High Power Backward Wave Oscillator Research Articles

High resolution rotation–inversion spectroscopy on doubly deuterated ammonia, ND 2H, up to 2.6 THz

Frequency multiplication of phase locked, high power backward wave oscillators (BWO) permitted to record high accuracy spectra of ND 2H up to frequencies of 2.6 THz. Novel superlattice (SL) devices were used as key-element to generate higher order harmonics for the first time in spectroscopic applications. In total, 240 measured ground state transitions of the pure rotation and inversion–rotation spectra of ND 2H were measured with accuracies of several kilohertz in the frequency region between 0.08 and 2.58 THz. Energy levels up to quantum numbers J=18 and K a=9 have been accessed. A greatly extended experimental dataset was obtained with significantly improved accuracies. Two sets of molecular parameters have been determined from which predictions can be generated that can be regarded as a reliable basis for future astronomical high resolution observations throughout the microwave to terahertz regions. These predictions are available in the Cologne database for molecular spectroscopy ( www.cdms.de).

Slow Cyclotron Instability in a High-Power Backward-Wave Oscillator

The linear dispersion relation of a backward-wave oscillator (BWO), derived earlier by the authors, is modified to include effects of RF surface current at the beam-vacuum interface. This modified dispersion relation results in an unstable interaction between the slow cyclotron mode (SCM) and the structure mode in addition to the conventional Cherenkov instability caused by the slow space charge mode. Numerical analysis is then carried out using parameters of a BWO experiment at University of Maryland. Fine structure of the SCM instability is elucidated. The analysis indicates that BWO radiation would not be suppressed near cyclotron absorption in an infinitely long system.

Range of validity of the Rayleigh hypothesis.

The parameter range over which the Rayleigh hypothesis (RH) for optical gratings might be validly applied to analysis of high power backward wave oscillators has been investigated numerically. It had been pointed out that from a rigorous mathematical viewpoint, RH was only valid for a shallow corrugation of slow wave structure (SWS) such that h K0 <0.448; here, h and K0 are, respectively, the amplitude and wave number of the periodicity in a sinusoidal planar grating. We numerically analyze the electromagnetic fields in the axisymmetric SWS with and without use of RH. The field patterns and eigenfrequency for the SWS are solved numerically for a given k(z) by using the code HIDM (higher order implicit difference method) that is free from the RH. It is found that, for a deep corrugation, h K0 =5 x 0.448, using RH is still valid for obtaining the dispersion relation, although the Floquet harmonic expansion (FHE) fails to correctly represent the field patterns inside the corrugation. Accordingly, there exists a discrepancy between the validity of using RH for obtaining dispersion relations and for an exact convergence of FHE everywhere in the SWS.

Comment on “Experimental Study of a High-Power Backward-Wave Oscillator Operating Far from the Upper Cutoff”

Comment on “Experimental Study of a High-Power Backward-Wave Oscillator Operating Far from the Upper Cutoff”

Efficiency enhancement in high power backward-wave oscillators

High power microwave (HPM) sources based on the backward-wave oscillator (BWO) have been investigated for the past two decades primarily because of their potential for very high efficiency (15 to 40%) operation. Several different effects have been proposed to explain this high efficiency compared to conventional BWOs. One of the major contributors to the high efficiency of the plasma-filled Pasotron HPM BWO source is the presence of optimally phased end reflections. The Pasotron uses a long-pulse (/spl ges/100 /spl mu/s) plasma-cathode electron-gun and plasma filled slow-wave structure to produce microwave pulses in the range of 1 to 10 MW without the use of externally produced magnetic fields. The efficiency of the Pasotron can be enhanced by up to a factor of two when the device is configured as a standing-wave oscillator in which properly phased reflections from the downstream collector end of the finite length SWS constructively interfere with the fundamental backward-wave modes and improve the coupling of the beam to the circuit. Operation in this configuration increases the efficiency up to 30% but causes the frequency to vary in discrete steps and the output power to change strongly with beam parameters and oscillation frequency.

Experimental Study on Quasi-TE Mode Radiation from a High Power Backward Wave Oscillator

The dispersion relation of quasi- TE l s modes in sinusoidally corrugated metal wall slow wave structure with an annular electron beam is derived and analyzed numerically. Results of the numerical analysis for quasi- TE 11 mode and conventional TM 01 mode are compared with experimental results. TE 11 mode in addition to TM 01 mode microwave oscillation is observed near 2π mode region of operation. It is confirmed that oscillation property of both modes agree fairly well with numerically predicted values. The study of quasi-TE mode oscillation is required for an optimum design of a high power backward wave oscillator that works in a single frequency over a wide range of operating conditions.

Electron density measurements during microwave generation in a high power backward-wave oscillator

Laser interferometry is used for the first time to measure plasma electron density along the slow wave structure (SWS) wall during microwave generation in a vacuum, long pulse, high power backward-wave oscillator (BWO). The University of New Mexico long pulse backward-wave oscillator, which displays the characteristic pulse shortening phenomenon, is investigated in these studies. Although pulse shortening is observed across a wide class of high power microwave devices, its origin is not definitively understood. Many hypotheses suggest that the unintentional introduction of plasma into the interaction region near the walls of the SWS is one of several likely causes of pulse shortening in intense electron beam driven slow wave devices. This article presents initial measurements of the line-integrated, temporally resolved plasma density between an intense, relativistic, annular electron beam and SWS walls for a variety of radiated microwave peak power levels. Line-integrated electron densities, , between 9.10/sup 15/ and 2.410/sup 16/ cm/sup -2/ for radiated microwave powers between 20 and 120 MW have been measured. The two main sources of the measured electron density are postulated to be (i) plasma generated from the cutoff neck due to beam scrape off, and (ii) material removed and ionized from the SWS walls during microwave generation.

Pulse shortening causes in high power BWO and TWT microwave sources

Pulse shortening, an effect where the microwave output power from a high-power tube terminates or significantly degrades well before the end of the electron beam pulse, severely limits the energy per pulse and average power capability of many high power microwave (HPM) sources. The cause of pulse shortening varies from device to device, and different causes can simultaneously contribute to the observed power reduction behavior which tends to obscure the underlying mechanisms and possible solutions. In this paper, we show a variety of experimental situations that lead to pulse shortening in HPM sources. The mechanisms of the different pulse shortening triggers are examined in detail in high-vacuum traveling wave tubes (TWT) and plasma-filled backward-wave oscillators (BWO). We find that there are many different causes of pulse shortening such as arcing, mode competition, beam instability, etc. However, the most commonly observed situation that leads to pulse shortening is the combination of sufficiently high power electron beams and poor vacuum conditions that lead to plasma generation. The presence of plasma significantly modifies the beam coupling to the circuit, which can affect the microwave production efficiency on very short time scales. The situations lending to pulse shortening and possible solutions are presented.

Electromagnetic properties of periodic cavities coupled to a radiating antenna

The electromagnetic properties of spatially periodic cavities determines both the linear and nonlinear interaction between the waves and the electron beam in high-power backward-wave oscillators. A corrugated cavity is usually left open at one end for extraction of the useful microwave energy; however, reflections at the open end are large so that a cavity is still formed. In contrast to a previously studied N period closed cavity where the number of axial modes with frequencies falling in the lowest pass band of the structure is equal to N+1, an open cavity was found to support only N axial modes. In this paper the resonance frequencies, quality factors and the field patterns of the axial modes in an open cavity were all investigated experimentally and the results are in very good agreement with those obtained using a new, time-dependent, quasi-three dimensional code. It was also demonstrated that a short interface section between the periodic cavity and the radiating antenna can drastically reduce the quality factors to the diffraction limit over a very wide frequency band. This is expected to substantially increase the starting current and allow operation at high-beam current without degradation of spectral purity.

Toward "smart tubes" using iterative learning control

In the paper, we present our progress toward designing a "smart" high-peak power microwave (HPM) tube. We use iterative learning control (ILC) methodologies in order to control a repetitively pulsed high-power backward-wave oscillator (BWO). The learning-control algorithm is used to drive the error between the actual output and its desired value to zero. The desired output may be a given power level, a given frequency, or a combination of both. The learning-control methodology is then verified in simulation. This methodology is applicable to a wide variety of HPM sources.

Propagating quasi-TE modes in a vacuum axisymmetric corrugated-wall waveguide

The nonaxisymmetric quasi-TE modes in an axisymmetric corrugated-wall metal slow-wave structure are analyzed numerically and confirmed to coincide with test measurement. In our experiment of high-power backward-wave oscillators (BWOs), the oscillation of TE/sub 11/ mode was observed below the starting condition of the conventional axisymmetric TM/sub 01/ mode that has been mostly assumed in previous reports. The existence of nonaxisymmetric TE/sub 11/ mode must not be ignored for improved performance with a single frequency operation of high-power BWOs.

Initial plasma-filled backward-wave oscillator experiments using a cathode-mounted plasma prefill source

The introduction of plasma into the slow-wave structure (SWS) of a backward-wave oscillator (BWO) has been shown to increase microwave power output and generation efficiency, as well as provide several other benefits. Researchers at Niigata University in Japan have performed a linear analysis that has shown that the optimal plasma-filled BWO configuration is one in which the plasma is confined close to the axis of the SWS, while the electron beam driving the device is kept close to the walls. Previous schemes for preinjecting plasma have utilized sources mounted downstream of the SWS and have relied upon the converging field lines of the electron beam's guiding magnetic field to compress the plasma close to the axis as it entered the SWS. This paper presents data from experiments using a plasma source mounted internal to the cathode structure of a high-power BWO, thus providing injection of the plasma directly on axis in the strong, uniform field region of the BWO. Data describing plasma prefill density at different axial positions within the SWS prior to operation is presented. The plasma prefill has been found to both enhance and reduce the microwave generation efficiency, depending on the plasma density. Details of the novel plasma injection system are also provided.

Experimental Study of a High-Power Backward-Wave Oscillator Operating Far from the Upper Cutoff

The characteristics of a high-power backward-wave oscillator (BWO) operating far from the upper cutoff, i.e., “π” mode of a corrugated waveguide, are investigated experimentally. The high spectral purity of output power of the TM 01 mode is maintained for large beam currents, because the starting currents of device have large values in this regime. It is observed that, when the beam currents are less than the starting current of the TM 01 mode, hybrid TE 11 mode is excited in the BWO. Peak output powers of 5.3 MW at 7.2 GHz in TE 11 mode and 6.8 MW at 8.5 GHz in TM 01 mode are generated. The conversion efficiencies are about 13% and 11%, respectively. These experimental results indicate a possibility of improved performance in high-power and high efficiency relativistic BWO by controlling the operation regime.

Electron emission from slow-wave structure walls in a long-pulse, high-power backward wave oscillator

Pulse shortening is a phenomenon common among all long-pulse, electron-beam-driven high-power microwave sources. Although the electron beam driving the source may continue to propagate through the interaction region of the device for several microseconds or more, the duration of the emitted microwave pulse is typically no more than /spl sim/100 ns. Most explanations of this phenomenon put forth involve the introduction of plasma into the interaction region and/or the degradation of beam quality. This paper describes experiments conducted on the University of New Mexico's Long-Pulse Backward Wave Oscillator (BWO) Experiment which investigate the behavior of beam electrons in the slow-wave structure (SWS) during microwave generation. A current probe having a small aperture at a variable radius is placed within the SWS to monitor the beam current profile at different radii as a function of time. Results from these experiments reveal the appearance of electrons between SWS ripples at times corresponding to the peaks of the microwave pulses in separate experiments. A drop in the main beam current is observed shortly thereafter. The source of the electrons within the ripples is thought to be field emission or secondary electron emission from the SWS walls or emission from plasmas generated there.

Linear Dispersion Relation of Double Period Slow Wave Structure for High-Power Backward Wave Oscillators

Linear dispersion relation for an axisymmetric metal-wall slow wave structure (SWS) with double period corrugations has been analyzed numerically. The double period SWS is here proposed for improving the oscillation frequency performance of high-power backward wave oscillators that may sustain stable oscillation despite the energy exhaustion of the driving electron beam from upstream to downstream in the SWS. It is shown that conventional TM 01 mode in a single period SWS is split into a number of isolated branches. Oscillation frequencies are kept comparatively unchanged rather than the case of single period SWS.

Enhanced frequency agility of high-power relativistic backward wave oscillators

This paper describes how finite length effects in high-power backward wave oscillators can be exploited in a controlled manner to achieve enhanced frequency agility. Experiments were performed using a Sinus-6 high-power relativistic repetitively pulsed electron beam accelerator. A uniform slow wave structure was used in these studies and its parameters were fixed. Sections of smooth-walled circular waveguide of varying lengths were inserted both before and after the slow wave structure. Variations in the length of smooth-walled waveguide on the order of a quarter-wavelength of the generated electromagnetic radiation were found to significantly affect both microwave frequency and radiation efficiency in a periodic-like manner. The experimental results were reproduced in TWOQUICK electromagnetic particle-in-cell simulations. A bandwidth of about 500 MHz centered around 9.5 GHz at hundreds of MW power levels has been achieved with constant beam and slow wave structure parameters.

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.

A neural-network model of the input/output characteristics of a high-power backward wave oscillator

This paper discusses an approach to model the input/output characteristics of the Sinus-6 electron beam accelerator-driven backward wave oscillator. Since the Sinus-6 is extremely fast to warrant the inclusion of dynamical effects, and since the sampling interval in the experiment is not fixed, a static continuous neural network model is used to fit the experimental data. Simulation results show that such a simple nonlinear model is sufficient to accurately describe the input/output behavior of the Sinus-6-driven backward wave oscillator (BWO) and that the fitted output waveforms are basically noiseless. This model will be used to control the BWO in order to maximize the radiated power and the efficiency. This paper is also intended to introduce high-power microwave researchers to control concepts that may enhance the outputs of a wide spectrum of sources.

High-power microwave-induced TM/sub 01/ plasma ring

Open-shutter photography was used to capture the air breakdown pattern induced by a TM/sub 01/ mode radiated by a high-power backward wave oscillator. The resultant plasma ring was formed in air adjacent to a conical horn antenna fitted with a membrane to keep the experiment under vacuum. This image was digitized and further processed using Khoros 2.0 software to obtain the dimensions of the plasma ring. This information was used in an air breakdown analysis to estimate the radiated power, and agrees within 10% with the power measured using field mapping with an open-ended WR-90 waveguide.

A Quasi-Optical Multiplier for Terahertz Spectroscopy

Abstract The rotational K-structure (K = 0, ... 6) of the a-type transition (J = 65 - 64) of the symmetric top molecule methylcyanide, CH3CN, has been recorded at 1.2 THz employing a newly designed quasi-optical frequency multiplier. A planar Schottky diode is used as non-linear element for producing the harmonic power and is mounted in the focus of an off-axis antenna across a short single ridged waveguide. The 2nd harmonic of the fundamental frequency is filtered with adichroic plate and is focused onto the InSb bolometer after passing an absorption cell. The multiplier is driven by a high power backward wave oscillator with a fundamental frequency near 600 GHz.