Donald A. Durran

Performance of cw HF Chemical Laser with N2 or He Diluent

Recent experimental results on the performance of a continuous HF (or DF) chemical laser are reported. In this device, an arc‐heated diluent is mixed in a plenum with SF6 to provide F atoms. The mixture is expanded to form a supersonic jet into which H2 (or D2) is diffused. Population inversion and lasing are due to the reaction H2+F=HF(v)+H; v≤3 and Δ H=−31.7 kcal. Power levels above 1 kW have been obtained in a N2 diluent gas flow, and levels above 1.7 kW have been obtained in a He diluent gas flow. Specific power yields are 43 and 108 kW sec/lb for the N2 and He systems, respectively. The variation of laser power and efficiency with cavity optical axis location and SF6 mass flow are reported for both N2 and He systems. A spectrum of the HF laser operating in a He flow is also presented that gives evidence of v=3 partial inversion.

Performance of a 500-kjoule MHD wind tunnel

The operation of a pulsed MHD accelerator is described for a MHD wind-tunnel application. The accelerator is designed to provide a 50% velocity increase (from 15.5 kft/sec to 24 kft/sec), at constant enthalpy, of a gas at approximately atmospheric pressure. The accelerator gas source is a hot shot that heats potassium seeded nitrogen to 9000 °K and 1400 atm. The gas is expanded into a segmented electrode (94 pair) MHD accelerator channel. After tunnel startup, a 3-msec constant power pulse is switched on the electrodes so that current flow is approximately perpendicular to a quasi-steady 4-wb/m2 magnetic field. The power-on to power-off velocity ratio (=un/u/) at the accelerator exit is determined from changes in the stagnation pressure and open circuit induced potential. The corresponding enthalpy ratio (==hn/h/) is determined from the static pressure change and un/Uf. Results 0.5 msec after switch-on indicate un/Uf « 1.5 (un ~ 24 kft/sec) and hn/hf « 1.0. As a result of constant volumetric power addition and a decaying mass flow, un/u/ and hn/hf increase to 1.7 and 1.2, respectively, just prior to channel power switch-off. Although these results are essentially the design values, the detailed performance of the accelerator appears to be significantly influenced by spatial nonuniformities in the flow.

Continuous‐Chemical‐Laser Cavity Studies

The extent of the lasing medium along the jet axis in a continuous chemical laser is variable under changes in the jet temperature, pressure, velocity, reactant concentration, etc. The laser cavity must be readily tunable to accommodate the changes in lasing length in order to maintain maximum multimode laser power operation in a high‐Fresnel‐number cavity under variation in the jet parameters. In addition, as an aid in understanding laser performance, it is desirable to have the means to determine the multimode laser intensity distribution in the lasing region for various conditions in the jet. Techniques are presented to determine the optimum optic axis position for maximum multimode power operation, and to measure the intensity distribution along the jet and extent of the lasing medium. A flat and spherical mirror tuned during laser operation determine peak laser power position and the lasing length of the jet (typically 1. 6 in. in N2 flows). Axial local power surveys are made with this same cavity ar...

Hot-Shot Gas Source for Pulsed MHD Accelerator

Abstract : A hot-shot type nitrogen source has been developed that will produce the desired gasdynamic properties required for pulse-type MHD accelerator experiments. The source stagnation conditions are T3 = 9000 K and P = 1400 atm. Usable flow time of the source is approximately 3 msec, and the source pressure decay rate is 15%/(msec). A real-gas expansion (N2) can be made for typical gasdynamic parameters that agrees with measured channel values within experimental accuracy by accounting for contamination and seed material (KN3). Evidence presented supports the conclusion that highly ablative liners (Delrin, Teflon, etc.) considerably degrade the performance of high-pressure, high-temperature discharge devices. BN liners have an average contamination of 4% by weight; lower contamination is probably possible with lower stagnation temperatures. (Author)

PRESSURE GAUGE FOR PLASMAS SUBJECTED TO LARGE DYNAMIC ELECTRIC AND MAGNETIC FIELDS.

Abstract : A static pressure gauge, comprised of a transducer and a resistance bridge, for use in high-temperature (4000K), high-pressure (0.5 to 10 atm), plasma flow of short duration (10 msec) is described. The transducer is electrically insulated from the flow and used when the plasma is 1 kV above the instrumentation reference. The gauge output is free of negative pickup when used in a 4. 0-Wb/sq m time-varying, crow-barred sinusoidal negative field of 4-msec quarter wavelength. (Author)

Variable-aperture calorimeter for an unstable resonator.

A new technique for measuring the output power from a cw edge-coupled unstable resonator has been developed and applied to a cw DF laser. An internal-cavity calorimeter was used that permits measurement of output power with a variable rectangular mode geometry while containing the radiation inside the resonator. The variable aperture calorimeter absorbing scraper (VACAS) device consists of an absorbing copper plate calorimeter assembly, which replaces the 45 degrees output coupling mirror of a conventional edge-coupled unstable cavity. A rectangular hole in the plate of variable width w and height h defines a rectangular mode of variable width Mw and height Mh within the geometric optics approximation, where M is the magnification. Two important applications of VACAS are (1) the determination of the optimum mode geometry for a cw diffusion-type chemical laser where there is significant variation in gain across the unstable resonator mode and (2) the determination of the power tradeoff between mode width and the number of folds in a folded unstable resonator configuration.

Gauge for Measuring Impulsive Pressure in a Container Subjected to Large, Time‐Varying Applied Voltages

A pressure gauge consisting of a strain‐gauge transducer, bridge, and decoupler is described for the measurement of time‐varying pressures in the presence of an electrical noise environment typical of capacitor discharge systems. In particular, the transducer of the gauge has been subjected to large, time‐varying applied voltages, Vmax≈103 V and [(dv/dt)]max≈5×106 V/sec, and large, time‐varying magnetic fields, Bmax≈2×104 G and [(dB/dt)]max≈5×106 G/sec, with no ill effect on the output of the gauge. At least an order of magnitude increase in these quantities (i.e., Vmax, etc.) is believed possible without serious effect on the gauge output. The gauge can be statically calibrated, has a rise time of ∼4 μsec, and can be compensated for electronic drift.