  Using high resolution x-ray scattering, we have measured the structure of the
Q_1 CDW in Ta-doped NbSe_3. Detailed line shape analysis of the data
demonstrates that two length scales are required to describe the phase-phase
correlation function. Phase fluctuations with wavelengths less than a new
length scale $a$ are suppressed and this $a$ is identified with the amplitude
coherence length. We find that xi_a* = 34.4 \pm 10.3 angstroms. Implications
for the physical mechanisms responsible for pinning are discussed.
