Showing papers by "Phillip Sprangle published in 2012"

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

Abstract: 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.

On the sensitivity of terahertz gyrotron based systems for remote detection of concealed radioactive materials

Abstract: This paper analyzes some features of systems intended to remotely detect concealed radioactive materials by using a focused THz radiation. This concept is based on possibility to focus high-power THz radiation in a small spot where the wave field exceeds the breakdown threshold. However, in the absence of any sources of ionization, the probability to have in this breakdown-prone volume any seed electrons is very low. Thus, high breakdown rate in a series of THz pulses will indicate the presence of concealed radioactive materials in the vicinity of a focused wave beam. The goal of the present paper is to determine by using the statistical theory THz pulse duration required for reliable initiation of the discharge. Then, the detectable mass of the radioactive material is determined as the function of distance and of the THz wave power and pulse duration. Lastly, possible benefits from using pulse compressors, which shorten the pulse duration but increase the wave power and, hence, the breakdown-prone volume, are analyzed. It is shown that the use of pulse compressors can significantly improve the sensitivity of THz gyrotron based systems for remote detection of concealed radioactive materials.

Propagation of gamma rays and production of free electrons in air

Abstract: A new concept of remote detection of concealed radioactive materials has been recently proposed \cite{Gr.Nusin.2010}-\cite{NusinSprangle}. It is based on the breakdown in air at the focal point of a high-power beam of electromagnetic waves produced by a THz gyrotron. To initiate the avalanche breakdown, seed free electrons should be present in this focal region during the electromagnetic pulse. This paper is devoted to the analysis of production of free electrons by gamma rays leaking from radioactive materials. Within a hundred meters from the radiation source, the fluctuating free electrons appear with the rate that may exceed significantly the natural background ionization rate. During the gyrotron pulse of about 10 microsecond length, such electrons may seed the electric breakdown and create sufficiently dense plasma at the focal region to be detected as an unambiguous effect of the concealed radioactive material.

Propagation of gamma rays and production of free electrons in air

Abstract: This paper is devoted to the analysis of production of free electrons in air by gamma-rays leaking from radioactive materials. A model based on the Klein-Nishina scattering theory is used to calculate scattering cross sections and approximate the electron production rate. The model includes the effects of primary gamma-quanta radiated by the source as well as that scattered in air. Comparison of the model with the mcnpx kinetic code (http://mcnpx.lanl.gov/) in a sample problem shows excellent agreement. The motivation for this research comes from the recently proposed concept of remote detection of concealed radioactive materials [V. L. Granatstein and G. S. Nusinovich, J. Appl. Phys. 108, 063304 (2010)]. The concept is based on the breakdown in air at the focal point of a high-power beam of electromagnetic waves produced by a THz gyrotron with a 10-20 μs pulse. The presence of a radioactive material can greatly exceed the production rate of free electrons over the natural background rate. Additional elec...

Laser heating of uncoated optics in a convective medium

Abstract: Powerful, long-pulse lasers have a variety of applications. In many applications, optical elements are employed to direct, focus, or collimate the beam. Typically the optic is suspended in a gaseous environment (e.g., air) and can cool by convection. The variation of the optic temperature with time is obtained by combining the effects of laser heating, thermal conduction, and convective loss. Characteristics of the solutions in terms of the properties of the optic material, laser beam parameters, and the environment are discussed and compared with measurements at the Naval Research Laboratory, employing kW-class, 1 µm wavelength, continuous wave lasers and optical elements made of fused silica or BK7 glass. The calculated results are in good agreement with the measurements, given the approximations in the analysis and the expected variation in the absorption coefficients of the glasses used in the experiments.

Quasi-remote laser pulse compression and generation of radiation and particle beams

Abstract: A system and methods for the quasi-remote compression and focusing of a moderate-intensity laser pulse to form a much higher intensity beam that can be directed at a target and used as a probe beam or used in a probe beam converter to generate other forms of electromagnetic radiation or energetic particles A system for the quasi-remote propagation of high-intensity laser beams in accordance with the present invention comprises a main platform on which a first, “seed” laser pulse is generated, stretched, and amplified, and a remote platform, located at a distance from the main platform, which is configured to receive the amplified and stretched pulse and convert it into the high-intensity laser beam The high-intensity laser beam in turn can then be converted into one or more probe beams directed at a target object

A Gridded Thermionic Injector Gun for High-Average-Power Free-Electron Lasers

Abstract: High-average-power free-electron lasers (FELs) require high-average-current injectors capable of generating high-quality short-duration electron bunches with a repetition rate equal to the frequency of the energy-recovery linac (~700 MHz). We present an injector configuration that utilizes an RF-gated gridded thermionic cathode inset into a 700-MHz RF cavity. The grid is independently modulated using RF power at one or more harmonics of the main cavity frequency, in order to gate emission from the cathode surface. Two- and three-dimensional particle-in-cell simulations of this configuration were performed using the gun/collector modeling code MICHELLE. Simulations indicate that this approach can provide the necessary charge per bunch, bunch duration, beam quality, and repetition rate for high-average-power FELs operating in the infrared regime.

Beacon Beams for Deep Turbulence High Energy Laser Beam Directors

Abstract: : The objective of this report is to outline a new type of beam director for deep turbulence propagation of high energy lasers (HELs). The proposed beam director is based on a new and innovative approach employing a Brillouin enhanced four-wave mixing mechanism for generating a tight (small spot size) beacon beam on a remote target. This mechanism results in amplification and complete conjugation (phase and amplitude) of the beacon beam without the need for wavefront sensors, deformable mirrors or predictive feedback algorithms. Complete phase conjugation is critical for beam control in the presence of strong turbulence. Conventional AO techniques do not have this capability. The beacon beam phase information from the beacon beam can be used in conjunction with an AO system to propagate HEL beams in deep turbulence.

Remote detection of radioactive materials using a near-terahertz gyrotron

Abstract: Summary form only given. We are studying new concepts for remote detection of concealed radioactive materials.1–3 The research is a balanced effort consisting of theory, simulations and experiments.

Possible standoff detection of ionizing radiation Using high-power THz electromagnetic waves

Abstract: Recently, a new method of remote detection of concealed radioactive materials was proposed This method is based on focusing high-power short wavelength electromagnetic radiation in a small volume where the wave electric field exceeds the breakdown threshold In the presence of free electrons caused by ionizing radiation, in this volume an avalanche discharge can then be initiated When the wavelength is short enough, the probability of having even one free electron in this small volume in the absence of additional sources of ionization is low Hence, a high breakdown rate will indicate that in the vicinity of this volume there are some materials causing ionization of air To prove this concept a 067 THz gyrotron delivering 200-300 kW power in 10 microsecond pulses is under development This method of standoff detection of concealed sources of ionizing radiation requires a wide range of studies, viz, evaluation of possible range, THz power and pulse duration, production of free electrons in air by gamma rays penetrating through container walls, statistical delay time in initiation of the breakdown in the case of low electron density, temporal evolution of plasma structure in the breakdown and scattering of THz radiation from small plasma objects Most of these issues are discussed in the paper