Radiofrequency Pumping Research Articles

Matching of generators and gas-discharge loads with radio frequency pumping

The article considers the problem of matching power high-frequency generators and gas discharge loads. Variants of various types of tunable matching circuits are proposed. A method of adaptive continuous tuning is described and a variant of implementing tunable elements on varicaps is proposed.

Bose—Einstein Condensation and Spin Superfluidity of Magnons in a Perpendicularly Magnetized Yttrium Iron Garnet Film

The formation of a Bose condensate of magnons in a perpendicularly magnetized film of yttrium iron garnet under radio-frequency pumping in a strip line is studied experimentally. The characteristics of nonlinear magnetic resonance and the spatial distribution of the Bose condensate of magnons in the magnetic field gradient are investigated. In these experiments, the Bosonic system of magnons behaves similarly to the Bose condensate of magnons in the antiferromagnetic superfluid 3He-B, which was studied in detail earlier. Magnonic BEC forms a coherently precessing state with the properties of magnonic superfluidity. Its stability is determined by the repulsive potential between excited magnons, which compensates for the inhomogeneity of the magnetic field.

Phase coherence of parametric-decay modes during high-harmonic fast-wave heating in the National Spherical Torus Experiment

Third-order spectral analysis, in particular, the auto bicoherence, was applied to probe signals from high-harmonic fast-wave heating experiments in the National Spherical Torus Experiment. Strong evidence was found for parametric decay of the 30 MHz radio-frequency (RF) pump wave, with a low-frequency daughter wave at 2.7 MHz, the local majority-ion cyclotron frequency. The primary decay modes have auto bicoherence values around 0.85, very close to the theoretical value of one, which corresponds to total phase coherence with the pump wave. The threshold RF pump power for onset of parametric decay was found to be between 200 kW and 400 kW.

Planar Xe laser with cw radio-frequency pumping

The characteristics of the radiation of a planar Xe laser excited by transverse RF discharge at a frequency of 40.68 MHz with diffusive cooling of the active medium and a nonselective optical cavity have been investigated. The spectral and power characteristics of the Xe laser have been experimentally studied as functions of the interelectrode-gap width (1.4 — 2.5 mm), composition and pressure (30 — 100 Torr) of working gas mixture, input power density (0.4 — 34 W cm-3), and the electrode temperature. The output laser power was found to decrease at an interelectrode gap above 1.6 mm. The threshold pump power was 12 W (power density 0.4 W cm-3). Addition of butane to the working mixture increases the output power by approximately 50 % at interelectrode gaps of 2.0 — 2.5 mm. A cw lasing power of 4 W and a maximum efficiency of ∼0.7 % are obtained. Good prospects of the Xe laser operatingat wavelengths of 2.03, 2.65, and 3.51 μm are shown.

Resonant dislocation motions in NaCl crystals under EPR conditions in the Earth’s magnetic field with a radio-frequency pump field

Three resonant peaks l(ν) of dislocation paths, appearing in NaCl crystals under simultaneous exposure to the Earth’s magnetic field B Earth and an ac pump field \(\tilde B\) have been observed in the radio-frequency range (ν ∼ 1 MHz). Each peak corresponds to a specific experimental geometry. Resonance frequencies correspond to ordinary EPR conditions in which, however, instead of the total Earth’s field, its projections onto particular crystallographic directions play part. These directions are probably related to orientations of magnetic moments at impurity centers in dislocation cores. A physical model describing well the positions all three EPR peaks l(ν) is proposed. The peak height l m is proportional to the squared amplitude of the pump field and the time t of sample exposure to crossed fields: \(l_m \propto \tilde B_m^2 t\). The threshold amplitude \(\tilde B_m^{th} \sim 1 \mu {\rm T}\) of the pump field is detected, below which the effect rapidly disappears. The time of spin-lattice relaxation of radical pairs on dislocations is estimated by the threshold field \(\tilde B_m^{th}\) level.

Experimental Design for Free Space Nonlinear Magnetic Material Characterization Using Photonic Structures

ABSTRACTMeasuring nonlinear AC dielectric or magnetic properties of ferro and ferri magnetic materials has often required large, extremely high power and bulky equipment configurations for production of the required intense electric and/or magnetic fields. Multiple RF cavities, striplines or waveguide test fixtures may be required. The techniques often require 10s of cubic millimeter too centimeter material volumes. This paper presents results from an initial experimental design of a free space based measurement configuration for small material volumes such as nano and micro particulates or particulate composites. The technique uses wideband radiators and modest variable power radio frequency pumping sources. A second design uses, in concert with a higher power pump, a variable frequency low power probe source at frequencies other than that of the pump. A two dimensional photonic bandgap (PBG) structure is common to both configurations. The photonic structure acts to enhance power density at frequencies associated with localized electromagnetic fields constrained to small volumes of the photonic structures. Field localization is recognized and has been applied in biological diagnostics and treatment [Phys. Rev. v109, 1492; Phys. Rev. B, v55 and 62, n. 19 and 16, pp 13234 and 11230 and Chem.Soc.News, 1998, v27, p241].Modeling of various measurement configurations is based upon expansion of incident and propagating fields in characteristic modes external and within the structure. A proper choice of photonic structure, material and pump frequency is found to localize fields in air, between the structural dielectric members of the photonic structure. This will allow small magnetic or electric samples to be inserted for exposure in these regions. The electromagnetic reflection, transmission, absorption and field/power density multiplier of the photonic structure can be measured at multiple frequencies and reflect the dielectric or magnetic nonlinearity and changed dispersion induced at the pump frequency. One PBG-free space combination will be presented that finds power density multipliers of 10 6. Thus a 10 watt pump source will produce RF magnetic field strengths near 10 Oe. This magnetic field should be sufficient to exceed critical fields for many ferri and ferromagnetic samples inserted in the volume of field localization.

Understanding laser stabilization using spectral hole burning

There have recently been several studies of the performance of laser frequency stabilization using spectral holes in solids, instead of an external cavity, as a frequency reference. Here an analytical theory for Pound-Drever-Hall laser frequency stabilization using spectral hole-burning is developed. The interaction between the atomic medium and the phase modulated light is described using a linearized model of the Maxwell-Bloch equations. The interplay between the carrier and modulation sidebands reveals significant differences from the case of locking to a cavity. These include a different optimum modulation index, an optimum sample absorption, and the possibility to lock the laser in an inherent linear frequency drift mode. Spectral holes in solids can be permanent or transient. For the materials normally used, the dynamics and time scales of transient holes often depend on population relaxation processes between ground state hyperfine levels. These relaxation rates can be very different for different solid state materials. We demonstrate, using radio-frequency pumping, that the hyperfine population dynamics may be controlled and tailored to give optimum frequency stabilization performance. In this way also materials with initially non-optimum performance can be used for stabilization. The theoretical predictions regarding the inherent linear frequency drift is compared to experimental data from a dye laser stabilized to a spectral hole in a Pr(3+)3+:Y(2)SiO(5) crystal.

An interferometer experiment to explore the aspect angle dependence of stimulated electromagnetic emission spectra

Abstract. When the Earth's ionosphere is irradiated by a radiofrequency (RF) electromagnetic wave of sufficiently high power density and tuned to match a natural E- or F-region plasma frequency, ionospheric magnetoionic wave modes may be excited and may generate RF electromagnetic sideband waves via nonlinear interactions. These secondary emissions, which may then escape from the ionosphere, have been termed stimulated electromagnetic emission or SEE. The frequency spectra of this radiation has been studied extensively, and a number of characteristic spectral features have been identified and in some cases related to particular plasma processes. The separation in frequency between the RF pump and the harmonics of the local electron gyrofrequency is critical in determining the amount of anomalous absorption suffered by the pump wave and the spectral properties of the stimulated sidebands. The pump can excite electrostatic waves which do not propagate away but can in some cases be observed via radio-wave scattering from the electron density fluctuations associated with them. These enhanced density fluctuations are created by processes commonly referred to as upper-hybrid and Langmuir turbulence. Langmuir turbulence has been the subject of 930-MHz scattering observations with antenna scanning through several pre-selected angles between the geographic and geomagnetic zenith directions, and a preference for pointing angles between the Spitze angle and geomagnetic field-aligned was identified. Other phenomena, such as the generation of enhanced electron temperatures and artificial aurora, have more recently been shown to have special behavior at similar angles, near but apparently not quite at field-aligned. In view of this evidence for angular structure in several pump-induced effects, in light of the rich variety of SEE phenomena strongly dependent on the geomagnetic field via the frequency interval between the pump and the gyrofrequency harmonics, and in view of the not yet understood but complex relationship between electrostatic fluctuations and SEE, it is of interest to investigate experimentally whether a similar angular structure is present in the various spectral features of the SEE signals and to compare the results with radar and other observations of RF-pump-induced effects. To this end we describe a simple two-element radio interferometer designed to search for aspect angle dependence of SEE features. We present an example of the initial data produced by this system, and draw preliminary conclusions based on the example data.

Efficiency of caviton formation as a function of plasma density gradient

The effect of a zeroth-order density gradient on the development of cavitons has been investigated experimentally and numerically. The cavitons were produced via excitation of electron plasma waves (EPW) with a modest (E2/4πnTe≪1) resonant radio-frequency pump. The location of the resonance, on an inverse-parabolic density profile, was varied, with all other parameters being held constant. The depth of the caviton, and the strength of its associated trapped electric fields, are found to depend strongly on the density gradient scale length at the critical layer, with a maximum occurring when this length is infinite, at the flat top of the density profile. The results are accounted for by the dependence on the density gradient of the EPW convection rate and wave-breaking time. The study helps illuminate recent large-scale ionospheric density modification experiments.

ChemInform Abstract: Radiofrequency‐Microwave Double Resonance with a Dipole Moment Component Induced by Asymmetric Isotopic Substitution: Microwave Spectrum of C4O2Cl2

Abstract The microwave spectrum of C4O2Cl2 (1,2-dichlorocyclobuten-3,4-dione) has been investigated for both the C4O235Cl2 and the C4O235Cl37Cl isotopomers. The spectrum of the latter has been studied in greater detail because a number of μb rotational transitions were measured with the radiofrequency-microwave double-resonance technique by radiofrequency pumping of some K−1 doublets. This pumping has been made possible since a small μa dipole moment component appears owing to the asymmetric isotopic substitution.

Radiofrequency-microwave double resonance with a dipole moment component induced by asymmetric isotopic substitution: Microwave spectrum of C 4O 2Cl 2

The microwave spectrum of C 4O 2Cl 2 (1,2-dichlorocyclobuten-3,4-dione) has been investigated for both the C 4O 2 35Cl 2 and the C 4O 2 35Cl 37Cl isotopomers. The spectrum of the latter has been studied in greater detail because a number of μ b rotational transitions were measured with the radiofrequency-microwave double-resonance technique by radiofrequency pumping of some K −1 doublets. This pumping has been made possible since a small μ a dipole moment component appears owing to the asymmetric isotopic substitution.

Microwave spectrum of methylmercury chloride: gas-phase determination of the electric dipole moment and 201Hg quadrupole coupling

The nuclear quadrupole coupling constant of 201Hg ( I = 3 2 ) has been determined for CH 3HgCl in the vapour by microwave spectroscopy. A value of eQq( 201Hg) = −1025(3) MHz, along the axis of the symmetric top, has been determined for the vibrational ground state. An initial determination of the quadrupole coupling constant was made possible by radiofrequency pumping of the l-type doublets of the lowest bending mode, ν 8, for which eQq( 201Hg) = − 1016(6) MHz and the l-type doublet constant q 8 = 2.90(1) MHz. eQq( 35Cl) was confirmed to be −42.2(2) MHz. The dipole moment of CH 3HgCl has been determined from Stark effect measurements in the ground state as 3.411(11) D.

Theory of the free-electron laser in combined helical pump and axial guide fields

The linearized theory of a free-electron-laser amplifier consisting of a relativistic electron beam transported along the axis of a helical wiggler in the presence of an axial guide field is solved exactly. With suitable re-identification of parameters, the theory also applies to the case where the wiggler is replaced by a circularly polarized subluminous radio-frequency pump. The dispersion relation is derived and numerical examples of solutions are presented. These indicate (a) that the use of an axial field permits operation of a laser of given high frequency and undulatory transverse velocity of the unperturbed electron beam at lower values of the pump field, (b) that the gain can be enhanced by approaching the condition of resonance between the effective frequency of the pump and the cyclotron frequency, and (c) that the breadth in frequency of the region corresponding to spatially exponentially growing operation can be much extended.

NMR spectra of short-lived free radicals, detected from the CPN effects for radiofrequency pumping

NMR spectra of short-lived free radicals, detected from the CPN effects for radiofrequency pumping

Microwave spectra of CF3Br and CF3I. Structures and dipole moments

The microwave spectra of the 13C-species of CF3Br and CF3I have been measured with the aid of radiofrequency pumping of the quadrupole sub-levels. In addition, rotational data for the main species have been refined. The following zero-point average structures (rz) have been determined: CF3Br: C—F = 1.3265(23), C—Br = 1.9234(31)A, ∠FCF = 108.81(25)°, CF3I: C—F = 1.3285(23), C—I = 2.1438(27)A, ∠FCF = 108.42(23)°.Particular attention has been paid to the effect of isotopic variation in bond length in the structural calculations. The structures agree well with those recently obtained by combining electron diffraction and microwave data.Accurate Stark measurements have been performed for a number of transitions including the J= 3 → 4, K= 3, F= 7/2 → 9/2 transition of CF379Br and the J= 4 → 5, K= 3, F= 3/2 → 5/2 transition of CF3I. The results have been fitted using a diagonalization treatment of the quadrupole-Stark matrix, set up in the uncoupled basis. The dipole moment of CF3Br was determined to be 0.639(2) D and that of CF3I, 1.048(3) D. The structural properties of CF3Br and CF3I are shown to fit some trends with related molecules.

Enhancement of 235U isotope separation with radiofrequency hyperfine transitions

It is pointed out that the efficiency of isotope separation of 235U by laser/Hg lamp two-step ionization can be enhanced by the use of radiofrequency pumping between hyperfine levels.