Liling Xiao

Design of a Low Loss SRF Cavity for the ILC

An international team comprising SLAC, KEK, FNAL, JLAB and DESY is collaborating on the design, fabrication and test of a low loss, 1.3 GHz 9-cell SRF structure as a potential improvement for the ILC main linac. The advantages of this structure over the TESLA structure include lower cryogenic loss, shorter rise time, and less stored energy. Among the issues to be addressed in this design are HOM damping, Lorentz force detuning and multipacting. We will report on HOM damping calculations using the parallel finite element eigenmode solver Omega3P and the progress made towards an optimized design. Studies on multipacting and estimates of the Lorentz force detuning will also be presented.

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.

Nb-Pb Superconducting RF Gun

We report on the status of an electron RF-gun made of two superconductors: niobium and lead. The presented design combines the advantages of the RF performance of bulk niobium superconducting cavities and the reasonably high quantum efficiency of lead, as compared to other superconducting metals. The concept, mentioned in a previous paper, follows the attractive approach of all niobium superconducting RF-gun as it has been proposed by the BNL group. Measured values of quantum efficiency for lead at various photon energies, analysis of recombination time of photon-broken Cooper pairs for lead and niobium, and preliminary cold test results are discussed in this paper.

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.

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.

Optimization of the low loss SRF cavity for the ILC

The low-loss shape cavity design has been proposed as a possible alternative to the baseline TESLA cavity design for the ILC main linacs. The advantages of this design over the TESLA cavity are its lower cryogenic loss, and higher achievable gradient due to lower surface fields. High gradient prototypes for such designs have been tested at KEK (ICHIRO) and TJNAF (LL). However, issues related to HOM damping and multipacting still need to be addressed. Preliminary numerical studies of the prototype cavities have shown unacceptable damping factors for some higher-order dipole modes if the typical TESLA HOM couplers are directly adapted to the design. The resulting wakefield will dilute the beam emittance thus reducing the machine luminosity. Furthermore, high gradient tests on a 9-cell prototype at KEK have experienced multipacting barriers although a single LL cell had achieved a high gradient. From simulations, multipacting activities are found to occur in the end-groups of the cavity. In this paper, we will present the optimization results of the end-groups for the low-loss designs for effective HOM damping and alleviation of multipacting.

Large scale shape optimization for accelerator cavities

We present a shape optimization method for designing accelerator cavities with large scale computations. The objective is to find the best accelerator cavity shape with the desired spectral response, such as with the specified frequencies of resonant modes, field profiles, and external Q values. The forward problem is the large scale Maxwell equation in the frequency domain. The design parameters are the CAD parameters defining the cavity shape. We develop scalable algorithms with a discrete adjoint approach and use the quasi-Newton method to solve the nonlinear optimization problem. Two realistic accelerator cavity design examples are presented.

HOM and LOM coupler optimizations for the ILC crab cavity

The FNAL 9-cell 3.9 GHz deflecting cavity designed for the CKM experiment was chosen as the baseline design for the ILC BDS crab cavity. Effective damping is required for the lower-order TM010 modes (LOM), the same-order TM110 pi-mode (SOM) as well as the higher-order modes (HOM) to minimize the beam loading and beam centroid steering due to wakefields. Simulation results of the original CKM design using the eigensolver Omega3P showed that both the notch filters of the HOM/LOM couplers are too sensitive to the notch gap, and the damping of the SOM is insufficient for the ILC. To meet the ILC requirements, the couplers were redesigned to improve the damping and tuning sensitivity. With the new design, the damping of the LOM/SOM/HOM modes is significantly improved, the sensitivity of the notch filter for the HOM coupler is reduced by one order of magnitude and mechanically feasible, and the LOM coupler is simplified by aligning it on the same plane as the SOM coupler and by eliminating the notch filter. In this paper, we will present the coupler optimization, tolerance studies and multipacting analysis for the crab cavity.

Shape Determination for Deformed Cavities

A realistic superconducting RF cavity has its shape deformed comparing to its designed shape due to the loose tolerance in the fabrication process and the frequency tuning for its accelerating mode. A PDE-constrained optimization problem is proposed to determine the deformation of the cavity. A reduce space method is used to solve the PDE-constrained optimization problem where design sensitivities were computed using a continuous adjoint approach. A proof-of-concept example is given in which the deformation parameters of a single cavity-cell with two different types of deformation were computed.