Melissa K. Hornstein

Enhanced Stability of Second- and Fourth-Harmonic Gyrotrons Driven by a Frequency-Doubled Prebunched Beam

There is currently considerable interest in operating gyrotrons at the second and higher cyclotron harmonics in order to access the near-terahertz regime while reducing magnetic field requirements. High-frequency gyrotrons have successfully operated at the second harmonic. However, competition from the fundamental harmonic increasingly limits operation in the higher order modes needed for the near-terahertz regime. Savilov recently proposed a scheme for frequency-doubled phase bunching of gyrating electron beams in a waveguide resonator formed from Bragg reflectors and with the drive frequency equal to the cyclotron frequency. The advantages of phase prebunching at twice the cyclotron frequency include suppression of the fundamental harmonic, enhanced second-harmonic operation, and increased likelihood of fourth-harmonic operation. We have investigated the use of this phase bunching technique to enhance higher harmonic operation in gyrotron oscillators with annular beams. We compute the frequency-doubled bunching produced by a Bragg-type prebunching cavity and use a large-signal, multimode, and multiharmonic gyrotron oscillator code to simulate the effect of this bunching on a highly overmoded output cavity. Regimes of stable operation are predicted for the second- and fourth-harmonic point designs.

Long-range thermal imaging using a millimeter-wave source

Summary form only given. This paper describes a remote sensing concept that involves active thermal imaging with a millimeter-wave source (ATIMS). High frequency (HF) microwave or millimeter wave radiation can be beamed onto a target, thus generating rapid transient temperature increases in different portions of the target area. The time-dependent thermal contrast can be measured using sensitive infrared (IR) imagers. The technique can in principle be used in many situations where passive infrared imaging is currently used. This talk will include a description of the concept, a discussion of potential advantages and issues, model calculations of the predicted time-dependent temperature profile for various source intensities and materials, and results from preliminary laboratory proof-of-principle experiments. The experiments were performed at the HF microwave materials processing facility at NRL, where an 83 GHz gyrotron has been used to rapidly heat a variety of simple and complex targets. Thermal imaging with a sensitive mid-wavelength IR camera reveals clear signatures in a variety of target configurations. Potential applications include long range detection of explosive devices. Although a variety of heating sources have been used for active thermal imaging, including lasers, flashlamps, and longer wavelength microwaves, millimeter waves appear to be particularly well-suited for long range applications

Intense underwater laser acoustic source for Navy applications.

An intense remote underwater laser acoustic source is under development at the Naval Research Laboratory. In a novel configuration, a tailored intense laser pulse can be designed to propagate many meters underwater and compress at a predetermined remote location. Controlled compression of these optical pulses is governed by a combination of optical group velocity dispersion and nonlinear Kerr self‐focusing. Optical compression can result in laser‐induced breakdown, localized heating, and acoustic shock generation. Recent experiments include near‐field acoustic source characterization using lens‐focused 400 and 800 nm pulses of a Ti:sapphire laser, as well as 532 and 1064 nm pulses of a YAG laser. Sound pressure levels over 210 dB were achieved using a compact laser. Acoustic source characterization includes measurements of photoacoustic energy conversion efficiency, acoustic power spectrum, and directivity. Nonlinear optical studies included the precise measurement of the Kerr index of water at 400 and 80...

Design of a multi-kW, 600 GHz, second-harmonic gyrotron

We present a design study of a pulsed 600 GHz gyrotron operating at the second harmonic for a planned experiment based on a 12 T cryogen-free superconducting magnet and a Varian VUW-8010 electron gun. Threshold current studies and multimode simulations indicate that stable single operation at output powers over 10 kW is achievable.

Consolidation of Polycrystalline Yttria Powder By Millimeter-Wave Sintering for Laser Host Applications

Summary form only given. We report recent results of an investigation of millimeter-wave processing of yttria (Y2O3) for fabrication of transparent, high strength polycrystalline ceramic laser hosts for high energy laser (HEL) applications. The objective is to produce polycrystalline materials with optical quality comparable to that of a single crystal. It is difficult to produce yttria single crystals because of the phase transformation around 2000degC and the high melting temperature which is over 2400degC. While single crystals have high thermal conductivity and can operate at high powers, they are costly and limited in size and dopant concentration. Significant advantages of polycrystalline materials compared to single-crystals, are lower processing temperature, higher gain as a result of higher dopant concentrations, faster and less expensive fabrication, and the possibility of larger devices. Millimeter-wave processing has been proposed as an alternative method to solve the problems of both conventional vacuum sintering and low frequency microwave sintering, such as low heating rates, poor coupling, and unfavorable thermal gradients. A major component of the NRL millimeter-wave processing facility is a 20-kW, continuous-wave (CW), 83-GHz gyrotron oscillator (GYCOM, Ltd.). Translucent yttria has been successfully sintered with millimeter-wave beams with up to 99% theoretical density. A partially transparent yttria ceramic sample has also been achieved using the millimeter-wave sintering process. Several factors impact the quality of the sintered material including the presence of agglomerates, impurities, processing atmosphere, sintering aids, and thermal gradients. Efforts to improve the transparency are in progress.

Demonstration experiments for active thermal imaging using a millimeter-wave source (ATIMS)

Summary form only given. Thermal infrared imaging provides a powerful means for analyzing and identifying objects. While passive thermal imaging is useful in many situations where there is a large temperature differential between the target and background, it is less useful in situations where the target is at nearly the same temperature as the background. A method for enhancing the thermal contrast in targets or objects at long ranges is presented by R. Hubbard, et al. in the 33rd International Conference on Plasma Science. This paper describes several demonstration experiments for active thermal imaging using a millimeter-wave source (ATIMS). Our test setup relies upon the NRL millimeter-wave materials processing laboratory featuring a 20 kW, 83 GHz gyrotron oscillator and a mid-wavelength infrared (IR) camera. During and after the targets are irradiated with power levels of 50-200 W over an area of approximately 100 cm2, the IR sensitive camera captures a time-resolved history of the targets temperature. The target was located 1.6 m from the source while the IR camera was located 1.2 m from the target. Experiments were also performed where liquid crystal paper in thermal contact with the target scenario was used as the detector in lieu of the IR camera. We present data taken for a variety of objects and configurations, including situations where metallic and dielectric objects have been obscured, buried in sand, and covered by several layers of cloth. Since temperature differences of a few hundredths of a degree can be detected, temperature changes are often visible almost immediately after the radiation has been initiated. Initial results show that objects can be imaged underneath several layers of cloth or buried under sand

Underwater laser filamentation and guiding of electrical discharges

A technique to laser trigger and guide electrical discharges in an underwater environment is currently being developed at the Naval Research Laboratory. This work has potential applications in advanced micromachining, similar to that currently performed using electric discharge machining and water-assisted femtosecond laser machining, and low-jitter pulsed power generator switches1.

Directivity and frequency control of an intense underwater laser acoustic source for navy applications.

We develop an intense laser acoustic source, in which a tailored laser pulse can compress underwater at a predetermined remote location. Optical compression results in laser‐induced breakdown (LIB), localized heating, and acoustic shock generation. Recent experiments include near‐field acoustic source characterization using lens‐focused 400‐, 800‐, 532‐, and 1064‐nm pulses of Ti:sapphire and Nd:YAG lasers. Sound pressure levels over 215‐dB were achieved using a compact laser. We have demonstrated control of the shape of the LIB plasma volume, and thereby control of the acoustic frequency spectrum and acoustic source directivity. By superposition of volume elements within the LIB, the acoustic pulse duration in a given propagation direction is determined by the parallel dimension of the LIB, divided by the parallel acoustic transit time across the LIB. Thus, the shape of the LIB strongly affects the acoustic pulse duration and directivity, and aspherical LIB volumes result in strongly anisotropic acoustic ...

Optical bandwidth and focusing dynamics effects on an underwater laser acoustic source

Both femtosecond and nanosecond laser pulses can produce nonlinear effects in water, including filamentation and laser-induced breakdown resulting in acoustic generation. We examine the effects of GVD, varying wavelength, bandwidth, energy, and focusing configurations.

Characterization of underwater laser acoustic source for navy applications

An intense remote underwater laser acoustic source, utilizing laser-induced breakdown (LIB), is under development at the Naval Research Lab. In a novel configuration, a tailored intense laser pulse can be designed to propagate many meters underwater and compress at a predetermined remote location. Controlled compression of these optical pulses is governed by a combination of optical group velocity dispersion (GVD) and nonlinear Kerr self-focusing (NSF). Optical compression can result in underwater LIB, localized heating, and acoustic shock generation. Recent experiments include near-field acoustic source characterization using lens-focused 400 and 800 nm pulses of a femtosecond Ti:sapphire laser, as well as 532 and 1064 nm pulses of a Q-switched YAG laser. Sound pressure levels over 210 dB were achieved using a compact laser. Acoustic source characterization included measurements of: the acoustic power spectrum from a few Hz to 15 MHz, photoacoustic energy conversion efficiency, and directivity. Governing optical parameters of water have all been directly measured as part of this work, including the GVD of water at 400 nm, and Kerr index of water at 400 and 800 nm. Optical compression was demonstrated separately using GVD and NSF. Nonlinear optical studies included measurement of conditions for optical filament generation, and the filaments effects on acoustic signals. Ongoing and planned experiments include simultaneous optical compression using GVD and NSF, and optical and acoustic propagation studies in a bubbly salt water tank. Experimental results will be presented, and laser sources and techniques for underwater acoustic generation will be compared.