Probe Particles

Section author: Axel Huebl

Probe particles (“probes”) can be used to record field quantities at selected positions over time.

As a geometric data-reduction technique, analyzing the discrete, regular field of a particle-in-cell simulation only at selected points over time can greatly reduce the need for I/O. Such particles are often arranged at isolated points, regularly as along lines, in planes or in any other user-defined manner.

Probe particles are usually neutral, non-interacting test particles that are statically placed in the simulation or co-moving with along pre-defined path. Self-consistently interacting particles are usually called tracer particles.

Workflow

  • speciesDefinition.param: create a stationary probe species, add probeE and probeB attributes to it for storing interpolated fields

using ParticleFlagsProbes = MakeSeq_t<
    particlePusher< particles::pusher::Probe >,
    shape< UsedParticleShape >,
    interpolation< UsedField2Particle >
>;

using Probes = Particles<
    PMACC_CSTRING( "probe" ),
    ParticleFlagsProbes,
    MakeSeq_t<
        position< position_pic >,
        probeB,
        probeE
    >
>;

and add it to VectorAllSpecies:

using VectorAllSpecies = MakeSeq_t<
    Probes,
    // ...
>;

  • density.param: select in which cell a probe particle shall be placed, e.g. in each 4th cell per direction:

// put probe particles every 4th cell in X, Y(, Z)
using ProbeEveryFourthCell = EveryNthCellImpl<
    mCT::UInt32<
        4,
        4,
        4
    >
>;

  • particle.param: initialize the individual probe particles in-cell, e.g. always in the left-lower corner and only one per selected cell

CONST_VECTOR(
    float_X,
    3,
    InCellOffset,
    /* each x, y, z in-cell position component
     * in range [0.0, 1.0) */
    0.0,
    0.0,
    0.0
);
struct OnePositionParameter
{
    static constexpr uint32_t numParticlesPerCell = 1u;
    const InCellOffset_t inCellOffset;
};

using OnePosition = OnePositionImpl< OnePositionParameter >;

  • speciesInitialization.param: initialize particles for the probe just as with regular particles

using InitPipeline = pmacc::mp_list<
    // ... ,
    CreateDensity<
        densityProfiles::ProbeEveryFourthCell,
        startPosition::OnePosition,
        Probes
    >
>;

  • fileOutput.param: make sure the the probe particles are part of FileOutputParticles

// either all via VectorAllSpecies or just select
using FileOutputParticles = MakeSeq_t< Probes >;

Known Limitations

Note

currently, only the electric field \(\vec E\) and the magnetic field \(\vec B\) can be recorded

Note

we currently do not support time averaging

Warning

If the probe particles are dumped in the file output, the instantaneous fields they recorded will be one time step behind the last field update (since our runOneStep pushed the particles first and then calls the field solver). Adding the attributes probeE or probeB to a species will increase the particle memory footprint only for the corresponding species by 3 * sizeof(float_X) byte per attribute and particle.