  This paper presents the first detailed investigation of the characteristics
of mm/submm phase fluctuation and phase correction methods obtained using ALMA
with baseline lengths up to ~15 km. Most of the spatial structure functions
(SSFs) show that the phase fluctuation increases as a function of baseline
length, with a power-law slope of ~0.6. In many cases, we find that the slope
becomes shallower (average of ~0.2-0.3) at baseline lengths longer than ~1 km,
namely showing a turn-over in SSF. The phase correction method using water
vapor radiometers (WVRs) works well, especially for the cases where PWV >1 mm,
which reduces the degree of phase fluctuations by a factor of two in many
cases. However, phase fluctuations still remain after the WVR phase correction,
suggesting the existence of other turbulent constituent that cause the phase
fluctuation. This is supported by occasional SSFs that do not exhibit any
turn-over; these are only seen when the PWV is low or after WVR phase
correction. This means that the phase fluctuation caused by this turbulent
constituent is inherently smaller than that caused by water vapor. Since there
is no turn-over in the SSF up to the maximum baseline length of ~15 km, this
turbulent constituent must have scale height of 10 km or more, and thus cannot
be water vapor, whose scale height is around 1 km. This large scale height
turbulent constituent is likely to be water ice or a dry component. Excess path
length fluctuation after the WVR phase correction at a baseline length of 10 km
is large (>200 micron), which is significant for high frequency (>450 GHz or
<700 micron) observations. These results suggest the need for an additional
phase correction method, such as fast switching, in addition to the WVR phase
correction. We simulated the fast switching, and the result suggests that it
works well, with shorter cycle times linearly improving the coherence.
