D. Riley

Finite temperature dense matter studies on next-generation light sources

The construction of short-pulse tunable soft x-ray free electron laser sources based on the self-amplified spontaneous emission process will provide a major advance in capability for dense plasma-related and warm dense matter (WDM) research. The sources will provide 1013xa0photons in a 200-fs duration pulse that is tunable from approximately 6 to 100 nm. Here we discuss only two of the many applications made possible for WDM that has been severely hampered by the fact that laser-based methods have been unavailable because visible light will not propagate at electron densities of ne≥1022xa0cm-3. The next-generation light sources will remove these restrictions.

Creation of hot dense matter in short-pulse laser-plasma interaction with tamped titanium foils

Dense titanium plasma has been heated to an electron temperature up to 1300eV with a 100TW, high intensity short-pulse laser. The experiments were conducted using Ti foils (5μm thick) sandwiched between layers of either aluminum (1 or 2μm thick) or plastic (2μm thick) to prevent the effects of prepulse. Targets of two different sizes, i.e., 250×250μm2 and 1×1mm2 were used. Spectral measurements of the Ti inner-shell emission, in the region between 4and5keV, were taken from the front-side (i.e., the laser illuminated side) of the target. The data show large shifts in the Kα emission from open-shell ions, suggesting bulk heating of the sample at near solid density, which was largest for reduced mass targets. Comparison with collisional radiative and 2D radiation hydrodynamics codes indicates a peak temperature of Te,peak=1300eV of solid titanium plasma in ∼0.2μm thin layer. Higher bulk temperature (Te,bulk=100eV) for aluminum tamped compared to CH tamped targets (Te,bulk=40eV) was observed. A possible expla...

The complex ion structure of warm dense carbon measured by spectrally resolved x-ray scatteringa)

We present measurements of the complex ion structure of warm dense carbon close to the melting line at pressures around 100u2009GPa. High-pressure samples were created by laser-driven shock compression of graphite and probed by intense laser-generated x-ray sources with photon energies of 4.75u2009keV and 4.95u2009keV. High-efficiency crystal spectrometers allow for spectrally resolving the scattered radiation. Comparing the ratio of elastically and inelastically scattered radiation, we find evidence for a complex bonded liquid that is predicted by ab-initio quantum simulations showing the influence of chemical bonds under these conditions. Using graphite samples of different initial densities we demonstrate the capability of spectrally resolved x-ray scattering to monitor the carbon solid-liquid transition at relatively constant pressure of 150u2009GPa. Showing first single-pulse scattering spectra from cold graphite of unprecedented quality recorded at the Linac Coherent Light Source, we demonstrate the outstanding pos...

Evidence of plasma polarization shift of Ti He-α resonance line in high density laser produced plasmas

A spectroscopic study of the He-α (1s2 1s0 - ls2p 1p1) line emission (4749.73 eV) from high density plasma was conducted. The plasma was produced by irradiating Ti targets with intense (I ≈ l×l019 W/cm2), 400nm wavelength high contrast, short (45fs) p-polarized laser pulses at an angle of 45°. A line shift up to 3.4+1.0 eV (1.9±0.55 mA) was observed in the He-α line. The line width of the resonance line at FWHM was measured to be 12.1±0.6 eV (6.7±0.35 mA). For comparison, we looked into the emission of the same spectral line from plasma produced by irradiating the same target with laser pulses of reduced intensities (≈1017 W/cm2): we observed a spectral shift of only 1.8+1.0 eV (0.9+0.55mA) and the line-width measures up to 5.8+0.25 eV (2.7+0.35 mA). These data provide evidence of plasma polarization shift of the Ti He-α line.

Mapping neutral, ion, and electron number densities within laser-ablated plasma plumes

Spatially and temporally varying neutral, ion and electron number densities have been mapped out within laser ablated plasma plumes expanding into vacuum. Ablation of a magnesium target was performed using a KrF laser, 30 ns pulse duration and 248 nm wavelength. During the initial stage of plasma expansion (t <EQ 100 ns) interferometry has been used to obtain line averaged electron number densities, for laser power densities on target in the range 1.3 - 3.0 X 108 W/cm2. Later in the plasma expansion (t equals 1 microsecond(s) ) simultaneous absorption and laser induced fluorescence spectroscopy has been used to determine 3D neutral and ion number densities, for a power density equal to 6.7 X 107 W/cm2. Two distinct regions within the plume were identified. One is a fast component (approximately 106 cm-1) consisting of ions and neutrals with maximum number densities observed to be approximately 30 and 4 X 1012 cm-3 respectively, and the second consists of slow moving neutral material at a number density of up to 1015 cm-3. Additionally a Langmuir probe has been used to obtain ion and electron number densities at very late times in the plasma expansion (1 microsecond(s) <EQ t <EQ 15 microsecond(s) ). A copper target was ablated using a Nd:YAG laser, 7.5 ns duration and 532 nm (2 (omega) ) wavelength, with a power density on target equal to 6 X 108 W/cm2. Two regions within the plume with different velocities were observed. Within a fast component (approximately 3 X 106 cms-1) electron and ion number densities of the order 5 X 1012 cm-3 were observed and within the second slower component (approximately 106 cms-1) electron and ion number densities of the order 1 - 2 X 1013 cm-3 were determined.

Plasma-based studies on 4th generation light sources

The construction of a short pulse tunable x-ray laser source will be a watershed for plasma-based and warm dense matter research. The areas we will discuss below can be separated broadly into warm dense matter (WDM) research, laser probing of near solid density plasmas, and laser-plasma spectroscopy of ions in plasmas. The area of WDM refers to that part of the density-temperature phase space where the standard theories of condensed matter physics and/or plasma statistical physics are invalid. Warm dense matter, therefore, defines a region between solids and plasmas, a regime that is found in planetary interiors, cool dense stars, and in every plasma device where one starts from a solid, e.g., laser-solid matter produced plasma as well as all inertial fusion schemes. The study of dense plasmas has been severely hampered by the fact that laser-based methods have been unavailable. The single most useful diagnostic of local plasma conditions, e.g., the temperature (Te), the density (ne), and the ionization (...

Laser induced sparks in atmospheric helium and helium mixtures, probbed with thomson scattering

Summary form only given. In the plasma community there is widespread interest in atmospheric plasmas as they have various applications such as medicine, surface treatment and lighting for example. Helium is a commonly used gas in such plasmas. Emission spectroscopy is usually used to obtain information about the plasma parameters1. However, most of the emission line studies rely on Stark broadening theories where self-absorption of lines can be an issue and result in incorrect values of n e . A diagnostic such as Thomson scattering can be used to test the reliability of emission based methods.

Thomson scattering from a laser induced breakdown in 1 atmosphere of helium

Studies of the shape and separation of He I allowed and forbidden lines can give valuable information about the electron density in astrophysical and laboratory plasmas [1]. Most of these studies rely on Stark broadening theories where self-absorption of lines can be quite limiting and result in incorrect values of ne. Using diagnostics such as Thomson scattering can help in overcoming this problem, as well as test the Stark broadening approaches.

X-ray Thomson scattering

Summary form only given, as follows. A series of experiments have been carried out at the Rutherford-Appleton laboratory to investigate X-ray scattering from laser produced plasmas. Most experiments shave been based on Rayleigh scattering from bound electrons and are sensitive to ion-ion correlations which lead to a scattering structure factor with a peak at a distinct angle of scatter, which depends on the plasma density and temperature. At the time of writing an experiment is being carried out to investigate the possibility of soft X-ray Thomson scattering with a Ne-like Ni X-ray laser operating at 23.1 nm. We will report the experimental set up and any results that are obtained.