chapter one

1.14i High Power System. (FOLDING)

The use of very log tube can introduce problems not encountered in smaller systems.practically log tube are usually divided into sections(with alternate anodes and cathodes )to keep the excitation potential to reasonably low value .However ,great care must be taken to eliminate all possibly reflections ,particularly wall reflections. It is necessary to break the system up into optically isolated sections each with smaller gain .Further problem encountered with very long systems is the negative lens effect produced by the refractive index gradients in the discharge plasma [30][31] .i

The folding methods which have been used are illustrated in figure 1.8 ,the most widely used is a pair of plan mirrors each set at 45˚ to the primary beam direction which reverse and laterally displace the beam .A variant of this idea is to use a corner cube in place of pair of mirrors .This configuration is exceptionally stable against mechanical vibrations ,through it dose involve an extra reflecting surface


(a)Two mirror or rooftop prism fold



(b)Corner cub fold

Figure (1.8):Schematic diagram of two-mirror and corner-cube folds[31].


1.15.i Cooling of CO2 Laser:
There are two basic ways of disposing of the waste heat .The first is to use the heat conductive properties of the gas and the structure containing it to carry off the heat Conductive –cooling is simple but ineffective because of the low thermal conductivity of the gas .Laser that use this cooling method generate an output of about 50 W/m of active length and therefor, must be relatively long to produce industrially interesting power level ..i

The second way to remove waste heat is to hot gas and replace it with cool gas by forced convection. Such flow –cooled laser are often called often called fast-flow lasers. Where as almost no heat leaves the active region of conduction –cooled lasers except through the tube walls with flow cooled lasers no significant heat flows to or through the tube walls compared with the amount of heat carried out of the active region of the gas flow .Power output for this type is not dependent on the length of the active region .It depend only on the mass flow rate of the gas and can be as high as 120 to 150 W of power output for each gram per second of mass flow [33][34]..i


A. Diffusion – Cooled Laser..i
In a diffusion cooled laser, waste energy is rejected in a characteristic time approximately that of the diffusion time (τD). If an electrical discharge column of diameter D is assumed, the number of mean free paths during which the energy diffuses is given by D/ where is the mean free path of the CO2 molecules in the gas mixture .The mean free time between collision is given by the ratio of D to the molecular speed. Since diffusion is a random- walk process, is not equal to but is equal to



Since the power achievable from a laser is approximately inversely proportional to such characteristic cooling time ,the power (PL ) of a diffusion- cooled laser is proportional to


Where


is the gas density.


B.Connective –Cooled Laser..i
If the gas is moved at speed is rejected in a characteristic time (τF ) which is given [35]..i




For the same active volume and gas density ,the ratio of the laser power achievable with a diffusion-cooled (PLd ) and with convection–cooled(PLc)laser is simply proportional to the ratio of the characteristic cooling times :.i



Since eq. (1-8) is much than unity for even relatively slow gas flow velocity, the advantages of convective cooling over diffusion cooling are readily apparent. .i

Assume that we have a rectangular volume of gas as schematically illustrated in figure(1-9),having a cross-sectional area A and a thickness χ .We dissipate PE watts of electrical power in the volume .We extract PL watts of laser power from the volume and PH watts of heat by convectively flowing the gas through the volume .If the laser extraction efficiency is η ,then



And



Where,.i



Where
[IMG][/IMG] is the gas density,
is the specific heat of the gas ,
is the velocity of the flow, and is the volume .Substituting (1-7) into (1-8 ) we obtain:





Figure (1.9): Schematic of Volume of Gas of Length x and Area A[36] .i