Transverse Phase Space Research Articles

4D transverse phase space tomography of an operational hydrogen ion beam via noninvasive 2D measurements using laser wires

Utilizing noninvasive 2D scans with laser wires, we tomographically reconstructed the 4D transverse phase space distribution of a 1-GeV hydrogen ion (i.e., ${\mathrm{H}}^{\ensuremath{-}}$) beam during operation. The 4D tomography is based on two new advances. First, we extended the formulation of maximum entropy phase space tomography to take 2D projections as input and derived theoretical results relevant to our application. Second, we introduced the method of ``perpendicular scans'' to obtain cross-plane information from a laser wire emittance scanner. Perpendicular scans are two unconventional 2D measurements performed with perpendicular front and back wires. In contrast, only parallel front and back wires (i.e., ``parallel scans'') are utilized in ordinary measurements of the horizontal and vertical 2D phase space distributions. When we applied the technique to the laser emittance station in the high-energy beam transport of the Spallation Neutron Source, experimental results showed that perpendicular scans can provide significant new information to the reconstructed 4D phase space distribution.

Construction of self-consistent longitudinal matches in multipass energy recovery linacs

Any proposal for an accelerator facility based upon a multipass energy recovery linac (ERL) must possess a self-consistent match in longitudinal phase space, not just transverse phase space. We therefore present a semianalytic method to determine self-consistent longitudinal matches in any multipass ERL. We apply this method in collider scenarios (embodying an energy spread minimizing match) and FEL scenarios (embodying a compressive match), and discuss the consequences of each. As an example of the utility of the method, we prove that the choice of common or separate recirculation transport determines the feasibility of longitudinal matches in cases where disruption, such as synchrotron radiation loss, exists. We show that any high energy multipass ERL collider based upon common recirculation transport will require special care to produce a self-consistent longitudinal match, but that one based upon separate transport is readily available. Furthermore we show that any high energy multipass ERL FEL driver based upon common recirculation transport requires a larger resultant rf beam load than the one based on separate transport, favoring the separate transport designs.

Analysis of slice transverse emittance evolution in a very-high-frequency gun photoinjector

Emittance is a key figure-of-merit for high brightness electron beams that enable the success of a broad range of modern accelerator-based instruments. With the successful implementation of the emittance compensation scheme and beam-shaping techniques to align each temporal beam slice in phase space, slice emittance starts to play a more dominating role in determining the beam quality. In this paper, we develop an approach, combining simulations and analytical calculations, to systematically study the evolution of the slice emittance in a very high frequency gun photoinjector, which is under intense R worldwide for driving the next generation continuous-wave accelerators. Our approach is capable of tracing various sources that introduce curvature in the transverse phase space, corresponding to the growth of the slice emittance. It is shown that nonlinear transverse space charge forces in the vicinity of cathodes and the spherical aberration of the focusing solenoid are the main sources of nonlinear forces, while in a long drift section space charge forces can actually compensate for nonlinear transverse position-momentum correlations and hence recover the emittance. We further demonstrate that the sources of nonlinearities can be controlled by tailoring the transverse beam density profile and the design of solenoid lenses.

Multi-parameters optimization and matching analysis simulation studies of the linac for high-energy electron radiography

The electron linear accelerator, dedicated for high-energy electron radiography (HEER) experimental studies, was developed by the Institute of Modern Physics (IMP), Chinese Academy of Sciences (CAS). At present, some HEER experiments have been carried out in the operation stage and further requirements are put forward for lower beam emittance, lower energy spread and better uniformity at the exit of LINAC to enhance radiography quality. This paper uses the General Particle Tracer (GPT) code to perform global optimization studies based on the current beam line setup. The beam injection angle of alpha magnet and the model of the beam scraper are re-optimized. The influence of each factor on the beam parameters is comprehensively considered, and especially the influence of the space charge effect was analyzed by simulation studies. In order to suppress the deterioration of the beam quality in the alpha magnet, this paper take the lead in researching the relationship of the transverse phase space characteristics and the emittance growth and summarizes the rules for beam matching at the alpha magnet. The detailed simulation and analysis studies are presented in this paper.

Restoring betatron phase coherence in a beam-loaded laser-wakefield accelerator

Matched beam loading in laser wakefield acceleration, characterizing the state of flattening the accelerating electric field along the bunch, leads to the minimization of energy spread at high-bunch charges. Here, we experimentally demonstrate by independently controlling injected charge and accelerating gradients, using the self-truncated ionization injection scheme, that minimal energy spread coincides with a reduction of the normalized beam divergence. With the simultaneous confirmation of the micrometer-small beam radius at the plasma exit, deduced from betatron radiation spectroscopy, we attribute this effect to the minimization of chromatic betatron decoherence. These findings are supported by rigorous three-dimensional particle-in-cell simulations tracking self-consistently particle trajectories from injection, acceleration until beam extraction to vacuum. We conclude that beam-loaded laser wakefield acceleration enables highest longitudinal and transverse phase space densities.

Point-to-point Coulomb effects in high brightness photoelectron beam lines for ultrafast electron diffraction

In an effort to increase spatial and temporal resolution of ultrafast electron diffraction and microscopy, ultra-high brightness photocathodes are actively sought to improve electron beam quality. Beam dynamics codes often approximate the Coulomb interaction with mean-field space charge, which is a good approximation in traditional beams. However, point-to-point Coulomb effects, such as disorder induced heating and the Boersch effect, can not be neglected in cold, dense beams produced by such photocathodes. In this paper, we introduce two new numerical methods to calculate the important effects of the photocathode image charge when using a point-to-point interaction model. Equipped with an accurate model of the image charge, we calculate the effects of point-to-point interactions on two high brightness photoemission beamlines for ultrafast diffraction. The first beamline uses a 200 keV gun, whereas the second uses a 5 MeV gun, each operating in the single-shot diffraction regime with $10^5$ electrons/pulse. Assuming a zero photoemission temperature, it is shown that including stochastic Coulomb effects increases the final emittance of these beamlines by over a factor of 2 and decreases the peak transverse phase space density by over a factor of 3 as compared to mean-field simulations. We then introduce a method to compute the energy released by disorder induced heating using the pair correlation function. This disorder induced heating energy was found to scale very near the theoretical result for stationary ultracold plasmas, and it accounts for over half of the emittance growth above mean-field simulations.

Transverse 2d Phase-Space Tomography Using Beam Position Monitor Data of Kicked Beams

Transverse 2d Phase-Space Tomography Using Beam Position Monitor Data of Kicked Beams

Characterization of the Full Transverse Phase Space of Electron Bunches at ARES

Characterization of the Full Transverse Phase Space of Electron Bunches at ARES

Maximum Entropy Reconstruction of 4D Transverse Phase Space from 2D Projections: with Application to Laser Wire Measurements in the SNS HEBT

Maximum Entropy Reconstruction of 4D Transverse Phase Space from 2D Projections: with Application to Laser Wire Measurements in the SNS HEBT

Electron beam transverse phase space tomography using nanofabricated wire scanners with submicrometer resolution

Characterization and control of the transverse phase space of high-brightness electron beams is required at free-electron lasers or electron diffraction experiments for emittance measurement and beam optimization as well as at advanced acceleration experiments. Dielectric laser accelerators or plasma accelerators with external injection indeed require beam sizes at the micron level and below. We present a method using nano-fabricated metallic wires oriented at different angles to obtain projections of the transverse phase space by scanning the wires through the beam and detecting the amount of scattered particles. Performing this measurement at several locations along the waist allows assessing the transverse distribution at different phase advances. By applying a novel tomographic algorithm the transverse phase space density can be reconstructed. Measurements at the ACHIP chamber at SwissFEL confirm that the transverse phase space of micrometer-sized electron beams can be reliably characterized using this method.

Single-shot measurement of transverse second moments using the projection method

We introduce a diagnostic for measuring the transverse phase space (TPS) and transverse second moments of a particle beam in a single shot. The diagnostic consists of a slit followed by a simple projection beam line to project the TPS onto an imaging screen. The beam line is composed of a skew quadrupole magnet followed by a normal quadrupole magnet with an imaging screen at the end. The skew quadrupole generates a coupling between the initial $x$ and the final $y$, while the normal quadrupole removes the correlation between the initial $x$ and the initial ${x}^{\ensuremath{'}}$. Thus, the initial $(x,{x}^{\ensuremath{'}})$ is mapped onto the final $(x,y)$. The slit is needed before the projection beam line to remove the initial $(y,{y}^{\ensuremath{'}})$ contribution from the final $(x,y)$ image on the screen. We present simulation studies and a demonstration of the diagnostic with an experiment performed at Argonne Wakefield Accelerator facility. We also discuss the method's limitations.

Implementation of a Compact Spot-Scanning Proton Therapy System in a GPU Monte Carlo Code to Support Clinical Routine

The purpose of this work was to implement a fast Monte Carlo dose calculation tool, Fred, in the Maastro proton therapy center in Maastricht (Netherlands) to complement the clinical treatment planning system. Fred achieves high accuracy and computation speed by using physics models optimized for radiotherapy and extensive use of GPU technology for parallelization. We implemented the beam model of the Mevion S250i proton beam and validated it against data measured during commissioning and calculated with the clinical TPS. The beam exits the accelerator with a pristine energy of around 230 MeV and then travels through the dynamically extendable nozzle of the device. The nozzle contains the range modulation system and the multi-leaf collimator system named adaptive aperture. The latter trims the spots laterally over the 20 × 20 cm2 area at the isocenter plane. We use a single model to parameterize the longitudinal (energy and energy spread) and transverse (beam shape) phase space of the non-degraded beam in the default nozzle position. The range modulation plates and the adaptive aperture are simulated explicitly and moved in and out of the simulation geometry dynamically by Fred. Patient dose distributions recalculated with Fred were comparable with the TPS and met the clinical criteria. Calculation time was on the order of 10–15 min for typical patient cases, and future optimization of the simulation statistics is likely to improve this further. Already now, Fred is fast enough to be used as a tool for plan verification based on machine log files and daily (on-the-fly) dose recalculations in our facility.

An ultra-compact x-ray free-electron laser

In the field of beam physics, two frontier topics have taken center stage due to their potential to enable new approaches to discovery in a wide swath of science. These areas are: advanced, high gradient acceleration techniques, and x-ray free electron lasers (XFELs). Further, there is intense interest in the marriage of these two fields, with the goal of producing a very compact XFEL. In this context, recent advances in high gradient radio-frequency cryogenic copper structure research have opened the door to the use of surface electric fields between 250 and 500 MV m−1. Such an approach is foreseen to enable a new generation of photoinjectors with six-dimensional beam brightness beyond the current state-of-the-art by well over an order of magnitude. This advance is an essential ingredient enabling an ultra-compact XFEL (UC-XFEL). In addition, one may accelerate these bright beams to GeV scale in less than 10 m. Such an injector, when combined with inverse free electron laser-based bunching techniques can produce multi-kA beams with unprecedented beam quality, quantified by 50 nm-rad normalized emittances. The emittance, we note, is the effective area in transverse phase space (x, p x /m e c) or (y, p y /m e c) occupied by the beam distribution, and it is relevant to achievable beam sizes as well as setting a limit on FEL wavelength. These beams, when injected into innovative, short-period (1–10 mm) undulators uniquely enable UC-XFELs having footprints consistent with university-scale laboratories. We describe the architecture and predicted performance of this novel light source, which promises photon production per pulse of a few percent of existing XFEL sources. We review implementation issues including collective beam effects, compact x-ray optics systems, and other relevant technical challenges. To illustrate the potential of such a light source to fundamentally change the current paradigm of XFELs with their limited access, we examine possible applications in biology, chemistry, materials, atomic physics, industry, and medicine—including the imaging of virus particles—which may profit from this new model of performing XFEL science.

Measurements of a 2.1 MeV H- beam with an Allison scanner.

Transverse 2D phase space distributions of a 2.1 MeV, 5 mA H- beam are measured at the Proton Improvement Plan II Injector Test accelerator at Fermilab with an Allison scanner. This paper describes the design, calibration, and performance of the scanner along with the main results from beam measurements. Analyses of the recorded phase portraits are performed primarily in action-phase coordinates. The stability of the action under linear optics makes it easier to compare measurements taken under different beamline conditions. The amplitude of a single measured point ("pixel") is proportional to the phase density in the corresponding portion of the beam. When the Twiss parameters are calculated using only the high-phase density part of the beam, the pixel amplitude in the beam core is found to be decreasing approximately exponentially with action and to be phase-independent. Outside of the core, the amplitudes decrease with action at a slower rate than in the core. This "tail" comprises 10%-30% of the beam, with 0.1% of the total measured intensity extending beyond action 10-20 times larger than the rms emittance. The transition from the core to the tail is accompanied by the appearance of two "branches" that are separated in phase and extend beyond the core. A set of selected measurements shows that there is no measurable emittance dilution along the beamline; the beam parameters are practically constant over a 0.5 ms pulse; and scraping in various parts of the beamline is an effective way to decrease the transverse tails by removing the branches.

Bifurcation analysis of nonlinear Hamiltonian dynamics in the Fermilab Integrable Optics Test Accelerator

The Integrable Optics Test Accelerator (IOTA) is a novel storage ring at Fermi National Accelerator Laboratory designed (in part) to investigate the dynamics of beams in the presence of highly nonlinear transverse focusing fields that generate integrable single-particle motion with a large spread in the intrinsic betatron tunes. We describe how contemporary geometrical methods from the theory of integrable Hamiltonian systems may be used to locate all critical separatrixlike structures in the 4D transverse phase space, and to construct a complete analysis of the dynamical bifurcations of the system. Application of these techniques results in a global picture of the nominal on-energy transverse dynamics, revealing a rich diversity of accessible dynamical behavior. Similar techniques may be applied to future facilities that exploit the concept of nonlinear integrable optics.

Transverse phase space characterization in an accelerator test facility

We compare three techniques for characterising the transverse phase space distribution of the beam in CLARA FE (the Compact Linear Accelerator for Research and Applications Front End, at Daresbury Laboratory, UK): emittance and optics measurements using screens at three separate beamline locations; quadrupole scans; and phase space tomography. We find that where the beam distribution has significant structure (as in the case of CLARA FE at the time the measurements presented here were made) tomography analysis is the most reliable way to obtain a meaningful characterisation of the transverse beam properties. We present the first experimental results from four-dimensional phase space tomography: our results show that this technique can provide an insight into beam properties that are of importance for optimising machine performance.

Extended transfer matrix through aligned solenoidal fields using the traditional formalism

The traditional transfer matrix used in particle accelerators has been modified to easily take into account a non-zero angular canonical momentum, without the hard edge approximation of the field. To this end, we reformulate the transfer matrix based on an existing method, but by using both the explicit real transformation matrix in the Larmor frame of the 4D transverse phase space, and the real 4D transfer matrix of a solenoidal field in this frame and in the laboratory frame. This reformulation allows to clearly identify differences with the traditional matrix when performing analytic inter-comparisons. The implementation is also easier to perform. A comparison with computer simulation further illustrates its usefulness.

Emittance study of the high intensity highly charged heavy ion beams extracted from electron cyclotron resonance ion source

According to the requirements of ion beams extracted from an electron cyclotron resonance ion source transverse phase space coupling research and the afterglow beam property effective measurement, a pepper pot type meter called PEMiL (Pepper Pot Emittance Meter in Lanzhou) has been designed, fabricated, and commissioned to obtain the emittance of high intensity highly charged heavy ion beams. The direct current beam emittance measurement results verify the coupling property caused by the semisolenoid field. This paper also describes the scheme of multiple exposure accumulation which was applied to measure the afterglow beam property, and the transverse phase space distribution of the oxygen afterglow beam which was measured for the first time is presented.

Numerical simulations of electron cloud build-up in circular accelerators in the presence of multimode distribution beams

Electron cloud effects have become one of the main performance limitations for circular particle accelerators operating with positively charged beams. Among other machines worldwide, the CERN Super Proton Synchrotron (SPS) as well as the Large Hadron Collider (LHC) are affected by these phenomena. Intense efforts have been devoted in recent years to improving the understanding of electron cloud (EC) generation with the aim of finding efficient mitigation measures. In a different domain of accelerator physics, non-linear resonances in the transverse phase space have been proposed as novel means of manipulating charged particle beams. While the original goal was to perform multi-turn extraction from the CERN Proton Synchrotron (PS), several other applications have been proposed. In this paper, the study of EC generation in the presence of charged particle beams with multimode horizontal distribution is presented. Such a peculiar distribution can be generated by different approaches, one of which consists in splitting the initial Gaussian beam distribution by crossing a non-linear resonance. In this paper, the outcome of detailed numerical simulations is presented and discussed.

Development of a Pepper Pot probe to measure the Four-dimensional emittance of low energy beam of electron cyclotron resonance ion source at IMP

The ion beams extracted from an Electron Cyclotron Resonance (ECR) ion source always exist strong transverse coupling effect that is caused by the field of the axis mirror magnets and the extraction solenoid. In order to obtain the four-dimensional (4D) characteristic matrix with coupling elements, a Pepper Pot probe was developed and used to obtain the full 4D transverse phase space distribution of the low energy beam extracted from the ECR ion source at IMP. This paper describes the detailed design and image processing procedure of Pepper Pot probe, especially the analysis results verification compared to another type emittance meter. The first 4D transverse phase space distribution measurement data of oxygen beams from the LECR4 experimental platform are presented and discussed.