Modulated Electron Beam Research Articles

A Compact V-Band Transit Time Oscillator with Reflective Modulation Cavity

Improving compactness is essential for high-power microwave (HPM) sources. In this paper, a novel reflective modulation cavity is proposed and investigated in a V-band relativistic coaxial transit-time oscillator (RCTTO). The cold cavity analyses and particle-in-cell simulations show that the reflective modulation cavity has larger reflection coefficients of TEM mode and stronger electron beam modulation capability when compared with a uniform modulation cavity. When the input diode voltage is 391 kV, the beam current is 4.91 kA, and when the guiding magnetic field is 0.6 T, the compact V-band RCTTO produces an output microwave power of 518 MW (conversion efficiency of 27.0%). Compared with the original RCTTO, the compact V-band RCTTO featuring a reflective modulation cavity exhibits a 24.8% increase in output power and a 5.4% improvement in efficiency, and the axial length of the magnetic field uniform region is reduced by 24.2%. The compact V-band RCTTO also demonstrates a broad operation voltage range, indicating potential for stable operation with voltage fluctuations in experiments. Furthermore, the reflective modulation cavity can be integrated into other high-frequency O-type HPM devices to enhance compactness, thereby diminishing the demands on the magnetic field region, which is advantageous for the future permanent packaging of HPM sources.

Use of Electron Beam-Induced Current Technique to Characterize Transport Properties of Narrow-Gap-Energy Materials for IR Detection

The electron beam-induced current technique (EBIC) developed at CEA-Leti has been a valuable tool for studying the transport properties of minority carriers in HdCdTe. Indeed, previous work has demonstrated the use of this technique for estimating diffusion length from the exponential decay of EBIC as a function of junction distance, as well as direct measurement of the modulation transfer function (MTF) of small pixel pitch diodes. In this work, a modulated electron beam and a lock-in amplifier are used to measure the variation in current and phase shift with the scan distance to the junction. This work is focused on the estimation of the minority carrier lifetime from the linear evolution of the phase shift as a function of junction distance.

An inverse free electron laser-bunching driven shallow-angle superradiant Compton source

A coherent soft x-ray Compton source driven by a density modulated electron beam is investigated. The density modulation is attained by an inverse free electron laser prebuncher, which offers strong energy modulation at the relevant beam energies for Compton scattering in an shallow-angle oblique scattering geometry using relatively low laser beam power. In particular, we study the case of a 101.7 MeV electron beam produced by a state-of-the-art high brightness accelerator beamline. Calculations show that 1011 coherent soft x-ray photons per second can be generated in an ultra-narrow relative bandwidth of 0.002% from a 250 pC microbunched electron bunch colliding with a 23 mJ laser pulse at 10 kHz.

Development and verification of a Geant4 model of the electron beam mode in a clinical linear accelerator

At present, a significant number of studies are focused on the development of novel methodologies for the fabrication of dosimetry phantoms. One of these methods is to produce heterogeneous samples by 3D printing. In order to select the most appropriate parameters for such products, it is necessary to conduct numerical simulations. In this work, we developed the model of a beam-forming system using a medical linear accelerator as a reference. This model was used to determine simulation parameters and corresponding dose distributions of an electron beam with nominal energies of 6, 12, and 15 MeV in a homogeneous water phantom. These parameters were, in fact, adapted to provide maximum agreement between simulated distributions and those experimentally obtained with the clinical linear accelerator. The beam simulation was performed using the Geant4 Monte Carlo toolkit. The simulation geometry of the accelerator treatment head includes scattering foil and a flattening filter, which are designed for electron beam broadening. Additionally, the beam-forming system was incorporated to collimate the beam to the required size. A metal applicator was included to reduce the contribution of electron scattering in air. The main simulation parameters were iteratively tuned by comparing simulation results with experimentally obtained data. It is shown that the simulated percentage depth dose and transverse profiles for electron beams in water phantom are in good agreement with the experimental data obtained with a cylindrical ionization chamber. This demonstrates that the methodology employed in the development of the numerical model of the medical linear accelerator is vendor-independent, readily implementable, and allows for rapid calculations. Furthermore, the model can be applied for a variety of purposes, including the selection of parameters for the fabrication of heterogeneous dosimetry phantoms.

Twisted terahertz excitation using pre-bunched relativistic electron beam in magnetic wiggler

This work presents a theoretical analysis of the generation of twisted terahertz (THz) radiation using laser-bunched relativistic electron beams in a magnetic wiggler. By employing a laser-bunched relativistic electron beam, which introduces a transverse modulation to the electron beam density, and a magnetic wiggler, which induces a transverse deflection to the electron trajectories, the generation of twisted THz radiation is achieved. The interaction between the modulated electron beam and the magnetic field leads to the emission of THz photons with a twisted phase structure. The findings of this study provide valuable insights into the generation and manipulation of twisted THz radiation contributing to the advancement of THz technology and its diverse applications.

Self-enhanced coherent harmonic amplification in seeded free-electron lasers

High-intensity, ultrashort, fully coherent X-ray pulses hold great potential for advancing spectroscopic techniques to unprecedented levels. Here, we propose a novel scheme for generating high-brightness and femtosecond-scale soft X-ray radiation within a seeded free-electron laser (FEL) operating at MHz repetition rates. This scheme relies on the principles of self-modulation and superradiance. A relatively weak energy modulation of the pre-bunched electron beam is significantly amplified by the coherent radiation emitted in the self-modulator. Consequently, a coherent signal at ultra-high harmonics of the seed is achieved, and this signal is further amplified in the subsequent radiator through the fresh bunch and superradiant processes. Based on the parameters of the Shanghai soft X-ray FEL facility, three-dimensional simulations have been performed. The simulation results demonstrate that an electron beam with a laser-induced energy modulation as small as 2.3 times the slice energy spread can generate ultrashort coherent radiation pulses of around 2 GW within the water window spectral range. Moreover, the experimental results demonstrate that self-enhanced coherent energy modulation enables the production of coherent signals up to the 15th harmonic of a 266-nm seed laser. These findings indicate that the proposed scheme promises to generate ultrashort and coherent soft X-ray pulses at MHz repetition rates.

Electronic excitation of high-order spoof surface plasmons on metallic grating at terahertz frequencies

High-order spoof surface plasmon (SSP) mode on corrugated metallic surfaces can find many interesting applications, such as in imaging, sensing, transmission and enhanced radiation source, etc. In this paper, an efficient excitation method of the high-order SSP mode by using an injected electron beam on the uniform rectangular metallic grating is proposed and investigated numerically. Based on the matched wave momentum between the SSP mode and the electron beam, both the fundamental and high-order SSP modes can be excited on the structure by using a single injected electron beam. Numerical simulation results indicate that the maximum electric field intensity of the generated high-order SSP mode is about two orders higher than that of the fundamental SSP mode. In addition, the peak power of the excited high-order SSP mode is almost two times that of the fundamental SSP mode power by the same energy electron beam, which demonstrates the obvious advantage of the high-order SSP electronic excitation approach compared to the previous fundamental SSP mode excitation on the structure. The central working frequency of high-order SSP power spectrum is about three times that of the fundamental SSP power spectrum. Moreover, the influences of the injected electron beam energy on the excited SSP power spectrum are analyzed specifically. It is shown that the generated SSP power spectrum demonstrates a blue shift with the decreased working voltage of the electron beam simultaneously, with its peak power increasing. However, the working bandwidth is narrowed with decreased beam voltage, which further reveals its working mechanism of presented electronic excitation of the SSP mode. The presented studies provide a new method to excite a high-order SSP mode on the metallic grating, which can find some potential applications in high-sensitivity sensing, deep sub-wavelength waveguide, and many others in terahertz spectra.

Research on a Ka‐band large‐orbit gyro‐TWT with periodic dielectric‐loaded structure

AbstractThis paper presents a design of a gyro‐TWT operating in the large‐orbit electron beam mode, with the aim of reducing the working magnetic field while ensuring operational efficiency. The periodic dielectric‐loaded structure, which has been successfully applied in the development of small‐orbit devices, is adopted as the high‐frequency interaction circuit. This paper conducts a comprehensive study and analysis of this structure and achieves stable operation in the Ka‐band after the optimization of the tube. This tube works at second harmonic of electron frequency in the mode of large‐orbit electron beam. The required magnetic field is only 5100 gauss, which can be generated using electromagnetic coils instead of superconducting magnets. The operational parameters include voltage of 75 kV, current of 9A, and velocity spread of 3.5%. Under these conditions, the device presents stable operation, with −3 dB bandwidth of 4.3 GHz, and maximum output power of 165 kW. This result meets the expected requirements for magnetic field and operational efficiency, thus validating the feasibility of practical fabrication of large‐orbit gyro‐TWT with periodic dielectric‐loaded structure.

Characterization and Coherent Control of Spin Qubits with Modulated Electron Beam and Resonator

AbstractThe coherent dynamics and control of spin qubits are fundamental requirements for the development and implementation of quantum technology. However, controlling a single spin qubit within a set of qubits is challenging due to the spatial extent of the applied magnetic field, which can negatively impact the coherent dynamics of neighboring qubits. In this study, a scheme is proposed for characterizing and controlling the coherent dynamics of spin qubits. The proposed scheme comprises a resonator that encloses the desired quantum system and a modulated electron beam that passes through the resonator in close proximity to the qubit of interest. The scheme involves solving the quantum master equation with Lindblad terms to obtain the system's dynamics. Reliability of the proposed model is validated by testing it on a potassium atom, 41K, and an NV‐ center in diamond. The results demonstrate that by controlling the parameters of the resonator and electron beam, the coherence and decoherence rates of these quantum systems can be enhanced. The proposed scheme has the potential to characterize various types of spin‐based quantum systems and implement quantum logic gates for quantum computation .

Electrostatic Waves Around a Magnetopause Reconnection Secondary Electron Diffusion Region Modulated by Whistler and Lower‐Hybrid Waves

AbstractWe investigate electrostatic waves in a magnetopause reconnection event around a secondary electron diffusion region. Near the current sheet mid‐plane, parallel electron beam‐mode waves are modulated by whistler waves. We conclude that the anisotropy of energized electrons in the reconnection exhaust excites whistler waves, which produce spatially modulated electron beams through nonlinear Landau resonance, and these beams excite beam‐mode electrostatic waves. In the separatrix region, parallel propagating electrostatic waves associated with field‐aligned electron beams and perpendicular propagating electron cyclotron harmonic waves with loss cone distributions exhibit modulation frequencies in the lower‐hybrid wave (LHW) frequency range. We infer that LHWs scatter electrons to produce beams and alter loss cones to modulate electrostatic waves. The results advance our understanding about the regimes and mechanisms of electrostatic waves in reconnection, with an emphasis on their coupling with lower‐frequency electromagnetic waves.

Instability of Langmuir-beam waves: Kappa-distributed electrons

In space plasmas, electron populations exhibit non-equilibrium velocity distributions with high-energy tails that are reproduced by the Kappa power-laws and contrast with the Maxwellian distributions often used in theoretical and numerical analyses. In this work, we investigate typical electron beam-plasma systems and show the influence of Kappa tails on the linear dispersion and stability spectra of Langmuir-beam waves. The most common scenarios invoke instabilities of Langmuir waves at the origin of radio emissions in solar flares and interplanetary shocks. However, the parametric domain of these instabilities is narrow (i.e., energetic beams but with very low density, nb/ne≲10−3), making their analytical and numerical characterization not straightforward, while the approximations used may lead to inconclusive results. Here, we provide exact numerical solutions of the Langmuir-beam mode, which distinguish from the classical ones (unaffected by the beam), and also from electron beam modes destabilized by more energetic and/or denser beams. Langmuir-beam solutions are only slightly modified by the Kappa distribution of the beam component, due to its very low density. However, if the main (core) population is Kappa distributed, the instability of the Langmuir-beam mode is strongly inhibited, if not suppressed. New analytical solutions are derived taking into account the more or less resonant involvement of the electron core and beam populations. As a result, the analytical solutions show an improved match with the exact solutions, making them applicable in advanced modeling of weak (weakly nonlinear) turbulence.

The quality improvement of welded joints of die-cast Al alloys using oscillation and current modulation of the electron beam

The issues of the difficult weldability of the high-pressure die-cast aluminum alloys - AlSi10Mg(Fe) were studied within the presented paper. The metallographic, CT, and EDX analyses of castings were performed to identify potential origins of weld joint imperfections and deteriorated weldability. A comparative welding test of welding gravitation die-cast alloys was accomplished to review the influence of the melting process on the weld quality. The casting process was analyzed and several mediums were evaluated as potential sources of material contamination. The selected contaminants were intentionally introduced into the weld joints. The research on welding process improvement and the reduction of the susceptibility to the presence of impurities was accomplished in the next step. The aim of the experiments was the use of dynamic deflection of the electron beam and pulse modulation of its power. The influence of different oscillation frequencies, amplitudes, and patterns was examined. The verified technology was developed, based on the achieved results.

Tunable photon-induced spatial modulation of free electrons

Spatial modulation of electron beams is an essential tool for various applications such as nanolithography and imaging, yet its conventional implementations are severely limited and inherently non-tunable. Conversely, proposals of light-driven electron spatial modulation promise tunable electron wavefront shaping, for example, using the mechanism of photon-induced near-field electron microscopy. Here we present tunable photon-induced spatial modulation of electrons through their interaction with externally controlled surface plasmon polaritons (SPPs). Using recently developed methods of shaping SPP patterns, we demonstrate a dynamic control of the electron beam with a variety of electron distributions and verify their coherence through electron diffraction. Finally, the nonlinearity stemming from energy post-selection provides us with another avenue for controlling the electron shape, generating electron features far below the SPP wavelength. Our work paves the way to on-demand electron wavefront shaping at ultrafast timescales, with prospects for aberration correction, nanofabrication and material characterization.

Concept of Monobloc, Traveling Wave, Space Charge Current Limited Linear Accelerator for Radiotherapy in Oncology

The number of cancer cases will grow annually and according to WHO it will reach 25 million cases a year by 2035. Radiotherapy (RT) is a key element for the treatment of the 80 % of the cases [1-3] and its development and accessibility are the main routes for further improvement. At the current moment the large percentage of the negative outcomes of the cancer treatment is attributed to either lack of the RT machines or technical personal capable to maintain it. A modular approach to structure such an equipment is one of the ways to resolve the issues. The aim of the studies is to develop a conceptual design of a single module compact accelerator for medical applications and specifically RT of cancers. Development of such a machine is an important step to resolve the RT availability and challenging task from research and design point of view. The studies carried out using analytical and numerical (CST MW studio) approaches. In this paper the conceptual design of such a monobloc traveling wave (12 GHz) accelerator with the space charge limited electron beam current is presented and discussed. The accelerating section made of set of specially designed cell with average constant accelerating potential around 40 MV/m is demonstrated and its properties are discussed. It is shown that the low-relativistic electron beam can reach energy of 10 MeV on the length of the section less than 30 cm. It is shown that the electron beam capture, modulation and transportation takes place inside the accelerating section with the beam transportation efficiency above 80 %. It is illustrated that the main beam losses are taking place at the initial stage of beam formation and ways to optimise the system and minimise the beam losses are discussed. The results of the studies are compared and good agreement is demonstrated.

Modification of the Surface Properties of Hypereutectic Silumin by Irradiation with a Modulated Electron Beam

Modification of the Surface Properties of Hypereutectic Silumin by Irradiation with a Modulated Electron Beam

Modification of Commercial Radiotherapy System for Ultra-High Dose Rate Electron Beam Irradiation

<h3>Purpose/Objective(s)</h3> Because electron generation in X-ray mode needs to be about 1000 times more than electron beam mode, modifying commercial radiotherapy system make it possible to irradiate with ultra-high dose rate. This study aimed to show the feasibility of ultra-high dose rate irradiation using a commercial linear accelerator. <h3>Materials/Methods</h3> Removing targets and flattening filters, adjusting source-surface-distance (SSD), and controlling RF driver and electron gun were performed in Simens Oncor® and Varian Clinac iX® radiotherapy system. The irradiation time was adjusted in microseconds through the Arduino-based gating system, and dose rate was confirmed with EBT3 and EBT-XD films. <h3>Results</h3> In 6 MV X-ray mode of Siemens® system, 100.9±3.78 Gy/s electron beam was irradiated at SSD of 60cm. After modifying 10MV X-ray mode in Varian® radiotherapy system, dose rate was 43.1Gy/s at SSD of 100cm. By manually controlling the parameters of the RF driver and electron gun, 318 Gy/s rate of electron beam was irradiated within 15 × 15 cm² of irradiation field at SSD of 100cm. Acquired ultra-high dose rate electron beam exhibits the following characteristics at SSD of 100cm with full opened jaw irradiation: R₅₀ 4.1cm, W₈₀ 9.8cm (vertical) and 9.3cm (horizontal). <h3>Conclusion</h3> We confirmed that FLASH irradiation was feasible by modifying a commercial radiotherapy system, which made further research possible.

Intensity Modulation in Electron FLASH Radiotherapy

<h3>Purpose/Objective(s)</h3> Combining methods of conformal dose delivery with ultra-high dose rate (UHDR) beams is advantageous for realizing the benefits of the FLASH effect in clinical treatments. We hypothesize that deployment of intensity modulation in passive electron FLASH radiotherapy through treatment plan optimization is feasible. <h3>Materials/Methods</h3> A beam model of an electron FLASH irradiator (delivering ∼300Gy/s at the isocenter) in the decimal ElectronRT treatment planning system (TPS) was validated with film measured lateral and percent-depth-dose (PDD) profiles. Plans were developed comparing collimated open fields and intensity modulated electron beams achieved with cylindrical passive metal compensators that vary in spacing and size. Homogeneity and conformity were quantified for plans developed in a water phantom and anonymized patient cases considering constraints in prescribed dose to the target volume and minimizing dose to organs-at-risk (OAR). <h3>Results</h3> The film measured profiles and TPS beam model agreed on average to within 1% and 2% for lateral profile and PDD, respectively. For a large 15 × 15 cm² field in a water phantom the intensity modulation improved the beam flatness (∼30% to <5%) and penumbra (35 mm to 15mm) at 3 cm depth while retaining UHDR in the treatment field with a 38% reduction in central axis output while still retaining the UHDR conditions (∼180Gy/sec). The PDD exhibited <2% difference along the central axis and minimal changes to symmetry, shift, and practical range. In the rib metastasis case, the unmodulated and modulated plans treat the tumor volume with a homogeneity index (HI), improvement by 0.2. In the facial orbital plan, the conformity index improved by 8% with an improvement in HI by 0.05 and comparable dose to OARs. <h3>Conclusion</h3> Intensity modulation of electrons FLASH is achievable and improves homogeneity of prescribed dose with proper treatment constraints while reducing hot spots for clinical plans. Depending on the treatment case intensity modulation can also generate superior dose distributions while retaining the UHDR conditions to best exploit the FLASH effect. Future study will focus on demonstrating dosimetric benefits quantified by tumor control probability and normal tissue complication probability models in a larger number of relevant cases.

Ultrafast Transverse Modulation of Free Electrons by Interaction with Shaped Optical Fields.

Spatiotemporal electron-beam shaping is a bold frontier of electron microscopy. Over the past decade, shaping methods evolved from static phase plates to low-speed electrostatic and magnetostatic displays. Recently, a swift change of paradigm utilizing light to control free electrons has emerged. Here, we experimentally demonstrate arbitrary transverse modulation of electron beams without complicated electron-optics elements or material nanostructures, but rather using shaped light beams. On-demand spatial modulation of electron wavepackets is obtained via inelastic interaction with transversely shaped ultrafast light fields controlled by an external spatial light modulator. We illustrate this method for the cases of Hermite-Gaussian and Laguerre-Gaussian modulation and discuss their use in enhancing microscope sensitivity. Our approach dramatically widens the range of patterns that can be imprinted on the electron profile and greatly facilitates tailored electron-beam shaping.

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.

A Sub-THz High-Order Mode Backward Wave Oscillator Driven by Pseudospark Sourced Multiple Sheet Electron Beams

A novel concept of combining the advantages of pseudospark (PS)-sourced electron beam (high beam current density), multiple-sheet-electron-beams (large total beam cross-sectional area), and high-order mode (HOM) slow wave structure (SWS) (high power capacity) to produce high-power terahertz signals is presented in this article. As an example, a sub-terahertz backward wave oscillator (BEO) driven by PS-sourced dual-sheet-electron-beams is designed. Measured results of the interaction circuit, including transmission/reflection coefficients and the dispersion relation, were in good agreement with simulation predictions. Beam-wave interaction simulations having considered the plasma effect and conductor loss predicted stable output signals with radiation power of 167.4–255.4 W over a tunable bandwidth of 234.4–241.0 GHz.