  We present a theory for describing the inner structure of the electron bunch
in the bubble regime starting from a random distribution of electrons inside
the bubble and subsequently minimizing the system's energy. Consequently, we
find a filament-like structure in the direction of propagation that is
surrounded by various shells consisting of further electrons. If we specify a
2D initial structure, we observe a hexagonal structure for a high number of
particles, corresponding to the close-packing of spheres in two dimensions. The
two-dimensional structures are in agreement with the equilibrium slice model
(ESM).
