Guest >> Sign In

 
DIRECTED ENERGY PROFESSIONAL SOCIETY

Journal of Directed Energy
Papers to be Published

The papers listed below will be published in an upcoming issue of the Journal of Directed Energy. Pre-published versions of these submissions are available below.If you wish to reference these works, include the authors and title, plus "Journal of Directed Energy, forthcoming"'.

DEPS members enjoy access to the complete technical paper(s) through links in the paper titles. Members should sign in to their online account and return to this page to access this additional content.
Join DEPS



These thirteen papers will appear in Volume 7, Number 3, a special edition DPAL CUI issue.
Pre-published versions of these submissions are available. Please contact Damion Bradford at Damion@deps.org for access to these manuscripts.
Quenching of Rb 5 2D and 6 2P States by Helium and Nitrogen
Beth Komives, Blake A. Galloway, Christopher A. Rice and Glen P. Perram, Department of Engineering Physics, Air Force Institute of Technology

Temporally resolved, spectrally resolved laser induced fluorescence techniques have been employed to observe the collisional dynamics of the Rb 52D5/2,3/2 and 62P3/2,1/2 fine structure split states. The quenching rates for nitrogen are rapid, 5.9 ± 0.2 x 10-10 cm3/s and 5.7 ± 0.3 x 10-10 cm3/s for 52D and 62P, respectively. The quenching rates for helium are slow, 1.7 ± 0.4 x 10-14 cm3/s and 0.7 ± 1.2 x 10-14 cm3/s. Energy transfer between the fine structure split states are near gas kinetic for all cases, and scale with adiabaticity. Both red and blue emission is prompt due to bleaching of the two-photon pump transition and subsequent mid infrared stimulated emission.
KEYWORDS: diode pumped alkali lasers, potassium, collisional quenching, lifetimes

Performance of a Flowing Rb-He DPAL Modeled as a Nine-Level System
Grady T. Phillips and Glen P. Perram*, Department of Engineering Physics, Air Force Institute of Technology

The power scaling and beam quality of a Rb-He, longitudinally diode-pumped, alkali laser with transverse flow is predicted using an analytic model with a kinetic rate package developed to describe a system of nine Rb energy levels. The model includes processes that affect high-lying Rb energy levels, and lead to Rb ionization. Excitations to high-lying levels deplete population in the three-level laser system, increase heat load, and diminish output laser intensity. The departure from nearly ideal, quasi-two-level performance occurs at diode-pump intensities exceeding 50 kW/cm2 and alkali densities greater than 1 × 1014 atoms/cm3; it depends principally on the cross sections for far-wing absorption. Mitigation of decreased output laser intensity can be partially achieved by replenishing the alkali density in the three lowest levels and increasing the gas flow speed. A kinetic sensitivity analysis indicates far-wing absorption initiates the departure from ideal performance. For a flow residence time less than 1.5 ms, the optical-to-optical efficiency can exceed 80%, and nearly diffraction-limited beam quality can be maintained. The predictions depend on the included rate package in which several, crucial rates require further study.
KEYWORDS: diode pumped alkali laser, kinetics, high power transverse flow, device mode

Population Distributions in Cs Rydberg States after Pulsed Excitation of Cs 6 2P3/2
Jordan W. Joo, Christopher A. Rice, Daniel J. Emmons, and Glen P. Perram*, Department of Engineering Physics, Air Force Institute of Technology

Time averaged, spectrally resolved fluorescence from cesium vapor in helium after pulsed laser excitation on the D2 62 S1/2 – 62P3/2 transition has been observed to characterize the distribution of population in high lying states. At Cs melt pool temperatures of 373-523 K, visible emission from more than 50 lines and 35 states leads to fluorescence that evolves from primarily near infrared emission at low temperature, to a purple glow and eventually a white excitation volume, at higher temperature and Cs density. For states above the 82S1/2 level, the populations are statistically distributed with characteristic temperatures of T= 3340 ± 370 K, independent of Cs density or helium pressure to 600 Torr. More precise temperature determinations are hindered by poor availability of spontaneous emission rates. These statistical distributions partially support the modeling of Diode Pumped Alkali Lasers (DPAL) using a single population for the Rydberg states.
KEYWORDS: Diode Pumped Alkali Lasers, kinetics, electronic states, emission spectra

The Production of Higher Lying States after Pulsed Excitation of Cs 62S1/2 – 62P3/2 Part I: Emission Spectra
Jordan W. Joo, Daniel J. Emmons, Christopher A. Rice and Glen P. Perram*, Department of Engineering Physics, Air Force Institute of Technology

Pulsed laser excitation on the Cs D2 62S1/2 – 62P3/2 transition at pump intensities of ~ 107 W/cm2, and subsequent energy pooling and far wing absorption on the 62P3/2 – 6 2D5/2,3/2, 82S1/2 transitions produce atomic emission from 387 – 920 nm. As alkali density increases from 1013 to 1016 atoms/cm3, the side fluorescence evolves from mainly near infrared emission to include strong UV and blue lines, and eventually to emission from more than 35 states. Photoionization from the intermediate 62D5/2,3/2, 72P3/2,1/2 and 82S1/2 states during the laser pulse is rapid and subsequent ion recombination likely produces the highest lying states. While no ion emission is observed, the neutral Cs 92F7/2 – 52D5/2 line at 647.18 nm is Stark broadened and sensitive to electron density with time averaged densities of 2.1 – 14 x 1013 cm-3. At high pressure, molecular emission is observed near 870 nm. The emission from Rydberg states is strong at higher Cs density. By comparing the time averaged relative intensity from various lines as a function of Cs density and helium pressure, 1-800 Torr, a rate mechanism for Diode Pumped Alkali Lasers may be anchored.
KEYWORDS: cesium DPAL, pulsed excitation, ionization kinetics, side fluorescence

The Production of Higher Lying States after Pulsed Excitation of Cs 62S1/2 – 62P3/2 Part II: Kinetic Model
Daniel J. Emmons, Jordan W. Joo, and Glen P. Perram*, Department of Engineering Physics, Air Force Institute of Technology

A kinetic model of Cs 62S1/2 – 62P3/2 pulsed laser excitation is developed and compared to experiment. Wing absorption from 62P3/2 to 62D5/2,3/2 plays a dominant role is creating intermediate species, with a minor contribution from 62P3/2 energy pooling. Photoionization from 62D5/2,3/2 produces peak electron densities ranging from 5-18% of the total Cs density, depending on the He pressure and melt pool temperature. After the pulse, the principle kinetic pathways are modeled by electron-ion recombination to the Rydberg states and subsequent cascading down to lower energy states from He quenching. Predicted peak electron densities increase with increasing pressure and Cs density from the pressure dependence of the wing absorption cross sections. Comparison of relative integrated densities with experiment show general agreement in the trends as a function of pressure and Cs density, providing a partial validation of the kinetic mechanism.
KEYWORDS: diode pumped alklai laser, ionization kinetics, reaction mechansim, cesium

Fluorescence from Higher Lying States in a Potassium DPAL
A.J. Wallerstein,* Greg A. Pitz, Christopher A. Rice, and Glen P. Perram, Department of Engineering Physics, Air Force Institute of Technology

Fluorescence emitted by a transverse flow potassium Diode Pumped Alkali Laser (DPAL) pumped at 1.23 kW (37.4 kW/cm2) but without lasing was collected to characterize the highly excited state (n = 4 - 11) populations at total alkali densities of N = 0.63 – 1.87 x 1014 cm-3 and He and methane buffer gas pressures of P = 250 - 1200 Torr. Emission from 32 visible (404-697 nm) neutral potassium atomic lines was recorded at 0.036 nm resolution. The quenching of the intermediate states by helium is moderately fast, k = 0.11-1.11 x 10-11 cm3/atom-s, and primarily yields the 52P3/2,1/2 states. The higher lying, Rydberg excited states are statistically distributed with temperatures of T = 1,680 – 2,530 K. The population in these states was less than 5% of the total alkali density for all cases. The observations are compared to a nine-level kinetic model.
KEYWORDS: diode pumped alkali lasers, potassium, emission spectra, populations

Review of Alkali Atom Excited State Ionization Cross Sections Relevant to DPAL
Gary S. Kedziora and Glen P. Perram*, Department of Engineering Physics, Air Force Institute of Technology

Evidence of electrons and ions in diode pumped alkali laser (DPAL) systems has motivated the need to detail the various mechanisms for producing free electrons and alkali atom ions. In this review we focus on photoionization processes of excited state alkali atoms and dimers. There is a lack of reliable experimental cross sections at the DPAL pump frequencies for excited state atoms, but there is ample evidence of excited states and free electrons. Here we review theoretical cross sections relying on recently reported computed cross sections of alkali atom excited states of Singor and coworkers1 and compare with some experimental results justifying their use. We also suggest consideration of the excited state alkali dimer photoionization processes, photon-assisted associative ionization and photon-assisted Penning ionization, which have been discussed in the sodium laser induced 3S-3P resonant ionization literature. These processes should potentially be added to DPAL models, but there is a lack of rate information. Estimates of the rates of the cross sections and probabilities of dimer formation are needed.
KEYWORDS: Diode pumped alklai laser, photo-ionization cross-sections, atomic and dimer ions

Impact of Temperature-dependent Energy Pooling Reactions on Cs Alkali Laser Performance
David L. Carroll, CU Aerospace, L.L.C

The exciplex pumped alkali laser (XPAL) is a variant of the more common diode pumped alkali laser (DPAL). Experimental measurements of pulsed output energy from the four-level Cs-Ar XPAL as a function of input pump energy and temperature show a strong dependence on temperature. The D2 line laser performance initially increases with temperature, followed by a decrease as temperature is further increased. This paper presents BLAZE Multiphysics™ simulations using temperature-dependent energy pooling reaction rates baselined to available experimental rate data. Also included are photoionization and Penning ionization reactions. These calculations show that the inclusion of temperature-dependent energy pooling rates and the subsequent onset of significant ionization can explain the rise and fall of XPAL performance with temperature with reasonable accuracy.
KEYWORDS: Exciplex pumped alkali laser, XPAL, DPAL, Cesium, Cs-Ar

Excitation of the 52 D3/2, 52D5/2, and 72S1/2 Rubidium Energy Levels via Wing Absorption from the 52P1/2 and 52P3/2 Energy Levels
D. E. Weeks and G. P. Perram, Department of Engineering Physics, Air Force Institute of Technology

The semiclassical Anderson-Talman model is used to compute non-Lorentzian Rb and K excited state line shapes pressure broadened by helium. Far wing absorption cross sections for transitions from the diode pumped Rb 52 P1/2,3/2 levels to the 52 D3/2,5/2 and 72 S1/2 levels are evaluated at the D2 diode pump and D1 lasing wavelengths and range from 2.6 ×10-23 m2 to 97.0 ×10-23 m2. The excited Rb 52 D3/2, 52 D5/2, and 72 S1/2 levels are detuned from twice the D2 energy by 67.4 cm-1, 70.4 cm-1, and 678.3 cm-1 respectively. While this is a fairly significant detuning at low pressures, DPAL systems are often operated at high pressure and the absorption rates may compete favorably with energy pooling for production of higher lying states. The detuning is larger for the K + He system, suggesting better laser performance at elevated pump intensities and alkali densities.
KEYWORDS: Diode Pumped Alkali Laser, line shape, pressure broadening, far wing absorption

Recent Advances in Diode Pumped Alkali Laser at the Air Force Research Laboratory
G. A. Pitz*, Ryan A. Lane, David A. Hostutler, Air Force Research Laboratory - Directed Energy Directorate; and Donald M. Stalnaker and Stephen A. Shock, Leidos, Inc.

Diode Pumped Alkali Lasers (DPAL) were invented by William Krupke in 2004 and have been studied at the Air Force Research Laboratory (AFRL) since 2005. Over the past 15 years, AFRL has successfully scaled a rubidium based DPAL from a few milliwatts to just over 500 Watts. At which time, AFRL transitioned to a potassium based DPAL which achieved over 3 kW in less than a year. More recently with the use of methane, the K-DPAL has achieved a power of 4.2 kw with an optical-to-optical efficiency of 60% by beginning to area scale the pumped volume. However, when using only helium as the buffer gas the K-DPAL only achieved a power of 2.2 kW with an optical-to-optical efficiency of 30%. Implying additional study is needed to understand the limitation of ionization within an operational DPAL and the role methane plays enhancing performance over helium-only systems.
KEYWORDS: DPAL, Alkali, Laser, pressure broadening

Determination of the Pressure Broadening Rates of the 52DJ ← 52P3/2 Transition in Rubidium for He, Ar, CH4, and C2H6
G. A. Pitz, Air Force Research Laboratory - Directed Energy Directorate; P.G. Christodoulou, USRA - Directed Energy Scholars;and E.M. Guild, Boeing, Inc.

The pressure broadening rates for the 52DJ←52P3/2 transitions in rubidium were determined for helium, argon, methane, and ethane using a two-photon excitation process. Broadening rates for He, Ar, CH4, and C2H6 were measured to be 45.55±0.10, 38.93±0.10, 59.66±0.12, 51.35±0.13, and 47.35±0.10, 43.02±0.09, 55.60±0.12, 59.30±0.12 MHz/Torr for J=3/2 and 5/2, respectively. This work was attempted to improve numeric models for the diode pumped alkali laser (DPAL) and investigate this transition as a potential pathway to ionization within these laser systems.
KEYWORDS: DPAL, Alkali, Laser, pressure broadening

Modeling a DPAL system with the Laser Model Tool Kit v1.1
R. L. Lloyd, D. E. Weeks*, and G. P. Perram, Department of Engineering Physics, Air Force Institute of Technology

Diode pumped alkali lasers (DPAL) hold significant promise for high power laser applications. Current DPAL designs typically vaporize a small concentration of an alkali metal into a rare gas buffer. This mixture then flows through a gain cell positioned within an unstable resonator where it is pumped with diode lasers at the alkali D2 wavelength and subsequently lases at the alkali D1 wavelength. The complex modal structure of the unstable resonator together with the turbulent gas flow dynamics represent two significant challenges when modeling a DPAL system. The Laser Model Tool Kit (LMTK) developed by Creare, LLC meets these challenges by modeling the laser field in the unstable resonator with a Fourier transform based solution to the paraxial wave equation. The laser and pump fields are coupled to a rate package that governs the alkali populations that are in turn coupled with Reynolds-averaged Navier-Stokes equations solved using ANSYS Fluent. An early version of the code, LMTK v1.1 is benchmarked for input parameters that vary about a benchmark set given by average flow velocity, vavg=30 m/s, inlet temperature T=450 K, operating pressure Pop=10 bar, inlet rubidium concentration nRb=5×1013 atoms/cm3, output mirror coupling O=80%, and pump bandwidth,Δνpump=100 GHz.
KEYWORDS: Diode Pumped Alkali Laser, fluid model, turbulence, modeling and simulation

Modeling a DPAL system with the Laser Model Tool Kit v1.1High Fidelity Model of Nonlinear Losses in Potassium Diode Pumped Alkali Lasers
Ryan A. Lane, Hal Cambier, Greg Pitz, Air Force Research Laboratory-Directed Energy Directorate; and Benjamin Oliker, Deshawn Coombs, and Jay del Barga, Ball Aerospace

Diode Pumped Alkali Lasers (DPAL) use a gas gain media comprised of alkali atoms and a buffer gas to lase at visible wavelengths. DPAL were first demonstrated in 2004 by William Krupke and are a potential source for high energy laser weapons systems. The Air Force Research Laboratory (AFRL) has studied DPALs using experiment and modeling since 2005. AFRL currently uses a potassium DPAL with a 'triply-transverse' design and a buffer gas comprised of methane and helium. This design has the gas flow, pump light, and laser light in orthogonal directions and has been used to demonstrate world record powers and performance. The AFRL's model for DPAL includes thermal dynamics, fluid dynamics, optics, and laser gain. The AFRL model has been shown to align with the measured performance of the potassium DPAL system except when the buffer gas is comprised only of helium. It is hypothesized that this discrepancy is due to nonlinear processes involving the alkali atoms. These nonlinearities have the potential for large discrepancies under unknown conditions which limits the ability of modeling to guide efforts to power scale DPAL to mission relevant powers. A high fidelity model including these processes and their effects on DPAL performance has been developed. A summary of the model and a limited comparison of the model predictions and experimental measurements are provided.
KEYWORDS: potassium DPAL, non-equilibrium ionization


The following papers are awaiting publication in the next standard issue, CUI Volume of the Journal of Directed Energy.
Pre-published versions of these submission are available. Please contact Damion Bradford at Damion@deps.org for access to these manuscripts.
Coatings for Femtosecond and Continuous Wave Lasers: Applications for Directed Energy
Sandeep Kohli, Michael Gentile, Katy Csády Zadrovicz, and Michael L. Albrecht, Zygo Corporation

Ultra-high intensity lasers are gaining attention for use in Directed Energy (DE) applications and offer a unique opportunity to exploit the resultant laser-matter interactions on a relativistic scale. Femtosecond lasers employ ultra-short laser pulses (USLPs) which can produce energy more than petawatt levels. The intense power discharged in a very short time offers a multi-faceted approach to disabling a threat: ablating material from the target, blinding its sensors, and overloading its internal electronics by producing localized interference. In contrast, a continuous-wave (CW) laser operates at lower power and greater pulse widths to destroy a target primarily by melting. The optical coating design plays a key role in system effectiveness. But equally important - and sometimes overlooked - is possessing the ability to apply the coating design to large substrates with various surface geometries. We present spectral performance and laser damage properties of both high-reflectors (HR) and antireflection (AR) coatings applied to large optics for both USLP and CW laser applications.
KEYWORDS: Coatings, Laser damage, Femtosecond, Optics, Directed energy, Ultrashort laser pulse, High-reflector, Continuous wave, Electron beam deposition

Applications of Particle Beams
J.R. Harris, J.M. Connelly, R.H. Cooksey, E.L. Ruden, P.J. Mardahl, J.A. Elle, D.A. Shiffler, Directed Energy Directorate, AFRL; C.N. Harris, USAFA; N. Montgomery, R. Danley, Z. Olson, Airman Systems Directorate, AFRL; N.P. Lockwood, Air Force Office of Scientific Research; P. McChesney, Raytheon Missile Systems; J.W. Lewellen, Los Alamos National Laboratory; W. Sommars, Leidos; R.B. Miller, EBM LLC; N.T. Myers, Verus Research; J. Watrous, Confluent Sciences; W.F. Blakely, D.L. Bolduc, Armed Forces Radiobiology Research Institute and A. Romanyukha, Naval Dosimetry Center

For several years, a team led by the Air Force Research Laboratory's Directed Energy Directorate has been investigating the use of particle beams in an endoatmospheric, counter-electronics role. The primary advantages of particle beams in this role come from their ability to penetrate into target interiors, aided by their production of a shower of low-energy secondary particles, and therefore to upset electronic components in protected targets which conventional high power microwave (HPM) systems would be unable to affect. This paper will report on some of our recent results with an emphasis on understanding how beam-matter interactions affect system-level weapon design tradeoffs such as range and effects on target.
KEYWORDS: Particle Beam, Directed Energy Application, counter-electronics, electron beam, X-ray

To Be Published, Journal of Directed Energy

 
Copyright © 2024 Directed Energy Professional Society   DHTML/JavaScript Menus by OpenCube
DEPS Policies and Terms of Use

Last updated: 24 September 2023