Charles A. Helms

A simplified analytic model for gain saturation and power extraction in the flowing chemical oxygen-iodine laser

This paper describes the development of a simplified saturation model (SSM) for predicting power extraction from a chemical oxygen-iodine laser (COIL). Using the Fabry-Perot gain saturation assumption, analytic expressions for COIL extraction efficiency are presented for both constant-density and variable-density cavity conditions. The model treats mirror scattering, nonsaturable distributed losses, and diffractive losses from the mode-limiting aperture and is shown to be in excellent agreement with experimental COIL power extraction data. A comparison of the model with the Rigrod power extraction model is presented showing that the Rigrod model accurately predicts COIL extraction efficiency only in the limit that the COIL device no longer behaves as a transfer laser.

Performance of a high-efficiency 5-cm gain length supersonic chemical oxygen-iodine laser

The Air Force Phillips Laboratory has developed a small-scale supersonic chemical oxygen-iodine laser (COIL) test stand (VertiCOIL) to rapidly evaluate emerging potential technology improvements. VertiCOIL was designed to address issues relevant to military and commercial applications such as long run time, high-efficiency operation, and compact design. VertiCOIL demonstrated an overall chemical efficiency of nearly 27%, one of the highest chemical efficiencies ever achieved by a COIL. In addition, VertiCOIL was able to lase for greater than 1 h by employing a continuously cooled basic hydrogen peroxide (BHP) flowing loop. Measured resonator parameters were used with a simplified saturation model to calculate the optical extraction efficiency. The peak optical extraction efficiency predicted by the model was 0.82. The dependence of the predicted extraction efficiency on outcoupling fraction is in good agreement with experimental results. Combining the predicted extraction efficiency with the COIL heuristic equation results in good agreement between measured and predicted overall device chemical efficiency.

Determination of singlet-oxygen generator efficiency on a 10-kW class supersonic chemical oxygen-iodine laser (RADICL)

With the advent of the Air Force Airborne Laser program, the emphasis of chemical oxggen-iodine laser (COIL) research has shifted toward improving the overall efficiency. A key component of the COIL is the singlet-oxygen generator (SOG). To assess the efficiency of the SOG, an accurate method of determining the yield of O/sub 2/(a/sup 1//spl Delta//sub g/), [O/sub 2/(a/sup 1//spl Delta//sub g/)]/[O/sub 2/(total)] where [O/sub 2/(total)]=[O/sub 2/(a/sup 1//spl Delta//sub g/)]+[O/sub 2/(X/sup 3//spl Sigma//sub g//sup -/)], has been developed. Absorption measurements of ground-state oxygen utilizing the magnetic-dipole transition, O/sub 2/(X/sup 3//spl Sigma//sub g//sup -/)/sub (/spl nu/=0)//spl rarr/O/sub 2/(b/sup 1//spl Sigma//sub g//sup +/)/sub (/spl nu/=0)/ centered at 762 nm, have been obtained using a scanning diode laser in conjunction with a multiple-pass Herriott cell and balanced-ratiometric detection on the 10-kW class supersonic COIL (RADICL). A series of experiments was designed and conducted to test the efficiency of the SOG under various conditions.

High-efficiency operation of a 5-cm gain length supersonic chemical oxygen-iodine laser

The Air Force Phillips Laboratory has developed a small-scale supersonic Chemical Oxygen- Iodine Laser (COIL) test stand (VertiCOIL) in order to acquire COIL performance data quickly and inexpensively. The VertiCOIL device has demonstrated a chemical efficiency of 26.7%, the highest efficiency ever reported for a supersonic COIL. VertiCOIL uses a continuously-cooled basic hydrogen peroxide flowing loop which allows run times of greater than one hour.

Iodine dissociation in chemical oxygen-iodine lasers (COILs)

Iodine dissociation has been measured in the supersonic cavity of a chemical oxygen-iodine laser during lasing under a wide variety of flow conditions. By varying flow conditions, measured dissociations from 0 to 100 percent were observed. A simple model of the initial step in the dissociation process was developed that adequately rationalizes the measurements.

Compact cw supersonic chemical oxygen-iodine laser (COIL)

A closed-loop flowing basic hydrogen peroxide (BHP) system with real-time cooling was constructed and coupled to a supersonic COIL, resulting in a 20-min. continuous run at an average power of 500 W. An overall BHP heat transfer coefficient of 150 BTU/(hr(DOT)ft2(DOT) degree(s)F) was measured.

History of chemical oxygen-iodine laser (COIL) development in the USA

This is an overview of the development of Chemical Oxygen-Iodine Laser (COIL) technology in the United States. Key technical developments will be reviewed, beginning in 1960 and culminating in 1977 with the first COIL lasing demonstration at the Air Force Weapons Laboratory (now the Phillips Laboratory). The discussion will then turn to subsonic laser development, supersonic lasing demonstration and efficiency improvements, and finishing with a brief discussion of some spin off COIL technologies. Particular emphasis will be placed on how the O2 (1(Delta) ) generator and O2-I2 mixing nozzle technologies evolved.

COIL performance modeling and recent advances in diagnostic measurements

This paper describes the analysis of power extraction from a chemical oxygen iodine laser (COIL) using a simplified saturation model (SSM). Previously our COIL modeling efforts have been limited by an inability to accurately measure O2(1(Delta) ) concentrations which in turn is a measure of the power available in the laser. Earlier application of the SSM to RotoCOIL data implied that our measured O2(1(Delta) ) could not be correct. In this paper we show how a new method for experimentally inferring O2(1(Delta) ) by measuring O2(3(Sigma) ) leads to better agreement between experiment and theory. These results strongly imply that if a COIL model is anchored to literature O2(1(Delta) ) measurements, caution needs to be applied when using the model for predicting performance.

Experimental investigation of homogeneous and heterogeneous nucleation condensation processes and products in COIL

This paper describes the preliminary results of an ongoing study to characterize the nature and sources of sub-micron aerosols in the Phillips Laboratory small scale supersonic COIL device. Heterogeneously nucleated aerosols from both sub- and supersonic flow regimes were sampled and characterized using the University of Missouri-Rolla, Mobile Aerosol Sampling System (MASS). Under all operating conditions where the oxygen generator discs were rotating, significant concentrations of aerosols were detected. Typically these aerosols had peak dry diameters of < 0.05 micrometers and nascent wet diameters of < 0.08 micrometers . Their total number density increased with increasing rotating disc velocity and with the addition of chlorine. A maximum number density of < 3000/cc was observed at maximum chloride flow rates when the initial generator mixture had been heavily depleted (i.e. neutralized with chlorine). The chemical compositions of these aerosols were found to be exclusively solution droplets of KOH and KCl. The critical supersaturation spectra for a KOH, KCl, I2 have been measured and compared to theoretical calculations thus permitting growth calculations based on laser operating conditions. Experiments to simulate, in a pure gas phase supersonic nozzle facility, cavity pressure increases indicative of homogeneous nucleation of aerosols were successful. Experiments to directly detect the presence of homogeneously nucleated aerosol under these clean simulation conditions are planned for the next phase of this program.

Extraction efficiency of a 5-cm gain length supersonic chemical oxygen-iodine laser

Gain saturation and diffractive loss data have been collected on the Phillips Laboratorys VertiCOIL laser. These data have been applied to the COIL simplified saturation model to estimate the optical extraction efficiency of VertiCOIL.