Temporally resolved, spectrally resolved laser induced fluorescence techniques have been employed to observe the collisional dynamics of the Rb 5²D_5/2,3/2 and 6²P_3/2,1/2 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

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

Time averaged, spectrally resolved fluorescence from cesium vapor in helium after pulsed laser excitation on the D₂ 6² S_1/2 – 6²P_3/2 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

Pulsed laser excitation on the Cs D₂ 6²S_1/2 – 6²P_3/2 transition at pump intensities of ~ 10⁷ W/cm², and subsequent energy pooling and far wing absorption on the 6²P_3/2 – 6 ²D_5/2,3/2, 8²S_1/2 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 6²D_{5/2,3/2, 7²P3/2,1/2} and 8²S_1/2 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 9²F_7/2 – 5²D_5/2 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

A kinetic model of Cs 6²S_1/2 – 6²P_3/2 pulsed laser excitation is developed and compared to experiment. Wing absorption from 6²P_3/2 to 6²D_5/2,3/2 plays a dominant role is creating intermediate species, with a minor contribution from 6²P_3/2 energy pooling. Photoionization from 6²D_5/2,3/2 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 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 52P_3/2,1/2 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

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

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

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 P_1/2,3/2 levels to the 5² D_3/2,5/2 and 7² S_1/2 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 D_3/2, 5² D_5/2, and 7² S_1/2 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

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

The pressure broadening rates for the 5²D_J←5²P_3/2 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

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, v_avg=30 m/s, inlet temperature T=450 K, operating pressure P_op=10 bar, inlet rubidium concentration n_Rb=5×10¹³ atoms/cm³, output mirror coupling O=80%, and pump bandwidth,Δν_pump=100 GHz.
KEYWORDS: Diode Pumped Alkali Laser, fluid model, turbulence, modeling and simulation

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

We demonstrated continuous wave operation of a hydrocarbon free Potassium Diode Pumped Alkali Laser using a closed cycle system with flowing gain medium containing potassium atomic vapor and pure helium as a buffer gas. The laser demonstrated continuous wave operation at 6 Watts, 14% slope efficiency with a 7 m/s flow speed for more than seven minutes maintaining constant power output. Continuous wave operation of this system without power degradation was achieved by using increased flow speed of the gain medium and cell windows protection from contact with alkali metal that mitigated the parasitic effect of ionization of potassium atoms.
KEYWORDS: Atomic gas laser, Laser, Diode-pumped, Potassium

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

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

The purpose of a directed energy strike on a computer network is to degrade or disable the target for some period of time. Directed energy capabilities have limited usefulness without some confidence that the repair time (i.e., “Time Out of Action” or “TOA”) will be long enough to meet mission requirements. Clearly, the repair time depends on the number of target computers disrupted, but it also depends on the effectiveness of the personnel conducting the repairs. A response with the theoretical minimum repair time is highly unlikely due to constraints on planning, communication, information gathering, and multiple solution attempts based on incomplete information. In this paper we describe TOA Sim, a computer simulation which incorporates live field test data to generate estimates of repair times for systems targeted by a directed energy attack, emphasizing the human impacts on the process. This emphasis stems from a cognitive simulation of the repair personnel, who must work as a team using ambiguous information to perform repairs in a dynamic environment. The agents are built using the ACT-R Cognitive Architecture, and they operate on a high-level representation of the computer network that simulates system faults and interdependencies between components. The simulation outcome depends partly on random/unpredictable factors, so we include results from sampling the model to generate the distribution of outcomes.
KEYWORDS: directed energy, weapons effects, time out of action, recuperation, reconstitution, microwave, nonleathal, simulation

Finite element modeling is used to estimate the dwell time required to melt through target materials, for specified laser irradiance and spot size. A previous Naval Postgraduate School thesis developed a finite element model in COMSOL to perform this analysis on 7075-aluminum. To expand upon this work, melt-through times were estimated for 309 stainless steel and titanium-6Al-4V. To validate these finite element models, an analytic model was developed and implemented in MATLAB. The results of the analytic model were used to verify the COMSOL estimates, and for each metal it was observed that the COMSOL model predicted erosion times intermediate between the analytic model and empirical data obtained from tests conducted by the Naval Surface Warfare Center, Dahlgren Division.
KEYWORDS: laser damage, directed energy, continuous wave, finite element, COMSOL, analytic, modeling and simulation

A method to model and predict the conducted susceptibility of a commercial integrated circuit (IC) pin to narrowband radio frequency (RF) aggressors using only measurements and no prior information about the internal structure of the IC is presented. To demonstrate this method the clock pin of a simple D-Flip-Flop (DFF) was used as the test IC pin. The linear and non-linear responses of the clock pin were characterized via impedance and transmission line pulse (TLP) measurements, respectively. A passive model of the IC’s clock pin was developed from these measurements and is shown to accurately predict the peak level of an applied waveform within 2 dB up to 2 GHz. The susceptibility of the clock pin was then measured for three narrowband pulsed aggressors up to 2 GHz and modeled using a behavioral filter in MATLAB. The IC pin susceptibility model was then combined with the IC pin passive model and the overall susceptibility model is shown to predict the susceptibility level of the IC within 2 dB and predict the aggressor amplitude required to cause a susceptibility within 6 dB for the three different pulsed aggressors used in this work. The presented model is limited to only narrowband signals below 2 GHz. Future work is focused on expanding the presented model to accurately predict the susceptibility of the DFF to higher frequency broadband signals and further reducing the prediction error of the model.
KEYWORDS: Conducted susceptibility, direct pulse injection, integrated circuit modeling, immunity, radio frequency interference

Next-generation high power microwave (HPM) weapon systems, such as the Air Force Research Laboratory’s Defending Aircraft with Next-Generation Targeted Electromagnetics (DANTE) program, place requirements on the pulsed power that deviate from traditional drivers for HPM applications. This manuscript provides an overview of GW-class magnetic pulse compression (MPC) and step-up pulse transformers to achieve DANTE’s final desired short pulse width of 40 ns, high pulse repetition frequency (PRF) of 1 kHz and burst lengths of 100 or more pulses per burst. An overview of the magnetic pulse compression equations and design considerations are detailed. Additionally, initial data from a surrogate system designed to evaluate various magnetic cores at similar parameters to the first stage of the final design is presented. This research pushes compact pulsed power drivers for HPM into a new regime of operation to allow for enhanced mission capability for the warfighter.
KEYWORDS: pulsed power, magnetic switching, solid-state modulator

Information warfare is on the rise as more sensors, increased analytics, data fusion, and artificial intelligence offer more opportunities for improved battlespace awareness and decision-making advantage. This places a higher priority on communication between distributed military platforms, such as ships, aircraft, and forward-deployed forces. In parallel, with the need for more communication, are several rising constraints: more demands on the radio frequency spectrum (with military and commercial use), a greater need for emission control (to operate in communication denied environments), and increased bandwidth requirements. This paper explores an alternative communication concept using directed energy in the form of free space optics and space-based relays, specifically for maritime over-the-horizon communications. This study conducted a systems analysis that included exploring naval stakeholder needs, identifying requirements, developing functional and physical architectures, and modeling system behavior. Analysis results indicate that in nominal conditions, a free space optical space-based relay system of systems could provide over-the-horizon maritime communications with high reliability to support naval missions.
KEYWORDS: directed energy, free space optics, lasers, military communications, space-based relays, systems analysis, emergent behavior

Ionization breakdown (also known as microwave generated plasma) may occur when an insulating medium such as air is ionized due to the presence of high electric fields. These high electric fields exist locally on and around passive microwave transmission lines, antennas, and other components that are excited with high radiofrequency (RF) powers. The amplitude of the local electric fields may be increased due to reflections or backwards waves that create a standing wave. In a phased array antenna, mutual coupling between elements during scanning of the phased array beam can cause backwards waves to exist on the antenna and transmission lines, a phenomenon commonly known as "active reflection" or "array active reflection". In this paper, experimental observations of array active reflection causing the local electric fields on the antenna and transmission lines to exceed the RF breakdown threshold of air at atmospheric pressures and create local regions of plasma are qualitatively discussed.
KEYWORDS: radio frequency (RF) microwave ionization breakdown, phased array antenna, high power microwave, array active reflection, microwave plasma, Townsend avalanche discharge

The U.S. Naval Research Laboratory (NRL) defined Space-Based Solar Power (SBSP) as the collection of energy from the sun in space and its wireless transmission for use on Earth. The current modus operandi for the Department of Defense (DoD) to power their forward operating bases (FOBs) requires the delivery of energy by physical transport of fossil fuels or its derivatives, which can be dangerous. SBSP is regarded as technically possible with varying degrees of success by state actors, from ground-based wireless power transmission demonstrations; integrated solar collection, conversion, and transmission systems development; and microwave conversion and rectifying efficiency studies. However, a full end-to-end SBSP system has yet to be realized. This study explored SBSP’s utility in DoD operations at the tactical edge, serving the warfighters at FOBs as well as expeditionary forces where power infrastructure is problematic. A collection of SBSP research, studies, and articles was used to identify FOBs as the major stakeholder for such a system, analyze their requirements, and identify a reference architecture to maximize the SBSP solution space. The results serve as a baseline for further research in this field. Near-peer nations are already aggressively underway in SBSP system design, and the results herein highlight the need for the DoD to act as a leader in this space.
KEYWORDS: space solar power, space-based solar power, power beaming, forward operating base, microwave energy

This research addresses the temporal and vertical variability of small-scale turbulence in a coastal environment represented by the structure function parameter for the index of refraction, C_n², used to quantify the impact of atmospheric turbulence on electromagnetic and electro-optical wave (EM/EO) propagation through atmospheric scintillation. The measurements were made on a coastal 12-m scaffold tower during the Coastal Land-Air-Sea Interaction (CLASI) field campaigns in Monterey Bay, California, from June to October 2021. The results from this research show a diurnal variability of C_n² unique to the coastal region and the impact of wind direction associated with sea breeze/land breeze circulations. The results also reveal the vertical variations in C_n² seen from the three measurement levels on the tower and how such variability changes diurnally. The vertical variation was also found to be affected by the mean wind direction, particularly for the water vapor contribution to the total C_n².
KEYWORDS: index of refraction, structure function parameter, diurnal variability, vertical variability, coastal winds