Beam Bunch Research Articles

Real-time sextupole tuning for a long in-plane polarization at storage rings

A long in-plane beam polarization can be a desired feature for spin measurement experiments in storage rings. The spin precession of the particles within a beam can be controlled by means of the frozen spin method and beam bunching via RF cavities, eventually yielding a polarization lifetime of 10–100 s. Previous studies have shown that it can be further improved by sextupoles, which correct the second order effects related to the chromaticity of the beam. However, sextupoles can require readjustment after slight changes in ring parameters. This work presents a real-time sextupole tuning method that relies on a feedback algorithm. It adjusts the sextupole strength during storage, targeting a zero average radial spin component. Satisfying this condition results in a longer polarization lifetime. Simulation studies show that roughly determined feedback coefficients in this method work effectively for a wide range of ring parameters, with practical field imperfections and measurement errors taken into account. Alternatively, this technique can be used to optimize sextupole strengths in a test run without intervening the measurement.

Neutron and [formula omitted] background measurements of the experimental halls at the CSNS back-streaming white neutron source

Accurate nuclear data measurements require a good background control of the experimental environment. At the back-streaming white neutron source (the so-called Back-n White Neutron Source) at China Spallation Neutron Source that is dedicated for nuclear data measurements, comprehensive shielding measures have been employed around the neutron beam line to reduce the neutron and γ backgrounds in the experimental end-stations. The basic background generated by the neutron beam without samples was simulated by the Monte Carlo method and correspondingly measured with a Bonner sphere spectrometer. The results show that the experimental measurements and simulations are generally in good agreement, and the neutron background level meets the requirements of the nuclear data experiments. Moreover, the γ background that was measured by using liquid BC501A and NaI scintillator detectors is mainly from the natural radioactivity of environment, and the beam induced γ background is very weak, whereas the natural γ background can be removed by a coincidence in time with beam bunch or be deducted by background measurement without samples. While the present measurements prove that the neutron/γ basic background has been well controlled, the experimentalists can devote the effort to solve the background problem caused by the samples.

Development of monolithic SOI pixel sensors capable of fine measurements of space and time

Silicon-on-insulator (SOI) for fine measurement of space and time (SOFIST) is designed taking full advantage of advanced SOI monolithic pixel detector fabrication technology to record the charge and time information of hit pixels. The aim is to achieve 3 μm position resolution and ultimately identify the International Linear Collider beam bunches of 554 ns separation. SOFIST Ver. 3 includes full circuits to read out the charge and time for each pixel of size 30×30μm. Its performance is evaluated using a 120 GeV proton beam. The time resolution is evaluated to be 1.92 μs (including calibration error), with an intrinsic time resolution of 1.34–1.55 μs. Additionally, multi-memory readout functionality is successfully examined. With SOFIST Ver. 4, the same functionality is to be realized with a 20×20μm pixel size by 3D stacking. The first chip was successfully tested to image β rays. The connection yield of Au micro-bumps is greater than 99.9%.

Estimation of longitudinal bunch characteristics in the LHC using Schottky-based diagnostics

The Large Hadron Collider (LHC) Schottky monitors have been designed to measure various parameters of relevance to beam quality, namely tune, momentum spread, and chromaticity. In this work, we present how this instrument can be used to estimate longitudinal bunch characteristics, such as longitudinal bunch profile or synchrotron frequency distribution. Under the assumption of bunched beams with no intrabunch coherent motion, we start by deriving the relation between the distribution of synchrotron amplitudes within the bunch population and the longitudinal bunch profile from probabilistic principles. Subsequently, we fit the cumulative power density of acquired Schottky spectra with the underlying distribution of synchrotron amplitudes. Finally, the result of this fit is used to reconstruct the bunch profile using the derived model. The results obtained with this method are verified by comparison with longitudinal profile measurements from the LHC wall current monitors.

First experiments with a laser ablation source at the COALA setup

A laser ablation ion source has been built and tested at the COALA beamline. Time-of-flight measurements for various elements are sufficiently accurate to confirm the production of the intended ion species. Furthermore, the isotope shift between 40Ca+ and 44Ca+ has been determined on these bunched beams by collinear laser spectroscopy and compared to previous measurements. The frequency of the 4s{}{}^{2}textit {S}_{1/2} rightarrow 4p{}{}^{2}textit {P}_{1/2} transition has been determined to nu ^{mathrm {D1}}_{0}=755 222 766.8(15) MHz for 40Ca+, as well as the corresponding isotope shift of 44Ca+ to 848.1(23) MHz. This is in a good agreement to more accurate measurements performed in a recent ion trap experiment and demonstrates the strength of the collinear-anticollinear approach to reduce systematic effects caused by the broad velocity distribution generated in the ablation process.

Control of intense-laser ion acceleration

An ion beam has a unique feature to deposit its main energy inside a human body to kill cancer cells or inside material. In this paper, a future intense laser ion accelerator is discussed to make the laser-based ion accelerator compact and controllable. The issues in the laser ion accelerator include the energy efficiency from the laser to the ions, the ion beam collimation, the ion energy spectrum control, the ion beam bunching, and the ion particle energy control. The energy efficiency from the laser to ions is improved by using a solid target with a fine sub-wavelength structure. The ion beam collimation is performed by larger holes behind the solid target. An ion beam bunching and a post-acceleration are successfully realized by a multi-stage laser-target interaction. The controllable laser ion acceleration would become viable based on the study discussed in this paper. In this paper, we present 3D PIC simulation results for the controllability of laser ion acceleration system.

Efficient continuous wave accelerating structure for ion beams

The Facility for Rare Isotope Beams driver linac was designed for acceleration of multiple-charge-state beams after the stripping at ion beam energies of 17 to $20\text{ }\text{ }\mathrm{MeV}\mathrm{/u}$ depending on the ion species. The linac includes a 180\ifmmode^\circ\else\textdegree\fi{} achromatic bend to select multiple charge states for further acceleration and to dump unwanted charge states after the first 45\ifmmode^\circ\else\textdegree\fi{} magnet. Two rf rebunchers between the stripper and the linac segment 2 (LS2) are required to minimize the effective emittance growth of the multiple-charge-state beams and provide matching to the LS2. Recent studies have shown that the best choice for these two rebunchers is a room-temperature (RT) accelerating structure capable of providing robust operation in the presence of the stripped heavy ion beam contaminants. The latter can result in uncontrolled losses of contaminant ions after the passage off the stripper. Therefore, using a superconducting (SC) rebuncher after the stripper is not rational due to possible contamination and performance degradation of SC cavities over a long period of operation. For the beam bunching in the energy range from 13 to $22\text{ }\text{ }\mathrm{MeV}\mathrm{/u}$, two 161 MHz rebunchers based on interdigital H-type (IH) structure were developed, built, installed and commissioned with beam at the FRIB linac. This is the first experimental demonstration for application of a cw RT drift tube accelerating structure in this energy range.

High-brightness electron beams for linac-based bunched beam electron cooling

A high-current high-brightness electron accelerator for low-energy RHIC electron cooling (LEReC) was successfully commissioned at Brookhaven National Laboratory. The LEReC accelerator includes a dc photoemission gun, a laser system, a photocathode delivery system, magnets, beam diagnostics, a superconducting rf booster cavity, and a set of normal conducting rf cavities to provide enough flexibility to tune the beam in the longitudinal phase space. Cooling with nonmagnetized rf accelerated electron beams requires longitudinal corrections to obtain a small momentum spread while preserving the transverse emittances. Electron beams with kinetic energies of 1.6 and 2.0 MeV with a beam quality suitable for cooling were successfully propagated through 100 m of beam lines, including dispersion sections, maintained through both cooling sections in RHIC and used for cooling ions in both RHIC rings simultaneously. The beam quality suitable for cooling RHIC beams was achieved in 2018, which led to the first experimental demonstration of bunched beam electron cooling of hadron beams in 2019.7 MoreReceived 22 November 2019Accepted 23 January 2020DOI:https://doi.org/10.1103/PhysRevAccelBeams.23.021003Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBeam coolingBeam opticsHigh intensity beam dynamicsAccelerators & Beams

Electron beam probe: The analysis of the electron beam spot expansion and a prototype development at IMP

Non-interceptive diagnostic techniques are essential for high beam power accelerators. An Electron Beam Probe (EBP) is preferable to measure the beam distribution of a dense beam bunch in which a low-intensity and low-energy electron beam sweeps across the ion beam and the deflection angle exerted on each electron by the space charge field of ion beam reflecting the local density distribution of the ion beam. In this paper, the design and setup of an EBP system, together with preliminary beam experiment result, are presented. A dedicated multi-particle simulation algorithm is developed which reproduces the expansion effect of electron beam spot in a dense ion bunch as observed by several Labs, and a theoretical interpretation is given with a preliminary solution being proposed. In the future, an improved version of EBP will supply non-interceptive diagnostics for Booster Ring (BRing) of the High Intensity heavy ion Accelerator Facility (HIAF), of which a fast scanning system will be implemented with the parallel electron beam optics configured.

Development of a prototype radio-frequency system for a radio-frequency quadrupole cooler buncher in the rare isotope science project.

Radioactive ion beams produced using the isotope separation on-line method in the Rare isotope Accelerator complex for ON-line (RAON) experiment are to be delivered with a beam emittance of around 3 π mm mrad, an energy spread of less than 10 eV, and a short beam bunch width of around 10 µs to meet the requirements of an electron beam ion source charge breeder. A radio frequency quadrupole cooler buncher (RFQ-CB) will be used to meet the beam quality requirements mentioned above. Our target bunching capacity of RFQ-CB is 108 ions/bunch for various ion species. Such a high bunching capacity requires an RF amplitude of ∼3 kV and a frequency range of 1.5-4.5 MHz in our RFQ-CB design. We designed and tested the prototype RF system composed of a helical resonator, a high-power RF amplifier, and high-voltage probes. To reduce heat load to the high voltage probes, we employed vacuum capacitors serially connected to the ends of helical resonators. In the experiment, we confirmed that our 4.5-µH helical resonators made of a 12-mm copper tube and variable vacuum capacitor with a capacitance range of 120-1120 pF can produce required voltages and frequencies using a 100-W RF amplifier. As a result, with 2.5-W RF output power, we obtained the maximum voltage amplitude of 1 kV at 4.5 MHz, which is equivalent to 6.4 kV with 100-W RF output power.

Higher order mode coupler for the circular electron positron collider

A conceptual design report (CDR) for the Circular Electron Positron Collider (CEPC) was published in September 2018. CEPC will use a 650 MHz radiofrequency (RF) system with 240 two-cell cavities. The collider will be a double ring with shared cavities for Higgs operation and separate cavities for W and Z operations. The higher order modes (HOMs) excited by the intense beam bunches must be damped to avoid additional cryogenic loss and multi-bunch instabilities. This study examined the impedance budget and HOM damping requirements for the CEPC ring to keep the beam stable, and a coaxial HOM coupler was considered for extracting the beam-induced HOM power. A double-notch coupler was chosen because of its wide bandwidth for the fundamental mode. Each cavity has two detachable coaxial HOM couplers mounted on the cavity beam pipe with a HOM power handling capacity of 1 kW. This paper presents the design process of the HOM coupler: the RF design, thermal analysis, and mechanical aspects. To validate the design, a prototype of the HOM coupler and a coaxial line test bench were fabricated and tested, and the test results agreed well with the simulation results. The HOM damping results for a real 650 MHz two-cell cavity were also measured and compared with the damping requirements.

HOM damping requirements and measurements for CEPC 650-MHz 2-cell cavity

The circular electron–positron collider (CEPC) will use a 650-MHz RF system with 240 two-cell cavities for the collider. The collider is a double ring with shared cavities for Higgs operation and separate cavities for W and Z operations. The higher-order modes (HOM) excited by the intense beam bunches must be damped to avoid additional cryogenic loss and multi-bunch instabilities. To get the real damping results, two prototypes of HOM coupler have been fabricated and installed on the 650-MHz two-cell cavity. The HOMs have been verified by bead pulling method. A test bench with two 2-cell cavities is used to measure the real damping results and study HOM propagating properties for a cavity string. In this paper, the impedance budget, HOM damping and HOM power requirements for the CEPC collider ring are given. The damping results measured for the fundamental mode and HOMs seem good compared with the simulated results. The absorbing efficiency of the absorber and the extraction power efficiency of HOM couplers were also achieved.

Study on beam bunching and rotation characteristics of low power laser in permanent magnetic field

In this paper, a test platform based on the Faraday Effect was designed to investigate the beam bunching and rotation characteristics of low power laser in permanent magnetic field. By changing the structural parameters of the permanent magnetic confined cavity, the effects of permanent magnetic field on the rotation characteristics of linearly polarized light and the beam bunching characteristics of low power laser were studied. In the tests of the rotation characteristics of the linearly polarized light, it was found that the rotation angle of linearly polarized light tended to decrease with the increase of the diameter of the magnetic cavity and the decrease of the number of magnets. The maximum rotation angle of linearly polarized light was 142.6°, and the minimum rotation angle was 13.6°. In the tests of beam bunching characteristics of low power laser, it was found that the laser spot area had a slight reduction and the laser power was slightly enhanced when the magnetic flux density increased from 0.4563T to 0.8243T, which proved that the strong magnetic field had a bunching effect on the low power laser.

Luminosity of an Ion Collider

A formula is obtained for the luminosity of a collider at the collision of two beams that differ, generally speaking, by their parameters (asymmetric colliders). The formula is valid for counterpropagating and merging beams with coincident longitudinal axes. Three special cases of the formula are considered: collision of two identical axially symmetric bunched beams, collision of a bunch with a coasting beam, and collision of two coasting beams. Collision of intersecting beams is briefly considered, and the method for the luminosity calculation is formulated. The synchronization problem is considered for collisions of asymmetric beams. A method is presented for optimizing parameters of a cyclic collider by minimizing betatron frequency shifts caused by the effect of the space charge of the beams. Numerical examples of luminosity calculations for several types of asymmetric colliders are given.

Electron Diagnostics for Extreme High Brightness Nano-Blade Field Emission Cathodes

Electron beams are essential tools in modern science. They are ubiquitous in fields ranging from microscopy to the creation of coherent ultra-fast X-rays to lithography. To keep pace with demand, electron beam brightness must be continually increased. One of the main strategic aims of the Center for Bright Beams (CBB), a National Science Foundation Science and Technology Center, is to increase brightness from photocathodes by two orders of magnitude. Improving the state-of-the-art for photoemission-based cathodes is one possibility. Several factors have led to an alternative design becoming an increasing necessity; the nanoscale structure. Field emission sources from nano-tips would be an ideal candidate were it not for their low current and damage threshold. A 1-dimensional extended nano-fabricated blade, i.e., a projected tip, can solve the problems inherent in both designs. The novel geometry has been demonstrated to produce extremely high brightness electron beam bunches and is significantly more robust and easier to manufacture than traditional photocathodes. Theory indicates electron emission up to keV energies. We thus present a system of diagnostics capable of analyzing the cathodes and assessing their viability. The diagnostics are designed to measure the electron spectrum up to keV energies, with sub meV resolution at <100 eV, mean transverse energy (MTE), emission uniformity, and cathode lifetime. We also report preliminary data on total extracted charge and maximum detectable electron energy with a simplified retarding field spectrometer.

A new fast time of flight detector for single ion counting to high flux radioactive beams at ISOLDE

Beam diagnostics at the Isotope Separator On-Line DEvice (ISOLDE) consists mainly of wire SCanners and Faraday Cups (FC) coupled to charge sensitive picoammeters. Presented here is the design, simulations, and results of a new FC combined Secondary Electron Emission (SEE) multiplication detector for the beam energies found at ISOLDE (up to 60 keV). It expands the region of sensitivity from the current noise limit (≃0.25 pA) to individual ion counting. The detector is capable of counting rates of 105 ion ms−1 (≃10 pA) for DC or bunched beams with a high sensitivity timing resolution of σt = 0.10 ± 0.03 ns for individual ions. The device also operates as a conventional FC with integrated current measurements on the SEE plate for non-time sensitive operation with efficiency ε = 94% ± 1%. The detector was tested both at the ISOLDE Offline 2 facility and the high resolution separator beam line at the ISOLDE on-line facility where bunched and DC beams from ISCOOL were used.

Beam-beam effects on the luminosity measurement at FCC-ee

The first part of the physics programme of the integrated FCC (Future Circular Colliders) proposal includes measurements of Standard Model processes in e+e− collisions (FCC-ee) with an unprecedented precision. In particular, the potential precision of the Z lineshape determination calls for a very precise measurement of the absolute luminosity, at the level of 10−4, and the precision on the relative luminosity between energy scan points around the Z pole should be an order of magnitude better. The luminosity is principally determined from the rate of low-angle Bhabha interactions, e+e−→ e+e−, where the final state electrons and positrons are detected in dedicated calorimeters covering small angles from the outgoing beam directions. Electromagnetic effects caused by the very large charge density of the beam bunches affect the effective acceptance of these luminometers in a nontrivial way. If not corrected for, these effects would lead, at the Z pole, to a systematic bias of the measured luminosity that is more than one order of magnitude larger than the desired precision. In this note, these effects are studied in detail, and methods to measure and correct for them are proposed.

Offline 2, ISOLDE’s target, laser and beams development facility

Offline 2 is an entirely new research and development mass separator laboratory for the ISOLDE facility located on the Meyrin site of CERN. It closely resembles beam production (for non-radioactive beams) of the on-line (radioactive) systems at ISOLDE with machine versatility and flexibility at the core of its design. The beam optics and down stream beam preparation can deliver bunched beams or continuous beams of negative or positive ions with kinetic energies up to 60 keV. The mass resolving power of the separator operating with a typical ISOLDE ion source is R∼500. All of this is housed within a highly accessible 125 m2 work area with multiple beam optics instruments to determine the beam quality, quantify modifications, develop new technologies and create alternate beam tunes, with the purpose of improving the beams and targets used during on-line operations.

Parallel-plate waveguides for terahertz-driven MeV electron bunch compression.

We demonstrate the electromagnetic performance of waveguides for femtosecond electron beam bunch manipulation and compression with strong-field terahertz (THz) pulses. The compressor structure is a dispersion-free exponentially-tapered parallel-plate waveguide (PPWG) that can focus single-cycle THz pulses along one dimension. We show test results of the tapered PPWG structure using electro-optic sampling (EOS) at the interaction region with peak fields of at least 300 kV/cm, given 0.9 µJ of incoming THz energy. We also present a modified shorted design of the tapered PPWG for better beam manipulation and reduced magnetic field as an alternative to a dual-feed approach. As an example, we demonstrate that with 5 µJ of THz energy, the PPWG compresses a 2.5 MeV electron bunch by a compression factor of more than 4, achieving a bunch length of about 18 fs.

Beam optics studies and lattice design of the 1 GeV H- injector linac for ISNS

Abstract In this paper, we discuss a general methodology to make a compact design of the 1 GeV, 1 MW H- superconducting injector linac for the Indian Spallation Neutron Source (ISNS) project, by following the n o n - e q u i p a r t i t i o n e d design approach, where the emittance growth of the beam is controlled through appropriate adjustment of the strength of lattice elements. Analytical studies of the stability of mismatched envelope oscillations have been performed for a three dimensional beam bunch, which, to the best of our knowledge, is not reported earlier. A rigorous beam halo analysis based on Particle Core Model (PCM) is also presented, considering the actual periodicity of the ISNS lattice, in order to estimate the maximum radial extent of the halo particle.