**4.2. Photodissociation excimer XeF(C-A) amplifier**

XeF(C-A) amplifier and its schematic cross section are shown in **Figures 12** and **13**, respectively. It consists of two high-voltage pulse generators, a vacuum chamber with six electron cathodes, a xenon gas chamber, and a laser cell. For multipass laser beam amplification, the laser cell has a mirror unit (32 mirrors).

Two high-voltage pulse generators are line transformers, which have twelve transformer stages. Each stage has 320 nF capacitors and multigap spark switch. A secondary turn of the transformer is a vacuum coaxial line, which is connected with an e-beam diode. The diode has six explosive emission cathodes (flock-coated metal). The anode of vacuum diode is 45 cm-dia gas chamber closed by 40-μm-thick titanium foil (**Figure 14**). The vacuum diode forms six radially converging electron beams, each of 15 × 100 cm<sup>2</sup> area, that are injected into gas chamber filled by xenon at a 3 atm pressure. The capacitors of each stage can be charged up to 90–100 kV.

At charge voltage of 95 kV, the energy stored in the capacitor of a line transformer is 34.6 kJ, the energy of electron beam in the vacuum diode is 21 kJ, and the energy delivered to the gas chamber is 7 kJ. E-beam in the vacuum diode has following parameters: peak voltage of 550 kV, peak current of 200 kA, and current pulse duration of 150 ns (FWHM). The electron beam is injected into the gas chamber and one almost completely is absorbed by xenon at its thickness of 8 cm. An efficiency of the electron beam energy converting to VUV radiation of Xe<sup>2</sup> \* excimers is 30–40% [27].

The laser cell (**Figure 15**) is housed into the gas chamber along its axis and contains a gas mixture of XeF<sup>2</sup> and high-purity nitrogen at partial pressures of 0.2–0.4 and 190–380 torr, respectively. The active medium of amplifier on XeF(C-A) molecules is excited by the VUV radiation (172 nm) due to photodissociation of XeF<sup>2</sup> molecules with the XeF(B) molecules formation. The

XeF(C) state of the laser (C-A) transition is formed due to relaxation of the XeF(B) molecules in

cell against the foils through which the electron beams are injected into the gas chamber. This ensures the highest geometric coupling of the pumping source with the active medium. The windows are vacuum sealed by Viton gaskets. The hexagon laser cell has a clear aperture of 25 cm. The laser cells are sealed by the end flanges with 30 cm-dia fused silica windows.

. The active medium of 110 cm long is pumped through arrays

) located on the side walls of the hexahedral laser

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collisions with buffer gas of N<sup>2</sup>

windows (square of 12 × 12 cm<sup>2</sup>

**Figure 13.** Schematic of the pump circuit of the XeF(C-A) amplifier active medium.

**Figure 14.** Gas chamber of the XeF(C-A) amplifier with windows for electron beam injection.

of 54 CaF<sup>2</sup>

**Figure 12.** Photo of the XeF(C-A) amplifier.

**Figure 13.** Schematic of the pump circuit of the XeF(C-A) amplifier active medium.

**4.2. Photodissociation excimer XeF(C-A) amplifier**

ally converging electron beams, each of 15 × 100 cm<sup>2</sup>

ciency of the electron beam energy converting to VUV radiation of Xe<sup>2</sup>

laser cell has a mirror unit (32 mirrors).

18 High Power Laser Systems

(172 nm) due to photodissociation of XeF<sup>2</sup>

**Figure 12.** Photo of the XeF(C-A) amplifier.

ture of XeF<sup>2</sup>

XeF(C-A) amplifier and its schematic cross section are shown in **Figures 12** and **13**, respectively. It consists of two high-voltage pulse generators, a vacuum chamber with six electron cathodes, a xenon gas chamber, and a laser cell. For multipass laser beam amplification, the

Two high-voltage pulse generators are line transformers, which have twelve transformer stages. Each stage has 320 nF capacitors and multigap spark switch. A secondary turn of the transformer is a vacuum coaxial line, which is connected with an e-beam diode. The diode has six explosive emission cathodes (flock-coated metal). The anode of vacuum diode is 45 cm-dia gas chamber closed by 40-μm-thick titanium foil (**Figure 14**). The vacuum diode forms six radi-

filled by xenon at a 3 atm pressure. The capacitors of each stage can be charged up to 90–100 kV. At charge voltage of 95 kV, the energy stored in the capacitor of a line transformer is 34.6 kJ, the energy of electron beam in the vacuum diode is 21 kJ, and the energy delivered to the gas chamber is 7 kJ. E-beam in the vacuum diode has following parameters: peak voltage of 550 kV, peak current of 200 kA, and current pulse duration of 150 ns (FWHM). The electron beam is injected into the gas chamber and one almost completely is absorbed by xenon at its thickness of 8 cm. An effi-

The laser cell (**Figure 15**) is housed into the gas chamber along its axis and contains a gas mix-

tively. The active medium of amplifier on XeF(C-A) molecules is excited by the VUV radiation

and high-purity nitrogen at partial pressures of 0.2–0.4 and 190–380 torr, respec-

area, that are injected into gas chamber

excimers is 30–40% [27].

\*

molecules with the XeF(B) molecules formation. The

XeF(C) state of the laser (C-A) transition is formed due to relaxation of the XeF(B) molecules in collisions with buffer gas of N<sup>2</sup> . The active medium of 110 cm long is pumped through arrays of 54 CaF<sup>2</sup> windows (square of 12 × 12 cm<sup>2</sup> ) located on the side walls of the hexahedral laser cell against the foils through which the electron beams are injected into the gas chamber. This ensures the highest geometric coupling of the pumping source with the active medium. The windows are vacuum sealed by Viton gaskets. The hexagon laser cell has a clear aperture of 25 cm. The laser cells are sealed by the end flanges with 30 cm-dia fused silica windows.

**Figure 14.** Gas chamber of the XeF(C-A) amplifier with windows for electron beam injection.

The energies of the e-beams in the converter and VUV radiation in the laser chamber were measured by TPI-2-7 calorimeter. The output energy ща XeF(C-A) amplifier was measured with an OPHIR energy meter placed in the output beam, which was attenuated with a fused silica wedge and focused to a spot of 2.5 cm. The part of the beam passed through the wedge plate was used to record an image of laser beam on a photographic paper. The small signal gain of active medium was measured using a Sapphire-488 CW semiconductor laser after four passes of its probe beam through the active medium. This laser emits at 488 nm wavelength

was measured. Its value was 240–260 J. In view of the quantum efficiency of laser transition and 100% quantum yield of XeF(C) state production, the integral value of the energy stored in the active medium is ~ 90 J. In actuality, the actual lifetime of the XeF(C) state, which is determined by radiative decay and quenching, is much shorter than the pump pulse width. This makes the maximum current value of the energy stored on the XeF(C-A) transition 10

results correlate well with those found in the experiments on femtosecond pulse amplification. As can be seen in the **Figure 16**, the maximum gain is 0.004 cm−1 and the FWHM of the amplified signal is ~ 200 ns. The amplification of picosecond pulses was performed within the

windows into the laser cell

windows with the continuous

windows is shown in **Figure 16**. These

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coinciding with the amplification band maximum of the XeF(C-A) transition.

First of all, the VUV radiation energy transmitted through the CaF<sup>2</sup>

time interval of 146 ns (33 passes) close to the gain profile maximum.

**Figure 16.** Time profile of the e-beam current in the diode and gain measured near the CaF<sup>2</sup>

vapor pressure of 0.25 torr.

The time profile of the small-signal gain near the CaF<sup>2</sup>

**4.4. Experimental results**

times less than the integral value.

laser at 488 nm for a XeF<sup>2</sup>

**Figure 15.** Laser cell of the XeF(C-A) amplifier.

A high-purity xenon (99.9997%) was supplied to the gas chamber to provide maximum efficiency of the e-beam to VUV radiation conversion. The chamber preliminary evacuated to a pressure of 10−4 torr. During operation of the amplifier, the intensity of xenon VUV radiation gradually decreased due to outgassing from the stainless steel walls of the gas chamber, exposed to the electron beam. Xenon was circulated through a Sircal MP-2000 purifier to support the gas purity recovery.

In our experiments, the mixture in the laser cell was replaced after each shot, because repeated pumping of the mixture decreased the output energy by 20–30%.
