chapter one

1.7i Glow Discharge in a CO2Laser
Brown`s classification of discharge [27 ]is shown in fig.(1-4) .A glow discharge is characterized by a relatively high specific electric power (1-10 W/cm3 )and by “gab” between the electron temperature Te (Te 1-2 eV) and the gas temperature Tg (Tg=300 –600 K).These two feature are important for the use of glow discharge to excited nitrogen –molecule vibrations and to pump the vibrational (001) level of the CO2 molecule.i
The glow discharge in a tube, the from investigated detail before the invention of the CO2 laser, is divided in to three regions: the cathode drop re (its value in N2 is 340 V, and its length d=0.4 /p, where d is in cm and p in Torr), the anode drop region (the voltage drop is several times ten volts definite (for the given gas ) electric –field strength E proportional to the pressure and weakly dependent on the tube diameter and on the current flowing through the column .For nitrogen we have E/P= 5 V/cm. Torr..i

The current density in the column has a maximum at the tube center and drops to zero at the wall .In a round tube of radius a it varies with the column radius in proportion to the Basal function J0 (2.4 r/a)[26]..i

1.7.1.i Normal Density of Current at Cathode
The current distribution on the cathode surface is different: at low currents the cathode emission (observed at those where current flows) covers a cathode area fraction proportional to the total current ,i.e., the current density on the cathode is constant –this is called the normal density , proportional to the esquire of the gas pressure and equal to 0.240 mA/(cm.torr)2 for nitrogen and copper cathode .As the current is increased and the emission covers the entire cathode area the current density increases in proportion to the current , and the cathode drop increases slowly (it doubles approximately when the current density is increased by two orders above the normal value )[26]..i

1.7.2.iNormal Anode Current Density
Glowing spots were observed also on a solid anode at place s where current flowed into the anode . This phenomenon was observed in animmobile gas both at low pressure and at pressures of several times ten torr in air and in nitrogen. Measurements of the normal current density ja in a stainless steel anode in this pressure range yielded ja =4.2 10-4p2 ja in A/(cm.torr)2 and p in torr for nitrogen ,and ja =2.6*10-4 p2 for air .glowing layer were observed on an anode placed in a laminar produced by introducing gauze grids (at a velocity 30 m/sec and pressures 20-70 torr of the air ,N2, and CO2 )[27]..i