D.C. Speirs

Gyro-BWO experiments using a helical interaction waveguide

A helically corrugated waveguide was used for a gyrotron backward-wave oscillator (gyro-BWO) experiment. A thermionic cathode was used to produce an electron beam of 90-215 keV in energy, 2-3 A in current, and pitch alpha of up to 1.6. The oscillator achieved high-efficiency frequency-tunable operation. At a fixed beam voltage of 185 kV and a current of 2 A, the output frequency was tuned by adjusting the magnetic field in the interaction cavity. A maximum power of 62 kW and a 3-dB frequency-tuning band of 8.0-9.5 GHz (17% relative tuning range) with a maximum electronic efficiency of 16.5% were measured. In addition, the interaction frequency could be tuned by varying the electron beam energy. At a fixed cavity magnetic field of 0.195 T, the output frequency and power from the gyro-BWO were measured as a function of tuning electron beam energy while the beam current was maintained at 2.5 A. A 3-dB relative frequency tuning range of 8% was measured when the electron beam voltage was changed from 215 to 110 kV.

Axis-encircling electron beam generation using a smooth magnetic cusp for gyrodevices

The generation of an annular-shaped axis-encircling electron beam using a smooth magnetic cusp was studied through numerical simulations and experiments for harmonic operation of a gyrodevice. Two magnetic coils were used to form a magnetic cusp located just downstream from the velvet cathode of an accelerator diode. An electron beam of current 34 A and voltage 130 kV with an adjustable velocity ratio α up to 1.2 was fully transported to the downstream uniform magnetic field region and used to drive a gyrotron traveling wave amplifier into saturation.

Demonstration of auroral radio emission mechanisms by laboratory experiment

Auroral kilometric radiation occurs in regions of depleted plasma density in the polar magnetosphere. These emissions are close to the electron cyclotron frequency and appear to be connected to the formation of high pitch angle electron populations due to the conservation of the magnetic moment. This results in a horseshoe type distribution function being formed in velocity space where electrons are magnetically compressed as they descend towards the Earths atmosphere. Satellites have observed that radio emissions occur in conjunction with the formation of this distribution and show the radiation to have propagation and polarization characteristics of the extraordinary (X-mode) plasma mode with emission efficiency observed at ~1–2%. To investigate this phenomenon a laboratory experiment, scaled to microwave frequencies and lab dimensions by increasing the cyclotron frequency, was constructed whereby an electron beam propagated through a region of increasing magnetic field created by five independently variable solenoids. Results are presented for two experimental regimes of resonant coupling, 11.7 and 4.42 GHz, achieved by varying the peak magnetic field. Measurements of the experimental radiation frequency, power and efficiency were undertaken as a function of the magnetic compression. Results showed the radiation to be polarized in the near cut-off transverse electric radiation modes, with efficiency of emission ~1–2%, peak power outputs of ~19–30 kW and frequency close to the cyclotron frequency. This represented close correlation between the laboratory radiation efficiency, spectra, polarization and propagation with that of numerical predictions and the magnetospheric observations.

Gyro-BWO experiment using a helical interaction waveguide

By using a helical corrugation on the inner surface of a cylindrical waveguide, it has been demonstrated that the wave dispersion has a near constant group velocity in the region of small axial wave number (G.G. Denisov et al., IEEE Trans. Plasma Sci. vol. 26, p. 508, 1998). This allows broadband microwave amplification to be achieved in a gyrotron travelling wave amplifier (G.G. Denisov et al., Phys. Rev. Lett. vol. 81, p. 5680, 1998; G.G. Denisov et al., ibid. vol. 81, p. 2746, 2000). Such a system is also favourable when used in a gyrotron backward wave oscillator (S.V. Samsonov et al., PS Spec. Issue on High Power Microwave Gen.; G.G. Denisov et al., Proc. IEEE IVEC 2003, pp. 338-339, 2003.) (gyro-BWO) in two ways. Firstly due to its larger group velocity as compared to normal gyro-BWOs, it has a wider frequency tuning range. Secondly due to the nonsymmetrical geometry of the helical interaction region, the electron beam interacts only with the wave in one direction (either backward or forward wave), and hence has a pure frequency component. In our experiment, a thermionic cathode electron gun in a Pierce-type geometry was used to generate a rectilinear electron beam of 1 /spl mu/s pulse duration, 90-185 keV energy and /spl sim/2.5 A in current.

Numerical simulation of auroral cyclotron maser processes

Results are presented from a numerical investigation of radiation emission from an electron beam with a horseshoe-shaped velocity distribution. This process is relevant to the phenomenon of auroral kilometric radiation (AKR) which occurs in the polar regions of the Earths magnetosphere. In these regions of the auroral zone, particles accelerated into the increasing magnetic field of the Earths dipole develop a horseshoe-shaped velocity distribution through conservation of magnetic moment. It has been shown theoretically that this distribution is unstable to a cyclotron maser instability. A 2D particle-in-cell (PIC) code model was constructed to simulate a scaled laboratory experiment in which an electron beam subject to significant magnetic compression may be studied and brought into resonance with TE modes of an interaction waveguide. Results were obtained for electron beam energies of 75-85 keV, magnetic compression factors of up to 30 and electron cyclotron frequencies of 4.42 and 11.7 GHz. At 11.7 GHz, beam-wave coupling was observed with the TE03 mode and an RF output power of 20 kW was obtained corresponding to an RF conversion efficiency of 1.3%. At 4.42 GHz, excitation of the TE01 mode was observed with an RF output power of 35 kW for a cyclotron-wave detuning of 2%. This corresponds to an RF conversion efficiency of 2.6%. In both cases PiC particle velocity distributions show the clear formation of a horseshoe-shaped velocity distribution and subsequent action of a cyclotron maser instability. The RF conversion efficiencies obtained are also comparable with estimates for the AKR generation efficiency. (Abstract from: http://iopscience.iop.org/0741-3335/50/7/074011/)

Radio frequency resonator structure and diagnostic measurements for a laboratory simulation of Auroral Kilometric Radiation

Auroral Kilometric Radiation is emitted from regions of depleted plasma density in the Earth’s polar magnetosphere. The radiation frequency is close to the local electron cyclotron frequency, polarized in the X-mode with an efficiency of ∼1%, with power up to 1GW. Kinetic analysis of the instability in the descending auroral flux indicated that the phenomena scaled with the cyclotron frequency. Therefore, an experimental reproduction of the auroral geometry has been created scaled to laboratory dimensions by raising the radiation frequency to the microwave range. The experiment transports a 75–85keV electron beam through a region of increasing magnetic flux density, with a mirror ratio of up to 30. The experiments measured the mode, spectrum, power, and conversion efficiency of the emitted radiation as a function of the mirror ratio in two resonance regimes, with frequencies of 4.42 and 11.7GHz. The microwave diagnostics and measurements will be presented in this paper.

3D PiC code simulations for a laboratory experimental investigation of Auroral Kilometric Radiation mechanisms

Auroral Kilometric Radiation (AKR), occurs naturally in the polar regions of the Earth’s magnetosphere where electrons are accelerated by electric fields into the increasing planetary magnetic dipole. Here conservation of the magnetic moment converts axial to rotational momentum forming a horseshoe distribution in velocity phase space. This distribution is unstable to cyclotron emission with radiation emitted in the X-mode. In a scaled laboratory reproduction of this process, a 75–85 keV electron beam of 5–40 A was magnetically compressed by a system of solenoids and emissions were observed for cyclotron frequencies of 4.42 GHz and 11.7 GHz resonating with near cut-off TE0,1 and TE0,3 modes, respectively. Here we compare these measurements with numerical predictions from the 3D PiC code KARAT. The 3D simulations accurately predicted the radiation modes and frequencies produced by the experiment. The predicted conversion efficiency between electron kinetic and wave field energy of around 1% is close to the experimental measurements and broadly consistent with quasi-linear theoretical analysis and geophysical observations. (Some figures in this article are in colour only in the electronic version)

A laboratory experiment to investigate auroral kilometric radiation emission mechanisms

If an initially mainly rectilinear electron beam is subject to significant magnetic compression, the conservation of the magnetic moment results in the ultimate formation of a horseshoe distribution in phase space. A similar situation occurs where particles are accelerated into the auroral region of the Earths magnetic dipole. Such a distribution has been shown to be unstable to a cyclotron resonance maser type of instability and it has been postulated that this may be the mechanism required to explain the production in these regions of auroral kilometric radiation (AKR) and also possibly radiation from other astrophysical objects such as stars with a suitable magnetic field configuration. In this paper we describe a laboratory experiment to investigate the evolution of an electron beam subject to a magnetic compression of up to a factor of 30.

Electron beam measurements for a laboratory simulation of auroral kilometric radiation

Efficient (~1%) electron cyclotron radio emissions are produced in the X-mode from regions of locally depleted plasma in the Earths polar magnetosphere. These emissions are commonly referred to as auroral kilometric radiation. Two populations of electrons exist with rotational kinetic energy to contribute to this effect, the downward propagating auroral electron flux which acquires transverse momentum due to conservation of the magnetic moment as it experiences an increasing magnetic field and the mirrored component of this flux. This paper demonstrates the production of an electron beam having a controlled velocity spread for use in an experiment to investigate the available free energy in the earthbound electron flux. The experiment was scaled to microwave frequencies and used an electron gun to inject an electron beam into a controlled region of increasing magnetic field produced by a set of solenoids reproducing the magnetospheric situation. Results are presented of the measurements of diode voltage, beam current as a function of magnetic mirror ratio and estimates of the line density versus electron pitch angle consistent with the formation of a horseshoe velocity distribution and demonstrating control of the electron distribution in velocity space.

Numerical investigation of auroral cyclotron maser processes

When a mainly rectilinear electron beam is subject to significant magnetic compression, conservation of magnetic moment results in the formation of a horseshoe shaped velocity distribution. It has been shown that such a distribution is unstable to cyclotron emission and may be responsible for the generation of auroral kilometric radiation—an intense rf emission sourced at high altitudes in the terrestrial auroral magnetosphere. Particle-in-cell code simulations have been undertaken to investigate the dynamics of the cyclotron emission process in the absence of cavity boundaries with particular consideration of the spatial growth rate, spectral output and rf conversion efficiency. Computations reveal that a well-defined cyclotron emission process occurs albeit with a low spatial growth rate compared with waveguide bounded simulations. The rf output is near perpendicular to the electron beam with a slight backward-wave character reflected in the spectral output with a well defined peak at 2.68 GHz, just bel...