Linac Coherent Light Source Facility Research Articles

Plasma density effects on electron impact ionization.

We present new results on ionization by electron impacts in a dense plasma. We are interested in the density effect known as ionization potential depression and its role in atomic structure. Rather than using the well-known Stewart-Pyatt or Ecker-Kröll formulas for the ionization potential depression, we consider a distribution function of the ionization energy, which involves the plasma fluctuations due to ion dynamics. This distribution is calculated within classical molecular dynamics. The removal of the noise yields a new distribution which is composed of a small set of Gaussian peaks among which one peak is selected by considering the signal-to-noise ratio. This approach provides an ionization potential depression in good agreement with experimental results obtained at the Linac Coherent Light Source facility. Our results are also compared with other calculations. In a second part, we investigate the effects of the ionization potential depression and the fluctuations on ionization by electron impacts. We propose an expression of the cross sectionthat is based on an average over the ionization energy distribution. This cross sectioncan be calculated analytically. The main strength of our work is to account for the fluctuations due to ion dynamics.

Consistent approach for electrical resistivity within Ziman's theory from solid state to hot dense plasma: Application to aluminum.

The approach presented in this work allows a consistent calculation of electrical conductivity of dense matter from the solid state to the hot plasma using the same procedure, consisting in dropping elastic scattering contributions to solid's and liquid's structure factors in the framework of the Ziman theory. The solid's structure factor was computed using a multiphonon expansion. The elastic part is the zero-phonon term and corresponds to Bragg peaks, thermally damped by Debye-Waller attenuation factors. For the liquid, a similar elastic contribution to the structure factor results from a long-range order persisting during the characteristic electron-ion scattering time. All the quantities required for the calculation of the resistivities are obtained from our average-atom model, including the total hypernetted-chain structure factor used from the liquid state to the plasma. No interpolation between two limiting structure factors is required. We derive the correction to apply to the resistivity in order to account for the transient long-range order in the liquid and show that it improves considerably the agreement with quantum-molecular dynamics simulations and experimental aluminum's isochoric and isobaric conductivities. Our results suggest that the long-range order in liquid aluminum could be a slightly compressed fcc one. Two series of ultrafast experiments performed on aluminum were also considered, the first one by Milchberg etal. using short laser pulses and the second one by Sperling etal. involving x-ray heating and carried out on the Linac Coherent Light Source facility. Our attempts to explain the latter assuming an initial liquid state at an ion temperature much smaller than the electron one suggest that the actual initial state before main heating is neither perfectly solid nor a normal liquid.

Imaging at an x-ray absorption edge using free electron laser pulses for interface dynamics in high energy density systems.

Tuning the energy of an x-ray probe to an absorption line or edge can provide material-specific measurements that are particularly useful for interfaces. Simulated hard x-ray images above the Fe K-edge are presented to examine ion diffusion across an interface between Fe2O3 and SiO2 aerogel foam materials. The simulations demonstrate the feasibility of such a technique for measurements of density scale lengths near the interface with submicron spatial resolution. A proof-of-principle experiment is designed and performed at the Linac coherent light source facility. Preliminary data show the change of the interface after shock compression and heating with simultaneous fluorescence spectra for temperature determination. The results provide the first demonstration of using x-ray imaging at an absorption edge as a diagnostic to detect ultrafast phenomena for interface physics in high-energy-density systems.

NON-EQUILIBRIUM MODELING OF THE FE XVII 3C/3D LINE RATIO IN AN INTENSE X-RAY FREE-ELECTRON LASER EXCITED PLASMA

Recent measurements using an X-ray Free Electron Laser (XFEL) and an Electron Beam Ion Trap at the Linac Coherent Light Source facility highlighted large discrepancies between the observed and theoretical values for the Fe xvii 3C/3D line intensity ratio. This result raised the question of whether the theoretical oscillator strengths may be significantly in error, due to insufficiencies in the atomic structure calculations. We present time-dependent spectral modeling of this experiment and show that non-equilibrium effects can dramatically reduce the predicted 3C/3D line intensity ratio, compared with that obtained by simply taking the ratio of oscillator strengths. Once these non-equilibrium effects are accounted for, the measured line intensity ratio can be used to determine a revised value for the 3C/3D oscillator strength ratio, giving a range from 3.0 to 3.5. We also provide a framework to narrow this range further, if more precise information about the pulse parameters can be determined. We discuss the implications of the new results for the use of Fe xvii spectral features as astrophysical diagnostics and investigate the importance of time-dependent effects in interpreting XFEL-excited plasmas.

The creation of large-volume, gradient-free warm dense matter with an x-ray free-electron laser

The efficiency and uniformity of heating induced by hard x-ray free-electron laser pulse is investigated for 0.5 μm silver foils using the X-ray Pump Probe instrument at the Linac Coherent Light Source facility. Intense 8.9 keV x-ray pulses of 60 fs duration deposit energy predominantly via inner-shell ionization to create a non-equilibrium Ag solid density plasma. The x-ray pulses are focused to 14 × 17 μm2 by means of beryllium lenses and by varying the total beam energy, the energy deposition is varied over a range of irradiances from 4.4 to 6.5 × 1015 W/cm2. Two time-and-space resolved interferometers simultaneously probed the expansion of the front and rear sample surfaces and find evidence of a nearly symmetric expansion pointing to the uniformity of energy deposition over the full target thickness. The experimental results are compared with two different hydrodynamic simulations of the sample expansion. The agreement between experimental and theoretical results yields an estimate of the temperature evolution as a function of x-ray irradiance that varies from 8 to 10 eV for the x-ray irradiances studied.

Thermoelastic effects as a way of creating transient renewable reflective optics

A technique for creating renewable reflective optics suitable for focusing of pulsed laser beams is proposed. It is based on the heating of the surface of a planar reflecting slab by an auxiliary heating source that causes thermal expansion of the slab material and creates a desired surface relief, e.g., a focusing mirror. The presence of this relief is a transient phenomenon, but, for a short-enough main pulse, this does not cause any problems. If the surface is damaged by the main pulse, the shifting of the slab and repeating the whole cycle allows recreating the transient mirror. Favorable features of this approach include controlling the optics “at a distance,” without any direct mechanical contact. A detailed discussion of the possibilities provided by this technique for the focusing of x-ray beams at the planned Linac Coherent Light Source facility at Stanford is presented. It is concluded that a 20-fold increase of intensity of an 8 keV beam and 100-fold increase of intensity of a 0.8 keV beam is possible. A set of design equations and constraints is formulated. The analysis presented can be used as a template for analyses of similar transient optical systems for the UV and optical range.

Status of the LCLS x-ray FEL program (invited)

The Linac Coherent Light Source (LCLS) program involves a collaboration of several U.S. National Laboratories and universities with the goal of designing and building the first fourth-generation hard x-ray source, an x-ray free-electron laser (FEL). This FEL will utilize extremely short, intense, low-emittance electron pulses created by the high-energy linear accelerator at the Stanford Linear Accelerator Center. The FEL radiation produced will feature unprecedented peak brightness, short pulse length, and spatial coherence, tunable over an energy range of 0.8–8 keV. With favorable funding, major construction will begin by 2004 and the LCLS will be operating late in 2006. The LCLS facility will include experimental stations for carrying out groundbreaking experiments in several scientific fields. Current research and development efforts are directed at experimentally studying the physics of high-gain FELs, and refining the details of the plan for the LCLS facility. The FEL experiments, at Argonne and Brookhaven National Labs (along with experiments carried out at the German laboratory DESY), have confirmed the basic physical concepts upon which LCLS is based, and have demonstrated that many of the stringent technical requirements can already be met.