Design And Construction Of Axial Slow Flow Cw Co2 Laser

السلام عليكم ورحمة الله وبركاته

استكمالا للبحث سوف نبدأ باذن الله بالفصل الثاني:

chapter two

Co2 laser system

2.1i Introduction:

The first start to design CO2 laser system that we should know the relative optical output power that we wont to get it and the efficiency of the system that we should work with it .i

In this project the target optical power was 500W and the efficiency that we assume to work in 10%.So, the input electrical power should be of about 5000W and because we work in slow flow this mean that the available output power with respect to the length of the discharge tube of about 50 W/m.. So for 500W the overall active medium length of about 10m.i

The use of very long tube can introduce problem not encountered in smaller systems. So, long tube were divided into section with alternate anode and cathode to keep the excitation potential to reasonably low value .It divided into seven folds each folds consists from two discharge double jacket tubes with parallel input discharge voltage ( see figure 2.1) .By using stability condition of the optical resonator we found that the radius of curvature of the higher reflector mirror should be greater than or equal to 10m we chose hemispherical stable resonator to provide higher gain and easy in the alignment process..i

Using the above input parameters to the system and by using eq: (1-16) ,(1-17),(1-18),(1-22) and (1.23). at different variable parameter a compromise has been made between these parameters of the optical resonator using forward and backward calculation to specified the optimum values of diameter ,the best radius of curvature of higher reflector mirror ,the beam waist of the output window gain coefficient , Frenal number and the best output power that we can get from all these variable parameter (see table 2-1)..i

We found from the above calculation that the best discharge tube length equal to 0.75m, tube inner diameter equal to 0.17m. the best mirror reflectivity of 60%. Assume that we work at an output beam of TEM00 mode ..i

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Fig.(2.1):Multi Folds Structure for Our Design System ..i

Table (2-1): Theoretical Calculation for Output Power at Different Variable..i

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In the present work five different electrodes shapes and discharge tubes have been designed and constructed .The details for these terminals, discharge tubes and power supply unit, cooling system, vacuum circuit and gas supply have also been described. .i

chapter two

2.2i Mechanical Design:
Mechanical design represents the base for constructing the system because we take all basic principle for CO2 laser in to a count and related on it to design the system which include the size of system, type of material that must be used in each part of the system. Make sure that all quit good for optimal performance and min cost.i

The mechanical design includes five different forms of electrodes configuration. One made from Bakelite and the other from Aluminum..i

1.i . The first Type Aluminum electrode used as the output couplers mirror holder that consists from four parts. The first part use as mirror holder, which has central hole of 20 mm in diameter for laser output. The output window has been placed in the hole of 52mm diameter and thickness equal 5mm and water surrounding the mirror from every direction except the face for cooling the mirror. Tape screw holes of 8mm in diameter for inlet and outlet cooling water have been used. .i

An O-ring groove of 4mm thickness is made in front of the mirror for fitting and safety the mirror from broken and not let water to drain in front of mirror that because ZnSe mirror coating affected by water. Another O-ring groove of 4mm thickness has been made and used between first part and second part. This O-ring is important in the alignment of mirror. .i


The second part is the mirror adjustment plates Three screw holes are placed 120 degree apart is made for mirror adjustment and thick O-ring groove to provide both high vacuum seal and enough play for mirror adjustment .It has a central hole of 21 mm in diameter..i


The third part represents the cathode and tube holder that has central hole of 21 mm in diameter and hole of 10 mm in diameter from top of view of the electrode to enter the gasses to the plasma discharge tube. There is also groove of 4mm thickness for O-ring from one side and from far side there is cut of 45º for O-ring.An O-ring has been used for tightening the connection between the tube flange which is the fourth part of this electrode and with cathode to provide both high vacuum seal and decrease the pressure surrounds the tube..i


The fourth part is the circular flange of central hole of 21 mm in diameter and three screw holes placed 120º for easy work and to providing high vacuum seal ..i

2.i . The second electrode is the aluminum rear mirror (HR) electrode which consist also from four parts .All parts of this electrode like the first electrode except the first part has one different from first electrode that the mirror cooled from back and also from sides..i

3.i . The third electrode is also made from Aluminum and have the same parts of second electrode except that it has the central hole of 61mm in diameter in third and fourth that because we use Pyrex tube of 60 mm in diameter and 80 mm length for joining these parts with the fourth electrode. .i

4.i . The fourth electrode (see figure 2.2) is the corner holder tubes electrode that represents new design shape for multi fold CO2 laser. Designing this shape depend on corner of 20º .It consist form main part that is used as cathode and holed two discharge tube with one connecting tube. At top view of this part there is hole of 12 mm in diameter for outlet the gas..i

Also, there are three holes (one of 61mm in diameter and two of 21mm in diameter), the center of last one make a corner of 10º with strata line from face of the mirror on the third electrode..i

Light should enter from center of one hole of inner diameter (ID) of 20 mm pass through tube ID of 60mm touch the center of the mirror (which represent the center of the corner) and then reflected back to the second hole of ID of 21mm. .i

5.i . The last and fifth electrode (see figure 2.3) is Bakelite electrode which is made from Bakelite material .It is very high electrical insulation .The main part (A) of this electrode has a cylindrical shape of diminution (100×100) mm with central hole of 21 mm inner diameter for holding two discharge tubes [one from each side]. Both tubes enter to 10 mm distance in the Bakelite. .i

This part also has tip hole of 8mm inner diameter as inlet of gas from top view of electrode and there are two tips holes of 6mm diameter use for discharge pins where the last are made from stainless steel for high conductor and corrosion resistance..i

The discharge pine have length of 60 mm. About10mm of it length left on top of the electrode used for connected anode wire comes from power supply with system..i

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Fig.2.2: Top View Section with Dimension for Holder Tubes Electrode
(New Design)..i

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Fig.2.3: Sides View Section with Dimension of the Bakelite Electrode.

chapter two

2.3i Optics:
The optical cavity has two main cavity mirrors (output window and rear mirror). A ****l coated gold mirror has been used as the higher reflector mirror, The radius of curvature of it is 10m, 98% reflectivity and dimension of (50 ×10) mm. Another ****l gold coated mirror, plane, 98% reflectivity and has been used just as reflector.i
[’’’’The output widow is made from more exotic material Zinc Solenoid (ZnSe) for its low absorption at 10.6 μm and high thermal resistance .It is reflectivity was 60%, absorption coefficient was 0.005cm-1 and its plane which made with HR mirror hemispherical resonator that represents the best type for slow flow CO2 laser.i

2.4i Gas and Vacuum System:
We have been used premixed CO2 gas of the ratio (8%CO2: 8%N2: 82%He:i
2%CO) and flow meter of the maximum scale of flow rate equal to 50 liter /min.i
Oil sealed mechanical pumps has been used in which the gas is trapped, compressed and removed bodily from low pressure to the high pressure side of the pump, hence it is expelled to the atmosphere .The rotary that has been used have speed of (50 L/min) type Leybold –Heaves S8A and we measured the pressure by using absolute gage of maximum rang 27 mbar and tolerance of 0.9 percent. i
2.5i Cooling System :

In order to find the best radius of the jacket tube we have studied the effect of change tube radius on the water flow rate and instead of that it effect on the water temperature degree by using few relations as shown below:i

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From above equation we found that the best radius of the jacket tube equal to 0.02 m at flow rate equal to 10 liter /min (1.6666×10-4m3/s) and rate of change of water temperature equal to 8.556*10-3˚C (see table 2-2).i

Table (2.2): Effects of variation radius of jacket tube on the water velocity and temperature. i

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2.6i Laser Discharge Tube Fabrication:
Pyrex glass double jacket has been used as plasma tube or discharge tube. Several tubes have been made. Each double jacket tube has two concentric tubes, one of them has 17 mm inner diameter and 750mm length used as a plasma discharge tube and the second one of 40mm inner diameter and 670 mm length used for getting rid of excess heat (see figure 2.11) i

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Fig.2.11: Dimension of One of Tubes used as Laser Discharge Tube.i
A comprehensive check was regularly done to be sure that water circulation paths are well insulated from high voltage and that all fittings are securely grounded.i

Cooling system consists of chiller type (Leybold 20 AR ) having flow rate of 20 L/min was used. A connection of plastic tubes to let the water reaching the double jacket tubes and to the mirror cooling area.i

chapter two

2.7i The CO2 laser Power Supply:
Gas mixture pressure plays a dominant role in determining the gas laser output. This parameter may affect directly on the lasing action. The gas pressure is a function of the applied discharge voltage.i

So, the increase of the gas pressure means increase the discharge voltage and that lead to increase the output laser power to a certain limit..i

Calculations by using equations (1-3), (1-4) and (1-6) have been observed to find the optimum discharge voltage for each pressure (see table 2-3). Figure (2-4), which depicts the increasing of the discharge voltage with the pressure inside the cavity of the present system which has discharge length of 0.75m and total active length of 1 m at room temperature..i

The figure reflects the depend of the pressure inside the active medium on the discharge voltage within the working conditions. There is a linear increase between the pressures with the discharge voltage..i

Table2.3: The effect of increasing molecule density on the discharge voltage at different pressure..i

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Fig. 2.4: Theoretical Calculation of the Ignition Supply Voltage (kV) Variation with Pressure..i

The power supply has been designed and constructed to supply high and variable D.C.voltage required to break down the gas impedance and to and to provide glow discharge voltage. The circuit diagram is shown in figure 2.5 and the operation concept can be summarized as follows:.i

The input AC voltage connected to a variac device, in order to control the input voltage to the transformer. On the other hand it will control the output D.C voltage. This device has been chosen to allow high output current..i

The output of the variac was connected in parallel to the transformer that will raise the input voltage from 220V to 30 kV and it has turn ratio equal 1:135. The transformer specification has been selected to give maximum current of 100 mA and maximum voltage to 30 kV..i

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Fig.2.5: Block Diagram of the D.C. Power Supply for Laser CO2 Constructed in Our Work..i

The transformer is connected to a set of three H.V diodes of 10kV and 1mA for each one, which represent half wave bridge rectifier, which is used to remove the ripple of the A.C. compound and make it right for D.C electronic circuit. The output of the rectifier is connected to a C-filter of the characteristic of 30 kV and 2.42μf in order to modify the output voltage..i

Also the output cathode of the rectifier is connected to the Ballast resistor or limiting resistor use for limiting the output current in order to protect the power supply from high current may be delivered due to discharge process and has been used to control the input current to the discharge tube (see chapter one (1.9)). The value of ballast resistance 500kΩ and it has maximum power of 1000W..i

chapter two

2.8i CO2 Laser System Configuration:

Three CO2 laser system configurations have been built in this project . The first configuration in our work (as shown in figure 2.6) consist from higher reflected electrode works as an anode terminal and output coupler electrode works as cathode terminal with one plasma discharge tube 0.75m long joining between them. The above system represented only one fold laser type.i

The second configuration consists from three blocks HR electrode and OC electrode both of them works as cathode terminal and the third block is the Bakelite electrode, which has two-anode pins terminal..i

There are two double jacket discharge tubes represent optical active medium joining between the two aluminum electrode with Bakelite terminal (see figure 2.7)..i

In both of the above system we have been used the same hemispherical resonator consist from plane, 60% ZnSe (OC)output coupler mirror and 10 m radius of curvature gold coated mirror as HR back mirror..i

The third and last configuration is the (v-configuration) which consists of two parallel folds, which consist from four electrodes. OC electrode and HR electrode both of them works as anode and joining the corner holder tubes electrode, which represent cathode by two discharge tubes..i

Then the corner holder tubes electrode connected with corner electrode by using Pyrex tube of 0.6m outer diameter and length of 0.8m(see figure 2.8).In this system we use the same hemispherical resonator with adding Gold coated plane mirror in the corner electrode use just as reflector..i

In the above three systems configuration we use the same CO2 gas mixture and cooled by the same chiller..i

HeNe laser has been used to perform the mirror alignment and we do the initial alignment once every thing for each part of the system and after evacuating the system..i

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.Figure 2.6: Picture of One Fold of The System

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Fig.2.7: Two Folds CO2 Laser System as Being Builds in the Lab..i

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Figure 2.8: Corner Electrode Joining with Holder Tubes Electrode..i


انتهى الفصل الثاني

ان شاء الله الفصل الثالث ياتيكم لاحقا

سلامي لكم

chapter three

3.1i Introduction:

In this chapter, the results for the design and construction have been analyzed and discussed where different parameters are varied.i
3.2 Effect of The Pressure on The Discharge Voltage:i

Gas mixture pressure plays a dominant role in determining the gas laser output. This parameter may affect drastically on the lasing action. The gas pressure is a function of the applied discharge voltage. So, the increase of the pressure means increase the discharge voltage and that lead to increase the output laser power to a certain limit.i

Figure 3.1 presents the practical results obtained in the laboratory for the present designed at an active length 0.75 m. The required discharge voltage for normal glow discharge increases as the pressure increases. The normal glow discharge obtained at high vacuum (pressure in the tube less than 0.1mbar) for 0.75m tube length was 3.5kV and at steady state was 2kV. For both one and two fold and because the input voltage to the folded system are parallel but the current is series , So the input current for each fold was 10mA.The discharge in the two fold system configuration was successful obtained in both tubes (see figure 3.2)i

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Fig. 3.1: Practical Result for One Fold and Two Folds Discharge Voltage Variation with Pressure at Active Laser Media Length 0.75m.)i

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Figure 3.2:Picture of the Two Folds at Discharge.)i

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Fig.3.3:The Behavior of The Discharge Current as Function of Discharge Voltage For Tube Length 0.75 m at Different Pressure and Flow Rate 50liter/min.)i


As we can see from the chart that increasing the pressure for a certain value will decrease the knee of discharge point, which obey the Pachen curve. )i


3.3)i Effect of the Pressure and Gas Flow Rate on The Output Power:)i

The variation of the output power of our conductive cooled CO2 laser as function of pressure and gas flow rate is illustrated by figure 3.4.)i

To optimize the output power ,we fixed the gas flow rate and change the pressure. The maximum output power obtained was at 16 mbar with 50 liter /min flow rate as shown in figure 3.4 This means that by increasing the flow rate , the power increases
The above result represent the practical result obtained in the lap where, the maximum output power obtained for only one fold =23.5W,discharge voltage =8 kV, discharge current =28mA at flow rate =50 liter /min and at pressure = 16 mbar.)i

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Fig.3.4: Relative Output Power of CO2 Laser as Function of Pressure for Several Gas Flow Rates.)i

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Fig. 3.5: Output Power Behavior as Function of Pressure at Different Discharge Current for Only One Fold.
As we can see from the fig.3.5, increasing the pressure of the inner gas will decrease the knee point at the pachen curve which will reduce the input power , the thing that affect the output power positively.)i

3.4)i Effect of The Discharge Current on The Power Output:)i

All CO2 laser mixtures show current saturation of the output power, as the current is increased the laser output increases steadily to maximum and the decreases slowly as the current rises above the optimum value.)i

In general the small signal gain of the convectively cooled CO2 laser increases linearly as a function of discharge current for given gas flow rate .A point is finally reached where the small signal gain saturates, and as the discharge current increased beyond this point , the small signal gain begins to decrease .)i


[The decrease of the small signal gain with increasing discharge current an indication of excessive gas heating due to the inability of the gas flow to sufficiently cool the discharge .As the gas flow rate increased , the saturation of the small signal gain peaks successively reaches higher values and at higher discharge currents. )i

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Fig. 3.6: Output Power Behavior as A function of Discharge Current at Different Gas Pressure.)i

The small signal gain is relatively insensitive to gas flow rate variation at low –discharge current. Fig.3.6 illustrates experimental results of output power as a function of discharge current at various gas pressure. we can notice that as soon as the ignition start, the voltage will drop and the current will build up and therefor the output power will increase till a saturation point where there is no longer increasing in the power, and after a while the power will start to fall reaching the sustain current condition.)i

The maximum output power obtained at discharge current of 28 mA, flow rate of 50 liter /min and pressure=16mbar, for only on fold is 23.5W and for the two fold (V-configuration see fig.3.2) at current 40 mA and pressure of 16mA at flow rate of 50liter/min the maximum output power is 37W.

chapter three

3.5i Efficiency of the System:

The efficiency of the system can be defined as the ratio of the output power to the input power at 100%.The output power of our system has been measured by using IR power meter [heat sink detector( coherent ) ]of type LTD, model 542]maximum scale power measurement of 1000 W.i
The efficiency of the laser varies with the current flowing through the tube..i

Maximum efficiency and output power might not be obtained at the same time, the current which gives greatest efficiency might being lower than that which gives the best output. .i


The maximum efficiency comes out to be in our work equal 16.2% at input power equal of 90W, discharge current of about 20 mA, discharge voltage 11kV and output power equal 14.6 W at a plasma tube pressure of 15mbar.These measurement were carried out for only one fold design ..i

But the maximum output power has been recorded at an efficiency equal 10% and input power equal 235W which is equal 23.5Wat pressure of 12mbar input discharge current 28mA Figure 3.7 shows the variation of the efficiency with input power at different pressure..i

It is clear from the figure that the efficiency depends on the input current being used..i
In two fold (V- configuration) the maximum laser output power was 37 W at input power of 400 W, discharge voltage was 13kV at pressure 16mbar and discharge current 30mA. The efficiency of such design is about 9% the total discharge length of the system equal 1.5m and cavity length of 2m .This value considered to be very low with the expected value which is about 75 W because the available input current compare with the current that we need is very low .The min required value of the current should be not less than 100mA..i

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Fig.3.7: The Effect of Efficiency by Input Power at Different Pressure..i

So, the resultant of our work slow axial flow CO2 laser multi folds work in TEM01 mode (as we have see in the lap when the laser drop at the different material basis) .i

Conclusion :.i

Usually, from every experiment, there are results. These results lead to conclusions that consider as rules of thumb.Our experiments guided us to our own rules of thumb as follow:-.i

1.Seeking for reliability of CO2 system starts from its power supply. The ballast resistance that should be used to control the amount of the current that enter to the discharge tube and to protect power supply from the high current delivered due to discharge processes..i

2.Three main parameters affecting on the output power of CO2 laser are pressure, current and discharge voltage, where the current affects by the cross-section area of the discharge tube and the voltage affects by the active discharge length. Controlling the discharge needs to control gas pressure..i

3.Its possible to get uniform excitation in a multi folds CO2, V-configuration so this design proved to be quite reliable as the have no vacuum troubles and an other excepted problems during the construction and operation as well as minimize size and cost of the system..i

4.It is possible to get uniform excitation of the V-shape multi fold..i


Proposal for Future Work:-.i

1.A compact power supply should take the priority of the future work,due to fact that ,controlling the power supply and its current will control the whole system easily .There fore anew power supply of higher current(1A-2A) should be design and implemented before any farther improvements in the CO2 laser system ..i

2.Completing the system by groups the different configuration that have been design into one system and increase the number of folds to five tube at each fold to increase the power to 500 W..i

3.Using this laser in the industrial application like wielding, drilling, melting and cutting for ****l and non****l..i

4.Use Teflon material instead of Bakelite for higher thermal resistive..i