chapter one

1.10i Effect of E/N parameter:

An important parameter in the electron –induced population of both CO2 and N2 levels in a discharge is the ratio of the electric field E to the total density of neutrals N.The average electron energy in creases steadily as E/N increases from 10-16 V-cm2 , reaching ~3eV at E/N~6*10-15V-cm2 in atypical CO2,N2,He laser mixture .This value of E/N is that at which part of the incident discharge power first starts to be used in producing electronic excitation of N2 in pure N2 .i

Optimum pumping of the CO2 (001) level in pure CO2 occurs when E/N s~2.5*10-16 V-cm2 .It should be noted that E/N in typical high-pressure pulsed discharges is usually in the range (2*10-16ˉ─8*10-16 ) V-cm2.In electron –beam –controlled lasers E/N may be (1*10-16 -2*10-16) V-cm2 .The possibility of controlling E/N to tailor this ratio to the value most effective, since with optimized E/N ,as much as possible is being done to adjust the discharge to yield direct electron impact excitation of CO2 (001).Thus laser in which E/N can be controlled will be inherently more efficient that those in which the values of E/N tend to be relatively uncontrollable and high[28].i
The following equation shows the relation between the E/N ratio to the pressure gas temperature , gas density and discharge voltage at optimum pumping in pure CO2 when E/N equal 2.5*10-16 V.cm2





where






1.11 Parameters Affecting the CW Gain in CO2 Systems (D.C Excitation.].i

It is difficult to separate completely the influence of the various parameters on the gain of the CO2 laser, even when it is operating in the CW, D.C. excitation regime. This is primarily because many of the parameters are inter-related, e.g. change the current in the discharge will also change the balance of the components in the optimum mixture in a given tube, etc. Although one can predict the optimum operating parameters of the a system moderately well, it is almost always necessary to experimentally trim them to achieve the peak performance of the system [28].].i

].i 1.11.1 Current:

All CO2 laser mixtures show current saturation of output; as the current increases the laser output increases steadily to maximum and then decreases slowly as the current rises above the optimum .The current for maximum CW output depends on gas pressures and tube diameter .The axial gain behaves in a similar manner but the optimum current is much lower, generally (10-20) mA.].i

As the current is increased above this value the center of the gas becomes too hot for the gain to remain optimum so the radial gain profile in the tube dip in the center .The larger the current, the deeper is the dip and nearer to the walls are the points of maximum gain .].i

The efficiency of the laser also varies with current flowing through the tube, maximum efficiency and maximum output power cannot be obtained at the same time, the current which gives greatest efficiency being lower than that which gives the best output [28].[].i




Figure (1.6): Optimum Current and Total Pressure for Maximum CW Oscillator Output Power as A function of Tube Diameter [28].].i

].i1.11.4 Tube Diameter.

It already been mentioned that the tube diameter influences the optimum gas pressures and the currents (The larger the diameter, the lower the optimum gas pressures and the larger the current). ].i

The maximum output power obtainable is, however, virtually independent of tube diameter (up to 2-inch diameter at least) if all the other parameters are properly optimized for each tube diameter [28]. ].i

The peak gain attainable in the laser dose depends on tube diameter (see figure 1.6) [30]. The precise manner depends on the gas mixture, but the peak gain always decreases as the diameter increases.].i

To a first approximation the maximum axial gain for N2-CO2-He mixture varies inversely with the diameter.].i

1.11.5].i Gas Composition.
A wide variety of different three –component gas mixtures have been investigated in conventional flowing CO2 laser systems but the highest output power is obtained from a mixture of CO2 ,N2 and He .Generally ,the output power is not too sensitive to changes in the [CO2,N2]:[He] ratio but will depend quite strongly on CO2 pressure and the [CO2]:[N2] ratio .In practice ,to determine optimum operating conditions one would set the [CO2]:[N2] ratio constant at about 0.8:1 and then vary [CO2,N2]:[H2] until peak power was obtained .The [CO2]:[N2]ratio would then be “fine tuned” by varying the N2 flow rate to find a point at which the system was optimized[11].].i

1.11.6].i Efficiency:

The efficiency of conventional CW CO2 laser discharges may approach 30% at low laser output .it is shown that the efficiency depends strongly on wall temperature and the flow rate. It is also usually true that maximum efficiency of practical system may not occur at the same discharge parameters as the point at which maximum power output is optioned [30]. ].i