Gary E. Thomas

Operating modes of relativistic rising-sun and A6 magnetrons

The operating characteristics of a relativistic 16-vane rising-sun magnetron were investigated with particular emphasis on determining the operating regimes of different modes. The magnetron performance was studied as a function of voltage, magnetic field, cathode geometry, axial boundary conditions, and output coupling. Operation was observed in the 3 pi /8 mode at 3.3 GHz, in the pi /2 or 3 pi /8 mode at 3.5 GHz, and in the pi or 7 pi /8 mode at 4.6 GHz. A maximum power of 80 MW was emitted in the 3 pi /8 mode with an efficiency of 4.5%. Typical pulse lengths were 40-50 ns. Cold tests were performed to measure the resonant frequencies and azimuthal electric fields in the interaction space which agreed within 1-4% of theoretical calculations. The operating modes were inferred from close agreement between hot-test frequencies and cold-test results and because high-power RF emission occurred at, or just above, the Buneman-Hartree threshold calculated for these modes. The characteristics of a six-vane A6-magnetron operating in the pi and 2 pi modes were also studied. A unique transmitting-receiving system, which was used as a microwave diagnostic, is described. >

Temporal study of long-pulse relativistic magnetron operation

A streak camera was used to make observations of apparent plasma motion in the interaction space of an A6 relativistic magnetron with an explosive emission cathode. The anode plasma luminosity velocity was measured to be 0.8 cm/ mu s, comparable with the gap closure velocity, which was indirectly measured to be 1.0-1.6 cm/ mu s, depending upon the magnetic field. The gap closure velocity appeared to be independent of the microwave oscillation. Long-pulse operation (hundreds of nanoseconds) as a function of the magnetron-modulator interaction is also presented. At 250 kV the A6 emitted microwave pulses out of a single resonator for up to 250 ns in duration, with peak power up to 90 MW and a characteristic single-spike pulse length of 150 ns. >

Gas-Breakdown Transmit-Receive Tube Turn-On Times

The first measured sub-nanosecond gas-breakdown microwave receiver protector turn-on times are reported. A technique to measure turn-on times as short as 0.35 ns is described. The turn-on time of an X-band gas-breakdown transmit-receive (TR) tube, both alone and when backed by a solid-state diode limiter (DL), is measured to be a small fraction of a nanosecond. The spike leakage energy and transmitted peak power are measured as functions of incident power up to 2 kW. Extrapolation of the results indicates that effective receiver protection might be achieved at gigawatt incident power levels.

Solitons and non-linear gyro-TWT theory

A description of two new approaches to non-linear gyro-TWT theory is given. Both approaches are based upon the soliton concept. A soliton is a solitary wave in a non-linear, dispersive medium where the non-linearity and dispersion counteract each other to prevent, wave break up. The application of tile soliton concept to microwave, crossed-field amplifiers (i.e., the applied DC magnetic and electric fields arc perpendicular) has shown great success. Close agreement between theory and experiment has been demonstrated. This success is the motivation for applying the soliton concept to gyro-TWTs, The first approach is a non-linear, coupled-mode theory. Only a brief discussion is given. The second approach differs from the first in that the coupling between modes is assumed strong with a weak non-linearity. A non-linear, partial differential equation is derived and solved for soliton solutions. Amplification regimes are discussed as well as saturation mechanism. Velocity spread effects are briefly analysed b...

Chaotic instabilities and density profiles in a crossed-field electron vacuum device

Crossed-field electron vacuum devices are resonant devices. When properly tuned, they operate at a single frequency and have an average background distribution. Thus one can use the cold-fluid equations and a Fourier decomposition to separate the physical quantities into a background (DC) mode and a pump (RF) mode. We have improved our previous calculations on these devices and can now understand how the background plasma density varies and evolves as the RF wave travels down the slow-wave structure. We study the evolution of an RF pump wave through the device and find that in general, chaotic (period-2) instabilities can occur if the device is too long. We also present results for the high magnetic field case, (typical CFA/magnetron regime), for the moderate magnetic field case (ultra-low noise regime), and discuss how these solutions correspond to device operation. Lastly, we discuss our results and point out future work in need of study.

Linearized Vlasov-Poisson equations for the planar magnetron

The linearized Vlasov–Poisson equations are obtained for the planar magnetron in the form ∂2yφ1+(V−k2)φ1=0, where V∝(∂yn0)Z(ζ), (∂yn0) is the density gradient and Z(ζ) is usual plasma dispersion function. The form for ζ, the ratio of the shifted phase velocity to the thermal velocity, is very strongly density dependent. One may understand this by considering the thermal velocity to be density dependent. In the interior of the sheath, the wave–particle resonance is very broad, corresponding to a high effective temperature. Just outside the sheath, this effective temperature drops to very low values (proportional to the electron density), leaving very narrow wave–particle resonances. The cold‐fluid limit of this equation is also obtained.

A strongly nonlinear analytic model for microwave electron devices

In this paper, a description of a new method for obtaining a strongly nonlinear analytic model for nonlinear electron devices is given. Specific examples include crossed‐field amplifiers and gyro‐travelling wave tubes. This method is an extension of previous work where a nonlinear partial differential equation is derived and reduced by a multiple time scales perturbation technique to a nonlinear Schrodinger equation. The extension of the above approach to a strongly nonlinear regime is accomplished by including ErfxB0 (rf electric and dc magnetic field) effects in the linear dispersion relation in an approximate manner. The inclusion of these ErfxB0 terms results in nonlinear coefficients (i.e., dependence upon Erf ) of the nonlinear Schrodinger equation. The importance of these rf related terms is discussed for the crossed‐field amplifier and the gyro‐travelling wave tube.

Relativistic density profiles and current flow in a cross-field relativistic electron vacuum device

We use the cold-fluid plasma equations to consider the nonlinear effects of a strong, relativistic RF electric field (with a frequency, w, and a wavevector, k) which is propagating on the background electron density profile in a relativistic crossed-field, electron vacuum device. Earlier, we had shown that in the nonrelativistic case, when k and w are such that a wave-particle resonance, w equals vdk, can occur at the edge of a Brillouin sheath, then the Brillouin sheath becomes strongly unstable to a Rayleigh instability, with the instability being driven by the strong negative density gradient at the edge of the Brillouin sheath. As a consequence of this instability, the average DC density profile becomes strongly modified and is driven away from the classical Brillouin flow by the RF field, and is driven toward stationary solutions of a nonlinear diffusion equation. From this nonlinear diffusion equation, one can predict the DC current flow through a device and also can predict the shape of the stationary DC electron density profile. Also we have demonstrated that such stationary solutions do exist and can be calculated. Further, we showed that when one combined these stationary solutions with the RF field solutions, then the total solution would generate the standard spoke structure, long seen in numerical simulations. Here, we shall extend these calculations into the relativistic regime and discuss their form.

Experimental results of power combining and phase-locking magnetrons for accelerator applications

It is demonstrated that injection-locked magnetrons can be used to drive a moderate Q cavity without a circulator with excellent interpulse phase coherency. A 3-dB hybrid coupler provides the avenue for both injection locking and power combining of magnetron pairs. Better intrapulse phase coherency than has been demonstrated is anticipated when lower Q magnetrons or lower gains are used. The cavity transient impedance does not preclude the magnetrons from filling the cavity, at least when the cavity fill time is less than the magnetron phase lock time.<<ETX>>

Operational theory of crossed-field devices based on double-box density profiles

Studies of the nonlinear evolution of the electron‐density profile in crossed‐field devices indicate the classical Brillouin profile to be inadequate for the explanation of the operation of such devices. Instead, the existence of a density plateau at the edge of the electron sheath is found to be essential in the operation of such a device. Such a plateau placed at the edge of a classical Brillouin profile creates a double‐box profile. The operational theory for crossed‐field devices based on this double‐box density profile is presented. It is shown that this profile generates an operating voltage range that agrees quite well with the actual voltage operating range of such devices, except for high magnetic fields.