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In general, material processing with high-power ultra-short-pulsed lasers yields cleaner surfaces, as long as the intensity profile of the laser beam is well shaped. However, the beam suffers distortions during propagation through ambient atmospheric media such as air. Passage through such media causes the beam to self-focus, increasing the intensity further and causing the breakdown of the gas. The resulting plasma distorts the beam’s original profile and the ablated surface conforms to the beam profile. A numerical scheme is developed here to calculate the intensity profile of an optical beam propagating through a medium. Intensity distribution of the beam is then used to determine the profile of the processed surface by a geometrical method developed recently. The calculated profile is compared with the experimentally obtained surface with good agreement. For medium spot sizes, the self-focusing and plasma effects tend to cancel each other, maintaining the intensity profile of the beam similar to the original Gaussian distribution. For small spot sizes when the intensity is high, the plasma effects are found to distort the beam profile. This indicates that the experimental parameters can be adjusted to improve the quality of the machined surface. [ABSTRACT FROM AUTHOR]

Recent advances in laser machining technology have made it possible to fabricate parts and features with high accuracy and precision, using high-powered, short-pulsed, Q-switched lasers. To determine the machining parameters to obtain the desired geometrical quality, an understanding of the relationship between the process parameters and the resulting surface profile is necessary. In the present study, we adopt a geometrical approach which, coupled with the material properties and machining process parameters, yields a method to determine the surface profile of the material ablated by a laser pulse. It is reasoned that the energy incident upon an infinitesimal area of the surface at a given time is transferred in the outward normal direction to the surface, and the volume of ablation, centered about the normal, is determined by the laser-material interaction and the process parameters. The direction and depth of ablation determine the modified surface profile an infinitesimal time later, yielding a nonlinear partial differential equation, which is then integrated starting with the initial known surface to determine the profile at an arbitrary time. Theoretical predictions and the experimental results are compared for a test case of metals. The agreement between the two is satisfactory indicating the adequacy of the approach. [ABSTRACT FROM AUTHOR]

A study of the dependence of the laser-induced avalanche behavior on the intensity of irradiation in dielectrics is carried out. The avalanche rate is computed directly from the reduced equations, instead of the time dependent kinetic equations, thereby requiring less computational effort. For comparison, the calculations were carried out for both the flux-doubling model and the linear Fokker–Planck type equation for fused silica material. The flux-doubling model overestimates the avalanche rate by about 8%–10%. The properties of the equations describing the avalanche behavior indicate a nonlinear dependence of the avalanche rate on the intensity. The results for fused silica show an almost linear dependence in the high intensity range from about 1 TW/cm2 to infinity, and in the low intensity range up to about 0.01 TW/cm2, but a significant departure from linearity at the intermediate intensities. For a Gaussian pulse, the exponential growth rate of the electron density distribution in the avalanche regime is still almost directly proportional to the instantaneous fluence. Although the proportionality constant increases with increasing peak intensity, the difference is negligible in the high and low intensity ranges. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Features a study that described the operational characteristics of a gas discharge under the influence of a transverse rotating magnetic field. Cause of the discharge of instabilities that eventually result in serious plasma restriction; Main features of the experimental system; Effects of the rotating magnetic field on the gas-discharge V-I characteristic.

Presents measurements of the electron density within a photo-initiated, impulse-enhanced, electrically excited laser gas discharge. Effect of external ionization source on the electron density; Ion current measurements through the external circuit.

The incorporation of a gating signal into the trigger circuit of a photoinitiated, impulse-enhanced, electrically excited (PIE) laser system has permitted high-power, pulsed operation of a normally cw CO2 discharge. The 40 liter gain medium has been run at repetition rates approaching 1 kHz utilizing this approach. Plasma uniformity and stability have been significantly enhanced, such that a factor of two increase in electrical power deposition into the excited volume has been achieved. Results suggest that pulsed performance considerably in excess of that achievable under cw operating conditions can be realized through the adoption of this simple modification to the PIE ionization process. [ABSTRACT FROM AUTHOR]

Background: The limitations of the conventional methods for diagnosing tuberculosis (TB) have spurred multi-faceted research activities throughout the world. This study aims to explore the levels of adenosine deaminase (ADA) and interleukins in pleural effusion of tuberculous, malignant, and miscellaneous origin for differential diagnosis of tubercular and non-tubercular effusion., Method: Adenosine deaminase was estimated by kinetic method employing xanthine oxidase while interleukins were measured using commercially available ELISA kits in pleural fluids of tubercular and non-tubercular origin., Results: Pleural fluids INF-γ, sIL-2R, TNF-α and ADA were significantly higher in TB group (n = 48) as compared to the non-TB group (n = 33) (mean ± SD: INF-γ; 1,958.7 ± 896.5 pg/mL vs 356.9 ± 733.6 pg/mL, sIL-2R; 6,101 ± 1,753.8 pg/mL vs 3,166 ± 2,611.1 ± pg/mL, TNF-α; 195.5 ± 292.1 pg/mL vs 59.7 ± 128.9 pg/mL, ADA; 123.6 ± 81.8 IU/L vs 48 ± 48.5 IU/L, P < 0.01)., Conclusion: INF-(is more sensitive and specific than ADA for the diagnosis of TB and should be added to the armamentarium of the diagnostic workup of pleural fluids for timely and accurate diagnosis of TB and differentiation of tubercular pleural effusion from non-tubercular effusion.

The output characteristics of a toric unstable resonator fitted to a multichannel stripline excitation system are presented. The resonator is shown to possess the usual advantages of a conventional unstable resonator plus the ability to modify the profile of the output beam by a simple change in the coupling aperture. Laser output parameters have been studied as a function of coupling fraction, magnification, and internal loss factors. Variations in the focal spot size as a function of the coupling aperture as well as resonator alignment sensitivity and polarization properties have been investigated.

An electronic laser controller, based on a computer vision system, has been developed for automatic mode and power control of a high powered CO(2) laser. The cross-sectional image of a sample of the output beam is continually analyzed and used to servo regulate both the laser cavity and the excitation level of the discharge. The technique stabilizes average laser output power to within 4%, while maintaining the best output power distribution achievable by the particular laser. Control system design also provides important on-line beam intensity profile data to assist in maximization of operating efficiency.

Changes in dietary protein quality and high altitude stress alter the serum urea cycle enzyme activities. To find a dietary protein which will facilitate quicker acclimatization to altitude stress, groups of rats were fed two diets, isocaloric and isonitrogenous, but differing in amino acid make up. The rats were exposed to simulated altitude stress of 0.5 atmospheric pressure for various periods up to 65 h. When serum arginase and ornithine transcarbamylase activities were assayed, it was seen that the group on the unbalanced protein (Cajanus cajan) diet showed less increase on exposure and a tendency to return to normal level earlier than the group on the balanced protein (egg) diet.

The design, construction, and operational characteristics of an optical resonator for an annular gain media are described. The system, developed for laser power extraction investigations in a new type of coaxial discharge geometry, features a folded multipass unstable resonator concept, fabricated from lightweight uncoated diamond-turned aluminum substrates. The resulting cw CO2 device incorporates excitation aspects of the nonself-sustained PIE excitation process in addition to a new magnetic discharge stabilization technique. Laser performance and output beam characteristics are presented.

The optical performance of a cw PIE CO(2) laser has been substantially improved through the adoption of a burst-mode gain switching technique. The approach has provided a doubling of the average beam power extractable from the device. With appropriate optimization, the process could possibly permit the attainment of pulsed energy extraction in the kilohertz range, and with average optical powers within the several tens of kilowatt category.

Groups of rats were fed diets containing 15% protein from arhar dal (Cajanus cajan), bengal gram dal (Cicer arietinum), urad dal (Phaseolus mungo) and also isolated proteins from casein, egg, soya bean, gluten and gelatin for a period of 10 days. Rats maintained on the casein diet had the highest liver arginase activity and those having egg the lowest. All the leguminous proteins gave similar levels of arginase activity, the values falling between the casein and egg groups. Serum arginase level was found to be low only in rats on egg and gluten diets. Liver ornithine transcarbamylase (OTC) activity was significantly higher when legumes provided the protein as compared with the diet containing egg. Serum OTC level was lowest in the egg and arhar dal groups. Though the levels of urea cycle enzymes were altered by the quality of dietary proteins, no simple relationship could be established between them.

A theoretical investigation of a stable concave-convex resonator configuration, which appears suitable for single-mode high-power energy extraction from large volume gain media, is presented. The design features annular output coupling with a surprisingly uniform near-field intensity distribution. The computer-based analysis, supported by preliminary experimental results, suggests that acceptable alignment tolerances are provided along with an unusually small beam divergence. With proper design, a far-field divergence of 0.5 mrad, encompassing near 80% of the total laser energy, appears feasible. Operational data, recently obtained with this optical extraction approach, have revealed a further important practical advantage over an unstable resonator, being far less prone to mode degradation and hot spot formation on optical component misalignment.