**3. Laser system with an output aperture of 40 cm**

The laser system consists of five excimer lasers (Photon-1–Photon-5), the synchronization and starting system, and the matching optical elements. In three lasers, the working mixture is excited by an electric discharge. In two lasers, electron-beam excitation is used. In the experiments, the parameters of the laser radiation are measured using conventional methods and devices. To measure the time and energy characteristics of the laser pulses, we employ an FEK-22 vacuum photodiode, TPI and IKT-2 N calorimeters, and an OPHIR calorimeter with an L30A-EX head. The signals are detected with Tektronix oscilloscopes.

#### **3.1. Discharge lasers**

or an increase in the diameter of the beam amplified in it leads to a decrease in the energy concentrated inside the angle Qd to a level of no greater than 50%. Using single-pass amplification in the passive part of the active medium of the first stage, we additionally increase the output

**Parameters First stage Second stage Third stage Fourth stage** A, cm Ø = 0.6 1.2 × 1.8 5 × 6 21 × 25 Ein, J – 0.003 0.03 1.2 Eout, J 0.005 0.08 1.5 25 Ed/Eout 0.82 0.77 0.5 –

For the second stage, an input energy of 3 mJ is sufficient for the saturation of the amplifier active medium at two passes. A minor increase in the divergence after the second amplification stage in comparison to the divergence after the first stage is related to the presence of the ASE. The active medium of the preamplifier does not contribute to the observed increase in the divergence that is related to the distortions in the remaining part of the optical path (air and optical elements). The atmospheric turbulent flows impede the measurements of the divergence at the output of the main amplifier. In particular, the position and structure of the focal spot are unstable when the radiation of the master oscillator passes through the optical system in the absence of amplification. The instability strongly depends on the presence of heat sources in the vicinity of the optical path and on the time interval after the operation of the amplifiers. This is the reason for the approximate value of the ratio Ed/Eout for a beam size of 10 × 12 cm. We may state that, in this case, nearly 50% of the energy is concentrated in an angle of 5 × 10−5 rad. This result is in agreement with the results of the alternative measurements in which the output radiation is focused by a lens with *F* = 1.5 m on the titanium foil with a thickness of 50 μm: a

hole diameter of 100 μm corresponds to a divergence of about 6.5 × 10−5 rad.

spot is broken into a few spots, so that the divergence is significantly higher than Qd.

To more thoroughly study the effect of the heterogeneities in the active medium of the main amplifier and the optical path, we amplify the radiation of the master oscillator at three beam diameters: 35, 75, and 150 mm, respectively. **Figure 5** demonstrates the intensity distributions of the original radiation (the amplifier is switched off) and the amplified radiation measured in the far-field region. The first three panels correspond to the single-pass amplification in the active medium. It is seen that, for beam diameters of 35 and 75 mm, the divergence of the original and amplified radiation is close to the diffraction-limited divergence Qd. At a beam diameter of 150 mm, the focal

The most probable reason for this lies in the fluctuations of the air density in the optical path, since, in the case under consideration, the distance between lens *5* and the focal spot is about 25 m. We change the optical scheme to decrease the possible effect of air. In the new scheme, the amplified radiation is expanded with a telescope in front of the amplifier, amplified, reflected by the mirror, amplified on the return pass, compressed by the same telescope, and detected. For detection, the reflection mirror was slightly misaligned relative to the optical axis

energy of the master oscillator by a factor of about 20.

8 High Power Laser Systems

**Table 1.** Parameters of the radiation amplified with an MELS-4 k laser system.

**Figure 6** shows the exterior view of the first laser (Photon-1) [34]. The electrodes of the discharge gap are mounted inside the steel discharge chamber of the laser, which has a diameter

**Figure 6.** Photograph of the Photon-1 laser setup.

of 35 cm. The electrode length is 107 cm, its active length is 102 cm, and the width of the interelectrode gap is 4 cm. A rectangular hole on one of the sides of the laser chamber is covered with an insulator. The elements of the system for the laser excitation are placed on the insulator surface and are covered with a metal housing.

is 80 (100) cm. The x-ray radiation is transmitted to the discharge gap through the electrode window, which is covered with 80-μm-thick titanium foil. The anode of the discharge gap is connected to the pump generator located outside the chamber via an insulator with metal studs. The design of the elements that connect the generator to the anode and the backward

The vacuum diode of the x-ray source has a cylindrical housing that accommodates the anode and the cold cathode working upon explosive electron emission. The emission surface of the cathode consists of strips of foil-clad fiberglass plastic. The strips are attached to a separating grid that is covered with a 40-μm-thick titanium foil for the sealing of the vacuum diode and is used for the out coupling of the x-ray radiation. A tantalum foil serves as the anode of the vacuum diode. The vacuum diode is evacuated with an oil-diffusion pump to a residual pres-

A three-stage Arkad'ev-Marx generator with a fast capacitance of 15 nF serves as the power supply for the vacuum diode. The generator is connected to the vacuum diode with a KVI-120 high-voltage cable. A positive voltage pulse with an amplitude of 50–55 kV and a duration of 700 ns is fed to the anode. The x-ray doze in the cathode region of the discharge gap is 20–30 mR. The storage capacitor, the switching unit, and the peaking capacitor are the main elements of the

FL-300 lines connected in parallel. The electric length of the line is 300 ns, its capacitance is 150 nF, and the impedance is 1 Ω. The pulsed charging of the line employs an IK-100 capacitor that is connected to the line with a KVI-120 cable. This capacitor can be charged to a voltage of 40–65 kV. The peaking capacitors C<sup>2</sup> = 4.9 (Photon-2) and 6.9 nF (Photon-3) that form the space discharge in the gap represent the batteries of KVI-3 ceramic capacitors (20 kV and 680 pF).

consists of two (Photon-2) or three (Photon-3)

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wire-ways provides for the minimum inductance of the discharge circuit.

sure of about 10−4 torr.

**Figure 7.** Photograph of the Photon-3 laser setup.

laser pump generator. The storage capacitor *CL*

For the excitation, we employ an electric circuit with inductive energy storage and a semiconductor current breaker [35]. In this circuit, the high-voltage pulse with a short leading edge that is needed for discharge ignition is formed with a peaking capacitor *C* = 3.2 nF that is charged by the inductive energy storage with 12 SOS diodes [36]. The discharge preionization is realized upon the charging of the peaking capacitors owing to the radiation of 90 spark gaps that are uniformly distributed on the two sides of the anode. The main energy is deposited to the active medium at a low optimal voltage supplied by the storage capacitor (550 nF) that consists of K15–10 ceramic capacitors. The laser cavity consists of dielectric mirrors with reflectances of 98% and 7%. For the mixture Ne:Xe:HCl = 1520:10:1 with a pressure of 2 atm, the maximum laser energy is higher than 1.5 J at a pulse duration of 300 ns (FWHM) and a total efficiency of 1.35%. The low efficiency was due to the low content of HCl in the mixture, which was necessary to ignite the bulk discharge with large duration (~ 500 ns).

The two remaining electric-discharge lasers (Photon-2 and Photon-3) [37, 38] are virtually identical with respect to the design and the pumping scheme. **Figure 7** demonstrates the exterior view of Photon-3. Each of these lasers consists of three units that contain the gas-discharge chamber with the pump generator and electric and pneumatic control panels. The chamber contains the vacuum diode of the soft x-ray source, the electrodes of the laser discharge gap, and the insulator of the high-voltage input. The electrodes are made of stainless steel. For the Photon-2 (Photon-3) laser, the interelectrode distance is 5.4 (9) cm and the electrode length High-Power Laser Systems of UV and Visible Spectral Ranges http://dx.doi.org/10.5772/intechopen.71455 11

**Figure 7.** Photograph of the Photon-3 laser setup.

of 35 cm. The electrode length is 107 cm, its active length is 102 cm, and the width of the interelectrode gap is 4 cm. A rectangular hole on one of the sides of the laser chamber is covered with an insulator. The elements of the system for the laser excitation are placed on the insula-

For the excitation, we employ an electric circuit with inductive energy storage and a semiconductor current breaker [35]. In this circuit, the high-voltage pulse with a short leading edge that is needed for discharge ignition is formed with a peaking capacitor *C* = 3.2 nF that is charged by the inductive energy storage with 12 SOS diodes [36]. The discharge preionization is realized upon the charging of the peaking capacitors owing to the radiation of 90 spark gaps that are uniformly distributed on the two sides of the anode. The main energy is deposited to the active medium at a low optimal voltage supplied by the storage capacitor (550 nF) that consists of K15–10 ceramic capacitors. The laser cavity consists of dielectric mirrors with reflectances of 98% and 7%. For the mixture Ne:Xe:HCl = 1520:10:1 with a pressure of 2 atm, the maximum laser energy is higher than 1.5 J at a pulse duration of 300 ns (FWHM) and a total efficiency of 1.35%. The low efficiency was due to the low content of HCl in the mixture,

which was necessary to ignite the bulk discharge with large duration (~ 500 ns).

The two remaining electric-discharge lasers (Photon-2 and Photon-3) [37, 38] are virtually identical with respect to the design and the pumping scheme. **Figure 7** demonstrates the exterior view of Photon-3. Each of these lasers consists of three units that contain the gas-discharge chamber with the pump generator and electric and pneumatic control panels. The chamber contains the vacuum diode of the soft x-ray source, the electrodes of the laser discharge gap, and the insulator of the high-voltage input. The electrodes are made of stainless steel. For the Photon-2 (Photon-3) laser, the interelectrode distance is 5.4 (9) cm and the electrode length

tor surface and are covered with a metal housing.

**Figure 6.** Photograph of the Photon-1 laser setup.

10 High Power Laser Systems

is 80 (100) cm. The x-ray radiation is transmitted to the discharge gap through the electrode window, which is covered with 80-μm-thick titanium foil. The anode of the discharge gap is connected to the pump generator located outside the chamber via an insulator with metal studs. The design of the elements that connect the generator to the anode and the backward wire-ways provides for the minimum inductance of the discharge circuit.

The vacuum diode of the x-ray source has a cylindrical housing that accommodates the anode and the cold cathode working upon explosive electron emission. The emission surface of the cathode consists of strips of foil-clad fiberglass plastic. The strips are attached to a separating grid that is covered with a 40-μm-thick titanium foil for the sealing of the vacuum diode and is used for the out coupling of the x-ray radiation. A tantalum foil serves as the anode of the vacuum diode. The vacuum diode is evacuated with an oil-diffusion pump to a residual pressure of about 10−4 torr.

A three-stage Arkad'ev-Marx generator with a fast capacitance of 15 nF serves as the power supply for the vacuum diode. The generator is connected to the vacuum diode with a KVI-120 high-voltage cable. A positive voltage pulse with an amplitude of 50–55 kV and a duration of 700 ns is fed to the anode. The x-ray doze in the cathode region of the discharge gap is 20–30 mR.

The storage capacitor, the switching unit, and the peaking capacitor are the main elements of the laser pump generator. The storage capacitor *CL* consists of two (Photon-2) or three (Photon-3) FL-300 lines connected in parallel. The electric length of the line is 300 ns, its capacitance is 150 nF, and the impedance is 1 Ω. The pulsed charging of the line employs an IK-100 capacitor that is connected to the line with a KVI-120 cable. This capacitor can be charged to a voltage of 40–65 kV. The peaking capacitors C<sup>2</sup> = 4.9 (Photon-2) and 6.9 nF (Photon-3) that form the space discharge in the gap represent the batteries of KVI-3 ceramic capacitors (20 kV and 680 pF).

A rail-gap switch exhibits a low inductance. Its electrodes are made of stainless steel. For the Photon-2 (Photon-3) laser, the electrode length is 80 (100) cm and the interelectrode distance is 4 (6) mm. A starting electrode made up of foil leaves is located in the vicinity of the cathode along its entire length. The gaps are activated when a high-voltage pulse is fed to the starting electrode. The gap case represents a dielectric tube with an outer diameter of 65 mm. In the working mode, the gaps are filled with dry air at a pressure of 4–6.6 atm.

The high-voltage pulse needed for the activation of the spark gaps is produced by a high-voltage impulse generator based on a TGI-1-1000/25 thyratron. The artificial delay lines of the synchronization system provide for a sequential switching on of the pump generator and the x-ray source.

Plane-parallel plates made of fused quartz serve as the windows of the laser chamber. In the lasing mode, the laser cavity is formed by the external dielectric mirror with a reflectance of 98% at a wavelength of 308 nm and the window of the laser chamber. The laser mixture has a pressure of 3.5–4 atm and consists of the gases Ne, Xe, and HCl.

The free-running Photon-2 and Photon-3 lasers generate pulses with a duration of 250–300 ns (FWHM) and an energy of 3.5 and 10 J, respectively.

> At a charging voltage of 85 kV, the voltage pulse formed by the generator across the diode has a duration of 1000 ns, an amplitude of 480 kV, and a total current of 74 kA. The electron beam generated in the diode provides for a relatively homogeneous excitation of the laser mixture. The plane-parallel plates, which have a diameter of 300 mm and are made of fused quartz, serve as the windows of the laser chamber. In the lasing mode, the laser cavity is formed by

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The laser mixture consists of argon, xenon, and HCl. For the mixture Ar:Xe:HCl = 760:20:1 with a pressure of 2 atm, the radiation pulse energy amounts to 120 J at a charging voltage of 85 kV. The FWHM of the laser pulse is about 250 ns. **Figure 9** shows the exterior view of the Photon-5 laser. The laser gas mixture is excited by a radially converging electron beam from six directions [8]. The beam is generated in a vacuum diode that contains 18 cathodes. The cathode profile is chosen in accordance with the results of the numerical calculations of the beam parameters using the original 2D code. The emitting cathode surface is made of carbotextim (graphite-fiber material with a resistivity of about [5–50] × 10−2 Ωm), and it is covered with velvet. The width of the emitting surface is 120 mm and the total area of the cathode-emitting

structure of the outcoupling window is 6 cm. The supporting structure contains 18 windows (three windows in a row with a total length of 150 cm). The geometrical transparency of the beam outcoupling system is about 75%. The beam is outcoupled to the laser chamber through the Ti foil with a thickness of 40 μm. The laser cell diameter is 41 cm and the cell volume is 200 l. The voltage pulse across the diode is generated using two paralleled linear transformers with the vacuum insulation of the secondary coil. Each of the transformers consists of ten stages,

. The interelectrode gap between the emitting surface and the supporting

the Al-coated external mirror and the window of the laser chamber.

**Figure 8.** Photograph of the Photon-4 laser setup.

surface is 0.95 m2
