Kevin B. Hewett

Singlet oxygen generators - the heart of chemical oxygen iodine lasers: past, present, and future

Since the initial demonstration of chemical oxygen iodine lasers in 1977, researchers have realized that the heart of the COIL system is the singlet oxygen generator. This drives the performance of the system in terms of output power, mass efficiency, engineering complexity, reliability and maintainability. For this reason the singlet oxygen generator has been the focus of intense research and development efforts over the last 30 years. This paper reports on the history of singlet oxygen generators - starting with the simple sparger design used in the initial COIL demonstration and ending with current jet or droplet generators used in laboratories around the world. The relative performance of the different generator types will naturally lead to performance goals for the research efforts of the future.

Weight scaling model for high-power DPAL system

Since Krupke et.al. proposed and demonstrated pumping alkali atoms using diode lasers in 2003, there has been lot of interest in the diode pumped alkali laser (DPAL) systems. Several researchers have been able to scale the DPAL system to powers in the tens of watts. We have conducted a systems-level, weight-scaling study of a notional medium power, CW DPAL system. Three different modes of operation are considered: (i) very high pressure operation (over 25 atmospheres of He) in which the absorption and emission lines of the alkali atoms are broadened sufficiently to allow for efficient pumping with off-the-shelf diodes that have line width of 2 to 3 nm, (ii) intermediate pressure regime (~ 5 atmospheres) that requires diodes that are line narrowed to ~0.4 nm, and (iii) low pressure operation (~ 1 atmosphere) that requires diodes that are line narrowed to < 0.1 nm for efficient pumping of pump radiation into the alkali vapor. In the latter two cases some amount of methane, ethane, or some other gas would be needed to mix the two upper states rapidly; while in the first case, helium is used to broaden the transition and to mix the upper states. We have considered closed-cycle transverse flowing systems with the transverse length limited by medium inhomogeneity caused by heat deposition into the gas. Weight models have been developed for each of the following sub-systems: Pump Diodes, Fluid Flow System, Thermal Management System, Optics and Diagnostics System, Instrumentation & Control System, and Electrical Power system. The results of our weight estimates for a notional 100 kW DPAL system are presented.

Non-Intrusive mach number measurement in a supersonic hydrogen fluoride laser

An experimental technique has been developed to directly measure flow velocity and Mach number in a supersonic hydrogen fluoride laser. The technique uses a tunable diode laser source to probe the laser cavity at an angle to the flow creating a Doppler shifted lineshape. The amount of Doppler shift can be related to the flow velocity. The diode laser was traversed in the vertical direction to produce velocity and Mach number profiles.

Multiphase Reacting Flow Modeling of Singlet Oxygen Generators for Chemical Oxygen Iodine Lasers

Singlet oxygen generators are multiphase flow chemical reactors that produce energetic oxygen to be used as a fuel for chemical oxygen iodine lasers. In this paper, a theoretical model of the generator is presented that consists of a twophase reacting flow model that treats both the gas phase and dispersed (liquid droplet) phase. The model includes the discretization over droplet size distribution as well. Algorithms for the robust solution of the large set of coupled, nonlinear, partial differential equations enable the investigation of a wide range of operating conditions and even geometric design choices.

Hydrogen fluoride overtone laser: experimental methods of characterization

The uncertainty in both the fluorine atom concentration and the gain length has inhibited the development of accurate and device independent models of HF overtone lasers. Furthermore, previous methods of measuring the small signal gain were cumbersome and could not easily generate spatial maps of the gain in the cavity. Experimental techniques have been developed to directly measure the concentration of fluorine atoms, the gain length and the small signal gain in a hydrogen fluoride 5 cm slit nozzle laser. A gas phase titration technique was utilized to measure the fluorine atom concentration using HCl as the titrant. The gain length was measured using a pitot probe to locate the interface of the primary flow with the high Mach number shroud flows. A tunable diode laser was utilized to perform small signal gain measurements on HF overtone (ν=2→0) transitions.

Hydrogen fluoride overtone laser: 2D CFD modeling of the small signal gain

The small signal gain of a small-scale HF overtone laser was measured using a sub-Doppler tunable diode laser system. The spatially resolved, two-dimensional small signal gain maps that were generated show a highly inhomogeneous gain medium indicating the dominant role played by mixing of the H2 and F streams in HF laser performance. The measured gain data were analyzed with the aid of a two-dimensional computational fluid dynamics model. The results show that reactant mixing mechanisms have a large effect on the gain averaged over a vertical profile while kinetic rate mechanisms, including reaction rate constants and reactant concentration, have a greater effect on the maximum system gain.

Measuring the yield of singlet oxygen in a chemical oxygen iodine laser

A critical parameter for understanding the performance of Chemical Oxygen Iodine Lasers is the yield of singlet oxygen produced by the generator. Off-Axis Integrated Cavity Output Spectroscopy (Off-Axis ICOS) has been utilized to measure the absolute density of both ground-state and singlet oxygen in the cavity of a COIL laser.

Spatially resolved sub-Doppler overtone gain measurements on a small-scale supersonic HF laser

The small signal gain of a small-scale HF overtone laser was measured using a sub-Doppler tunable diode laser system. Measurements of reactant concentration, flow velocity and gain length were also made. The spatially resolved, two-dimensional small signal gain and temperature maps that were generated show a highly inhomogeneous gain medium indicating the dominant role played by mixing of the H2 and F streams in HF laser performance. The measured gain and temperature data were analyzed with the aid of a two-dimensional computational fluid dynamics model. The results show that reactant mixing mechanisms have a large effect on the gain averaged over a vertical profile while kinetic rate mechanisms, including reaction rate constants and reactant concentration, have a greater effect on the maximum system gain.

On the presence of rotational nonequilibrium in a supersonic HF laser

Overtone small signal gain data measured while operating a small-scale HF laser saturated on the fundamental transitions are compared with fundamental lasing output spectra and spontaneous overtone emission spectra measured orthogonal to the lasing axis. In all cases, the data are consistent with an equilibrium rotational distribution. These results are discussed in terms of their applicability to the question of rotational nonequilibrium in cw HF lasers.

Advanced chemical lasers

A review of recent advances in chemical laser technology is presented. New technology and concepts related to the Chemical Oxygen Iodine Laser (COIL), All Gas-phase Iodine Laser (AGIL), and HF Overtone Laser are discussed.