  Optimizing the confinement and transport of fast ions is an important
consideration in the design of modern fusion reactors. For spherical tokamaks
in particular, fast ions can significantly influence global plasma behavior
because their large drift orbits often sample both core and scrape-off-layer
(SOL) plasma conditions. Their Larmor radii are also comparable to the SOL
width, rendering the commonly chosen guiding center approximations
inappropriate. Accurately modeling the behavior of fast ions therefore requires
retaining a complete description of the fast ion orbit including its Larmor
motion. Here, we introduce the Scrape-Off-Layer Fast Ion (SOLFI) code, which is
a new and versatile full-orbit Monte Carlo particle tracer being developed to
follow fast ion orbits inside and outside the separatrix. We benchmark SOLFI in
a simple straight mirror geometry and show that the code (i) conserves particle
energy and magnetic moment, (ii) obtains the correct passing boundary for
particles moving in magnetic mirror field with an imposed electrostatic field,
and (iii) correctly observes equal ion and electron current at the ambipolar
potential predicted from analytical theory.
