B. Hafizi

Incoherent Combining and Atmospheric Propagation of High-Power Fiber Lasers for Directed-Energy Applications

High-power fiber lasers can be incoherently combined to form the basis of a directed high-energy laser system which is highly efficient, compact, robust, low-maintenance and has a long operating lifetime. This approach has a number of advantages over other beam combining methods. We present results of the first field demonstration of incoherent beam combining using kilowatt-class, single-mode fiber lasers. The experiment combined four fiber lasers using a beam director consisting of individually controlled steering mirrors. Propagation efficiencies of ~90%, at a range of 1.2 km, with transmitted continious-wave power levels of 3 kW were demonstrated in moderate atmospheric turbulence. We analyze the propagation of combined single-mode and multimode beams in atmospheric turbulence and find good agreement between theory, simulations and experiments.

Propagation of ultra-short, intense laser pulses in air

Recent theoretical, computational, and experimental work carried out at the Naval Research Laboratory on the propagation of ultra-short laser pulses in air is presented. Fully time-dependent, three-dimensional, nonlinear equations describing the propagation of laser pulses in air under the influence of diffraction, group velocity dispersion,Kerr nonlinearity, stimulated Raman scattering,ionization, and plasma wakefield excitation are presented and analyzed. The propagation code, HELCAP [P. Sprangle, J. R. Penano, and B. Hafizi, Phys. Rev. E 66, 046418 (2002)], is used to simulate the propagation of laser pulses in air under the influence of the physical processes mentioned above. Simulations of laser filamentation together with experimental measurements are used to confirm that the filamentation process is dependent on pulse duration. An equilibrium configuration for optical and plasma filaments in air is derived and the dynamic guiding and spectral broadening of a laser pulse is modeled. The effect of atmospheric turbulence on nonlinear self-focusing is demonstrated. Simulations of a recent electromagnetic pulse (EMP) generation experiment are also presented and the efficiency of EMP generation is determined and found to be extremely small.

Relativistic effects on intense laser beam propagation in plasma channels

Propagation characteristics of a radiation beam in a preformed, tapered plasma channel are analyzed by means of an envelope equation for the beam spot size. The model allows for relativistic focusing and ponderomotive channeling, radial and axial density gradients, and is valid for arbitrary intensity. The characteristics of laser beam propagation are shown to be governed by two parameters, the ratio of laser power to the critical power for relativistic focusing, and a dimensionless focusing strength parameter that includes contributions from both relativistic and channel focusing. The envelope equation provides a unified approach for exploring diverse applications such as designing a tapered laser wakefield accelerator or a plasma lens. The model is employed in interpretation of pump–probe laser propagation experiments and an x-ray source experiment. Full-scale simulations of a plasma channel lens are presented and shown to be in excellent agreement with the analytical results.

Remote lasing in air by recombination and electron impact excitation of molecular nitrogen

We analyze and simulate the physical mechanisms for a remote atmospheric lasing configuration which utilizes a combination of an ultrashort pulse laser to form a plasma filament of seed electrons, and a heater beam to heat the seed electrons. Nitrogen molecules are excited by electron impact and recombination processes to induce lasing in the ultraviolet. Recombination excitation, thermal excitation, gain, and saturation are analyzed and simulated. The lasing gain is sufficiently high to reach saturation within the length of the plasma filament. A remotely generated ultraviolet source may have applications for standoff detection of biological and chemical agents.

Analysis of the deflection system for a magnetic-field-immersed magnicon amplifier

A linear analysis of the electron-beam deflection system in a magnicon amplifier is presented. The system consists of identical cavities, one driven and the remainder passive, separated by a drift space and immersed in an axial magnetic field. The cavities contain a rotating TM/sub 110/ mode. The length of each cavity is pi nu /sub z// omega , and that of the drift space is pi nu /sub z// omega /sub c/, where omega is the RF frequency, omega /sub c/ is the relativistic gyrofrequency in the guide field, and nu /sub z/ is the mean axial velocity of the beam electrons. The linearized electron orbits are obtained for arbitrary initial axial velocity, radial coordinate, and magnetic field. The small-signal gain and the phase shift are determined. The special case where omega /sub c// omega =2 has unique features and is discussed in detail. For the NRL magnicon design, a power gain of 10 dB per passive cavity is feasible. Results from numerical modeling of a magnicon with two passive cavities are presented. Operation of the output cavity at the fundamental and higher harmonics of the input drive frequency is briefly discussed. >

GeV acceleration in tapered plasma channels

To achieve multi GeV electron energies in the laser wakefield accelerator (LWFA) it is necessary to propagate an intense laser pulse long distances in a plasma without disruption. A three-dimensional envelope equation for the laser field is derived that includes nonparaxial effects, wakefields, and relativistic nonlinearities. In the broad beam, short pulse limit the nonlinear terms in the wave equation that lead to Raman and modulation instabilities cancel. Long pulses (several plasma wavelengths) experience substantial modification due to these instabilities. The short pulse LWFA, although having smaller accelerating fields, can provide acceleration for longer distances in a plasma channel. By allowing the plasma density to increase along the propagation path electron dephasing can be deferred, increasing the energy gain. A simulation example of a GeV channel guided LWFA accelerator is presented. Simulations also show that multi-GeV energies can be achieved by optimally tapering the plasma channel.

Optical guiding of a radially polarized laser beam for inverse Cherenkov acceleration in a plasma channel

In a conventional inverse Cherenkov accelerator (ICA), the background neutral gas provides the necessary dispersion to maintain the synchronism between the drive laser and the accelerated electrons. A laser-driven ICA is susceptible to diffraction, and the acceleration length is limited to approximately a Rayleigh range (for a Gaussian beam). In this paper, an ICA configuration is proposed that avoids the laser diffraction limitation by employing a preformed plasma channel. It is shown that a radially polarized laser beam can be optically guided if the plasma density increases with radius-like r/sup 2/. Expressions for the guided axial and radial components of the laser field are derived, and a numerical example is discussed.

Initial operation of a high-power frequency-doubling X-band magnicon amplifier

This paper reports the initial high-power operation of a frequency-doubling magnicon amplifier at 11.120 GHz. The deflection cavities operate at 5.560 GHz. The device is operating in a single-pulse mode at 650 kV and /spl sim/225 A, using a 5.5-mm diameter beam from a plasma cathode, at a magnetic field of 6.7-8.2 kG. In order to overcome a gain saturation problem in the deflection cavities caused by plasma loading, the penultimate deflection cavity is operated very close to self-oscillation. The typical output pulselength is 100 ns full width at half maximum (FWHM), and is limited by RF breakdown of the penultimate cavity. Based on the measured far-field antenna pattern and absolute calibration of all microwave components, the measured output power is 14 MW (/spl plusmn/3 dB), corresponding to an efficiency of /spl sim/10%.

Design of a high power X-band magnicon amplifier

We present a design study for an X-band frequency-doubling magnicon amplifier driven by a 500 keV, 172 A beam from a field-emission diode. This study makes use of steady-state particle simulations employing the realistic fields of magnicon cavities connected by beam tunnels, and includes the effects of finite electron beam diameter. The simulations propagate an electron beam through a sequence of deflection cavities at 5.7 GHz, followed by an output cavity that operates at 11.4 GHz. The deflection cavities and the output cavity contain synchronously rotating TM modes. The deflection cavities progressively spin up the beam transverse momentum, until /spl alpha//spl equiv/v/sub /spl perp///v/sub z/>1, where v/sub /spl perp// and v/sub z/ are the velocity components perpendicular and parallel to the axial magnetic field. The output cavity uses this synchronously gyrating beam to generate microwave radiation at twice the drive frequency. Self-consistency of the simulation is achieved by iteration until power balance exists in each cavity, and until the optimum RF phase in each cavity is determined. The final magnicon circuit should produce 20 to 50 MW at 11.4 GHz, depending on initial beam diameter, with a drive power of 1 kW at 5.7 GHz. >

Measurements of energetic electrons from the high-intensity laser ionization of gases

Electrons ionized from tightly bound atomic states by a high-intensity laser pulse can gain energies from one to millions of electron volts dependent on the intensity of the pulse. We have currently been investigating hundreds of kilovolt to megavolt electrons produced by ionization of krypton and argon with terawatt laser pulses. Angular and energy distributions have been measured to determine the usability of this electron source as an injector for higher energy accelerators. Studies have included pressure dependence, angular ejection angle energy dependence, and polarization dependence. In particular, the energy-dependent ejection angle of electrons has been used to produce electron beams with energies peaked at 600 keV. Numerical simulations of these electrons show that 4 MV electron beams with excellent beam quality and femtosecond pulse widths can be produced from this electron source using higher power laser pulses.