Flashlamp design theory has not kept pace with design requirements for short pulse (<10 μs) operation. Recent tests have shed some light on changes that can be made to existing design equations to extend their validity into the short pulse domain but more experiments are needed before these results can be formalized. Present trial and error designs have performed well for some applications and some of the limits of operation have been extended by state-of-the-art advances in materials and design. Even for long pulse operation (50 μs - 3 ms), the design of a flashlamp to meet a specific application has not been straightforward. To a large extent, past experience plus iterative use of computer optimization codes forms the basis for a new lamp design. For short pulse operation even use of computer codes is of limited value.
This paper describes flow techniques developed recently for pulsed chemical laser applications. These techniques support development of a high power, repetitively pulsed DF chemical laser that has high beam quality and high mass flow utilization, and eliminate flow issues as major technical obstacles to such laser development. Three coupled flow problems are identified, and mutally compatible solutions to these problems are described. These problems are 1) extinguishing chemical reaction following each laser pulse (i.e. "flame-out") so that fresh D2-F2 mixture can flow into the cavity, 2) supplying a homogeneous reagent mixture with the required 99% individual species uniformity, and 3) rapid suppression and clearing of pressure disturbances produced by pulsed reaction to provide required density uniformity in the fresh mixture.
Work dealing with the development of a repetitively pulsed, high average power DF chemical laser is discussed. Primary emphasis is on demonstration of a scalable fuel/diluent feed system which allows fuel combustion to be extinguished between pulses. A valving concept is described where fluorine and diluent flow through the cavity on a steady-state basis and deuterium flow occurs through a valve which opens only when the lasing mixture is filling the cavity. A flow device utilizing this valving concept has been built and tested. Flame-out was achieved at 50 pps using fuel mixtures consisting of 70% He/20% F2 /8% D2 /1-2% 02 with fuel-to-purge ratios of 1:1.
A series of experiments using fast photography and interferometry have been performed to investigate the non-steady aero-acoustic effects in large scale, atmospheric UV-preionized CO2 laser discharges. Of primary concern were the density disturbances resulting from the sudden temperature and pressure excursions created by the electric discharge. A short discussion of the nature of the discharge generated disturbances is included and experimental results are presented.
The creation of a population inversion in large-volume high-pressure gas via an electrical discharge poses numerous engineering problems, not only concerning details of various components, but also in overall system performance or optimization. The Laser Division at Los Alamos has developed a base of engineering data and computational techniques which addresses the engineering design problems for our short-pulse, large, high-pressure CO2 amplifiers. The various aspects of the problem considered individually and in total are: 1. Selection of gas mixture for specific application; 2. Physical specifications (size, pressure, voltage, current, etc.) of discharge to achieve desired energy output. This involves the use of computer codes to account for gas kinetics relevant to the production of a population inversion and energy-extraction codes; 3. Specification of energy supply and delivery system to provide power to the discharge. This involves computer codes to predict discharge current and voltage waveforms, taking account of various circuit elements (capacitors, inductances, and transmission lines) as well as the nonlinear character of the discharge; 4. Nonuniformities in discharge and population inversion resulting from magnetic field effects, finite geometry, etc. This involves computer codes which consider electron trajectories in the high-pressure gas in the presence of electric and magnetic fields as well as the nonlinear characteristic of the discharge. The calculational techniques have been used to design a prototype amplifier for the Los Alamos 100-kJ Antares system. The prototype was built and the design calculations confirmed by measurements.
This paper reviews the recent development of mercury halide lasers. Comparison of the different pumping mechanisms is presented in terms of the efficiency for producing the excited state and the permissible operating conditions associated with individual techniques. A simple model for the optical extraction efficiency is discussed. Finally, the spectroscopy and some of the key issues for the related kinetics are briefly mentioned.
In this paper we briefly review some of the efficiency and energy scaling issues that are important in rare gas halide lasers. We use the KrF laser as a specific example and show that for this laser these issues are now sufficiently well understood so that optimum laser operating regimes can be chosen and theoretical performance predictions can be made. The other rare gas halide systems as well as the mercury halides are currently being actively investigated.
The recent emergence of efficient high-power lasers in the visible-UV region has led to their serious consideration for a number of applications. However, for some of these applications, while the lasers' output power and efficiency are quite adequate, their output wavelengths are inappropriate. In these instances, then, means of shifting the laser wavelengths without undue sacrifices, in the overall efficiency of the system and the spatial quality of the beam must be found. Two properties that any wavelength conversion scheme in this connection must have are simplicity and scalability.
The growth in high energy pulsed electric laser technology has depended heavily on the development of electron beam sources. In response to this need, various electron beam technologies have been developed and are in a continuing state of evolution. Gun technologies related to moderate and high current densities and their applicable parameter ranges will be described in this paper. Primary power conditioning requirements will also be indicated.
Criteria for fluid and thermal properties delineating the baseline flow homogeneity, such as those for ΔT/T, ΔP/P, Δp/p, ΔU/U will be established. These are based on medium optical characteristics and requirements in terms of wavelength, pressure, and temperature of the gas medium associate,- with visible wave-length lasers. To meet these criteria, various methods for conditioning the baseline flow from the stand-point of gas dynamic and acoustic technologies will be presented and discussed. The merits of open and closed cycles for the baseline flow will be evaluated through consideration of gas utilization efficiency and component requirements. These criteria become acoustic dominated for pulsed lasers. The pulse repetition frequency required for a given laser defines the clearing time allowed to restore the original baseline flow. A parallel consideration becomes necessary to account for the pulse input perturbation. Solutions to acoustic management problems must be obtained by considering all sources of reflection, acoustic characteristics of open and closed cycles in terms of wave propagation, attenuation, and dissipation.
A general nonlinear treatment of mode medium interaction in CW flowing gas lasers is presented. Mode medium interaction is the term which has been used to refer to the dynamic process of extracting optical energy from an excited laser medium. The nonlinear theory describes the coupling between the excited medium perturbation due to optical power extraction and the optical system controlling the power extraction. The nonlinear theory shows that, while a significant mode medium interaction can exist in large scale devices, no acoustic instability is predicted. This prediction is contrasted with results obtained from a conventional linear instability analysis of the same system which erroneously suggests an acoustic instability such that beyond a small threshold value, medium and optical power disturbances are amplified to instability. The underlying reasons for the breakdown of the linearized theory are described. Scaling relations developed from the nonlinear theory are also presented to establish that high quality large scale CW flowing gas lasers are feasible. The paper is theoretically oriented; however, some supporting experimental data is presented. In addition to the nonlinear stability analysis, an accurate description of cross-flow medium acoustics is developed, which properly accounts for the bulk heat input into the flowing medium. It is shown that the pressure gradient, produced in the flow direction by the medium bulk heating, strongly damps the buildup of cross-flow acoustic disturbances.
The design and performance of a CO2 CW EDL are described. Particular emphasis is given to the effects of medium inhomogeneities and mode medium interaction on the properties of the output beam. Experimental results show medium density profiles, predominantly linear, quadratic and cubic, arising from the heating due to power loading of the gas. Fluctuations in the output flux are also shown. These arise from the interaction of the mode flux with the medium. The variation of this interaction with device parameters on beam quality is discussed. Experimental results showing a minimal impact of mode-medium effects on beam quality are shown.
Gasdynamic lasers (GDLs) have been recognized for the last decade as prime candidates for very high power CW infrared laser systems, but in general, have been considered to have very limited performance potential. Recent analytical and experimental work indicates considerable improvement in the specific power of GDLs that can be achieved. This paper presents a summary of these results.
A detailed fluid mechanics and optical analysis has been completed for the TRW CLXV chemical laser nozzle. This nozzle configuration incorporates contoured supersonic nozzles for both the fuel and oxidizer streams. Both oxidizer and fuel nozzles incorporate "trip flows" to enhance the mixing of the two streams. The results presented were obtained from a two part study, the first of which investigated the detailed fluid mechanics and chemistry of the gain regions with simplified optics and the second of which included a detailed physical optics analysis with a simplified fluid mechanics model. In the former, the LAMP computer code was utilized and in the latter the MRO and BLAZER codes were used. The chief interaction between the two parts was the development of the fluid mechanics model in the MRO/BLAZER computer codes from the results of the detailed fluid mechanics analysis. The results obtained compare favorably with measurements which have been made in experiments at TRW. These include small signal gain profiles, chemlum results, power distributions and flow field pressure measurements.
Variation in coupling of unstable resonators is analyzed for the case of low order medium inhomogeneities. The analysis examines one-dimensional confocal unstable resonators with linear or quadratic index profiles in the resonator medium. These types of effects can arise from the bulk and wall heating present in gas lasers. The analysis shows that both the effective Fresnel number and the geometric magnification influence the local stability of the design point coupling. Choosing the Fresnel number to maximize mode loss separation may not be consistent with the choice of Fresnel number to minimize the influence of medium variations.
A highly accurate gain diagnostic system has been developed which will reliably measure gain coefficients as a function of spatial position and laser transition for specific application to high energy chemical lasers. The system consists of a state-of-the-art single line, single mode, frequency-stabilized DF/HF chemical laser, an advanced programmable optical system which provides a two dimensional raster scan through the lasing medium, and a mini-computer based data processing and control subsystem. The DF/HF chemical laser and scanning optics have been designed for both remote and local operation. The data processing electronics are capable of providing a "quick look" real time gain profile of the medium. The dedicated mini-computer subsystem has been specifically designed to provide a complete analysis of the gain data in both tabular and graphic forms; and, by storing the data as complete digitized waveforms, dramatically reduces the data analysis time compared to conventional techniques. A detailed discussion of the operation and capabilities of the gain diagnostic system will be presented with representative results of the system.
Typically, for a high energy laser experimental program numerous parameters such as power, irradiance, phase, etc. must be measured in order to adequately characterize the system performance. Correlation between effect and cause in a timely manner becomes very important in minimizing experimental effort. To this end, an on-line minicomputer controlled measuring system was developed. The interfaces to this system necessary to accommodate a wide variety of instruments (i. e. , calorimeters, IR cameras, spectrometers, etc. ) for a specific experiment are few, with most additions only requiring a software change. Some system features discussed include: time indexing of data f or search, retrieval and correlation; real time data acquisition and control of diagnostic instruments; graphical display capabilities such as isometric and contour beam intensity plots, beam jitter, etc. ; and software capabilities such as cross and auto-correlations, fast Fourier transforms, and signal averaging.
Evaluation of continuous wave chemical laser performance has been accomplished through the investigation of key operating parameters. Various optical diagnostics have been employed to measure output beam quality, jitter, spectral content, total power and power spectral density. Near- and far-field intensity distributions have been characterized. Flow field visualization systems to monitor index variations, small signal gain, and vibrational populations have been examined. Techniques utilizing grating rhombs, scanning spectrometers and infrared cameras will be discussed. Methods employing Fizeau interferometers, probe lasers and chemiluminescence spectroscopy will also be addressed. Data reduction and interpretation of results from these diagnostic techniques will be described.