arXiv subjects

Andreas Adelmann

Publications and source records attributed to Andreas Adelmann.

54 records · Page 3Linked to original sources

arXiv2019

On Architecture and Performance of Adaptive Mesh Refinement in an Electrostatics Particle-In-Cell Code

This article presents a hardware architecture independent implementation of an adaptive mesh refinement Poisson solver that is integrated into the electrostatic Particle-In-Cell beam dynamics code OPAL. The Poisson solver is solely based on second generation Trilinos packages to ensure the desired hardware portability. Based on the massively parallel framework AMReX, formerly known as BoxLib, the new adaptive mesh refinement interface provides several refinement policies in order to enable precise large-scale neighbouring bunch simulations in high intensity cyclotrons. The solver is validated with a built-in multigrid solver of AMReX and a test problem with analytical solution. The parallel scalability is presented as well as an example of a neighbouring bunch simulation that covers the scale of the later anticipated physics simulation.

physics.comp-ph↗

arXiv2019

Matching of turn pattern measurements for cyclotrons using multi-objective optimization

The usage of numerical models to study the evolution of particle beams is an essential step in the design process of particle accelerators However, uncertainties of input quantities such as beam energy and magnetic field lead to simulation results that do not fully agree with measurements, hence the final machine will behave slightly differently than the simulations In case of cyclotrons such discrepancies affect the overall turn pattern or may even alter the number of turns in the machine Inaccuracies at the PSI Ring cyclotron facility that may harm the isochronism are compensated by additional magnetic fields provided by 18 trim coils These are often absent from simulations or their implementation is very simplistic In this paper a newly developed realistic trim coil model within the particle accelerator framework OPAL is presented that was used to match the turn pattern of the PSI Ring cyclotron Due to the high-dimensional search space consisting of 48 design variables (simulation input parameters) and 182 objectives (i.e turns) simulation and measurement cannot be matched in a straightforward manner Instead, an evolutionary multi-objective optimisation with a population size of more than 8000 individuals per generation together with a local search approach were applied that reduced the maximum absolute error to 4.54 mm over all 182 turns.

physics.acc-ph↗

arXiv2019

Constrained multi-objective shape optimization of superconducting RF cavities considering robustness against geometric perturbations

High current storage rings, such as the Z-pole operating mode of the FCC-ee, require accelerating cavities that are optimized with respect to both the fundamental mode and the higher order modes. Furthermore, the cavity shape needs to be robust against geometric perturbations which could, for example, arise from manufacturing inaccuracies or harsh operating conditions at cryogenic temperatures. This leads to a constrained multi-objective shape optimization problem which is computationally expensive even for axisymmetric cavity shapes. In order to decrease the computation cost, a global sensitivity analysis is performed and its results are used to reduce the search space and redefine the objective functions. A massively parallel implementation of an evolutionary algorithm, combined with a fast axisymmetric Maxwell eigensolver and a frequency-tuning method is used to find an approximation of the Pareto front. The computed Pareto front approximation and a cavity shape with desired properties are shown. Further, the approach is generalized and applied to another type of cavity.

physics.acc-ph↗

arXiv2019

OPAL a Versatile Tool for Charged Particle Accelerator Simulations

Many sophisticated computer models have been developed to understand the behaviour of particle accelerators. Even these complex models often do not describe the measured data. Interactions of the beam with external fields, other particles in the same beam and the beam walls all present modelling challenges. These can be challenging to model correctly even with modern supercomputers. This paper describes OPAL (Object Oriented Parallel Accelerator Library), a parallel open source tool for charged-particle optics in linear accelerators and rings, including 3D space charge. OPAL is built from the ground up as a parallel application exemplifying the fact that high performance computing is the third leg of science, complementing theory and experiment. Using the MAD language with extensions, OPAL can run on a laptop as well as on the largest high performance computing systems. The OPAL framework makes it easy to add new features in the form of new C++ classes, enabling the modelling of many physics processes and field types. OPAL comes in two flavours: OPAL-cycl: tracks particles with 3D space charge including neighbouring turns in cyclotrons and FFAs with time as the independent variable. OPAL-t: models beam lines, linacs, rf-photo injectors and complete XFELs excluding the undulator. The code is managed through the git distributed version control system. A suite of unit tests have been developed for various parts of OPAL, validating each part of the code independently. System tests validate the overall integration of different elements.

physics.acc-ph↗

arXiv2019

Multi-objective shape optimization of radio frequency cavities using an evolutionary algorithm

Radio frequency (RF) cavities are commonly used to accelerate charged particle beams. The shape of the RF cavity determines the resonant electromagnetic fields and frequencies, which need to satisfy a variety of requirements for a stable and efficient acceleration of the beam. For example, the accelerating frequency has to match a given target frequency, the shunt impedance usually has to be maximized, and the interaction of higher order modes with the beam minimized. In this paper we formulate such problems as constrained multi-objective shape optimization problems, use a massively parallel implementation of an evolutionary algorithm to find an approximation of the Pareto front, and employ a penalty method to deal with the constraint on the accelerating frequency. Considering vacuated axisymmetric RF cavities, we parameterize and mesh their cross section and then solve time-harmonic Maxwell's equations with perfectly electrically conducting boundary conditions using a fast 2D Maxwell eigensolver. The specific problem we focus on is the hypothetical problem of optimizing the shape of the main RF cavity of the planned upgrade of the Swiss Synchrotron Light Source (SLS), called SLS-2. We consider different objectives and geometry types and show the obtained results, i.e. the computed Pareto front approximations and the RF cavity shapes with desired properties. Finally, we compare these newfound cavity shapes with the current cavity of SLS.

physics.acc-ph↗

arXiv2018

On Uncertainty Quantification in Particle Accelerators Modelling

Using a cyclotron based model problem, we demonstrate for the first time the applicability and usefulness of a uncertainty quantification (UQ) approach in order to construct surrogate models for quantities such as emittance, energy spread but also the halo parameter, and construct a global sensitivity analysis together with error propagation and $L_{2}$ error analysis. The model problem is selected in a way that it represents a template for general high intensity particle accelerator modelling tasks. The presented physics problem has to be seen as hypothetical, with the aim to demonstrate the usefulness and applicability of the presented UQ approach and not solving a particulate problem. The proposed UQ approach is based on sparse polynomial chaos expansions and relies on a small number of high fidelity particle accelerator simulations. Within this UQ framework, the identification of most important uncertainty sources is achieved by performing a global sensitivity analysis via computing the so-called Sobols' indices.

physics.acc-ph↗

arXiv2018

Opportunities in Machine Learning for Particle Accelerators

Machine learning (ML) is a subfield of artificial intelligence. The term applies broadly to a collection of computational algorithms and techniques that train systems from raw data rather than a priori models. ML techniques are now technologically mature enough to be applied to particle accelerators, and we expect that ML will become an increasingly valuable tool to meet new demands for beam energy, brightness, and stability. The intent of this white paper is to provide a high-level introduction to problems in accelerator science and operation where incorporating ML-based approaches may provide significant benefit. We review ML techniques currently being investigated at particle accelerator facilities, and we place specific emphasis on active research efforts and promising exploratory results. We also identify new applications and discuss their feasibility, along with the required data and infrastructure strategies. We conclude with a set of guidelines and recommendations for laboratory managers and administrators, emphasizing the logistical and technological requirements for successfully adopting this technology. This white paper also serves as a summary of the discussion from a recent workshop held at SLAC on ML for particle accelerators.

physics.acc-ph↗

arXiv2018

Calculation of Longitudinal Collective Instabilities with mbtrack-cuda

Macro-particle tracking is a prominent method to study the collective beam instabilities in accelerators. However, the heavy computation load often limits the capability of the tracking codes. One widely used macro-particle tracking code to simulate collective instabilities in storage rings is mbtrack. The Message Passing Interface (MPI) is already implemented in the original mbtrack to accelerate the simulations. However, many CPU threads are requested in mbtrack for the analysis of the coupled-bunch instabilities. Therefore, computer clusters or desktops with many CPU cores are needed. Since these are not always available, we employ as alternative a Graphics Processing Unit (GPU) with CUDA programming interface to run such simulations in a stand-alone workstation. All the heavy computations have been moved to the GPU. The benchmarks confirm that mbtrack-cuda can be used to analyze coupled bunch instabilities up to at least 484 bunches. Compared to mbtrack on an 8-core CPU, 36-core CPU and a cluster, mbtrack-cuda is faster for simulations of up to 3 bunches. For 363 bunches, mbtrack-cuda needs about six times the execution time of the cluster and twice of the 36-core CPU. The multi-bunch instability analysis shows that the length of the ion-cleaning gap has no big influence, at least at filling to 3/4.

physics.acc-ph↗

arXiv2017

Intensity limits of the PSI Injector II cyclotron

We investigate limits on the current of the PSI Injector II high intensity separate-sector isochronous cyclotron, in its present configuration and after a proposed upgrade. Accelerator Driven Subcritical Reactors, neutron and neutrino experiments, and medical isotope production all benefit from increases in current, even at the ~ 10% level: the PSI cyclotrons provide relevant experience. As space charge dominates at low beam energy, the injector is critical. Understanding space charge effects and halo formation through detailed numerical modelling gives clues on how to maximise the extracted current. Simulation of a space-charge dominated low energy high intensity (9.5 mA DC) machine, with a complex collimator set up in the central region shaping the bunch, is not trivial. We use the OPAL code, a tool for charged-particle optics calculations in large accelerator structures and beam lines, including 3D space charge. We have a precise model of the present production) Injector II, operating at 2.2 mA current. A simple model of the proposed future (upgraded) configuration of the cyclotron is also investigated. We estimate intensity limits based on the developed models, supported by fitted scaling laws and measurements. We have been able to perform more detailed analysis of the bunch parameters and halo development than any previous study. Optimisation techniques enable better matching of the simulation set-up with Injector II parameters and measurements. We show that in the production configuration the beam current scales to the power of three with the beam size. However, at higher intensities, 4th power scaling is a better fit, setting the limit of approximately 3 mA. Currents of over 5 mA, higher than have been achieved to date, can be produced if the collimation scheme is adjusted.

physics.acc-ph↗

arXiv2016

Realistic Injection Simulations of a Cyclotron Spiral Inflector using OPAL

We present an upgrade to the particle-in-cell ion beam simulation code OPAL that enables us to run highly realistic simulations of the spiral inflector system of a compact cyclotron. This upgrade includes a new geometry class and field solver that can handle the complicated boundary conditions posed by the electrode system in the central region of the cyclotron both in terms of particle termination, and calculation of self-fields. Results are benchmarked against the analytical solution of a coasting beam. As a practical example, the spiral inflector and the first revolution in a 1 MeV/amu test cyclotron, located at Best Cyclotron Systems, Inc., are modeled and compared to the simulation results. We find that OPAL can now handle arbitrary boundary geometries with relative ease. Comparison of simulated injection efficiencies, and beam shape compare well with measured efficiencies and a preliminary measurement of the beam distribution after injection.

physics.acc-ph↗

arXiv2016

Real-Time Computation of Parameter Fitting and Image Reconstruction Using Graphical Processing Units

In recent years graphical processing units (GPUs) have become a powerful tool in scientific computing. Their potential to speed up highly parallel applications brings the power of high performance computing to a wider range of users. However, programming these devices and integrating their use in existing applications is still a challenging task. In this paper we examined the potential of GPUs for two different applications. The first application, created at Paul Scherrer Institut (PSI), is used for parameter fitting during data analysis of muSR (muon spin rotation, relaxation and resonance) experiments. The second application, developed at ETH, is used for PET (Positron Emission Tomography) image reconstruction and analysis. Applications currently in use were examined to identify parts of the algorithms in need of optimization. Efficient GPU kernels were created in order to allow applications to use a GPU, to speed up the previously identified parts. Benchmarking tests were performed in order to measure the achieved speedup During this work, we focused on single GPU systems to show that real time data analysis of these problems can be achieved without the need for large computing clusters. The results show that the currently used application for parameter fitting, which uses OpenMP to parallelize calculations over multiple CPU cores, can be accelerated around 40 times through the use of a GPU. The speedup may vary depending on the size and complexity of the problem. For PET image analysis, the obtained speedups of the GPU version was more than x40 larger compared to a single core CPU implementation. The achieved results show that it is possible to improve the execution time by orders of magnitude.

cs.DC↗

arXiv2016

A Response to arXiv:1512.09181, "Space Charge Limits in the DAEdALUS DIC Compact Cyclotron"

This document addresses concerns raised about possible limits, due to space charge, to the maximum H2+ ion beam current that can be injected into and accepted by a compact cyclotron. The discussion of the compact cyclotron is primarily within the context of the proposed DAEdALUS and IsoDAR neutrino experiments. These concerns are examined by the collaboration and addressed individually. While some of the concerns are valid, and present serious challenges to the proposed program, the collaboration sees no immediate showstoppers. However, some of the issues raised clearly need to be addressed carefully--analytically, through simulation, and through experiments. In this report, the matter is discussed, references are given to work already done and future plans are outlined.

physics.acc-ph↗

arXiv2016

Examination of the Plasma located in PSI Ring Cyclotron

A plasma has been observed inside the vacuum chamber of the PSI Ring Cyclotron. This ionized gas cloud maybe a substantial contributor to several interior components having reduced lifetimes. The plasma's generation has been directly linked to the voltage that is applied to the Flat Top Cavity through visual confirmation using CCD cameras. A spectrometer was used to correlate the plasma's intensity and ignition to the Flat Top Cavity voltage as well as to determine the composition of the plasma. This paper reports on the analysis of the plasma using spectroscopy. The spectrometer data was analyzed to determine the composition of the plasma and that the plasma intensity (luminosity) directly corresponds to the Flat Top voltage. The results showed that the plasma was comprised of elements consistent with the cyclotrons vacuum interior

physics.acc-ph↗

arXiv2015

The Dynamical Kernel Scheduler - Part 1

Emerging processor architectures such as GPUs and Intel MICs provide a huge performance potential for high performance computing. However developing software using these hardware accelerators introduces additional challenges for the developer such as exposing additional parallelism, dealing with different hardware designs and using multiple development frameworks in order to use devices from different vendors. The Dynamic Kernel Scheduler (DKS) is being developed in order to provide a software layer between host application and different hardware accelerators. DKS handles the communication between the host and device, schedules task execution, and provides a library of built-in algorithms. Algorithms available in the DKS library will be written in CUDA, OpenCL and OpenMP. Depending on the available hardware, the DKS can select the appropriate implementation of the algorithm. The first DKS version was created using CUDA for the Nvidia GPUs and OpenMP for Intel MIC. DKS was further integrated in OPAL (Object-oriented Parallel Accelerator Library) to speed up a parallel FFT based Poisson solver and Monte Carlo simulations for particle matter interaction used for proton therapy degrader modeling. DKS was also used together with Minuit2 for parameter fitting, where $χ^2$ and $max-log-likelihood$ functions were offloaded to the hardwareaccelerator. The concepts of the DKS, first results together with plans for the future will be shown in this paper.

physics.comp-ph↗

arXiv2015

A Homotopy Method for Large-Scale Multi-Objective Optimization

A homotopy method for multi-objective optimization that produces uniformly sampled Pareto fronts by construction is presented. While the algorithm is general, of particular interest is application to simulation-based engineering optimization problems where economy of function evaluations, smoothness of result, and time-to-solution are critical. The presented algorithm achieves an order of magnitude improvement over other geometrically motivated methods, like Normal Boundary Intersection and Normal Constraint, with respect to solution evenness for similar computational expense. Furthermore, the resulting uniformity of solutions extends even to more difficult problems, such as those appearing in common Evolutionary Algorithm test cases.

math.OC↗

arXiv2014

Multilevel Monte Carlo for the Feynman-Kac Formula for the Laplace Equation

Since its formulation in the late 1940s, the Feynman-Kac formula has proven to be an effective tool for both theoretical reformulations and practical simulations of differential equations. The link it establishes between such equations and stochastic processes can be exploited to develop Monte Carlo sampling methods that are effective, especially in high dimensions. There exist many techniques of improving standard Monte Carlo sampling methods, a relatively new development being the so-called Multilevel Monte Carlo method. This paper investigates the applicability of multilevel ideas to the stochastic representation of partial differential equations by the Feynman- Kac formula, using the Walk on Sphere algorithm to generate the required random paths. We focus on the Laplace equation, the simplest elliptic PDE, while mentioning some extension possibilities.

math.PR↗

arXiv2012

A novel adaptive time stepping variant of the Boris-Buneman integrator for the simulation of particle accelerators with space charge

We show that adaptive time stepping in particle accelerator simulation is an enhancement for certain problems. The new algorithm has been implemented in the OPAL (Object Oriented Parallel Accelerator Library) framework, and is compared to the existing code. The idea is to adjust the frequency of costly self field calculations, which are needed to model Coulomb interaction (space charge) effects. In analogy to a Kepler orbit simulation that requires a higher time step resolution at the close encounter, we propose to choose the time step based on the magnitude of the space charge forces. Inspired by geometric integration techniques, our algorithm chooses the time step proportional to a function of the current phase space state instead of calculating a local error estimate like a conventional adaptive procedure. In this paper we build up on first observations made in recent work. A more profound argument is given on how exactly the time step should be chosen. An intermediate algorithm, initially built to allow a clearer analysis by introducing separate time steps for external field and self field integration, turned out to be useful in itself already for a large class of problems.

math-ph↗

arXiv2011

Perfectly Matched Layers in a Divergence Preserving ADI Scheme for Electromagnetics

For numerical simulations of highly relativistic and transversely accelerated charged particles including radiation fast algorithms are needed. While the radiation in particle accelerators has wavelengths in the order of 100 um the computational domain has dimensions roughly 5 orders of magnitude larger resulting in very large mesh sizes. The particles are confined to a small area of this domain only. To resolve the smallest scales close to the particles subgrids are envisioned. For reasons of stability the alternating direction implicit (ADI) scheme by D. N. Smithe et al. (J. Comput. Phys. 228 (2009) pp.7289-7299) for Maxwell equations has been adopted. At the boundary of the domain absorbing boundary conditions have to be employed to prevent reflection of the radiation. In this paper we show how the divergence preserving ADI scheme has to be formulated in perfectly matched layers (PML) and compare the performance in several scenarios.

physics.comp-ph↗