INCITE Allocation Helping Drive Research in Future Accelerator Design

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.