Dennis K. Remelius

First operation of a photocathode radio frequency gun injector at high duty factor

Tests of the electron beam injector for the Boeing/Los Alamos Average Power Laser Experiment have demonstrated first time operation of a photocathode radio frequency gun accelerator at 25% duty factor, exceeding previous photocathode operating parameters by three orders of magnitude. The macropulse format was 30 Hz and 8.3 ms with a micropulse frequency of 27 MHz. Average beam currents of up to 32 mA have been accelerated to 5 MeV for an average beam power of 160 kW. The macropulse peak current was 128 mA. The 32 mA average beam current exceeds previous cathode performance by a factor of 1000. Emittance measurements demonstrate excellent electron beam quality.

High-speed random access laser tuning

We have developed a technique for laser tuning at rates of 100 kHz or more using a pair of acousto-optic modulators. In addition to all-electronic wavelength control, the same modulators also can provide electronically variable Q-switching, cavity length and power stabilization, chirp and linewidth control, and variable output coupling, all at rates far beyond what is possible with conventional mechanically tuned components. Tuning rates of 70 kHz have been demonstrated on a radio-frequency-pumped CO2 laser, with random access to over 50 laser lines spanning a 17% range in wavelength and with wavelength discrimination better than 1 part in 1000. A compact tuner and Q-switch has been deployed in a 5-10-kHz pulsed lidar system. The modulators each operate at a fixed Bragg angle, with the acoustic frequency determining the selected wavelength. This arrangement doubles the wavelength resolution without introducing an undesirable frequency shift.

Argon arc lamps

The spectra of several Ar arc lamps were measured in the 0.4–0.9-μm range. The voltage and current characteristics of these lamps filled with various Ar pressures were also measured. One of the lamps was then used to pump a Nd-YAG laser, and the same laser was also pumped with a common Xe arc lamp. A comparison of the laser performance with both arc lamps is presented.

Acousto-optically tuned isotopic CO2 lasers for long-range differential absorption lidar

We are developing 2-100 kHz repetition rate CO2 lasers with milliJoule pulse energies, rapid acousto-optic tuning and isotopic gas mixes, for differential absorption LIDAR applications. We explain the tuning method, which uses a pair of acousto-optic modulators and is capable of random access to CO2 laser lines at rates of 100 kHz or more. The laser system is also described, and we report on performance with both normal and isotopic gas mixes.

Integrated rheology model: Explosive Composition B-3

ABSTRACT Composition B-3 (Comp B-3) is a high explosive formulation composed of 60/40wt% RDX (1,3,5-trinitroperhydro-1,3,5-triazine) /TNT (2,4,6 trinitrotoluene). Above approximately 78°C this formulation partially melts to form a multiphase system with solid RDX particles in a molten TNT matrix. This multiphase system presents a number of phenomena that influence its apparent viscosity. In an earlier study explosive Composition B-3 (Comp B-3, 60/40wt% RDX/TNT) was examined for evidence of yield stress using a non-isothermal falling ball viscometer and a yield stress model was proposed. An integrated viscosity model suitable for use in computational fluid dynamics (CFD) simulations is developed to capture the transition from a heterogeneous solid to a Bingham viscoplastic fluid. This viscosity model is used to simulate the motion of imbedded spheres falling through molten Comp B-3. Comparison of the simulations to physical tests show agreement between the positions predicted by the model and the measured locations of the spheres as a function of temperature between 90C and 165C.

Internal sub-sonic burning during an explosion viewed via dynamic X-ray radiography

We observe internal convective and conductive burn front propagation and solid consumption subsequent to thermal ignition for plastic bonded formulations of the solid organic secondary explosives octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine and 1,3,5-triamino-2,4,6-trinitrobenzene. This work describes x-ray radiographic diagnostics enabling the study of solid density in a fully encased explosive during internal burning subsequent to ignition. The result of this study is the ability to directly observe and measure rates of energy release during a thermal explosion.

Cross delay line sensor characterization

There exists a wealth of information in the scientific literature on the physical properties and device characterization procedures for complementary metal oxide semiconductor (CMOS), charge coupled device (CCD) and avalanche photodiode (APD) format detectors. Numerous papers and books have also treated photocathode operation in the context of photomultiplier tube (PMT) operation for either non imaging applications or limited night vision capability. However, much less information has been reported in the literature about the characterization procedures and properties of photocathode detectors with novel cross delay line (XDL) anode structures. These allow one to detect single photons and create images by recording space and time coordinate (X, Y & T) information. In this paper, we report on the physical characteristics and performance of a cross delay line anode sensor with an enhanced near infrared wavelength response photocathode and high dynamic range micro channel plate (MCP) gain (> 106 ) multiplier stage. Measurement procedures and results including the device dark event rate (DER), pulse height distribution, quantum and electronic device efficiency (QE & DQE) and spatial resolution per effective pixel region in a 25 mm sensor array are presented. The overall knowledge and information obtained from XDL sensor characterization allow us to optimize device performance and assess capability. These device performance properties and capabilities make XDL detectors ideal for remote sensing field applications that require single photon detection, imaging, sub nano-second timing response, high spatial resolution (10s of microns) and large effective image format.ÿ

First operation of a photoinjector at high-average power

Tests of the Boeing Average Power Laser Experiment injector have demonstrated first-time operation of a photocathode RF gun electron accelerator at 25% duty factor. The multi-alkali photocathode was illuminated by a frequency-doubled, mode-locked Nd:YLF laser. The cathode was placed in the first cell of four single-cell cavities resonant at 433 MHz. The 4 cavities accelerated the beam to 5 MeV. The pulse duration was 8.3 ms and the repetition rate was 30 Hz. True average beam currents of up to 35 mA have been accelerated to 5 MeV for an average beam power of 170 kilowatts. The 35 mA beam current exceeded previous photocathode performance by a factor of 1000.

Heterodyne lidar for chemical sensing

The overall objective is to assess the detection performance of LWIR (long wavelength infrared) coherent Lidar systems that potentially possess enhanced effluent detection capabilities. Previous work conducted by Los Alamos has demonstrated that infrared DIfferential Absorption Lidar (DIAL) is capable of detecting chemicals in plumes from long standoff ranges. Our DIAL approach relied on the reflectivity of topographical targets to provide a strong return signal. With the inherent advantage of applying heterodyne transceivers to approach single-photon detection in LWIR, it is projected that marked improvements in detection range or in spatial coverage can be attained. In some cases, the added photon detection sensitivity could be utilized for sensing “soft targets”, such as atmospheric and threat aerosols where return signal strength is drastically reduced, as opposed to topographical targets. This would allow range resolved measurements and could lead to the mitigation of the limiting source of noise due to spectral/spatial/temporal variability of the ground scene. The ability to distinguish normal variations in the background from true chemical signatures is crucial to the further development of sensitive remote chemical sensing technologies. One main difficulty in demonstrating coherent DIAL detection is the development of suitable heterodyne transceivers that can achieve rapid multi-wavelength tuning required for obtaining spectral signature information. LANL has recently devised a novel multi-wavelength heterodyne transceiver concept that addresses this issue. A 5-KHz prototype coherent CO2 transceiver has been constructed and is being now used to help address important issues in remote CBW agent standoff detection. Laboratory measurements of signal-to-noise ratio (SNR) will be reported. Since the heterodyne detection scheme fundamentally has poor shot-to-shot signal statistics, in order to achieve sensitive detection limits, favorable averaging statistics have to be validated. The baseline coherent DIAL detection sensitivity that can be achieved averaging multiple laser pulses and by comparisons of different wavelengths will be demonstrated. Factors that are presently limiting performance and attempts to circumvent these issues will be discussed.

Development of CO2 waveguide lasers and laser technology for remote sensing

We have developed rapidly tuned RF-pumped CO2 waveguide laser transmitters for remote sensing in the 9 - 11 micrometers spectral range. The small size, high power and efficiency, and tunability of these lasers offer significant advantages over other laser sources in this spectral region. Employing acousto-optic modulators to achieve random-access tuning at pulse rates up to 100 kHz permits rapid gathering of data on time scales short compared to times for change in atmospheric turbulence and absorption effects, thereby improving the signal-to-noise ratios that can be achieved. Laser system design and performance characteristics of present systems are described, along with proposed concepts to increase optical bandwidths and extend the tuning range to cover the full long-wave atmospheric transmission window from 8 - 12 micrometers .