  The PICOSEC Micromegas detector can time the arrival of Minimum Ionizing
Particles with a sub-25 ps precision. A very good timing resolution in
detecting single photons is also demonstrated in laser beams. The PICOSEC
timing resolution is determined mainly by the drift field. The arrival time of
the signal and the timing resolution vary with the size of the pulse amplitude.
Detailed simulations based on GARFIELD++ reproduce the experimental PICOSEC
timing characteristics. This agreement is exploited to identify the microscopic
physical variables, which determine the observed timing properties. In these
studies, several counter-intuitive observations are made for the behavior of
such microscopic variables. In order to gain insight on the main physical
mechanisms causing the observed behavior, a phenomenological model is
constructed and presented. The model is based on a simple mechanism of
"time-gain per interaction" and it employs a statistical description of the
avalanche evolution. It describes quantitatively the dynamical and statistical
properties of the microscopic quantities, which determine the PICOSEC timing
characteristics, in excellent agreement with the simulations. In parallel, it
offers phenomenological explanations for the behavior of these microscopic
variables. The formulae expressing this model can be used as a tool for fast
and reliable predictions, provided that the input parameter values (e.g. drift
velocities) are known for the considered operating conditions.
