Beam Halo Research Articles

The high performance microstrip silicon detector tracking system for an innovative crystal based collimation experiment

In order to increase the LHC luminosity, an innovative crystal based collimation system is being developed in the UA9 experiment: exploiting the well-known effect of channeling and the recently discovered volume reflection phenomenon, a short silicon bent crystal will be used to replace the traditional multi-stage collimation system to remove the beam halo particles. This idea will be tested with a 120 GeV / c proton beam on the LSS5 straight section at the SPS accelerator ring (CERN) using several crystal technologies and a high performance tracking system based on high resolution ( 5 μ m ) double-side microstrip silicon detectors. The first silicon detector will be positioned in a roman pot at about 50 m from the crystal, where the phase difference of 90 ∘ maximizes the distance of the steered particles (due to the crystal) from the beam core; after around 20 m a second tracking station will be able to measure the steering angle. Each detector side is readout by three 128 channel VA1TA self-triggering ASICs, which contain a preamplifier, two shapers and a sample and hold circuit; the analog signals are digitized by one 10 MHz ADC (per ASIC) and are carried to the DAQ system (which is located on ground) by a series of fiber links (GOH). The DAQ will be completed by a Slow Controls system which will check the power supply voltages and currents and the roman pot pressures and temperatures. This paper gives an insight of the UA9 experiment, underlining the tracking system features and the first results of a silicon detector prototype in the SPS.

Tracking studies of the Compact Linear Collider collimation system

A collimation system performance study includes several types of computations performed by different codes. Optics calculations are performed with codes such as MADX, tracking studies including additional effects such as wakefields, halo and tail generation, and dynamical machine alignment are done with codes such as PLACET, and energy deposition can be studied with BDSIM. More detailed studies of hadron production in the beam halo interaction with collimators are better performed with Geant4 and FLUKA. A procedure has been developed that allows one to perform a single tracking study using several codes simultaneously. In this paper we study the performance of the Compact Linear Collider collimation system using such a procedure.

MO‐D‐BRD‐05: The Proton Beam Therapy Facility at PSI

The centre for proton therapy at the Paul Scherrer Institute was expanded recently following the commissioning of a new superconducting‐magnet cyclotron (COMET) dedicated to patient treatment. The accelerator was built by ACCEL Instruments GmbH (Varian), based on a design from the National Superconducting Cyclotron Laboratory (Michigan State University). The cyclotron is specified to provide a 500 nano‐Ampere beam of protons at a fixed energy of 250 MeV. The large magnetic field (∼ 3T) required at extraction is provided by the superconducting coils. A high power radio‐frequency system, operating at 72.8 MHz (second harmonic of the cyclotron frequency) provides the required acceleration of protons following their emission from a cold‐cathode ion source. The facility has also been up‐graded with the addition of new gantry (Gantry‐2) for patient treatment and a new beam line dedicated to eye treatments (OPTIS2). A novel feature of COMET is the high beam extraction efficiency (> 80%). This limits component irradiation and facilitates the regular maintenance periods which are an important consideration for any patient treatment center where high reliability and availability are required. We will describe the modes of operation of PROSCAN and, in particular, the spot‐scanning technique used to irradiate tumours. Magnetic elements permit rapid transverse deflection of a pencil beam for two dimensional scanning. An energy “degrader” allows rapid scanning in energy over the range of 70 – 238 MeV. This results in a variable depth of penetration of the beam, thus permitting a complete three dimensional scan. The beamlines which transport protons from COMET towards the two patient gantries and to OPTIS2 will also be discussed. They require sophisticated diagnostics for safe and reliable use of the facility. The various diagnostics include ionization chambers, secondary emission monitors and multi‐leaf Faraday cups. They are used to measure beam current, profile, position, halo and loss. The role of the diagnostics as interlocks for safe operation of the beamlines will also be described.Learning Objectives:1. Understanding the principle of the spot scanning technique.2. Understanding the issues concerning diagnostics for safe beam‐line operation.

Heavy ion microbeam vacuum requirements

Transport of heavy ions through an ion microbeam focusing system can be affected by insufficient vacuum within the beam transport tube. Due to interactions of heavy ions with atoms of residual gas in the vacuum tube of a microbeam facility, the angular, lateral and energy spreading of an ion beam increases prior to focusing, creating a beam halo. This beam halo can produce undesirable effects in some applications of ion microbeam techniques. In order to model this effect, the ion beam angular spread in residual gas has been approximated by Sigmund’s theoretical predictions for small-angle ion multiple scattering (MS), while ion energy loss straggling distributions have been applied for studying the energy spread. The extent of the beam halo has been estimated by combining the results of these calculations with ion optics calculations. Recommendations concerning microbeam focusing due to the vacuum conditions are given for different heavy ions in the MeV energy range.

Theoretical Study of the Electrostatic Lens Aberrations of a Negative Ion Accelerator for a Neutral Beam Injector

Aberrations due to the electrostatic lenses of a negative ion accelerator for a neutral beam injector and the space charge effect are theoretically investigated. A multi-stage extractor/accelerator is modeled and the aberration coefficients are numerically calculated using the eikonal method, which is conventionally used in electron optics. The aberrations are compared with the radii of a beam core with good beam divergence and a beam halo with poor beam divergence. H- beamlet profile measurements give the 1/e radii of the beam core and beam halo of 5.8 mm (beam divergence angle: 6 mrad) and 11.5 mm (beam divergence angle: 12 mrad), respectively. When the beam divergence angle of the beam core is 5 mrad and the beam energy is 406 keV, the aberrations due to the electrostatic lenses are less than a few millimeters, thus are less than the radii of the beam core and beam halo. The geometrical aberrations due to the space charge effect (negative ion current density: 10 mA/cm2 ), however, are estimated to be much larger than the radius of the beam halo. Although the aperture radii of the grids are not taken into account in this estimation, the results indicate that the space charge effect is an important factor in the aberration or beam halo in a negative ion accelerator.

Efficiency simulations for the beam collimation system of the Japan Proton Accelerator Research Complex rapid-cycling synchrotron

The 3 GeV rapid-cycling synchrotron (RCS) of the Japan Proton Accelerator Research Complex (J-PARC) project generates a 1 MW proton beam. In operation of a high intensity hadron accelerator, the most important issue is to minimize the uncontrolled loss. The RCS beam collimation system is designed for this purpose and the performance is confirmed. In the present design, the physical aperture is 1.5 times wider than the primary collimator aperture, so that the beam loss can be sufficiently localized under this condition. The influence of positioning errors of the collimators is also estimated. Furthermore, the dependence of the collimation efficiency on the impact parameter is investigated taking the growth process of the beam halo into consideration. As a result, the beam loss mechanism changes according to the impact parameter.

Control of Beam Halo-Chaos for an Intense Charged-Particle Beam Propagating Through Double Periodic Focusing Field by Soliton

We study an intense beam propagating through the double periodic focusing channel by the particle-core model, and obtain the beam envelope equation. According to the Poincare–Lyapunov theorem, we analyze the stability of beam envelope equation and find the beam halo. The soliton control method for controlling the beam halo-chaos is put forward based on mechanism of halo formation and strategy of controlling beam halo-chaos, and we also prove the validity of the control method, and furthermore, the feasible experimental project is given. We perform multiparticle simulation to control the halo by using the soliton controller. It is shown that our control method is effective. We also find the radial ion density changes when the ion beam is in the channel, not only the halo-chaos and its regeneration can be eliminated by using the nonlinear control method, but also the density uniformity can be found at beam's centre as long as an appropriate control method is chosen.

Measurement of longitudinal acceptance and emittance of the Oak Ridge Spallation Neutron Source Superconducting Linac

The longitudinal acceptance of the Spallation Neutron Source superconducting linac is computed with a longitudinal model. A beam current monitor and beam loss monitors are utilized in a new beam acceptance measurement technique, and the measured results show close agreement with the model. Based on the simulations and on the measurements of the superconducting linac acceptance, we developed a novel method to measure beam bunch shape, beam energy profile, and the longitudinal emittance at the entrance of the linac. The experimental measurements reveal that a large longitudinal beam halo exists in the injected beam to the superconducting linac, and the longitudinal rms emittance is approximately twice that of the nominal design. The simple measurement method is applicable to other superconducting linacs.

Halo Formation and Emittance Growth of Positron Beams in Plasmas

An ultrarelativistic 28.5 GeV, 700-microm-long positron bunch is focused near the entrance of a 1.4-m-long plasma with a density n(e) between approximately equal to 10(13) and approximately equal to 5 x 10(14) cm(-3). Partial neutralization of the bunch space charge by the mobile plasma electrons results in a reduction in transverse size by a factor of approximately equal to 3 in the high emittance plane of the beam approximately equal to 1 m downstream from the plasma exit. As n(e) increases, the formation of a beam halo containing approximately 40% of the total charge is observed, indicating that the plasma focusing force is nonlinear. Numerical simulations confirm these observations. The bunch with an incoming transverse size ratio of approximately 3 and emittance ratio of approximately 5 suffers emittance growth and exits the plasma with approximately equal sizes and emittances.

An analytical model to determine equilibrium quantities of azimuthally symmetric and mismatched charged particle beams under linear focusing

The model developed here analytically allows to obtain equilibrium quantities of interest from high-intensity charged particle beams such as the emittance, beam envelope, and the number of beam halo particles. The results obtained in this work have been particularized to the case of initially homogeneous beams, with azimuthal symmetry, and focused by a constant magnetic field while confined in a linear channel. For validation, full self-consistent N-particle beam simulations have been carried out and its results compared with the predictions supplied by the developed hybrid numerical-analytical model. The agreement has been reasonable. Also, the model revealed to be useful to understand the basic physical aspects of the problem.

Improvements of the DRAGON recoil separator at ISAC

The DRAGON (Detector of Recoils And Gammas Of Nuclear reactions) is used to measure radiative proton and alpha capture reaction rates involving both stable and radioactive, heavy-ion reactants at the TRIUMF-ISAC high intensity radioactive beam facility. Completed in 2001 it has been used for several challenging studies for nuclear astrophysics, e.g. 12C(α, γ)16O, 21Na(p, γ)22Mg, 26gAl(p, γ)27Si and 40Ca(α, γ)44Ti. Since initial operation, a number of improvements have been incorporated which are described here. These include a beam centering monitor based on a CCD camera, a mechanical iris to skim of beam halo, a solid state stripper acting as a charge state booster for beams with A≳30, beta and gamma detectors to monitor beam intensity and to determine beam contamination in experiments with radioactive beam and the ionization chamber for both recoil identification and isobar separation.

Controlling beam halo-chaos via backstepping design

A backstepping control method is proposed for controlling beam halo-chaos in the periodic focusing channels (PFCs) of high-current ion accelerator. The analysis and numerical results show that the method, via adjusting an exterior magnetic field, is effective to control beam halo chaos with five types of initial distribution ion beams, all statistical quantities of the beam halo-chaos are largely reduced, and the uniformity of ion beam is improved. This control method has an important value of application, for the exterior magnetic field can be easily adjusted in the periodical magnetic focusing channels in experiment.

A new analogue sampling readout system for the COMPASS RICH-1 detector

A new electronic readout for CsI-coated multiwire proportional chambers (MWPC), used as photon detectors in the COMPASS ring imaging Cherenkov (RICH) detector, is described. A prototype system comprising more than 5000 channels has been built and tested in high-intensity beam conditions. It is based on the APV25-S1 analogue sampling chip, and replaces the GASSIPLEX chip readout used previously. The APV25 chip, although originally designed for Silicon microstrip detectors, is shown to perform well even with “slow” signals from an MWPC, maintaining a signal-to-noise ratio (SNR) of 9. For every trigger the system reads out three consecutive amplitudes in time, thus allowing to extract information on both the signal amplitude and its timing. This information is used to reduce pile-up events in a high-rate environment. Prototype tests of the new readout electronics on a central RICH photocathode in nominal COMPASS beam conditions showed that the effective time window is reduced from more than 3μs for the GASSIPLEX to less than 400ns for the APV25 chip. This leads to a significant improvement of the signal-to-background ratio (SBR) with respect to the original readout. A gain by a factor of 5–6 was experimentally verified in the very forward region of phase space, where pile-up due to the muon beam halo is most significant. Owing to its pipelined architecture, the new readout system also considerably reduces the dead time per event, thus allowing to make use of trigger rates exceeding 50kHz.

Spatial and temporal beam profile monitor with nanosecond resolution for CERN's Linac4 and Superconducting Proton Linac

The Linac4, now being developed at CERN, will provide 160-MeV H - beams of high intensity N = 2 × 10 14 ions s - 1 . Before this beam can be injected into the CERN Proton Synchrotron Booster or future Superconducting Proton Linac for further acceleration, some sequences of 500-ps-long micro-bunches must be removed from it, using a beam chopper. These bunches, if left in the beam, would fall outside the longitudinal acceptance of the accelerators and make them radioactive. We developed a monitor to measure the time structure and spatial profile of this chopped beam, with respective resolutions Δ t ∼ 1 ns and Δ x ∼ 2 mm . Its large active area 40 mm × 40 mm and dynamic range also allows investigations of beam halos. The ion beam first struck a carbon foil, and secondary electrons emerging from the foil were accelerated by a series of parallel grid electrodes. These electrons struck a phosphor screen, and the resulting image of the scintillation light was guided to a thermoelectrically cooled, charge-coupled device camera. The time resolution was attained by applying high-voltage pulses of sub-nanosecond rise and fall times to the grids. The monitor has been tested with 700-ps-long UV laser pulses, and a 3-MeV proton beam. Its response over a wide range of beam intensities between N e ∼ 5 and 4 × 10 8 electrons emitted from the foil per pulse was studied. The monitor can also be used to measure the profiles of antiproton beams in the future facilities of Facility for Low-energy Antiproton Ion Research (FLAIR) or Extra Low Energy Antiproton Ring (ELENA).

Electron cloud generation and trapping in a quadrupole magnet at the Los Alamos proton storage ring

Recent beam physics studies on the two-stream e-p instability at the LANL proton storage ring (PSR) have focused on the role of the electron cloud generated in quadrupole magnets where primary electrons, which seed beam-induced multipacting, are expected to be largest due to grazing angle losses from the beam halo. A new diagnostic to measure electron cloud formation and trapping in a quadrupole magnet has been developed, installed, and successfully tested at PSR. Beam studies using this diagnostic show that the ``prompt'' electron flux striking the wall in a quadrupole is comparable to the prompt signal in the adjacent drift space. In addition, the ``swept'' electron signal, obtained using the sweeping feature of the diagnostic after the beam was extracted from the ring, was larger than expected and decayed slowly with an exponential time constant of 50 to $100\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$. Other measurements include the cumulative energy spectra of prompt electrons and the variation of both prompt and swept electron signals with beam intensity. Experimental results were also obtained which suggest that a good fraction of the electrons observed in the adjacent drift space for the typical beam conditions in the 2006 run cycle were seeded by electrons ejected from the quadrupole.

Power transmission of the neutral beam heating beams at TJ-II

A carbon fiber composite (CFC) target calorimeter has been installed at TJ-II to study in situ the power density distribution of the neutral beams. The thermographic print of the beam can be recorded and analysed in a reliable way due to the highly anisotropic thermal conductivity of the target material. Direct conversion from temperature to power density is possible due to the independent thermocouple measurements on two adiabatically mounted copper buttons. With the combined thermographic and calorimetric measurements it has been possible to determine the power density distribution of the beams. It has been found that a large beam halo is present, which can be explained by the extreme misalignment of the grids. This kind of halo has a deleterious effect on beam transport and must be minimized in order to improve the plasma heating capability of the beams.

A technique to improve crystal channeling efficiency of charged particles

It is shown that a narrow plane cut near the crystal surfaceconsiderably increases the probability of capture into the stablechanneling motion of positively charged particles entering acrystal at angles smaller than a quarter of the criticalchanneling angle with respect to the crystal planes. At smallestincidence angles the capture probability reaches 99 percent. Apair of crystals bent in orthogonal planes and provided with thecuts allows to reach a 99.9 percent efficiency of single-passdeflection of a proton beam with an ultra small divergence.Conditions necessary for efficient single-pass deflection ofprotons from the LHC beam halo are also discussed.

Plans for longitudinal and transverse neutralized beam compression experiments, and initial results from solenoid transport experiments

This paper presents plans for neutralized drift compression experiments, precursors to future target heating experiments. The target-physics objective is to study warm dense matter (WDM) using short-duration (∼1 ns) ion beams that enter the targets at energies just above that at which d E/d x is maximal. High intensity on target is to be achieved by a combination of longitudinal compression and transverse focusing. This work will build upon recent success in longitudinal compression, where the ion beam was compressed lengthwise by a factor of more than 50 by first applying a linear head-to-tail velocity tilt to the beam, and then allowing the beam to drift through a dense, neutralizing background plasma. Studies on a novel pulse line ion accelerator were also carried out. It is planned to demonstrate simultaneous transverse focusing and longitudinal compression in a series of future experiments, thereby achieving conditions suitable for future WDM target experiments. Future experiments may use solenoids for transverse focusing of un-neutralized ion beams during acceleration. Recent results are reported in the transport of a high-perveance heavy ion beam in a solenoid transport channel. The principal objectives of this solenoid transport experiment are to match and transport a space-charge-dominated ion beam, and to study associated electron-cloud and gas effects that may limit the beam quality in a solenoid transport system. Ideally, the beam will establish a Brillouin-flow condition (rotation at one-half the cyclotron frequency). Other mechanisms that potentially degrade beam quality are being studied, such as focusing-field aberrations, beam halo, and separation of lattice focusing elements.

Vibrating wires for beam diagnostics

A new approach to the technique of particle beam scanning by wires has been developed. The novelty of the method is that the wire heating quantity is used as a source of information about the number of interacting particles. The wire heating measurement is performed as a change of the wire natural oscillation frequency. The excitation of wire natural oscillations is provided by interaction of a current through the wire with a permanent magnetic field. A shift in the wire natural oscillation frequency characterizes the change in the conditions of wire irradiation by the measured beam. By the rigid fixing of the wire ends on the support an unprecedented sensitivity of the frequency to the temperature and to the corresponding flux of colliding particles is obtained. The range of frequencies used (about 10 kHz) and characteristic time of heat transfer limit the speed characteristics of the proposed scanning method; however, the high sensitivity makes it a prospective one for investigation of beam halo and weak beam scanning. Traditional beam profile monitors generally focus on the beam core and loose sensitivity in the halo region where a large dynamic range of detection is necessary. The scanning by a vibrating wire can also be used in profiling and detecting of neutron and photon beams.

Collimation of H− beam transverse halo by triplets and foil scrapers

Transverse beam halo from a high current linac is an important source of beam losses in a ring as the next stage accelerator. A method is introduced that uses periodic triplet cells and foil scrapers to collimate the beam halo in the beam transport line from the linac to the ring and transports the collimated particles to a beam dump. It has good properties such as being low beam loss in the beam line and avoiding local collimators. Comparisons are made with doublet cells and FODO cells as well as with other collimation methods. In the paper, applying the method to the Chinese Spallation Neutron Source (CSNS) is also presented, together with the simulation results, using macro-particles and taking into account nuclear elastic scattering effect and foil stripping efficiency.