John W. Daiber

An Electrically Excited Gas‐Dynamic Carbon Monoxide Laser

A carbon monoxide laser is reported which utilizes a glow discharge in the plenum of a supersonic nozzle. The discharge selectively excites the CO vibrational mode, while the gas translational temperature remains relatively cold. Continuous output is obtained from optical cavities established transverse to the flow at two nozzle area ratios. Maximum laser power obtained to date is 6.8 W corresponding to an efficiency of 0.6%, based on electrical power input.

Performance of a large, CW, preexcited CO supersonic laser

The performance of a CO laser having electrical excitation by a continuously operating, self-sustained glow discharge in a high-pressure gas mixture followed by a supersonic expansion to the cavity region has been studied. A threshold for lasing was found which corresponded to 0.035 eV of input energy per CO molecule. Beyond this, nearly 15 percent of the additional energy was extracted by the resonator. The total power reached 940 W with a maximum electrooptical efficiency of 13.7 percent and a specific power of 17 kJ/lb. The vibrational population distribution of the CO in the cavity, with and without lasing, was determined from the overtone emission spectrum. The data indicated a significant loss of vibrational energy during expansion.

Laser‐Driven Waves in a Freely Expanding Gas Jet

The focused output of a ruby laser has been used to drive waves which travel in the same direction as the laser beam. The density gradient produced by expanding high‐pressure air or hydrogen into a vacuum was irradiated by the laser from the vacuum side of the jet. A luminous plasma was observed to propagate up the jet, through the orifice, and into the gas reservoir. Observations of this type have application to the study of comparable waves generated at the surface of a solid. Measurements of the luminous front velocity are reported over the gas density range of 1–7 amagats and laser power range of 100–500 MW. The scaling relations for this front velocity with laser intensity and gas density are derived for many of the interaction models proposed in the literature. Only the gasdynamic model with fluid motions induced during laser heating compares favorably with the data.

Two‐dimensional growth of laser‐driven waves in a hydrogen free jet

A Q‐switched ruby laser has been used to irradiate a free jet of hydrogen. The propagation of the luminous wave which originated in the breakdown spark has been monitored using an optical system which permitted simultaneous observation of the motion along and perpendicular to the coincident axes of the focused laser beam and the free jet. Correlation of the luminous frontal wave and the late‐time shock wave (made visible with a schlieren system) with the third root of the ratio of laser power to free‐jet plenum pressure was found to hold over a range of several orders of magnitude.

Electron‐temperature and spontaneous‐magnetic‐field measurements in a laser‐irradiated free jet

The output of a Q‐switched ruby laser irradiated a freely expanding jet of nitrogen gas. The laser optical axis was collinear with that of the jet. Gaseous breakdown occurred and the remaining energy within the laser pulse heated the plasma and drove forward‐travelling waves through the orifice and into the plenum of the jet. The temperature of the electrons within the plasma was measured using relative transmittance of x rays through thin metal‐foil spectrometers. The temperature varied as the square of the wave velocity from 60 to 160 eV. Measurements were also made of the spontaneous magnetic field generated in the vicinity of the plasma.

The efficiency of CO vibrational excitation in a self-sustained CW glow discharge

The fraction of the electrical power which is placed into the vibrational energy mode of CO by a CW, aerodynamically stabilized, glow discharge has been found to decrease from 76 to 30 percent as the input energy increases from zero to 0.8 eV per CO molecule. The remaining energy is measured as being lost to gas heating within the discharge. In the first of the two experiments reported here, the vibrational energy was transferred into stream heating by collisions of the excited CO molecules with aluminum screening. The resulting heating was used to determine the energy in CO vibration. In the second experiment, the relative intensities of the spontaneous emission from the overtone bands of CO were used to determine the vibrational energy content of the CO. The results of the two measurements agreed well. The fraction of power entering vibration was found to depend on the energy loading per CO molecule, independent of He diluent concentration, total pressure, or nearness to the arcing limit. The maximum energy which can be stored in vibration for this self-sustained discharge was found to be approximately 0.25 eV/CO molecule.