Cho-Kuen Ng

Shape determination for deformed electromagnetic cavities

The measured physical parameters of a superconducting cavity differ from those of the designed ideal cavity. This is due to shape deviations caused by both loose machine tolerances during fabrication and by the tuning process for the accelerating mode. We present a shape determination algorithm to solve for the unknown deviations from the ideal cavity using experimentally measured cavity data. The objective is to match the results of the deformed cavity model to experimental data through least-squares minimization. The inversion variables are unknown shape deformation parameters that describe perturbations of the ideal cavity. The constraint is the Maxwell eigenvalue problem. We solve the nonlinear optimization problem using a line-search based reduced space Gauss-Newton method where we compute shape sensitivities with a discrete adjoint approach. We present two shape determination examples, one from synthetic and the other from experimental data. The results demonstrate that the proposed algorithm is very effective in determining the deformed cavity shape.

Towards simulation of electromagnetics and beam physics at the petascale

Under the support of the U.S. DOE SciDAC program, SLAC has been developing a suite of 3D parallel finite- element codes aimed at high-accuracy, high-fidelity electromagnetic and beam physics simulations for the design and optimization of next-generation particle accelerators. Running on the latest supercomputers, these codes have made great strides in advancing the state of the art in applied math and computer science at the petascale that enable the integrated modeling of electromagnetics, self-consistent Particle-In-Cell (PIC) particle dynamics as well as thermal, mechanical, and multi-physics effects. This paper will present 3D results of trapped mode calculations in an ILC cryomodule and the modeling of the ILC Sheet Beam klystron, shape determination of superconducting RF (SCRF) cavities and multipacting studies of SCRF HOM couplers, as well as PIC simulation results of the LCLS RF gun.

Solving large sparse linear systems in end-to-end accelerator structure simulations

Summary form only given. We present a case study of solving very large sparse linear systems in end-to-end accelerator structure simulations. Both direct solvers and iterative solvers are investigated. A parallel multilevel preconditioner based on hierarchical finite element basis functions is considered and has been implemented to accelerate the convergence of iterative solvers. A linear system with matrix size 93,147,736 and with 3,964,961,944 nonzeros from 3D electromagnetic finite element discretization has been solved in less than 8 minutes with 1024 CPUs on the NERSC IBM SP. The resource utilization as well as the application performance for these solvers is discussed.

Modeling imperfection effects on dipole modes in TESLA cavity

The actual cell shapes of the TESLA cavities differ from the ideal due to fabrication errors, the addition of stiffening rings and the frequency tuning process. Cavity imperfection shifts the dipole mode frequencies and alters the Qexts from those computed for the ideal cavity. Qext increase could be problematic if its value exceeds the limit required for ILC beam stability. To study these effects, a cavity imperfection model was established using a mesh distortion method. The eigensolver Omega3P was then used to find the critical dimensions that contribute to the Qext spread and frequency shift by comparing predictions to TESLA cavity measurement data. Using the imperfection parameters obtained from these studies, fiducial cavity imperfection models are generated for the studies of wake fields.

X-BAND LINEAR COLLIDER R&D IN ACCELERATING STRUCTURES THROUGH ADVANCED COMPUTING ∗

This paper describes a major computational effort that addresses key design issues in the high gradient accelerating structures for the proposed X-band linear collider, GLC/NLC. Supported by the US DOE’s Accelerator Simulation Project, SLAC is developing a suite of parallel electromagnetic codes based on unstructured grids for modeling RF structures with higher accuracy and on a scale previously not possible. The new simulation tools have played an important role in the R&D of X-Band accelerating structures, in cell design, wakefield analysis and dark current studies.

An RF bunch-length monitor for the SLC final focus

In preparation for the 1997 SLC run, a novel RF bunch-length monitor has been installed in the SLC South Final Focus. The monitor consists of a ceramic gap in the beam pipe, a 160-ft long X-band waveguide (WR90), and a set of dividers, tapers and microwave detectors. Electromagnetic fields radiated through the ceramic gap excite modes in the nearby open-ended X-band waveguide, which transmits the beam-induced signal to a radiation-free shack outside of the beamline vault. There, a combination of power dividers, tapers, waveguides, and crystal detectors is used to measure the signal power in 4 separate frequency channels between 7 and 110 GHz. For typical rms bunch lengths of 0.5-2 mm in the SLC, the bunch frequency spectrum can extend up to 100 GHz. In this paper, we present the overall monitor layout, describe MAFIA calculations of the signal coupled into the waveguide based on a detailed model of the complex beam-pipe geometry, estimate the final power level at the RF conversion points, and report the measured transmission properties of the installed waveguide system.

Parallel finite element particle-in-cell code for simulations of space-charge dominated beam-cavity interactions

Over the past years,SLACs Advanced Computations Department (ACD) has developed the parallel finite element (FE) particle-in-cell code Pic3P (Pic2P) for simulations of beam-cavity interactions dominated by space- charge effects. As opposed to standard space-charge dominated beam transport codes, which are based on the electrostatic approximation,Pic3P (Pic2P) includes space-charge, retardation and boundary effects as it self-consistently solves the complete set of Maxwell-Lorentz equations using higher-order FE methods on conformal meshes. Use of efficient, large-scale parallel processing allows for the modeling of photoinjectors with unprecedented accuracy, aiding the design and operation of the next-generation of accelerator facilities. Applications to the Linac Coherent Light Source (LCLS) RF gun are presented.

Computational science research in support of petascale electromagnetic modeling

Computational science research components were vital parts of the SciDAC-1 accelerator project and are continuing to play a critical role in newly-funded SciDAC-2 accelerator project, the Community Petascale Project for Accelerator Science and Simulation (ComPASS). Recent advances and achievements in the area of computational science research in support of petascale electromagnetic modeling for accelerator design analysis are presented, which include shape determination of superconducting RF cavities, mesh-based multilevel preconditioner in solving highly-indefinite linear systems, moving window using h- or p- refinement for time-domain short-range wakefield calculations, and improved scalable application I/O.

Status of X-band standing wave structure studies at SLAC

Accelerating gradient is one of the major parameters of a linear accelerator. It determines the length of the accelerator and its power consumption. The SLAC two-mile linear accelerator uses 3 meter long S-band traveling wave (TW) accelerating structures. The average gradient in the linac is about 20 MV/m. This gradient corresponds to a maximum surface electric field of about 40 MV/m. An operational gradient of 40 MV/m was reported for a 1.5 m constant impedance TW structure for the SLC positron injector. This corresponds to a maximum surface field of 80 MV/m. A typical operational gradient for standing wave (SW) structures of a medical linear accelerator is 30 MV/m, with surface electric fields of 130 MV/m at a pulse width of several microseconds (longer than the working pulse width for SLAC TW structures). SW structures for S-band rf guns routinely operate at maximum surface fields of 130 MV/m (/spl sim/2 /spl mu/s pulse width). We emphasize an operational gradient with a very low fault rate in comparison to much higher gradients obtained in dedicated high gradient test structures. The operational surface fields in the above mentioned SW structures are obviously higher than in TW, S-band structures. Design considerations, results of high power tests and future plans are discussed in this paper.

Impedance spectrum for the PEP-II RF cavity

The impedance spectrum presented by the PEP-II RF cavity to the beam is calculated using a 3D MAFIA model which includes the damping waveguides and the input coupler. The simulation assumes that all the ports leading out of the cavity, including the beam pipes, are terminated in matched loads. The effect of the external loading on the longitudinal impedances will be examined. This study takes into account the input coupler damping which has not been considered in previous calculations.