• G.R. Werner, C.A. Bauer, J.R. Cary, T. Munsat
  CIPS, Boulder, Colorado

Funding: This work is supported by the U.S. Department of Energy grant DE-FG02-04ER41317.

The RF properties of photonic crystals (PhCs) can be exploited to avoid the parasitic higher order modes (HOMs) that degrade beam quality in accelerator cavities and reduce efficiency and power in RF generators. Computer simulations show that long-range wake fields are significantly reduced in accelerator structures based on dielectric PhC cavities, which can be designed to trap only those modes within a narrow frequency range. A 2D PhC structure can be used to create a 3D accelerator cavity by using metal end-plates to confine the fields in the third dimension; however, even when the 2D photonic structure allows only a single mode, the 3D structure may trap HOMs, such as guided modes in the dielectric rods, that increase wake fields. For a 3D cavity based on a triangular lattice of dielectric rods, the rod positions can be optimized (breaking the lattice symmetry) to reduce radiation leakage using a fixed number of rods; moreover, the optimized structure has reduced wake fields. Using computer simulation, wake fields in pillbox, PhC, and optimized photonic cavities are calculated; a design for a klystron using the optimized photonic cavity structure is presented.

• D.L. Bruhwiler, B.M. Cowan, K. Paul
  Tech-X, Boulder, Colorado
• J.R. Cary
  CIPS, Boulder, Colorado
• E. Cormier-Michel, C.G.R. Geddes, C.B. Schroeder
  LBNL, Berkeley, California
• E. Esarey, W. Leemans
  University of Nevada, Reno, Reno, Nevada

Funding: Supported by the US DOE Office of Science, Office of High Energy Physics under grant No. DE-FC02-07ER41499; used NERSC resources under grant DE-AC02-05CH11231.

Laser wakefield accelerators (LWFA) have accelerated ~100 pC electron bunches to GeV energies over cm scale distances, via self-trapping from the plasma. Self-trapping cannot be tolerated in staged LWFA modules for high-energy physics applications. The ~1% energy spread of self-trapped electron bunches is too large for light source applications. Both difficulties could be resolved via external injection of a low-emittance electron bunch into a quasilinear LWFA, for which the dimensionless laser amplitude is less than two. However, significant beam charge will result in nonlinear beam loading effects, which will make it challenging to preserve the low emittance. The cold, relativistic fluid model of the parallel VORPAL framework* will be used to simulate the laser-driven electron wake, in the presence of an idealized electron beam. Profiles of the electron beam density, laser pulse envelope and plasma channel will be varied to find a nonlinear beam loading configuration that approximately flattens the electric fields across the beam. Hybrid fluid-PIC simulations will be used to measure the self-consistent emittance growth of the beam.

* C. Nieter and J.R Cary, J. Comp. Phys. 196 (2004), p. 448.

• T.M. Austin
  Tech-X, Boulder, Colorado
• L. Bellantoni
  Fermilab, Batavia
• J.R. Cary
  CIPS, Boulder, Colorado

Funding: US DOE, COMPASS SciDAC-2, Grant Number DE-FC02-07ER41499

We have applied the Werner-Cary method* for extracting modes and mode frequencies from time-domain simulations of crab cavities, as are needed for the ILC and the beam delivery system of the LHC. This method for frequency extraction relies on a small number of simulations and post-processing using the SVD algorithm with Tikhonov regularization. The time domain simulations were carried out using the VORPAL computational framework, which is based on the eminently scalable finite-difference time-domain algorithm. A validation study was performed on an aluminum model of the 3.9 GHz RF separators built originally at Fermi National Accelerator Laboratory in the US. Comparisons with measurements of the A15 cavity show that this method can provide accuracy to within 0.01% of experimental results after accounting for manufacturing imperfections. To capture the near degeneracies two simulations requiring in total a few hours on 600 processors were employed. This method has applications across many areas including obtaining MHD spectra from time-domain simulations.

*J. Comp. Phys. 227, 5200-5214 (2008)

• C.A. Bauer, J.R. Cary, G.R. Werner
  CIPS, Boulder, Colorado

Funding: This work is supported by the U.S. Department of Energy grant DE-FG02-04ER41317.

Through computer simulation, a 2D photonic crystal (PhC) cavity formed from a truncated triangular lattice of dielectric rods is optimized to confine a single accelerating mode efficiently. Photonic crystals have the ability to reflect radiation within only certain frequency ranges, called bandgaps; the bandgaps are determined by the geometry and material of the PhC and so are tunable. For truncated PhCs, reflection is incomplete. Therefore, the confinement of bandgap frequencies to a cavity within a truncated PhC is weakened by the severity of the truncation. For a cavity made of 18 dielectric rods in a truncated triangular lattice arrangement, the desired accelerating cavity mode is weakly confined. Adjusting the positions and sizes of the dielectric rods away from the best lattice configuration within an optimization procedure gives unintuitive structures, ultimately increasing the confinement of the accelerating mode by a factor of 100. Confinement of higher-order modes is also dramatically reduced by the optimization. Similar increases in confinement of the fundamental accelerating mode are found for a 24-rod structure.

• C.G.R. Geddes, E. Cormier-Michel, E. Esarey, W. Leemans, C.B. Schroeder
  LBNL, Berkeley, California
• D.L. Bruhwiler, J.R. Cary, B.M. Cowan, C. Nieter, K. Paul
  Tech-X, Boulder, Colorado

Funding: Funded by the U.S. DOE Office of Science HEP including contract DE-AC02-05CH11231 and SciDAC, and by U.S. DOE NA-22, DARPA, and NSF

Collider and light source applications of laser wakefield accelerators will likely require staging of controlled injection with multi-GeV accelerator modules to produce and maintain the required low emittance and energy spread. We present simulations of upcoming 10 GeV-class LWFA stages, towards eventual collider modules for both electrons and positrons*.  Laser and structure propagation are controlled through a combination of laser channeling and self guiding.  Electron beam evolution is controlled through laser pulse and plasma density shaping, and beam loading. This can result in efficient stages which preserve high quality beams.  We also present results on controlled injection of electrons into the structure to produce the required low emittance bunches using plasma density gradient** and colliding laser pulses.  Tools for accurately modeling emittance and energy spread will be discussed***.

*E. Cormier-Michel et al., Proc. Adv Accel. Workshop 2008.
**C.G.R. Geddes et al., PRL 2008.
***E. Cormier-Michel et al, PRE 2008; C.G.R. Geddes et al, Proc. Adv Accel. Workshop 2008.

• R. Busby, J.R. Cary, D.A. Dimitrov
  Tech-X, Boulder, Colorado
• I. Ben-Zvi, X. Chang, J.W. Keister, E.M. Muller, T. Rao, J. Smedley, Q. Wu
  BNL, Upton, Long Island, New York

Funding: The work at Tech-X Corp. is supported by the U. S. Department of Energy under the DE-FG02-06ER84509 SBIR grant.

The Relativistic Heavy Ion Collider (RHIC) contributes fundamental advances to nuclear physics by colliding a wide range of ions. A novel electron cooling section, which is a key component of the proposed luminosity upgrade for RHIC, requires the acceleration of high-charge electron bunches with low emittance and energy spread. A promising candidate for the electron source is the recently developed concept of a high quantum efficiency photoinjector with a diamond amplifier. To assist in the development of such an electron source, we have implemented algorithms within the VORPAL particle-in-cell framework for modeling secondary electron and hole generation, and for charge transport in diamond. The algorithms include elastic, phonon, and impurity scattering processes over a wide range of charge carrier energies. Results from simulations using the implemented capabilities will be presented and discussed.

• D.A. Dimitrov, R. Busby, J.R. Cary
  Tech-X, Boulder, Colorado
• I. Ben-Zvi, X. Chang, J.W. Keister, E.M. Muller, T. Rao, J. Smedley, Q. Wu
  BNL, Upton, Long Island, New York

Funding: The work at Tech-X Corp. is supported by the US DoE under grant DE-FG02-06ER84509.

A promising new concept of a diamond amplified photocathode for generation of high-current, high-brightness, and low thermal emittance electron beams was recently proposed* and is currently under active development. To better understand the different effects involved in the generation of electron beams from diamond, we have been developing models (within the VORPAL computational framework) to simulate secondary electron generation and charge transport. The currently implemented models include inelastic scattering of electrons and holes for generation of electron-hole pairs, elastic, phonon, and charge impurity scattering. We will present results from 3D VORPAL simulations with these capabilities on charge gain as a function of primary electron energy and applied electric field. Moreover, we consider effects of electron and hole cloud expansion (initiated by primary electrons) and separation in a surface domain of diamond.

*I. Ben-Zvi et al., Secondary emission enhanced photoinjector, C-AD Accel. Phys. Rep. C-A/AP/149, BNL (2004).

• P. Messmer, T.M. Austin, J.R. Cary
  Tech-X, Boulder, Colorado

Funding: This project was in part supported by DOE Office of Advanced Scientific Computing Research SBIR Phase II grant #DE-FG02-07ER84731, SciDAC Grant #DE-FC02-07ER41499, and Tech-X Corporation.

In order to meet the design and budget constraints of next generation particle accelerators, individual components have to be optimized using numerical simulations. Among the optimizations are the geometric shape of RF cavities and the placement of coupler and dampers, requiring large numbers of simulations. It is therefore desirable to accelerate individual cavity simulations. Finite-Difference Time-Domain (FDTD) is a widely used algorithm for modeling electromagnetic fields. While being a time-domain algorithm, it can also be used to determine cavity modes and their frequencies. Weak scaling of parallel FDTD yields good results due to the algorithm locality, but the maximum speedup is determined by the usually small problem size. Graphics Processing Units (GPUs) offer a huge amount of processing power and memory bandwidth, well suited for accelerating FDTD simulations. We therefore developed an FDTD solver on GPUs and incorporated it into the plasma simulation code VORPAL. We will present GPU accelerated VORPAL simulations, provide speedup figures and address the effect of running these simulations in single precision.