Source Of Microwave Radiation Research Articles

A model of the atmosphere above a sunspot from radio observations

The results of VLA observations of an unusual source of microwave radiation, associated with the sunspot NOAA 7789 on October, 15, 1994, are presented. The fine structure of the source, which is a ring structure in intensity and polarization at frequencies 4.5 and 8.0 GHz, is discussed. It is shown that the features observed can be explained by a thermal cyclotron mechanism if the magnetic field is approximated by a vertical dipole buried under the photosphere, but the spatial distributions of kinetic temperature and electron density in the atmosphere above the sunspot differ considerably from the standard model. A two-dimensional source model (the dependences of the parameters on the height and distance from the center of the sunspot), which fits the observations at the above frequencies, is evolved. The principal physical result is that the data observed are explainable by the presence of an unexpectedly dense cool plasma in the atmosphere over the center of the umbra.

Arrays of Josephson junctions closed into a superconducting loop as a source of coherent radiation

We have studied via computer simulations the dynamical properties of a circuit consisting of two arrays of Josephson junctions closed into a superconducting loop. Due to an internal circuit resonance the phase locking of Josephson junctions is enhanced, which makes possible the application of such a circuit as a source of coherent microwave radiation. The optimal circuit parameters and the influence of thermal noise are discussed. It is shown that the critical spread in junction parameters is about twice that of linear Josephson junction arrays. The possibility of tuning such a device is demonstrated.

Intensity interferometry for measurement of electron temperature fluctuations in fusion plasmas

Transport-relevant fluctuations of the electron temperature can in principle be measured by radiometry of the electron cyclotron emission (ECE), but they might be buried completely in natural fluctuations of the ECE due to the thermal nature of this radiation. The spatial coherence properties of thermal radiation can be utilized in correlation experiments to overcome the principal difficulties and to make temperature fluctuations accessible to measurement. On the basis of these considerations an intensity interferometer was built and installed on the Wendelstein 7-AS stellarator. The interferometer consists of two identical but independent multichannel heterodyne radiometers viewing the same emitting plasma volume along crossed lines of sight. The angle between the sightlines is chosen sufficiently large to decorrelate the natural fluctuations of the radiation field. A quasithermal microwave radiation source is used to test the intensity interferometer and to simulate the correlation experiment. Measurements of the spatial coherence and the statistical properties are in agreement with theoretical predictions.

Electromagnetic radiation curing of an epoxy/fibre glass reinforced composite

Conventional curing processes for producing thermoset composites uses a thermal approach, which suffers from numerous disadvantages, including a long curing cycle time. This paper reports on an alternative curing process using an electromagnectic microwave radiation source operated at 2450 MHz. Results indicate that a significantly faster curing time was achieved, with the cured specimens having strength properties at least as high as those cured in a thermal curing process, with the modulus value being significantly higher. The effects of the irradiation was also found to be uniform throughout the thickness of the material, for a 100 mm thick specimen.

Analytical process in which materials to be analyzed are directly and indirectly heated and dried by microwave radiation

An apparatus for controllably heating material, such as a material to be analyzed for volatiles or non-volatiles contents, directly and indirectly by microwave radiation, includes a chamber, in which the material to be heated is placed, a source of microwave radiation, for radiating onto materials in the chamber and for controllably heating such a material, and a support for such material, which support includes a matrix of a material which transmits microwave radiation, and also includes particulate material therein which absorbs microwave radiation. The absorptive material is one that has a Curie temperature, above which it is transmissive of microwave radiation, so as thereby to prevent excessive heating of the support and the supported material which is to be heated. The matrix material is stable and form-retaining at the Curie temperature of the dispersed material. Preferably the support matrix will be of a silicone rubber or of a polyfluoroalkylene polymer, such as polytetrafluoroethylene, and the dispersed material will be a ferrite, garnet or a mixture thereof. The apparatus may include a turntable for one or more supports, an automatic electronic balance, computerized controls and readout means. Also within the invention are the described support, and processes for heating and drying, in a preferred embodiment of which processes a particulate diluent or dispersant for the sample being treated is employed to prevent overheating of the sample.

Applications of pulsed microwave-microwave double resonance: I. Relaxation studies in gases

The time-resolved response of a gas absorber exposed to two independent sources of microwave radiation, one steady and the other pulsed, is described using a formalism based on the Bloch equations for a three-level system. This predicts that a series of experiments can be made on the resonant and off-resonant recoveries of the absorptive and dispersive properties of a single sample by which the individual collisional relaxation parameters that characterise its rotational states may be separately measured. Some of these can be determined only by double-resonance experiments. The construction and operation of amicrowave double-resonance bridge spectrometer capable of carrying out these experiments is described, and results obtained for the (J=2 from 1)-(J=1 from 0) system of OCS are discussed in detail. The polarisation relaxation rate for the 2 from 1 transition is found to be 38.2(2) mu s-1 Torr-1 and its population relaxation rate, including cross-relaxation effects, is found to be 38.9(1.9) mu s-1 Torr-2. Values for the cross-relaxation parameters are obtained for the first time.

A theory of multiwave devices on coherent magnetic bremsstrahlung of relativistic electron beams

Presented are the results of numerical simulation of coherent microwave radiation sources, based on magnetic bremsstrahlung of the relativistic electron beam. For the description of electromagnetic field the Fourier analysis of delayed potentials is used. Angular characteristics of collective radiation were studied. The possibility of high efficiency of multiwave interaction is demonstrated.

Design considerations for the gyro-peniotron oscillator

The gyro-peniotron shows great promise as a source of microwave radiation. In this paper we investigate the stability and the non-linear saturated efficiency of a gyropeniotron oscillatro with an axis encircling beam operating at the TE211 and the TE311 cylindrical cavity modes. Beam and cavity parameters for stable high efficiency operation are presented from a linear theory analysis of the start-oscillation condition. Using design parameters taken from the linear analysis, the strong field non-linear emission efficiencies are studied numerically. The sensitivity of the saturated efficiency to beam axial velocity and guiding center spread is also considered. The results of our modeling indicate that high efficiency (70–90%) operation of the gyro-peniotron oscillator is possible, even with large guiding center\((\delta r_{gc_ \circ } \mathop< \limits_ \sim 0.2 - 0.4r_{L_ \circ } )\) and large axial velocity\((\delta \beta _{||} \mathop< \limits_ \sim 0.3\beta _{||_ \circ } )\) spread.

Improved Use of Görlich Photocathodes in the Lasertron

The next generation of particle accelerators will require the development of powerful microwave radiation sources, in addition to the use of free electron lasers for high microwave fields or various concepts of laser acceleration of particles. One of these concepts is the lasertron in which recent experimental studies indicated limitations by pre-discharges at the photoemitter irradiation by a mode locked laser. Improvements are discussed within the framework of the accelerator requirements of efficiency and cost. The possibility of two-photon emission, the reduction of the involved surface traps (increasing the exit potential of the cathodes) and alternative production of a Cs3Sb compound for the Görlich cathode using the chemistry of water-like solvents as liquid ammonia is evaluated.

Radiofrequency and microwave radiation

This paper deals with the controversy and disagreement surrounding the issue of harm from radiofrequency (RF) and microwaves. The radiation standards adopted by different countries are quite divergent with the least strict standard for microwave exposure differing from the most strict by a factor of 100. Among the most powerful sources of RF and microwave radiation are radar systems used for tracking and guidance purposes, as well as transmiters used in satellite communication systems. RF and microwaves are nonionizing because the energy of each photon is relatively low. Biological systems exposed to RF and microwaves acquire induced electric and magnetic fields which can be focused by a combination of high refractive index within an animal and convex body contours. The effects on animals and humans are summarized. (KRM)

Plasma microwave electronics

The theory of Cherenkov and cyclotron plasma sources of coherent microwave radiation excited by intense electron beams is reviewed systematically. The linear approximation of the theory yields the output frequency spectra, the wave growth rates, and the threshold electron beam currents required for exciting these sources. The general theory is illustrated for some particular devices: the forward-wave plasma Cherenkov source, foward-and backward-wave cyclotron-resonance masers, and foward-and backward-wave Cherenkov sources in a slightly corrugated slow-wave structure. The nonlinear theory of plasma microwave sources leans heavily on a numerical solution of the dynamic equations of the electromagnetic field and the charged particles in the system. The nonlinear operation of a forward-wave plasma Cherenkov source is analyzed. Under optimum conditions, the efficiency of this device can reach 30–35%. The efficiencies of other high-current plasma sources are comparable in magnitude. Experimental progress toward the development of high-power pulsed microwave sources of the Cherenkov and cyclotron types using intense relativistic electron beams is reviewed briefly.

Potentially Hazardous Microwave Radiation Sources—A Review

An increasing proliferation of microwave sources and their continuously broadening scope of applications result in the necessity of an assessment of the potential radiation hazard in various situations. Recently, there has appeared quite a large number of references dealing with specific sources. This paper provides a review of various microwave radiation sources to assist in a general understanding of the basic characteristics of the sources which are pertinent to microwave radiation hazards.

Fast-wave interaction with a relativistic electron beam

The excitation of a relativistic electron beam, by means of a fast waveguide structure, is examined. Here the beam is injected into a modified waveguide, and interacts with the modes of the guide in such a way as to transform some of its energy into microwave radiation. This microwave generation device, called the Ubitron, is based upon a fast-wave excitation of a magnetically modulated relativistic electron beam. The beam is modulated by injecting it into a small spatially periodic magnetic field region within the guide. Analysis of this interaction shows that the slow space charge beam mode couples actively to the fast transverse electric guide mode. The result is parametric instability of the coupled modes. Synchronism between the doppler-shifted transverse travelling wave and the undulating electron beam results in a transfer of energy from the beam to the transverse field. The parametrically growing field can be a source of microwave radiation. The period magnetic field, together with the beam density, provide the coupling media between the unstable waves. The growth rate of the instability is shown to depend, in a nonlinear manner, on the product of the beam plasma frequency and the strength of the applied rippled magnetic field. The growth rate is obtained as a function of the system parameters.

Nonsteady Response of Plasmas to Microwave Radiation

A theoretical study of some nonsteady aspects of microwave radiation on ionized gases is presented. For weakly ionized gases, the concept of ``stabilized'' electron temperature is introduced. It is shown that under certain conditions, ionizations due to microwave radiation may be assumed to take place with the electron temperature remaining at this stabilized value. A simple expression for the ionization rate can then be obtained. Several time scales pertinent to these nonsteady processes have been obtained, and their relative order of magnitude is seen to be crucial in determining the nature of the nonsteady response. One type of nonsteady solutions for the ionization front of the gas under microwave radiation has been considered in some detail. Three different modes of response are possible depending on the initial electron density distributions. In particular, when the initial distribution is of the form ba exp (aξ)/[1 + b exp (aξ)], for ξ ≤ 0, where a and b are constants, the ionization front will propagate in the negative ξ direction toward the source of microwave radiation at a constant speed and with the same profile of electron density. This nonsteady response characteristic has some similarity with those observed in shock tube experiments.