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The implementation of an innovative technique for measuring the propagation of intense laser pulses through plasma channels is described. At high laser intensities, temporally resolved stimulated Raman backscattering can be used to diagnose both the electron density and the laser intensity inside the plasma channel, observations which are not possible using other techniques. This diagnostic is demonstrated in experiments using an open-ended capillary in which a plasma channel was created. The plasma channel was generated using either an electrical discharge or laser ablation by a second laser pulse.

What is the relationship between variability in ongoing brain activity preceding a sensory stimulus and subsequent perception of that stimulus? A challenge in the study of this key topic in systems neuroscience is the relative rarity of certain brain ‘states’—left to chance, they may seldom align with sensory presentation. We developed a novel method for studying the influence of targeted brain states on subsequent perceptual performance by online identification of spatiotemporal brain activity patterns of interest, and brain-state triggered presentation of subsequent stimuli. This general method was applied to an electroencephalography study of human auditory selective listening. We obtained online, time-varying estimates of the instantaneous direction of neural bias (towards processing left or right ear sounds). Detection of target sounds was influenced by pre-target fluctuations in neural bias, within and across trials. We propose that brain state-triggered stimulus delivery will enable efficient, statistically tractable studies of rare patterns of ongoing activity in single neurons and distributed neural circuits, and their influence on subsequent behavioral and neural responses.

We have observed longitudinal acceleration of free electrons by photon scattering in the low-energy Thomson regime. This is the first observation of the transition between the Thomson and Compton regimes of electron scattering due to higher-order photon interactions.

Although functional magnetic resonance imaging (fMRI) methods yield rich temporal and spatial data for even a single subject, universally accepted data analysis techniques have not been developed that use all the potential information from fMRI of the brain. Specifically, temporal correlations and confounds are a problem in assessing change within pixels. Spatial correlations across pixels are a problem in determining regions of activation and in correcting for multiple significance tests. We propose methods that address these issues in the analysis of task-related changes in mean signal intensity for individual subjects. Our approach to temporally based problems within pixels is to employ a model based on autoregressive-moving average (ARMA or "Box-Jenkins") time series methods, which we call CARMA (Contrasts and ARMA). To adjust for performing multiple significance tests across pixels, taking into account between-pixel correlations, we propose adjustment of P values with "resampling methods." Our objective is to produce two- or three-dimensional brain maps that provide, at each pixel in the map, an estimated P value with absolute meaning. That is, each P value approximates the probability of having obtained by chance the observed signal effect at that pixel, given that the null hypothesis is true. Simulated and real data examples are provided.

We present the results of numerical experiments in which electron-atom scattering occurs in the presence of a strong laser field. We compare our results for both forward and backward scattering with previous theoretical work in both strong-field and weak-field regimes. Theoretische Experimente zu vielphotonenunterstutzter Elektron-Atomstreuung Wir stellen die Ergebnisse numerischer Experimente, in welchen eine Elektron-Atomstreuung bei Anwesenheit eines starken Laserfeldes auftritt, vor. Die Ergebnisse, fur sowohl Vorwarts- als auch Ruckwartsstreuung, werden mit alteren theoretischen Arbeiten in Regimen mit starkem und schwachem Feld verglichen.

The quiver motion of an electron in a high-intensity laser field becomes relativistic (β= v/c ~ 1) at high-field strengths. This relativistic motion can lead to the emission of second and higher harmonics of the laser field1 and to forward ponderomotive acceleration of the electrons emerging from the laser focus. We will present the first observations of forward ponderomotive acceleration of the electrons born by field ionization at the center of the laser focus.

Production of laser harmonics occurs when a high-intensity laser interacts with a non-linear medium. At sufficiently high intensities any medium will show non-linear effects. We have observed high-order odd harmonic generation using a l-µm, 1-ps laser pulse at intensities between 1013 and 1016 W/cm2, and we have measured the spatial profiles of the harmonics. The existence of high order odd harmonics has previously been demonstrated1 in noble gases using intensities between 1013 and 1014 W/cm2.

Observations of multiphoton scattering of electrons born in a high-intensity laser focus are extended to intensities at which the relativistic mass shift associated with the quiver motion must be taken into account.

Extended propagation of terawatt (TW) laser pulses has been shown in plasma channels formed with a capillary discharge. Capillary optical guiding experiments at NRL utilize plasma channels formed using both laser ablation and electrical discharge. Present experiments aim to develop laser preionization and ablation from capillary walls as a formation technique for the guiding plasma. Electron density measurements of laser-formed plasmas within the capillary yield values as low as 10/sup 18/ cm/sup -3/. These densities hold promise for optimization of plasma conditions for standard laser wakefield acceleration (LWFA). Experiments also aim to characterize plasma ablation dynamics and plasma channel conditions within the capillary using a variety of imaging and other diagnostics. Imaging diagnostics include axial shadowgraphy. Capillary guiding diagnostics such as transmission efficiency, output beam mode imaging, time resolved backward-scattered Raman spectra, forward Raman spectra, and Thomson scattering imaging are also employed.