Extended Interaction Klystrons Research Articles

An Angular Radial Extended Interaction Amplifier at the W Band

In this paper, an angular radial extended interaction amplifier (AREIA) that consists of a pair of angular extended interaction cavities is proposed. Both the convergence angle cavity and the divergence angle cavity, which are designed for the converging beam and diverging beam, respectively, are investigated to present the potential of the proposed AREIA. They are proposed and explored to improve the beam-wave interaction capability of W-band extended interaction klystrons (EIKs). Compared to conventional radial cavities, the angular cavities have greatly decreased the ohmic loss area and increased the characteristic impedance. Compared to the sheet beam (0°) cavity, it has been found that the convergence angle cavity has a higher effective impedance and the diverging beam has a weaker space-charge effect under the same ideal electron beam area; the advantages become more obvious as the propagation distance increases. Particle-in-cell (PIC) results have shown that the diverging beam (8°) EIA performs better at an output power of 94 GHz under the condition of lossless, while the converging beam (-2°) EIA has a higher output power of 6.24 kW under the conditions of ohmic loss, an input power of 0.5 W, and an ideal electron beam of 20.5 kV and 1.5 A. When the loss increases and the beam current decreases, the output power of the -2° EIA can be improved by nearly 30% compared to the 0° EIA, and the -2° EIA has a greatly improved beam-wave interaction capacity than conventional EIAs under those conditions. In addition, an angular radial electron gun is designed.

Analysis and Improvement of Performance Instability in Extended Interaction Klystrons With Random Geometrical Perturbations

This article describes and evaluates the effect of random geometrical perturbations on resonance characteristics and output performance of extended interaction klystrons (EIKs). The perturbations, assuming a normal distribution, will result in random variation in resonance characteristics, including resonant frequency shift, quality factor shift, and field distortion. The results are demonstrated in a 220-GHz extended interaction cavity by combining theory, simulations, and experimental measurements. In addition, 3-D particle-in-cell (PIC) simulation and small-signal theory are employed to gain insights into the output performance instabilities caused by variation in the resonance characteristics. The comprehension of the factors that contribute to performance instability is fundamental to avoid excess costs in the fabrication process. Aided by the small-signal theory code with a calculation speed that is several orders faster than PIC simulation, we address the instability by applying cathode voltage adjustment technology, frequency tuning technology, and multiparameter optimization based on a genetic algorithm. Such analysis and improvement patterns are general for the EIKs operating in the terahertz regime.

Overlapping-Mode Extended Interaction Klystrons for Broadband Terahertz Power Amplifiers

The design approaches of the overlapping-mode extended interaction klystrons (EIKs) are presented to satisfy the requirement for broadband terahertz power amplifiers. Since all the cavities are designed and tuned to operate in multiple-cavity modes, the overlapping-mode EIKs can provide high output power over a wider frequency band. The circuit characteristics of the overlapping-mode EIKs are studied, including the dispersion curve, the coupling coefficient, and the frequency interval between cavity modes. Moreover, a six-cavity EIK operating at 0.34 THz is designed to demonstrate the broadband output capability of the overlapping-mode EIKs. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\pi $ </tex-math></inline-formula> -mode and its adjacent axial mode, the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> /13-mode, are chosen as the operating modes of the designed EIK. The 3-D particle-in-cell (PIC) simulation predicts a 3-dB bandwidth of 2.9 GHz for the designed power amplifier at a small-signal level or in saturation. Driven by an input power of 10 mW, the amplifier can provide the output power of 3.9 W, and the corresponding gain is 25.9 dB. When the input power is 150 mW, the amplifier is partially saturated and can provide the output power of 33 W with a corresponding electronic efficiency of 1.23%.

Design and Experiment of a Hollow Beam Electron Optics System for Ka-Band Extended Interaction Klystrons

In this article, the design and experiment of a hollow electron beam (HEB) electron optics system (EOS) for Ka-band extended interaction klystrons (EIKs) are presented. To realize the fast switch of the current emission at a low cutoff voltage, an electron gun with two focus electrodes (FEs) is chosen. The emitting surface of cathode is a ringed spherical surface. The first focus electrode (FE1) is around the cathode, and the second focus electrode (FE2) is set in the central hole of the cathode. The current emitted from the cathode surface is about 2A, when the operating voltage is 25 kV. Three simulation codes, Egun, Superfish, and CST, are used for designing the electron gun and the permanent magnet focusing system (PMFS). Some sensitivity analyses with respect to the PMFS’s critical parameters are presented. The simulation shows the dc beam transmission is 100%. The electron beam, confined by 6000 Gauss uniform field, propagates through the drift tube of 37 mm with good laminarity and small ripple. The emission current can be suppressed when the voltage of the FEs is −3.4 kV. According to the results of simulation, the Ka-band EIK had been constructed and the hot-test experiments had been fulfilled in succession. The experimental results exhibit that the dc beam transmission is about 99% and the beam can sustain 98% transmission at high frequency (HF) operation. In addition, the output power is higher than 8 kW in the bandwidth of over 100 MHz.

General electron conductance expression of the multi-gap extended interaction cavity

To predict the stability of a multi-gap extended interaction cavity used in the klystrons, a general electron conductance expression has been rigorously derived from the small-signal space charge wave theory, where the field shape at the gap edge is described by a uniform distribution or a hyperbolic cosine function. Moreover, it is also pointed out that a simplified expression is acceptable in applications. This formula can be readily applied to the solid and annular beams, and its validity has been demonstrated through the stability analysis of a triple-gap extended interaction cavity, where the effect of adopting different gap field shapes is presented. This general expression is very useful for the parameter choice during the cavity design of extended interaction klystrons.

Design and Simulation of High-Aspect-Ratio Sheet Beam EIK at 0.22 THz

To improve the output performance of 0.22 THz extended-interaction klystrons (EIKs), we utilized a 20:1-aspect-ratio sheet beam to drive the circuit. The applicable high-frequency structure is discussed to obtain effective characteristic impedance, which is conducive to stronger beam-wave interaction. In addition, the loaded waveguide and the tuning scheme in the input cavity are proposed to provide good electric field uniformity over the whole beamwidth. Moreover, the nonuniform period output circuit based on the phase dynamic synchronization technology is optimized to further enhance power. Meanwhile, considering the feasibility of cathode performance, the beam current density in the tunnel is limited to 330 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The designed four-cavity interaction circuit is comprised of two 9-gap idler cavities and 11-gap input and output cavities. With the operating parameters of 16.2 kV beam voltage and 0.66 A beam current, the particle-in-cell (PIC) simulation predicts a stable output power of 700 W with a 39 mW input excitation. The corresponding gain and 3 dB bandwidth are 42.5 dB and 500 MHz, respectively. An asymmetric single-stage depressed collector is also designed to recover the spent beam energy from the interaction circuit. The collector efficiency is 81.1% with zero electron reflux rate while taking secondary electrons into consideration, and the overall tube efficiency is 26.9%.

Recent advances in metamaterial klystrons

As a kind of artificially structured media, electromagnetic metamaterials (MTMs) have exotic electromagnetic properties that are not found or difficult to achieve in natural materials. This class of metal/dielectric-structured artificial media has attracted great attention during the past two decades and made important breakthroughs. A variety of passive and active devices based on MTMs have been developed rapidly. Especially MTM klystrons, which show very remarkable advantages, including miniaturization, high gain, and high efficiency in the microwave band. MTM extended interaction klystrons creatively combine the advantages of MTMs, extended interaction technology, and klystrons. It provides a new design idea for the development of brand-new klystrons with high performance. In this review paper, we report the recent advances in MTM klystrons including MTM extended interaction oscillator and MTM extended interaction klystron amplifier. Furthermore, the prospects and challenges of MTM klystrons are discussed. Finally, the development trend is concluded.

Demonstration of a High-Power Ka-Band Extended Interaction Klystron

The Ka-band extended interaction klystrons (EIK) have been developed at the Institute of Electronics, Chinese Academy of Sciences (IECAS), to satisfy the requirement for the high-power millimeter-wave sources in scientific researches. In this work, two kinds of high compression electron gun with the solid or hollow beams, the derivative electron optics systems using the compact permanent magnet uniform focusing and the shared efficient interaction circuit based on the multigap ladder cavity, have been studied in detail and well designed through a mass of calculations. After the mechanical design, finely manufacturing, and a series of technical processes, the Ka-band EIK prototype tubes have been successfully built and tested. For the solid beam scheme, the maximum output power achieves 20 kW in the bandwidth of 60 MHz, and, correspondingly, the gain of 53 dB and the efficiency of 24% are observed. The measured -1 and -3 dB bandwidths are about 220 and 350 MHz, respectively. Meanwhile, the beam transmission is excellent, that is over 97% in dc state and over 90% in RF operation. For the hollow beam case, an over 91% dc transmission test has finished.

Novel Coupling Cavities for Improving the Performance of G-Band Ladder-Type Multigap Extended Interaction Klystrons

A pair of ladder-type multigap cavities, namely, wide-coupling cavity and narrow-coupling cavity, were comparatively studied to improve the performance of $G$ -band extended interaction klystrons (EIKs). It is found that the narrow-coupling cavity has higher effective characteristic impedance and stronger electric field distribution which lead to relatively stronger beam–wave interaction strength and obviously better output characteristics. Particle-in-cell (PIC) simulation confirmed this conclusion. Under an working voltage of 16.5 kV, current of 0.3 A, and operating mode of $\pi $ mode, the narrow-coupling cavity structure has greater output power and wider bandwidth. Specifically, when the ratio of long and short slots is 1.13, 1.21, and 1.26, compared with the wide coupling cavity structure, the output power of the narrow coupling cavity structure is increased by 40, 90, and 50 W, and the bandwidth is significantly increased by 50, 100, and 200 MHz, respectively. Then a maximum output power of 400 W, a gain of 39 dB, a bandwidth of 500 MHz can be achieved with our narrow-coupling cavity structure EIK in $G$ -band. Thus, such structures show potential performance improvement of millimeter-wave and terahertz EIK.

Guest Editorial for the special issue on devices and circuits for millimeter‐wave and THz applications

Guest Editorial for the special issue on devices and circuits for millimeter‐wave and THz applications

Design and experimental analysis of W-band extended interaction klystron amplifier

Beam loading is an important parameter in extended interaction klystron, which can be used to analyze the influence of beam on resonant frequency and ohm loss &lt;i&gt;Q&lt;/i&gt;, and study the match condition between input cavity and external power source, etc. Based on the kinematical theory, law of induce current, principle of charge conservation under the small signal condition, and one-dimensional (1D) mode, the transit time effect of electron in &lt;inline-formula&gt;&lt;tex-math id="M1"&gt;\begin{document}${\text{π}}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20182194_M1.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20182194_M1.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; mode standing wave electric treld in a multiple-gap resonator is analyzed, and the expressions of electron load conductance and electron load susceptance in the multiple-gap resonator are presented. The results show that the electron load conductance of extended interaction cavity can change in a bigger extension than that of traditional single gap cavity, which means that the loaded &lt;i&gt;Q&lt;/i&gt; of extended interaction cavity can be adjusted in a bigger extension to realize a desired &lt;i&gt;Q&lt;/i&gt;. And the results also show that the electron load susceptance of extended interaction cavity can change in a bigger extension than that of traditional single gap cavity, which means that the loaded frequency of extended interaction cavity can also be easily adjusted to a desired value. The influence of gap number on the power exchange between beam and microwave is also investigated, which shows that the maximum power exchange between beam and microwave electric field increases with the number of resonator gaps increasing, and so does the efficiency of klystron. A W band extended interaction klystron amplifoer is designed by the above theory analysis and three-dimensional (3D) PIC code. The simulation results show that when beam voltage is 20.8 kV, current is 0.28 A, input power is 30 mW at a frequency of 94.77 GHz, the extended interaction klystron can produce 443 W output power. The responding electron efficiency is 7.6%, the gain is 41.7 dB, and the 3 dB bandwidth exceeds 150 MHz. The extended interaction klystron is machined and tested, and the experimental results show that the maximum output power of 175 W is obtained with a beam of 300 mA, a voltage of 20.8 kV, and an input microwave power of 30 mW at a frequency of 95.37 GHz in a magnetic field of 0.62 T. The responding electron efficiency is 2.8%, the gain is 34.6 dB, the 3-dB bandwidth exceeds 90 MHz. This study is meaningful for designing and developing greater power extended interaction klystrons.

Multi-resonant photonic band-gap/saddle coil DNP probehead for static solid state NMR of microliter volume samples.

The most critical condition for performing Dynamic Nuclear Polarization (DNP) NMR experiments is achieving sufficiently high electronic B1e fields over the sample at the matched EPR frequencies, which for modern high-resolution NMR instruments fall into the millimeter wave (mmW) range. Typically, mmWs are generated by powerful gyrotrons and/or extended interaction klystrons (EIKs) sources and then focused onto the sample by dielectric lenses. However, further development of DNP methods including new DNP pulse sequences may require B1e fields higher than one could achieve with the current mmW technology. In order to address the challenge of significantly enhancing the mmW field at the sample, we have constructed and tested one-dimensional photonic band-gap (PBG) mmW resonator that was incorporated inside a double-tuned radiofrequency (rf) NMR saddle coil. The photonic crystal is formed by stacking ceramic discs with alternating high and low dielectric constants and thicknesses of λ/4 or 3λ/4, where λ is the wavelength of the incident mmW field in the corresponding dielectric material. When the mmW frequency is within the band gap of the photonic crystal, a defect created in the middle of the crystal confines the mmW energy, thus forming a resonant structure. An aluminum mirror in the middle of the defect has been used to substitute one-half of the structure with its mirror image in order to reduce the resonator size and simplify its tuning. The latter is achieved by adjusting the width of the defect by moving the aluminum mirror with respect to the dielectric stack using a gear mechanism. The 1D PBG resonator was the key element for constructing a multi-resonant integrated DNP/NMR probehead operating at 190-199 GHz EPR/300 MHz 1H/75.5 MHz 13C NMR frequencies. Initial tests of the multi-resonant DNP/NMR probehead were carried out using a quasioptical mmW bridge and a Bruker Biospin Avance II spectrometer equipped with a standard Bruker 7 T wide-bore 89 mm magnet parked at 300.13 MHz 1H NMR frequency. The mmW bridge built with all solid-state active components allows for the frequency tuning between ca. 190 and ca. 199 GHz with the output power up to 27 dBm (0.5 W) at 192 GHz and up to 23 dBm (0.2 W) at 197.5 GHz. Room temperature DNP experiments with a synthetic single crystal high-pressure high-temperature (HPHT) diamond (0.3 × 0.3 × 3.0 mm3) demonstrated dramatic 1500-fold enhancement of 13C natural abundance NMR signal at full incident mmW power. Significant 13C DNP enhancement (of about 90) have been obtained at incident mmW powers of as low as <100 μW. Further tests of the resonator performance have been carried out with a thin (ca. 100 μm thickness) composite polystyrene-microdiamond film by controlling the average mmW power at the optimal DNP conditions via a gated mode of operation. From these experiments, the PBG resonator with loaded Q ≃ 250 and finesse F≈75 provides up to 12-fold or 11 db gain in the average mmW power vs. the non-resonant probehead configuration employing only a reflective mirror.

Study of Electronic Switching Between Multiple Backward-Wave Modes in a W-Band Extended Interaction Oscillator

The performance of electronic switching between multiple backward-wave modes is studied in a designed extended interaction oscillator (EIO) based on a ladder circuit with finite number of periods to overcome electronic tuning range limits of EIOs operated in standing-wave mode. The dispersion characteristic of the circuit with finite number of periods, which is constructed by a series of discrete modes, is investigated. The mode separation is analyzed and reduced to support continuous switching between multiple different modes by increasing the number of periods as compared with the standing-wave EIO approach. An output circuit is designed to extract the power of backward wave. The electronic switching between nine backward-wave modes has been achieved by changing the beam voltage from 4.1 to 10.5 kV, where the maximum output power over 58 W is obtained at 5.3 kV from the simulation prediction. The EIO can operate over an electronic tuning range of 3.53 GHz from 89.65 to 93.18 GHz in ensuring the output power no less than 20 W. This technique can be extensively applied to increase operating band for extended interaction klystrons (EIKs) and electronic tuning range for EIOs, making them more suitable for many potential applications.

Practical Aspects of EIK Technology

Significant progress in modeling and manufacturing technologies open wide possibilities for performance improvements of millimeter-wave vacuum electron devices. However, many practical aspects should be considered to realize reliable long-life high-power sources. These are: thermal and RF stability, materials and assembly sensitivity to manufacturing, electrical stresses, cathode poisoning prevention, thermal beam effects, and many others. We address the full spectrum of design and manufacturing aspects while developing state-of-the-art extended interaction klystrons (EIKs). EIKs provide unprecedented RF performance and reliability in a compact user-friendly package. This paper discusses EIK design methodology and manufacturing concepts stating self-imposed restrictions and design modifications enhancing power capability, bandwidth, and extending operating frequencies into the terahertz region.

Nonperiodic Perturbations in Periodic RF Structures

This paper presents analysis of the effects of nonperiodic perturbations in periodic RF structures. Small departures from periodicity change the phase shift experienced by a wave as it traverses an imperfect cell. Cell imperfections may be introduced intentionally or due to fabrication errors or material properties variations. The cumulative effect produces distortions of the fields. If the errors are large enough, the structure modes become localized. This is the first demonstration of field localization in a periodic RF structure. Mechanisms, theory, and analytical relations describing the field distortion and localization phenomena are discussed. The results are demonstrated in periodic structures being investigated for submillimeter wave extended interaction klystrons. In an ancillary result, we discover a fundamental limitation of the finite element as well as other mesh-based codes when applied to long periodic structures.

Latest Advancements in High-Power Millimeter-Wave Helix TWTs

The millimeter-wave spectrum, generally considered to range from 30 to 300 GHz, is one of the most actively used spectrums today, with applications such as high-data-rate terrestrial and space communications, radar, and electronic warfare. The helix traveling-wave tube (TWT) amplifier has long been a critical component for enabling modern millimeter-wave systems. This paper presents an overview of several recently developed high-power millimeter-wave helix TWTs which have extended the helix TWT performance envelope. In particular, the power-frequency-bandwidth product for these devices has increased by a factor of 4 to over 100 depending upon the application. These helix TWTs operate at power levels up to 1-kW peak power or 500-W average or continuous-wave power and over frequencies from 18 to 46 GHz, with efficiencies up to 60%. The helix TWT performance improvements discussed here have not only kept pace with improvements in rival technologies such as GaN solid-state power amplifiers but also pushed helix TWTs into competition with traditionally higher power and higher frequency vacuum electron devices such as extended-interaction klystrons and coupled-cavity TWTs. Indeed, the modern high-power millimeter-wave helix TWT offers system engineers a singularly attractive amplifier technology with an outstanding combination of small size, light weight, high efficiency, good linearity, high reliability, and large bandwidth.

Millimeter-wave extended interaction klystron

The interaction of the HF fields in the distributed resonance system of a high-power millimeter-wave klystron with an electron beam is theoretically and experimentally studied under limitations imposed on the length of interaction space and moderate values of the beam perveance. A possibility of amplifying and generating extended interaction klystrons (EIKs) is demonstrated. The EIKs with the distributed extraction of energy exhibit relatively high efficiencies, output powers, and gains at an acceptable gain band. In the experimental prototypes of the four-cavity packaged EIK working in the mid-millimeter-wavelength range, the output pulse power is no less than 2.5–3 kW, the efficiency is 30%, the gain is 25–30 dB, and the gain band at a level of 3 dB is 0.25–0.35% at an anode voltage of 18 kV and a beam current of 0.5 A.

State-of-the-Art W-Band Extended Interaction Klystron for the CloudSat Program

This paper reviews the National Aeronautics and Space Administration (NASA)/Jet Propulsion Laboratory (JPL) CloudSat Extended Interaction Klystron requirements, design, performance, and development of a long life model for the European Space Agency (ESA)/Japan Aerospace Exploration Agency EarthCARE program plus Ka-band Extended Interaction Klystrons for the NASA/JPL Jupiter Icy Moons Orbiter and ESA European Global Precipitation Measurement Missions.

The Ubitron, a high-power traveling-wave tube based on a periodic beam interaction in unloaded waveguide

The Ubitron is a high-power traveling-wave tube which makes use of the interaction between a magnetically undulated periodic electron beam and the TE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</inf> mode in unloaded waveguide. The electron-wave interaction exhibits the same type of first-order axial beam bunching characteristic of the conventional slow-wave traveling-wave tube; hence, it can be used in place of conventional O-type interaction in extended interaction klystrons and electron accelerators, as well as traveling-wave tubes. Experimental results are presented for the simplest physical embodiment of the Ubitron, which consists of an undulated pencil beam in a rectangular waveguide. Two of the unique features of this tube are very broad interaction bandwidth which results from the absence of a dispersive slow-wave circuit, and variable interaction phase velocity--hence, variable saturation power level. Among the physical embodiments of the Ubitron are a number of higher-order mode waveguide and beam configurations. These include plane, coaxial, and circular waveguides, all supporting the TE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</inf> mode, interacting with magnetically undulated sheet, hollow and cylindrical beams, respectively. The advantage of these configurations, which have not yet been tested experimentally, is that they provide a very large interaction area for beam placement. This property, plus the fact that the peak interacting field is far from the waveguide walls, makes the Ubitron an interesting prospect for high-power millimeter wave amplification.