  Evolution of the single-mode ablative Rayleigh-Taylor instability in the
presence of preheating is investigated numerically. It is found that the
instability evolution depends strongly on the preheating length $L$ in front of
the ablation-front interface. For weak preheating ($L/L_{\rm SH}\leq f_0$,
where $L_{\rm SH}$ is the Spitzer-Harm length and $f_0\sim 20$ is a critical
value), the linear growth rate has a sharp maximum and a secondary spike-bubble
structure is generated. Interaction of this growing structure with the master
spike-bubble leads to rupture of the latter. However, for strong preheating
($L/L_{\rm SH}>20$), the linear growth rate has no sharp maximum and no
secondary spike-bubble is generated. Instead, the master spike-bubble evolves
into an elongated jet. These results are interpreted in terms of the evolution
and interaction of the instability-generated harmonic modes.
