Craig T. Walters

Nd-glass burst laser with kW average power output

Demonstrations of operation of a compact neodymium glass laser with average output powers greater than 1 kW for several seconds are reported. The laser is based on the thermal inertia laser (TIL) concept wherein a neodymium-doped glass rod is pumped uniformly without cooling during a burst mode of laser operation. Design principles for TIL devices and scaling to 100 kW class lasers are discussed. Experimental results for a low repetition-rate proof-of-concept pulsed device (30 J, 0.2 Hz) and a high repetition-rate pulsed prototype (40 J, 36 Hz) are presented and compared to numerical solutions for the laser rate equations with temperature dependent cross-sections. >

Laser cleaning of metal surfaces

There is a critical need to replace ozone-depleting substances and hazardous chemicals that, in the past, have been used routinely in aerospace maintenance operations such as precision cleaning of metal surfaces. Lasers now offer the potential for removal of many organic materials from metals without the use of any solvent or aqueous cleaning agents. This paper presents quantitative results of laser-cleaning process-development research with a pulsed Nd:YAG laser and several common metals and organic contaminants. Metal coupons of Stainless Steel 304, Aluminum 5052, and Titanium were contaminated with known amounts of organic oils and greases at contamination levels in the 5 to 200 (mu) g/cm2 range. A fiber-optic-delivered 1064-nm pulsed laser beam (20-Hz repetition rate) was scanned over the coupons with different overlap and pulse fluence conditions. Measurements of mass loss revealed that all levels of initial contamination could be removed to final cleanliness levels less than 3 (mu) g/cm2, at which point the mass loss measurements became uncertain. Pulse fluence thresholds for initial cleaning effects and practical cleaning rates for several metal and contaminant combinations are reported. From the totality of the results, an overall picture of the contaminant removal mechanism is emerging. For semi-transparent films, it is conjectured that a thermo-mechanical effect occurs wherein the laser energy is absorbed predominantly in the metal substrate which expands on the nanosecond time scale. This rapid expansion, in combination with some material evaporation at the film/metal interface, is believed to eject the contaminant film directly into aerosol droplets/particles which can be swept away and collected for recycle or cost- effective disposal in a compact form. Evidence for this mechanism will be presented.

Flat-field postobjective polygon scanner.

A general two-dimensional ray-trace analysis is presented for the motion of a geometric focal point over a flat surface provided by a postobjective rotating polygon laser beam scanner. The exact defocus equation is derived for any value of the neutral scan position deflection angle and the polygon rotation angle. The scan nonlinearity is derived for the special case of a zero neutral scan deflection angle. Geometric parameters were found that reduce the peak-to-peak defocus by more than an order of magnitude from that found in previous design approaches. Conditions were also found that reduce scan nonlinearity to less than 2 × 10(-4). Practical limitations, such as large polygons and beam obscurations, encountered in the implementation of postobjective scanning are discussed.

LASER GENERATION OF 100-kbar SHOCK WAVES IN SOLIDS

Neodymium-glass laser pulses (1.06-μm wavelength, 25-ns pulse width) have been used to generate shock waves with peak pressures in the 5- to 120-kbar range at the front surface of solids. Relatively uniform irradiance levels were employed with circular beam areas in the 0.4- to 1.5-cm2 range and single pulse energy up to 800 J (fluences ranged from 200-2000 J/cm2). At 1000 J/cm2, the resulting peak shock pressure is about 35 kbar. By confining the plasma with a transparent glass overlay, this peak pressure was raised to 120 kbar. The nature of the plasma initiation process has been revealed through careful simultaneous temporal resolution of the beam-power, temperature, and stress-wave details.

Heat-transfer analysis in loss-of-coolant accidents

Abstract A computer program has been written for the comprehensive analysis of core-pressure-vessel heat transfer during a loss-of-coolant accident. The program, NURLOC-1.0, is based upon a generalized procedure for numerically solving the two-dimensional heat-conduction equation over multi-connected regions with various complicating factors specific to this application. The numerical procedure consists of solving the explicit finite-difference form of the conduction equation at mesh intersections. Boundary conditions and heat-generation rates are set by models that characterize various complex chemical and physical processes which can occur during an accident. The processes considered in NURLOC-1.0 include boil-off of residual coolant water, metal-water reactions, radiative exchange, and material transport as a result of melting in addition to the usual conditions of convection, fission-product-decay heating, and delayed-neutron heating.

Laser removal of contaminant films from metal surfaces

In precision cleaning operations, it is essential to remove all traces of organic films and other contaminants, such as particles and fibers, from critical aerospace components. In the past, this was accomplished effectively with powerful solvents such as CFC-113, which can no longer be used because of its adverse effects on the environment. Among many alternative cleaning technologies under investigation, use of lasers to remove contaminants has recently shown promise in several applications, particularly in the area of particle removal from semiconductor surfaces for microcircuit manufacture. We present here, results of some of the first definitive studies of removal of organic films from metal surfaces using pulsed lasers. The substrate metals included aluminum and stainless steel, test coupons of which were contaminated with controlled amounts of organic substances (oils and greases) that might be present from normal use or handling of parts made from these materials. The test coupons were laser cleaned with short pulses having wavelengths selected to span a range of physical removal mechanisms from photo-chemical ablation to pure thermal effects (248, 355, and 1064 nm). Cleaning thresholds were measured using sloped irradiance profiles and post-test SEM observation of the position of the boundary between cleaned and uncleaned zones. These test results were combined with engineering studies of optical fiber beam delivery approaches in the design of a system for precision cleaning of the inside surfaces of metal tubes. Quantitative results of the laser cleaning tests and their implications for optical fiber-based laser cleaning systems are presented.In precision cleaning operations, it is essential to remove all traces of organic films and other contaminants, such as particles and fibers, from critical aerospace components. In the past, this was accomplished effectively with powerful solvents such as CFC-113, which can no longer be used because of its adverse effects on the environment. Among many alternative cleaning technologies under investigation, use of lasers to remove contaminants has recently shown promise in several applications, particularly in the area of particle removal from semiconductor surfaces for microcircuit manufacture. We present here, results of some of the first definitive studies of removal of organic films from metal surfaces using pulsed lasers. The substrate metals included aluminum and stainless steel, test coupons of which were contaminated with controlled amounts of organic substances (oils and greases) that might be present from normal use or handling of parts made from these materials. The test coupons were laser clean...

Anomalous Behavior Of The Reflectivity Of Metals Irradiated With 10.6 µm Laser Pulses

Anomalous behavior of the specular reflectivity of polished metal surfaces subjected to intense CO2 laser pulses is reported. Irradiations of several metallic elements and alloys were conducted in hard vacuum and time-resolved measurements of specular reflectivity were performed during the 60-ns pulse. Results indicated in many cases a transient drop in reflectivity to a level in the 10-40% range followed by recovery to near unity by the end of the pulse for a stabilized surface. Metal vapor plasma was not detected in most cases and is not believed to be the dominant mechanism operative. Theoretical estimates of surface temperature transients are presented along with correlations of observed surface damage. Mechanisms which may explain the observed anomaly are discussed.