Page 51 - High Power Laser Handbook

22 G a s , C h e m i c a l , a n d F r e e - E l e c t r o n L a s e r s Excimer Lasers 23

C 3 − C p
Current

Laser
C 2 − C 3
pulse
Trigger
C − C
C − C 1 1 2
0
Time
Figure 2.4 Compression of the discharge pulse from C to discharge C .
1
p
For pulsed operation of gas-discharge lasers, the laser pulser’s pri-
mary storage capacitor must be charged to the desired high-voltage
level before energy transfer to the discharge. Resonant charging schemes
are widely used in high-repetition-rate, high-power systems because of
the high efficiency of direct current resonant charging (>90%). The
advancement of switched-mode power supply (SMPS) technology
makes this the method of choice for commercial excimer lasers.

Gas Circulation, Cooling, and Replenishment
Because the gas volume in the discharge area is no longer thermally
homogenous after the discharge, it is completely exchanged between
two successive laser pulses. A transverse circulation fan positioned
within the laser tube completely replaces the gas volume between the
main electrodes after each laser pulse, thereby providing a homoge-
neous gas flow over the entire electrode length. Between consecutive
laser pulses, the gas exchange in the discharge must provide clearing
of at least factor 2. Figure 2.5 shows the gas flow within the discharge
region of a high-repetition-rate excimer laser.

Laser repetition rate: 4 kHz
Gas circulation speed
40 Hz 50 Hz 60 Hz 70 Hz
4. Gas flow
c a
3.
2.
1.
Frame 1 Frame 2 Frame 3 Frame 4

Figure 2.5 Principle of gas flow between electrodes.