Laser fluence distribution (arbitrary units) of a megawatt-class beam in target plane at 5 km range after propagating through a maritime environment. Insets show spatial wander of the laser centroid in time for various power levels.
Laser fluence distribution (arbitrary units) of a megawatt-class beam in target plane at 5 km range after propagating through a maritime environment. Insets show spatial wander of the laser centroid in time for various power levels.

Background: The propagation of high-energy laser (HEL) beams in the atmosphere is rich in fundamental physics and of paramount importance to the Navy’s directed energy research program. Laser beams with hundreds of kilowatts to megawatts of average power are affected by numerous interrelated linear and nonlinear phenomena such as molecular and aerosol absorption and scattering, atmospheric turbulence, and thermal blooming. Aerosol scattering and absorption are often the major limiting factors in HEL propagation. In particular, aerosol absorption has been shown to be a major factor leading to thermal blooming. Properties of aerosols are often found in outdated tables using methodologies that may not be consistent with HEL propagation. In particular, under nonlinear conditions attained by HELs, aerosols can change their scattering and absorption properties during the HEL engagement time. In a typical directed energy engagement scenario at multi-kilometer ranges, approximately half of the laser power can be lost to aerosol scattering. In addition, the maritime propagation environment is characterized by strong turbulence which causes beams to wander and spread. Standard adaptive optics methods fail to compensate for the effects of deep turbulence, which can result in significant power loss at the target. Thus it is important to characterize and understand the interaction of HEL beams with aerosols and the effects of deep turbulence on beam propagation.

Accomplishment: NRL has developed the HELCAP atmospheric propagation simulation. The HELCAP code is a unique 3D computer simulation that models a wide parameter space of laser propagation from high-intensity (TWs/cm2 ), femtosecond laser pulses to high-average power, megawatt-class beams. The HELCAP code has been used in several award-winning studies that solidified NRL’s role as a world leader in high-energy laser propagation modeling. Recently, the HELCAP code was used in conjunction with atmospheric measurements taken by NRL Monterey to model the propagation of megawatt-class laser beams through highly stressing maritime environments. The HELCAP code is presently being used to support experiments at NRL to demonstrate these nonlinear aerosol effects in the laboratory.

Significance: The characterization and modeling of atmospheric effects on the propagation of high-energy laser beams is of critical importance to directed energy research programs throughout the Department of Defense. The HELCAP code provides NRL with a unique capability to study the complex physics of HEL propagation and provide guidance for these programs.

Application: A major goal of this research is the development of next-generation directed energy systems for ship defense against a variety of asymmetric threats, as well as other military applications.