LLE Review 127

Review 127


This volume of the LLE Review, covering April–June 2011, features "Langmuir Turbulence and Suprathermal Electron Production from the Two-Plasmon-Decay Instability Driven by Crossed Laser Beams in an Inhomogeneous Plasma." In this report, the fully kinetic reduced particle-in-cell method, utilizing novel diagnostics, has been applied to simulations of the two-plasmon-decay (TPD) instability in inhomogeneous plasma for parameters consistent with recent direct-drive experiments. The nonlinear saturated state of TPD is one of Langmuir turbulence involving the coexistence of the Langmuir cavitation and collapse, the Langmuir decay instability, and ponderomotive density-profile modification. The saturated state is characterized by very spiky electric fields, and Langmuir cavitation occurs preferentially inside density channels produced by the ponderomotive beating of the crossed laser beams. Statistical analyses show that cavitons follow Gaussian statistics. At times exceeding 10 ps, the excited Langmuir turbulence moves away from the quarter-critical surface to lower densities. The heated electron-distribution function is, in all cases, bi-Maxwellian, with hot-electron temperatures in the range of 60 keV to 100 keV. In all cases considered, Langmuir cavitation and collapse provide dissipation by producing suprathermal electrons that stabilize the system in saturation and drive the Langmuir wave spectrum to the small dissipation scales at the Landau cutoff. The net hot-electron energy flux out of the system is a small fraction (+0.5% to 2%) of the input laser power in these simulations.

Additional highlights of research presented in this issue include the following:

  • A closed-loop, high-resolution beam-shaping system based on a liquid-crystal-on-silicon (LCOS) spatial-light modulator (SLM) in a multiterawatt laser system and in the OMEGA EP long-pulse front end is demonstrated. The closed-loop algorithm is based on the linearity of image transformation between the control device and the measured image, where miscalibration of the linear parameters or blurring of the image affects the stability of the algorithm. One of the main causes of blurring is ascribed to the presence of tilted plates and wedges in the imaging system. These are common elements in complex laser systems. Such effects can be either compensated for or avoided by careful design. The procedure and results of damage-threshold measurement for LCOS-SLM are presented to help determine a safe operation regime for this device in high-power laser systems.
  • The impact of high-frequency spectral phase modulation on the temporal contrast of ultrafast pulses are evaluated. Expressions are derived for the low-intensity pedestal produced by optical component surface roughness within pulse stretchers and compressors. Phase noise, added across the near field of a spectrally dispersed beam, produces space–time coupling in the far field or focal plane. The pedestal is swept across an area in the focal plane many times the size of the diffraction-limited spot. Simulations are performed for generic stretchers and compressors that show fundamentally different forms of temporal contrast degradation at focus.
  • New wavefront reconstruction algorithms for high-spatial-resolution applications have been developed. Analyzing wavefront reconstructors in the frequency domain lends new insight into ways to improve frequency response and to understand noise propagation. The mathematical tools required to analyze the frequency domain are first developed for discrete band-limited signals. These tools are shown to improve frequency response in either spatial- or frequency-domain reconstruction algorithms. A new spatial-domain iterative reconstruction algorithm based on the Simpson rule is presented. The previously developed rectangular-geometry band-limited algorithm in frequency domain is adapted to hexagonal geometry, which adds flexibility when applying frequency-domain algorithms. Finally, a generalized analytic error propagation formula is found for different types of reconstructors and compared with numerical simulations.
  • Femtosecond pump–probe spectroscopy studies of time-resolved optical reflectivity of all-oxide, YBa2Cu3O7/La0.7Sr0.3MnO3 superconductor/ferromagnet nano-bilayers are presented. The temperature dependence of the nonequilibrium carrier dynamics is investigated down to 4 K. The photoresponse of bilayers has two characteristic relaxation times that are shorter than that of the YBa2Cu3O7 film and their superconducting properties are revealed in sharp peaks near the superconducting transition. The bilayer dynamics cannot be interpreted as an incoherent sum of contributions from the two layers; instead, the results point to an active role of an interface layer, where the electronic charge transfer from La0.7Sr0.3MnO3 to YBa2Cu3O7 takes place.

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