D. S. Whittaker

Temperatures following x-ray free-electron-laser heating of thin low- and medium-Z solid targets

The absorption of ultrashort (100 fs or shorter) bursts of x-ray free-electron laser (XFEL) radiation by thin (less than an attenuation length) solid carbon and iron target is modeled. Photon energies of several hundred eV to several keV and intensities of 1016 to 1018 W cm−2 are considered. We calculate carbon and iron temperatures at sufficient times after XFEL irradiation that local thermodynamic equilibrium has been established and electron-ion thermalization has occurred, assuming classical heat capacities. It is shown that there is an optimum photon energy for maximizing temperatures. Constant irradiation at 1018 W cm−2 for 100 fs with photons of 2 and 3 keV, for example, is predicted to result in a temperature of over 500 eV in solid density carbon and over 1 keV in solid density iron, respectively.

Development and applications of multimillijoule soft X-ray lasers

We review development of multimillijoule X-ray lasers and of applications of these new laboratory sources carried out recently at the PALS facility. A backbone of this development is the neon-like zinc laser providing saturated output at 21.2 nm, with up to 10 mJ of energy per pulse. This represents currently the most energetic soft X-ray laboratory source. Recent improvements in its operation include better control of the beam shape, and more complete understanding of the prepulse pumping. The laser at 21.2 nm has been employed for a number of application experiments reviewed in this paper. They include transmission measurements of intense soft X-ray radiation, studies of fundamental processes of soft X-ray ablation, ablation micropatterning, feasibility study of soft X-ray Thomson scattering from dense plasmas, visualization of nanometric transient perturbation of optical surfaces, measurements of ablation rates of foils heated by IR pulses, and studies of 2D plasma hydrodynamics in the regime of sequential illumination.

Opacity and temperature profiles through thick iron targets irradiated by short bursts of intense x-rays

We model x-ray free-electron laser (XFEL) interactions of pulses of 100 fs duration or less with thick (many attenuation lengths) solid iron targets assuming the instantaneous target opacity is determined solely by the energy absorbed for a given photon energy. Examples of the bound-free opacity dependence on energy absorbed for iron targets at photon energies of 750–2000 eV are presented. This is utilized to model XFEL pulse propagation through solid iron and to predict the resulting iron plasma opacity as the pulse progresses. Assuming the establishment of local thermodynamic equilibrium and electron-ion thermalization after a sufficiently long time interval, we calculate the temperature profiles to be expected in solid iron targets.

X-ray lasers as probes to measure plasma ablation rates

The rate of laser ablation at irradiances of ~2x1014 Wcm-2 of solid iron and aluminum has been measured using the transmission of a neon-like zinc X-ray laser at 21.2 nm through thin iron and aluminum targets. It is shown that the opacity of ablated material falls rapidly with increasing temperatures and decreasing density from the solid value. As ablated plasma becomes transparent to the X-ray laser flux, the thickness of solid, unablated material and hence the rate of ablation can be measured from time resolved X-ray laser transmission. A self-regulating model of laser ablation and fluid code simulations with absorption to thermal plasma of 5-10% show agreement with our measured ablation rates.

Plasma Opacity and Laser Ablation Measurements Using X-Ray Lasers

The use of x-ray lasers as probes of the opacity of hot dense plasma and rates of laser ablation is considered. It is shown that x-ray lasers are sufficiently bright to overcome plasma emission and enable plasma opacity to be measured. A demonstration experiment is presented where the temporal evolution of the opacity of a thin iron plasma at high temperature (30 – 250 eV) formed from an initially 50 nm thick solid tamped with a plastic overlay after heating by a laser pulse has been measured using the transmission of a nickel-like silver x-ray laser at 13.9 nm. The experimental results are compared to transmission calculations based on the iron opacity evaluated in a post-processor from predictions of the plasma conditions using a fluid and atomic physics code (EHYBRID). In another experiment, it is shown that laser ablation of a solid iron layer that is not tamped can be determined by the change in transmission of a 21.2 nm x-ray laser.

Hot dense plasma opacity measurements using x-ray lasers

Experimental measurements of the opacity of plasmas at densities close to solid state and temperatures ~ 60 - 300 eV using a probing X-ray laser are presented. Utilizing thin targets, opacities of iron have been measured using x-ray lasers of photon energy 89 eV created by pumping with the VULCAN RAL laser. The thin targets are separately heated by spot focus laser pulses. We have demonstrated that X-ray laser brightness is sufficient to overcome the self-emission of hot plasma so that useful opacity measurements can be made. Due to their high brightness, x-ray lasers can fulfill a useful niche in measuring opacity and other phenomena associated with laser-plasma interactions. Quantities such as opacity measured in laser-plasmas are useful elsewhere. For example, plasma opacity is important in understanding radiative transfer in the sun.

Heating of high energy density plasmas using EUV and x-ray lasers

Heating of high energy density plasmas using extreme ultra-violet (EUV) and x-ray lasers is examined. Our modeling studies show that solid carbon and iron can be heated by focused X-ray laser pulses of irradiance 1017 Wcm-2, duration 100 fs so that after a picosecond or so equilibration, LTE plasmas of temperatures up to 400 eV are produced in a uniform solid density of thickness close to one micron. Solid target heating experiments can also be carried out with laboratory based EUV lasers, but the temperatures achieved are < 20 eV. The equilibrium temperature reached with EUV and X-ray laser heating is strongly dependent on the photon energy, while the focused irradiance determines the thickness of heated material.

Probing high energy density plasmas with X-ray lasers

X-ray lasers can probe solid density plasmas as the critical density is well above the electron densities produced. We consider an experiment where interferometry has been utilized to provide phase information and transmission measurements for an extreme ultra-violet (EUV) laser beam at 21.2 nm probing longitudinally through a laser irradiated plastic (parylene-N) target. The transmission provides a good measure of ablation as hot plasma becomes transparent. We show that refractive indices significantly below the solid parylene-N and plasma refractive indices are produced in the warm dense plasma produced by laser irradiation.

X-Ray Lasers as Probes of Plasma Parameters

General issues relevant to the probing of plasmas with backlighters are first discussed. It is shown that soft x-ray wavelengths (> 10 nm) can generally probe thicker plasmas than harder radiation ( 100 eV). Demonstration experiments are discussed where the transmission of x-ray lasers through other laser-heated solid targets have been used (i) to measure the rate of laser ablation of solid targets and (ii) to probe the opacity of a plasma material. It is shown that narrow bandwidth radiation, such as from an x-ray laser, offers the ability to probe much more optically thick plasmas than backlighters composed of moderate to wide bandwidth spectral lines.

Collisionally excited extreme ultra-violet lasers created by laser irradiation of plasmas

Short wavelength extreme ultra-violet (EUV) lasers pumped by electron collisional excitation in plasmas irradiated by infra-red lasers have achieved saturated output down to wavelengths as short as 5.9 nm and are now used for applications. These lasers operate without mirrors with output due to amplified spontaneous emission (ASE). The beam profiles consist typically of a large number of individual coherent beamlets with a total near-field profile that is typically crescent shaped. Gain profiles are spectrally narrow dependent on thermal Doppler broadening of ions of temperature ~ 100-200 eV. With gain narrowing, this result in pulses of spectral width v/Deltav > 104. The shortest pulse durations of the output EUV lasers have consequently been close to Fourier transform limited at ap 3 ps.