Uncontrolled Beam Loss Research Articles

Technological developments and accelerator improvements for the FRIB beam power ramp-up

The Facility for Rare Isotope Beams (FRIB) began operation with 1 kW beam power for scientific users in May 2022 upon completion of 8 years of project construction. The ramp-up to the ultimate beam power of 400 kW, planned over a 6-year period, will enable the facility to reach its full potential for scientific discovery in isotope science and applications. In December 2023, a record-high beam power of 10.4 kW uranium was delivered to the target. Technological developments and accelerator improvements are being made over the entire facility and are key to completion of the power ramp-up. Major technological developments entail the phased deployment of high-power beam-intercepting systems, including the charge strippers, the charge selection systems, the production target, and the beam dump, along with support systems, including non-conventional utilities (NCU) and remote handling facilities. Major accelerator improvements include renovations to aging legacy systems associated with experimental beam lines and system automation for improved operational efficiency and better machine availability. Experience must be gained to safely handle the increased radiological impacts associated with high beam power; extensive machine studies and advanced beam tuning procedures are needed to minimize uncontrolled beam losses for the desired operating conditions. This paper discusses the technological developments and accelerator improvements with emphasis on major R&D efforts.

Commissioning and operation of the transverse collimation system at the RCS of CSNS

In high-intensity proton synchrotrons, controlling beam loss during machine operation is crucial to prevent radiation damage. Uncontrolled beam loss poses a significant challenge in achieving higher beam intensity and power in these synchrotrons. The beam collimation system plays a vital role in removing halo particles and localizing beam loss, serving as a critical tool for controlling uncontrolled beam loss in high-intensity proton accelerators. To address the issue of uncontrolled beam loss, a two-stage transverse collimation system was designed at the rapid cycling synchrotron (RCS) of China Spallation Neutron Source (CSNS). However, during the beam commissioning, it was found that the collimation efficiency was compromised due to the small ratio of the physical acceptance to the collimator acceptance and insufficient phase advances between the primary and secondary collimators. To address this issue, the designed two-stage collimator was modified to a one-stage collimator. Through optimization of the collimation system, the beam loss was effectively localized within the collimator area. As a result, CSNS achieved the design power of 100 kW with minimal uncontrolled beam loss.

Recent results of beam loss mitigation and extremely low beam loss operation of J-PARC RCS

In the 3-GeV RCS (Rapid Cycling Synchrotron) at J-PARC (Japan Proton Accelerator Research Complex), multi-turn H− charge-exchange injection is performed by using a thin stripped foil. The residual radiation at the injection area caused by the uncontrolled beam loss occurred by foil scattering of the circulating beam is a serious issue for regular maintenance works. In addition, the beam loss at the collimator section and its downstream caused for a large emittance beam also should be reduced, especially at high intensity operation. For that purpose we have minimized injection beam size and implemented a smaller size stripper foil. The circulating beam hitting rate is reduced by using a smaller foil, while an optimized vertical angle of the smaller injection beam for vertical transverse painting also gave a reduction of the circulating beam emittances and resulted a further significant beam loss mitigation at the collimator section and its downstream. As a result, the residual radiation after user operation at 700 kW beam power was also measured to be significantly reduced.

Fast failures in the LHC and the future high luminosity LHC

An energy of $362\:\text{MJ}$ is stored in each of the two LHC proton beams for nominal beam parameters. This will be further increased to about $700\:\text{MJ}$ in the future High Luminosity LHC (HL-LHC) and uncontrolled beam losses represent a significant hazard for the integrity and safe operation of the machine. In this paper, a number of failure mechanisms that can lead to a fast increase of beam losses are analyzed. Most critical are failures in the magnet protection system, namely the quench heaters and a novel protection system called Coupling-Loss Induced Quench (CLIQ). An important outcome is that magnet protection has to be evaluated for its impact on the beam and designed accordingly. In particular, CLIQ, which is to protect the new HL-LHC triplet magnets, constitutes the fastest known failure in the LHC if triggered spuriously. A schematic change of CLIQ to mitigate the hazard is presented. A loss of the Beam-Beam Kick due to the extraction of one beam is another source of beam losses with a fast onset. A significantly stronger impact is expected in the upcoming LHC Run III and HL-LHC as compared to the current LHC, mainly due to the increased bunch intensity. Its criticality and mitigation methods are discussed. It is shown that symmetric quenches in the superconducting magnets for the final focusing triplet can have a significant impact on the beam on short timescales. The impact on the beam due to failures of the Beam-Beam Compensating Wires as well as coherent excitations by the transverse beam damper are also discussed.

Beam commissioning and beam loss control for CSNS accelerators

The China Spallation Neutron Source (CSNS) accelerators consist of an 80 MeV H− linac and a 1.6 GeV proton rapid cycling synchrotron (RCS), which provides 1.6 GeV proton beam to the target at a repetition rate of 25 Hz. The beam commissioning of CSNS accelerators had been started since April 2017, and to the end of February 2020, the CSNS reached the design beam power of 100 kW. The most important issue in high-power beam commissioning is the beam loss control, as well as the control of induced activities, to meet the requirement of manual maintenance. A series of beam loss optimization work had been done to reduce the uncontrolled beam loss. Transverse and longitudinal matching in the MEBT was performed. For RCS commissioning, the orbit and lattice correction, the matching between the rf frequency and the dipole field, the tuning of the injection beam were performed. Many efforts have beam made to depress the effects of space charge and collective instability for beam loss control, such as injection optimization, longitudinal beam dynamics optimization, tune optimization. The beam commissioning procedure and results for CSNS accelerators are introduced in this paper.

NSLS-II beam loss control and monitoring system

One of the most challenging issues for accelerator operations is why and where the beam is lost. This is a major concern for radiation protection at facilities with a large user base, since uncontrolled beam losses could potentially result in unwanted radiation exposures to occupied areas. For NSLS-II the tunnel radiation shield wall has increased attenuation over two lattice cells after the injection region, to shield for the possibility of higher injection losses. This raised the concern over exceeding radiation exposure limits in lesser shielded regions, if similar losses occur in those regions. To address this issue a beam loss control system was designed to limit most beam losses to the more heavily shielded (injection) region. A beam loss monitor system was also designed to verify the fraction of the beam lost in that region and thereby ensure lower levels of exposure around the rest of the ring. The design of these beam loss control and monitoring systems and the NSLS-II operational experience is presented.

Prediction of beam losses during crab cavity quenches at the high luminosity LHC

Studies of the crab cavities at KEKB revealed that the RF phase could shift by up to 50o within ~50 us during a quench; while the cavity voltage is still at approximately 75% of its nominal amplitude. If such a failure were to occur on the HL-LHC crab cavities, it is likely that the machine would sustain substantial damage to the beam line and surrounding infrastructure due to uncontrolled beam loss before the machine protection system could dump the beam. We have developed a low-level RF system model, including detuning mechanisms and beam loading, and use this to simulate the behaviour of a crab cavity during a quench, modeling the low-level RF system, detuning mechanisms and beam loading. We supplement this with measurement data of the actual RF response of the proof of principle Double-Quarter Wave Crab Cravity during a quench. Extrapolating these measurements to the HL-LHC, we show that Lorentz Force detuning is the dominant effect leading to phase shifts in the crab cavity during quenches; rather than pressure detuning which is expected to be dominant for the KEKB crab cavities. The total frequency shift for the HL-LHC crab cavities during quenches is expected to be about 460 Hz, leading to a phase shift of no more than 3o. The results of the quench model are read into a particle tracking simulation, SixTrack, and used to determine the effect of quenches on the HL-LHC beam. The quench model has been benchmarked against the KEKB experimental measurements. In this paper we present the results of the simulations on a crab cavity failure for HL-LHC as well as for the SPS and show that beam loss is negligible when using a realistic low-level RF response.

Beam Dynamics for High-Power Superconducting Heavy-Ion Linear Accelerator of RAON

We investigated optimum choices of accelerator parameters for superconducting-technology-based heavy-ion linear accelerator (linac) to achieve the high beam quality at an in-flight target with a high beam power of 400 kW. The superconducting linac system is designed to provide a stable-ion beams from protons to uranium with energies of 600 MeV and 200 MeV/u, respectively. The linac optics is also designed such that the effects of envelope instability, parametric resonance between the transverse and longitudinal planes, beam steering effect due to asymmetric field in quarter-wave resonance (QWR) cavities, and emittance growth in charge strippers due to the straggling effect, parametric resonance, and envelope instability become as small as possible. The dimensions of the warm and cold sections in the low-energy section which mainly determine the longitudinal acceptance of the linac are optimized to achieve a longitudinal acceptance larger than 27 keV/u-ns enabling stable operation. The tracking simulation for the uranium beam was performed and we achieved the transverse normalized rms emittance of 0.095 mm-mrad and longitudinal rms emittance of 2.10 keV/u-ns at the end of designed linac without uncontrolled beam losses along the linac. In order to estimate the degradation of the machine performance due to imperfections such as misalignment, ripple of the magnet power source, and phase and power variation of the RF source, the particle tracking simulation was performed and the uncontrolled beam loss with the machine imperfection without the orbit and optics correction was less than 1 W/m which is a condition required for machine operation. The optimum scheme for orbit correction in the low-energy linac is investigated based on the transfer matrix of the elements and the validation of the scheme is confirmed by the particle tracking simulation using TRACK code.

Measured and simulated heavy-ion beam loss patterns at the CERN Large Hadron Collider

The Large Hadron Collider (LHC) at CERN pushes forward to new regimes in terms of beam energy and intensity. In view of the combination of very energetic and intense beams together with sensitive machine components, in particular the superconducting magnets, the LHC is equipped with a collimation system to provide protection and intercept uncontrolled beam losses. Beam losses could cause a superconducting magnet to quench, or in the worst case, damage the hardware. The collimation system, which is optimized to provide a good protection with proton beams, has shown a cleaning efficiency with heavy-ion beams which is worse by up to two orders of magnitude. The reason for this reduced cleaning efficiency is the fragmentation of heavy-ion beams into isotopes with a different mass to charge ratios because of the interaction with the collimator material. In order to ensure sufficient collimation performance in future ion runs, a detailed theoretical understanding of ion collimation is needed. The simulation of heavy-ion collimation must include processes in which Pb82+208 ions fragment into dozens of new isotopes. The ions and their fragments must be tracked inside the magnetic lattice of the LHC to determine their loss positions. This paper gives an overview of physical processes important for the description of heavy-ion loss patterns. Loss maps simulated by means of the two tools ICOSIM [1,2] and the newly developed STIER (SixTrack with Ion-Equivalent Rigidities) are compared with experimental data measured during LHC operation. The comparison shows that the tool STIER is in better agreement.

Towards quantitative simulations of high power proton cyclotrons

We describe a large scale simulation effort using Object Oriented Parallel Accelerator Library, that leads to a better quantitative understanding of the existing Paul Scherrer Institut high power proton cyclotron facility. The 1.3 MW of beam power on target poses stringent constraints on the controlled and uncontrolled beam losses. We present initial conditions for the Ring simulation, obtained from the new time-structure measurement and the many profile monitors of the 72 MeV transfer line. A trim coil model is developed, including trim coil TC15, which is needed to avoid the dangerous ${\ensuremath{\nu}}_{r}=2{\ensuremath{\nu}}_{z}$ resonance. By properly selecting the injection position and angle (eccentric injection), the flattop voltage, and phase, very good agreement between simulations and measurements at the radial probe RRE4 is obtained. We report on $3--4$ orders of magnitude in dynamic range when comparing simulations with measurements. The relation between beam intensity, rms beam size, and accelerating voltage is studied and compared with measurement. The demonstrated capabilities are mandatory in the design and operation of the next generation high power proton drivers. In an outlook we discuss our future plans to include more physics into the model, which eventually leads to an even larger dynamic range in the simulation.

Beam Collimation in PEFP RCS

This work is related with the beam collimation in the injection period of the proton engineering frontier project (PEFP) rapid cycling synchrotron (RCS).The two-stage collimation scheme was used with two rectangular collimators. We studied the collimation efficiency in various collimator conditions and optimized the collimation systemin order to minimize the uncontrolled beam losses.

Estimated Limits on Uncontrolled Beam Losses of Heavy Ions for Allowing Hands-On Maintenance at an Exotic Beam Facility Linac

The purpose of the study is to obtain estimates of limits on uncontrolled beam losses of heavy ions for allowing hands-on maintenance at a heavy-ion linac for a rare isotope beam facility. Semiempirical formulas are used to estimate dose equivalent rates from activated accelerator components for 1 W/m uncontrolled losses of protons up to 1 GeV. The estimated dose rates after a 100-day irradiation time, 4-h postshutdown cooling time are compared to a hands-on maintenance limit of 1 mSv/h (100 mrem/h) at 30 cm. The transport codes PHITS and MCNP5 and activation code DCHAIN-SP 2001 are used to verify the estimate for proton losses and to obtain limits on heavy-ion beam losses that will satisfy the hands-on maintenance dose rate limit.

Longitudinal RF capture and acceleration simulation in CSNS RCS

China Spallation Neutron Source (CSNS) is a high power proton accelerator-based facility. Uncontrolled beam loss is a major concern in designing the CSNS to control the radioactivation level. For the Rapid Cycling Synchrotron (RCS) of the CSNS, the repetition frequency is too high for the longitudinal motion to be fully adiabatic. Significant beam loss happens during the RF capture and initial acceleration of the injection period. To reduce the longitudinal beam loss, beam chopping and momentum offset painting methods are used in the RCS injection. This paper presents detailed studies on the longitudinal motion in the RCS by using the ORBIT simulations, which include different beam chopping factors, momentum offsets and RF voltage optimization. With a trade-off between the longitudinal beam loss and transverse incoherent tune shift that will also result in beam losses, optimized longitudinal painting schemes are obtained.

Macro-particle simulation study on transverse halo collimator for J-PARC linac

To realize the transverse emittance required for the ring injection, a transverse halo collimation system of the charge-exchange type has been developed for J-PARC (Japan Proton Accelerator Research Complex) linac. In this type of collimation system, negative hydrogen ions constituting a tail portion are charge-exchanged to protons with thin foil collimators. Then, the generated protons are separated by a bending magnet to be delivered to a dedicated beam dump. Because a two-species beam is transported along the collimation system, a special care is to be taken to avoid the uncontrolled beam loss along the beam transport. To this end, it is of practical importance to develop an accurate numerical model that has the capability of both particle tracking with space-charge effects and modeling of scattering at the charge-exchange foil. In this paper, a numerical model is developed for the collimation system using a three-dimensional particle-in-cell code, IMPACT, and a Monte-Carlo simulation tool kit, GEANT. Uncontrolled beam loss along the collimation system has been evaluated with this model, and the optimum foil thickness has been determined for the collimation system.

Proof-of-principle demonstration of high efficiency laser-assistedH−beam conversion to protons

Thin carbon foils are used as strippers for charge exchange injection into high intensity proton rings. However, the stripping foils become radioactive and produce uncontrolled beam loss, which is one of the main factors limiting beam power in high intensity proton rings. Recently, we presented a scheme for laser stripping an ${\mathrm{H}}^{\ensuremath{-}}$ beam for the Spallation Neutron Source (SNS) ring. First, ${\mathrm{H}}^{\ensuremath{-}}$ atoms are converted to ${\mathrm{H}}^{0}$ by a magnetic field, then ${\mathrm{H}}^{0}$ atoms are excited from the ground state to the upper levels by a laser, and the excited states are converted to protons by a magnetic field. In this paper we report on the proof-of-principle demonstration of this scheme to give high efficiency (around 90%) conversion of ${\mathrm{H}}^{\ensuremath{-}}$ beam into protons at SNS in Oak Ridge. The experimental setup is described, and comparison of the experimental data with simulations is presented.

Protection of the CERN Large Hadron Collider

The Large Hadron Collider (LHC) at CERN will collide two counter-rotating proton beams, each with an energy of 7 TeV. The energy stored in the superconducting magnet system will exceed 10 GJ, and each beam has a stored energy of 362 MJ which could cause major damage to accelerator equipment in the case of uncontrolled beam loss. Safe operation of the LHC will therefore rely on a complex system for equipment protection. The systems for protection of the superconducting magnets in case of quench must be fully operational before powering the magnets. For safe injection of the 450 GeV beam into the LHC, beam absorbers must be in their correct positions and specific procedures must be applied. Requirements for safe operation throughout the cycle necessitate early detection of failures within the equipment, and active monitoring of the beam with fast and reliable beam instrumentation, mainly beam loss monitors (BLM). When operating with circulating beams, the time constant for beam loss after a failure extends from ≈ms to a few minutes—failures must be detected sufficiently early and transmitted to the beam interlock system that triggers a beam dump. It is essential that the beams are properly extracted on to the dump blocks at the end of a fill and in case of emergency, since the beam dump blocks are the only elements of the LHC that can withstand the impact of the full beam.

Beam halo formation in high-intensity beams

Studies of beam halo became an unavoidable feature of high-intensity machines where uncontrolled beam loss should be kept to extremely small level. For a well-controlled stable beam such a loss is typically associated with the low-density halo surrounding beam core. In order to minimize uncontrolled beam loss or improve performance of an accelerator, it is very important to understand what are the sources of halo formation in a specific machine of interest. The dominant mechanisms are, in fact, different in linear accelerators, circular machines or energy recovering linacs (ERL). In this paper, we summarize basic mechanisms of halo formation in high-intensity beams and discuss their application to various types of accelerators of interest, such as linacs, rings and ERL.

Low-loss design for the high-intensity accumulator ring of the Spallation Neutron Source

This paper summarizes the low-loss design for the Spallation Neutron Source accumulator ring [``Spallation Neutron Source Design Manual'' (unpublished)]. A hybrid lattice consisting of FODO arcs and doublet straights provides optimum matching and flexibility for injection and collimation. For this lattice, optimization focuses on six design goals: a space-charge tune shift low enough (below 0.15) to avoid strong resonances, adequate transverse and momentum acceptance for efficient beam collimation, injection optimized for desired target beam shape and minimal halo development, compensation of magnet field errors, control of impedance and instability, and prevention against accidental system malfunction. With an expected collimation efficiency of more than 90%, the uncontrolled fractional beam loss is expected to be at the ${10}^{\ensuremath{-}4}$ level.

Space charge dynamics in high intensity rings

Uncontrolled beam losses due to space-charge-induced halo generation are a concern in high intensity rings, which are characterized by high beam intensities and low uncontrolled beam loss requirements. It is therefore important to investigate the dynamics of space charge in high intensity rings. We report here the results of extensive calculations using a particle-tracking approach with a self-consistent particle-in-cell model and alternatively with a particle core model. We find that the inclusion of space charge forces provides agreement between calculated and experimentally observed beam profile shapes in the high intensity proton storage ring. We also confirm computationally the extension to rings of the accepted dynamics of halo generation with rms beam mismatch exciting the parametric resonance. In addition, we propose a new two-stage mechanism for halo production in rings in which space-charge-driven lattice resonances generate beam mismatch that excites the parametric resonance. Because of its dependence on lattice resonances, this mechanism is peculiar to rings and is capable of generating halo even from initially matched beams. It is also very sensitive to the operating point in tune space, as we show in the results of a vertical tune scan simulating injection into the Spallation Neutron Source accumulator ring. Our results extend and enhance the understanding of fundamental space charge physics, which has been developed for linear accelerators, to rings.

SNS accumulator ring collimator optimization with beam dynamics

Abstract It is required to have an uncontrolled beam loss less than 1 nA/m at 1 GeV energy for hands-on maintenance of the Spallation Neutron Source (SNS) Accumulator Ring. Collimators will be used for this end. Various beam dynamics questions related with the collimators have been studied. Various factors are optimized with the given ring lattice and error study is done. Numerical simulations also indicate that movable shielding is necessary for a few hot places downstream of the primary collimator. These simulations indicate that with properly designed collimators the uncontrolled beam-loss requirements of the SNS accumulator ring may be achievable.