Periodic index-modulated plasma waveguide.

We demonstrate a wire-obstructed cluster flow technique for making periodically modulated plasma waveguides in hydrogen, nitrogen, and argon with sharp, stable voids as short as 50 microm with a period as small as 200 microm. These gaps persist as the plasma expands for the full lifetime of the waveguide. We demonstrate guided propagation at intensities up to 2 x 10(17) W/cm(2), limited by our laser energy currently available. This technique is useful for quasi-phase matching applications where index-modulated guides are superior to diameter modulated guides.

Ultrahigh-intensity optical slow-wave structure.

We report the development of corrugated "slow-wave" plasma guiding structures with application to quasiphase-matched direct laser acceleration of charged particles and generation of a wide spectrum of electromagnetic radiation. These structures support guided propagation at intensities up to 2 x 10(17) W/cm(2), limited by our current laser energy and side leakage. Hydrogen and argon plasma waveguides up to 1.5 cm in length with corrugation period as short as 35 microm are generated in a cryogenic cluster jet. Experimental data are consistent with simulations showing periodic modulations of the laser pulse intensity.

Clustered gases as a medium for efficient plasma waveguide generation.

Clustered gas jets are shown to be an efficient means for plasma waveguide generation, for both femtosecond and picosecond generation pulses. These waveguides enable significantly lower on-axis plasma density (less than 10(18) cm(-3)) than in conventional hydrodynamic plasma waveguides generated in unclustered gases. Using femtosecond pump pulses, self-guided propagation and strong absorption (more than 70%) are used to produce long centimetre scale channels in an argon cluster jet, and a subsequent intense pulse is coupled into the guide with 50% efficiency and guided at above 10(17)W cm(-2) intensity over 40 Rayleigh lengths. We also demonstrate efficient generation of waveguides using 100 ps axicon-generated Bessel-beam pump pulses. Despite the expected sub-picosecond cluster disassembly time, we observe long pulse absorption efficiencies up to a maximum of 35%. Simulations show that in the far leading edge of the long laser pulse, the volume of heated clusters evolves to a locally uniform and cool plasma already near ionization saturation, which is then efficiently heated by the remainder of the pulse.

Plasma waveguides efficiently generated by Bessel beams in elongated cluster gas jets.

We demonstrate the efficient generation of plasma waveguides in elongated cluster gas jets using 100 ps axicon-generated Bessel beam pump pulses. The plasma waveguide space and time evolution is measured using picosecond interferometry. Small radius waveguides with central densities as low as approximately 10(18) cm(-3) can be generated with this technique. Despite the expected subpicosecond cluster disassembly time, we observe long pulse absorption efficiencies that can be more than a factor of 10 greater than in unclustered gas targets of the same volume average atomic density. The maximum long pulse absorption observed in cluster jets under our range of conditions was 35%. The explanation for the enhanced absorption is that in the far-leading edge of the laser pulse, the volume of heated clusters evolves to a locally uniform and cool plasma already near ionization saturation, which is then heated by the remainder of the pulse. From this perspective, the use of clustered gases is equivalent to a supercharged preionization scheme for long duration laser pulses.

[Functional magnetic resonance tomography in psychopathological research]. / Funktionelle Magnetresonanztomographie in der psychopathologischen Forschung.

Mental disorders are characterised by psychopathological symptoms which correspond to functional brain states. Functional magnetic resonance imaging (fMRI) is used for the non-invasive study of cerebral activation patterns in man. First of all, the neurobiological principles and presuppositions of the method are outlined. Results from the Heidelberg imaging lab on several simple sensorimotor tasks as well as higher cognitive functions, such as working and semantic memory, are then presented. Thereafter, results from preliminary fMRI studies of psychopathological symptoms are discussed, with emphasis on hallucinations, psychomotoric phenomena, emotions, as well as obsessions and compulsions. Functional MRI is limited by the physics underlying the method, as well as by practical constraints regarding its use in conjunction with mentally ill patients. Within this framework, the problems of signal-to-noise ratio, data analysis strategies, motion correction, and neurovascular coupling are considered. Because of the rapid development of the field of fMRI, maps of higher cognitive functions and their respective pathology seem to be coming within easy reach.