  We present novel roulette schemes for rare-event sampling that are both
structure-preserving and unbiased. The boundaries where Monte Carlo markers are
split and deleted are placed automatically and adapted during runtime.
Extending existing codes with the new schemes is possible without severe
changes because the equation of motion for the markers is not altered. These
schemes can also be applied in the presence of nonlinear and nonlocal coupling
between markers. As an illustrative application, we have implemented this
method in the ASCOT-RFOF code, used to simulate fast-ion generation by
radio-frequency waves in fusion plasmas. In this application, with this method
the Monte-Carlo noise level for typical fast-ion energies can be reduced at
least of one order of magnitude without increasing the computational effort.
