AXIAL LASER DISCHARGE STABILIZATION WITH ROTATING MAGNETIC FIELDS.

摘要

Efforts to increase the discharge specific power loading in high power carbon dioxide lasers have been hampered by the onset of electrothermal instabilities. Previous research, performed at the University of Alberta's Laser Laboratory, showed that a stabilization technique, using static magnetic fields, was effective in suppressing these instabilites, in both transverse and coaxial geometries, through the generation of sheared mixing velocities in the bulk gas and magnetic stabilization of the charged sheath regions.;A theoretical analysis showed that the stabilization mechanism in this axial discharge geometry was not a result of the generation of mixing velocities in the bulk gas, as was the case for the transverse and coaxial geometries. In fact, high speed photography revealed that the plasma column was deflected away from the centerline position when the transverse rotating magnetic field was applied. Further theoretical analysis showed that this deflection was the result of a Lorentz force acting on the plasma column. Consequently, as the magnetic field rotated, the plasma column swept around the discharge cross-section. As the magnetic field strength was increased, the Lorentz force grew, thereby deflecting the plasma still closer to the chamber wall.;Experimental studies were performed with the discharge test section, now 50 cm in length, mounted into a recirculating gas loop. Diagnostics, done with a 20 Torr laser gas mixture, included: terminal characteristics, temperature measurements, electron density assessments and gain studies. Laser power output is discussed briefly. The results indicated that a transverse rotating magnetic field can indeed improve gas discharge stability in an axial discharge geometry. A 25% increase in discharge power loading was demonstrated, using this discharge stabilization technique.;The purpose of this investigation was to adapt this magnetic stabilization technique to an axial discharge, with the use of a transverse rotating magnetic field. Preliminary work, done with a sealed cylindrical discharge chamber, 10 cm in diameter and 25 cm in length, showed that electrothermal instabilities were indeed preventable in such an axial discharge, through a judicious choice of magnetic field profile and strength.