W.W. Destler

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. >

Relativistic plasma microwave electronics : studies of high-power plasma-filled backward-wave oscillators

The area of relativistic plasma microwave electronics has only recently generated renewed interest. New experimental data are presented demonstrating that the presence of a low‐density background plasma in a relativistic backward‐wave oscillator leads to several beneficial effects, including (a) enhanced interaction efficiency (40%), (b) operation at very low and possibly zero guiding magnetic field, (c) tunability by controlling the plasma density, (d) high degree of spectral coherency, and (e) operation well above the vacuum limiting current.

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. >

Design of high-average-power near-millimeter free electron laser oscillators using short period wigglers and sheet electron beams

The design and feasibility of a 1-MW continuous-wave (CW) free electron laser (FEL) oscillator are reviewed. The proposed configuration includes a short-period (I/sub w/ approximately 1 cm) planar wiggler, a sheet electron beam, a 0.5-1.0-MV thermionic electron gun, a hybrid waveguide/quasi-optical resonator, commercial DC power supplies, and a depressed collector. Cavity ohmic RF losses are estimated to be extremely low ( >

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. >

Experimental studies of overmoded high power microwave generators

Summary form only given. Experiments with overmoded (Dspl lambda/ /spl sim/3) high-power microwave generators have been performed. Two classes of devices were investigated: (1) single-stage backward wave oscillators (BWOs) with varying electrodynamic structure lengths; and (2) multiwave Cerenkov generators (MWCGs), consisting of two stages of seven sinusoidal periods each, connected by a variable length drift section. Both types of devices were driven by large-diameter, magnetized, annular, intense relativistic electron beams (250-900 keV, 2.5-9 kA). High-power microwave radiation in the 5.5 to 6 GHz frequency range was generated with both types of structures. A maximum radiated power of 320 MW was obtained with an overmoded BWO structure, and a maximum power of 102 MW was obtained with an MWCG. Start oscillation currents in the range of 204 kA were observed for both classes of devices.

Overmoded backward-wave oscillator: a comparison of experimental results with non-linear analysis

Summary form only given, as follows. Overmoded structures have the potential for increasing the power handling capability of linear beam, high power microwave (HPM) devices, reducing internal rf electric fields to below field-emission breakdown levels. However, potential detrimental effects include multi-mode generation and a reduction in efficiency and power. To study the feasibility of using overmoded structures in HPM generation, we conducted a series of experiments with an overmoded (D//spl lambda/-3), relativistic backward-wave oscillator (BWO) in the 5.4 to 5.6 GHz frequency range. The experiment, which was designed to operate near the edge of the passband of the TM/sub 01/ mode, produced highly coherent radiation (/spl Delta/f/f/spl les/0.5%) with a maximum power of 320 MW. Power, efficiency, and frequency were measured as a function of both electron beam parameters and electrodynamic structure length. In parallel, we developed theoretical models of BWOs operating near cutoff, which included end-reflections and finite magnetic field effects. For most of the experiment, the measured frequencies were within 5% of the model, which correctly tracked the trend of efficiency with structure length. We present a comparison of the theoretical and experimental results and an interpretation of the physical processes.

Electromagnetic properties of corrugated and smooth waveguides filled with radially inhomogeneous plasma

Summary form only given. The creation and diagnosis of plasma in high power microwave devices remains one of the primary challenges of plasma microwave electronics. In the present work we deal with (a) diagnostics techniques for characterization of radially nonuniform plasma columns suitable for use in high power microwave sources and (b) the effects of such plasmas on the electromagnetic properties of finite length, spatially periodic slow wave structures. Experimental studies were performed both for a strong and weak guiding magnetic field. A single Langmuir probe technique cannot be used alone for precise characterization of magnetized plasma columns, especially for measurements of the electron plasma density. However, using a combination of a novel cylindrical resonant cavity technique supported by accurate numerical calculations of the plasma influence on cavity resonances of a long, thin plasma, column we were able to characterize pulsed plasma columns. The peak density (higher than 10/sup 12/ cm/sup -3/) and the spatial distributions (transverse and axial) of the plasma was measured as a function of the applied magnetic field (2 to 10 KG) and the plasma gun operating conditions. By applying a combined probe-microwave resonator (X band) technique to plasma-filled, open, corrugated cavities we were able to measure in situ for the first time the complete dispersion curve of the TM/sub 01n/ modes of a plasma-loaded, finite length, corrugated cavities immersed in guiding magnetic field.

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.