Fast Cyclotron Wave Research Articles

On Using Cyclotron Waves for Output of Radiation from High-Power Relativistic Backward-Wave Oscillators

We discuss the scheme of a high-power relativistic backward-wave oscillator operating with a high-current explosive-emission electron beam. The radiation output from the tube is based on reflection of the operating wave into a fast cyclotron wave which transmits the energy of the high-frequency field towards the collector. At the collector, reverse conversion into the output electromagnetic wave takes place. The proposed scheme allows one to increase electric strength, as well as the transverse size of the working space of the backward-wave oscillator. It is shown that within the proposed scheme, one can reduce the focusing magnetic field without decreasing the power of the output radiation.

Roles of Fast-Cyclotron and Alfvén-Cyclotron Waves for the Multi-Ion Solar Wind

Using linear Vlasov theory of plasma waves and quasi-linear theory of resonant wave-particle interaction, the dispersion relations and the electromagnetic field fluctuations of fast and Alfven waves are studied for a low-beta multi-ion plasma in the inner corona. Their probable roles in heating and accelerating the solar wind via Landau and cyclotron resonances are quantified. We assume that (1) low-frequency Alfven and fast waves have the same spectral shape and the same amplitude of power spectral density; (2) these waves eventually reach ion cyclotron frequencies due to a turbulence cascade; (3) kinetic wave-particle interaction powers the solar wind. The existence of alpha particles in a dominant proton/electron plasma can trigger linear mode conversion between oblique fast-whistler and hybrid alpha-proton cyclotron waves. The fast-cyclotron waves undergo both alpha and proton cyclotron resonances. The alpha cyclotron resonance in fast-cyclotron waves is much stronger than that in Alfven-cyclotron waves. For alpha cyclotron resonance, an oblique fast-cyclotron wave has a larger left-handed electric field fluctuation, a smaller wave number, a larger local wave amplitude, and a greater energization capability than a corresponding Alfven-cyclotron wave at the same wave propagation angle \theta, particularly at $80^\circ$ < \theta < $90^\circ$. When Alfven-cyclotron or fast-cyclotron waves are present, alpha particles are the chief energy recipient. The transition of preferential energization from alpha particles to protons may be self-modulated by differential speed and temperature anisotropy of alpha particles via the self-consistently evolving wave-particle interaction. Therefore, fast-cyclotron waves as a result of linear mode coupling is a potentially important mechanism for preferential energization of minor ions in the main acceleration region of the solar wind.

Theory of nearly perpendicular electrostatic plasma waves and comparison to Freja satellite observations

The linear and nonlinear two‐fluid nature of oblique electrostatic plasma waves is explored and clarified. It is found that two distinct wave regimes exist corresponding to propagation angles greater than or less than with respect to the perpendicular direction of the magnetic field. Propagation angles greater than ε correspond to either the electrostatic ion cyclotron wave or the shorter wavelength oblique ion acoustic wave, which are termed the fast ion cyclotron and fast ion acoustic waves respectively. Angles less than ϵ correspond to the inertial Alfvén wave which is called the slow ion cyclotron wave and in the short wavelength limit is called the slow ion acoustic wave. The slow ion‐acoustic wave is found to have a predominately ion‐Boltzmann response. Initial value simulations having fast or slow ion cyclotron wave initial conditions of modest amplitudes result in nonlinear steepening and subsequent breakup into fast or slow ion acoustic waves respectively. This mechanism of ion acoustic wave generation may partially explain the origin of ion acoustic activity detected by the Freja satellite within regions of ion and electron acceleration.

Experimental demonstration of longitudinal wiggler free-electron laser

The lowbitron—a longitudinal wiggler beam interaction device—as a novel source of submillimeter wave radiation was proposed and analyzed theoretically by McMullin and Bekefi [Phys. Rev. A 25, 1826 (1982)]. This letter reports the first experimental measurements of lowbitron radiation obtained with a 740-kV, 400-A electron beam. The measured power spectra in the W-band range for different wiggler field periods agree with the lowbitron interaction theory. They match the intersection points of the shifted fast cyclotron wave dispersion relation ω−(k+kw)v∥−Ω0/γ=0 and the TE01 electromagnetic waveguide mode.

THEORETICAL ANALYSIS OF FREE ELECTRON LASER AMPLIFIER

In this paper, a detailed analysis on the operating mechanism and parameter properties of free electron laser amplifier (FELA) in the case of small signal is presented. The results show that FELA is in essence the interaction between the electron beam and the fast cyclotron wave. The cyclotron resonance condition of electron motion in the uniform axial and periodic transverse static magnetic fields significantly influences the interaction process. We have calculated numerically the central operating frequency, band width, small signal gain and wave number of the amplified electromagnetic wave as functions of the parameters of the amplifier (such as the voltage and current density of the electron beam, the intensities of axial and transverse static magnetic field). The way to optimize the design of FELA is also suggested. It is possible to obtain a small signal gain of 10-2 cm-1 with enough band width in the millimeter and submillimeter range of weve length following a proper design.

A dispersion equation for gyrotron TWTs

A linear theory of gyrotron travelling wave tubes (gyro-TWTs) is presented in this paper. It is shown that the main features of operation are adequately revealed by considering the interaction between a relativistically corrected fast cyclotron wave of an RK excited electron beam and a TK waveguide mode. The corresponding coupled differential equations lead to a dispersion equation which characterizes the behaviour of the system in a simple and straightforward manner. Numerical solutions of the dispersion equation form the basis of the concluding remarks.

Behavior of the fast cyclotron wave in a nonuniform magnetic field

The fast cyclotron wave becomes unstable and is excited in a spiral electron beam-plasma system when the perpendicular energy component of the beam is sufficiently large. When a nonuniform magnetic field is applied to the system, the cyclotron frequency as well as the parallel velocity component of the beam vary spatially. It is confirmed experimentally, that the variation affects the excited wave and results in a spatial variation of its wavenumber in the way predicted by the dispersion relation of the fast cyclotron wave.

Observation of the simultaneous amplification of space charge and fast cyclotron waves in a spiral beam-plasma system

The amplification of the fast cyclotron wave is observed in a spiral beam-plasma system, overlapping the amplifying space charge wave. The interferometer system output shows the amplifying wave pattern with an amplifying beat wave component or that overlapping another amplifying one with a larger wavelength. The wave numbers and the amplification factors of both waves are determined from these wave patterns.

Fast Cyclotron Wave Excitation in a Spiral Electron Beam-Plasma System

The fast cyclotron wave excitation resulting from the coupling with the electron Bernstein wave is observed in a magnetized plasma penetrated by a spiral electron beam. For the excitation to occur, the beam energy component perpendicular to the magnetic field is larger than a critical value. From measurements of the wave number and the growth rate of the excited cyclotron wave, its dispersion relation is determined for various values of beam and plasma parameters. The experimental results are compared with the theoretical considerations.

Instability of the fast cyclotron wave in a spiral beam-plasma system

The excitation of the fast cyclotron wave is observed in a magnetized plasma penetrated by a spiral electron beam. For the excitation to occur, the beam energy component perpendicular to the magnetic field should be larger than about 20 percent of the total energy, which is consistent with theoretical considerations.

Ion Bernstein Wave in an Ion Beam-Plasma System

The dispersion relation of ion Bernstein wave is analyzed numerically, for Maxwellian plasma and an ion beam-plasma system. In an ion beam-plasma system, the coupling of the wave with the slow space charge wave and the slow cyclotron wave results in electrostatic instabilities, while those with the fast space charge wave and the fast cyclotron wave do in strong damping of waves. The behaviors of the wave are studied in detail for both cases of a Maxwellian plasma and an ion beam-plasma system, the results of which are compared with the electron Bernstein wave.

Electrostatic Instability of Electron Bernstein Wave in a Beam-Plasma System

The dispersion relation of Bernstein wave in an electron beam-plasma system is analyzed numerically. Electrostatic instability of the wave occurs as the result of interaction with a slow space charge wave or a slow cyclotron wave of beam, while interactions with a fast space charge wave and a fast cyclotron wave of beam result in a strong damping of waves. The behavior of the instability is investigated in datail as function of plasma and beam parameters. It is shown that these are consistent with the previous experimental results.

Double-stream cyclotron wave amplifier

Experimental confirmation is reported of the theoretically predicted two-stream interactions (for circularly symmetric modes, n=0) between 1) the "fast cyclotron wave" of the slower beam and the "slow space-charge wave" of the faster beam; and between 2) the "fast space-charge wave" of the slower beam and the "slow space-charge wave" of the faster beam. The experiments were carried out at S-band. In the case of "cyclotron wave"-"space-charge wave" interaction, an electronic gain of 10 dB was measured. This double-stream interaction mode has not previously been observed. It affords the possibility of fast-wave coupling, by use of a nonuniform magnetic field along the tube, which offers size advantages at millimeter wavelengths. In the case of "space-charge wave"-"space-charge wave" interaction, the maximum electronic gain measured was 17 dB. In seeking interactions for n = ± 1 modes, none were found, due primarily to opposite polarizations, hence orthogonality, of the waves involved.

Phase distortion in d.c.-pumped electrostatic ring cyclotron-wave amplifiers

Computations are given of the phase distortion of the fast cyclotron wave in a d.c.-pumped electrostatic ring microwave amplifier for a range of input power levels in the strong-signal region. The degenerate case (ω=ωc) is considered for a range of pump strengths and beam velocities at and above the small-signal small-pump synchronous value.

A small-signal analysis of the electron cyclotron backward-wave oscillator

In an earlier report on the construction and performance of the electron cyclotron backward-wave oscillator, it was shown, through physical arguments, that in an unloaded waveguide supporting the dominant mode, an electron having transverse rotation at its cyclotron frequency will interact with RF fields of approximately equal frequency. This transverse motion will deliver energy to the RF E fields and interact with the RF H fields, thus producing longitudinal bunching. A small-signal analysis is presented in this paper. With the use of the normal mode expansion analysis, the circuit equation is obtained by considering the normal mode in approximate synchronism with the beam. The RF current is computed by considering electron motion under the dc and circuit fields, but neglecting RF space-charge fields. Combining these equations leads to a sixth-order equation of propagation constants. Two are far from synchronism and are therefore neglected; the remaining four are two which originate from the fast cyclotron waves and two which originate from the forward and reflected circuit waves. The fast cyclotron wave so obtained has a different meaning from the usual definition and is discussed in detail. Theoretical start-oscillation current is found to depend critically on the reflection coefficient at the electron gun end. Proper adjustment of this parameter leads to excellent agreement between the theoretical and experimental start-oscillation currents.

Beam Refrigeration by Means of Large Magnetic Fields

The resistive loading which an electron beam produces in an adjacent structure can be made to exhibit a very low noise temperature. This is achieved by coupling to the fast cyclotron wave in a large magnetic field; the noise temperature at a given signal frequency is shown to be equal to the cathode temperature times the ratio of signal frequency to cyclotron frequency. An experiment is described in which this ratio is 19. The coupling structure interacts with the fast cyclotron wave but rejects the slow cyclotron wave. A noise temperature of 186°K is measured. In conclusion, it is shown that the large magnetic field required for beam cooling need not extend throughout the tube.

Waves on a Filamentary Electron Beam in a Transverse-Field Slow-Wave Circuit

The waves on a filamentary electron beam in a longitudinal dc magnetic field, and their interaction with a transverse-field slow-wave circuit, are studied in detail. All quantities are expressed in terms of circular polarization, with the circuit fields having arbitrary polarization. The beam is found to carry four waves: a positively polarized negative-energy slow cyclotron wave, a negatively polarized positive-energy fast cyclotron wave, and two synchronous (β=βe) waves, one with positive polarization and positive energy, one with negative polarization and negative energy. The coupling of these waves to the circuit is described both in the manner of Pierce's longitudinal traveling wave tube (TWT) analysis and in a coupled-mode description. For the two special cases of positive or negative circularly polarized fields, only the appropriately polarized beam waves couple. A third special case of linear polarization is more complicated, but essentially only the two cyclotron waves couple. In each of the three cases one positive-energy and one negative-energy beam wave is involved. In each case the equations can be made identical with the longitudinal TWT equations. As one example, a positively polarized circuit can be used as a fast-wave coupler for an Adler-type parametric amplifier, and its design becomes formally identical with the longitudinal Kompfner dip problem. Published Kompfner dip data can be used. The beam-circuit interaction is found to be correctly described by considering only the apparent transverse motion of the beam's position, although the actual interaction mechanism involves both the true transverse interaction with the actual transverse electron velocities, and the longitudinal interaction of the dc beam velocity with the longitudinal ac fields off the dc beam axis. The latter leads to changes in longitudinal dc velocity of the electrons and thus accounts for the negative rf energies of two of the waves.

The Quadrupole Amplifier, a Low-Noise Parametric Device

Unusually low noise, combined with high stable gain over fairly wide bands, has been obtained with electron beam amplifiers of a new kind. This paper explains how this performance is achieved by the action of a transverse quadrupole field upon a fast cyclotron wave. The first two sections give a qualitative description of the device and of the amplifying mechanism. A physical picture of the fast cyclotron wave is used to explain the interchange of signal and noise in the input coupler and the mechanism of parametric amplification in the quadrupole region. The third section presents a detailed analysis of the amplification process. It shows that the fast cyclotron wave is amplified in accordance with a cosh function of distance traveled through the quadrupole, and that a new cyclotron wave at idler frequency (difference between pump and signal frequencies) is generated which grows as a sinh function of distance. The fourth section describes experimental tubes built to date. These operate on frequency bands between 400 and 800 mc. Typical bandwidth is 40 to 50 mc independent of gain, which may be adjusted to 20 or 30 db. Residual noise temperature of the electron beam in good specimens within this experimental lot is 70°K; input coupler loss raises this figure to about 100°K. This is equivalent to a noise figure of 1.4 db if the device is used, for instance, in radio astronomy.