Backward Wave Oscillation Power Research Articles

Experimental study on performance change in F-band surface-wave oscillator due to slow-wave structure length

A surface-wave oscillator (SWO) is a high-power terahertz-wave source based on a slow-wave structure (SWS). A cylindrical corrugated waveguide is used as the SWS. The length of SWS is an important factor in the oscillation. Experiments of the SWO with various length structures are conducted. An instability responsible for the SWO changes depending on the SWS length. For the 200-period SWS, the radiation is due to the absolute instability and depends on the beam current like the temporal growth rate. By contrast, for the 80-period SWS, the radiation is due to the global instability having the current dependence like the spatial growth rate. Compared to the 80-period SWS, the 200-period SWS can improve the radiation power more than seven times in the backward wave oscillation (BWO) region. The maximum BWO power reaches up to about 10 kW and the maximum electric efficiency of 1.5% is obtained.

Impact of Thermal State on Backward-Wave Oscillation in Helix TWT Under Operation

The impact of thermal state on backward-wave oscillation (BWO) in a helix traveling-wave tube (TWT) under operation is explored, considering total thermal losses induced by high-frequency electromagnetic field power dissipation on helix, output port, carbon-coating attenuators, support rods, inner wall of the barrel, and the electron beam collision on the helix. By numerically solving the Maxwell equations, the electron motion equation, the heat conduction equation, and the thermal expansion equation, we analyze integrally the beam-wave interaction process and the resulting thermal distribution as well as thermal deformation of both slow wave structure and the output port. The simulation results for a broadband (8–18 GHz) helix TWT show the temperature on the last 20 turns of the helix is the highest, with a peak value of 347 °C at the end of helix. The greatest thermal deformation (0.0151 mm) is located on the inner conductor of the output port. The experimental result for the saturated output power of the TWT prototype shows a BWO power hole at 11 GHz, which is not found in simulation without considering thermal deformation. We find also that, considering the thermal deformation of helix and output port, BWO occurs at 22.4 GHz with the amplitude of 0.0708 V, which is over 5 times of that without considering thermal deformation, and it induces the same power hole as the experimental one at 11 GHz. Thermal deformation is one of the contributing factors for BWO in the helix TWT.

Extremely High Peak Power Obtained at 29 GHZ Microwave Pulse Generation

The paper presents research results on enhancing the peak power of microwave pulses with sub- and nanosecond length using a backward-wave oscillator (BWO) operating at 29 GHz frequency and possessing a reproducible phase structure. Experiments are conducted in two modes on a high-current electron accelerator with the required electron beam power. In the first (superradiation) mode, which utilizes the elongated slow-wave structure, the BWO peak power is 3 GW at 180 ns pulse duration (full width at halfmaximum, FWHM). In the second (quasi-stationary) mode, the BWO peak power reaches 600 MW at ~2 ns pulse duration (FWHM). The phase spread from pulse to pulse can vary from units to several tens of percent in a nanosecond pulse mode. The experiments do not show any influence of microwave breakdown on the BWO power generation and radiation pulse duration.

Suppression of In-Band Power Holes in Helix Traveling-Wave Tubes

Slow-wave structures employed in forward-wave linear beam amplifiers such as wideband helix traveling-wave tubes (TWTs) always support backward waves. A backward-wave space harmonic of a signal at a harmonic of a drive signal can effectively interact with a slow space-charge wave of an electron beam. In this case, the backward wave can compete with a forward wave, which results in backward-wave oscillation (BWO) power holes under large signal conditions. Sufficiently large power holes have serious consequences for the performance of a system in which the TWT is employed. In this paper, we report a study of BWO power holes simulated using a 2.5-D large signal helix TWT code and demonstrate an effective method, the pitch step method, to suppress BWO power holes.

Nonlinear Analyses of the Parasitic Backward-Wave Oscillation Power in the Magnetically Focused Pulsed Helix Traveling-Wave Tube Amplifier in the Absence of the Amplified Signal

This paper analyzes parasitic backward-wave oscillation power as a function of interaction length and focusing magnetic field parameters in a generic helix traveling-wave tube (TWT) amplifier in the absence of amplified signal. The permanent periodic magnetic (PPM) focusing of the electron beam, the relatively narrowband (less than one octave) TWT, the electron gun with no control grid, the pulsed full beam current cutoff operating mode of the TWT, and the accelerating voltage pulse wider than the amplified RF pulse are considered. Under such conditions, the parasitic backward-wave oscillation can build up at the leading and trailing edge and at the top of the accelerating pulse before and after the input RF signal is applied when the instantaneous accelerating voltage provides for the synchronism condition. The parasitic backward-wave oscillation, although nondesirable in general, can be tolerated if its power does not exceed some allowable level. In this paper, the nonlinear (large-signal) theory of beam-wave interaction in the TWT in the specified case is developed. The theory accounts for the interaction of the multiple harmonics of the backward wave of the slow-wave circuit with the electron beam that alternatively changes the direction of its rotation on each half period of the focusing magnetic field. A system of equations, which makes accessible the start oscillation length and the starting Pierce relative velocity parameter as a function of the electrical parameters of the TWT, the PPM focusing field period, and the magnetic flux density distribution in the large-signal regime, is obtained. A particular numerical example reveals the relation between interaction length, PPM focusing field period, and parasitic backward-wave oscillation power. The approach permits one to design a TWT having the maximum possible interaction length under the allowable parasitic backward-wave oscillation power. Also, the results demonstrate that the focusing magnetic field parameters have a significant effect on the interaction of the rotating electron beam with the backward wave in the nonlinear regime, as they have in the linear regime

Twin traveling-wave tube amplifiers driven by a relativistic backward-wave oscillator

Experiments demonstrate stable frequency and relative-phase angle output from twin traveling-wave tube (TWT) amplifiers driven with the redirected signal from a high-power backward-wave oscillator (BWO). The experimental X-band apparatus employs a single generator to produce three independent electron beams which simultaneously drive the BWO and TWT sources. The BWO spontaneously generates up to 14.1 MW peak, 25 to 15-ns long pulses over a current-tunable bandwidth of 9.6%. The BWO power extracted upstream is split and redirected into twin TWT's for amplification. The TWT's produce up to 9.0 MW pulses over an 800 MHz instantaneous bandwidth. Across the amplifier's full-width half-maximum pulse duration of 10 to 20 ns, a relative-phase angle of better than 15/spl deg/ is maintained between TWT's for an 11.0 to 11.7 GHz range. Experiments characterize the gain, relative-phase angle, and efficiency of twin-TWT output as a function of RF-drive frequency and beam current. These experiments are the first to demonstrate the feasibility of relativistic TWT's for phased-array applications, and increase the limited data base documenting relativistic-TWT operation.

Electric field measurement in a plasma-filled X-band backward wave oscillator

The electric field in a plasma-filled X-band backward wave oscillator (BWO) was measured by the Stark-effect method. Field strengths were as high as 110 kV/cm when the BWO power level was ∼75 MW. Observed electric fields lasted throughout the ∼60 ns high power microwave pulse. After this pulse, fields 30 to 40 kV/cm persisted until beam shutoff, presumably arising from beam-plasma turbulence and low power level electromagnetic radiation.

Experimental study of a plasma-filled backward wave oscillator

Experimental studies of a plasma-filled X-band backward-wave oscillator (BWO) are presented. Depending on the background gas pressure, microwave frequency upshifts of up to 1 GHz appeared along with an enhancement by a factor of 7 in the total microwave power emission. The bandwidth of the microwave emission increased from <or=0.5 GHz to 2 GHz when the BWO was working at the RF power enhancement pressure region. The RF power enhancement appeared over a much wider pressure range in a high beam current case (10-100 mT for 3 kA) than in a lower beam case (80-115 mT for 1.6 kA). The plasma-filled BWO has higher power output than the vacuum BWO over a broader region of magnetic guide field strength. Trivelpiece-Gould modes (T-G modes) are observed with frequencies up to the background plasma frequency in a plasma-filled BWO. Mode competition between the T-G modes and the X-band Tm/sub 01/ mode prevailed when the background plasma density was below 6*10/sup 11/ cm/sup -3/. At a critical background plasma density of approximately=8*10/sup 11/ cm/sup -3/ power enhancement appeared in both X-band and the T-G modes. Power enhancement of the S-band in this mode collaboration region reached up to 8 dB. Electric fields measured by the Stark-effect method were as high as 34 kV/cm while the BWO power level was 80 MW. These electric fields lasted throughout the high-power microwave pulse.<<ETX>>