Stabilization Scheme for Hot-Electron Bolometer Receivers Using Microwave Radiation

We present the results of a stabilization scheme for terahertz receivers based on NbN hot-electron bolometer (HEB) mixers that uses microwave radiation with a frequency much lower than the gap frequency of NbN to compensate for mixer current fluctuations. A feedback control loop, which actively controls the power level of the injected microwave radiation, has successfully been implemented to stabilize the operating point of the HEB mixer. This allows us to increase the receiver Allan time to 10 s and also improve the temperature resolution of the receiver by about 30% in the total power mode of operation.     

短毫米波和亚毫米波接收机的发展现状

本文概述了短毫米和亚毫米波接收机的发展现状，介绍了接收机的重要组成部件：混频器、振荡器和放大器的发展水平以及毫米波单片集成电路的现状，给出了接收机的最新性能指标。     

Application of the Gyrotron FU II Submillimeter Wave Radiation Source to Plasma Scattering Measurements

Gyrotron FU II has been successfully applied as a submillimeter wave radiation source to plasma scattering measurements on the Compact Helical System (CHS) in National Institute for Fusion Science (NIFS) in Japan. The gyrotron operates in a long pulse mode (the pulse width is about 600 ms) at a frequency of about 350 GHz (the corresponding wavelength is 0.85 mm). The output power is about 110 W. The output power is transmitted along a circular waveguide system and converted to a Gaussian-like beam by a quasi-optical antenna. After that, the beam is directed onto the CHS plasma and the scattered signal is detected by a homodyne detection system. The frequency and the wave number of the scattered signal are analyzed. The results suggest that a broad band low frequency (several tens to several hundreds kHz) density fluctuation is excited in the CHS plasma only during neutral beam injection (NBI) or ion cyclotron resonance (ICR) heating.     

Production of an Intense, Well-Collimated Submillimeter Wave Beam and Its Application to Plasma Scattering Measurements

Intense submillimeter wavelength radiation (f=354 GHz, P=110 W) from Gyrotron FU II is converted into a well-collimated, linearly-polarized beam by a quasi-optical transmission line consisting of a quasi-optical antenna and several mirrors. The observed focusing of the beam agrees with calculations based on the Huygens equation. We have used this intense beam to study low frequency density fluctuations by plasma scattering in Compact Helical System (CHS) plasmas at the National Institute for Fusion Science.     

A Racetrack Microtron for Millimeter and Submillimeter Wave Generation

The accelerator described here is an eight-orbit four-sector racetrack microtron possessing strong focusing action. The magnet gap is only 7mm, and the accelerating cavity is placed in one of the field-free regions. The energy gain per traversal of the cavity can be varied from 0.4 to 1.5 Mev and synchronism obtained by adjustment of the magnetic field strength and the length of the main straight section. A theoretical analysis of the synchrotrons oscillations in energy and phase shows that tight bunching can be achieved at almost any point in any desired orbit by changing the frequency of the synchrotrons oscillations. This can be accomplished by varying the RF power and therefore the accelerating voltage. One particularly attractive operating region gives tight bunching in the third orbit, allowing the construction of a compact machine if desired. For the RF accelerating source used (frequency 2800 Mc) one obtains 20 per cent of the third orbit current in a bunch length of 0.1 mm using dc gun injection (no prebunching). A current of 20 ma was obtained in the third orbit (2.2 Mev) which should be sufficient for the production of milliwatt power in the submillimeter region.     

Dynamics of Short Sea Wave Spectrum Estimated From Microwave Radiometric Measurements

This paper presents some results of the experiment CAPMOS'05 performed on an oceanographic research platform in the Black Sea. The platform, located 600 m offshore, was equipped with a set of contact and remote sensing instruments. Conventional contact sensors were used for direct measurements of atmosphere and sea boundary layer parameters (wind speed and direction, air temperature, water temperature and salinity profiles, etc.), whereas microwave and infrared band radiometers were used for remote sensing measurements of surface temperature and wave parameters. In particular, microwave Ka-band radiometer measurements were applied for gravity-capillary wave spectrum retrieval using the original techniques based on angular measurements. The spectrum component evolution under unstable wind conditions has been investigated. It has been demonstrated that the spectral components in the vicinity of the maximum of the wave curvature are the most sensitive to the wind-velocity variations.      

Probes for Microwave Near-Field Measurements

To be satisfactory for microwave near-field measurements, a probe must have desirable polarization characteristics, must have an aperture small enough to indicate the field at a point, must deliver sufficient signal voltage to permit accurate measurement, and yet must not seriously distort the fields. The design of a probe may be simplified if the fields to be measured are known to be almost linearly polarized or to consist only of a traveling wave. Comparison of measurements made with various probes has led to the development of a small open-ended waveguide probe which is simple to construct and has given excellent results.     

Novel Applications of Millimeter and Submillimeter Wave Gyro-Devices

The possible applications of high-power millimeter (mm) and submillimeter waves from gyro-devices span a wide range of technologies. The plasma physics community has already taken advantage of recent advances in applying high-power micro- and mm-waves generated by gyrotron oscillators in the areas of RF-plasma production, heating, noninductive current drive, plasma stabilization and active plasma diagnostics for magnetic confinement thermonuclear fusion research, such as lower hybrid current drive (8 GHz), electron cyclotron resonance heating (ECRH) (28–170 GHz), electron cyclotron current drive (ECCD), collective Thomson scattering and heat-wave propagation experiments. Other important applications of gyrotrons are electron cyclotron resonance (ECR) discharges for generation of multi-charged ions and soft X-rays, as well as industrial materials processing and plasma chemistry. Submillimeter wave gyrotrons are employed in high-frequency broadband electron paramagnetic resonance (EPR) spectroscopy. Future applications which await the development of novel high-power gyro-amplifiers include high resolution radar ranging and imaging in atmospheric and planetary science as well as deep-space and specialized satellite communications and RF drivers for next-generation high-gradient linear accelerators (supercolliders). The present paper reviews the state-of-the-art and future prospects of these recent applications of gyro-devices.     

Modelling and Simulation of Magnetron Infection Guns for Submillimeter Wave Gyrotrons

The software package GUN-MIG has been developed for computer simulation of beam formation in magnetron injection guns (MIG). It is based on a fully relativistic self-consistent physical model which takes into account the specific problems and requirements that are being encountered in the analysis and design of MIG for gyrotrons. Besides providing a general outline of the physical models and the program implementation of the code, several illustrative examples of the numerical experiments with real MIG for submillimeter wave gyrotrons are presented and discussed.     

Standoff Detection Using Millimeter and Submillimeter Wave Spectroscopy

The millimeter (MM) wave and sub-MM wave (30-600 GHz) frequency band contains fundamental rotational and vibrational resonances of many molecular gases composed of carbon, nitrogen, oxygen, and sulphur. The high specificity of rotational spectra to organic molecules affords MM wave spectroscopy having potential use in remotely sensing atmospheric pollutants and the detection of airborne chemicals is important for arms control treaty verification, intelligence collection, and environmental monitoring. This paper considers the sensitivity requirements of radiofrequency receiver systems for measuring MM wave absorption/emission signatures. The significance of receiver sensitivity and material optical depth to sensing is highlighted. A background to the technology needed for sensing at MM and sub-MM wavelengths then provides the basis for a review of MM wave spectroscopy and its role on profiling the concentrations of trace polar molecules and ionized radicals in the high altitude atmosphere. The application of the MM wave spectroscopic technique in ambient conditions is then reviewed and the issues associated with developing the technique for standoff remote sensing is discussed.     

A Shunt Technique for Microwave Measurements

A new method of measuring low-loss quantities at microwave frequencies, which employs a Iossy shunt structure, is described. The additional loss introduced thereby reduces the excessively high VSWR's to values lying in the measurable range. The relevant information is abstracted from the data in a precision manner independent of the parameters of the shunt structure itself. Applications are made to the measurement of low-loss dielectric constants, structures with shunt representations such as certain bolometer elements, attenuation constants, and VSWR's of slightly Iossy variable short circuits. Physical realizations of such shunt structures are discussed.     

Microwave Measurements Part II Non-linear Measurements

This paper addresses the problems in microwave non-linear measurements. It discusses techniques to synthesize loads, the most used non-linear measurement techniques, and harmonic load-pulling. An experimental setup for characterizing power amplifiers must be able to measure the complex spectrum of the waves at the amplifier ports as a function of frequency, input power, and source and load termination at the fundamental and harmonic frequencies. The vector network analyzer (VNA) is the core instrument used in the non-linear characterization scenario. The basic idea is to keep the operations of VNA/mixers linear, diverting to them only a small portion of the signal present at the device under test (DUT) ports, therefore keeping unaltered the VNA capabilities already exhibited for small signal measurements.     

Design of Transmission Line for Submillimeter Wave Gyrotron Using Dielectric-Lined Waveguide

The excitation and transmission losses of a dielectric-lined waveguide have been calculated with real, experimentally measured output beams of a submillimeter wave gyrotron. It is shown that for ‘not-so-long’ transmission lines the coupling losses far exceed the transmission losses in the waveguide.     

超辐射双纵模亚毫米波激光的多位曼效应

在半经典密度矩阵理论的基础上,理论研究了超辐射双纵模CO2－9R（30）光泵NH3分子激光过程中的多拉曼相互作用,并研究了不同泵浦功率、工作气压和激光管长等工作参数条件下多拉曼相互作用规律和对超辐射亚毫米波激光频谱特性的影响。     

Precision Resonator Microwave Spectroscopy in Millimeter and Submillimeter Range

By the use of Fabry–Perot resonator with quality factor ? 600 000 and fast precision (down to one Hertz) frequency control of coherent millimeterwave radiation source the 20 Hz accuracy in measurements of the width of resonance curve is obtained. This accuracy corresponds to the detection of 1.8 × 10^?3 dB/km absorption in the sample filling the resonator and exceeds the before known sensitivity more than by an order of magnitude. The example of precise measurement of 60 GHz oxygen absorption band in the real atmosphere is presented. The new possibilities of application of precision resonator microwave spectroscopy to the atmospheric problems as well as to the ultra-low absorptions measurements in dielectrics and metal surfaces up to Terahertz frequencies are discussed.     

Observation of Frequency Down-Shift in a Submillimeter Wave Gyrotron

Frequency down-shift in the initial stage of CW or long pulse operation of a submillimeter wave gyrotron, Gyrotron FU IV, is observed. The shift occurs in a few minutes after turning on the operation and the amount of the shift attains even 0.1 GHz. The observation results are analyzed on the basis of a simple model for heat conduction in the region of a resonant cavity. The frequency shift is explained consistently by expansion of the cavity.     

Voltage Induction Method for Microwave Susceptibility Measurements

A voltage induction method of microwave susceptibility measurement has been proposed. A signal from a one-turn coil wrapped directly on a magnetic sample is phase sensitively detected, and the real and imaginary parts of susceptibility as a function of magnetic field are directly indicated on an oscilloscope. Analysis shows this method to have a much higher sensitivity for small samples than the conventional cavity perturbation method. Also, this method is characterized by a higher stability, since the measurement is not for the perturbation of the resonant curve of high Q cavity, but for RF magnetic flux in a sample which is relatively stable for a frequency fluctuation of the signal source, independent of the sample volume. This method has been checked with good agreement against a perturbation method for a sample where both methods could be used. A YIG film, which is too small (4 mm by 4 mm by 0.3/spl mu/) to obtain any detectable perturbation of cavity parameters, was successfully measured by this method. The new method allows very rapid measurement combined with a very simple procedure for the relative measurement of susceptibility.