Single High-intensity Pulses Research Articles

Millijoule femtosecond micro-Bessel beams for ultra-high aspect ratio machining.

We report on a functional experimental design for Bessel beam generation capable of handling high-energy ultrashort pulses (up to 1.2mJ per pulse of 50fs duration). This allows us to deliver intensities exceeding the breakdown threshold for air or any dielectric along controlled micro-filaments with lengths exceeding 4mm. It represents an unprecedented upscaling in comparison to recent femtosecond Bessel beam micromachining experiments. We produce void microchannels through glass substrates to demonstrate that aspect ratios exceeding 1200∶1 can be achieved by using single high-intensity pulses. This demonstration must lead to new methodologies for deep-drilling and high-speed cutting applications.

Spectroscopic analysis using a hybrid LIBS-Raman system

A novel setup, combining two spectroscopic techniques, laser induced breakdown spectroscopy (LIBS) and Raman spectroscopy in a hybrid unit, is described. The work presented herein is part of a broader project that aims to demonstrate the applicability of the hybrid LIBS-Raman unit as an analytical tool for the investigation of samples and objects of cultural heritage. The system utilizes a nanosecond pulsed Nd:YAG laser (532 nm) for both LIBS and Raman analysis. In the Raman mode, a low intensity beam from the laser probes the sample surface and the scattering signal is collected into a grating spectrograph coupled to an intensified charge-coupled device (ICCD) detector, which records the Raman spectrum. In the LIBS mode a single high intensity pulse from the laser irradiates the sample surface and the time- and spectrally-resolved emission from the resulting laser ablation plume yields the LIBS spectrum. The use of a non-gated CCD detector was found to produce similar quality data (in terms of S/N ratio and fluorescence background) in the Raman mode, while in the LIBS mode spectral features were clearly broader but did not prevent identification of prominent atomic emission lines. Several model pigment samples were examined and the data obtained show the ability of the hybrid unit to record both Raman and LIBS spectra from the same point on the sample, a clear advantage over the use of different analytical setups.

Improving limb flexion in FES gait using the flexion withdrawal response for the spinal cord injured person

In the restoration of gait for paraplegics using functional electrical stimulation, the method most commonly used to produce hip flexion for the swing phase of gait has been the elicitation of the flexion withdrawal response. Several problems have been noted with the response: there is a decrease in the magnitude of the hip flexion to repeated stimuli (habituation); long latency; and inhibition of the response when stimulated bilaterally. These have been characterized and methods for overcoming the problems tested. Results show that increasing stimulation frequency reduces latency. Habituation can be reduced in some subjects by multiplexing two sites of stimulation. Habituation can further be reduced by applying single high-intensity pulses and this has been used in a one-step-ahead controller for regulating hip flexion angle. Inhibition due to bilateral stimulation had been significantly reduced by altering the timing of the stimulation to the two legs.

A high performance excimer pumped Raman laser

An electron beam pumped large aperture KrF laser operating in a short pulse multiplexed mode has been used to pump a methane Raman laser to produce a single high intensity pulse at 268 nm. With an output beam divergence of 20 μrad and final amplifier conversion efficiency of greater than 50%, intensity at the focus of an F/3 lens was greater than 10 17 W/cm 2. Prepulse intensity was less than 10 −10 of peak intensity.

A new experimental technique for the study of the surface reactions of extreme pressure additives at elevated temperatures

A single-pulse heating method for studying chemical surface reactions of extreme pressure additives is described. The method uses a single high intensity pulse of short duration in conjunction with highly sensitive surface analytical tools such as Auger electron spectroscopy and X-ray photoelectron spectroscopy for monitoring reaction products. The method was applied to the study of the reaction of dibenzyl disulphide with iron.

Detailed activation sequence in the region of electrically induced ventricular fibrillation in dogs.

We determined the details of ventricular activation sequence during the onset of ventricular fibrillation in 6 dogs. Unipolar electrograms were simultaneously recorded from 192 ventricular epicardial sites evenly spaced in a 3 by 3 cm square area surrounding 2 central sites to which stimuli for inducing fibrillation were delivered. In 5 dogs, 2 to 10 msec duration pulses with amplitudes of 150 to 220 v were used to induce fibrillation, and in 1 dog, 3 successive permature stimuli of 3 v intensity were used to induce fibrillation. In 2 dogs the pattern of activation at the onset of fibrillation was compatible with local reentrant paths near the stimulus site. Fibrillation was induced with a single high intensity pulse in both of these dogs. In the other 4 dogs, the pattern of activation at the onset of fibrillation was highly suggestive of reentrant paths distant from the stimulating electrodes. In 3 of these dogs fibrillation was induced by a single high intensity pulse and in the other it was induced by 3 repetitive stimuli. Activation patterns resembling supraventricular activation occurred prior to fibrillation in the latter animal. The data demonstrate 2 distinct modes of onset of electrically induced ventricular fibrillation. The findings support local reentry in the region of the stimulus site as the mode of onset of fibrillation in some animals and possible reentry at sites distant from the stimulus site with participation of the specialized conduction system in other animals. The differences in mode of onset of electrically induced fibrillation may be responsible for instability of fibrillation threshold measurements in some animals. In addition the role of the specialized conduction system in the onset of fibrillation may be responsible for the effects of sympathetic and vagal stimulation on fibrillation threshold.

Irradiation of Serratia marcescens by Single and Double Pulses of High-Intensity Electrons: Oxygen Diffusion Kinetics and Lifetime of Oxygen-Sensitive Species

Breaking survival curves have been measured for Serratia marcescens on Millipore filters equilibrated with known oxygen concentrations and irradiated by single pulses at ultrahigh dose rates. This phenomenon is attributed to the radiochemical depletion of intracellular oxygen and provides a basis for double-pulse experiments conducted to measure diffusion of oxygen in irradiated cells. A first pulse with sufficient dose to deplete intracellular oxygen precedes a similar second pulse by an accurately known interpulse time, variable from 10-6 sec to 30 sec. The amount of oxygen diffusing to critical sites in the cell during the interpulse time is inferred by comparison to cellular response measured under various oxygen concentrations with single high-intensity pulses. Oxygen-diffusion curves obtained show that a significant amount of oxygen diffuses to these sites by 10-4 sec. This can be interpreted as an upper limit to the lifetime of the radiation-induced oxygen-dependent damage. This limit is in agreeme...

The Radiosensitivity of Cultured Mammalian Cells Exposed to Single High Intensity Pulses of Electrons in Various Concentrations of Oxygen

The oxygen effect in cultured HeLa cells exposed to single short (3 nsec) pulses of electrons from a field emission source has been investigated. Cellular radiosensitivity as measured by the abilit...

Near-Ultraviolet and Visible Radiation from a Dynamic Pinch

The utility of a magnetically compressed plasma as a high-intensity light source was experimentally investigated in a dynamic pinch. The motion of the confined plasma column was studied both experimentally and theoretically throughout a wide range of parameters. It was found that the peak intensity of the emitted light in the 2000–6000 Å spectral region is maximum when the plasma reaches minimum radius at times comparable to the quarter cycle period of the discharge current. When the parameters are adjusted to give this mode of operation, a single high-intensity pulse, lasting approximately 2×10−6 sec, is emitted. The peak intensity was greatest with xenon as the filling gas. The absolute intensity of the light emitted from helium, air, argon, and xenon plasmas was also measured. The results indicate that a dynamic pinch arrangement can be employed to obtain peak intensities significantly greater than those available from conventional flashlamps. The time-integrated intensity, however, is limited by the inherently unstable nature of the pinched plasma, which restricts the pulse duration to a few microseconds.