The Next Generation PIC Simulation Tool

Troels Haugbølle1)

Jacob Trier Frederiksen2) Christian Hededal1) Åke Nordlund1)

trier@astro.su.se hededal@astro.ku.dk aake@astro.ku.dk

troels_h@astro.ku.dk

1) Niels Bohr Institute / Dept. of Astronomy, Copenhagen2) Stockholm Observatory, Stockholm

Thinkshop on Modelling and Simulation of Photon-Plasma InteractionStockholm University, February 2005

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!

Standard PIC code of today

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)

Based on original 2-D, non-relativistic code by Michael Hesse, GSF

3-D, relativistic version developed by Jacob Trier Frederiksen, Stockholm University

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.

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

Maxwell Solver

Fields on mesh

Sampledparticles

Passed basic tests: wave propagation, etc

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

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

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

neutron

Electron

t + dt

Proton

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

photons Positron

Electron

t + dt

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

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 scaleableMPI is the right way to synchronize things

Initialization

Main Loop

Timestep

Photon/particle splitting/fusing?

Neutron decay ?

Move charged particles/Gather sources

Move neutral particles

Move photons

Main Program

Push Magnetic field

Push Electric field

MPI Communication

IO routines

Interpolation Routines

Sort a specie

Analysis Routines(Calculate spectra etc)

Sort particles

Exchange particles

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.

Utillities

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)