Showing papers by "John R. Cary published in 2006"

Low-noise electromagnetic δf particle-in-cell simulation of electron Bernstein waves

Abstract: The conversion of the extraordinary (X) mode to an electron Bernstein wave (EBW) is one way to get rf energy into an overdense plasma. Analysis of this is complex, as the EBW is a fully kinetic wave, and so its linear propagation is described by an intractable integro-differential equation. Nonlinear effects cannot be calculated within this rubric at all. Full particle-in-cell (PIC) simulations cannot be used for these analyses, as the noise levels for reasonable simulation parameters are much greater than the typical rf amplitudes. It is shown that the delta-f computations are effective for this analysis. In particular, the accuracy of those computations has been verified by comparison with full PIC, cold plasma theory, and small gyroradius theory. This computational method is then used to analyze mode conversion in different frequency regimes. In particular, reasonable agreement with the theoretical predictions of Ram and Schultz [Phys. Plasmas 7, 4084 (2000)] in the linear regime is found, where 100% X...

Numerical study of the magnetized friction force

Abstract: (Received 14 November 2005; published 7 July 2006)Fundamental advances in experimental nuclear physics will require ion beams with orders of magnitudeluminosity increase and temperature reduction. One of the most promising particle accelerator techniquesfor achieving these goals is electron cooling, where the ion beam repeatedly transfers thermal energy to acopropagating electron beam. The dynamical friction force on a fully ionized gold ion moving throughmagnetized and unmagnetized electron distributions has been simulated, using molecular dynamicstechniques that resolve close binary collisions. We present a comprehensive examination of theoreticalmodels in use by the electron cooling community. Differences in these models are clarified, enabling theaccurate design of future electron cooling systems for relativistic ion accelerators.

Petascale self-consistent electromagnetic computations using scalable and accurate algorithms for complex structures

Abstract: As the size and cost of particle accelerators escalate, high-performance computing plays an increasingly important role; optimization through accurate, detailed computermodeling increases performance and reduces costs. But consequently, computer simulations face enormous challenges. Early approximation methods, such as expansions in distance from the design orbit, were unable to supply detailed accurate results, such as in the computation of wake fields in complex cavities. Since the advent of message-passing supercomputers with thousands of processors, earlier approximations are no longer necessary, and it is now possible to compute wake fields, the effects of dampers, and self-consistent dynamics in cavities accurately. In this environment, the focus has shifted towards the development and implementation of algorithms that scale to large numbers of processors. So-called charge-conserving algorithms evolve the electromagnetic fields without the need for any global solves (which are difficult to scale up to many processors). Using cut-cell (or embedded) boundaries, these algorithms can simulate the fields in complex accelerator cavities with curved walls. New implicit algorithms, which are stable for any time-step, conserve charge as well, allowing faster simulation of structures with details small compared to the characteristic wavelength. These algorithmic and computational advances have been implemented in the VORPAL7 Framework, a flexible, object-oriented, massively parallel computational application that allows run-time assembly of algorithms and objects, thus composing an application on the fly.

Simulations of Dynamical Friction Including Spatially-Varying Magnetic Fields

Abstract: A proposed luminosity upgrade to the Relativistic Heavy Ion Collider (RHIC) includes a novel electron cooling section, which would use ∼55 MeV electrons to cool fully‐ionized 100 GeV/nucleon gold ions. We consider the dynamical friction force exerted on individual ions due to a relevant electron distribution. The electrons may be focussed by a strong solenoid field, with sensitive dependence on errors, or by a wiggler field. In the rest frame of the relativistic co‐propagating electron and ion beams, where the friction force can be simulated for nonrelativistic motion and electrostatic fields, the Lorentz transform of these spatially‐varying magnetic fields includes strong, rapidly‐varying electric fields. Previous friction force simulations for unmagnetized electrons or error‐free solenoids used a 4th‐order Hermite algorithm, which is not well‐suited for the inclusion of strong, rapidly‐varying external fields. We present here a new algorithm for friction force simulations, using an exact two‐body collision model to accurately resolve close interactions between electron/ion pairs. This field‐free binary‐collision model is combined with a modified Boris push, using an operator‐splitting approach, to include the effects of external fields. The algorithm has been implemented in the VORPAL code and successfully benchmarked.

Towards the petascale in electromagnetic modeling of plasma-based accelerators for high-energy physics

Abstract: Plasma-based lepton acceleration concepts are a key element of the long-term R&D portfolio for the U.S. Office of High Energy Physics. There are many such concepts, but we consider only the laser (LWFA) and plasma (PWFA) wakefield accelerators. We present a summary of electromagnetic particle-in-cell (PIC) simulations for recent LWFA and PWFA experiments. These simulations, including both time explicit algorithms and reduced models, have effectively used terascale computing resources to support and guide experiments in this rapidly developing field. We briefly discuss the challenges and opportunities posed by the near-term availability of petascale computing hardware.

Simulations of electron effects in superconducting cavities with the vorpal code

Abstract: Modeling the complex boundaries of superconducting radio frequency (SRF) accelerating cavities on a Cartesian grid is a challenge for many Finite Difference Time Domain (FDTD) electromagnetic PIC codes. The simulation of such cavities requires conformal (curve fitting) boundaries. Modelingthe full cavity includingcouplers and ports is fundamentallya three dimensional problemrequiringthe capability to run in parallel on large numbersof processors. We have recently added conformal boundaries using the method of Dey and Mittra to the plasma simulation code VORPAL. Using this higher order boundary algorithm and the surface physics package TxPhysics, we have begun studies of self-consistent electron effects in SRF cavities. We have modeled the beam excitation of cavity modes and the effects of electron multipacting. Results from these studies will be presented using the new user-friendly visualization tool developed specifically for VORPAL.