Particle Merger Probabilistic Version¶

Merges macro particles that are close in phase space to reduce computational load. Voronoi-based probalistic variative algorithm. The difference between Base Voronoi algorothm and probabilistic version in parameters: instead of threshold of spread in position and momentum use ratio of deleted particles.

.param file¶

In particleMerging.param is currently one compile-time parameter:

Compile-Time Option	Description
`MAX_VORONOI_CELLS`	Maximum number of active Voronoi cells per supercell. If the number of active Voronoi cells reaches this limit merging events are dropped.

.cfg file¶

PIConGPU command line option	Description
`--<species>_randomizedMerger.period`	The ouput periodicity of the plugin. A value of `100` would mean an output at simulation time step 0, 100, 200, ….
`--<species>_randomizedMerger.ratioDeletedParticles`	The ratio of particles to delete. The parameter have to be in Range [0:1].
`--<species>_randomizedMerger.maxParticlesToMerge`	Maximum number of macroparticles that can be merged into a single macroparticle.
`--<species>_randomizedMerger.posSpreadThreshold`	Below this threshold of spread in position macroparticles can be merged [unit: cell edge length].
`--<species>_randomizedMerger.momSpreadThreshold`	Below this absolute threshold of spread in momentum macroparticles can be merged [unit: \(m_{e-} \cdot c\)].

Memory Complexity¶

Accelerator¶

no extra allocations, but requires an extra particle attribute per species, voronoiCellId.

Host¶

no extra allocations.

Known Limitations¶

this plugin is only available with the CUDA backend
this plugin might take a significant amount of time due to not being fully parallelized.

Reference¶

The particle merger implements a macro particle merging algorithm based on:

Luu, P. T., Tueckmantel, T., & Pukhov, A. (2016). Voronoi particle merging algorithm for PIC codes. Computer Physics Communications, 202, 165-174.

There is a slight deviation from the paper in determining the next subdivision. The implementation always tries to subdivide a Voronoi cell by positions first; momentums are only checked in case the spreads in the positions satisfy the threshold.