Compact circular waveguide TM 02 ‐TE 11 mode converter

In recent years, R&D for pulse compression and power distribution systems for the Next Linear Collider has led to the invention of many novel rf components, some of which must handle up to 600 MW of pulsed power at X‐band. These include passive waveguide components, active switch designs, and non‐reciprocal devices. Among the former is a class of multi‐moded, highly efficient rf components based on planar geometries with overmoded rectangular ports. Multi‐moding allows us, by means of input phasing, to direct power to different locations through the same waveguide. Planar symmetry allows the height to be increased to improve power handling capacity. Features that invite breakdown, such as coupling slots, irises and H‐plane septa, are avoided. This class includes hybrids, directional couplers, an eight‐port superhybrid/dual‐mode launcher, a mode‐selective extractor, mode‐preserving bends, a rectangular mode converter, and mode‐mixers. We are able to utilize such rectangular waveguide components in systems incorporating low‐loss, circular waveguide delay lines by means of specially designed tapers that efficiently transform multiple rectangular waveguide modes into their corresponding circular waveguide modes, specifically TE10 and TE20 into circular TE11 and TE01. These extremely compact tapers can replace well‐known mode converters such as the Marié type. Another component, a reflective TE01‐TE02 mode converter in circular waveguide, allows us to double the delay in reflective or resonant delay lines. Ideas for multi‐megawatt active components, such as switches, have also been pursued. Power‐handling capacity for these is increased by making them also highly overmoded. We present a design methodology for active rf magnetic components which are suitable for pulse compression systems of future X‐band linear colliders. We also present an active switch based on a PIN diode array. This component comprises an array of active elements arranged so that the electric fields are reduced and the power handling capability is increased. Novel designs allow these components to operate in the low‐loss circular waveguide TE01 mode. We describe the switching elements and circuits.

Research is in progress on a TeV-scale linear collider that will operate at 5-10 times the energy of present-generation accelerators. This will require development of high power RF sources generating of 50-100 MW per source. Transmission of power at this level requires overmoded waveguide to avoid breakdown. In particular, the TE{sub 01} circular waveguide mode is currently the mode of choice for waveguide transmission at Stanford Linear Accelerator Center (SLAC) in the Multimode Delay Line Distribution System (MDLDS). A common device for protecting an RF source from reflected power is the waveguide circulator. A circulator is typically a three-port device that allows low loss power transmission from the source to the load, but diverts power coming from the load (reflected power) to a third terminated port. To achieve a low loss, matched, three port junction requires nonreciprocal behavior. This is achieved using ferrites in a static magnetic field which introduces a propagation constant dependent on RF field direction relative to the static magnetic field. Circulators are currently available at X-Band for power levels up to 1 MW in fundamental rectangular waveguide; however, the next generation of RF sources for TeV-level accelerators will require circulators in the 50-100 MW range. Clearly,more » conventional technology is not capable of reaching the power level required. In this paper, we discuss the development of an X-Band circulator operating at multi-megawatt power levels in overmoded waveguide. The circulator will employ an innovative coaxial geometry using the TE{sub 01} mode. Difficulties in maintaining mode purity in oversized rectangular guide preclude increasing guide area to allow increasing the power level to the desired 50-100 MW range. The TE{sub 01} mode in circular waveguide is very robust mode for transmission of high power in overmoded waveguide. The mode is ideal for transmission of high power microwaves because of its low-losses, zero tangential field on the guide (which minimizes arcing problems) and reduced propensity for mode conversion compared to non-asymmetric circular waveguide modes. Unfortunately, no current designs exist for circulators using the circular TE{sub 01} mode. The basic building block for all low-loss circulators and isolators is a nonreciprocal element with a phase shift dependent on the propagation direction in the guide. Such an element can be constructed by placement of a hollow ferrite rod in a cylindrical waveguide. An inner conductor placed inside the ferrite rod conducts a current pulse that induces an azimuthal magnetic field inside the ferrite. This configuration is depicted in Figure 1a. An alternate configuration using permanent magnets is shown in Figure 1b. Either of these configurations will create a different phase shift for waves propagating in opposite directions along the waveguide axis. This feature can be used to develop a high power circulator. We are currently testing a TE{sub 01} nonreciprocal phase shifter in a 50 MW test stand. This device is in the configuration shown in Figure 1a. The induced differential phase shift and loss will be measured and compared to calculations.« less

In this paper, a dielectric-filled circular waveguide TM01-TE11 mode converter is proposed, which has high conversion efficiency and a wide operating bandwidth. Filling the circular waveguide with dielectric material changes the local propagation characteristics, thus achieving a phase difference between the TE11 modes in the two halves of the circular waveguide during propagation. This, in turn, facilitates the completion of mode conversion with high efficiency. Compared with the conventional radial dielectric plate, this paper improves the method of filling the dielectric inside the circular waveguide by transforming it into a coaxial structure. This is followed by the incorporation of a radial dielectric plate, a modification that has been proven to enhance the conversion efficiency and extend the operational bandwidth. The mode converter operates at 9.7 GHz, and when the dielectric filler material is polytetrafluoroethylene (PTFE), both simulation and practical studies are carried out. The simulation results demonstrate that the maximum conversion efficiency of this mode converter is 99.2%, and the bandwidth with conversion efficiency greater than 90% is nearly 21.1%. The maximum conversion efficiency in the actual test is essentially consistent with the simulation results. The validity of the design scheme of this converter and the accuracy of the simulation study are demonstrated.

Small circular waveguide mode converters used in the 28 GHz, 200 kW ECH system for the Hitachi tokamak HT-1 have been developed. In order to convert from the TE02 to the TE11 mode, TE0202-to-TE01 and TE01-to-TE11 mode converters are used. Both converters use periodic waveguide wall perturbations, the case of the former, circularly symmetric perturbations, and in the case of the latter, curvature perturbations. The lengths of the mode converters were successfully shortened with high mode conversion efficiencies by decreasing the number of periods and then optimizing the periods using quite a small diameter for the converters. The 30ϕ TE02-to-TE01 mode converter is 103 · 34 mm long and its mode conversion efficiency measured by a scalar network analyser is 96% at 28GHz. The TE01-to-TE11 mode converter is 427 · 38 mm long and has an efficiency of 79%, which is 10% lower than the design value. This discrepancy resulted mainly from mode conversion from the TE11 to the TE12 mode, caused by the smaller number of...

The characteristics of a 35-GHz oscillator operating with the TE/sub 01/ circular waveguide mode are described. The device produced 147 kW, with an efficiency of 31 percent at 100 kW. The total radiated energy was 2 kJ/pulse. The spectral coherence appears to be equal to those of other high-quality microwave tubes. The mode purity is greater than 95 percent.

In this paper, a compact, high efficiency TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> to TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode converter is presented. In the proposed structure, dielectric material is loaded below a tapered centre conductor, in lower section of a circular waveguide. The input TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode is initially transformed to TEM mode by the tapered centre conductor. The loaded dielectric material is designed to provide the desired phase shift for the mode conversion. Different dielectric materials are analyzed for loading to achieve high conversion efficiency and wide bandwidth. The proposed mode converter is fabricated with PTFE as dielectric for mode conversion and the results are verified through far field radiation pattern measurements. The mode conversion efficiency is more than 90% for 2.95-3.60 GHz band. Due to compactness and high conversion efficiency for wide bandwidth, it is suitable for S-band high power microwave (HPM) applications.

The paper presents the results of the computation of the propagation parameters (eigenvalue and attenuation constant) of the modes in circular waveguide partially filled with a dielectric ring. A new classification of the modes considering their propagating characteristics is introduced. The numerical results are presented under a normalized form which allows a general applicability for designing resonators and filters. Ohmic and dielectric contributions to the attenuation constant have been represented separately, again suitably normalized; criteria for selecting geometrical dimensions in order to minimize losses are discussed.

The use of TE12 in circular waveguide with smooth walls was suggested for low-loss transport of rf signals in multimoded systems [S. G. Tantawi et al., in Advanced Accelerator Concepts: Eighth Workshop, edited by Wes Lawson, AIP Conf. Proc. No. 472 (AIP, New York, 1999), pp. 967–974]. Such systems use the same waveguide to transport different signals over different modes. In this report we detail a series of experiments designed to measure the characteristics of this mode. We also describe the different techniques used to generate it and receive it. The experiments were done at X band around a frequency of 11.424 GHz, the frequency of choice for future linear colliders at X band [The NLC Design Group, Report No. LBNL-PUB-5424, SLAC Report No. 474, Report No. UCRL-ID 124161, 1996; The JLC Design Group, KEK-REPORT-97-1, 1997]. The transportation medium is 55 m of highly overmoded circular waveguide. The design of the joining flanges is also presented.

This paper presents the design of a unique multimode extractor and converter of higher order modes of a circular waveguide. The multi-mode extractor couples first five circular waveguide propagation modes TE11, orthogonal TE11(o), TM01, TE21, and TE21(o) that includes the three tracking signals (i.e. sum signal, elevation signal, and azimuth signal) used in a high-frequency monopulse tracking system and two communication channels for transmitting and receiving signal at Ka band. This design is based on amplitude and phase characteristics of the higher-order modes TM01, TE21, and TE21(o) excited in circular waveguide. TE21 and TE21 (o) modes are extracted from circular waveguide to TE10 mode in rectangular waveguide by two longitudinal slots milled in the circular waveguide. TE11 mode is coupled using two coupling slots and H-plane power combiner. A turnstile junction is used to extract TM01 and TE11(o) mode. This multimode tracking system is designed both for linear or circular polarized ground earth station antennas to track orbiting satellites.

A compact mode converter that transforms TM01 circular waveguide mode to TE10 rectangular waveguide mode is investigated. It consists of a circular waveguide with a short circuit terminal and a rectangular waveguide which is perpendicular to the circular waveguide. Simulation results show that conversion efficiency of the mode converter is about 99.8% at central frequency of 1.75 GHz, and the total return loss is approximately -30 dB. The experimental results are well consistent with the computer simulation, which demonstrates the feasibility and high power handling capacity of the mode converter.

The results of a detailed theoretical study on collinear guided wave to leaky wave acoustooptic (AO) interactions in proton-exchanged LiNbO(3) (PE:LiNbO(3)) planar waveguides are presented. The guided-to-leaky mode conversion for an input optical beam at the wavelength of 632.8 nm by the induced diffraction grating from a collinear surface acoustic wave (SAW) is analyzed using a generalized multimode formulation of the coupled mode theory. Mode conversion efficiency and AO bandwidth have been calculated as functions of acoustic frequency, interaction length, guiding layer thickness, and acoustic drive power density for three cuts of the LiNbO(3) substrate. High performance configurations that are desirable for application to demultiplexing and switching in optical communication systems are identified, and the corresponding channel capacity and frequency resolution are determined. For example, it was shown that the X-cut configuration features the highest mode conversion efficiency. However, a relatively small AO bandwidth is associated with this configuration. Both high mode conversion efficiency and large AO bandwidth can be accomplished at the guiding layer thickness of 1.0 microm. A TM(o)-->TE(nu) mode conversion efficiency as high as 42% together with an AO bandwidth of approximately 70 MHz can be achieved in the Z-cut waveguide at the guiding layer thickness of 1.0 microm, acoustic drive power density of 50 mW/mm, interaction length of 40 mm, and acoustic frequency of 460 MHz. The corresponding channel capacity and frequency resolution are 745 and 0.09 MHz, respectively. Measured mode conversion efficiencies as high as 90 and 78% obtained at the acoustic frequencies of 107 and 367 MHz using the X-cut substrate and the Y-propagation SAW have verified the theoretical prediction on the mode conversion efficiencies.

A Fabry-Perot–like interferometer with two microwaveguide adaptors as reflectors creates a passive dielectric medium with a negative group delay time due to polarization shift. In that case rectangular with circular waveguides are adapted. A rotational strain of the polarization vector by one of the adaptors is coupled with the excitation of a drastic negative group velocity. The adapted rectangular and circular waveguides have the same dispersion. The input rectangular waveguide mode is linearly polarized, whereas the basic mode of the adapted circular waveguide is circularly polarized. A 20 m long circular waveguide connects the input with the output adaptor. Experiments are reproduced in the frequency and in the time domain. A polarization turn of the two different handed reflected circular wave modes causes the observed negative group velocity.

Research is in progress on a TeV-scale linear collider that will operate at 5-10 times the energy of present-generation accelerators. This will require development of high power RF sources generating of 50-100 MW per source. Transmission of power at this level requires overmoded waveguided to avoid breakdown. In particular, the TE/sub 01/ circular waveguide mode is currently the mode of choice for waveguide transmission at Stanford Linear Accelerator Center (SLAC) in the Multinode Delay Line Distribution System (MDLDS). In this paper, we discuss the development of an X-band circulator operating at multi-megawatt power levels in overmoded waveguide. The circulator will employ an innovative coaxial geometry using the TE/sub 01/ mode. Difficulties in maintaining mode purity in oversized rectangular guide preclude increasing guide area to allow increasing the power level to the desired 50-100 MW range.

The deliberate use of two or more propagating modes in a multimode waveguide, and a knowledge of associated control elements, has assumed renewed importance, particularly for millimeter wavelength applications. This paper presents a resonance measurement technique for the precise evaluation of the equivalent network for a lossless shunt discontinuity coupling two nondegenerate modes in a multimode waveguide. The discontinuity structure is placed into a cavity closed by adjustable plungers, and the data consists of those plunger positions which render the cavity resonant in the two modes of interest. This multipoint data is then transformed to permit an analysis of the two-port network in the discontinuity plane by conventional techniques. Computations and experimental results obtained at S band illustrative of the procedure are presented for shunt discontinuities coupling the E/sub 01/ and H/sub 01/ modes in circular waveguide. The accuracy achieved is comparable to that obtained in single mode precision measurements.

For each mode of propagation in waveguide, there is a critical (cutoff) frequency below which waves do not propagate. In practice, it is preferable to restrict the propagation to a single mode. If the dominant (TE-11) mode in circular waveguide is to be utilized, other modes may be excluded by operating at frequencies below the cutoff of the next mode (usually the TM-01). Thus the ratio of the TM-01 cutoff frequency over the TE- 11 limits the waveguide's useful bandwidth. This ratio is 1.31 for uniform filling, and may be increased to 1.41 by a dielectric lining in the waveguide. A much greater ratio can be achieved by periodically loading the waveguide with discs of high dielectric constant, as shown in Fig. 1. The waveguide diameter is decreased to raise the cutoff frequency of the TM-01 mode, and the dielectric discs are inserted to lower the TE-11 cutoff frequency. As will be shown, the discs have relatively little effect on the TM-01 mode.