Validated helioseismic inversions for 3-D vector flows

According to time-distance helioseismology, information about internal fluid motions is encoded in the travel times of solar waves. The inverse problem consists of inferring 3-D vector flows from a set of travel-time measurements. Here we investigate the potential of time-distance helioseismology to infer 3-D convective velocities in the near-surface layers of the Sun. We developed a new Subtractive Optimally Localised Averaging (SOLA) code suitable for pipeline pseudo-automatic processing. Compared to its predecessor, the code was improved by accounting for additional constraints in order to get the right answer within a given noise level. The main aim of this study is to validate results obtained by our inversion code. We simulate travel-time maps using a snapshot from a numerical simulation of solar convective flows, realistic Born travel-time sensitivity kernels, and a realistic model of travel-time noise. These synthetic travel times are inverted for flows and the results compared with the known input flow field. Additional constraints are implemented in the inversion: cross-talk minimization between flow components and spatial localization of inversion coefficients. Using modes f, p1 through p4, we show that horizontal convective flow velocities can be inferred without bias, at a signal-to-noise ratio greater than one in the top 3.5 Mm, provided that observations span at least four days. The vertical component of velocity (v_z), if it were to be weak, is more difficult to infer and is seriously affected by cross-talk from horizontal velocity components. We emphasise that this cross-talk must be explicitly minimised in order to retrieve v_z in the top 1 Mm. We also show that statistical averaging over many different areas of the Sun allows for reliably measuring of average properties of all three flow components in the top 5.5 Mm of the convection zone.Comment: 14 pages main paper, 9 pages electronic supplement, 28 figures. Accepted for publication in Astronomy & Astrophysic

Adaptive pulse compression for transform-limited 15-fs high-energy pulse generation

Includes bibliographical references (page 589).We demonstrate the use of a deformable-mirror pulse shaper, combined with an evolutionary optimization algorithm, to correct high-order residual phase aberrations in a 1-mJ, 1-kHz, 15-fs laser amplifier. Frequency resolved optical gating measurements reveal that the output pulse duration of 15.2 fs is within our measurement error of the theoretical transform limit. This technique significantly reduces the pulse duration and the temporal prepulse energy of the pulse while increasing the peak intensity by 26%. It is demonstrated, for what is believed to be the first time, that the problem of pedestals in laser amplifiers can be addressed by spectral-domain correction

Absolute determination of the wavelength and spectrum of an extreme-ultraviolet beam by a Young's double-slit measurement

Includes bibliographical references (page 709).The interference pattern produced by irradiation of a pair of pinholes with a beam contains information on both the spatial and the temporal coherence properties of the beam, as well as its power spectrum. We demonstrate experimentally for what is believed to be the first time that the spectrum of an extreme-ultraviolet (EUV) beam can be obtained from a measurement of the interference pattern produced by a pinhole pair. This approach offers a convenient method of making absolute wavelength and relative spectral intensity calibrations in the EUV

Vlasov Equation In Magnetic Field

The linearized Vlasov equation for a plasma system in a uniform magnetic field and the corresponding linear Vlasov operator are studied. The spectrum and the corresponding eigenfunctions of the Vlasov operator are found. The spectrum of this operator consists of two parts: one is continuous and real; the other is discrete and complex. Interestingly, the real eigenvalues are infinitely degenerate, which causes difficulty solving this initial value problem by using the conventional eigenfunction expansion method. Finally, the Vlasov equation is solved by the resolvent method.Comment: 15 page

Generation of broadband VUV light using third-order cascaded processes

Includes bibliographical references (pages 013601-4).We report the first demonstration of broadband VUV light generation through cascaded nonlinear wave mixing in a gas. Using a hollow-fiber geometry to achieve broad-bandwidth phase-matching, frequency conversion of ultrashort-pulse Ti:sapphire laser pulses from the visible into the deep UV around 200 and160 nm is achieved. A new type of quasi-phase-matching is also observed in the VUV for the first time. Conversion using cascaded processes exhibits higher efficiencies, shorter pulse durations, and broader bandwidths than other schemes for generating light in the deep UV, and will enable many applications in science and technology

Generation of broadband VUV light using third-order cascaded processes

Includes bibliographical references (pages 013601-4).We report the first demonstration of broadband VUV light generation through cascaded nonlinear wave mixing in a gas. Using a hollow-fiber geometry to achieve broad-bandwidth phase-matching, frequency conversion of ultrashort-pulse Ti:sapphire laser pulses from the visible into the deep UV around 200 and160 nm is achieved. A new type of quasi-phase-matching is also observed in the VUV for the first time. Conversion using cascaded processes exhibits higher efficiencies, shorter pulse durations, and broader bandwidths than other schemes for generating light in the deep UV, and will enable many applications in science and technology