  We present a study of the spatial and spectral evolution of the loop-top (LT)
sources in a sample of 6 flares near the solar limb observed by {\it RHESSI}. A
distinct coronal source, which we identify as the LT source, was seen in each
of these flares from the early ``pre-heating'' phase through the late decay
phase. Spectral analyses reveal an evident steep power-law component in the
pre-heating and impulsive phases, suggesting that the particle acceleration
starts upon the onset of the flares. In the late decay phase the LT source has
a thermal spectrum and appears to be confined within a small region near the
top of the flare loop, and does not spread throughout the loop, as is observed
at lower energies. The total energy of this source decreases usually faster
than expected from the radiative cooling but much slower than that due to the
classical Spitzer conductive cooling along the flare loop. These results
indicate the presence of a distinct LT region, where the thermal conductivity
is suppressed significantly and/or there is a continuous energy input. We
suggest that plasma wave turbulence could play important roles in both heating
the plasma and suppressing the conduction during the decay phase of solar
flares. With a simple quasi-steady loop model we show that the energy input in
the gradual phase can be comparable to that in the impulsive phase and
demonstrate how the observed cooling and confinement of the LT source can be
used to constrain the wave-particle interaction.
