24 pages, contribution to the 2014 Joint International Accelerator School: Beam Loss and Accelerator Protection, Newport Beach, CA, USA , 5-14 Nov 2014
Beam loss is a critical issue in high-intensity accelerators, and much effort is expended during both the design and operation phases to minimize the loss and to keep it to manageable levels. As new accelerators become ever more powerful, beam loss becomes even more critical. Linacs for H- ion beams, such as the one at the Oak Ridge Spallation Neutron Source, have many more loss mechanisms compared to H+ (proton) linacs, such as the one being designed for the European Spallation Neutron Source. Interesting H- beam loss mechanisms include residual gas stripping, H+ capture and acceleration, field stripping, black-body radiation and the recently discovered intra-beam stripping mechanism. Beam halo formation, and ion source or RF turn on/off transients, are examples of beam loss mechanisms that are common for both H+ and H- accelerators. Machine protection systems play an important role in limiting the beam loss.
Different limitations on luminosity of linac-ring type colliders coming from beam dynamics issues are considered for the examples of the $ep$ options of the THERA and LinacXLHC proposals. It is shown that $L=1.3\times 10^{31}$ for THERA and $L=2.7\times 10^{32}$ for LinacXLHC can be achieved for round beams case. Corresponding sets of parameters are given with the explanations.
To realize and test advanced accelerator concepts and hardware, a beamline is being reconfigured in the Linac Extension Area (LEA) of APS linac. A photo-cathode RF gun installed at the beginning of the APS linac will provide a low emittance electron beam into the LEA beamline. The thermionic RF gun beam for the APS storage ring, and the photo-cathode RF gun beam for LEA beamline will be accelerated through the linac in an interleaved fashion. In this paper, the design studies for interleaving lattice realization in APS linac is described with initial experiment result
The superconducting (SC) High Intensity and Energy (HIE) ISOLDE linac will replace most of the existing accelerating infrastructure of the Radioactive ion beam EXperiment (REX) at CERN, however, the 101.28 MHz RFQ and 5 MV IH cavity will remain in the role of injector for the upgrade, boosting the beam up to an energy of 1.2 MeV/u. We present the results of a beam dynamics investigation of the injector focused most critically on matching the longitudinal beam parameters from the RFQ to the SC machine, which is complicated largely by the IH cavity employing a Combined Zero Degree (KONUS) beam d…
The characteristics of the high-intensity single-bunch electron beam of the 38 MeV L-band electron linac at the Institute of Scientific and Industrial Research in Osaka University were investigated.
Compensation of multi-bunch beam loading is of great importance in the main linac of the Compact Linear Collider (CLIC). The bunch-to-bunch energy variation has to stay below 1 part in 1000. In CLIC, the RF power is obtained by decelerating a drive beam which is formed by merging a number of short bunch trains. A promising scheme for tackling beam loading in the main linac is based on varying the lengths of the bunch trains in the drive beam. The scheme and its expected performance are presented.
The CSR-LINAC injector has been proposed in Heavy Ion Research Facility in Lanzhou (HIRFL). The linac mainly consists of two parts, the RFQ and the IH-DTL. The KONUS (Kombinierte Null Grad Struktur) concept has been introduced into the DTL section. In this paper, the re-matching of the main accelerating section will be finished in the 3.7 MeV/u scheme and the new beam dynamics design up to 7 MeV/u will be also shown. Through the beam re-matching, the relative emittance growth has been suppressed greatly along the linac.
As the pre-injector of the LHC injector chain, the proton linac at CERN is required to provide a high-intensity (180mA) beam to the Proton Synchrotron Booster. The results of measurements at this intensity will be presented. Furthermore, the linac is now equipped with bunch shape monitors from INR, Moscow, which have allowed the comparison of the Alvarez tank RF settings with simulations.
The MTA beam line has been specifically designed to facilitate measurements of the Fermilab Linac beam emittance and properties utilizing a long, 10m, element-free straight. Linac beam is extracted downstream of the 400-MeV electrostatic chopper located in the Booster injection line. This chopper cannot be utilized for MTA beam, and therefore the entire Linac beam pulse is directed into the MTA beamline. Pulse length manipulation is provided by the 750-keV electrostatic chopper at the upstream end of the Linac and, using this device, beam can be delivered from 8 μsec up to the full 50 μsec Lin…
The new method to stabilize the beam energy of the KEK 40MeV proton linac, is developed now. In this method, the signal of the velocity monitor installed upstream the debuncher in the 40MeV beam line, is processed and then fed to the phase shifter of the debuncher rf system so as to cancel the fluctuation of the beam energy. In this article, the beam tests to prove the validity of this method and the system are described.
This paper describes beam energy online measurement of BEPCII linac, presents the calculation formula and some of the results. The method mentioned here measures the beam energy by acquiring beam positions in the horizontal direction with three beam position monitors (BPM) eliminating the effect of orbit fluctuation, which is much better than the one using the single BPM. The error analysis indicates that this online measurement has further potential usage such as a part of beam energy feedback system. The reliability of this method is also discussed and demonstrated in the end of this paper.
The ADS accelerator in China is a CW (Continuous-Wave) proton linac with 1.5 GeV in beam energy, 10 mA in beam current, and 15 MW in beam power. To meet the extremely low beam loss rate requirement and high reliability, it is very important to study the beam halo caused by beam mismatch, which is one major source of beam loss. To avoid the envelope instability, the phase advances per period are all smaller than 90 degree in the main linac design. In this paper, the results of the emittance growth and the envelope oscillations caused by mismatch in the main linac section are presented. To meet …
A superconducting (SC) linac is expected to lead to outstanding discoveries in various scientific fields because its beam current is a few orders of magnitude larger than in a normal-conducting linac. However, the widespread use of SC linac is limited by the high construction and operation costs. To resolve this problem, we propose a continuous wave (CW) operation of a SC linac shared by electron/positron beams for effective multi-purposes utilization. A high current positron/electron beam is required for high-energy physics projects such as linear collider and muon collider while high-current…
The KEK 8-GeV electron / 3.5-GeV positron linac has been operated with very different beam specifications for downstream rings, KEKB, PF and PF-AR. For the reliable operation among these beam modes intelligent beam switching and beam feedback systems have been developed and used since its commissioning. A software panel is used to choose one of four beam modes and a switching sequence is executed in about two minutes. Most items in a sequence are simple operations followed by failure recoveries. The magnet standardization part consumes most of the time. The sequence can be re-arranged easily b…
A dielectric-loaded linac powered by THz-pulses is one of the key parts of the "Attosecond X-ray Science: Imaging and Spectroscopy" (AXSIS) project at DESY, Hamburg. As in conventional accelerators, the AXSIS linac is designed to have phase velocity equal to the speed of light which, in this case, is realized by tuning the thickness of the dielectric layer and the radius of the vacuum channel. Therefore, structure fabrication errors will lead to a change in the beam dynamics and beam quality. Additionally, errors in the bunch injection will also affect the acceleration process and can cause be…
There is demand for the construction of a medium-energy ion linear accelerator based on superconducting rf (SRF) technology. It must be capable of producing several hundred kilowatts of CW beams ranging from protons to uranium. A considerable amount of power is required in order to generate intense beams of rare isotopes for subsequent acceleration. At present, however, the beam power available for the heavier ions would be limited by ion source performance. To overcome this limit, we have studied the possibility of accelerating multiple-charge-state (multi-Q) beams through a linac. We show th…
The implications of long-range wakefields on the beam quality are investigated through a detailed beam dynamics study. Injection offsets are considered and the resulting emittance dilution recorded, including systematic sources of error. These simulations have been conducted for damped and detuned structures (DDS) and for waveguide damped structures-both for the CLIC collider.
This paper reports on the feasibility study of a proton Super-Conducting Linac (SCL) as a new injector to the Alternating Gradient Synchrotron (AGS) of the Brookhaven National Laboratory (BNL). The Linac beam energy is in the range of 1.5 to 2.4 GeV. The Beam intensity is adjusted to provide an average beam power of 4 MW at the top energy of 24 GeV. The repetition rate of the SCL-AGS facility is 5 beam pulses per second.
In designing linear accelerator structures for multi-bunch applications we are often interested in estimating the effect of relatively weak multi-bunch beam break-up (BBU), due to the somewhat complicated wakefields of detuned structures. This, for example, is the case for the injector linacs of the JLC/NLC linear collider project. Deriving an analytical formula for such a problem is the subject of this report.
The first phase of the HIE-ISOLDE project at CERN consists of a superconducting (SC) linac upgrade in order to increase the energy of post-accelerated radioactive ion beams from 2.8 MeV/u to over 10 MeV/u (for A/q = 4.5). In preparation for the upgrade, we present beam dynamics studies of the booster section of the normal conducting (NC) REX-ISOLDE linac, focused on the longitudinal development of the beam in the 101.28 MHz Interdigital H-mode Structure (IHS), employing a Combined Zero Degree Structure (KONUS), pulsing at a high gradient of over 3 MV/m. The evolution of the transverse emittanc…
We outline the design of beam experiments for the electron linac at the Fermilab Accelerator Science and Technology (FAST) facility and for the Integrable Optics Test Accelerator (IOTA), based on synchrotron light emitted by the electrons in bend dipoles, detected with gated microchannel-plate photomultipliers (MCP-PMTs). The system can be used both for beam diagnostics (e.g., beam intensity with full dynamic range, turn-by-turn beam vibrations, etc.) and for scientific experiments, such as the direct observation of the time structure of the radiation emitted by single electrons in a storage r…
FAST linear accelerator has been commissioned in 2017. Experimental program of the facility requires high quality beams with well-defined properties. Solenoidal fields at photoinjector, laser spot shape, space charge forces and other effects can distort beam distribution and introduce coupling. This work presents results of a beam phase space tomography for a coupled 4D case. Beam was rotated in two planes with seven quads by 180 degrees and images from YaG screen were used to perform SVD based reconstruction of the beam distribution.
PIP-II is an essential upgrade of the Fermilab complex that will enable the worlds most intense high-energy beam of neutrinos for the international Deep Underground Neutrino Experiment at LBNF and support a broad physics program at Fermilab. Ultimately, the PIP-II superconducting linac will be capable of accelerating the $H-$ CW beam to 800 MeV with an average power of 1.6 MW. To operate the linac with such high power, beam losses and beam emittance growth must be tightly controlled. In this paper, we present the results of global optimization of the Linac options towards a robust and efficien…
Project-X is the proposed high intensity proton facility to be built at Fermilab, US. First stage of the Project-X consists of superconducting linac which will be operated in continuous wave (CW) mode to accelerate the beam from 2.5 MeV to 3 GeV. The operation at CW mode puts high tolerances on the beam line components, particularly on radiofrequency (RF) cavity. The failure of beam line elements at low energy is very critical as it results in mis-match of the beam with the following sections due to different beam parameters than designed parameter. It makes the beam unstable which causes emit…
It is difficult for us to measure the Booster ring impedance up to the GHz range due to the instrumentation limit. Since the de-bunching process in the Linac to Booster transfer is determined by the complex impedance of the Booster ring, one can obtain an impedance model, which has the best match to the machine impedance, just by searching the optimal fit between the simulated and measured turn-by-turn resistive wall signal (RWS).
The RAON, a superconducting linear accelerator for RISP (Rare Isotope Science Project), will use a charge stripper in order to increase the charge states of the heavy ions for effective acceleration in the higher energy part of the linac. The charge stripper affects the beam qualities by scattering when the heavy ions go through the charge stripper. Moreover we have to select and accelerate proper charge states between 77+ and 81+ for uranium beam case in order to satisfy the beam power requirement at an IF (Inflight Fragmentation) target. This work focuses on the beam optics affected by the c…
To achieve the physics goals of future Linear Colliders, it is important that electron and positron beams are polarized. The positron source planned for the International Linear Collider (ILC) is based on a helical undulator system and can deliver a polarised beam with positron polarization of 60%. To ensure that no significant polarization is lost during the transport of the electron and positron beams from the source to the interaction region, spin tracking has to be included in all transport elements which can contribute to a loss of polarization. These are the positron source, the damping …
We explore the NLC linac optics compatible with a low energy scenario where initially only part of the full linac is installed. Optics modification suitable for a low energy beam running and upgrade to the nominal energy is discussed. Linac parameters and beam tolerances in the modified lattice are compared to the nominal design.
In this paper we present results of using parallel supercomputers to simulate beam dynamics in next-generation high intensity ion linacs. Our approach uses a three-dimensional space charge calculation with six types of boundary conditions. The simulations use a hybrid approach involving transfer maps to treat externally applied fields (including rf cavities) and parallel particle-in-cell techniques to treat the space-charge fields. The large-scale simulation results presented here represent a three order of magnitude improvement in simulation capability, in terms of problem size and speed of e…
The CLIC linear collider aims at accelerating multiple bunches of electrons and positrons and colliding them at a centre of mass energy of 3 TeV. These bunches will be accelerated through X-band linacs, operating at an accelerating frequency of 12 GHz. Each beam readily excites wake-fields within the accelerating cavities of each linac. The transverse components of the wake-fields, if left unchecked, can dilute the beam emittance. The present CLIC design relies on heavy damping of these wake-fields in order to ameliorate the effects of the wake-fields on the beam emittance. Here we present ini…
Femtosecond bunch is a hot topic in the present world accelerator research community. The high peak current bunch will lead to the beam breakup phenomenon. In this paper, the scaling law with current for the single ultrashort bunch beam breakup in the radio frequency linear accelerator is proposed and obtained. The scaling power factor is roughly estimated to be 1/6, 1/6 and 1/3 for the geometry wake, the surface roughness wake and the resistance wake respectively.
The conception of High Power Linac developed in Russian accelerator centres is based on the use of independently phased SC resonators with quadrupole lenses between them. The type and parameters of the resonators as well as focusing structure are varied along the linac to optimise beam dynamics and the characteristics of the linac. The beam evolution in the linac was studied by simulation in 3D accelerating and focusing fields by computer code DYNAMION. The simulation includes all nonlinearities of external fields and space charge forces. Estimations of particle losses in the beam based on ana…
The China spallation neutron source (CSNS) linac is designed with beam energy of 81MeV and a peak current of 15mA in the first phase. The RF power system for the 81 MeV Linac requires 8 units of RF power sources, each unit has one independent digital low level RF (LLRF) control system which is used to stabilize the amplitude and phase of the RF accelerating field along the linac, and to minimize beam loss. During beam commissioning, all of 8 on-line units of LLRF control system were operating stably and reliable, the control precision of accelerating fields met requirement of beam commissionin…
Electromagnetic modeling with MAFIA of the combined beam position-phase monitors (BPPMs) for the Spallation Neutron Source (SNS) linac has been performed. Time-domain 3-D simulations are used to compute the signal amplitudes and phases on the BPPM electrodes for a given processing frequency, 402.5 MHz or 805 MHz, as functions of the beam transverse position. Working with a summed signal from all the BPPM electrodes provides a good way to measure accurately the beam phase. While for an off-axis beam the signal phases on the individual electrodes can differ from those for a centered beam by a fe…
This paper reports on the feasibility study of a proton Super-Conducting Linac (SCL) as a driver gor the High-Flux Breader Reactor (HFBR) at Brookhaven National Laboratory (BNL). The Linac operates in Continuos Wave (CW) mode to produce an average 10 MW of beam power. The Linac energy is 1.0 GeV. The average proton beam intensity is 10 mA.
In the framework of the Eupraxia Design Study an advanced accelerator facility EUPRAXIA@SPARC_LAB has been proposed to be realized at Frascati (Italy) Laboratories of INFN. Two advanced acceleration schemes will be applied, namely an ultimate high gradient 1 GeV X-band linac together with a plasma acceleration stage to provide accelerating gradients of the GeV/m order. A FEL scheme is foreseen to produce X-ray beams within 3-10 nm range. A 500-TW Laser system is also foreseen for electron and ion production experiments and a Compton backscattering Interaction is planned together with extractio…
Recirculating energy recovery linacs are a promising technology for delivering high power particle beams ($\sim$GW) while only requiring low power ($\sim$kW) RF sources. This is achieved by decelerating the used bunches and using the energy they deposit in the accelerating structures to accelerate new bunches. We present studies of the impact of the bunch packet filling pattern on the performance of the accelerating RF system. We perform RF beam loading simulations under various noise levels and beam loading phases with different injection schemes. We also present a mathematical description of…
The peak current limit for the Fermilab Linac was recently studied. The purpose was to learn what components of the present Linac can be used for the first stage of a proposed proton driver[1]. For this application the Linac must provide a H- beam in excess of 5000 mA-msec per pulse. The original Fermilab Linac was designed for protons with a peak current of 75 mA and a pulse length of four Booster turns (~10 msec). The high energy replacement was designed for a peak current of 35 mA and a beam pulse length of 50 msec. The present H- source cannot deliver more than ~80 mA which produces 55 mA …
A 52MHz Radio Frequency Quadrupole (RFQ) linear accelerator (linac) is designed to serve as an initial structure for the SSC-linac system (injector into Separated Sector Cyclotron). The designed injection and output energy are 3.5 keV/u and 143 keV/u, respectively. Beam dynamics study in RFQ was done using 3-dimensional particle-in-cell code BEAMPATH [1]. Simulation results show that this RFQ structure is characterized by stable value of beam transmission efficiency (at least 95%) for both zero-current mode and for space charge dominated regime. The beam accelerated in RFQ has good quality in …
The Fermilab Linac delivers a variable intensity, 400-MeV beam to the The MuCool Test Area experimental hall via a beam line specifically designed to facilitate measurements of the Linac beam emittance and properties. A 10 m, dispersion-free and magnet-free straight utilizes an upstream quadrupole focusing triplet in combination with the necessary in-straight beam diagnostics to fully characterize the transverse beam properties. Since the Linac does not produce a strictly elliptical phase space, tomography must be performed on the profile data to retrieve the actual particle distribution in ph…
In a long-term planning for neutrino experiments in China, a medium baseline neutrino beam is proposed which uses a CW superconducting linac of 15 MW in beam power as the proton driver. The linac will be based on the technologies which are under development by the China-ADS project, but with much weaker requirement on reliability. It is composed of a 3.2-MeV normal conducting RFQ and 5 different types of superconducting cavities. The nominal design energy and current are 1.5 GeV and 10 mA, respectively. The general considerations and preliminary results on the physics design will be presented …
For injection of beams into circular machines with different horizontal and vertical emittance acceptance, the injection efficiency can be increased if these beams are flat, i.e. if they feature unequal transverse emittances. Generation of flat electron beams is well known and has been demonstrated already in beam experiments. It was proposed also for ion beams that were generated in an Electron Cyclotron-Resonance (ECR) source. We introduce an extension of the method to beams that underwent charge state stripping without requiring their generation inside an ECR source. Results from multi-part…
The paper proposes a method of finding the beam loss locations in a linac. If the beam is scraped at an aperture limitation, moving its centroid with two dipole correctors located upstream and oscillating in sync produces a line at the corresponding frequency in spectra of current-sensitive devices downstream of the loss point. The phase of this signal contains information about the location of the beam loss. Similar lines appear also in the position signals of Beam Position Monitors (BPMs). The phases of the BPM position lines change monotonically (within each 2π) along the linac and can be u…
The bremsstrahlung of accelerated electrons passing through a converter is used to study multiparticle photo-nuclear reactions. The results of calculations, numerical modeling, design, and testing of a special magnetic cleaning system to obtain a "pure" beam of bremsstrahlung quanta when studying the cross-sections of such reactions at the LUE-40 linac are presented. The system is based on commercially available permanent magnets of rectangular cross-sections. The maximum on-axis field is 0.9 T, which provides sufficient separation of the electron beam and gamma rays at a distance of more than…
Fermilab is developing a Proton Improvement Plan (PIP) to increase throughput of its proton source. The plan addresses hardware modifications to increase repetition rate and improve beam loss while ensuring viable operation of the proton source through 2025. The first phase of the PIP will enable the Fermilab proton source to deliver 1.8e17 protons per hour by mid-2013. As part of this initial upgrade, Fermilab plans to install a new front-end consisting of dual H- ion sources and a 201 MHz pulsed RFQ. This paper will present beam studies measurements of this new front-end and discuss new beam…
Beam breakup instability is a potential issue for all particle accelerators and is often the limiting factor for the maximum beam current that can be achieved. This is particularly relevant for Energy Recovery Linacs with multiple passes where a relatively small amount of charge can result in a large beam current. Recent studies have shown that the choice of filling pattern and recirculation scheme for a multi-pass energy recovery linac can drastically affect the interactions between the beam and RF system. In this paper we further explore this topic to study how filling patterns affect the be…
Extensive beam-based feedback systems are planned as an integral part of the Next Linear Collider (NLC) control system. Wakefield effects are a significant influence on the feedback design, imposing both architectural and algorithmic constraints. Studies are in progress to assure the optimal selection of devices and to refine and confirm the algorithms for the system design. We show the results of initial simulations, along with evaluations of system response for various conditions of ground motion and other operational disturbances.
A dedicated clinical synchrotron facility for cancer therapy using energetic proton and ion beams (C, He and O) has been designed at GSI for the Radiologische Universitaetsklinik at Heidelberg, Germany. The design of the injector linac is presented. Suitable ion sources are discussed and results of ion source test measurements are reported. The LEBT allows for switching between two ion sources. A short RFQ accelerates the ions from 8 keV/u to 400 keV/u. It is followed by a very compact beam matching section and a 3.8 m long IH-type drift tube linac for the acceleration to 7 MeV/u. Both rf stru…
Energy Recovery Linacs provide high-energy beams, but decelerate those beams before dumping them, so that their energy is available for the acceleration of new particles. During this deceleration, any relative energy spread that is created at high energy is amplified by the ratio between high energy and dump energy. Therefore, Energy Recovery Linacs are sensitive to energy spread acquired at high energy, e.g. from wake fields. One can compensate the time-correlated energy spread due to wakes via energy-dependent time-of-flight terms in appropriate sections of an Energy Recovery Linac, and via …
We present a new conceptual and optical design for the Next Linear Collider post-linac collimation system. Energy collimation and passive protection against off-energy beams are achieved in a system with large horizontal dispersion and vertical betatron functions. Betatron collimation is performed in a relatively low-beta (FODO-like) lattice in which only thin spoilers intercept particles near the beam core, while thick absorbers maintain a large stay-clear from the beam. Two possible schemes for the spoilers are considered: one in which the spoilers are capable of tolerating a certain number …
The new 1.4 MeV/u front end HSI (HochStromInjektor) of the Unilac accelerates ions with A/q ratios of up to 65 and with beam intensities in emA of up to 0.25 A/q. The maximum beam pulse power is up to 1300 kW. During the stepwise linac commissioning from April to Septem-ber 1999 the beam behind of each cavity was analysed within two weeks. A very stable Ar1+ beam out of a vol-ume plasma source MUCIS was used mainly. The meas-ured norm. 80 % emittance areas around 0.45 pi mm mrad are close to the results from beam simulations. Up to 80 % of the design intensity at the linac exit were achieved. …
In this paper, we propose to use the LWFA as the preinjector for XFEL driver linac. We can use LWFA to produce the femtosecond electron beam. The peak current of the produced beam can reach tens of $kA$ and is high enough to drive XFEL so that the present bunch compressing technique can be deserted. The output optical pusle length can be as short as $ 1 fs $.
The accelerator for the APT Project is a 100 mA CW proton linac with an output energy of 1030 MeV. A High Energy Beam Transport (HEBT) conveys the beam to a raster expander, that provides a large rectangular distribution at a target/blanket (T/B) assembly. Spallation neutrons generated by the proton beam in the T/B reacts with Helium-3 to produce tritium. The design of the APT linac is an integrated normal-conducting (NC)/superconducting (SC) proton linac; the machine architecture has been discussed elsewhere [1]. The NC linac consists of a 75 keV injector, a 6.7-MeV 350-MHz RFQ (radio frequen…
