Design And Construction Of Axial Slow Flow Cw Co2 Laser

[ماجستير هندسة ليزر]

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

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

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

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 Poweri

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

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

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 Outputi

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

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.

[ماجستير هندسة ليزر]

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

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

[ماجستير هندسة ليزر]

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