J. Rodgers

Overmoded GW-class surface-wave microwave oscillator

Results of theoretical and experimental studies of a GW-class, large diameter microwave oscillator are presented. The device consists of a large cross-section (overmoded), slow-wave structure with a unique profile of wall radius specifically designed to support surface waves and to provide a strong beam-wave coupling at moderate voltage (500 kV), an internal adjustable microwave reflector, a coaxial microwave extraction section, and a coaxial magnetically insulated field emission electron gun. In preliminary experiments carried out at 8.3 GHz, the power level exceeding 0.5 GW and efficiency of 15% have been measured calorimetrically.

A 670 GHz gyrotron with record power and efficiency

A 670 GHz gyrotron with record power and efficiency has been developed in joint experiments of the Institute of Applied Physics, Russian Academy of Sciences (Nizhny Novgord, Russia), and the University of Maryland (USA) teams. The magnetic field of 27–28 T required for operation at the 670 GHz at the fundamental cyclotron resonance is produced by a pulsed solenoid. The pulse duration of the magnetic field is several milliseconds. A gyrotron is driven by a 70 kV, 15 A electron beam, so the beam power is on the order of 1 MW in 10–20 ms pulses. The ratio of the orbital to axial electron velocity components is in the range of 1.2–1.3. The gyrotron is designed to operate in the TE31,8-mode. Operation in a so high-order mode results in relatively low ohmic losses (less than 10% of the radiated power). Achieved power of the outgoing radiation (210 kW) and corresponding efficiency (about 20%) represent record numbers for high-power sources of sub-THz radiation.

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.

A novel highly accurate synthetic technique for determination of the dispersive characteristics in periodic slow wave circuits

A highly accurate (0.1-0.5%) synthetic technique for determining the complete dispersive characteristics of electromagnetic modes in a spatially periodic structure is presented. It was successfully applied for the cases of the fundamental (TM/sub 0(1)/) as well as higher-order (TM/sub 0(2)/, TM/sub 0(3)/) passband modes in a corrugated waveguide. This structure is commonly used in relativistic backward wave oscillators, traveling wave tubes, extended interaction oscillators, and a variety of multiwave Cerenkov generators. An appropriately shorted periodic structure resonates at specific frequencies. To measure these frequencies accurately and unambiguously, the authors used unique antenna radiators to excite pure modes in the circuit under test. An analytical technique for deriving the complete dispersion relation using the experimentally measured resonances is presented. This technique, which is based on the intrinsic characteristics of spatially periodic structures, is applicable to slow wave structures of arbitrary geometry. >

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. >

Experimental studies of high‐power microwave reflection, transmission, and absorption from a plasma‐covered plane conducting boundary

Experimental studies of the reflection, transmission, and absorption of high‐power microwave pulses from a plasma‐covered plane conducting boundary are presented. Under optimum conditions, backscattered rf power is attenuated by more than 30 dB over values measured in the absence of the plasma. Measurements of the radial and axial plasma density profiles and the neutral gas pressure near the plane conductor indicate that collisional absorption processes are not the primary source of the observed attenuation in the backscattered microwave signal, and that the plasma density exceeds the critical density over much of the volume nearest the conductor. The effects of a tenfold reduction in the microwave power density on the reflection and absorption characteristics of the system are also reported.

Propagation of wiggler focused relativistic sheet electron beams

A recent design concept for millimeter‐wave free‐electron lasers [J. Appl. Phys. 60, 521 (1986)] would require the stable propagation of a sheet electron beam through a narrow waveguide channel. Experimental results reported in this article support the feasibility of such a configuration by demonstrating the stable propagation of relativistic sheet electron beams through a narrow waveguide gap (3.2 mm) using focusing by a short‐period electromagnet wiggler. 90% of the electron current in a 100‐keV sheet electron beam was transmitted through a 5‐cm‐long channel with peak wiggler fields of 800 G. Almost 80% of a 400‐keV beam was similarly confined with a 1600‐G wiggler field. The data were consistent with single electron trajectory models, indicating that space‐charge effects were minimal. No evidence of beam breakup or filamentation instabilities was observed.

Operation of a high performance, harmonic-multiplying, inverted gyrotwystron

Experiment on a new type of high-power, millimeter wave amplifier, (a harmonic-multiplying, inverted gyrotwystron), are reported. Superior stability resulted from two factors: (1) interaction between a relatively low order waveguide mode (TE/sub 22/) and the beam wave at the fundamental cyclotron frequency in the traveling wave input section; (2) an internal mode filter in the highly overmoded (TE/sub 42p/), second harmonic output cavity. Bandwidth was 1.3% with saturated gain of 33 dB around 31.8 GHz. The gain-band width performance represents a significant advance for gyrotron amplifiers operating in such high order modes, and is thought to be due in part to operation of the long output cavity in several modes with different axial eigennumber (p=3, 4, 5) but with overlapping spectral regions.

High power, high brightness electron beam generation in a pulse-line driven pseudospark discharge

High brightness (∼1010 A/m2 rad2), high power density (∼1010 W/cm2) electron beams have been generated by the mating of a hollow‐cathode discharge device operating in the pseudospark regime to the output of a high power pulse line accelerator. Very small diameter (∼1 mm) electron beams with currents in the range 500–1000 A and energies in the range 150–300 keV have been generated with effective emittances estimated to be at or below 170 mm mrad. Such emittances are comparable to those achieved in conventional electron beam sources at current densities several orders of magnitude lower than those observed in these experiments.

Short-pulse high-power microwave propagation in the atmosphere

The propagation of high‐power (10–200 kW/cm2 ) short‐burst (3–30 ns) microwave pulses in the atmosphere has been studied experimentally. Microwave power from a large orbit gyrotron operating at 9.6 GHz is focused by a large‐diameter parabolic reflector into a test cell. The ambient pressure in the test cell was varied over a wide range and the microwave power density necessary for atmospheric breakdown has been determined as a function of ambient pressure and pulse duration. Measurements of the microwave pulse duration before and after breakdown have been obtained to determine the extent to which microwave energy is absorbed or reflected by the breakdown plasma. Results are compared with available theory and previously reported experiments.