J. Weaver

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. >

Characterization of the plasma column used in studies of a plasma-loaded relativistic backward wave oscillator

Summary form only given. A series of experiments were made to characterize the electron component of a transient plasma column immersed in magnetic field. These long plasma columns (radius /spl ap/1 cm and length /spl ap/100 cm) were used to fill the slow wave structure of a relativistic backward wave oscillator (BWO). The addition of plasma increased the efficiency of microwave generation, the operating frequency, and the maximum allowable beam current. The plasma was generated by a coaxial hydrogen flashover gun located just outside the solenoid on the center axis. The plasma electron radial and axial density profiles and the plasma electron temperature were studied over a range of magnetic fields (2-15 kG), gun positions (24-40 cm from the solenoid end), and gun voltages (10-14 kV). This knowledge will allow more precise control of the operating conditions of plasma-loaded BWOs. A single movable Langmuir probe and a cylindrical resonant cavity were used.

Measurement of the dispersion relation of plasma-loaded slow wave structure

Summary form only given, as follows. Recent experiments with the plasma-loaded, 8.5 GHz, relativistic backward wave oscillator (BWO) showed an increase in the microwave interaction efficiency up to 40% and the possibility of operation at beam currents beyond the vacuum limit. Our goal is to analyze and optimize interaction between the electron beam and electromagnetic fields in plasma-loaded periodic slow wave structures filled with plasma. In this work, we measured electromagnetic dispersion characteristic of a plasma-loaded corrugated slow wave structure. A hydrogen flashover gun generated a plasma column which was guided by a magnetic field (2-15 kG) and filled a periodic slow wave structure. Since the structure is of finite length, resonances occur only for discrete values of the wavenumbers. Introduction of plasma into the slow wave structure was expected to cause upward frequency shifts of the resonances. The frequency upshifts associated with the TM/sub 01/ mode in the periodic slow wave structure were measured as a function of the background plasma density by a single port (S/sub 11/) method. In order to determine the background plasma density the same technique was used for a smooth wall cavity. We measured frequency shifts on the order of 0.1 GHz around a center frequency of 8.5 GHz for the plasma density of about 10/sup 11/ cm/sup -3/. The obtained resonance frequency upshifts in the plasma-loaded slow wave structure showed good agreement with theoretical calculations.

Investigations of the electromagnetic properties of finite length X-band slow-wave structures

Summary form only given. Cold tests have been performed on a slow-wave structure made from a cylindrical waveguide with sinusoidally varying radius. To study the dispersion relation, a cavity was formed by shorting a section of the structure between metal plates placed at planes of mirror symmetry. A simple axial wire antenna was used to launch azimuthally symmetric TM (transverse magnetic) modes. The end reflection was measured on the 8-period beam test structure to include the effect of the output taper. Since the input side was cutoff to all TM modes, one only needed to measure reflection for the output side of the structure. This reflection was found by measuring the diffractive quality factor of the structure.

Cold test measurements of a BWO slow-wave structure

Bead-on-string perturbation techniques have been primarily used to study particle accelerator cavities. These techniques are now extended to measure the dispersion relation and axial modes of several transverse magnetic (TM) modes in a slow-wave structure with sinusoidally varying radius. This type of slow-wave structure is commonly used in high power backward wave oscillators (BWO). The dispersion relation is found from the discrete measured resonant frequencies and wave numbers of a cavity containing six periods of the slow-wave structure. The measured axial and radial field profiles are compared with the results of a 2-D code specially developed for calculation of the fields in these generators. For the first three TM modes the frequency error was less than 0.7%.<<ETX>>