the Ni-, Co-, and Fe-like ionization stages of xenon. In addition, the model includes tunneling ionization rates that confirm an initial condition assumption that Ni-like ground states can be created almost instantaneously, on the order of a femtosecond or less, i.e., at laser intensities larger than 10¹⁹ W/cm², all of the N-shell, n = 4 electrons are striped from a xenon atom in less than a femtosecond. Because of the abundance of these ground states, large numbers of n = 2, inner-shell hole states and large population inversions can be created when the Ni-like ground states are photo- or collisionally ionized. Once the M-shell is entered, tunneling ionization slows down as does collisional ionization due to the fall in ion density as the cluster expands. Moreover, as the cluster density goes down, our combined MD and ID calculations show that so do the calculated population inversions. Thus, our calculations do not support the initial experimental data interpretations in which the measured gains have been associated with double holes in more highly ionized stages of xenon (Xe³²⁺, Xe³⁴⁺, Xe³⁵⁺, and Xe³⁷⁺), which our calculations suggest would require laser intensities in excess of 1.5 × 10²⁰ W/cm², for a 248 nm, ～250 fs laser pulse focused in a gas of xenon clusters. At laser intensities used in the experiment, such ionization stages would be reached, but only later in time when cluster densities have fallen by several orders of magnitude from their initial values to values where pumping rates are too low and gains cannot be generated.     

Atomic physics of relativistic high contrast laser-produced plasmas in experiments on Leopard laser facility at UNR

The results of the recent experiments focused on study of x-ray radiation from multicharged plasmas irradiated by relativistic (I > 10¹⁹ W/cm²) sub-ps laser pulses on Leopard laser facility at NTF/UNR are presented. These shots were done under different experimental conditions related to laser pulse and contrast. In particular, the duration of the laser pulse was 350 fs or 0.8 ns and the contrast was varied from high (10^?7) to moderate (10^?5). The thin laser targets (from 4 to 750 μm) made of a broad range of materials (from Teflon to iron and molybden to tungsten and gold) were utilized. Using the x-ray diagnostics including the high-precision spectrometer with resolution R ～ 3000 and a survey spectrometer, we have observed unique spectral features that are illustrated in this paper. Specifically, the observed L-shell spectra for Fe targets subject to high intensity lasers (～10¹⁹ W/cm²) indicate electron beams, while at lower intensities (～10¹⁶ W/cm²) or for Cu targets there is much less evidence for an electron beam. In addition, K-shell Mg features with dielectronic satellites from high-Rydberg states, and the new K-shell F features with dielectronic satellites including exotic transitions from hollow ions are highlighted.     

Measurements of niobium absorption spectra in plasmas with nearly full M-shell configurations

A systematic study has been carried out on the changes in the L-shell absorption structure of niobium as a result of changing the population of the n = 3 shell from full to having vacancies in the 3d level. The niobium spectra were measured in the 2–3 keV frequency range, which spanned the 2p-nd transitions where 3 ≤ n ≤ 11. In addition to the detailed structure in these arrays the data also show 2s-4p and 2p-4s transitions and the bound-free L edge. The frequencies and widths of transition arrays, transmission between arrays, and the absorption due to the bound-free edge, can be seen in the data. The sample conditions were found from a combination of two-dimensional radiation-hydrodynamics calculations using the AWE NYM code and flux measurements using X-ray diodes, measurements of 1s-2p absorption spectra in aluminium and mixed aluminium/niobium samples. The electron temperature error, inferred from the modelling, is ±2 eV, with a density error of 30%. The data were recorded over the temperature range from 28 to 45 eV and show marked changes in the spectra over this range.The data were compared to spectra predicted by the AWE CASSANDRA [B.J.B. Crowley, J.W.O. Harris, J. Quant. Spectrosc. Radiat. Transfer 71 (2000) p. 257] opacity code. The calculated spectra were able to reproduce the measurements reasonably well. However, there are some differences in line positions that cannot be accounted for by gradients and there are differences in the array structure in the prediction and the measurements, with additional structure predicted but not seen in the measurements. There is also lower transmission on the blue side of the 2p-3d transition arrays compared to prediction.     

Equation of state studies of warm dense matter samples heated by laser produced proton beams

Heating of matter by proton beams produced by short pulse, laser-solid target interaction has been demonstrated over the last ten years by a number of workers. In the work described in this paper heating by a pulse of laser produced protons has been combined with high-resolution soft x-ray radiography to record the expansion of thin wire targets. Analysis of the radiographs yields material properties in the warm dense matter regime. These measurements imply initial temperatures in the experimental samples over a range from 14 eV up to 40 eV; the sample densities varied from solid to a tenth solid density. Assuming an adiabatic expansion after the initial proton heating phase isentropes of the aluminium sample material were inferred and compared to tabulated data from the SESAME equation of state library. The proton spectrum was also measured using calibrated magnetic spectrometers and radiochromic film. The accuracy of the technique used to infer material data is discussed along with possible future development.     

Backlighter development at the National Ignition Facility (NIF): Zinc to zirconium

K-shell X-ray emission from laser-irradiated planar Zn, Ge, Br, and Zr foils was measured at the National Ignition Facility for laser irradiances in the range of 0.6–9.5 × 10¹⁵ W/cm². The incident laser power had a pre-pulse to enhance the laser-to-X-ray conversion efficiency (CE) of a 2–5 ns constant-intensity pulse used as the main laser drive. The measured CE into the 8–16 keV energy band ranged from 0.43% to 2%, while the measured CE into the He-like resonance 1s2–1s2p(1P) and intercombination 1s2–1s2p(3P) transitions, as well as from their 1s2(2s,2p)l–1s2p(2s,2p)l satellite transitions for l = 1, 2, 3, corresponding to the Li-, Be-, and B-like resonances, respectively, ranged from 0.3% to 1.5%. Absolute and relative CE measurements are consistent with X-ray energy scaling of (hν)^?3 to (hν)^?5, where hν is the X-ray energy. The temporal evolution of the broadband X-ray power was similar to the main laser drive for ablation plasmas having a critical density surface.     

Plasma jets produced by low energy laser pulse interaction with planar and cratered targets

Laser experiments of the plasma jet formation using nanosecond laser pulses with low energy, i.e., <20 J, are presented. Planar and cratered gadolinium and aluminum targets are irradiated with laser intensities of several 10¹⁴ W/cm². Spatially-resolved time-integrated X-ray spectra were recorded in the spectral range from 7 to 10 Å. A jet-like structure is obtained from aluminum targets with a preformed crater, which is not seen in planar target irradiation. For gadolinium, a jet is observed from both planar and preformed cratered targets, suggesting that the collimation is dominated by radiative cooling. A radiation-hydrodynamics code coupled to a non-LTE ionization code was used to model the plasma. The calculated plasma emission was found to be consistent with the experimental results.     

A computational study of x-ray emission from high-Z x-ray sources on the National Ignition Facility laser

We have begun to use 350–500 kJ of 1/3-micron laser light from the National Ignition Facility (NIF) laser to create millimeter-scale, bright multi-keV x-ray sources. In the first set of shots we achieved 15%–18% x-ray conversion efficiency into Xe M-shell (∼1.5–2.5 keV), Ar K-shell (∼3 keV) and Xe L-shell (∼4–5.5 keV) emission (Fournier et al., Phys. Plasmas 17, 082701, 2010), in good agreement with the emission modeled using a 2D radiation-hydrodynamics code incorporating a modern Detailed Configuration Accounting atomic model in non-LTE (Colvin et al., Phys. Plasmas, 17, 073111, 2010). In this paper we first briefly review details of the computational model and comparisons of the simulations with the Ar/Xe NIF data. We then discuss a computational study showing sensitivity of the x-ray emission to various beam illumination details (beam configuration, pointing, peak power, pulse shape, etc.) and target parameters (size, initial density, etc.), and finally make some predictions of how the x-ray conversion efficiency expected from NIF shots scales with atomic number of the emitting plasma.     

Measurements of emission spectra from hot,dense germanium plasma in short pulse laser experiments

Heating of thin foil targets by an high power laser at intensities of 10¹⁷–10¹⁹ W/cm² has been studied as a method for producing high temperature, high density samples to investigate X-ray opacity and equation of state. The targets were plastic (parylene-N) foils with a microdot made of a mixture of germanium and titanium buried at depth of 1.5 μm. The L-shell spectra from the germanium and the K-shell spectra from the titanium were taken using crystal spectrometers recording onto film and an ultra fast X-ray streak camera coupled to a conical focussing crystal with a time resolution of 1 ps. The conditions in the microdot were inferred by comparing the measured spectra to synthetic spectra produced by the time-dependent collisional–radiative (CR) models FLY and FLYCHK. The data were also compared to simulated spectra from a number of opacity codes assuming local thermodynamic equilibrium (LTE). Temperature and density gradients were taken into account in the comparisons. The sample conditions were inferred from the CR modelling using FLYCHK to be 800 ± 100 eV and 1.5 ± 0.5 g/cc. The best fit to the LTE models was at a temperature 20% lower than with the CR model. Though the sample departs from LTE significantly useful spectral comparisons can still be made. The results and comparisons are discussed along with improvements to the experimental technique to achieve conditions closer to LTE.