Beam Halo Research Articles

Beam diffusion measurements using collimator scans in the LHC

The time evolution of beam losses during a collimator scan provides information on halo diffusion and population. This is an essential input for machine performance characterization and for the design of collimation systems. Beam halo measurements in the CERN Large Hadron Collider were conducted through collimator scrapings in a dedicated beam study for the first time at 4 TeV. Four scans were performed with two collimators, in the vertical plane for beam 1 and horizontally for beam 2, before and after bringing the beams into collisions. Inward and outward steps were performed. A diffusion model was used to interpret the observed loss rate evolution in response to the collimator steps. With this technique, diffusion coefficients were estimated as a function of betatron oscillation amplitude from approximately 3 to 7 standard deviations of the transverse beam distribution. A comparison of halo diffusion and core emittance growth rates is also presented.

Study of beam optics and beam halo by integrated modeling of negative ion beams from plasma meniscus formation to beam acceleration

To understand the physical mechanism of the beam halo formation in negative ion beams, a two-dimensional particle-in-cell code for simulating the trajectories of negative ions created via surface production has been developed. The simulation code reproduces a beam halo observed in an actual negative ion beam. The negative ions extracted from the periphery of the plasma meniscus (an electro-static lens in a source plasma) are over-focused in the extractor due to large curvature of the meniscus.

Superconducting Hadron Linacs

This article discusses the main building blocks of a superconducting (SC) linac, the choice of SC resonators, their frequencies, accelerating gradients and apertures, focusing structures, practical aspects of cryomodule design, and concepts to minimize the heat load into the cryogenic system. It starts with an overview of design concepts for all types of hadron linacs differentiated by duty cycle (pulsed or continuous wave) or by the type of ion species (protons, H -, and ions) being accelerated. Design concepts are detailed for SC linacs in application to both light ion (proton, deuteron) and heavy ion linacs. The physics design of SC linacs, including transverse and longitudinal lattice designs, matching between different accelerating–focusing lattices, and transition from NC to SC sections, is detailed. Design of high-intensity SC linacs for light ions, methods for the reduction of beam losses, preventing beam halo formation, and the effect of HOMs and errors on beam quality are discussed. Examples are taken from existing designs of continuous wave (CW) heavy ion linacs and high-intensity pulsed or CW proton linacs. Finally, we review ongoing R&D work toward high-power SC linacs for various applications.

Beam halo imaging with a digital optical mask

Beam halo is an important factor in any high intensity accelerator. It can cause difficulties in the control of the beam, emittance growth, particle loss, and even damage to the accelerator. It is therefore essential to understand the mechanisms of halo formation and its dynamics. Experimental measurement of the halo distribution is a fundamental tool for such studies. In this paper, we present a new high dynamic range, adaptive masking method to image beam halo, which uses a digital micromirror-array device. This method has been thoroughly tested in the laboratory using standard optical techniques, and with an actual beam produced by the University of Maryland Electron Ring (UMER). A high dynamic range ($\mathrm{DR}\ensuremath{\sim}{10}^{5}$) has been demonstrated with this new method at UMER and recent studies, with more intense beams, indicate that this DR can be exceeded by more than an order of magnitude. The method is flexible, easy to implement, low cost, and can be used at any accelerator or light source. We present the results of our measurements of the performance of the method and illustrative images of beam halos produced under various experimental conditions.

Meniscus and beam halo formation in a tandem-type negative ion source with surface production

A meniscus of plasma-beam boundary in H− ion sources largely affects the extracted H− ion beam optics. Although it is hypothesized that the shape of the meniscus is one of the main reasons for the beam halo observed in experiments, a physical mechanism of the beam halo formation is not yet fully understood. In this letter, it is first shown by the 2D particle in cell simulation that the H− ions extracted from the periphery of the meniscus cause a beam halo since the surface produced H− ions penetrate into the bulk plasma, and, thus, the resultant meniscus has a relatively large curvature.

Spatial and temporal profile monitor with nanosecond resolution for CERN's Linac4

CERN's Linac4 will provide 160-MeV H− beams with an intensity of 2 × 1014 ions s−1. In the first section of Linac4, a beam chopper removes sequences of 500-ps-long micro-bunches from the H− beam. We developed a monitor to measure the time structure and spatial profile of this chopped beam with resolutions Δt ∼ 1 ns and Δx ∼ 2 mm. The large active area 40 mm × 40 mm and dynamic range of the detector also allows studies of beam halos. The H− beam first struck a carbon foil, and the resulting burst of secondary electrons were accelerated by a series of parallel grid electrodes. The electrons struck a phosphor screen, and the resulting image of the scintillation light was transported to a charge-coupled device camera. The time resolution was attained by applying high-voltage pulses of sub-nanosecond rise and fall times to the electrodes and phosphor screen.

USE OF CRYSTALS FOR BEAM DEFLECTION IN PARTICLE ACCELERATORS

Charged particles interacting with a bent crystal can be trapped in channeling states and deflected by the atomic planes of the crystal lattice. The use of bent crystals for beam manipulation in particle accelerators is a well assessed concept rapidly evolving into practical applications. In the last three decades, a large number of experimental findings have contributed to clarify our knowledge and to improve our control of crystal–particle interactions. Bent crystals can impart angular deflections to the incoming particles, through channeling or volume reflection mechanisms. The efficiency of the latter mechanism has been found to be intrinsically very large, whilst the channeling efficiency has been improved by the increased technological expertize in crystal cutting and bending. In this paper, we review the recent milestones of the worldwide effort to propose a routine use of bent crystals in particle accelerators, with a specific attention to the proposals of promoting the use of bent crystals as primary element in beam halo collimation systems.

Pulse-mode measurement of electron beam halo using diamond-based detector

Using a diamond-based detector, the electron beam halo in a high-energy accelerator can be measured with a lower detection limit than that using other instruments, such as a core monitor, a dose meter, or an optical fiber. We have successfully measured an electron beam halo using diamond-based detectors operating in the ionization mode, which were installed in the beam duct to measure the intensity of the beam halo directly. Pulse-by-pulse measurements were adopted to suppress the background noise efficiently. Feasibility tests on the diamond-based detector and beam halo monitor were performed in the beam dump area of the 8 GeV SPring-8 synchrotron booster and at the 250 MeV SPring-8 Compact SASE Source test accelerator for the SPring-8 Angstrom Compact free electron LAser (SACLA), respectively. We achieved a lower detection limit of $2\ifmmode\times\else\texttimes\fi{}{10}^{3}\text{ }\text{ }\mathrm{electrons}/\mathrm{pulse}$ for single-shot measurement, which corresponds to a ratio of about ${10}^{\ensuremath{-}6}$ relative to the typical charge of the beam core of 0.3 pC. We also confirmed the feasibility of the electron beam halo monitor for use as an interlock sensor to protect undulator permanent magnets used in SACLA from radiation damage.

The CMS Beam Conditions and Radiation Monitoring System

Abstract The Compact Muon Solenoid (CMS) is one of the two large, general purpose experiments situated at the LHC at CERN. As with all high energy physics experiments, knowledge of the beam conditions and luminosity is of vital importance. The Beam Conditions and Radiation Monitoring System (BRM) is installed in CMS to protect the detector and to provide feedback to LHC on beam conditions. It is composed of several sub-systems that measure the radiation level close to or inside all sub-detectors, monitor the beam halo conditions with different time resolution, support beam tuning and protect CMS in case of adverse beam conditions by firing a beam abort signal. This paper presents three of the BRM subsystems: the Fast Beam Conditions Monitor (BCM1F), which is designed for fast flux monitoring, measuring with nanosecond time resolution, both the beam halo and collision products; the Beam Scintillator Counters (BSC), that provide hit rates and time information of beam halo and collision products; and the Beam Conditions Monitors (BCM) used as a protection system that can trigger a beam dump when beam losses occur in order to prevent damage to the pixel and tracker detectors. A description of the systems and a characterization on the basis of data collected during LHC operation is presented.

A Gas-Filled Calorimeter for High Intensity Beam Environments

Abstract We describe a novel gas-Cherenkov calorimeter, which detects Cherenkov light showers emitted in an array of thin metal tubes or channels filled with gas. The materials are not vulnerable to radiation damage, and the detector is inherently fast and able to operate in high rate environments. Future accelerators such as the ILC and a muon collider will need fast, radiation-tolerant detectors for monitoring beams and beam halos, and detectors are needed that can operate in the presence of high particle rates. Such detectors will also be useful for high rate environments at upgraded facilities such as RHIC, CEBAF II, and at Fermilab's Project X.

The Large Angle Photon Veto System for the NA62 Experiment at CERN

The NA62 experiment at CERN SPS aims at measuring-100 events of the very rare decay K + →π + ννˉ (BR∼8.5xlO∼ 10 ). It poses stringent requirements on PID capabilities to reject the overwhelming π + π 0 (63%) and Kμ 2 (21%) backgrounds. The photon veto system must provide a rejection factor of 10∼ 8 on π 0 decays. As a main γ veto detector, the NA48 liquid Kripton calorimeter will be used. To have full geometrical acceptance up to 50 mr, a set of 12 veto stations should be placed along the vacuum decay tank, with an inefficiency <10∼ 4 in a wide energy range (200 MeV-35 GeV). Good energy resolution (∼10% at 1 GeV) for threshold definition, good time resolution (∼1 ns) to be used at the trigger level, sensitivity to MIP for calibration with muons of the beam halo are needed. A moderate segmentation in the azimuthal angle is desirable, for reducing the counting rate and providing information on the γ direction. We performed an intense R&D program on three solutions: “spaghetti” calorimeter, lead/scintillator sandwich calorimeter, and original re-use of the existing barrel of the OPAL lead-glass e.m. calorimeter. Studies have been performed at the Frascati BTF beam and all three meet the efficiency requirements. The final choice uses a peculiar radial arrangement of lead-glasses in rings. Front-end electronics has been designed to cover the tree orders of magnitude of the signal, contributing to the trigger, and integrated in the general TDAQ, while keeping low cost and simplicity. The first five full veto stations have been constructed. Two tests have been done and problems found fixed. We will discuss about R&D for the technology choice, LAV construction, test beams results and simulation performance.

Experiences with the Muon Alignment Systems of the Compact Muon Solenoid Detector

Abstract After briefly explaining the need for a precise muon chamber alignment, the different muon alignment systems implemented at CMS are described. Due to the tight spatial confinement and challenging large radiation and high magnetic field environment, unique alignment systems had to be developed that handle separately the Barrel and the Endcap regions. A third subsystem, called Link, connects these two together and to the Tracker in a common reference frame. The aligned chamber geometry obtained from the Hardware-based muon alignment is validated by comparisons with photogrammetry information and by studies of residuals of muon tracks extrapolated between chambers. Stability studies, for which the hardware systems are particularly well suited, are also discussed.Alignment methods based on tracks are also described. Muons from cosmic rays and from collisions are used to align the chambers relative to the inner tracker. In addition, beam halo muon tracks traversing overlapping endcap chambers are used for internal endcap alignment. A comparison between the track-based and hardware-based results is given, together with an explanation of the advantages and disadvantages of the different alignment strategies.

Performance and Calibration of CASTOR Calorimeter at CMS

Abstract CASTOR (Centauro And Strange Object Research) is a Cerenkov quartz-tungsten sampling calorimeter installed in the very forward region of the CMS experiment covering the pseudorapidity range −5.2 to −6.6. The location of CASTOR and current geometry of the shielding imply operation under relatively high radiation dose and magnetic field. Except for very particular regions, the calorimeter read-out with fine mesh PMTs demonstrates good performance under these rough conditions. Relative calibration of CASTOR is done using beam halo muons. To obtain relevant data, a dedicated CASTOR trigger was developed and activated during beam injections and ramp-ups. The triggered events correspond to an isolated particle penetrating the calorimeter parallel to the beam axis. Analysis of the obtained spectra provides relative response to a muon per individual read-out channel. An absolute CASTOR calibration is not discussed here.

Experimental characterization of lateral profiles of scanned proton and carbon ion pencil beams for improved beam models in ion therapy treatment planning

Scanned ion pencil beams carry a low-dose envelope which can extend up to several centimeters from the individual beam central axis. Depending on the energy and species of the beam, this halo consists mainly of secondary particles produced by nuclear interactions in the target or of particles undergoing multiple Coulomb scattering in the beam line components. This halo is often neglected by single Gaussian beam modeling in current treatment planning systems. One possibility of improving the accuracy of treatment planning is to upgrade the used pencil beam models by adding a description of the low-dose envelope. But at the same time it is crucial to keep the calculation time and the complexity for treatment planning in reasonable limits. As a first approach we measured the lateral beam profiles of scanned proton and carbon ion pencil beams at different energies and depths in water and air at the Heidelberg Ion Beam Therapy Center. Then we tried to describe their beam halo by adding a supplementary Gaussian function to the standard single Gauss modeling which is used at the moment by our treatment planning systems. This analysis helped to identify trends in the parameters describing the lateral beam broadening to support its modeling. Finally, it is shown that the accuracy of treatment planning could be improved by the proposed upgrade of the pencil beam model. In particular, the presented experimental data can be either used directly as input for dose calculation or serve for representative comparison with the results of calculation models such as Monte Carlo simulations for the generation of lateral basic data to be input in upgraded beam models of treatment planning systems.

Courant-Snyder invariant density screening method for emittance analysis

Emittance is an important characteristic of describing charged particle beams. In hadron accelerators, we often meet irregular beam distributions that are not appropriately described by a single rms emittance or 95% emittance or total emittance. In this paper, it is pointed out that in many cases a beam halo should be described with very different Courant-Snyder parameters from the ones used for the beam core. A new method — the Courant-Snyder invariant density screening method — is introduced for analyzing emittance data clearly and accurately. The method treats the emittance data from both measurements and numerical simulations. The method uses the statistical distribution of the beam around each particle in phase space to mark its local density parameter, and then uses the density distribution to calculate the beam parameters such as the Courant-Snyder parameters and emittance for different beam boundary definitions. The method has been used in the calculations for beams from different sources, and shows its advantages over other methods. An application code based on the method including the graphic interface has also been designed.

The UA9 experimental layout

The UA9 experimental equipment was installed in the CERN-SPS in March '09 with the aim of investigating crystal assisted collimation in coasting mode. Its basic layout comprises silicon bent crystals acting as primary collimators mounted inside two vacuum vessels. A movable 60 cm long block of tungsten located downstream at about 90 degrees phase advance intercepts the deflected beam.Scintillators, Gas Electron Multiplier chambers and other beam loss monitors measure nuclear loss rates induced by the interaction of the beam halo in the crystal. Two Roman pots installed in the path of the deflected particles are equipped with a Medipix detector to reconstruct the transverse distribution of the impinging beam. Finally UA9 takes advantage of an LHC-collimator prototype installed close to the first Roman pot to help in setting the beam conditions and to analyze the efficiency to deflect the beam. This paper describes in details the hardware installed to study the crystal collimation during 2010.

Comparative results on collimation of the SPS beam of protons and Pb ions with bent crystals

New experiments on crystal assisted collimation have been carried out at the CERN SPS with stored beams of 120 GeV/c protons and Pb ions. Bent silicon crystals of 2 mm long with about 170 μrad bend angle and a small residual torsion were used as primary collimators. In channeling conditions, the beam loss rate induced by inelastic interactions of particles with the crystal nuclei is minimal. The loss reduction was about 6 for protons and about 3 for Pb ions. Lower reduction value for Pb ions can be explained by their considerably larger ionization losses in the crystal. In one of the crystals, the measured fraction of the Pb ion beam halo deflected in channeling conditions was 74%, a value very close to that for protons. The intensity of the off-momentum halo leaking out from the collimation station was measured in the first high dispersion area downstream. The particle population in the shadow of the secondary collimator–absorber was considerably smaller in channeling conditions than for amorphous orientations of the crystal. The corresponding reduction was in the range of 2–5 for both protons and Pb ions.

Tevatron beam halo collimation system: design, operational experience and new methods11Work supported by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the US Department of Energy.

Collimation of proton and antiproton beams in the Tevatron collider is required to protect CDF and D0 detectors and minimize their background rates, to keep irradiation of superconducting magnets under control, to maintain long-term operational reliability, and to reduce the impact of beam-induced radiation on the environment. In this article we briefly describe the design, practical implementation and performance of the collider collimation system, methods to control transverse and longitudinal beam halo and two novel collimation techniques tested in theTevatron.

SU-E-T-723: Pencil Beam Depth-Dose Distributions in the Astroid TPS

Purpose: To describe the commissioning of a database of depth‐dose distributions for the dose calculation in Astroid. Methods: A database of Bragg peaks with zero energy width is generated by GEANT4 Monte Carlo. The Bragg peak distributions are smeared with a Gaussian in energy and tuned to match relative dose distributions measured with a Bragg peak chamber (BPC). BPC values are corrected for dose deposited outside of the chamber from scattered and secondary radiation in the pencil beam halo. Individual relative Bragg peaks are calibrated to match the absolute dose measured with a Markus ionization chamber. The uncertainty introduced by this halo correction is minimized by selecting a depth for calibration with the Markus chamber that minimizes the size of the correction. A Faraday cup gives the delivered dose per gigaproton. An SOBP is constructed from the database and the absolute dose in the plateau region is verified with an Exradin T1 ionization chamber. Results: The energy RMS spread decreases with energy from 0.9 to 0.2%. The correction for dose deposited outside the Bragg peak chamber is less than +/−4%. The uniformity of SOBPs generated between 10 to 30 g/cm2 is 2% with a global scaling factor of 1.01 to give the best agreement with the measured dose. Conclusions: It is important to have the correct Range, dose, and relative shape of Bragg peaks, otherwise the delivered SOBPs can be tilted or do not match the expected dose.Dose distributions in Astroid agree well with measurements in water. The simulated database with zero energy spread is renormalized per gigaproton following the same procedure and could simplify commissioning at different facilities. In the future one needs only the energy spread, a list of available proton Ranges, and a calibrated ionization chamber measurement to commission treatment planning depth‐dose distributions at other facilities.

A spectral method for halo particle definition in intense mismatched beams

An advanced spectral analysis of a mismatched charged particle beam propagating through a periodic focusing transport lattice is utilized in particle-in-cell (PIC) simulations. It is found that the betatron frequency distribution function of a mismatched space-charge-dominated beam has a bump-on-tail structure attributed to the beam halo particles. Based on this observation, a new spectral method for halo particle definition is proposed that provides the opportunity to carry out a quantitative analysis of halo particle production by a beam mismatch. In addition, it is shown that the spectral analysis of the mismatch relaxation process provides important insights into the emittance growth attributed to the halo formation and the core relaxation processes. Finally, the spectral method is applied to the problem of space-charge transport limits.