  Imaging microwave observations of an eruptive, partially occulted solar flare
on 18 April 2001 suggest that the global structure of the event can be
described by the helical kink instability of a twisted magnetic flux rope. This
model is suggested by the inverse gamma shape of the source exhibiting crossing
legs of a rising flux loop and by evidence that the legs interact at or near
the crossing point. The interaction is reflected by the location of peak
brightness near the crossing point and by the formation of superimposed compact
nonthermal sources most likely at or near the crossing point. These sources
propagate upward along both legs, merge into a single, bright source at the top
of the structure, and continue to rise at a velocity $>\!1000$~km\,s$^{-1}$.
The compact sources trap accelerated electrons which radiate in the radio and
hard X-ray ranges. This suggests that they are plasmoids, although their
internal structure is not revealed by the data. They exhibit variations of the
radio brightness temperature at a characteristic time scale of $\sim\!40$~s,
anti-correlated to their area, which also support their interpretation as
plasmoids. Their propagation path differs from the standard scenario of
plasmoid formation and propagation in the flare current sheet, suggesting the
helical current sheet formed by the instability instead.
