Search Results (15 total)

A laser wakefield acceleration study has been performed in the matched, self-guided, blowout regime producing 720±50  MeV quasimonoenergetic electrons with a divergence Δθ_FWHM of 2.85±0.15  mrad using a 10 J, 60 fs 0.8  μm laser. While maintaining a nearly constant plasma density (3×10¹⁸  cm^-3), the energy gain increased from 75 to 720 MeV when the plasma length was increased from 3 to 8 mm. Absolute charge measurements indicate that self-injection of electrons occurs when the laser power P exceeds 3 times the critical power P_cr for relativistic self-focusing and saturates around 100 pC for P/P_cr>5. The results are compared with both analytical scalings and full 3D particle-in-cell simulations.

Inertially confined, ignited thermonuclear D-T plasmas will produce intense blackbody radiation at temperatures T≳20  keV; it is shown that the injection of GeV electrons into the burning core can efficiently generate high-energy Compton scattering photons. Moreover, the spectrum scattered in a small solid angle can be remarkably monochromatic, due to kinematic pileup; a peak brightness in excess of 10³⁰  photons/(mm² mrad² s 0.1% bandwidth) is predicted. These results are discussed within the context of the Schwinger field and the Sunyaev-Zel’dovich effect.

No monochromatic (Δω_x/ω_x<1%), high peak brightness [>10²⁰   photons/(mm²×mrad²×s×0.1%   bandwidth)], tunable light sources currently exist above 100 keV. Important applications that would benefit from such new hard x-ray and γ-ray sources include the following: nuclear resonance fluorescence spectroscopy and isotopic imaging, time-resolved positron annihilation spectroscopy, and MeV flash radiography. In this paper, the peak brightness of Compton scattering light sources is derived for head-on collisions and found to scale quadratically with the normalized energy, γ; inversely with the electron beam duration, Δτ, and the square of its normalized emittance, ε; and linearly with the bunch charge, eN_e, and the number of photons in the laser pulse, N_γ:   B-^ _x∝γ²N_eN_γ/ε²Δτ. This γ² scaling shows that for low normalized emittance electron beams (1 nC, 1  mm·mrad, <1  ps, >100  MeV), and tabletop laser systems (1–10   J, 5 ps) the x-ray peak brightness can exceed 10²³   photons/(mm²×mrad²×s×0.1%   bandwidth) near ℏω_x=1   MeV; this is confirmed by three-dimensional codes that have been benchmarked against Compton scattering experiments performed at Lawrence Livermore National Laboratory. The interaction geometry under consideration is head-on collisions, where the x-ray flash duration is shown to be equal to that of the electron bunch, and which produce the highest peak brightness for compressed electron beams. Important nonlinear effects, including spectral broadening, are also taken into account in our analysis; they show that there is an optimum laser pulse duration in this geometry, of the order of a few picoseconds, in sharp contrast with the initial approach to laser-driven Compton scattering sources where femtosecond laser systems were thought to be mandatory. The analytical expression for the peak on-axis brightness derived here is a powerful tool to efficiently explore the 12-dimensional parameter space corresponding to the phase spaces of both the electron and incident laser beams and to determine optimum conditions for producing high-brightness x rays.

We present a detailed comparison of the measured characteristics of Thomson backscattered x rays produced at the Picosecond Laser-Electron Interaction for the Dynamic Evaluation of Structures facility at Lawrence Livermore National Laboratory to predicted results from a newly developed, fully three-dimensional time and frequency-domain code. Based on the relativistic differential cross section, this code has the capability to calculate time and space dependent spectra of the x-ray photons produced from linear Thomson scattering for both bandwidth-limited and chirped incident laser pulses. Spectral broadening of the scattered x-ray pulse resulting from the incident laser bandwidth, perpendicular wave vector components in the laser focus, and the transverse and longitudinal phase spaces of the electron beam are included. Electron beam energy, energy spread, and transverse phase space measurements of the electron beam at the interaction point are presented, and the corresponding predicted x-ray characteristics are determined. In addition, time-integrated measurements of the x rays produced from the interaction are presented and shown to agree well with the simulations.

The pulse-width dependence of thermal melting and ablation thresholds in germanium and gallium arsenide is correlated to direct, ultrafast x-ray measurements of laser-heated depths. The heating dynamics, determined by the interplay of nonlinear optical absorption, delayed Auger heating, and high-density carrier diffusion, explain the scaling laws of thermal melting thresholds in different semiconductors.

The field ionization cross sections for the L-shell states in argon are presented as measured with pulsed-laser radiation at an intensity of up to 10¹⁹ W/cm². For ultrahigh intensities, the photoelectron continuum dynamics will be relativistic. However, the measured charge-state yields for Ar⁹⁺ to Ar¹⁶⁺ compare favorably to numerical solutions of the nonrelativistic Schrödinger equation and a widely used Ammosov-Delone-Krainov/WKB tunneling ionization model. The results are interpreted within a two-step, strong-field ionization model, where the initial tunneling ionization process is dominated by nonrelativistic effects while the photoelectron continuum dynamics are strongly relativistic.

A new physical scheme for femtosecond x-ray lasers, where the upper lasing level (L₂₃ innershell vacancy level) is pumped by x-ray photons and the lower lasing level (M₁ innershell vacancy level) is depopulated via a Coster–Kronig radiationless transition, is analyzed for Ca. The transition wavelength is 4.1 nm, which is inside the water window (the wavelength range between the K absorption edges of oxygen and carbon). The peak spectral brightness of the x-ray laser output at 4.1 nm is predicted to be as large as 5×10²⁵ photons/s/(mm² mrad² 0.1% bandwidth), which is 4 to 5 orders of magnitude brighter than a typical undulator radiation in the similar spectral region. In addition to the high flux, the expected duration of x-ray lasing of ∼3 fs will be useful for the study of fast dynamics in physical and biological sciences.

Damping of impulsively generated coherent acoustic oscillations in a femtosecond laser-heated thin germanium film is measured as a function of fluence by means of ultrafast x-ray diffraction. By simultaneously measuring picosecond strain dynamics in the film and in the unexcited silicon substrate, we separate anharmonic damping from acoustic transmission through the buried interface. The measured damping rate and its dependence on the calculated temperature of the thermal bath is consistent with estimated four-body, elastic dephasing times (T₂) for 7-GHz longitudinal acoustic phonons in germanium.

Population dynamics of atomic inner-shell vacancy states are analyzed for the possibility of inversion in relation to keV x-ray laser schemes. Transitions between pairs of inner-shell vacancy states are considered in which the states are pumped via electron-impact inner-shell ionization by a femtosecond high-energy electron pulse. For appropriate atomic systems, transient inversion is predicted due to rapid lower state depopulation via Coster-Kronig decay.

The ionization dynamics of small rare-gas clusters in intense, ultrafast laser fields are studied via classical trajectory Monte Carlo simulations. Our results indicate that for similar laser pulses the charge states reached by atoms in a cluster can be significantly higher than those for atoms in the gas phase. The ionization enhancement is strongly dependent on the cluster density and exhibits a rapid increase in charge state once the laser intensity has reached the threshold for single ionization. This 'ionization ignition model' is driven by the combination of the laser field and the strong field from the ionized cluster atoms. Approximate atomic inner-shell ionization probabilities are calculated for several cluster densities and peak laser intensities and provide evidence for the generation of inner-shell holes on an ultrafast time scale. This is a necessary condition for the generation of x-ray pulses with temporal widths comparable to that of the driving laser pulses.

We report the observation of a gain of approximately exp(11) at 41.8 nm in 8-times-ionized xenon. This extreme ultraviolet (XUV) laser is driven by a 10-Hz, 70-mJ circularly polarized femtosecond laser pulse. The laser is focused into Xe at pressures ranging from 5 to 12 torr. The laser is collisionally excited, with both the ions and electrons produced by field induced tunneling.

Investigations of a 96.9-nm laser in neutral cesium are described. Theoretical and experimental evidence is presented for the laser level designation and pumping mechanism. Measurements of the laser output are given, including saturated pulse energy, temporal profile, spatial profile, transition wavelength, gain cross section, and the variation of small signal gain with operating parameters. Comparisons of the temporal and spatial behavior of the 96.9-nm laser emission with respect to resonance line emission from ionic Cs are also presented.

We report the use of a photoionization electron source to pump a 116-nm laser in the Werner band (C ¹Π_u→X ¹Σ_g⁺) of molecular hydrogen. The laser is pumped by free electrons which are created by photoionizing molecular hydrogen with soft x-rays from a traveling-wave laser plasma. We show that even though the free electrons have an average temperature of ∼10 eV, the lasing hydrogen molecules retain an ambient temeprature of ∼0.01 eV. This allows an extrapolated small-signal gain of exp(43), with a 1064-nm pumping energy of 580 mJ in 200 psec.

We report the operation of a saturated 96.9-nm laser in Cs vapor that has an extrapolated small-signal gain of exp(83) in a total length of 17 cm. We believe that lasing occurs from a core-excited level embedded in the continuum of the valence electron. The laser is pumped by soft x rays from a synchronous, traveling-wave, laser-produced (2.5 J, 20 ps, 1064 nm) plasma.

The paper describes the experimental investigation of a subclass of quartet levels of the alkali-metal atoms which retain metastability against autoionization and have large radiative yields. Using high-pulsed-power microwaves, we obtain emission spectra of Na, K, Rb, and Cs. In each case, the neutral emission spectrum is dominated by emission from these levels.