Beam Accelerator Research Articles

Design and low-power measurement of 1.5 GHz TM020-type harmonic cavity for KEK future synchrotron light source

We designed a normal-conducting (NC) 1.5 GHz harmonic cavity that can be used in a future synchrotron light source at High Energy Accelerator Research Organization (KEK). By utilizing a higher TM020 mode for beam acceleration, this cavity has lower R/Q and higher unloaded Q as compared to those of conventional NC cavities. These features are advantageous in reducing the fluctuations in harmonic RF voltages when the bunch gaps are introduced. Harmful parasitic modes other than the TM020 mode can be heavily damped using coaxial slots located at the node of the TM020 mode. We made a concerted effort to lower the coupling impedances of parasitic modes while maintaining high Q value of the accelerating mode. We found that maintaining the axial symmetry of the cavity is essential for minimizing the leakage power of the accelerating mode into the coaxial slots. In this article, we present the optimization of the cavity shape, low-power measurement, and the basic design of a high-power model.

A new point kinetics model for ADS-type reactor using the importance function associated to the fission rate as weight function

This paper describes the development of a new point kinetics model for sub-critical nuclear reactors, driven by an external neutron source of the ADS-type. The equations and kinetic parameters were obtained with the use of the importance function, associated to the fission rate, as the weight function. Based on the point kinetics equations of this new model it was possible to get a new definition for the multiplication factor for the ADS-type sub-critical system. The comparison of the results obtained by this new point kinetics model with those of spatial kinetics, for the ABI (Accelerator Beam Interruption) and ABO (Accelerator Beam Overpower) transients postulated showed outstanding agreement, its highest relative deviation being of −0.612%.

High-Bunch-Charge and High-Energy-Gain Inverse-Cherenkov Particle Accelerator With Prebunched Beam

Dielectric laser accelerator (DLA) is a promising candidate for next-generation on-chip particle accelerator. However, practical applications of DLA to date have been restricted due to low accelerated bunch charge and low energy gain, especially for subrelativistic particles. To break through these limitations, here we propose a multistage DLA scheme with inverse Cherenkov effect. The first stage, formed by a pair of symmetric dielectric prisms, is to prebunch a long (tens to hundreds of laser-wavelengths) electron beam with relatively high bunch charge (several to tens of femto-coulombs) via velocity bunching. The subsequent stages, consisting of pairs of dielectric prism stacks, are to fulfill cascade acceleration of the prebunched beam. The synchronization of subrelativistic particles with laser field is realized via adjusting the prism angles in each stack. With theoretical analyses and simulations, we obtain an energy gain of more than 400 keV for an electron beam with initial energy of 100 keV. Compared with that of a non-prebunched beam, the accelerated bunch charge of a prebunched beam is more than four times higher. This proposed scheme affords an effective way of developing an on-chip particle accelerator with high energy gain and high bunch charge.

Investigating of plasma diagnostics by utilizing spectroscopic measurements of Balmer emission

Plasma technology offers revolutionary potential for particle accelerators by enabling the acceleration of electron beams to ultra-relativistic velocities in a small-scale dimension. The compact nature of plasma-based accelerators permits the creation of accelerating gradients on the GV scale. Plasma acceleration structures are created by utilizing either ultra-short laser pulses (Laser Wakefield Acceleration, LWFA) or energetic particle beams (Particle Wakefield Acceleration, PWFA), which need to be tailored to the plasma parameters. However, both methods face the challenge of limited acceleration length, which is currently only a few centimeters. To overcome this challenge, one approach is to generate plasma within a capillary tube, which can extend the acceleration length up to approximately forty centimeters or more. Consequently, it is crucial to characterize the produced plasma in terms of density and geometric structure. Optical emission spectroscopy (EOS) methods can be employed to measure and characterize the plasma electron density by analyzing the emitted plasma light. This paper presents measurements of the plasma electron density distribution for a hydrogen-filled capillary tube using both Balmer alpha (Hα) and Balmer beta (Hβ) lines. Comparing the intensities of Hα and Hβ emissions enables more precise measurements of the plasma electron density and provides additional information about other plasma properties.

Dynamics of Two-dimensional Type III Electron Beams in Randomly Inhomogeneous Solar Wind Plasmas

The dynamics of a type III electron beam generating Langmuir wave turbulence and subsequent electromagnetic emissions is studied owing to two-dimensional Particle-In-Cell simulations performed in both homogeneous and randomly inhomogeneous solar wind plasmas. Important differences in the beam dynamics are highlighted between both cases, due to Langmuir waves’ transformations on the density fluctuations. This paper studies the dynamics of a weak beam interacting with Langmuir wave turbulence scattered by initially applied plasma density fluctuations, in terms of particle acceleration, non-Gaussian suprathermal electron tails, broadening and relaxation of velocity distributions, beam density localization, and electron diffusion or trapping in a turbulent plasma. Density fluctuations are the cause of beam acceleration during its relaxation stage; after Langmuir wave saturation, it gains up to half the energy lost during deceleration while wave turbulence is damping, exhibiting asymptotically a suprathermal tail of electrons carrying around 30% of its initial kinetic energy. Some important features observed for one-dimensional beams exciting Langmuir wave turbulence in randomly inhomogeneous plasmas can be recovered.

Crack identification driven by the fusion of mechanism and data for the Variable-Cross-Section cantilever beam

Crack identification for variable-cross-section cantilever structures is a main means of avoiding sudden failure caused by cracking and realizing the prognostics and health management (PHM) of such structures. This paper proposes an inverse crack identification method driven by mechanism and data for variable-cross-section cantilever beams. First, the Rayleigh method is revised for the fast, accurate calculation of the natural frequencies of a healthy cantilever beam, which is regarded as a basic criterion. Subsequently, for the rapid, accurate expression of the relationship between crack information and natural-frequency variations, a data-driven method based on the radial basis function is proposed to calculate the response variations accurately according to the crack information. Then, the obtained acceleration of the cantilever beam is decomposed to determine its natural frequencies, and the variations in these natural frequencies are obtained and compared with the results of the mechanism model. Finally, crack positions, lengths, and widths are identified based on the forward model and decomposition results. Particle swarm optimization is employed to minimize the discrepancy in the changes between the monitored and calculated natural frequencies. The proposed forward model is compared with other methods, and the superiority of the crack identification method is verified experimentally. Results indicate that the proposed method accurately identifies cracks on variable-cross-section cantilever beams and provides an effective PHM approach for beam-type structures.

Phase Space Reconstruction from Accelerator Beam Measurements Using Neural Networks and Differentiable Simulations.

Characterizing the phase space distribution of particle beams in accelerators is a central part of understanding beam dynamics and improving accelerator performance. However, conventional analysis methods either use simplifying assumptions or require specialized diagnostics to infer high-dimensional (>2D) beam properties. In this Letter, we introduce a general-purpose algorithm that combines neural networks with differentiable particle tracking to efficiently reconstruct high-dimensional phase space distributions without using specialized beam diagnostics or beam manipulations. We demonstrate that our algorithm accurately reconstructs detailed 4D phase space distributions with corresponding confidence intervals in both simulation and experiment using a limited number of measurements from a single focusing quadrupole and diagnostic screen. This technique allows for the measurement of multiple correlated phase spaces simultaneously, which will enable simplified 6D phase space distribution reconstructions in the future.

First Measurement of Λ Electroproduction off Nuclei in the Current and Target Fragmentation Regions.

We report results of Λ hyperon production in semi-inclusive deep-inelastic scattering off deuterium, carbon, iron, and lead targets obtained with the CLAS detector and the Continuous Electron Beam Accelerator Facility 5.014GeV electron beam. These results represent the first measurements of the Λ multiplicity ratio and transverse momentum broadening as a function of the energy fraction (z) in the current and target fragmentation regions. The multiplicity ratio exhibits a strong suppression at high z and an enhancement at low z. The measured transverse momentum broadening is an order of magnitude greater than that seen for light mesons. This indicates that the propagating entity interacts very strongly with the nuclear medium, which suggests that propagation of diquark configurations in the nuclear medium takes place at least part of the time, even at high z. The trends of these results are qualitatively described by the Giessen Boltzmann-Uehling-Uhlenbeck transport model, particularly for the multiplicity ratios. These observations will potentially open a new era of studies of the structure of the nucleon as well as of strange baryons.

Laser wakefield acceleration of electrons using Bessel–Gauss doughnut beams for accelerating beam guiding

A high-intensity laser pulse propagating through a gas target disturbs the uniform plasma distribution. Plasma density structures, created by high-order Bessel–Gauss beams for guiding the accelerating Gaussian beam and laser wakefield acceleration of electrons, are analysed using Wake-T and Fourier–Bessel particle-in-cell (FBPIC) simulation tools. The use of Bessel–Gauss doughnut beams increases the acceleration distance and energy of accelerated electrons up to 2.3 times at a 2 mm distance relative to the Gaussian beam of the same intensity.

Preliminary Experimental Research on Split-Cathode-Fed Relativistic Magnetron With Bidirectional Emission Structure

Simulations and experiments of an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}$ </tex-math></inline-formula> -band TEM-output relativistic magnetron (RM) driven by split cathode (SC) are proposed. In the previous researches on SC, as a result of the space-charge-screening effect of squeeze-state electrons accumulated nearby the emission surface, the diode current is limited, which finally results in low output power. To improve the output performance, a bidirectional emission SC (BSC) is introduced. Experimental results on EPA-90 high-voltage electron beam accelerator demonstrate that when the voltage is around 480 kV and the axial magnetic field is 0.49 T, comparing with a conventional unidirectional emission SC, the output power can be enhanced from 124 to 400 MW by adding an extra downstream emitter. This work lays a foundation for further researches to make the SC-fed RM more practical.

Synchronization of parallel intense electron beam accelerators based on single trigger generator

In this study, the authors provide results of the precisely synchronized triggering of an intense electron beam accelerator (IEBA). The trigger generator was composed of a fractional-turn ratio saturable-pulse transformer and a compact six-stage Marx generator. The main switch of the IEBA was a corona-stabilized triggered switch (CSTS) based on the stabilized corona mechanism. The output voltage of a single IEBA exceeded 500 kV with less than 4 ns of jitter on a 50 Ω dummy load. We also conducted an experiment on the synchronous triggering of two IEBAs by using two independent trigger generators. The synchronization-related jitter was 6.1 ns, while the average time difference was 1.3 ns. We used this to attempt to trigger two parallel IEBAs by using a single trigger generator. The results showed a reduction in the synchronization-related jitter of 31% to 4.2 ns. The designs of the CSTS and the trigger generator guaranteed a precisely synchronous trigger by using a single trigger generator. Thus, the proposed method appears to be promising for accurately and synchronously triggering multiple IEBAs by using a single trigger generator. This provides an effective method to generate pulses with high power.

The Beam Stability Study of Varian Linear Accelerator Based on MPC and Morning Check

Background: Our hospital has been using Varian VitalBeam linear accelerator since December 2020. Daily quality control of accelerator beam stability in precision radiotherapy is very important. In this paper, we compare and analyze the beam stability of the accelerator with two monitoring methods. Objective: The long-term stability analyze and study of VitalBeam linear accelerator by using machine Machine Performance Check (MPC) and Morning Check. Methods: The energies of 6MV and 10MV x-ray most commonly used by Varian Linear accelerator (VitalBeam) for patient treatment were analyzed and compared，Firstly, the absolute dose of 6MV and 10MV X-ray energies of VitalBeam was calibrated. Collected and analyzed the morning checking data of VitalBeam from November 2021 to May 2022, including the CAX, SYM and FLAT offset data; the MPC daily data was collected and analyzed at the same time, which include beam output change, uniformity change and beam center shift data. The beam output changes were statistically analyzed, and the stability of other data was analyzed. Results: The Morning Check and MPC can accurately detect the change of accelerator beam performance, and the measured values are stable and within the tolerance range，The comparison of 6MV and 10MV beam outputs between the morning detector and MPC was statistically significant (P<0.05). Conclusion: Both MPC and Morning Check can detecte the requirements of accelerator quality assurement, the beam output monitoring morning detector has advantages, but MPC has faster execution efficiency than morning detector instrument, and is more suitable for units with a large number of patients treated daily.

Performance Analysis of Learning-Based Disturbance Observer for Pulsed Superconducting Cavity Field Control

The use of Disturbance Observer-based (DOB) control is widespread in stabilizing the electromagnetic field in superconducting radio frequency cavities to facilitate beam acceleration in particle accelerators. Repetitive disturbances such as beam loading and Lorentz force cavity detuning are compensated by DOB control, and their suppression is enhanced through the incorporation of a learning scheme into the conventional disturbance observer. This paper evaluated the performance of a learning-based disturbance observer for compensating beam loading and cavity detuning in pulsed superconducting radio frequency cavities and proposes modifications for better field stability. A superconducting cavity baseband model for π-mode was simulated in Matlab/Simulink with a trapezoidal beam pulse as the input disturbance and different cavity detuning values to analyze the controllers’ performance. The simulations were conducted for multiple observer filter bandwidths to evaluate the performance of the learning-based disturbance observer under plant model uncertainties and different detuning values. The results demonstrate that the learning-based disturbance observer yields faster convergence to the reference input and lower tracking errors during the flat top of pulse voltage in comparison to conventional disturbance observer control.

Multistage Positron Acceleration by an Electron Beam-Driven Strong Terahertz Radiation

Laser–plasma accelerators (LPAs) have been demonstrated as one of the candidates for traditional accelerators and have attracted increasing attention due to their compact size, high acceleration gradients, low cost, etc. However, LPAs for positrons still face many challenges, such as the beam divergence controlling, large energy spread, and complicated plasma backgrounds. Here, we propose a possible multistage positron acceleration scheme for high energy positron beam acceleration and propagation. It is driven by the strong coherent THz radiation generated when an injected electron ring beam passes through one or more solid targets. Multidimensional particle-in-cell simulations demonstrated that each acceleration stage is able to provide nearly 200 MeV energy gain for the positrons. Meanwhile, the positron beam energy spread can be controlled within 2%, and the beam emittance can be maintained during the beam acceleration and propagation. This may attract one’s interests in potential experiments on both large laser facilities and a traditional accelerator together with a laser system.

Influence of Particle Beam and Accelerator Type on ADS Efficiency

Conditions that maximize the performance of an accelerator-driven system related to particle beam and energy and accelerator type are analyzed. The toolkit Geant4 simulated the interaction of protons and ions with masses up to 20Ne and energies from 0.2 to 2 GeV/n. The beam intensity considered is 1.5 × 1016 p/s. The core of the reactor is modeled as an assembly of fuel rods surrounding a cylindrical beryllium converter, with a criticality coefficient of 0.985 and lead-bismuth eutectic coolant. Lower enrichment generates better utilization of fuel (20% to 25% from the initial actinide mass can fission in a cycle keeping neutron damage in clad below 200 displacements per atom). Data on particle fluence and energy released obtained from the simulation are used to calculate total electric power produced and isotope evolution. Power spent to accelerate the beam depends on accelerator type and is calculated by scaling from data on accelerator efficiency for a reference particle. Optimal proton energy is ~1.5 GeV when the beam is accelerated in a linac with energy gain G ~ 14 and is 0.75 to 1 GeV in the case of a cyclotron (G ~ 12). Ion beams starting with 4He realize higher G values than protons: 20 to 50 in a linac and 15 to 35 in a cyclotron.

Observation of Monoenergetic Electrons from Two-Pulse Ionization Injection in Quasilinear Laser Wakefields

The generation of low emittance electron beams from laser-driven wakefields is crucial for the development of compact x-ray sources. Here, we show new results for the injection and acceleration of quasimonoenergetic electron beams in low amplitude wakefields experimentally and using simulations. This is achieved by using two laser pulses decoupling the wakefield generation from the electron trapping via ionization injection. The injection duration, which affects the beam charge and energy spread, is found to be tunable by adjusting the relative pulse delay. By changing the polarization of the injector pulse, reducing the ionization volume, the electron spectra of the accelerated electron bunches are improved.

Experimental Characterization of Photoemission from Plasmonic Nanogroove Arrays

Metal photocathodes are an important source of high-brightness electron beams, ubiquitous in the operation of both large-scale accelerators and table-top microscopes. When the surface of a metal is nanoengineered with patterns on the order of the optical wavelength, it can lead to the excitation and confinement of surface-plasmon-polariton waves that drive nonlinear photoemission. In this work, we aim to evaluate gold plasmonic nanogrooves as a concept for producing bright electron beams for accelerators via nonlinear photoemission. We do this by first comparing their optical properties to numerical calculations from first principles to confirm our ability to fabricate these nanoscale structures. Their nonlinear photoemission yield is found by measuring emitted photocurrent as the intensity of their driving laser is varied. Finally, the mean transverse energy of this electron source is found using the solenoid-scan technique. Our data demonstrate the ability of these cathodes to provide a tenfold enhancement in the efficiency of photoemission over flat metals driven with a linear process. We find that these cathodes are robust and capable of reaching sustained average currents over 100 nA at optical intensities larger than $2\phantom{\rule{0.2em}{0ex}}\mathrm{GW}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ with no degradation of performance. The emittance of the generated beam is found to be highly asymmetric, a fact we can explain with calculations involving the also asymmetric roughness of the patterned surface. These results demonstrate the use of nanoengineered surfaces as enhanced photocathodes, providing a robust air-stable source of high-average-current electron beams hopefully with great potential for industrial and scientific applications.

Seismic Reduction Performance of Wrap Rope Connection Devices for Continuous Girder Bridges

In this article, wrap rope connection device (WRCD), which considers the relative acceleration of piers and beam as a control variable, is proposed for improving the current situation of continuous girder bridges whose bearing force of a single pier is along the longitudinal direction and based on the synergy principle. The WRCD device, which meets the slow displacement requirements of temperature and vehicle load under normal operation, is implemented and used to improve the performance of the sliding bearing pier. During earthquakes, because of the amplification effect of the wrap rope, the instantaneous large stiffness state in the longitudinal force can be achieved. Based on the shaking table test of a typical continuous girder bridge for examining the performance of the WRCD during earthquakes, the dynamic characteristics, structural acceleration, displacement, and strain responses of the structure under different frequency spectra, and seismic input intensities are analyzed and the seismic reduction performance of the WRCD is demonstrated. This analysis demonstrated that, by activating WRCD, the ratio of the acceleration response of the fixed bearing pier to the sliding bearing pier increased from 10% to 57%; moreover, the force on each pier appeared more uniform. Furthermore, with an increase in the input intensity of the earthquake, the displacement of the primary beam and the seismic response of the fixed pier bottom considerably decreased and the synergy effect of each pier was more prominent. Under certain site conditions, the WRCD can effectively improve the synergy effect between the sliding bearing piers and fixed bearing pier; however, the improvement in the obtained result is directly associated with the seismic input characteristics. The design parameters of the WRCD should be determined as per different site conditions and the optimum application range of the WRCD.

Efficient plasma electron accelerator driven by linearly chirped multi-10-TW laser pulses

The temporal rearrangement of the spectral components of an ultrafast and intense laser pulse, i.e., the chirp of the pulse, offers significant possibilities for controlling its interaction with matter and plasma. In the propagation of ultra-strong laser pulses within the self-induced plasma, laser pulse chirp can play a major role in the dynamics of wakefield and plasma bubble formation, as well as in the electron injection and related electron acceleration. Here, we experimentally demonstrate the control of the generation efficiency of a relativistic electron beam, with respect to maximum electron energy and current, by accurately varying the chirp value of a multi-10-TW laser pulse. We explicitly show that positively chirped laser pulses, i.e., pulses with instantaneous frequency increasing with time, accelerate electrons in the order of 100 MeV much more efficiently in comparison to unchirped or negatively chirped pulses. Corresponding Particle-In-Cell simulations strongly support the experimental results, depicting a smoother plasma bubble density distribution and electron injection conditions that favor the maximum acceleration of the electron beam, when positively chirped laser pulses are used. Our results, aside from extending the validity of similar studies reported for PW laser pulses, provide the ground for understanding the subtle dynamics of an efficient plasma electron accelerator driven by chirped laser pulses.

A smart alarm for particle accelerator beamline operations

We present the initial results of a proof-of-concept ‘smart alarm’ for the Continuous Electron Beam Accelerator Facility injector beamline at Jefferson Lab. To minimize machine downtime and improve operational efficiency, an autonomous alarm system able to identify and diagnose unusual machine states is needed. Our approach leverages a trained neural network capable of alerting operators (a) when an anomalous condition exists in the beamline and (b) identifying the element setting that is the root cause. The tool is based on an inverse model that maps beamline readings (diagnostic readbacks) to settings (beamline attributes operators can modify). The model takes as input readings from the machine and computes machine settings which are compared to control setpoints. Instances where predictions differ from setpoints by a user-defined threshold are flagged as anomalous. Given data corresponding to 354 anomalous injector configurations, the model can narrow the root cause of an anomalous condition to three potential candidates with 94.6% accuracy. Furthermore, compared to the current method of identifying anomalous conditions which raises an alarm when machine parameters drift outside their normal tolerances, the data-driven model can identify 83% more anomalous conditions.