The Next Generation PIC Simulation Tool

1. Jacob Trier Frederiksen2) Christian Hededal1) Åke Nordlund1) trier@astro.su.sehededal@astro.ku.dkaake@astro.ku.dk The Next Generation PIC Simulation Tool Thinkshop on Modelling and Simulation of Photon-Plasma Interaction Stockholm University, February 2005 Troels Haugbølle1) troels_h@astro.ku.dk 1) Niels Bohr Institute / Dept. of Astronomy, Copenhagen2) Stockholm Observatory, Stockholm

2. Contents • Standard Particle-In-Cell code of today • Rationale: What physics is interesting and neccesary for understanding tomorrows problems • Internal shocks in GRBs  pair production, neutron decay • Photon-photon interaction  photon ”particles” • Realistic output spectra  frequency/intensity information • How do we implement the physics in a manageable and flexible manner? • Your input!

3. Standard PIC code of today Based on original 2-D, non-relativistic code by Michael Hesse, GSF 3-D, relativistic version developed by Jacob Trier Frederiksen, Stockholm University • Steps • Relativistic particle move, using B & E • Uses  - relativistic momenta • About 3 105 particle updates / sec on P4 laptop • Up to ~ 2.5 107 particle updates / sec on current Altix machine • Parallelizes with OpenMP on Origin,UltraSparc,Power4,Itanium,… • Gather fields; ni, ne , ji , je • 2nd order; Triangular Shaped Clouds (TSC) • Push B & E – staggered in space and time • Electrostatic solver • Optionally include radiative cooling in the particle move (Christian Hededal will talk more about that)

4. Move particles (E, B →xi, vi) • The fields are interpolated from the nearest 3x3 grid points according to the triangular shaped cloud (TSC) scheme. • The particles are then moved by the Lorentz force. • The TSC is an 2nd order scheme.

5. Gather source fields (ni, ne , ji , je) • The particles are interpolated to the nearest 3x3 grid points according to the triangular shaped cloud (TSC) scheme. • The currents and charge densities are then found from the interpolated particles. • Since we use the same scheme to interpolate to and from particles we have: • momentum conservation • minimal self interaction

6. Maxwell Solver Sampled particles Fields on mesh Passed basic tests: wave propagation, etc

7. The GRB fireball • An interesting case in its own. A laboratory of extreme relativistic physics • We need to understand the underlying microphysics • The internal and external shocks are suspectible to the Weibel instability → Magnetic fields depends on microphysics • Neutron decay is important • The internal shocks happens in an pairplasma rich environ-ment, we need to model ”hard” photons and their interactions • We need realistic output spectra to compare theory/modelling with observations

8. The GRB fireball • We need to understand the underlying microphysics • The internal and external shocks are suspectible to the Weibel instability • Micro physics may determine the magnetic field structure • The particle distribution is dependening on the tangled magnetic field

9. t + dt neutron Proton Electron The GRB fireball • We need to understand the underlying microphysics • Neutron decay is important • The new PIC code should be able to create and destroy particles and handle not only electrically charged particles

10. The GRB fireball • We need to understand the underlying microphysics • The internal shocks happens in an pairplasma rich environment, we need to model ”hard” photons • We have to implement a Monte Carlo model for the free streaming ”photon packets” • We should be able to ”renormalize” or fuse/split the packets and create electron/positron pairs depending on the local conditions in the cell t + dt photons Positron Electron

11. The GRB fireball • We need to understand the underlying microphysics • We need realistic output spectra to compare theory/modelling with observations • This comes for free as soon as we have implemented our monte carlo photons. • It is important to have enough photons to recontruct the spectrum reliably. Since there are relatively few high energy photons and many lower energy photons, care must be taken in the renormalization of the photon packets, to get full spectrum coverage

12. Implementing the physics • The current trend in supercomputing is massively parrallel machines. • The number of CPU’s per machine/cluster is going upwards almost as fast as single CPU performance. • The next couple of years we will have access to machines with 1000+ cpus, in the foreseeable future that will be 10000+ cpus • Our language of choice is Fortran 90 • The code has to be highly scaleable • MPI is the right way to synchronize things

13. Organizing things • We are going to make the code object oriented in the sense that you have a structure which is extendible. Everything are plugins, and you can reuse code modules. Main Program Initialization Main Loop Utillities Move charged particles/Gather sources Timestep MPI Communication Move neutral particles Photon/particle splitting/fusing? Interpolation Routines Move photons Neutron decay ? Sort a specie Push Magnetic field Sort particles Push Electric field Exchange particles IO routines Analysis Routines (Calculate spectra etc)

14. Your input / criticism • Is it a good idea to use a Monte Carlo model for photons ? • Can we actually predict reliable spectras ? • Are we focusing on the right / wrong physics ? • Do you have any experience with similar projects ? (Thanks for listening)