[en] A theoretical model for the production of high-power, high-frequency electromagnetic radiation from unmagnetized, relativistic beam-plasma interactions is studied. Emphasis is placed on the injected-beam system, for which the dominant portion of the radiation is emitted near the point where the beam enters the plasma. In such systems, frequencies much larger than the plasma frequency and power levels many orders of magnitude above that predicted by single-particle radiation have been observed experimentally. A two-step process is proposed to explain these observations: electrostatic bunching of the beam followed by coherent radiation by the bunches. The first step, beam bunching, produces large-amplitude electrostatic waves. A Green's function analysis is employed to understand the convective growth of those waves near the plasma boundary; their saturation amplitude is found by applying conservation of energy to the beam-plasma system. An azimuthally symmetric model is used to compute the saturated spectrum analytically, and a relatively simple expression is found. The second step, the interaction of the electron beam with the electrostatic spectrum, leads to the production of high-power, high-frequency electromagnetic radiation. From a detailed analysis of the phase-space evolution of the trapped beam, an analytic expression for the electromagnetic spectrum is found as a function of angle and frequency

[en] The binary power multiplier (BPM) recently proposed by Farkas requires a pair of RF inputs whose phases are set independently. In this note, a method is presented in which a single source may be used to drive a BPM. Phase coding occurs at the source input where the power is low and phase switching is straightforward. There is a loss in energy of around 25% but only a small reduction in peak power. 4 refs., 3 figs

[en] In multi-cavity gyroklystron amplifiers, the drift sections between cavities are generally designed to cutoff the mode of interest; it is the isolation between cavities that keeps the circuit from oscillating as a whole. Higher frequency modes, however, are not cutoff in the drift tubes. Because of their long effective length, these modes may have low start oscillation currents. Using the scattering matrix method, the authors find the modes in a complex, cylindrical cavity as an expansion in transverse eigenfunctions. They then compute the start oscillation current by numerically integrating the particles through the circuit. The start oscillation current determines the level of wall loading needed to suppress the mode. Results are presented for the two cavity gyroklystron amplifier to be used in an initial University of Maryland experiments. Curves of start oscillation current vs. magnetic field are shown for selected modes with azimuthal mode numbers of 0,1, and 2

[en] When ac space charge is included in the analysis of weakly relativistic multicavity gyroklystron amplifiers, the optimized nonlinear efficiency becomes a function of beam current. For a cold beam (no velocity spread), the efficiency is maximum at zero current and decreases monotonically as the current increases. The zero current limit of the optimized efficiency when ac space-charge effects are included is not the same as the optimized efficiency with no space charge; it is significantly higher. This rather strange behavior is regularized when velocity spread is taken into account; in that case, the nonlinear efficiency increases with beam current until it reaches a maximum, then falls off slowly. The increase in the efficiency is attributed to enhanced bunching associated with the saturation of the space-charge instability in the drift region; the reduction in efficiency at high current occurs because space charge induces an additional velocity spread in the beam

[en] In this paper, the authors apply mode conversion theory to 1-dimensional reflection in a time-varying inhomogenous plasma and present numerical results. Degenerate mode conversion, i.e. mode conversion between two eigenvectors of a plasma dispersion tensor with the same eigenvalue, may be analyzed using the same techniques as nondegenerate mode conversion by adding several Maxwell-Two Fluid unknowns and equations into an initial description of the problem. They apply this procedure to simple reflection in a slowly varying plasma with no magnetic field. Results for different cases, i.e. above and below the plasma critical density, are presented. The application to microwave breakdown in the atmosphere is discussed

[en] Relativistic gyrotwistrons, which can, in principle, achieve efficiencies in excess of 50%, are promising sources for driving particle accelerators. The optimum operating point for these devices represents a tradeoff between maximizing the available energy and minimizing the deleterious effects of velocity spread. The former increases with Doppler upshift and pitch ratio, while the effects of velocity spread are made worse by both of these quantities. The authors discuss the issues that go into deciding how to achieve maximum efficiency, with particular attention paid to the role of beam current

[en] The authors are studying the propagation of pulsed high-power microwaves in the atmosphere. Their first calculations examine the individual processes in thier problem. These include reflection, absorption, transmission of normally incident electromagnetic radiation onto a finite width resistive plasma and models of ionization of various gases by high-power RF. These results form the base for pulsed RF propagation calculations. They calculate the growth of a plasma in a one-dimensional gas cell using a low ionization rate, long-pulse model. Iterative numerical techniques permit simultaneous calculation of the electric field and plasma density profile as they evolve. They display results as a function of plasma density, plasma width and plasma neutral pressure. They study the validity of their model with the aid of a time domain analysis of the RF pulse response of a finite width resistive plasma. Time domain analysis is accomplished by Fast Fourier Transform methods. FFT results also illustrate transient effects associated with RF pulse propagation

[en] The realization of e^-e⁺ supercolliders will require advances in tehnology including the development of x-band microwave amplifiers with pulse energy > 60 J. Candidate microwave amplifiers include klystrons, lasertrons, free electron lasers (FEL's), and gyrotrons; gyrotron amplifiers employing a multicavity gyroklystron configuration appear advantageous at λ ≅ 3 cm. Measurements on a 50 kW, 1 μs gyroklystron show phase jitter < 0.75⁰ indicating compatibility of this type of amplifier with collider requirements. The University of Maryland is currently developing an x-band, TE⁰₀₁ mode gyroklystron driven by 500 keV, 160 A, 2 μs electron beam pulses; combining this tube with a TE⁰₀₁ binary pulse compression circuit under development at SLAC could produce 475 MW, 120 ns microwave pulses which imply the feasibility of achieving linac accelerating fields in the range 100-200 MV/m

[en] The stability of gyrotwistrons against the excitation of parasitic modes is studied. A method is described which can be used to compute the nonlinear saturation amplitude of the parasites when the operating mode is present. An example relevant to driving particle accelerators (near 10 GHz and 500 kV) is discussed

[en] The inherent periodicity of the strong magnetic focusing in a planar wiggler can lead to an FEL-like instability. This instability differs from the conventional FEL in a number of ways: 1. The EM electric field component responsible for the interaction is perpendicular to the wiggler plane. 2. The EM waves couples to the betatron oscillations instead of the plasma oscillations. As such, it relies on transverse gradients in the wiggler field. 3. The 'fundamental' mode is at the second harmonic: Δk=2k_w rather than Δk=k_w. 4. Coupling goes as the fourth power of the wiggler parameter. The dispersion relation for this instability is derived for two cases: A cold sheet beam and a cold string beam, both in a planar wiggler configuration. An absolute instability is found and its growth rate is given in terms of the beam plasma density and the wiggler parameter. (orig.)