INCITE Allocation Helping Drive Research in Future Accelerator Design
September 1, 2006
Using an allocation of 2.5 million processor hours on Seaborg at NERSC, a team led by Cameron Geddes of Lawrence Berkeley National Laboratory is creating detailed 3D simulations of laser-driven wakefield particle accelerators (LWFAs), providing crucial under- standing of the next generation of particle accelerators and ultrafast applications in chemistry and biology.
The allocation was awarded under DOE’s Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program, which provides large allocations to high-impact projects related to DOE’s mission areas. Plasmas are not subject to the electrical breakdown that limits conventional accelerators, and LWFAs have demonstrated accelerating gradients thousands of times those obtained in conventional accelerators using the electric field of a plasma wave (the wakefield) driven by an intense laser.
Plasma-based accelerators hence offer a path to more compact machines, and also to high-current ultrashort electron bunches, which may revolutionize applications of accelerators to radiation sources as well as applications in chemistry and biology.
“Future particle accelerators may use laser-driven plasmas to accelerate particles in as little as a thousandth of the length required by conventional machines, and our INCITE allocation is allowing us to create simulations with detailed three- dimensional modeling of such accelerators,” Geddes said. “These simulations are computationally intensive because the laser wavelength (micron) must be resolved over the acceleration length of centimeters. Coupled with experiments, these simulations are developing the detailed understanding of laser acceleration needed to apply this technology to future higher energy particle physics experiments and to compact machines for medicine and laboratory science.”
In addition to Geddes, the team includes Carl Schroeder, Eric Esarey and Wim Leemans of LBNL, and David Bruhwiler and John Cary of Tech-X Corp.
Recent experiments have demonstrated for the first time the production of high- quality electron beams in a high-gradient laser wakefield accelerator. This was achieved in an LBNL laboratory by extending the interaction distance using a pre-formed plasma density structure, or channel, to guide the drive laser pulse over many diffraction ranges. Such beams allow laser-plasma accelerators to be considered seriously as alternatives to conventional accelerators for a wide variety of applications that demand high-quality electron bunches, making simulations to understand their behavior imperative.
Particle-in-cell simulations are a crucial tool in interpreting these experiments and planning the next generation because they can resolve kinetics and particle trapping. Such simulations have shown that the important physics for production of narrow energy spread in recent experiments is that trapping of an initial bunch of electrons loads the wake, suppressing further injection and forming a bunch of electrons isolated in phase space. At the dephasing point, as the bunch begins to outrun the wake, the particles are then concentrated near a single energy and a high quality bunch is obtained. Only a single wake period contributes to the high energy bunch, and hence the electron bunch length is near 10 fs, indicating that a compact ultra- fast electron source with high beam quality has been developed.
While two-dimensional simulations showed the essential physics and demonstrated the applicability of the VORPAL code used in this project (and its scaling to thousands of processors), substantially higher resolution as well as three-dimensional effects are important in order to allow detailed understanding of the physics of these accelerators and the eventual construction of accelerators for applications, according to Geddes.
Scaling from existing runs, reasonable three dimensional modeling of current experiments (100 MeV class) as well as new GeV-class experiments requires a few hundred thousand to a million hours of Seaborg time per run, and the INCITE program is providing 2.5 million hours to allow several such runs.
“The ability to do such high-resolution runs with full particle models is also vital to the development of reduced models which may reduce computation methods in the future, but which require benchmarking against cases of experimental interest at sufficiently high resolution to give confi- dence in the results,” Geddes said.
About NERSC and Berkeley Lab
The National Energy Research Scientific Computing Center (NERSC) is a U.S. Department of Energy Office of Science User Facility that serves as the primary high-performance computing center for scientific research sponsored by the Office of Science. Located at Lawrence Berkeley National Laboratory, the NERSC Center serves more than 7,000 scientists at national laboratories and universities researching a wide range of problems in combustion, climate modeling, fusion energy, materials science, physics, chemistry, computational biology, and other disciplines. Berkeley Lab is a DOE national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California for the U.S. Department of Energy. »Learn more about computing sciences at Berkeley Lab.