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Experiments are performed to characterize the electrical properties of plasma filaments that are generated by self-guided femtosecond laser pulses propagating in air. A single plasma filament passes through a high-voltage sphere pulsed at--100 kV to a grounded electrode, which serves as a current monitor. The experiments utilize moderate electric fields to probe the filament conductivity, thereby avoiding the strong perturbations caused by electric discharges. The measured filament current decreases as ~1/[R.sup.2] as the separation R between the electrodes is increased up to 1.5 m. The pulselength of the filament current signal is 2 ns (full-width at half-maximum), but the time resolution is limited by the bandwidth of the oscilloscope. The typical plasma density in the conducting filament is 9 x [10.sup.15] [cm.sup.-3], which is inferred from the conductivity measurements and the size of the optical filaments. Comparisons are made with mobility values derived from electron swarm data, where the mobility depends upon the applied electric field. The conductivity of the filament is measured as the laser pulselength is varied from 50 fs to 1.5 ps. We find that relatively long laser pulses (1 ps) produce filaments with the largest conductivity. A model is used to predict the longitudinal position where the plasma filament forms and is in reasonably good agreement with measurements. Index Terms--Air propagation, filament conductivity, filamentation, laser--plasma interaction, ultrashort pulses.

The duration of HIV infection is usually unknown for most patients entering into HIV care. Data on the frequency at which resistance mutations are detected in these patients are needed to support practical guidance on the use of resistance testing in this clinical situation. Furthermore, little is known about HIV subtype diversity in much of the United States. Therefore, we analyzed the prevalence of drug resistance mutations and nonsub-type B strains of HIV among antiretroviral-naive individuals presenting for HIV care in New York State between September 2000 and January 2004. Sequences were obtained using a commercial HIV genotyping assay. Seventeen of 151 subjects (11.3%; 95% confidence interval 7.2%-17.3%) had at least one drug-resistance mutation, including 5 subjects with fewer than 200 [CD4.sup.+] T cells, indicative of advanced infection. Nucleoside reverse transcriptase inhibitor, non-nucleoside reverse transcriptase inhibitor, and protease inhibitor resistance mutations were detected in 6.6%, 5.3%, and 0.7% of subjects, respectively. Subjects from New York City-based clinics were less likely to have resistant virus than subjects from clinics elsewhere in New York State. Nonsubtype B strains of HIV were detected in 9 (6.0%) individuals and were associated with heterosexual contact. Two nonsubtype B strains from this cohort also carried drug-resistance mutations. These data indicate that drug-resistant virus is frequently detected in antiretroviral-naive individuals entering HIV care in New York State. Furthermore, a diverse set of nonsubtype B strains were identified and evidence suggests that nonsubtype B strains, including those carrying drug-resistance mutations, are being transmitted in New York State.

Imaging of femtosecond laser filaments is accomplished by utilizing a recently developed diagnostic which is not damaged by the filaments and which greatly reduces the nonlinearities in the detection system. This calibrated detection system allows quantities such as the filament energy and fluence distribution to be determined with greater detail accuracy, and confidence than was previously possible. The diagnostic is placed on a rail so that filament images can be obtained at a large number of positions along the propagation path. lt is found that filaments formed from 450-fs pulses carry higher fluence and propagate farther than filaments formed from 50-fs pulses. Index Terms--Air propagation, filamentation, laser-plasma interaction, ultrashort pulses.

A remote atmospheric breakdown is a very rich source of UV and broadband visible light that could provide an early warning of the presence of chemical-biological warfare agents at extended standoff distances. A negatively chirped laser pulse propagating in air compresses in time and focuses transversely, which results in a rapid laser intensity increase and ionization near the focal region that can be located kilometers away from the laser system. Proof-of-principle laboratory experiments are performed on the generation of remote atmospheric breakdown and the spectroscopic detection of mock biological warfare agents. We have generated third harmonics at 267 nm and UV broadband radiation in air from the compression and focusing of femtosecond laser pulses. Fluorescence emission from albumin aerosols as they were illuminated by the femtosecond laser pulse has been observed.

High-energy electrons were generated by the synchronized interaction of a 2-TW laser beam with a nitrogen gas jet and a 10-TW laser beam with helium gas jet. The jets centers are separated by 0.5 mm distance and the propagation of the lasers are collinear. Plasma electrons were trapped and accelerated by the wakefields generated in the gas jet plasmas to above 20 MeV. Index Terms--Electron accelerators, lasers, plasmas.

Most conceptual designs for future laser wakefield accelerators (LWFA) require external injection of precisely-phased, monoenergetic, ultrashort bunches of MeV electrons. This paper reports simulation and Hamiltonian models of several nonideal injection schemes that demonstrate strong phase bunching and good accelerated beam quality in a channel-guided LWFA. For the case of monoenergetic, unphased (long bunch) injection, there is an optimum range of injection energies for which the LWFA can trap a significant fraction of the injected pulse while producing an ultrashort, high-quality accelerated pulse. These favorable results are due to a combination of pruning of particles at unfavorable phases, rapid acceleration, and strong phase bunching. Also, the plasma channel introduces a favorable shift in the region of accelerating phase where electrons are focused, which can significantly reduce the required injection energy. Simulation results agree well with the predictions of the Hamiltonian model. Simulations of phased injection with a broad injected energy spread also exhibit final accelerated bunches with small energy spread. These results suggest that relatively poor quality injection pulses may still be useful in LWFA demonstration experiments. Index Terms--Accelerators, laser applications, optical propagation in plasma media, pulsed lasers.

High-power femtosecond laser pulses propagating in air form self-guided filaments that can persist for many meters. Characterizing these filaments has always been challenging owing to their high intensity. An apparently novel diagnostic is used to directly measure the fluence distribution of femtosecond laser pulses after they have formed self-guided optical filaments in air. The diagnostic is unique in that the information contained in the filaments is not lost owing to the interaction with the apparatus. This allows filament characteristics such as energy and size to be unambiguously determined for the first time. OCIS codes: 120.1880, 190.3270, 190.4420, 320.2250, 320.7110.

Living With the Genie: Essays on Technology and the Quest for Human Mastery, edited by Alan Lightman, Daniel Sarewitz, and Christina Desser. Washington, D.C.: Island Press, 2003, 347 pp In […]

Background: Cardiac magnetic resonance imaging (MRI) is becoming an alternative to right heart catheterization (RHC) for evaluating pulmonary hypertension (PH). A need exists to further evaluate cardiac MRI's ability to characterize PH. Purpose: To evaluate the potential for four‐dimensional (4D) flow MRI‐derived pulmonary artery velocities to characterize PH. Study Type: Prospective case–control. Population: Fifty‐four PH patients (56% female); 25 controls (36% female). Field Strength/Sequence: 1.5 T; gradient recalled echo 4D flow and balanced steady‐state free precession cardiac cine. Assessment: RHC was used to derive patients' pulmonary vascular resistance (PVR). 4D flow measured blood velocities at the main, left, and right pulmonary arteries (MPA, LPA, and RPA); cine measured ejection fraction, end diastolic, and end systolic volumes (EF, EDV, and ESV). EDV and ESV were normalized (indexed) to body surface area (ESVI and EDVI). Parameters were evaluated between, and within, PH subgroups: pulmonary arterial hypertension (PAH); PH due to left heart disease (PH‐LHD)/chronic lung disease (PH‐CLD)/or chronic thrombo‐emboli (CTE‐PH). Statistical Tests: Analysis of variance and Kruskal–Wallis tests compared parameters between subgroups. Pearson's r assessed velocity, PVR, and volume correlations. Significance definition: P < 0.05. Results: PAH peak and mean velocities were significantly lower than in controls at the LPA (36 ± 12 cm/second and 20 ± 4 cm/second vs. 59 ± 15 cm/second and 32 ± 9 cm/second). At the RPA, mean velocities were significantly lower in PAH vs. controls (27 ± 6 cm/second vs. 40 ± 9 cm/second). Peak velocities significantly correlated with right ventricular EF at the MPA (r = 0.286), RPA (r = 0.400), and LPA (r = 0.401). Peak velocity significantly correlated with right ventricular ESVI at the RPA (r = −0.355) and LPA (r = −0.316). Significant correlations between peak velocities and PVR were moderate at the LPA in PAH (r = −0.641) and in PH‐LHD (r = −0.606) patients, and at the MPA in PH‐CLD (r = −0.728). CTE‐PH showed non‐significant correlations between peak velocity and PVR at all locations. Data Conclusion: Preliminary findings suggest 4D flow can identify PAH and track PVR changes. Level of Evidence: 1 Technical Efficacy: Stage 5 [ABSTRACT FROM AUTHOR]

'The oldest hath borne most. We that are young Shall never see so much, nor live so long.' Elias was very old when he died in 1990 at the age [...]