Journal of Directed Energy, Volume 2, Number 3

Chemical oxygen-iodine laser (COIL)-based weapon systems that operate near the ground will experience thermal blooming due to atmospheric absorption if output power is sufficiently high. The thermal lens in the air-to-ground case is predominantly in the far field of the optical system, which puts the problem outside the envelope for most classical phase correction techniques. Focusing the laser beyond the target (defocus) in the air-to-ground regime is shown to improve irradiance at the target and can be thought of as reducing the thermal blooming distortion number ND rather than as phase correction. Improvement is shown in a baseline scenario presented and all variations from it are explored. The Breaux ND is examined for potential use in a defocus scaling law, and a correction factor due to Smith is proposed to address deficiencies. Optimal defocus settings and expected improvement are presented as a function of Breaux ND, and a discussion of the interaction between turbulence and thermal blooming that may limit performance in the air-to-ground case is given.

A characteristic nondimensional distortion number Nd was derived in the 1970's that allows inference of the degree of nonlinear refraction or thermal blooming asociated with an atmospheric laser path. For a continuous-wave (CW) laser with a Gaussian-shaped beam, the distortion number is a function of several variables including laser power and aperture size, optical wavelength, atmospheric absorption and extinction, index of refraction, temperature, air density, and the air speed or flow transverse to the laser beam. Scenario-dependent calculations of atmospheric distortion number Nd are developed for different geographic regions and seasons using the Air Force Research Laboratory's global thermosonde database, the HITRAN molecular spectroscopic database, and global climatological aerosol model extinction profiles. Tactical air-to-ground scenarios are described as a function of altitude, target distance, and laser-to-target azimuth angle for the COIL wavelength (1.314 microm). The results are interpreted in light of seasonal and geographical factors as well as path-integrated moisture.

Directed energy/impulse power (DE/IP) systems offer solutions for a wide range of emerging applications including sensors, electronic countermeasures, clearing of improvised explosive devices and mines, and weapons. However, it is becoming clear that the conventional electronic breakdown switches currently used in DE/IP systems are not well suited for the demands of the future because of their limited reliability and adaptability. Recent developments at Diversified Technologies, Inc. (DTI), in solid-state switch technology are yeilding significantly increased pulse energies and speeds. Solid-state switches now approach the performance required for DE/IP applications and offer the added benefits of inherent reliability, pulse flexibility, and operation at high pulse frequencies. In this paper, DTI will provide an overview of these developments and describe how system designers can apply them to new DE/IP designs.

A novel concept that utilizes an AC-Link (trademark) topology is presented for ac-dc power conditioning. It is applicable for charging pulse-forming networks, dc loads with highly regulated voltage requirements, or other pulsed applications. The power conditioner interfaces with any ac generator and produces less than 1% harmonics, therefore permitting the generator to operate at full rated power, minimizing heating and eliminating the low-frequency transformers necessary for isolation or voltage transformation. The AC-Link (trademark) power conditioner provides full galvanic isolation between the generator and the load, and no transformer is required for a dc load with voltage requirements similar to the ac supply voltage. For higher dc voltage applications, the power conditioner can be configured as an electronic transformer. This system includes a single-phase, high-frequency transformer that is operated in the 20-kHz range. This high-frequency operation reduces the transformer weight, volume, and losses to a minimum, yielding an overall compact power conditioning system. The inverter is scalable to any power level using present-day power electric components. The preliminary design of a 5-MW, 95-kV klystron power supply will be used as an illustration.

Photocathodes are a critical component in photoinjectors for free-electron lasers (FELs) and other accelerator applications requiring a high-current, low-emittance electron beam. An ideal photocathode would have high efficiency in the visible range, a long operations lifetime in a typical accelerator vacuum environment, and prompt electron emission. Efficiency is typically improved by adding a photosensitive, cesium-based compound to the cathode surface. Because this layer is chemically active, however, it is vulnerable to evaporation and contamination, causing the cathode to degrade with use. Reapplication of this surface layer requires operational downtime and motiviated the need for an in situ rejuvenation technique that would prevent the cathode from being a significant failure mode of an FEL system. This work determines the effect of surface cesium coverage on the efficincy of a metallic cathode and compares theory and experiment as precursory work toward a despenser photocathode. A prototype multialkali dispense photocathode is proposed, along with a measurement technique used to monitor its surface conditions during operation.

The particle swarm optimization algorithm is applied to the design of impulse dipole array antennas utilizing passive straight-wire reflectors. The goal is to maximize the peak squared electric field strength at a specified location in the near field of the antenna for various driving voltage waveforms. The algorithm relies on a rapid computational engine for evaluation of currents and near fields that is based on numberical solution of the Hallen time-domain integral equation. Convergence of the algorithm is shown with improvements in peak squared field exceeding 100% compared to a standard near-field focus array employing elliptical reflector placement.

Coherent and incoherent fiber laser beam combining for long-range directed energy applications is discussed. We present a configuration for incoherently combining fiber lasers that can be employed for these applications. Unlike coherent beam combining approaches, incoherent combining does not require phase locking between the fiber lasers, and the polarization of the individual lasers can be random. In addition, the linewidths of the fiber lasers can be large. These relaxed requirements on the incoherent beam combining configuration allow for the use of recently developed high-continuous-wave (CW) power (~2.5kW per fiber), single-mode (TEM00), high-quality (M2 less than 1.2) fiber lasers having relatively large linewidths (delta lambda/lambda ~1%). These high-power lasers cannot be used for coherent beam combining. The proposed incoherent combining configuration consists of an array of fiber lasers in which the beams diffract to a spot size of ~4 cm onto individual collimating lenses. The Rayleigh length associated with each beam is ~5 km. The collimated beams can be directed to a target at distances of more than 5 km by individually controlled steering mirrors that form the beam director. We present parameters for an incoherently combined high-energy laser system that can deliver 100 kW of CW power on a target of area 100 cm2 at a range of 5 km. The system has 49 fiber lasers and a beam director with transverse dimension 60 x 60 cm. In principle, this configuration is scalable to higher CW powers. The effects of atmospheric turbulence on the propagation efficiency are addressed for the incoherent beam combining configuration.