Radio Frequency Breakdown Research Articles

Experimental study on influence of temperature on breakdown in a waveguide cavity

Radio frequency (RF) breakdown is one of the crucial factors limiting the power capacity of high-power microwave (HPM) generators. In this paper, a waveguide cavity has been designed to study quantitatively the influence of temperature on high-gradient RF breakdown. The two planes of the waveguide cavity are divided into a strong-field side and a weak-field side with the feeding of microwave signals. The temperature of the strong-field side of the waveguide cavity can be adjusted from ambient temperature (25 °C) to 435 °C by loading a resistive wire heater. The breakdown threshold of the waveguide cavity decreases apparently with the increase in the temperature. The breakdown threshold at 408 °C is 751 kV/cm, which is about 150 kV/cm lower than that at ambient temperature. In addition, the higher the temperature, the more pronounced the pulse shortening. Under the same microwave power injection, the averaged pulse width of HPM after passing through the waveguide cavity at 408 °C is equal to 17.6 ns, shortened by about 1/3 compared to that at ambient temperature. The quantitative study of the influence of temperature on the RF breakdown provides a new guideline for exploring breakdown suppression methods in HPM generators.

Design Criteria for High-Gradient Radio-Frequency Linacs

This article will review methods used at the SLAC National Accelerator Laboratory and other world accelerator laboratories to design high-gradient normal conducting accelerating structures. A quest for compact radio-frequency linacs fueled decades of studies toward a higher accelerating gradient. A major phenomena limiting the increase of the gradient is vacuum radio-frequency breakdown; therefore, this paper will address the breakdown physics and discuss approaches that reduce the breakdown probability. This discussion will cover both the electrical design and fabrication technology of the accelerating structures to achieve practical operating accelerating gradients in excess of 100 MV/m. Most of the data described here were obtained during the development of 11 GHz linacs for electron–positron linear colliders, so extrapolation of the results to other frequencies should be performed cautiously.

Experimental study on the influence of grain boundary on breakdown in relativistic backward wave oscillator

The presence of radio frequency (RF) breakdown seriously degrades the performance of relativistic backward wave oscillator (RBWO) and affects its lifetime. In this paper, the influence of grain boundary on RF breakdown in an X-band RBWO has been experimentally investigated. Firstly, titanium with different grain sizes has been prepared using a high-temperature annealing method before its field emission characteristics are studied by a field emission test system. The experimental results indicate that the segregation effect of the impurities at the position of the grain boundary may become more pronounced as the grain size increases, which results in a significant improvement in the field emission performance of titanium. A high-power microwave experimental platform is then set up, and the influence of the grain boundary of titanium on RF breakdown has been revealed. The shortening of the microwave pulses is more serious with the increase of the grain size, and the corresponding breakdown traces appearing in the high-frequency structures become more apparent. Therefore, developing fine-grained and single-crystal metallic materials may be an important method for suppressing RF breakdown in RBWOs.

Starting Modes of Bi-Directional Plasma Thruster Utilizing Krypton

Multidirectional plasma thrusters are of particular interest for dynamic space missions due to the adjustability of their integral characteristics. One type of multidirectional plasma thrusters is -directional, consisting of a symmetric electromagnetic system surrounding the gas discharge chamber, capable of generating a propulsion minimum in two directions. The experimental results of this study of the starting modes of a multidirectional plasma thruster utilizing krypton as propellant are reported. The thruster is placed in a vacuum chamber. The magnetic field strength is adjusted in the range of 35 to 400 G in peaks. The current of 13.56 MHz frequency applied to the antenna is regulated in the range of 0 to 25 A. The diameter of the orifices is varied in the range of 3 to 10 mm. In contrast to the unidirectional electrodeless plasma thruster, the radiofrequency breakdown threshold of the multidirectional plasma thruster decreases with increasing static magnetic field due to the symmetry of the magnetic system and the gas discharge chamber. The influence of the magnetic field on the radiofrequency breakdown threshold in the multidirectional plasma thruster is shown theoretically by the classical diffusion theory and ponderomotive effects, and discussed in the electron circulation hypothesis.

Mechanism analysis of field electron emission of titanium

Field electron emission (FEE) is generally considered to be closely correlated with radio frequency (RF) breakdown in accelerating structures and high-power microwave (HPM) devices. In this study, the field emission characteristics of titanium are investigated by using a field emission test system. With the increase of the number of field emission tests and stability tests, the repeatability of the field emission characteristic curve of titanium tends to be better, and the amplitude of the current oscillation in the stability tests gradually decreases, indicating that the field emission performance of titanium gradually becomes stable. Subsequently, the field emission characteristic curve of titanium is analyzed by adopting the field emission theory of metal microprotrusions. Combined with the analysis of the electric field enhancement effects caused by the surface morphology, the dominant effect of the metal microprotrusions on the FEE of titanium is excluded from the two aspects of the electric field enhancement factor and local maximum emission current density. At last, the field emission theory of dielectric micropoints is introduced to analyze the field emission characteristics of titanium. The electric field enhancement factor of 102 ∼ 103 are explained theoretically. Simultaneously, the reasonable effective emission area and local emission current density are given, which can better explain the field emission phenomena of titanium, such as the sharp decrease in emission current and repeatability of the field emission curve. Hence, it is revealed that the key factor that dominates the FEE of titanium is the dielectric impurities on the surface, rather than the metal microprotrusions.

Experimental study on the influence of thermal accumulation on breakdown in slow-wave structures

Radio frequency (RF) breakdown in the slow-wave structures (SWSs) is a crucial bottleneck restricting relativistic backward wave oscillator (RBWO) to pursuing higher output power and longer pulse width. This paper has experimentally studied the influence of thermal accumulation during repetitive operation on RF breakdown in an X-band RBWO. A method for cooling the SWSs using water flow has been proposed to restrain the temperature rise to some extent. Under different heat dissipation conditions, the operating states of the RBWOs exhibit great differences. The greater the distance between the water-cooling heat transfer channel and the SWSs, the more serious the pulse shortening of high-power microwave. Moreover, the breakdown traces appearing in the SWSs become more obvious with the worse convective heat transfer capacity. The observed experimental phenomena provide a new guideline that helps to enrich the mechanism of RF breakdown and to explore corresponding suppression methods in RBWO.

Improved Power Capacity in a Low-Magnetic Field and High-Efficiency Transit-Time Oscillator by a Double-Gap Extractor

With an increase in the input power of a low-magnetic field and high-efficiency transit-time oscillator, the deceleration electric field in the single-gap extractor increases sharply to output a higher-power microwave, resulting in radio frequency breakdown on the surface of the single-gap extractor. Thus, a double-gap extractor with an operating mode of <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> mode is introduced to reduce the electric field in this type of device. The double-gap extractor can extract electron energy in two stages and lengthen the deceleration distance. Therefore, the electron beam loses less energy at the same deceleration distance in the double-gap extractor, and a weaker deceleration electric field is generated in the double-gap extractor. Additionally, the modulation electric field in the device with a double-gap extractor is stronger for modulating electrons and can further increase the fundamental harmonic current, resulting in higher extraction efficiency. In simulations, the output power is 6.3 GW and the maximum electric field strength is 1.06 MV/cm in the initial model with a single-gap extractor. When the single-gap extractor is replaced by the double-gap extractor, the output power increases to 6.5 GW with a conversion efficiency of 40% and a guiding magnetic field of 0.6 T. The maximum electric field strength decreases to 817 kV/cm, indicating an enhancement in the power capacity of the device.

Analysis and design of overmoded coaxial Ku-band Cerenkov high power microwave oscillator with high efficiency

An overmoded coaxial Ku-band Cerenkov high power microwave oscillator with high efficiency is proposed. The distance between the inner and outer conductors of the slow wave structure (SWS) in the Cerenkov oscillator is smaller than the space wavelength of output microwave. The overmoded structure can increase the power capacity of the oscillator by increasing the transversal size of the device. The higher-order modes are introduced into the beam-wave interaction region intentionally, so that the intense relativistic electron beams (IREB) interacts with the fundamental and higher-order modes. Dual-mode operation leads impure mode component to output microwave, so the simple trapezoid inner conductor is designed to achieve mode purification and output pure TEM mode. The extractor is added at the end of the SWS, which enhances the axial electric field near the location and increases the modulation depth of the electron beam current passing through the region. The particle-in-cell (PIC) simulation results show that under the input power of 3.5 GW, the output microwave power is 1.8 GW, and the corresponding working efficiency is 51.4%, which is much higher than that of similar periodic devices. The mode of output microwave is pure TEM mode, and the maximum surface electric field intensity is 0.96 MV cm−1, which means that the probability of radio-frequency breakdown is low.

Assessing the Role of Photon Processes in Facilitating Radio Frequency Breakdown of Air at Atmospheric Pressure in Millimeter Gaps

The behavior of the breakdown electric field versus gap lengths (in the 1–5-mm range) and at different frequencies in the 1–80-MHz span, has been studied numerically at atmospheric pressure. Unlike previous studies of radio frequency (RF) breakdown, the role of photon-emission processes is explicitly included and shown to be important for large-area electrode configurations. Numerical analysis based on Monte Carlo calculations is used to predict the breakdown thresholds. The simulations embed a statistical photon transport model, based on random selections of emission angles and times from excited atoms, as well as photoemission from the electrodes. Simulation results compare well with experimental data from our group, but only with the inclusion of photon processes. Though both photoemission and photoionization are included in the breakdown physics, the former is identified as the dominant process. The frequency behavior of breakdown fields is also assessed with the inclusion of photons, and the results reveal a U-shaped trend with increasing values for smaller gaps.

An X-band high-power and high-efficiency coaxial relativistic klystron oscillator with four-gap buncher and three-gap extractor

Because of the scaling invariance, the over-mode ratio of the coaxial resonator can be increased to increase the power handling capability. However, as the over-mode ratio increases, the characteristic impedance and external quality factor decrease, which causes the modulation of the electron beam to be weakened. Moreover, when the output microwave power increases, the double-gap output cavity will suffer from severe radio frequency breakdown. Therefore, an X-band high-power and high-efficiency coaxial relativistic klystron oscillator with a four-gap modulation cavity and a three-gap extraction cavity is proposed. First, a four-gap modulation cavity can increase the modulation depth of the electron beam to improve the beam-wave conversion efficiency. The operating mode of the modulation cavity is the 3π/4 mode of the coaxial TM01 mode. Second, a three-gap extraction cavity is adopted to enhance the microwave extraction energy and reduce the RF field strength. The simulation results show that when the diode voltage is 650 kV, the beam current is 15.4 kA, and the guiding magnetic field is 0.48 T, the device outputs a microwave power of 4.2 GW, a frequency of 8.4 GHz, and an efficiency of 42%.

Demonstration of sub- GV/m accelerating field in a photoemission electron gun powered by nanosecond X -band radio-frequency pulses

Radiofrequency (RF) electron guns operating at high accelerating gradients offer a pathway to producing bright electron bunches. Such beams are expected to revolutionize many areas of science: they could form the backbone of next-generation compact x-ray free-electron lasers or provide coherent ultrafast quantum electron probes. We report on the experimental demonstration of an RF photoemission electron source supporting an accelerating field close to 400~MV/m at the photocathode surface. The gun was operated in an RF transient mode driven by short $\sim 9$~ns X-band (\SI{11.7}{\giga\hertz}) RF pulses. We did not observe any major RF breakdown or significant dark current over a three-week experimental run at high accelerating fields. The demonstrated paradigm provides a viable path to forming relativistic electron beams with unprecedented brightness.

A Ku-band radial transit time oscillator with high-power capacity

In order to improve the power capacity of a high-power microwave (HPM) generator, a Ku-band radial transit time oscillator (RTTO) with a trapezoidal resonator is designed. Compared with the traditional RTTO, both the modulation cavity and the extraction cavity are trapezoidal, which can modulate the electron beam more effectively. At the same time, the conductor tips on both sides of the cavity are farther, which helps to avoid the enhancement of the local electric field at the conductor tip, so as to reduce the risk of radio frequency (RF) breakdown. In particle-in-cell (PIC) simulation, the proposed Ku-band RTTO can output HPMs with the power of 1.66 GW and the frequency of 14.33 GHz, and the working efficiency is 41%. The maximum radial electric field intensity in the extraction cavity is 0.76 MV cm−1, lower than the RF breakdown threshold of metallic materials, which can effectively improve the power capacity of the device.

In situ detection of RF breakdown on microfabricated surface ion traps

Microfabricated surface ion traps are a principal component of many ion-based quantum information science platforms. The operational parameters of these devices are pushed to the edge of their physical capabilities as the experiments strive for increasing performance. When the applied radio-frequency (RF) voltage is increased excessively, the devices can experience damaging electric discharge events known as RF breakdown. We introduce two novel techniques for in situ detection of RF breakdown, which we implemented while characterizing the breakdown threshold of surface ion traps produced at Sandia National Laboratories. In these traps, breakdown did not always occur immediately after increasing the RF voltage, but often minutes or even hours later. This result is surprising in the context of the suggested mechanisms for RF breakdown in vacuum. Additionally, the extent of visible damage caused by breakdown events increased with the applied voltage. To minimize the probability for damage when RF power is first applied to a device, our results strongly suggest that the voltage should be ramped up over the course of several hours and monitored for breakdown.

Experimental studies of a high-gradient X-band welded hard-copper split accelerating structure

Recent research on high-gradient radio frequency (RF) accelerating structures indicates that the use of hard copper alloys provides improvement in high gradient performance over annealed copper. Such structures are made by bonding individually manufactured parts. However, there are no well-established bonding techniques that preserve the hardness, surface finish and cleanliness required for high gradient operation. To preserve the copper hardness, RadiaBeam has developed a novel high-gradient split accelerating structure, based on electron beam welding joining technique. This technique provides efficient bonding with strong, clean welds and minimal thermal loading, while maintaining a clean inner RF environment. Our RF design and fabrication methodology limits the small heat affected zone to the outer cavity envelop, with virtually no distortions or thermal loading of critical RF surfaces. It also incorporates provisions to precisely control the gap despite conventional issues with weld joint shrinkage. To date we have manufactured and validated an RF accelerating structure joined by electron-beam welding that incorporates a novel open split design to significantly reduce the assembly complexity and cost. In this paper, we will present the electromagnetic design of this structure, discuss bonding, and present the results of high-power tests, where the accelerating gradients of 140 MV m−1 with surface peak fields of 400 MV m−1 were achieved for flat-top pulse length of 600 ns with an RF breakdown rate of 10−4 1/(pulseċm).

Analysis of Destructive Effects with Electron Bombardment in Slow-Wave Structures

Radio frequency (RF) breakdown can result in pulse shortening and seriously degrade the stability and reliability of relativistic backward wave oscillators (RBWOs). This paper discusses the energy range of electrons causing breakdown traces in slow-wave structures (SWSs) through particle-in-cell (PIC) simulation, numerical calculation, and experimental verification. The PIC simulation and numerical calculation results reveal that the energy of the majority of the field-induced electrons bombarding the SWS surfaces after being accelerated is less than 120 keV. Furthermore, the micro appearances of the breakdown traces in SWSs and the witness targets bombarded directly by electrons of various energy levels have been analyzed. Scanning electron microscope (SEM) shows that the breakdown traces are featured with corrugated morphologies with a wide range and a shallow depth. A mass of craters emerge in the vicinity of the corrugated morphologies. These appearances are quite similar to destructive traces impacted directly by low-energy electrons (around 160 keV). Thus, it is confirmed that the breakdown traces result from the bombardment of low-energy electrons. Therefore, the breakdown mechanism of field-emitted electrons impacting on the structure surfaces in RBWOs has been further improved.

High Power RF Discharge Detection Technique Based on the In-Phase and Quadrature Signals

High power radio frequency (RF) breakdown testing is a subject of great relevance in the space industry, due to the increasing need of higher transmission power and smaller devices. This work presents a novel RF breakdown detection system, which monitors the same parameters as the microwave nulling system but with several advantages. Where microwave nulling--a de facto standard in RF breakdown testing--is narrowband and requires continuous tuning to keep its sensitivity, the proposed technique is broadband and maintains its performance for any RF signal. On top of that, defining the detection threshold is cumbersome due to the lack of an international standardized criterion. Small responses may appear in the detection system during the test and, sometimes, it is not possible to determine if these are an actual RF breakdown or random noise. This new detection system uses a larger analysis bandwidth, thus reducing the cases in which a small response is difficult to be classified. The proposed detection method represents a major step forward in high power testing as it runs without human intervention, warning the operator or decreasing the RF power automatically much faster than any human operator.

Design and simulation of a compact Ku-band RTTO with power divider extraction structure

The radial transit time oscillator (RTTO) has attracted much attention because of its high power capacity and pure mode of output microwave. To make the high power microwave (HPM) source devices more compact and to enable it to measure the output microwave mode quantitatively, this paper proposed a compact Ku-band RTTO with the power divider extraction structure (PDES). The radial decreasing magnetic field is applied to decreasing the mass of excitation system. Compared the conventional uniform solenoids, it can reduce the mass by about 30%. In the coaxial output waveguide, the PDES is used instead of the traditional support rods connecting the inner and outer conductors so as to convert TEM mode into TE10 mode efficiently. This structure can not only help shorten the axial dimension of the device, but also make it possible to measure the output microwave mode more accurately online. In particle-in-cell (PIC) simulation, the proposed Ku-band RTTO can output HPMs with the power of 3.05 GW and the frequency of 14.36 GHz, and the working efficiency is 40.3%. The maximum radial electric field intensity in the extraction cavity is 0.92 MV/cm, and the maximum electric field intensity in the PDES is 0.52 MV/cm, both of which are lower than the radio frequency (RF) breakdown threshold of metal materials.

A Coaxial High Power Output Cavity Operating in Hybrid TM₀₁-TM₀₂ Modes for Repetitive Operation

An output cavity operating in hybrid TM01-TM02 modes is investigated for the sake of producing repetitive high power microwaves (HPMs) in coaxial electromagnetic structures. To suppress anode plasma formation, the generated microwave is changed to be extracted inwards, so that the intense electron beam can be collected on the outer conductor, which makes beam collector cooling feasible practically in this topology. The issue of radio frequency breakdown is addressed by using a large extraction waveguide, and particularly the external quality factor required for optimal beam wave interaction is achieved by adjusting mode coupling. The design is examined in an X-band triaxial klystron amplifier by particle-in-cell simulation, which shows HPM with a power of 2 GW can be generated with an efficiency of about 36%.

Mechanism of radio frequency breakdown on metal surfaces in relativistic backward wave oscillator

Radio frequency (RF) breakdown can result in pulse shortening and severely limits the performance of the relativistic backward wave oscillator (RBWO). In this study, segmented slow-wave structure (SWS) rings, which can be observed directly using a scanning electron microscope without any post-processing, are used to study the breakdown in a RBWO. After undergoing high-power microwave experiments, the working surface of the SWS rings was found to exhibit corrugated morphologies at the sub-mm to mm scale, and a large number of craters in the vicinity of the corrugated morphologies and a mass of scattered droplets were visible on the surface. It was deduced that the unusual morphologies resulted from electron bombardment of the working surface. The observed structures and droplets provide new evidence that helps to explain the mechanism for RF breakdown in the RBWO.

Design and preliminary experiment of a disk-beam relativistic klystron amplifier for Ku-band long-pulse high power microwave radiation

Spatial coherent combination of multiple high power microwave (HPM) sources is considered as a promising scheme to improve the equivalent radiation power of the HPM system dramatically. Relativistic klystron amplifier (RKA) is one of the most suitable sources for the coherent power combination owing to its specific capabilities of stable microwave frequency and controllable phase. However, the RKAs operating at high frequency-band are severely limited by the problems of the intense space-charge effect and radio frequency breakdown. The radial-line HPM sources driven by the disk-shape electron beam may provide the potential to alleviate this issue due to its attractive features of the weak space-charge effect, the high power handling capacity, and the strong electron collection ability. In this paper, a disk-beam relativistic klystron amplifier (DB-RKA) is proposed and physically designed aiming to generate long-pulse HPM radiation at Ku-band. The physical idea, design principles, and simulation results are presented in detail. In a preliminary experiment, the disk-shape intense electron beam is well focused with an axial-width of 1.2 mm by an improved magnetic-excited method. Furthermore, the DB-RKA is demonstrated to be capable of generating Ku-band HPMs typically with peak power of 320 MW, pulse duration of 100 ns, and gain of 42 dB.