Beam Loss Research Articles

Liouville Theory for Fully Analytic Studies of Transverse Beam Dynamics in Laser-Plasma Ion Accelerators

The exact solution of the Liouville equation expressed in terms of exponential operators can describe the phase space evolution of particle beams in transport lines. In this paper, we generalize the solution of the above equation for the case of beam losses induced by apertures and for particle beams with large spreads in the momentum space. We discuss the applicability of such approach to ion beams produced by high-intensity lasers interacting with critical plasmas, based on the comparison between theoretical findings and measurements.

Extremum Seeking-Based Control System for Particle Accelerator Beam Loss Minimization

Particle accelerators throughout the world vary widely in terms of age and availability of advanced noninvasive diagnostics that provide varying levels of detail about the accelerated beams. Beam loss measurements and current monitors are ubiquitous in the accelerator community, they are noninvasive, and they are some of the most important metrics in terms of preventing damage or irradiation of beam pipes and equipment for high-energy machines. However, beam loss and current measurements are difficult to use for feedback tuning because of a lack of a known analytic relationship between scalar loss measurements throughout an accelerator and the hundreds of thousands of parameters that influence the beam and time variation of the beam source itself. In this work, we present a model-independent extremum seeking (ES) controller, which has been implemented for automated tuning and optimization of the Los Alamos Neutron Science Center (LANSCE) linear ion accelerator based only on scalar quantities, such as beam loss or flux measurements. We demonstrate the approach on various accelerator subsystems, including groups of radio frequency (RF) accelerating cavities, quadrupole magnets, bending magnets, and steering magnets. This tool is now available as a use graphical user interface (GUI) in the LANSCE control room, allowing beam physicists to choose arbitrary groups of parameters and beam loss or current monitors for adaptive tuning.

Fabrication and Test of a 1:4 Scaled HTS Coil for the Defocusing Magnet of 2 GeV FFAG Accelerator

The basic design of a 2 GeV fixed field alternating gradient (FFAG) accelerator was proposed by China Institute of Atomic Energy (CIAE) in 2019. A preliminary study of high temperature superconducting (HTS) magnet for the 2 GeV FFAG accelerator lattice scheme was carried out, considering the beam loss of high energy proton beams and energy efficiency of the main machine. In order to demonstrate the feasibility of manufacturing a spiral-shaped HTS magnet with concave edges based on ReBCO tapes, a 1:4 scaled HTS model magnet is being developed in CIAE. First, the design and fabrication of the 1:4 scaled prototype HTS magnet with concave edges will be described in detail. Then, each double pancake (DP) was tested by liquid nitrogen (LN) immersion. The terminal voltage and magnetic field variation were measured and simulated. Finally, the whole assembled HTS magnet was cooled down to 23 K using helium gas pipeline contact cooling and operated on 300 A for over 24 hours. The central magnetic field reached 719.5 mT, without iron. More detailed electromagnetic characteristics of the magnet during the excitation will be illustrated, which is much helpful for the fabrication of the final full-size HTS magnets.

Construction of New Branching Point and Operation of New Primary Beam Line at the J-PARC Hadron Facility

Construction of a new primary beam line, called the B-line, started in 2013 at the J-PARC Hadron Experimental Facility. The B-line is branched at the middle of an existing primary proton beam line (A-line) in the beam switching yard (SY), which is the connecting tunnel between the Main Ring (MR) and the Hadron experimental hall (HD-hall). At the branching point, about 0.1% of the primary beam is kicked off at 5 degrees using a Lanbertson magnet and two septum magnets. The Lambertson magnet has a field free hole in its yoke. The proton beam that goes through a field region is separated from the A-line, and it is extracted to the B-line. The rest of the beam that goes through the field free hole is transported through the existing primary beam line. Since a significant beam loss as much as 420 W occurs at the edge of field free hole, the magnets near the Lambertson magnet are operated under a very high radioactive environment. We have developed a “mini-chimney system” regarding easy maintenance of those magnets. The mini-chimney means a vertical tower of approximately 1m in height. The tower is comprised of water pipes, power electrodes, and signal cables for safety interlock. Those can be easily connected and disconnected at the top of chimney on the ceiling iron shields. In this paper we summarize the maintenance scheme developed for the B-line, as well as the design of B-line.In May 2020, the first proton beam was successfully extracted to the B-line by means of the Lambertson and the septum magnets. Up to now, the 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> protons per 5.2 sec accelerator cycle shot have been available.

Evaluation of Eddy Currents Dependent on Excitation Pattern in Design of Pulse Electromagnets

The evaluation of a new shift bump magnet for replacement of the J-PARC (Japan Proton Accelerator Research Complex) RCS (Rapid Cycling Synchrotron) confirmed that the measured magnetic field waveform was different from the excitation-current waveform. The reason for this is the eddy-current effect due to the fast trapezoidal excitation pattern of approximately 1.5 ms. The same result was confirmed by OPERA-3D. A high-precision injection-beam orbit that suppresses device radiation due to beam loss was necessary. Therefore, in the excitation waveform of the injection bump magnet, the variation range at the flat-top must be less than ±1.0%. We evaluated the effectiveness of the dynamic-magnetic field analysis of OPERA-3D comparison with the measurement results. An agreement with a 0.06% deviation in the magnetic field distribution, the effect of eddy currents on the magnetic field waveform, and the potential of the analysis software for system design were confirmed. We also calculated the amount of heat generated by the bump-magnet coil and the necessary measures for operation. Furthermore, we verified that probes can accurately evaluate the pulsed excitation waveforms.

Design of a radio frequency quadrupole for a high intensity heavy-ion accelerator facility

We report a design work of a continuous-wave (cw) radio frequency quadrupole (RFQ) for the injector linac of the High Intensity heavy-ion Accelerator Facility project. The rf design and multiphysics analysis were conducted to enhance the long-term stability and reliability of the cavity. To minimize the peak modified Poynting vector and peak surface electric field associated with the rf breakdown rate, the cross section and the vanes segmentation of the RFQ cavity were optimized. Pi-mode stabilizing loops (PISLs) were adopted for the cavity to increase the mode separation between the quadrupole and the neighboring dipole modes, intending to reduce the emittance growth and beam loss caused by the field asymmetry of the quadrupole mode. Due to the lack of a universal criterion of acceptable mode separation, the field asymmetry of the quadrupole mode was directly used as the design criterion for the PISL. Tuners were designed to tune the cavity frequency and longitudinal field flatness. Thermal distributions on tuners with and without rf sealing were investigated, and we found that rf sealing was indispensable in decreasing the tuner's temperature. A multiphysics analysis was carried out to maintain the cavity frequency while running with different rf power, in an attempt to avoid frequently adjusting the cooling water temperature when accelerating different ions. The analysis also verified that a large wall thickness could make the cavity possess a low stress and a minor deformation.

Positioning of the coil and reduction of the first harmonic of the magnetic field in the superconducting cyclotron using support link forces

A high magnetic field in a superconducting cyclotron is associated with large magnetic forces acting on the magnet coils. These coils are placed in a position using support links that provide stability against the magnetic forces. However, the present configuration of our K500 superconducting cyclotron at VECC, Kolkata does not allow the superconducting coil to be centred with respect to pole tips. This causes field error and beam losses before extraction as observed in our superconducting cyclotron. An analytical model is developed to locate the optimal positioning of the superconducting coils based on support link forces. The present paper describes the methodology for positioning the superconducting coil that produces minimum field error and maintains a safe limit for link forces. Different analyses are presented and compared with experimental data of the superconducting cyclotron at VECC Kolkata.

Optimal injection voltage in the LHC

Finding the optimal rf injection voltage in the LHC is a trade-off between minimising capture and flat-bottom losses, which call for an increased voltage, and minimising rf power consumption as well as improving beam stability, both of which call for a reduced voltage. From the beam stability point of view, earlier particle-tracking simulations showed that the decrease of the injection voltage from 6MV to 4MV is beneficial. This reduction was performed in the LHC Run 2 over a three-week period in steps of 0.5MV. The impact of different voltages and injection energy errors on the evolution of beam parameters such as bunch length and beam losses are analysed in this paper. Operation at 4MV, maintained in the machine after the reduction period, is studied in more detail. The implications for the future High-Luminosity LHC operation with high-intensity beams are also discussed.

The Silicon Vertex Detector of the Belle II experiment

Since the start of data taking in spring 2019 at the SuperKEKB collider (KEK, Japan) the Belle II Silicon Vertex Detector (SVD) has been operating reliably and with high efficiency, while providing high quality data: high signal-to-noise ratio, greater than 99% hit efficiency, and precise spatial resolution. These attributes, combined with stability over time, result in good tracking efficiency. Currently the occupancy, dominated by beam-background hits, is quite low (about 0.5 % in the innermost layer), causing no problems to the SVD data reconstruction. In view of the operation at higher luminosity foreseen in the next years, specific strategies aiming to preserve the tracking performance have been developed and tested on data. The time stability of the trigger allows reducing sampling of the strip-amplifier waveform. The good hit-time resolution can be exploited to further improve the robustness against the higher level of beam background. First effects of radiation damage on strip noise, sensor currents and depletion voltage have been measured: they do not have any detrimental effect on the performance of the detector. Furthermore, no damage to the SVD is observed after sudden and intense bursts of radiation due to beam losses.

Simulation study of the space charge limit in heavy-ion synchrotrons

The SIS100 synchrotron as a part of the new Facility for Antiproton and Ion Research (FAIR) accelerator facility at GSI should be operated at the ``space charge limit'' for light- and heavy-ion beams. Beam losses due to space-charge-induced resonance crossing should not exceed a few percent during a full cycle. The recent advances in the performance of particle tracking tools with self-consistent solvers for the 3D space charge forces now allow us to reliably identify low-loss areas in tune space, considering the full SIS100 accumulation plateau of one second (160 000 turns) duration. A realistic magnet error model, extracted from precise bench measurements of the SIS100 main dipole and quadrupole magnets, is included in the simulations. Previously, such beam dynamics simulations required non-self-consistent space charge models. By comparing to the self-consistent simulations results, we are now able to demonstrate that the predictions from such faster space charge models can be used to identify low-loss regions with sufficient accuracy. The findings are applied by identifying a low-loss working point region in SIS100 for the design FAIR beam parameters. The bunch intensity at the space charge limit is determined. Several countermeasures to space charge are proposed to enlarge the low-loss area and to further increase the space charge limit.

Beam emittance growth due to the strong space-charge field at low energy of a high-intensity ion linac and its mitigation using an octupole magnetic field

Abstract In the low-energy region of a high-intensity ion linac, a strong space-charge field causes a rapid beam emittance growth over a short distance of only a few meters. The beam emittance growth leads to beam loss and machine activation, raising a serious issue for regular maintenance of the accelerator component and beam power ramp-up. We studied the mechanism of beam emittance growth due to the space-charge field based on three-dimensional particle-tracking simulation and theoretical considerations. Numerical simulations of the high-intensity H− (negative hydrogen) linac at the Japan Proton Accelerator Research Complex shows that the nonlinear terms in the space-charge field directly cause beam emittance growth and beam halo formation. We also propose a method to mitigate the beam emittance growth by using an octupole magnetic field, which arises as one of the nonlinear terms in the space-charge field. By applying this method in the simulation we have succeeded in mitigating the beam emittance growth.

Feasibility of crystal-assisted collimation in the CERN accelerator complex

Bent silicon crystals mounted on high-accuracy angular actuators were installed in the CERN Super Proton Synchrotron (SPS) and extensively tested to assess the feasibility of crystal-assisted collimation in circular hadron colliders. The adopted layout was exploited and regularly upgraded for about a decade by the UA9 Collaboration. The investigations provided the compelling evidence of a strong reduction of beam losses induced by nuclear inelastic interactions in the aligned crystals in comparison with amorphous orientation. A conceptually similar device, installed in the betatron cleaning insertion of CERN Large Hadron Collider (LHC), was operated through the complete acceleration and storage cycle and demonstrated a large reduction of the background leaking from the collimation region and radiated into the cold sections of the accelerator and the experimental detectors. The implemented layout and the relevant results of the beam tests performed in the SPS and in the LHC with stored proton and ion beams are extensively discussed.

Long-term mechanical behaviors of uncracked concrete beams prestressed with external basalt fiber-reinforced polymer tendons

In this study, an experiment was conducted on four uncracked concrete beams prestressed with external basalt fiber-reinforced polymer (BFRP) tendons or stress-relieved strands subjected to 300-day sustained loads. The experimental variables included tendon type, prestress level (40 kN and 50 kN), and concrete strength grade (C40 and C60). The effects of these variables on the long-term deformation and prestress loss of the beams were discussed and analyzed. Based on an existing approach using an age-adjusted effective modulus method (AEMM), a modified model was proposed for predicting the long-term deformation and prestress loss of concrete beams prestressed with external BFRP tendons. The results indicated that the prestress level had significant effects on the long-term deformation and prestress loss. The concrete strength grade significantly affected the long-term deformation, but had no effect on the prestress loss. The stress-relieved strands exhibited a prestress loss that was 16% greater than that of the BFRP tendons. The theoretical results based on the modified model were consistent with the experimental results. The results of this study contribute to the design and prediction of the long-term behaviors of concrete beams prestressed with external BFRP tendons.

Beam loss study for the implementation of corrugated structure at the European XFEL

The European XFEL is a free-electron laser facility based on superconducting linac with high repetition rate of up to 4.5 MHz. A corrugated structure is planned to be installed in front of the SASE beam line at the European XFEL, which can be used as a kicker for fresh-slice schemes or a dechirper to control the bandwidth of SASE radiation. When the beam passes through the corrugated structure module, strong longitudinal and transverse wakefields can be excited to introduce a correlated energy chirp and/or a kick along the bunch. However, due to the relatively small gap of the corrugated structure, beam halo particles experience large betatron oscillations after hitting the corrugated structure, which can lead to energy deposition in the undulator beam line. This may cause demagnetization of the undulators especially at high repetition rate. For this reason, we have performed beam loss simulations using BDSIM for two different configurations: double-plate corrugated structure with nominal FODO optics and single-plate corrugated structure with small beta optics. Our simulation results showed that the latter configuration can help us not only to relax the tolerance on beam halo extension, but also to reduce the peak radiation doses by more than factor 2. Besides, the possibility of adding a collimator downstream of the corrugated structure has also been studied, which can further help us to reduce the loss in the downstream cells by factor 100.

Beam loss monitoring system for the SKIF synchrotron light source

The Siberian ring source of photons (SKIF) is a new 3 GeV fourth-generation synchrotron light source being developed by the Budker Institute of Nuclear Physics (BINP). A beam loss monitoring system is necessary to ensure its reliable commissioning and operation. Two types of beam loss monitors will be installed in the SKIF: 5 fiber-based Cherenkov beam loss monitors (CBLMs) for the linac and transfer lines and 128 scintillator-based beam loss monitors (SBLMs) for the storage ring. Sophisticated electronic equipment is employed to use these monitors in different modes of SKIF operation. The paper describes the design of the SKIF beam loss monitoring system based on numerical simulations and experimental studies.

Emittance growth caused by phase modulation in bunched electron beam cooling

The demand for electron coolers operating at much higher energies than previously achieved necessitates the use of radio-frequency (RF) fields for electron acceleration resulting in the bunched electron beams. Differences in the time structure of ion and electron bunches and variations in their relative timing significantly affect the cooling process. Experiments on ion beam cooling with a short electron bunch have been carried out in the CSRm storage ring at IMP (Lanzhou). Longitudinal painting by the electron bunch phase modulation is found leading to a significant ion beam loss. We show that the space charge field variations of the bunched electron beam play a major role in the ion beam emittance growth while cooling. In this paper, we study the dependency of the ion beam emittance growth on the electron beam parameters and present results of numerical simulation which support experimental observations.

Fatigue Behavior of Heavy-Haul Railway Prestressed Concrete Beams Based on Vehicle-Bridge Coupling Vibration.

Due to the demand for increasing trainload and enhancing some existing heavy-haul railways, the low reserve value of bearing capacity is a problem for the 32 m-span simply supported beam. The fatigue behavior of prestressed concrete beams in a heavy-haul railway loaded by 33 t and larger axle weight of trains was experimentally investigated. The experimental results of the fatigue behaviors, including fatigue deformation, crack propagation behavior, and strains of classical materials were obtained and analyzed. A fatigue behavior assessment model was established to investigate the residual stiffness and yield point degradation of the beams loaded by the trainload. The effects of train fatigue cycles and prestress loss on the residual stiffness and yield point degradation models of the beams were analyzed. The results indicated that the crack development process had three stages during the fatigue process: the derivative stage, gradual development stage, and fatigue failure stage. Trainload was the main external factor influencing the fatigue behavior of prestressed concrete beams. The increase in trainload accelerated the degradation rate of the residual stiffness of the beams and yield point, reducing the fatigue life. The prestressing strand was primarily used to delay the concrete cracking in the tension zone. When the beam was not cracked, the prestressed concrete beam showed good fatigue performance, and the degree of prestressing did not affect the fatigue life of the beams. When the maximum fatigue load exceeded the cracking load of the beam, prestress loss in beams became a critical issue that accelerated the degradation rate of fatigue strength and reduced fatigue life. The higher the fatigue damage degree, the more pronounced the effect of prestress loss on the fatigue strength of the beams. The fatigue failure of prestressed concrete beams occurred in the bottom tensile steel bar. Therefore, when the trainload of a heavy-haul railway is greater than the cracking load of the beam, it is recommended to strengthen the beam by prestressing and strictly control the trainload to avoid yield failure.

Control of bending wave reflection at beam terminations by thermally tunable subwavelength resonators

The perfect absorption of bending waves by thermally tunable subwavelength absorbers is analytically and experimentally demonstrated in this work for a reflection problem in beams. These absorbers are local resonators located at the end of a semi-infinite beam and consist of thin blades covered with a layer of Shape Memory Polymer (SMP), the stiffness and damping of which strongly depend on the temperature. The resonators therefore present both energy leakage from the absorbers to the beam and inherent losses thanks to the presence of the SMP. In particular, the situation in which the inherent losses balances the energy leakage is known as the critical coupling conditions and leads to the perfect absorption of the incident wave in the reflection problem. It is shown that these conditions can be simultaneously fulfilled for the different blades of the resonating system by tuning the temperature of the SMP and the geometry of the blades. The analytical results are based on the Impedance Matrix Method. The experimental validation highlights the possibility of achieving multi-frequency subwavelength absorption in the case of multi-blades configurations.

Dust-induced beam losses in the cryogenic arcs of the CERN Large Hadron Collider

The interaction of dust particles with the LHC proton beams accounts for a major fraction of irregular beam loss events observed in LHC physics operation. The events cease after a few beam revolutions because of the expulsion of dust particles from the beam once they become ionized in the transverse beam tails. Despite the transient nature of these events, the resulting beam losses can trigger beam aborts or provoke quenches of superconducting magnets. In this paper, we study the characteristics of beam-dust particle interactions in the cryogenic arcs by reconstructing key observables like nuclear collision rates, loss durations and integral losses per event. The study is based on events recorded during 6.5 TeV operation with stored beam intensities of up to $\sim 3\times 10{^{14}}$ protons per beam. We show that inelastic collision rates can reach almost $10^{12}$ collisions per second, resulting in a loss of up to $\sim 1.6\times 10^{8}$ protons per event. We demonstrate that the experimental distributions and their dependence on beam parameters can be described quantitatively by a previously developed simulation model if dust particles are assumed to be attracted by the beam. The latter finding is consistent with recent time profile studies and yields further evidence that dust particles carry a negative charge when entering the beam. We also develop different hypotheses regarding the absence of higher-loss events in the measurements, although such events are theoretically not excluded by the simulation model. The results provide grounds for predicting dust-induced beam losses in presence of higher-intensity beams in future runs of the High-Luminosity LHC.

Vlasov description of the beam response to noise in the presence of wakefields in high-energy synchrotrons: beam transfer function, diffusion, and loss of Landau damping

Noise can have severe impacts on particle beams in high-energy synchrotrons. In particular, it has recently been discovered that noise combined with wakefields can cause a diffusion that leads to a loss of Landau damping after a latency. Such instabilities have been observed in the Large Hadron Collider. This paper, therefore, studies the beam response to noise in the presence of wakefields, within the framework of the Vlasov equation. First, a wakefield beam eigenmode transfer function (MTF) is derived, quantifying the amplitude of a wakefield eigenmode when excited by noise. Then, the MTFs of all the wakefield eigenmodes are combined to derive the beam transfer function (BTF) including the impact of wakefields. It is found to agree excellently with multi-particle tracking simulations. Finally, the MTFs are also used to derive the single-particle diffusion driven by the wakefield eigenmodes. This new Vlasov-based theory for the diffusion driven by noise-excited wakefields is found to be superior to an existing theory by comparing to multi-particle tracking simulations. Through sophisticated simulations that self-consistently model the evolution of the distribution and the stability diagram, the diffusion is found to lead to a loss of Landau damping after a latency. The most important technique to extend the latency and thereby mitigate these instabilities is to operate the synchrotron with a stability margin in detuning strength relative to the amount of detuning required to barely stabilize the beam with its initial distribution.