Intense Relativistic Electron Beam Research Articles

Design of a compact coaxial distributed intense relativistic electron beam focusing system

In order to achieve the objectives of compactness and miniaturization in high-power microwave systems, along with reducing system energy consumption, this paper presents a design of a compact coaxial distributed intense relativistic electron beam (IREB) focusing system based on the transit time oscillator. The design methodology integrates theoretical analysis with a 2.5-dimensional particle simulation method. The entire IREB focusing system is composed of three sets of distributed magnetic rings and front-end focusing structures. The layout of this magnetic system is optimized based on periodic permanent magnets, leading to the formation of a quasi-trapezoidal wave magnetic field configuration. This optimization reduces the required number of magnetic rings while ensuring the efficient transmission of the IREB. To prevent the breakdown of the loaded magnetic rings and to optimize the radial electric field in the diode region, an additional front-end focusing structure was added. Based on the aforementioned structural configuration, a 100% electron beam transmission rate was achieved within a 150 mm range.

Preliminary study of dual annular multiple-beam cathode for V-band coaxial transit-time oscillator

Since the research toward high-power millimeter-wave generator becomes a tendency in high-power microwave, overmoded structure with the high-order mode has been a considerable interest because of its potential to increase power handling capacity (PHC). To expand the PHC of V-band transit-time oscillator and excite higher mode TM03, a dual annular multiple-beam cathode has been proposed. In the geometric structure of the dual annular multiple-beam diode, cathode rods around two concentric circles are uniformly placed on cathode base, and each circle has several graphite rods. Because of space charge shielding effect and fluctuation of electron beam, explosive emission current of inner and outer cathode circles is difficult to be balanced, and the electron beam transmission rate is not very high. To solve those two problems, the relative length (ΔL) and the relative radius (Δr) between the inner and outer cathode circles are optimized to obtain balanced currents of inner and outer beams and a good transmission rate. In this paper, the preliminary study of a dual annular multiple-beam cathode is carried out by optimizing the cathode structure. When ΔL and Δr are equal to 2.5 and 4 mm, respectively, the dual annular multiple-beams cathode can provide 2.71 kA uniform intense relativistic electron beams under 421 kV, and the magnetic field is 1.2T. As a simulation result, explosively emitted and the 95.2% of total beam current transmission rate can be reached.

Chaotic dynamics in X-ray free-electron lasers with an optical undulator.

In this work, the chaotic motions of relativistic electrons in X-ray free-electron lasers are investigated using an optical undulator in the presence of a magnetized ion-channel background. To miniaturize X-ray light sources, the optical undulator is a promising concept. The optical undulator provides higher optical gain than conventional magnetostatic undulators due to its micrometer wavelength. In addition, it reduces the required electron beam energy from several GeV to the multi-MeV range to produce X-ray pulses. The interaction of an optical undulator with an intense relativistic electron beam is a highly non-linear phenomenon that can lead to chaotic dynamics. At synchrotron radiation sources, the possibility of chaos control for X-ray FELs can be critical for certain classes of experimental studies. The equations of motion for a relativistic electron propagating through the optical undulator in the presence of a magnetized ion-channel can be derived from the Hamiltonian of the interaction region. Simulation results revealed that the intensity of the perturbation route from orderly behavior to chaos depends on the beam density, axial magnetic field strength, ion-channel density parameter, and pump laser undulator. Specific values of parameters were obtained for the transition from regular to chaotic paths. Bifurcation diagrams of the system were plotted to demonstrate the origin of chaos at a critical point, and Poincaré maps were created to distinguish between chaotic and orderly motions of electrons. The proposed new scheme can help to improve X-ray FELs, which have potential usages in basic sciences, medicine, and industry.

Highly efficient superradiant oscillator with front-extraction output in TE11 mode operating at low magnetic field

With a low guiding magnetic field, a highly efficient X-band coaxial superradiant relativistic backward wave oscillator (SR-RBWO) with a front-extraction cavity and a built-in mode converter is proposed in this paper. The coaxial structure is used to reduce the guiding magnetic field of the intense relativistic electron beam. To decrease the transmission loss of microwave power in the slow wave structure (SWS), a front-extraction cavity is introduced. The inner conductor of the coaxial SWS is hollowed out as the output waveguide, and then, the microwave is extracted by the front-extraction cavity and enters the hollow waveguide rather than re-enter the SWS to output. With this improvement, the power conversion factor K can be increased by 0.44 even more. Making use of the feature of longer SWSs of SR-RBWOs, a built-in mode converter is designed, which can convert the circular TM01 mode propagating in the output waveguide into the circular TE11 radiation mode that can be radiated into free space directly. Therefore, the length of the radiation system can be reduced significantly, which is beneficial to the miniaturization of the entire high-power microwave system. Particle-in-cell simulation demonstrates that with a diode voltage of 180 kV and a guiding magnetic field of 0.37 T, the output SR pulse with an average power of 1.00 GW corresponding to power conversion factor K = 1.8 can be obtained.

GW-Level Ku-Band Compact Coaxial TTO With Permanent Magnet Packaging Potential

When the O-type high-power microwave (HPM) sources are extended to the high band, the current density of the intense relativistic electron beam will increase, which requires a higher intensity of the guiding magnetic field. The coil of the excitation system will be bulkier, which is not conducive to the development of the device toward the direction of light and small. The transit time oscillator (TTO) has attracted much attention due to its ability to output GW-level HPMs in the small number of cavities. In order to replace solenoid with permanent magnet, the compact coaxial TTO with a low magnetic field is designed in this article. The coaxial structure reduces the need for magnetic field strength. In addition, the familiar reflector is removed and the axial size of the beam-wave interaction region is reduced, which not only reduces the risk of electron beam diffusion under low magnetic field, but helps to reduce the weight of permanent magnets in the future. In particle-in-cell (PIC) simulation, the proposed coaxial TTO can output more than HPMs of 1.0 GW under the magnetic field less than 0.4 T with the efficiency is over 30%. Therefore, it is feasible to use permanent magnet packaging for the device.

A Compact Repetitive Marx Generator for Generating Intense Electron Beams for HPM Sources

A 500 kV, 80 Hz repetition rate compact Marx generator has been designed and developed for producing high intense relativistic electron beams for high-power microwave (HPM) sources. This Marx generator comprises eight bipolar stages (16 stages of unipolar) using energy storage capacitors for achieving improved performance in repetition mode operation. The maximum charging voltage per stage is 50 kV, producing an erected voltage of 800 kV. However, the load voltage will vary according to the impedance of the load connected across the generator. Custom-designed energy storage capacitors were developed to allow voltage reversal of up to 30%–50%, enabling the system to deliver a higher current. A small pulse transformer-based triggering unit with an output voltage of 30 kV was used for triggering the generator. The system’s performance was tested in pulse repetition frequency (PRF) mode with resistive and diode loads and achieved a repetition rate of 80 Hz. An energy density of ~460 J/m3 was achieved in the developed system. A vircator device was connected and tested to benchmark the Marx generator for its repetitive and burst mode operations. The test results show the measured RF output power of 100 MW.

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.

A RepetitiveKu-Band Coaxial Relativistic Klystron Amplifier Packaged With Permanent Magnets

A compact Ku-band coaxial relativistic klystron amplifier (RKA) with permanent magnets is proposed for the sake of producing repetitive high-power microwaves (HPMs) in this article. The improved input and output mode converters are designed to load the input signal at the end of this device to ensure the integrity of permanent magnets. Besides, the intense relativistic electron beam (IREB) is changed to be collected on the outer collector, which makes collector cooling feasible in the proposed device. The typical experimental results show that the Ku-band HPM with a power of 540 MW is generated, corresponding to a gain of 48 dB and an efficiency of 29%, when the guiding magnetic field is around 0.45 T produced by the permanent magnets. The result of continuous operation for 30 s at a repetition rate of 35 Hz indicates that the proposed coaxial RKA has the capability of operating at a high repetition rate.

The Characteristics of the Second and Third Virtual Cathodes in an Axial Vircator for the Generation of High-Power Microwaves

A virtual cathode oscillator or vircator is a vacuum tube for producing high-power microwaves (HPM). The efficiency of the vircator has been a difficult task for decades. The main reasons for low efficiency are intense relativistic electron beam (IREB) loss and few or no interactions between IREB and HPM. In this case, forming multiple virtual cathodes may be beneficial in overcoming these constraints. By reusing the axially propagating leaked electrons (LE), we could confine them and form multiple virtual cathodes (VCs). This article discussed the characteristics of newly formed VCs based on simulation results. The formation time of new VCs was discovered to be highly dependent on the reflector position and the density of LE approaching their surfaces. Furthermore, multiple VC formation in the waveguide region does not affect conventional VCs’ position or forming time. The emission mode of the generated HPM was TM01 with single and multiple VCs and remained unaffected. The formation of multiple VCs positively influenced the axial and radial electric fields. When compared to a single VC, the axial and radial electric field increased 25.5 and 18 times with multiple VCs. The findings suggested that forming multiple VCs could be a future hope for achieving high vircator efficiency.

A compact coaxial relativistic klystron amplifier with three cascaded single-gap bunching cavities for efficient output

A compact coaxial relativistic klystron amplifier (RKA) with three cascaded bunching cavities is investigated to obtain high efficiency output in this paper. When the injection power and drift tube length are both decreased for the compactness of the initial coaxial RKA, the conversion efficiency of this device decreases to only 20%, which is mainly due to the insufficient modulation of intense relativistic electron beam (IREB). To solve this issue, three cascaded bunching cavities are designed to strengthen the modulation of IREB, and each bunching cavity is designed to a non-uniform single-gap cavity with a high external quality factor. Moreover, a specific drift tube length is chosen for minimizing the TEM mode leakage between every two adjacent cavities. With these methods, the self-oscillation between input and three cascaded bunching cavities can be successfully suppressed without loading any reflectors. The proposed three cascaded single-gap bunching cavities are further examined in a Ku-band coaxial RKA, and the output efficiency of this device increases to 44%, which is more than twice that of the initial coaxial RKA. Furthermore, the injection power and axial length of the propagating IREB in the improved coaxial RKA are reduced to only 5 kW and 10 wavelengths, respectively.

Experimental research on a gigawatt-class Ku-band coaxial transit-time oscillator with low guiding magnetic field

A Ku-band relativistic transit-time oscillator with a three-cavity extractor and a distance-tunable reflector was proposed in our previous work [Ling et al., Phys. Plasmas 24(1), 013103 (2017)]. Recently, a new experiment was carried out on this device with an intense relativistic electron beam generator that produces electron beams with a duration of 55 ns and voltage of 0.4–1 MV. In preliminary experiments, mode competition was found with three main frequencies of which the reasons are analyzed theoretically and experimentally. The results show that plasmas generated from the collector's surface are most likely to cause this problem. To solve this problem, the collector structure is extended axially. With an input voltage of 600 kV and a current of 11.7 kA, a microwave output power 1.9 GW with pulse width 26 ns and frequency 12.42 GHz is obtained under a 0.7 T guiding magnetic field. These experimental results effectively confirm the potential of high power capacity of this proposed Ku band transit-time oscillator.

Soft X-ray radiography for measurements of a dense metal plasma created by intense relativistic electron beam on a tantalum target

A radiographic diagnostic has been developed to quantify the plume of erosion products after the interaction between a target and a relativistic electron beam. The diagnostic is based on a point Z-pinch (PZ-pinch) with a metallic plasma jet. It creates an X-ray source with a typical size of 10 μm, a photon energy of up to 3 keV and a flash duration of 3 ns. The X-ray visualization system is based on a thin scintillator and a CCD camera. The spatial resolution of the diagnostic was evaluated experimentally. The spatial frequency limit was found to be about 1.5 line pairs per millimeter. The diagnostic was tested in an experiment performed using an LIA-5 linear induction accelerator. An electron beam with a current of 400 A, an energy of 3.5 MeV and a pulse duration of 80 ns was focused on a plane tantalum target 1 mm in thickness. The FWHM of the focal spot was ∼ 2 mm. Radiographs of the plume were taken with different delays after different accelerator pulses. According to our data, the plume most commonly propagates toward the beam. After 70 μs its length was about 70 mm, i.e. the plume expansion rate was about 1 km/s. The area density of the semi-transparent regions of the plume was estimated as lying in the range 3 · 10-16–2 · 1017 cm-2.

Radial oscillation of intense relativistic electron beam in low-magnetic-field foil-less diode

The radial oscillation of an intense relativistic electron beam possesses two main features of the spatial period and the radial oscillation amplitude in a low-magnetic-field foil-less diode, and the large radial oscillation extremely limits the beam–wave conversion efficiency and stability of a high-power microwave device. Thus, the formation mechanism of the radial oscillation is analyzed in detail. The results show that the radial oscillation of an electron beam consists of a great number of electrons with different Larmor radii and guiding centers, and the large radial oscillation is mainly caused by the strong radial electric field and the directional difference between the electric field and the magnetic field in the anode–cathode gap. A low diode voltage or a proper large anode radius is beneficial to improve the beam quality. Considering that cathode plasmas have a dominant effect on the spatial distribution of electrons, the explosive emission model was improved with cathode plasmas, and the consistency between simulation and experimental results becomes better.

Coherent Combination of Power in Space With Two X-Band Gigawatt Coaxial Multi-Beam Relativistic Klystron Amplifiers

The power capacity of an individual high power microwave (HPM) generator is severely limited by internal electric field breakdown, and the problem becomes more serious for long pulse and high frequency bands. One of the promising solutions to overcome this limitation is coherent combination of power in space. To realize coherent combination of power in space, the HPM generators must operate at the same frequency and stable phase differences. We presented an improved X-band gigawatt coaxial multi-beam relativistic klystron amplifier (CMB-RKA) with high phase stability. Two double gap buncher cavities were designed to improve the beam wave interaction efficiency, and decrease the distance for the modulated intense relativistic electron beam to reach its maximum, which significantly decrease the phase jitter due to the fluctuation of electron beam voltage and improve the phase stability of the CMB-RKA. Experimentally, the two X-band gigawatt CMB-RKA demonstrated the capability of coherent combination of power in space, typically of the combined efficiency of about 95% with the combined pulse width of 150 ns.

Color centers in K–Na–Cl crystals induced by pulsed intense relativistic electron beam at 77 K

In order to reproduce the irradiation environment of Europa, NaCl-0-2mol%KCl crystals were grown and irradiated with pulsed intense relativistic electron beams at RT and 77 K in the optical absorbance spectra. The peak wavelength of the F-center was shortened by decreasing the measurement temperature and the KCl content. This phenomenon can be explained by the changes of the lattice constant and utilized to determine temperature and KCl content in the sea of Europa.

Multiple electron beam generation with different energies and comparable currents from a single cathode potential for high power traveling wave tubes (TWTs)

We present a technique for generating multiple electron beams with different energies and comparable currents from a single cathode stalk at a single potential using nested magnetically insulated coaxial diodes (MICDs). The application is for a multi-stream traveling wave tube. Particle-in-cell simulations are performed for an experimental parameter-based geometry where two thin-walled intense relativistic electron beams immersed in a strong uniform magnetic field propagate through a cylindrical vacuum channel. The analytically derived results are obtained by extending Fedosov’s solution for generating a hollow electron beam from an MICD on a cathode stalk in an infinite magnetic field. Two electron beams are generated and accelerated downstream assuming zero initial kinetic energy of the electrons from the cathodes. Results show both electron beam currents ranging from 66 to 2.8 kA with an energy difference ranging from 6% to 27% depending on voltages applied from 100 to 600 kV and the geometry of the two MICDs. An optimal geometry is a crucial factor in achieving the maximum energy difference between the electron beams for comparable currents. The analytical and numerical simulation results show good agreement. We are currently in the process of planning experiments using the electron beam accelerator (SINUS-6 at UNM) to validate the analytical and simulation results.

Role of Second Harmonic in the Optimization of Microwave Conversion Efficiency From an Intense Relativistic Electron Beam

In this article, the effect of second harmonic (SH) on the microwave conversion efficiency in a klystron-like relativistic backward wave oscillator (RBWO) is investigated by the nonlinear theory, particle-in-cell (PIC) simulations, and experiments. The nonlinear theory of beam-wave interaction is developed with both the fundamental frequency and SH taken into account. The calculation results show that the optimized conversion efficiency is increased from 50% to 73% as the SH is included. The PIC simulations reveal that with the introduction of a dual-premodulation (DPM) cavity to increase the SH current and then the fundamental current modulation coefficient in a klystron-like RBWO, the conversion efficiency can be enhanced from 59% to 72%. In the klystron-like RBWO experiments, a significant efficiency increase from 58% for the device without the DPM cavity to 66% with the DPM cavity has been realized. As the electron beam power is 4 GW, the out powers of the fundamental frequency and SH are measured to be 2.6 GW and 100 MW, respectively. This provides helpful guidance to further increase the microwave conversion efficiency in high-power microwave sources driven by intense relativistic electron beams.

A Large Signal Theory of Multiple Cascaded Bunching Cavities for High-Efficiency Triaxial Klystron Amplifier

This paper presents a large signal theory of multiple cascaded bunching cavities for the design of high-efficiency triaxial klystron amplifiers (TKAs). The theoretical analysis of multiple cascaded bunching cavities is presented, focusing on the relationship between gap voltage and first harmonic current and velocity dispersion, which can exactly describe the clustering state of intense relativistic electron beams. The theoretical results of the first harmonic current and velocity dispersion are basically consistent with its simulation results, which can justify a high degree of confidence in the validity of that theory. This theory can predict the possibility of deep modulation of intense relativistic electron beams when the depth of the first harmonic current is about 150% by multiple cascaded bunching cavities. By properly accounting for this theory, we can design a Ku-band TKA with nearly 60% microwave conversion efficiency, which can provide theoretical and simulation guidance for the design of high-efficiency TKAs. More importantly, when we increase the electron beam voltage from 300 kV to 600 kV and keep the relativistic perveance constant, this device also can obtain more than 50% efficiency and 40 dB gain. As a result, we can design a Ku-band TKA with high average output power of about 1.5 GW, 52% efficiency and 46 dB gain.

Relativistic Sub-THz Surface-Wave Oscillators With Transverse Gaussian-Like Radiation Output

Surface-wave oscillators driven by intense relativistic sheet electron beams can produce sub-terahertz waves at sub-gigawatt level of output power; however, evanescent nature of the operating wave hampers further radiation transmission and application. For these devices, we propose a method of efficient conversion of the surface operating wave into the output Gaussian-like wavebeam radiated in transverse direction with respect to the grating by applying an additional corrugation with a period twice larger than the main period. The analysis based both on the quasi-optical model and on the PIC simulations shows feasibility of 150-GHz surface-wave oscillator with 35 % efficiency and 100 MW output power. The transverse energy extraction significantly reduces the Ohmic losses down to 10% of the radiated power, which is essential for sub-THz range.

Optimal design and experimental research of several-gigawatt multiple electron beam diode

The relativistic klystron amplifier (RKA) is one of the most efficient sources to amplify a high-power microwave signal due to its intrinsic merit of high-power conversion efficiency, high gain and stable operating frequency. However, the transverse dimensions of the RKA dramatically decrease when the operating frequency increases to X band, and the power capacity of the RKA is limited by the transverse dimensions. An X-band multiple-beam relativistic klystron amplifier is proposed to overcome the radiation power limitation. Each electron beam propagates in separate drift tubes and shares the same coaxial interaction cavities in the multiple-beam relativistic klystron amplifier, and the transverse dimensions of the multiple-beam relativistic klystron amplifier are free from the operating frequency restriction and a microwave power of over 1 GW is generated in the experiment. For a high-power electron device, the transmission of electron beam is critical, and the power conversion efficiency of the device is affected. In this paper, we conduct an investigation into the transmission process of the intense relativistic multiple electron beams, and the number of the multiple electron beams is set to be 16. It is found that when the multiple electron beam is transmitted in the device, the electron beam rotates around the center of the whole device, causing the electron beam to deviate from the drift tube channel. At the same time, each electron beam rotates around itself, and the cross section of the electron beam is deformed and expanded. In the improper design of electron beam and drift tube parameters, two kinds of rotating motions cause beam to lose. A multiple-electron-beam diode structure is optimized by the particle-in-cell simulation to reduce beam loss, with the effects of the related factors taken into account. Each pole of the cathodes is made up of graphite and stainless steel. The cathode head is made up of graphite, for the graphite has a lower emission threshold. The cathode base and cathode pole are made up of stainless steel, for the stainless steel has a higher emission threshold. Also the shape and structure of cathode pole, cathode head and anode are optimized to reduce the electric field intensity on the cathode pole and enhance the electric field intensity on the end face of cathode head. At the same time, the electric field distribution of the cathode head is uniform to improve the electron beam emission uniformity. The simulation results demonstrate that the transmission efficiency of multiple electron beams can reach 99%. In the experiment, the transmission efficiency of multiple electron beams is 92% with a beam voltage and beam current of 801 kV and 9.3 kA, respectively.