Design And Construction Of Axial Slow Flow Cw Co2 Laser

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chapter one

1.8i Self –Sustained Glow Discharge:
When along cylindrical glass tube with plane electrodes at its ends is filled with a gas at a pressure of ≈mmHg and the potential difference “V” between the electrodes is slowly raised, then a small current of about 10-12 A can be observed to flow through the gas.i
This current causes the ionization process in the gas .As V increases ,the ionization by collision in the gas beings to increase as well as the current rises . When the potential difference across the discharge tube reach the ionization by collision in the gas being to increase as well as the current rises. When the potential difference across the discharge tube reaches the breakdown value VB as determined by the gas, its pressure ,and the electrode spacing ,the current jumps to about 10-6 through it varies with the potential difference in a complicated way. .i

A resistor, of high resistance R, is connected in series with the discharge tube to limit the current at value of the supply voltage E. The current then takes such value that the voltage drop across the series resistor is equal to the difference between the supply voltage and the potential difference VB across the tube..i

The self-sustained glow discharge is, used to pump the longitudinal and transverse CO2 laser systems for different pressure [27]. Fig.(1-5)shows the various regions of the self –sustained glow discharge ,,which can be classified into very narrow dark space (Aston )close to the cathode. This is followed by a thin relatively weak luminous layer (the cathode glow) ,which in turne is followed by the cathode dark space ..i

Aston’s dark boundary separates the cathode glow are not always clearly visible .A sharp boundary separates the cathode dark space from the negative glow, which becomes progressively dimmer towards the Faraday Dark Space. The positive end of this is the positive column. It is either the region of uniform luminosity or regularly striated. At the positive end of this positive column there is sometimes visible an anode dark space followed by the anode glow close to it [27]..i

Fig.(1-5)istribution of the visible and dark regions, electrical field ,charges and current density in the normal glow discharge[27]..i

Consider an electron emitted from the cathode, which is accelerated in a strong field, however it performs few ionizing collisions due to its sufficient energy. Further from the cathode, the field has become weaker, the electron ionizes more efficiently. Near the boundary between the cathode space and the negative glow , the field has become very weak ,thus only the fast electron , which have not lost energy by inelastic collisions, will be able to ionize in that region .However ,a large number of electrons will cross the boundary and enter the negative glow [24]..i

In the negative glow occurs the recombination process between the electrons, which have low energy with positive ion produced from the collision between the electrons of high energy and this recombination produce the electron ion pairs. The negative glow is not affected by discharge tube dimension but effected by the cathode cross-section and the discharge voltage and current density .In the positive column the axial component of the electrical field is found to be constant at any point, it follows that the net space charge is zero (i.e. n+=n- )[20]..i

At the anode side of the positive column the anode attracts the electrons ,and the positive ions are repelled . A negative space charge is set up in front of the anode this lead to an increase of electric field as well as a rise in potential. The anode is therefore covered with a luminous sheath “the anode glow “[24]. .i

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chapter one

1.9i Output Stabilisation by the Opto –Voltaic Effect:
The phenomenon of static resistance fluctuation in a CW laser discharge due to the intra-cavity coherent radiation is well known as opt. -Voltaic or opt. galvanic effect .It can be used successfully for stabilising a single –line CO 2 laser with high accuracy, the alignment of a laser cavity, and the detection of radiation in CO2 laser.i
In general, the resistance fluctions of the discharge due to stimulated emission in a current stabilised or ballast resistor regime result in fluctuations of the power that is dissipated in the discharge, and hence in temperature variation of the plasma and the discharge tube. The relative changes of static resistance of the laser tube due to changes of the tube voltage V (or longitudinal electric field) and the discharge current I..i

where:.i
:
Changes of the static resistance of the laser tube..i

:
Static resistance of the laser tube..i

:
Changes of the tube voltage..i

:
Discharge tube voltage..i

:
Change of the tube current..i

:
Tube current..i

We shall consider the discharge laser tube as a nonlinear resistance with parametrical change of its voltage –current characteristic by the coherent radiation in the discharge tube .The operating current and voltage on the characteristic are determined by the voltage Ua of the power supply and the serial ballast resistor Rb ,the value of which must be larger than the absolute value of the negative dynamic resistance of the discharge tube ..i
The operating values of V and I0 change with the radiation power along the working –line determined by ballast resistor Rb and supply voltage Ua. .i

The impedance of the discharge tube, however, increases with radiation produced. This means that the variation of the discharge voltage ,called the opt-galvanic effect ,is in phase with variation of the radiation power and simultaneously the variation of the current ,called the opt-galvanic effect,has the opposite phase[27]. .i

.i 1.9.2 Gas Kinetic Temperature.

It is important that the gas kinetic temperature of the CO2 laser plasma be kept as low as possible. This follows both from Patel’s treatment of the gain of molecular laser (Patel 1964) which shows that the gain αT-3/2.The wall temperature, gas pressure and discharge current all affect the gas kinetic temperature, which varies radically, being maximum on the tube axis [15]. .i



1.9.3.i Helium Molecule.
The cooling of discharge gas is effectively obtained by the addition of He .The first excited state of the helium occurs at 159850 cm-1 , which is above the upper laser level (001) in only 2349 cm-1 above the ground level .The thermal conductivity of the He is about six times as large as that of CO 2 and N2.The difference between the temperature of the gas and the wall of the tubes cased by the discharge heat is inversely proportional to the conductivity of the gas .The considerable increase of heat transfer obtained by the addition of He means that the radiation production of the system saturate at higher discharge current [29]..i

As a result of the laser action, population of the (0110) level increases steadily. Since this level is close to the ground state, and since (k T) at room temperature is 210 cm –1 (where k is Boltzman constant (k=1.3805 *10-23 JK-1),and T Is absolute temperature in Kelvin ),it acts as bottleneck which prevents molecules transition down to the ground level .Such problems are overcome by the addition of He to the CO2 ,N2 gas mixture .Its addition effects the rat of dissipation of heat in the discharge tube that consequently effects the gas temperature ,and the rat of the thermal relaxation of each laser level .the effects of adding helium can be summarized by the following reactions :.i

CO2 (10˚0) + He → CO2 (00˚0) + He + K.E

He are added to the gas mixture in order to:.i

1.Empty the lower laser energy level so that population inversion is maintained. .i
2.Stabilize the electrical discharge by taking heat away from the lasing area. .i
((Advanced: (The specific heat (which determines the thermal conductivity) of He (1.24 [cal/gr* 0K] is five times that of Nitrogen (0.249 [cal/gr* 0K]).)) .i
Gas pressure inside the CO2 laser tube is 5-30 [Torr], of which 10% CO2 gas, 10% N2 and the rest is He [36][37]. .i

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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