Robert B. Barat

Terahertz study of 1,3,5-trinitro-s-triazine by time-domain and Fourier transform infrared spectroscopy

This letter describes the use of THz time-domain spectroscopy (TDS) applied in transmission to the secondary explosive 1,3,5 trinitro-s-triazine. Samples were also subjected to Fourier transform infrared spectroscopy over the same range for comparison. A detailed spectroscopy study is presented. General agreement between results from both methods confirms the absorption features found. A comparison study with computer molecular simulations shows that THz-TDS is sensitive to collective modes or vibrational modes of material.

Thermal reactions of CH2Cl2 in H2/O2 mixtures: Implications for chlorine inhibition of CO conversion to CO2

The thermal decomposition of dichloromethane in hydrogen/oxygen mixtures and argon bath gas was carried out at 1 atm pressure in tubular flow reactors of varied surface-to-volume ratios. The degradation of dichloromethane plus intermediate and final product formation was analyzed from 873 to 1093 K, with average residence times of 0.1–2.0 s. A detailed kinetic reaction mechanism based upon fundamental thermochemical principles and Transition State Theory was developed and used to model our experimental results. Sensitivity analysis was used to determine important reactions effective in inhibiting CO conversion to CO2. The results indicate that the reaction: OH + HCl → H2O + Cl is a major cause of OH loss and this decrease in OH significantly reduces CO conversion by reaction with OH. Lower temperatures result from reduced CO reaction with OH, which increases the importance of HO2. Here, the reaction of HO2 + Cl to the HCl + O2 (termination) channel further inhibits combustion. A significant fraction of the CH2Cl2 conversion occurs through C2 chlorocarbon formation, which results from methyl and chloromethyl combination reactions.

Rapid-phase modulation of terahertz radiation for high-speed terahertz imaging and spectroscopy.

Rapid voltage-controlled phase modulation of cw terahertz (THz) radiation is demonstrated. By transmitting an infrared beam through a lithium niobate phase modulator the phase of the THz radiation, which is generated by the photomixing of two infrared beams, can be directly modulated through a 2pi phase shift. The 100 kHz modulation rate that is demonstrated with this technique is approximately 3 orders of magnitude faster than what can be achieved by mechanical scanning.

THz standoff detection and imaging of explosives and weapons

The application of near-field interferometric imaging to the Terahertz frequency range for detection of concealed objects is discussed. A circular array architecture can be employed to compensate for near-field distortions and increase the field of view and depth of focus. The lateral and focusing errors of this imaging method are discussed as well as the trade-offs of interferometric imaging compared to a focal plane array architecture.

Experimental comparison of performance degradation from terahertz and infrared wireless links in fog

We describe a lab setup for analyzing impairments of terahertz (THz) and infrared (IR) free space links caused by local refraction index changes in the signals propagation paths that could be induced by turbulence, particles, humidity, etc. A THz signal comprising a 2.5 Gb/s data load modulated on a carrier at 625 GHz, is launched through a weather emulating chamber, detected, and its performance analyzed. An IR beam at 1.5 um wavelength carrying the same data load is superposed with the THz beam, propagating through the same weather conditions and also performance analyzed. We modulate the IR channel with a usual non-return-to-zero (NRZ) format but use duobinary coding for driving our THz source, which enables signaling at high data rate and higher output power. As both beams pass through the same channel perturbations and as their degradations are recorded simultaneously we can simultaneously compare the weather impact on both. We investigate scintillation and fog attenuation effects for the THz and IR signals by measuring bit error rates (BER), signal power, and phase front distortions.

Experimental comparison of terahertz and infrared data signal attenuation in dust clouds

In order to study and compare propagation features of terahertz (THz) links with infrared (IR) links under different weather conditions such as turbulence, fog, and dust particles, THz and IR free space communication links at 625 GHz carrier frequency and 1.5 μm wavelength, respectively, with a maximum data rate of 2.5 Gb/s have been developed. After propagating through the same channel perturbation caused by dust, attenuation of the carrier frequencies by dust as well as scintillation effects on both channels are analyzed by measuring the power attenuation and bit error rates. Attenuation by the presence of dust degrades the IR channel but exhibits almost no measurable impact on the THz signal. Numerical simulations of THz attenuation with different dust concentrations are presented and agree with the measured results.

Chlorocarbon-lnduced Incomplete Combustion In A Jet-Stirred Reactor

Abstract The chemistry leading to products of incomplete combustion, as affected by chlorocarbons, was studied by modeling data from a jet-stirred reactor, verifield experimentally to be at nearly perfectly-stirred conditions. Premixed, fuel-rich mixtures of C2H4, CH3Cl, and air were burned in a toroidal jet-stirred combustor. Instantaneous measurements of local temperature were obtained using laser Rayleigh scattering. and extractive probe sampling followed by gas chromatography was used to determine concentrations of stable species. Reactor temperature, and equivalence ratio (φ) were held constant, while the chlorine content was varied by modifying the C2H4, CH3Cl, and diluent N2 flows. The instantaneous temperature measurements showed that the reactor remains nearly perfectly-stirred, even to the point of blowout. A perfectly stirred reactor (PSR) model indicated that the principal effect of chlorine addition was that of inhibiting the burnout of CO to CO2, consistent with the experimental observation ...

Jet-Stirred Combustor Behavior Near Blowout: Observations and Implications

Abstract The behavior of jet-stirred combustors near blowout has been examined. Such units nominally emulate perfectly stirred reactors (PSR). Previous experimental data on instantaneous temperatures and stable species from a toroidal design have suggested non-PSR behavior as overall extinction is approached. Detailed mechanistic modeling in this work shows that such combustors experience global blowout at significantly lower mass flow rates than predicted by a PSR model. It is suggested that, as overall blowout is approached, only a fraction of the physical reactor volume is under active combustion as localized extinction occurs while apparent global stability still exists. This hypothesis is used to successfully predict observed global extinction points for C2H4/air and CO/H2/ air in a toroidal unit, and CH4/air in a spherical unit. In order to judge whether a stirred combustor is experiencing significant departures from PSR behavior, a test is offered for carbon-containing fuels burned under fuel-lean ...

Two-dimensional interferometric and synthetic aperture imaging with a hybrid terahertz/ millimeter wave system

We have developed an interferometric synthetic aperture incoherent imaging system at 94 GHz, in which a high-power electronic millimeter wave source (Gunn Oscillator) is integrated with a continuous-wave terahertz (THz) photomixing detection system to achieve a high signal-to-noise ratio. Imaging of a point source located 10?m away from the detector array is presented. Two-dimensional THz reflective images of an extended object with different shapes are reconstructed with only four detectors by use of rotational synthesis.

T-rays vs. terrorists

This paper discusses terahertz radiation technology (T-ray) that will probably find its first big uses in security-related applications, an enormous fast-growing business because of recent high-profile terrorist attacks.The technology appeal here is undeniable: in a terahertz image, a gun or a knife shines through whatever clothing its concealed in - even a plastic knife shows up, because of the way its sharp edges scatter the radiation. And yet, unlike X-rays, T-ray screeners could be used routinely on people, because the radiation is harmless.