Journal of Directed Energy, Volume 3, Number 4

A laser beam propagating in the atmosphere undergoes scattering and absorption by atmospheric molecules and suspended particles. A portion of the scattered radiation is backscattered and propagates toward the location of the laser source. There is concern that the backscattered radiation produced by a high-energy laser may pose ocular or skin hazards to personnel in the vicinity of the laser source. To assess these hazards, equations are derived for the backscattered irradiance from both continuous-wave and pulsed lasers. The derivations are based on the definition of the atmospheric backscatter coefficient and the principles of beam propagation theory. The expression for the backscatter irradiance of a pulsed laser is shown to reduce to the LIDAR (LIght Detection And Ranging) equation under conditions that characterize typical remote sensing scenarios. The application of the derived equations to backscatter hazard assessments using the applicable ANSI standards for safe use of lasers is discussed.

A computational finite element/cohesive zone hybrid method is used to simulate the fracture failure of structural panels that are irradiated by a laser source. Results from the computations are compared to those of experiments of laser irradiation from a CO2 laser beam on pretensioned 2014 T6 aluminum specimens. Modified middle tension dogbone geometry specimens are subjected to a displacement-controlled boundary condition during irradiation. An iterative technique is employed that allows the key parameters used in the constitutive model for the cohesive elements to be related to the thermal field induce by the laser. The resulting computational technique accurately predicted the crack initiation and propagation in specimens subjected to a 200-W continuous beam. Peak temperatures in the range of 258 degrees C were realized prior to failure.

Previous reflected laser beam hazard evaluations have been based primarily on simplifying and overly conservative assumptions. In the case of high-energy-laser (HEL) field tests, such assumptions typically produce reflected nominal ocular hazard distance (RNOHD) results that cannot be contained within the boundaries of existing test ranges. This severely limits the ability to test HEL systems under operational conditions. A methodology was developed to address the need of the laser safety community for a more rigorous and accurate procedure to determine RNOHDs to support HEL field tests. The methodology consists of several physically based models and is applicable to scenarios in which a single laser illuminates a target while both move along independent trajectories. The salient features of the methodology are consideration of reflections only in the specular direction, the use of reflecting properties of actual materials, and calculation of exposure times for specific engagement scenarios. The equations and procedures of the methodology are demonstrated using an example engagement scenario.

Tests are described using fiber-attached, all-dielectric sensors for the noninvasive detection of electric and magnetic fields. The sensors utilize nonlinear optical materials (electro-optic and magneto-optic crystals) nd are tested in a variety of radio frequency and high-power microwave sources.

The analytical expression allowing one to calculate the anode current of a relativistic magnetron is derived. The anode current is described as a cathode-to-anode drift of electron guiding centers in crossed external dc and induced RF (radio frequency) electric and magnetic fields along equipotential lines forming a magnetron spoke. The drift velocity of the electron guiding centers is calculated in the frame of reference moving with the phase velocity of the traveling RF wave associated with the induced RF electromagnetic field. Electron charge density near the cathode is considered as a parameter determining the electron guiding center charge density within the magnetron spoke. Anode current is calculated as a product of the electron guiding center cathode-to-anode drift velocity, the electron guiding center space charge density within the magnetron spoke, and the number of magnetron spokes. Results of the analytical calculations of the anode current of the A6 relativistic magnetron are compared with computer PIC simulations of the anode current of this magnetron.

Beam control is essential for the development and operation of a directed energy system, especially when operating in the air or on the sea in a combat maritime environment. Directed energy beams are susceptible to jitter due to platform-induced vibrations and atmospheric effects. To correct jitter caused by mechanical vibrations without feedback from the target, the exact position and orientation of the platform must be determined in real time. A unique laser jitter control test bed is used to develop the necessary algorithms and sensor placements that are suitable, for calculating the platform-induced error in the directed energy beam in real time. This error can then be used in a feed-forward compensation technique to calculate the beam position that mitigates platform-induced jitter. The paper introduces the algorithm and initial results in predicting jitter for a complex pitch motion using a least-mean-squares adaptive filter.

Calculating the reflected irradiances produced by a specularly reflecting object at many observation points is computationally intensive, the total computational load proportional to the product of the number of facets times the number of observation points. To capture specular glints at all observation points, it is necessary to finely discretize the surface of the object into a large number of facets. This can result in a massive number of computations. The computational load can be reduced by approximating the surface of the object by curved triangular facets modeled as either quadric surfaces or point-normal triangles. Starting with a coarse discretation of the surface, a finer representation can be produced by subdividing the initial facets. For a single observation point, only a small fraction of the surface contributes to the specular glint; therefore only a few facets need to be significantly subdivided for accurate computations. By adaptively subdividing, the number of facets required per observation point is greatly reduced, resulting in fewer computations and thus increased overall computational speed. The speed increase is illustrated for a cylindrical object and different angular widths of the specular peak. As the width decreases, adaptive faceting increases the computational savings.