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Gas Detection Using Terahertz Waves

Gyrotrons operating in the millimeter and submillimeter wavelength ranges are the promising sources for applications that are requiring good spectral characteristics and a wide range of output power. We report the precise measurement results of gyrotron spectra. Transient downshift of the frequency by 12 MHz with a time constant of 3 s was observed.

After reaching equilibrium, the frequency was maintained within 1 ppm for over 20 s. Frequency pulling by the beam current was observed, but it was shown to be masked by the downward shift of the gyrotron frequency with temperature. The linewidth was measured to be much less than 1 MHz at 60 dB relative to the carrier power [in decibels relative to carrier dBc ] and 4. IN RECENT years, an intensive search for radiation sources with good spectral characteristics and reasonable output power at millimeter and submillimeter wavelengths has been conducted for a great number of research projects and practical applications [ 1 ], [ 2 ].

To satisfy these challenging requirements, Massachusetts Institute of Technology MIT has been developing gyrotrons [ 8 ]—[ 12 ] and measurement systems [ 13 ], [ 14 ]. The gyrotron is a vacuum electron device based on the cyclotron resonance interaction between an electron beam and electromagnetic waves in a resonant cavity [ 15 ]. Using a smooth wall interaction structure, whose transverse dimensions are many wavelengths at the operating frequency, gyrotrons have the ability to generate very high power when compared to the slow wave microwave tubes such as klystrons, backward wave oscillators, or solid-state devices at the millimeter and submillimeter wavelength regions.

For the applications listed previously, a knowledge of the spectral characteristics of gyrotrons is very important. For example, for the CTS to diagnose ion energy distribution and instabilities in plasmas, it requires high spectral purity and narrow linewidth over a large dynamic range to insure that the measured signals only originate from the plasma and not from a stray reflection [ 14 ].

Consequently, the DNP mechanism also needs high spectral resolution and stability to match the driving frequency to the electron-spin-resonance spectrum of the paramagnetic species in the sample [ 3 ]. Therefore, we report the precise measurements of gyrotron spectra bearing those applications in mind and discuss the physical mechanism and its implications. This paper is organized as follows. In Section II, the experimental setups, including the GHz long-pulse gyrotron and the heterodyne frequency measurement system, are briefly overviewed.

Detailed results on frequency downshifts, frequency control by cavity temperature, and frequency pulling by current are presented in Section III, followed by the linewidth measurement in the largest dynamic range. In the final section, the results are summarized. To study the spectral characteristics of gyrotron emission, we used a GHz gyrotron operating in pulses with a duration of 30 s Figs. The typical operating parameters are listed in Table I. For single pulse operation, an output pulse as long as 5 min is readily achievable.

The TE 03 operating mode in the cavity is converted into the TE 01 mode by an internal mode converter. Outside the gyrotron, the TE 01 mode is converted to the TE 11 mode using a serpentine rippled wall mode converter. Details of this gyrotron are given in [ 8 ]. Schematic diagram of the gyrotron used for this experiment. The basic components of the gyrotron include a magnet, an electron gun, and a vacuum tube which consists of a beam tunnel, a resonator, and a mode converter and collector.

Block diagram of the heterodyne system that is used to measure the spectral characteristics of the pulsed gyrotron. A trigger signal with a repetition rate of 0. A segmented sweep over the duration of the pulse was made by changing the delay of the time gate in the SA. The gyrotron output was sampled by a mirror and a 1.

This pickoff sample of the gyrotron beam was frequency downshifted by a heterodyne receiver. The harmonic heterodyne-receiver system employed a low-frequency local oscillator LO between 18 and The intermediate frequency IF from the mixer was amplified by a series of low noise amplifiers and detected by a spectrum analyzer SA with a dynamic range of 90 dB. A transient downward frequency shift of 12 MHz has been observed.

In comparison with the exponential fitting curve, the time constant is estimated to be approximately 3 s. After several tens of seconds, the frequency reaches equilibrium and remains stable to within 1 ppm for the rest of the pulse. Frequency shift over the duration of a pulse at Solid dots represent the measured data, and the solid line is a curve fitted to an exponential function.

The SA is set to a ms sweep time and kHz resolution bandwidth. Possible explanations for the frequency downshift in this system are the following: 1 thermal expansion of the resonant cavity due to ohmic heating of the cavity walls; 2 neutralization of dc space charge fields of electron beam by impact ionization; and 3 resonance frequency modification of the cavity by the background plasma.

The sensitivity of the frequency shift due to thermal expansion was measured by changing the temperature of the cavity coolant, which is water in our case Fig. This demonstrates that frequency fine tuning of a gyrotron could be possible with precise cavity-temperature control. The observed frequency downshift in Fig. The beam current was maintained by controlling the heater current. Gyrotron radiation has a finite linewidth which can be attributed to both intrinsic noise sources such as the shot effect of the electron beam and thermal noise, as well as the extrinsic technical noise sources such as fluctuations in the operating parameters [ 25 ].

Linewidth and lineshape measurements of the gyrotron are very important in applications that are utilizing a frequency shift by scattering, for example, radar [ 26 ] and CTS [ 5 ]—[ 7 ]. In order to measure the linewidth of the gyrotron over a large dynamic range, we used a low noise harmonic-mixer receiver.

The input power to the mixer was increased to maximize the observable dynamic range. As a consequence, we had a conservative observable dynamic range of up to 80 dB. Three separate frequency sweeps lasting 0.

The power level of the center frequency After converting the power scale into a linear ratio relative to the peak power, the full-width at half-maximum FWHM linewidth is calculated to be 72 kHz.

The true FWHM linewidth of the gyrotron could be narrower, as shown in [ 27 ] and [ 28 ], because the linewidth in the heterodyne measurement is the convolution of the LO with the gyrotron, i.

The linewidth of the LO was directly measured to be 0. Multiplication of the LO linewidth by the harmonic number 0. This means that the true linewidth of the gyrotron is likely to be narrower than the measured one.

For this measurement, the SA was set to 10 kHz of resolution bandwidth and ms of sweep time. Sweeping was delayed by 10 s to wait until the frequency was stabilized after the onset of radiation. As with other long-pulse gyrotrons, the frequency was down-shifted during the pulse. In our measurements using the W GHz gyrotron, the measured frequency downshift was 12 MHz and saturated after 10 s with a time constant estimated to be about 3 s. After reaching equilibrium, the gyrotron frequency remained stable to within 1 ppm for the remainder of the s pulse.

This observation supports the possibility of fine tuning the gyrotron frequency by cavity-temperature control. Frequency pulling by beam current was observed, but this effect was shown to be hidden by the frequency pulling due to thermal expansion or other effect such as background plasma formation.

In addition, the linewidth was shown to be much less than 1 and 4. These results show that gyrotrons operating in the millimeter and submillimeter wavelength ranges are the promising sources for applications requiring good spectral characteristics and reasonable output power. Seong-Tae Han received the B. As a Research Assistant, he participated in developing the ultrawideband TWTs for jamming and satellite communication.

In addition, he built the first-working lithographie galvanoformung und abformung LIGA -fabricated folded waveguide TWT and developed a delayed feedback oscillator at Ka-band.

He currently works on the development of subterahertz continuous-wave gyrotrons and their applications, including imaging and hyperthermia. Other research interests cover high-power RF applications such as electron cyclotron heating of fusion plasmas and charged particle acceleration for medical use. Robert G. Griffin received the B. Louis, MO, in He has published more than articles concerned with the magnetic resonance methodology and applications of magnetic resonance [nuclear magnetic resonance NMR and electron paramagnetic resonance EPR ] to studies of the structure and function of a variety of chemical, physical, and biological systems.

He has served on numerous advisory and review panels for the National Science Foundation and the National Institutes of Health. Kan-Nian Hu received the B. Chan-Gyu Joo received the B. His principal research interest is dynamic-nuclear-polarization NMR spectroscopy in liquid and solid state.

Colin D. Joye S'03 received the B. His current research interests include gyrotron oscillator and amplifier sources at the millimeter and submillimeter wavelengths. Jagadishwar R. Sirigiri S'98—M'02 received the B.

He is involved in the design and development of novel high-power gyrotrons and gyrotron amplifiers at millimeter-wave frequencies. His research interests include novel microwave sources and amplifiers in the millimeter and terahertz regime, quasi-optical structures and photonic-band-gap structures, and their applications in microwave vacuum electronics. Richard J. Temkin M'87—F'94 received the B. His research interests include novel vacuum electron devices such as the gyrotron and free electron laser, advanced high-gradient electron accelerators, quasi-optical waveguides and antennas at millimeter wavelengths, plasma heating, and electron spin resonance spectroscopy.

He has been the author or coauthor of over published journal articles and book chapters and has been the editor of six books and conference proceedings. He was the recipient of the Kenneth J. Antonio C. Torrezan S'04 received the B. He is currently working toward the Ph. From to , he was associated with the Brazilian Synchrotron Light Laboratory where his research was focused on the development of an X-band electron-paramagnetic-resonance spectrometer for samples with small number of spins.

In , he worked as an intern at the Brazilian Aeronautics Company Embraer in the area of control systems. His research interests include millimeter-wave technologies and submillimeter-wave sources such as the gyrotron. Paul P. His principal interests include plasma diagnostics, fusion energy, millimeter-wave technologies, and environmental applications of plasmas and millimeter waves.

National Center for Biotechnology Information , U.

Photonic Technologies for Millimeter- and Submillimeter-Wave Signals

You need Adobe Reader 7. If Adobe Reader is not installed on your computer, click the button below and go to the download site. We are utilizing the characteristics of terahertz electromagnetic waves to develop a system for remotely detecting whether dangerous gases are present at locations such as disaster sites. In this article, we give an overview of our system, which is based on technology for generating narrow-linewidth variable-frequency terahertz waves derived from optical techniques. We also present results of gas absorption spectra experiments performed using this technology. Terahertz electromagnetic waves have several characteristic features: 1 their frequency is an order of magnitude higher shorter wavelength than electromagnetic waves in the microwave- and millimeter-wave bands, so their energy can be focused onto a small spot relatively easily, 2 they are longer in wavelength than infrared and visible light, so they are scattered less by dust, soot, and smoke in the air, and have little effect on the human body, and 3 their absorption by many materials produces characteristic absorption spectra, or spectral fingerprints. It should be possible to use these properties of terahertz waves to detect sources of dangerous gases, such as carbon monoxide, hydrogen cyanide, or hydrogen chloride, from a distance at disaster sites such as large-scale earthquakes.

This website uses cookies to deliver some of our products and services as well as for analytics and to provide you a more personalized experience. Click here to learn more. By continuing to use this site, you agree to our use of cookies. We've also updated our Privacy Notice. Click here to see what's new. Ultrabroadband millimeter-wave and subterahertz waveforms offer significant potential, from ultrahigh-speed communications to high-resolution radar. Electronic generation of broadband arbitrary waveforms at these frequencies suffers from limited digital-to-analog converter speed and high timing jitter.

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S ν The full-power channel () P ν is represented by the solid curve. The result of developed in the microwave molecular spectroscopy. The first one is based on der to operate at millimeter and submillimeter waves. it is necessary to use converter (Analog devices, Inc.) [34], which contains. high-quality.

Polarization-sensitive THz-TDS and its Application to Anisotropy Sensing

Gyrotrons operating in the millimeter and submillimeter wavelength ranges are the promising sources for applications that are requiring good spectral characteristics and a wide range of output power. We report the precise measurement results of gyrotron spectra. Transient downshift of the frequency by 12 MHz with a time constant of 3 s was observed. After reaching equilibrium, the frequency was maintained within 1 ppm for over 20 s.

Vidal, T. Nagatsuma, N. Gomes, T. Fiber optic components offer a competitive implementation for applications exploiting the millimeter-wave and THz regimes due to their capability for implementing broadband, compact, and cost-effective systems.

Holloway, G. Dogiamis and R. Solid-State Circuit Conf.

Polarization-sensitive THz-TDS and its Application to Anisotropy Sensing

Gas Detection Using Terahertz Waves

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Request PDF | Millimeter-Wave and Submillimeter-Wave Imaging for Security GHz to 3 THz are discussed for some typical inorganic and organic solids. and security imaging, biomedicine, and chemical spectroscopy [1] [2] [3][4]. which consists of a lock-in amplifier and analogue to digital converter.

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Spectral Characteristics of a 140-GHz Long-Pulsed Gyrotron

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  1. Pansy F.

    Developments in spectroscopic polarimetry and ellipsometry and various polarization components targeting THz frequencies are introduced.

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