Design And Construction Of Axial Slow Flow Cw Co2 Laser

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chapter one

1.6:i
Energy Transfer in the Discharge:i

The most commonly observed laser transition in the CO2 molecule, barring the use of any frequency tuning mechanisms, are from the CO2 asymmetric stretch transitions, from the (00*1) to the (100*0)(10.6) micron and (02*0) (9.6) micron states, using the notation (v1v2*v3), where v1 refers to the symmetric stretch quantum number, v2 refers to the asymmetric stretch quantum number and v3 refers to the asymmetric stretch quantum number. There are literally dozens of other lasing transition. Which employing an interactivity grating .In a CO2 laser, depending on instantaneous gain medium and resonator conditions can easily choose.:i

Any single possible laser line can be forced through the use of an interactive grating. Rotational structure, having energies clustered very close to one another, may exist at any time. Nonradiative decay to short-lived lower lying states followed by Nonradiative decay to the ground state follows .N2 is added to the laser gas to more efficiently transfer energy from electron impact to the CO2 upper vibrations laser level. The glow discharge is a very effective mechanism for vibration excitation of nitrogen. Since N2 is ahomonuclear molecule, dipole radiate de-excited is forbidden. This allows for long-lived vibration states, which makes excited N2 molecules more readily available for collision excitation of CO2.:i

De-excitation is only accomplished collisionally with the wall or other gas constituents, the most beneficial of which is the CO2 molecule .The N2 (v=2) state is only 18cm-1 (2.2E-3 eV) from the upper laser level of the CO2 molecule .This makes resonant energy transfer between N2 and CO2 more likely.This energy is much smaller than the average kinetic energy of the molecules in the surrounding glow ,so vibration energy can easily be supplied to the CO2 molecules .:i

Energy transfer occurs from vibration levels up to v=4 in N2,because the ensuing anharmonicity of these states ,due to bond stretching ,is still well blow the average molecular kinetic energy .CO is isoelectronic with N2 and also has vibration levels easily excited in the glow discharge [24].:i

Thermal poisoning can occur, which is build up of lower lasing level populations in CO2.This results in a reduction in laser output power due to a clogging of the path from the upper lasing level to the ground state, where the CO2 upper lasing level is most efficiently populated through collisions with N2 these lower levels are cooled by the addition of He to the gas mix helium energy levels are much higher than the molecular energies of N2and CO2, above 20 eV.:i

For typical electron energies in the glow discharge of 1 to 3 eV, the discharge is not significantly affected by the addition of He other than to raise the electron temperature of the discharge [25].:i

Since the first ionization level of the He is higher than that of the other gas components, high energy impacts (higher “voltage”) is required to make it apart of the glow conducting path. Only a small amount of energy is lost from the discharge due to inelastic collisions with He and subsequent collisions with the walls. Thermal conductivity in gases is independent of pressure and since thermal conductivity of the He is roughly six times that of CO2 and N2, He makes an efficient transporter of waste heat to the walls of the discharge tube. The efficiency of heat transfer resulting from the addition of He to the mixture allows for higher discharge current before radiation saturation [23].:i

CO may also be added to the laser mix to improve efficiency, but it dose not transfer vibration energy as efficiently as N2, due to a difference between the CO v=1 level and the CO2 upper lasing level of 170 cm-1 . CO also has a dipole moment which creates a radiative decay channel to depopulate the electron impact excited CO, thus making CO less available for the job of CO2 excitation. CO is also a component in the dissociation equilibrium of CO2, so when using added CO with CO oxidation catalysis, larger concentrations of CO effect the CO2 concentration not always in a predictable manner. :i With these drawbacks, CO still adds to more efficient CO2 vibration excitation than electron impact alone. H2O can be added as a heat transfer enhance but is less efficient at cooling than He. H2O, in small concentrations, also has the beneficial side effect of homogeneous catalytic recombination of the dissociated CO2 products, CO and O.H2O in a larger concentration overwhelms the beneficial catalytic effects and effectively depopulates the upper lasing levels of CO2 .:i

Xe may also be added to a laser gas mix to effectively cool the electron temperature of the discharge for a given current, thereby reducing the amount of electron impact dissociation of CO2. The prohibitive cost of laboratory grade Xe prevented this investigator from utilizing it [25].:i

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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

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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