  An algorithm and numerical code for the up-sampling of a system of particles,
from a smaller to a larger number, is described. The method introduces a
Poissonian `shot-noise' to the up-sampled distribution, typical of the noise
statistics arising in a bunch of particles generated by a particle accelerator.
The algorithm is applied to a 6-Dimensional phase-space distribution of
relatively few simulation particles, representing an electron beam generated by
particle accelerator modelling software, for subsequent injection into an Free
Electron Laser (FEL) amplifier which is used here to describe the model. A much
larger number of particles is usually required to model the FEL lasing process
than is required to model the electron beam accelerators that drive it. FEL
modelling software usually requires a much greater number of simulation
particles than is required for modelling the acceleration stages and an
increase in simulation particles is required while introducing the correct
Poisson statistical properties of a real electron distribution. A numerical
code developed from the algorithm was then used to generate electron bunches
for injection into to an unaveraged 3D FEL simulation code, Puffin. Results
show good qualitative and quantitative agreement with analytical theory. The
program and user manual is available for download.
