  High-frequency long-baseline experiments with the Atacama Large
Millimeter/submillimeter Array were organized to test the high angular
resolution imaging capabilities in the submillimeter wave regime using
baselines up to 16 km. Four experiments were conducted, two Band 7 (289 GHz)
and two Band 8 (405 GHz) observations. Phase correction using band-to-band
(B2B) phase referencing was used with a phase calibrator only 0.7deg away
observed in Band 3 (96 GHz) and Band 4 (135 GHz), respectively. In Band 8, we
achieved the highest resolution of 14x11 mas. We compared the synthesis images
of the target quasar using 20 and 60 s switching cycle times in the phase
referencing. In Band 7, the atmosphere had good stability in phase rms (<0.5
rad over 2 minutes), and there was little difference in image coherence between
the 20 and 60 s switching cycle times. One Band 8 experiment was conducted
under a worse phase rms condition (>1 rad over 2 minutes), which led to a
significantly reduced coherence when using the 60 s switching cycle time. One
of our four experiments indicates that the residual phase rms error after phase
referencing can be reduced to 0.16 rad at 289 GHz in using the 20 s switching
cycle time. Such conditions would meet the phase correction requirement of
image coherence of >70% in Band 10, assuming a similar phase calibrator
separation angle, emphasizing the need for such B2B phase referencing observing
at high frequencies.
