R.A. Correa

Gyrotron coaxial cylindrical resonators with corrugated inner conductor: Theory and experiment

Gyrotron coaxial resonators with a longitudinally slotted inner cylinder are examined analytically using a surface impedance model, from which expressions for the electromagnetic field, ohmic quality (Q) factor, and characteristic equation of the transverse eigenvalues /spl chi//sub m,p/ are obtained. The major attributes of such resonators are expressed by the dependence of /spl chi//sub m,p/ on the parameter C-defined as the ratio of the outer to inner radii of the coaxial structure. In that connection, the effect of the corrugation parameters on /spl chi//sub m,p/ is particularly investigated on the basis of an expression derived for the slope function d/spl chi//sub m,p,p//dC. It is shown that the /spl chi//sub m,p/(C) curve may either exhibit oscillatory behavior or present a flat portion over a wide range of C depending on the corrugation parameters chosen. The theory is checked against experiment in which resonant frequencies and total Q factors were measured for TE modes operating in the range of 8-16 GHz in a coaxial cavity with 40 slots. Good agreement is found in that the magnitude of the relative error in frequency is less than 0.5%. Corrugated coaxial resonators prove to be relevant to megawatt gyrotrons where highly selective cavities are required to ensure high conversion efficiency.

Operation of a 32 GHz gyrotron

The design and operation of a 32 GHz pulsed gyrotron are reported. The device is step-tuned between the TE1,2 (24.16 GHz) and TE0,2 (31.78 GHz) modes with cathode voltages ranging from 30 to 40kV and beam current up to 5.0A. Experimental frequencies are in close agreement with the self-consistent calculated values and in the TE2,2 resonator mode an output peak power of 6kW corresponding to an 18% efficiency was measured by using a fast response calorimeter with a thermal sensitivity of 0.1°C/Wmin.

Ohmic selection in open coaxial resonators: An experimental study

A study of ohmic selective properties of open coaxial cylindrical resonators has been conducted experimentally and compared with theory. The resonator consists of an inner cylinder made of silicon carbide symmetrically located inside an outer cylindrical, tube shaped waveguide. Several fundamental TE modes were identified over the range 9 to 17 GHz through measurements of the resonant frequencies and the associated quality factors. Mode discrimination is achieved both by exploring selective ohmic effects and examining the electrodynamical properties of the coaxial cylindrical waveguide. The effectiveness of a silicon carbide coaxial insert in providing ohmic mode selection is demonstrated in that the totalQ factors of TEmp modes with radial indexp≥2 become well below the quality factors for surface TEm1 modes. It has been verified that both structure and number of resonant modes are strongly dependent on the diameter and the resistivity of the coaxial insert.

Experimental Study of a Ka-Band Cylindrical Open Resonator

A study of the electrodynamical properties of a Ka-band gyrotron open resonator was experimentally conducted. Experiments were accomplished to measure resonant frequencies and their respective loaded quality factors for TE modes in the frequency range from 26 to 40 GHz. In particular, a perturbation technique was used to determine the axial, radial and azimuthal electric field profiles, as an identification method of the TE021 mode operating around 35 GHz. In any experimental event, good agreement with the values predicted by theory was found.

Electromagnetic simulation of a 32-GHz, TE/sub 021/ gyrotron

A simulation study with experimental parameters of a 32-GHz gyrotron operating in the TE/sub 021/ mode is presented. Beam electrons with typical energy of 40 keV and transverse-to-axial velocity ratio ranging from /spl alpha/=0.8 to /spl alpha/=2.0 are injected into the cavity to drive electromagnetic oscillations from noise. On the basis of an electromagnetic particle-in-cell (PIC) code, a parameterization study is carried out to determine the dependence of output power upon pitch ratio and beam current. A comparison of the PIC simulation results with the predictions of the fixed-field approximation on considering Gaussian and asymmetric profiles has indicated close agreement between field parameters and conversion efficiency corresponding to both approaches.

Cold Tests of Open Coaxial Resonators in the Range 9-17 GHz

A study of selective properties of coaxial open cylindrical resonators have been conducted experimentally and compared with theory. The resonator consists of an inner circular cylinder symmetrically located inside an outer weakly irregular open waveguide. Several fundamental TE modes were identified over the range 9 to 17 GHz through measurements of the resonant frequencies and the associated loaded quality factors. It has been verified that the structure and the number of resonant modes are both strongly dependent on the diameter of the coaxial insert. Such an electrodynamical system proves to be useful in guided wave applications requiring, for example, filters, frequency-tunable resonators and devices for analyzing the modal composition of a signal.

Cold tests of A 10-GHz gyrotron cavity

Experiments have been conducted to characterize a gyrotron cavity designed to operate in theTE021 mode at 10 GHz. Cavity excitation was accomplished via a coupling hole introduced into the cavity wall and mode detection was carried out by means of two experimental arrangements. In the first, electromagnetic energy is coupled into a receiving waveguide through a small second hole drilled in the opposite side of the cavity. The other scheme uses a horn antenna to receive the power reradiated by the open resonator. Both schemes are discussed regarding mode detection, and measured data includes resonant frequency, loadedQ factor, axial electric field profile and farfield radiation pattern. Evaluation of the loadedQ factor is based on bandwidth measurements whereas standing-wave electric field profile is determined by using perturbation techniques. For severalTE modes, close agreeent between theory and experiment is found.

Numerical simulation of high-power virtual-cathode reflex triode driven by repetitive short pulse electron gun

A virtual-cathode reflex triode is investigated by numerical simulations. A trapezoidal in shape voltage pulse with an amplitude of 300 kV is applied to the solid cathode of the device to drive the cathode negative. The electron beam-to-microwave power conversion efficiency /spl epsiv/, calculated for the pulse flat top with a duration /spl tau//sub ft/=1.2 ns is approximately the same (about 1.5-2%) as well as for a long flat top (/spl tau//sub ft/=4 ns). The simulations show a 10-15% increase of /spl epsiv/ at /spl tau//sub ft/ shortening to 0.6 ns. However, this occurs when the anode mesh transparency is high (80-90%). Considerable enhancement of the efficiency (about four times) for /spl tau//sub ft/=0.6 ns has been calculated if the cathode side surface is brought near to the anode tube (from /spl ap/0.5% at cathode radius R/sub c/=1.6 cm to /spl ap/2% at R/sub c/=3.8 cm). The obtained results would find an application for the design of virtual-cathode reflex triode devices driven by a short pulse and high repetition rate electron gun.

High-quality electron beam with nonadiabatic electrostatic pumping

In this paper we report on the numerical results for a nonadiabatic beam pumping method by electrostatic modulation. A 700 kV, several hundred amperes nonadiabatic gun is able to produce a high-quality hollow electron beam with pitch ratio of 0.6, perpendicular spread of 1.5% and longitudinal spread of 0.1% after adiabatic magnetic compression. >

Electromagnetic Field and Cutoff Frequencies of the Azimuthally Rippled Wall Waveguide

The cylindrical waveguide with azimuthally periodic corrugations described by a sinusoidal function is analyzed. Relevant to magnetron and gyrotron interactions, TE-mode oscillations are treated by deriving a dispersion relation—linking the cut-off frequency and corrugation parameters—upon expansion of the field quantities in terms of space harmonics. From numerical examples, the present paper examines how the geometrical properties (area and perimeter) of the corrugated cross section relates with the cutoff frequencies of 2π and π modes having one radial variation. The waveguiding system studied here with continuously corrugated azimuthal profile looks attractive to relativistic, slow-wave electron devices for it is more resistant to RF breakdown in comparison with traditional magnetron cavities with vane-type side resonators.