^{1,a)}, J. L. Giuliani

^{1}, M. F. Wolford

^{1}, J. D. Sethian

^{1}, G. M. Petrov

^{1}, D. D. Hinshelwood

^{1}, M. C. Myers

^{1}, A. Dasgupta

^{1}, F. Hegeler

^{2}and Ts. Petrova

^{3}

### Abstract

The Ar–Xe infrared laser has been investigated in several series of experiments carried out on the Naval Research Laboratory’s Electra generator. Our primary goals were to optimize the efficiency of the laser (within Electra’s capabilities) and to gain understanding of the main physical processes underlying the laser’s output as a function of controllable parameters such as Xe fraction, power deposition, and gas pressure. We find that the intrinsic efficiency maximizes at at a total pressure of 2.5 atm, Xe fraction of 1%, and electron beam power deposition density of . We deployed an interferometer to measure the electron density during lasing; the ionization fractions of – that it detected well exceed previous theoretical estimates. Some trends in the data as a function of beam power and xenon fraction are not fully understood. The as-yet incomplete picture of Ar–Xe laser physics is likely traceable in large part to significant uncertainties still present in many important rates influencing the atomic and molecular kinetics.

This work was supported by the Office of Naval Research.

I. INTRODUCTION

II. EXPERIMENTAL SETUP

III. RESULTS

A. Modeling and kinetics issues

B. Efficiency, e-beam power deposition, and electron density

C. Gain, saturation intensity, and effects of xenon fraction

IV. CONCLUDING DISCUSSION

### Key Topics

- Electron beams
- 28.0
- Reflectivity
- 13.0
- Ionization
- 12.0
- Irradiance
- 8.0
- Photodiodes
- 8.0

## Figures

Main elements of the reaction kinetics of the Ar–Xe laser, as currently understood, are diagrammed. Vertical scale shows the energies of key states in eV.

Main elements of the reaction kinetics of the Ar–Xe laser, as currently understood, are diagrammed. Vertical scale shows the energies of key states in eV.

Schematic of the Ar–Xe laser cell as implemented on Electra, with associated diagnostic instruments. Sometimes, five photodiodes were deployed behind a single output coupler instead of two or three linked to individual output couplers as is shown in the figure.

Schematic of the Ar–Xe laser cell as implemented on Electra, with associated diagnostic instruments. Sometimes, five photodiodes were deployed behind a single output coupler instead of two or three linked to individual output couplers as is shown in the figure.

Photodiode signals (proportional to power at ) are shown as a function of time and space for an Electra shot in which the total energy deposited was 577 J, the Xe fraction 1%, and the total pressure, 2.0 atm. A full aperture output coupler had a reflectivity of 6.5%. The e-beam originated at 0.0 cm, depicted at the top of the graph section. Also shown on the same time scale is the diode power. Linear interpolation was used to fill in the grid between the five photodiodes which were located at 5, 10, 15, 20, and 25 cm from the entrance foil.

Photodiode signals (proportional to power at ) are shown as a function of time and space for an Electra shot in which the total energy deposited was 577 J, the Xe fraction 1%, and the total pressure, 2.0 atm. A full aperture output coupler had a reflectivity of 6.5%. The e-beam originated at 0.0 cm, depicted at the top of the graph section. Also shown on the same time scale is the diode power. Linear interpolation was used to fill in the grid between the five photodiodes which were located at 5, 10, 15, 20, and 25 cm from the entrance foil.

Intrinsic efficiency as a function of e-beam power deposition density for a 99:1 Ar–Xe mixture at a total pressure of 2.5 atm. Uncertainties associated with the measurements are shown as error bars. Output coupler reflectivity is 0.337. Open squares are measured data and triangles are model calculations.

Intrinsic efficiency as a function of e-beam power deposition density for a 99:1 Ar–Xe mixture at a total pressure of 2.5 atm. Uncertainties associated with the measurements are shown as error bars. Output coupler reflectivity is 0.337. Open squares are measured data and triangles are model calculations.

Head-on view of laser aperture under the experimentally optimized efficiency conditions of Fig. 4, depicting calculated e-beam absorption and measured laser emission in three 10 cm segments.

Head-on view of laser aperture under the experimentally optimized efficiency conditions of Fig. 4, depicting calculated e-beam absorption and measured laser emission in three 10 cm segments.

Interferometrically measured peak electron density at midpoint of the laser cell is plotted vs e-beam power deposition density. The corresponding model-calculated electron density is also shown. The total pressure is 2.5 atm and the Xe fraction is 1%.

Interferometrically measured peak electron density at midpoint of the laser cell is plotted vs e-beam power deposition density. The corresponding model-calculated electron density is also shown. The total pressure is 2.5 atm and the Xe fraction is 1%.

Measured laser irradiance as a function of output coupler reflectivity is plotted for various Xe fractions: (a) 0.5% Xe, (b) 1.0% Xe, and (c) 2.5% Xe. Also shown are least-squares fits to the Rigrod formula and the corresponding inferred saturation intensities and small signal gains. The pressure was 2.5 atm and average deposition power was .

Measured laser irradiance as a function of output coupler reflectivity is plotted for various Xe fractions: (a) 0.5% Xe, (b) 1.0% Xe, and (c) 2.5% Xe. Also shown are least-squares fits to the Rigrod formula and the corresponding inferred saturation intensities and small signal gains. The pressure was 2.5 atm and average deposition power was .

Gain in the laser transition, inferred from the least-squares fits of Fig. 7, and also calculated from the kinetics model, is plotted as a function of Xe fraction.

Gain in the laser transition, inferred from the least-squares fits of Fig. 7, and also calculated from the kinetics model, is plotted as a function of Xe fraction.

The saturation intensity of the laser transition, inferred from the least-squares fits of Fig. 7, and also calculated from the kinetics model, is plotted as a function of Xe fraction.

The saturation intensity of the laser transition, inferred from the least-squares fits of Fig. 7, and also calculated from the kinetics model, is plotted as a function of Xe fraction.

Measured and model-calculated fluences at are plotted as a function of Xe fraction, for the same conditions of Figs. 7–9.

Measured and model-calculated fluences at are plotted as a function of Xe fraction, for the same conditions of Figs. 7–9.

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