**3.2. Electron beam–pumped lasers**

**Figure 8** shows a photograph of the forth laser (Photon-4). The laser consists of the Arkad'ev-Marx generator (high-voltage generator), vacuum diode, laser chamber, gas-filling and evacuation system, and electric control panel. The vacuum diode and the generator that serves as its power supply are placed in a single case. Thus, vacuum insulation of the high-voltage components is realized in the high-voltage generator. This design makes it possible to minimize the inductance of the power-supply circuit of the vacuum diode, as well as the accelerator size and weight. The high-voltage generator has three parallel branches that make it possible to minimize the inductance and the erosion of the gap electrodes. In each branch, the spark-gap space is filled with dry air mixed with SF6 . The capacitance of each stage in one branch is 0.18 nF. The accelerator is started with a controlled gap switched to a high-voltage cable connected to the high-voltage generator. The gap is activated by a high-voltage pulse of the thyratron generator. The vacuum-diode cathodes with a total length of 110 cm are mounted on a holder fixed on the upper stage of the high-voltage generator. The laser chamber that serves as the anode is located in between the cathodes in the center of the vacuum diode. The inner diameter of the laser chamber is 25 cm. At the entire length, the chamber is attached to the diode housing with a metal plate. The plate facilitates the current flow and decreases the electron beam loss owing to the effect of the self-magnetic field. In the absence of the plate, when the current is closed only on the ends of the gas chamber, the loss may amount to 50%. Velvet serves as the electron emitter on the cathodes of the vacuum diode. The width of the anode-cathode gap is 7 cm. Thus, four radially converging beams formed in the diode are injected into the laser chamber through eight windows (two windows in a row with a total length of 120 cm). Each vacuum-sealed window is closed with a 40-μm-thick titanium foil attached to a metal grid. The residual gas pressure in the case that accommodates the high-voltage generator and the vacuum diode is 5 × 10−5 torr.

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

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

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

The free-running Photon-2 and Photon-3 lasers generate pulses with a duration of 250–300 ns

**Figure 8** shows a photograph of the forth laser (Photon-4). The laser consists of the Arkad'ev-Marx generator (high-voltage generator), vacuum diode, laser chamber, gas-filling and evacuation system, and electric control panel. The vacuum diode and the generator that serves as its power supply are placed in a single case. Thus, vacuum insulation of the high-voltage components is realized in the high-voltage generator. This design makes it possible to minimize the inductance of the power-supply circuit of the vacuum diode, as well as the accelerator size and weight. The high-voltage generator has three parallel branches that make it possible to minimize the inductance and the erosion of the gap electrodes. In each branch, the spark-gap space is filled with dry air mixed with SF6

capacitance of each stage in one branch is 0.18 nF. The accelerator is started with a controlled gap switched to a high-voltage cable connected to the high-voltage generator. The gap is activated by a high-voltage pulse of the thyratron generator. The vacuum-diode cathodes with a total length of 110 cm are mounted on a holder fixed on the upper stage of the high-voltage generator. The laser chamber that serves as the anode is located in between the cathodes in the center of the vacuum diode. The inner diameter of the laser chamber is 25 cm. At the entire length, the chamber is attached to the diode housing with a metal plate. The plate facilitates the current flow and decreases the electron beam loss owing to the effect of the self-magnetic field. In the absence of the plate, when the current is closed only on the ends of the gas chamber, the loss may amount to 50%. Velvet serves as the electron emitter on the cathodes of the vacuum diode. The width of the anode-cathode gap is 7 cm. Thus, four radially converging beams formed in the diode are injected into the laser chamber through eight windows (two windows in a row with a total length of 120 cm). Each vacuum-sealed window is closed with a 40-μm-thick titanium foil attached to a metal grid. The residual gas pressure in the case that accommodates the high-voltage

. The

working mode, the gaps are filled with dry air at a pressure of 4–6.6 atm.

pressure of 3.5–4 atm and consists of the gases Ne, Xe, and HCl.

(FWHM) and an energy of 3.5 and 10 J, respectively.

generator and the vacuum diode is 5 × 10−5 torr.

**3.2. Electron beam–pumped lasers**

12 High Power Laser Systems

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

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 surface is 0.95 m2 . The interelectrode gap between the emitting surface and the supporting 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,

**3.3. Laser system**

**Figure 10** demonstrates the block diagram of the synchronization and starting system that is used to synchronize the setups. The laser system is controlled with a PC that is interfaced with a synchronization pulse generator producing voltage pulses with an amplitude of 600 V and a variable interpulse delay. These pulses are used to start four thyratron generators and the magneticbias generator of the Photon-5 laser. The last generator produces two bias pulses that are fed to the linear transformers to premagnetize the transformer cores in the appropriate direction. The thyratron generators produce negative voltage pulses with an amplitude of about 20 kV. These pulses are fed to the spark gaps of the Photon-1, Photon-2, and Photon-3 lasers and to the inputs of the generators of the Photon-4 and Photon-5 lasers. Both generators produce negative pulses with an amplitude of about 85 kV, which are used to trigger the rail gaps of the Photon-2 and Photon-3 lasers and to start the Photon-4 laser. These pulses are also fed to the inputs of the trigger generators G-1 and G-2. The trigger generators produce 40 negative pulses with an amplitude of 85 kV that activate the gaps of the transformer stages of the Photon-5 laser. In addition, the PC controls the on-off switching of the capacitive storage charging in all of the lasers with allowance for the particular charging times, so that the storages are charged simultaneously.

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Thus, at the first stage of the laser system's operation, the storage capacitors of all of the lasers are charged. Then, the starting pulse is fed to the magnetic-bias generator of the Photon-5 laser and a certain sequence of the starting pulses is fed to the thyratron generators of the Photon-1, Photon-2, Photon-3, and Photon-4 lasers. Thus, we activate the system as a whole.

**Figure 10.** Block diagram of the starting and synchronization system.

**Figure 9.** Photograph of the Photon-5 laser setup.

and each of the stages consists of eight IK-100-0.17 capacitors (100 kV, 17 μF, and 50 nH). The output power of the stage is about 12 GW. The voltage is fed to the diode (the cathode collector) with vacuum lines that serve as the secondary coils of the transformer. The diameter of the diode vacuum chamber is 131 cm and its length is 210 cm. The collector is suspended coaxially to the vacuum chamber on two springs in the upper part of the chamber. The vacuum chamber is evacuated using two AVDM-250 vacuum devices with liquid-nitrogen traps to a residual pressure in the diode of (3–4) × 10−5 torr.

At a charging voltage of 85 kV, the amplitude of the voltage pulse across the vacuum diode amounts to 550 kV. In this case, the total current (the sum of two currents) is 320 kA, and the energy transferred from the transformer to the diode is 87 kJ. At a charging voltage of 80 kV, the voltage across the vacuum diode is 440 kV, the total current is 290 kA, and the energy transferred to the diode is 78 kJ. The energy input to the gas increases when the pressure increases to 2.5 atm. Then, the energy input remains virtually unchanged when the pressure increases to 3.5 atm. The maximum energy input to the gas from the electron beam is about 19 kJ. The efficiency of the energy transfer from the primary storage to the gas is about 19%. This result is close to the value realized in conventional accelerators with water lines [1–3].

Plane-parallel plates with a diameter of 400 mm made of fused quartz serve as the windows of the laser chamber. This laser is tested in lasing mode with the cavity formed by the Al-coated plane mirror and the laser chamber window. For the mixture Ar:Xe:HCl = 760:20:1 with a pressure of 2 atm, the radiation pulse energy amounts to 660 J at a charging voltage of 85 kV [8]. The radiation pulse FWHM is about 350 ns. The heterogeneity of the radiation energy density distribution over the cross section of the laser beam is less than 10%.

### **3.3. Laser system**

and each of the stages consists of eight IK-100-0.17 capacitors (100 kV, 17 μF, and 50 nH). The output power of the stage is about 12 GW. The voltage is fed to the diode (the cathode collector) with vacuum lines that serve as the secondary coils of the transformer. The diameter of the diode vacuum chamber is 131 cm and its length is 210 cm. The collector is suspended coaxially to the vacuum chamber on two springs in the upper part of the chamber. The vacuum chamber is evacuated using two AVDM-250 vacuum devices with liquid-nitrogen traps

At a charging voltage of 85 kV, the amplitude of the voltage pulse across the vacuum diode amounts to 550 kV. In this case, the total current (the sum of two currents) is 320 kA, and the energy transferred from the transformer to the diode is 87 kJ. At a charging voltage of 80 kV, the voltage across the vacuum diode is 440 kV, the total current is 290 kA, and the energy transferred to the diode is 78 kJ. The energy input to the gas increases when the pressure increases to 2.5 atm. Then, the energy input remains virtually unchanged when the pressure increases to 3.5 atm. The maximum energy input to the gas from the electron beam is about 19 kJ. The efficiency of the energy transfer from the primary storage to the gas is about 19%. This result is close to the value realized in conventional accelerators with water lines [1–3]. Plane-parallel plates with a diameter of 400 mm made of fused quartz serve as the windows of the laser chamber. This laser is tested in lasing mode with the cavity formed by the Al-coated plane mirror and the laser chamber window. For the mixture Ar:Xe:HCl = 760:20:1 with a pressure of 2 atm, the radiation pulse energy amounts to 660 J at a charging voltage of 85 kV [8]. The radiation pulse FWHM is about 350 ns. The heterogeneity of the radiation energy

density distribution over the cross section of the laser beam is less than 10%.

to a residual pressure in the diode of (3–4) × 10−5 torr.

**Figure 9.** Photograph of the Photon-5 laser setup.

14 High Power Laser Systems

**Figure 10** demonstrates the block diagram of the synchronization and starting system that is used to synchronize the setups. The laser system is controlled with a PC that is interfaced with a synchronization pulse generator producing voltage pulses with an amplitude of 600 V and a variable interpulse delay. These pulses are used to start four thyratron generators and the magneticbias generator of the Photon-5 laser. The last generator produces two bias pulses that are fed to the linear transformers to premagnetize the transformer cores in the appropriate direction. The thyratron generators produce negative voltage pulses with an amplitude of about 20 kV. These pulses are fed to the spark gaps of the Photon-1, Photon-2, and Photon-3 lasers and to the inputs of the generators of the Photon-4 and Photon-5 lasers. Both generators produce negative pulses with an amplitude of about 85 kV, which are used to trigger the rail gaps of the Photon-2 and Photon-3 lasers and to start the Photon-4 laser. These pulses are also fed to the inputs of the trigger generators G-1 and G-2. The trigger generators produce 40 negative pulses with an amplitude of 85 kV that activate the gaps of the transformer stages of the Photon-5 laser. In addition, the PC controls the on-off switching of the capacitive storage charging in all of the lasers with allowance for the particular charging times, so that the storages are charged simultaneously.

Thus, at the first stage of the laser system's operation, the storage capacitors of all of the lasers are charged. Then, the starting pulse is fed to the magnetic-bias generator of the Photon-5 laser and a certain sequence of the starting pulses is fed to the thyratron generators of the Photon-1, Photon-2, Photon-3, and Photon-4 lasers. Thus, we activate the system as a whole.

**Figure 10.** Block diagram of the starting and synchronization system.

In the laser system, the Photon-2, Photon-3, Photon-4, and Photon-5 lasers start working as slave amplifiers. In this case, the windows of all of the laser chambers are tilted at angles providing for the absence of feedback. The Photon-1 laser serves as the master oscillator [34]. Its optical scheme makes it possible to generate a high-quality beam in a certain part of the active volume and to amplify this beam in the remaining part. To improve the spatial structure of the radiation, we employ two pinholes with a diameter of 2 mm. In this case, the Fresnel number is *N* ~ 2 for a cavity with a length of 1.5 m. The spectral selection is realized with an auto-collimation diffraction grating (1800 mm−1). The feedback in the cavity is maintained via the first diffraction order. To decrease the contribution of the noise component to the output radiation, we outcouple the laser beam through a semitransparent mirror with a reflectance of *R* = 30%. Then, the low-power high-quality radiation of the master laser is additionally amplified at two passes in the same active medium, so that the output beam diameter increases to 7 mm. The output pulse of the Photon-1 laser has an energy of 50 mJ, a duration of 250 ns, and a spectral line width of 0.9 cm−1. The divergence of the laser beam, which contains 80% of the energy (0.13 mrad), is greater than the diffraction-limited divergence by a factor of 1.2.

This beam is expanded using a lens telescope with a magnification of *M* = 1.5 to match the beam diameter with the sizes of the active media of the Photon-2 and Photon-3 lasers. The beam is amplified at three passes in the Photon-2 laser and one pass in the Photon-3 laser. The output beam diameters of these lasers are 3 and 6 cm, respectively. For further matching of the beam diameter with the sizes of the active media of the Photon-4 and Photon-5 lasers, we employ a lens telescope with a fivefold magnification. After the beam expansion, the radiation is amplified at one pass in the active medium of the Photon-4 laser and at one or two passes in the active medium of the Photon-5 laser.

The spectral and spatial parameters of the radiation are measured only for the first three setups (Photon-1, Photon-2, and Photon-3). The divergence is measured using the spot size in the focal plane of a lens with a focal length of *F* = 13.5 m. The line width is measured with the Fabry-Perot interferometer. In both cases, the radiation intensity distribution is detected by a CCD array. The spectral measurements show that the line width remains unchanged (0.9 cm−1) after the amplification. In general, the divergence of the amplified radiation decreases with an increase in the beam diameter. However, the divergence is slightly greater than the diffraction-limited divergence due to the distortions in the active medium and the optical path. **Figure 11** shows the dependence of the output energy of the main amplifier on the input energy upon the double-pass amplification. The input energy is measured when one of the amplifiers (Photon-3 or Photon-4) is switched off. It is seen that the Photon-5 amplifier is saturated only when all of the amplifiers are switched on. In this case, the gain is about 10. The laser beam spot on photosensitive paper exhibits a relatively homogeneous distribution and a minor contribution of the diffraction rings related to the heterogeneities in the optical path.

**Figure 11.** Plot of the output radiation energy vs. the input signal energy for the double-pass amplification in the active

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The femtosecond pulse generator (front-end) of laser system consists of a femtosecond Ti:sapphire master oscillator, regenerative and multipass amplifiers, a pulse stretcher and a pulse compressor, and generator of second harmonic. It operates at a pulse repetition rate of 10 Hz and in a single pulse mode. Output energy of front-end is up to 5 mJ at 475 nm.

This indicates a high spatial coherence of the output radiation.

**4. Laser system THL-100**

medium of the Photon-5 laser.

**4.1. Femtosecond pulse generator**

Transform-limited pulse duration is 50 fs.

**Table 2** summarizes the experimental results for the laser system. The maximum energy (330 J) is obtained in the case of the single-pass amplification in the Photon-5 laser when the ASE flux and the absorption in the active medium are minimized. In the case of double pass amplification, both the absorption and the ASE contribution increase (the ASE intensity increases owing to the reflection from the rear mirror with *R* = 99%). This leads to a decrease in the energy of the amplified radiation to a level of 250 J.


**Table 2.** Parameters of the radiation of the photon lasers.

**Figure 11.** Plot of the output radiation energy vs. the input signal energy for the double-pass amplification in the active medium of the Photon-5 laser.

The spectral and spatial parameters of the radiation are measured only for the first three setups (Photon-1, Photon-2, and Photon-3). The divergence is measured using the spot size in the focal plane of a lens with a focal length of *F* = 13.5 m. The line width is measured with the Fabry-Perot interferometer. In both cases, the radiation intensity distribution is detected by a CCD array. The spectral measurements show that the line width remains unchanged (0.9 cm−1) after the amplification. In general, the divergence of the amplified radiation decreases with an increase in the beam diameter. However, the divergence is slightly greater than the diffraction-limited divergence due to the distortions in the active medium and the optical path.

**Figure 11** shows the dependence of the output energy of the main amplifier on the input energy upon the double-pass amplification. The input energy is measured when one of the amplifiers (Photon-3 or Photon-4) is switched off. It is seen that the Photon-5 amplifier is saturated only when all of the amplifiers are switched on. In this case, the gain is about 10. The laser beam spot on photosensitive paper exhibits a relatively homogeneous distribution and a minor contribution of the diffraction rings related to the heterogeneities in the optical path. This indicates a high spatial coherence of the output radiation.
