K. Minami

High-power microwave generation by excitation of a plasma-filled rippled boundary resonator

An experimental demonstration of a strong enhancement of the interaction efficiency in a high-power relativistic backward-wave oscillator when plasma is injected is presented. Controlled plasma injection enhances the interaction efficiency for the vacuum case by a factor of up to eight to a value of about 40%. A linear theory of electromagnetic wave generation in plasma-loaded corrugated wall resonators is reviewed. A number of physical mechanisms are considered to account for the enhanced interaction, including two variations of a three-wave interaction involving the electron-beam slow space-charge wave, the slow electromagnetic waves in the structure, and the quasi-electrostatic waves in the plasma. >

Electromagnetic properties of open and closed overmoded slow-wave resonators for interaction with relativistic electron beams

Specific slow wave structures are needed in order to produce coherent Cherenkov radiation in overmoded relativistic generators. The electromagnetic characteristics of such slow wave, resonant, finite length structures commonly used in relativistic backward wave oscillators have been studied both experimentally and theoretically. In experiments, perturbation techniques were used to study both the fundamental and higher order symmetric transverse magnetic (TM) modes. Finite length effects lead to end reflections and quantization of the wave number. The effects of end reflections in open slow wave structures were found from the spectral broadening of the discrete resonances of the different axial modes. The measured axial and radial field distributions are in excellent agreement with the results of a 2-D code developed for the calculation of the fields in these structures. >

Analysis of the electromagnetic waves in an overmoded finite length slow wave structure

The electromagnetic fields of the higher order axial resonant modes in a slow wave structure are analyzed and found to have considerably different characteristics from those of the conventional fundamental mode. Here, the reflections at both ends produce axial resonant modes corresponding to axisymmetric transverse magnetic (TM) modes. The period of field modulation of some of the higher order axial modes is shorter than that of the usual mode in a cylindrical waveguide, which could be of practical interest for higher power, higher frequency operation of backward wave oscillators. A perturbation technique is used to ascertain the field distribution inside the resonant cavity, and the numerical results thus obtained are compared to some experimental data. >

Starting energy and current for a large diameter finite length backward wave oscillator operated at the fundamental mode

We study the starting conditions for a large diameter (diameter/wavelength=4.8) finite length backward wave oscillator designed for 24-GHz operation at the fundamental TM/sub 01/ mode. This geometry is very promising for high power handling capability. We analyze two separate threshold conditions. First, finite length effects give rise to a threshold in electron beam energy below which oscillations cannot be sustained at any beam current. The second is the more familiar current threshold known as a start current. It is also found that the growth rate for the fundamental mode can be much larger than those of other higher order modes thus leading to coherent operation of large diameter sources free from mode competition. >

Experiment on a cold cathode gyrotron

Summary form only given. A gyrotron oscillator operating at frequencies less than 20 GHz and driven by an electron beam from a cold cathode has been designed, fabricated and tested. The cavity with length 168 mm was installed in a solenoid coil which produced a magnetic field up to 1.2 T for 1 sec duration. Three aluminum cavities with various radii (viz., 10, 15, and 19 mm) were prepared. The beam source was placed 270 mm from the upstream end of the cavity, where the magnetic field strength was 33 % of that at the cavity. The anode was a stainless-steel disk with a circular hole of radius greater than the radius of the edge of the aluminum annular cathode. The cathode edge was wrapped with thin velvet to improve electron emission. The anode was at ground potential and a -80 kV high voltage pulse with duration of 100 nsec was applied to the cathode. The output microwave pulse was observed using an X-band horn antenna located 230 mm from the output glass disk window. A waveguide directional coupler divided the signal into two parts. One was fed to a crystal detector to observe the prompt signal through semi-rigid cable and coaxial attenuators. The other was fed to a 18.75 m waveguide delay line to form a delayed signal. Both prompt and delayed signals were displayed on a digital oscilloscope to measure the frequency and power. Output pulses corresponding to TE[111], TE[211] and TE[011] modes were observed near the expected values of magnetic field for the three cavities. The frequency measurements were restricted to a range near 10 GHz, because we had available only X-band waveguide delay line. The radiation patterns indicated multi-mode oscillations which might have been expected since the values of beam current were large. For cavity radii 15 and 10 mm, the outputs reached 5 MW. The frequency range of the measurement system was limited to be less than 20 GHz. We are improving the system by extending its frequency range up to 30 GHz.

High-power backward wave oscillator driven by an intense relativistic electron beam with tapered slow wave structure for improved performances

Abstract A new theoretical and computational method, which can analyze uniformly the working of slow wave oscillators with various shapes of the waveguide, is developed. Time evolution of nonlinear boundary value problems in 2D space are reduced to 1D space boundary value problems by the power expansion method, and the reduced equations can be solved numerically by the new numerical scheme HIDM (higher order implicit difference method) with high accuracy. Dynamics of the backward wave oscillator driven by an intense relativistic electron beam are analyzed by this method. It is found that a throttled shape waveguide is preferable to obtain uniform output power of microwave.

Experimental studies of high power plasma filled backward wave oscillators

Summary form only given, as follows. It has previously been demonstrated that a plasma-loaded backward wave oscillator (BWO) powered by a relativistic electron beam can generate hundreds of megawatts of microwave radiation at high efficiency (about 40%). An experimental study of an 8.4-GHz BWO filled with an externally controlled background plasma was performed. It was found that the enhanced efficiency can be maintained even for large electron beam currents approaching the vacuum space-charge limiting current, and it is anticipated that this might hold even beyond the space-charge limiting current. A small frequency up-shift (a few percent) was detected for the plasma loaded BWO. A hydrogen flashover plasma gun was used, and its characteristics, including plasma density, drift velocity, and temperature, were investigated. Detailed studies of beam propagation in vacuum as well as in plasma loaded structures were performed. It appears that a slightly over-moded device will be needed for peak power handling capability of 5-10 GW

The range of validity of the Rayleigh hypothesis

Summary form only given. The parameter range over which the Rayleigh hypothesis (RH) for optical gratings might be validly applied to analysis of high power backward wave oscillators (BWOs) has been investigated numerically. A widely used method to analyze high power backward wave oscillators (BWOs) is to represent the electromagnetic (EM) fields in axisymmetric slow wave structure (SWS) in terms of a Floquet harmonic expansion (FHE). EM fields, E and B, are expressed in a form as, [E B]=/spl Sigma//sub n=-N//sup N/[E/sub n/(r) B/sub n/(r)]exp i(k/sub zn/z+l/spl theta/-/spl omega/t) where k/sub zn/=k/sub z/+nK/sub 0/, K/sub 0/ is wavenumber of SWS periodicity, and n=0, /spl plusmn/1, /spl plusmn/2,.../spl plusmn/N is the Floquet harmonic number. Expansion similar to (1) was firstly introduced by Lord Rayleigh for diffraction of waves from planar grating. He assumed that the expansion was applicable both outside and inside the corrugation, and this assumption is called as RH. It was argued by some mathematician that our numerical analysis was applied to deep corrugation of hK/sub 0/=1.67 which was beyond the limiting value hK/sub 0/=0.448 for validity in RH for planar sinusoidal grating, consequently the results were invalid. Here, h is the amplitude of corrugation. To respond the doubt, EM fields and dispersion relation in the SWS are numerically analyzed with and without RH for a given set of size parameters. The field patterns and eigen frequency for the SWS are solved for a given k/sub z/ numerically by using finite difference code HIDM (Higher Order Implicit Difference Method) that is free from RH. The results are compared with those using (1). For a deep corrugation, hK/sub 0/=5/spl times/0.448, using RH is still valid for obtaining the dispersion relation, although the Floquet Harmonic Expansion (FHE) fails to correctly represent the field patterns inside the corrugation. Accordingly, there exists a discrepancy between the validity of using RH for obtaining dispersion relations and for an exact convergence of FHE everywhere in the SWS.

Optical observation of localized plasma after pulsed discharges in liquid helium

Summary form only given, as follows. Transient localized high-density plasma, in liquid helium (LHe) has been studied. Stark broadening of a He spectral line reveals the spatial and temporal evolution in the decaying plasma with density above 10/sup 16/ cm/sup -3/ produced by pulsed discharge between a pair of W needle electrodes in gaseous bubble surrounded by LHe. When one of the electrodes is replaced by Mg rod, strong spectral lines of Mg atoms are observed. Measurements axe made for two cases that the common axis of the electrodes is horizontal and vertical. In these cases, the density distribution in the directions, respectively, along and across the common axis can be measured.

Analysis of a large-orbit backward wave oscillator

Summary form only given, as follows. The effect of finite axial magnetic field on excitation of a backward wave oscillator (BWO) is investigated. The driver beam is assumed to be a mono-energetic helical electron beam with all the constituent electrons having their gyration centers on the axis of the slow wave structure (SWS). Such a beam supports negative energy fast cyclotron modes (FCMs) that can excite structure modes in the SWS. This may contribute to microwave generation in the BWO, which is solely due to the Cherenkov mechanism.