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Atacama Large Millimeter/submillimeter Array (ALMA) will be the world largestmm/submm interferometer, and currently the Early Science is ongoing, togetherwith the commissioning and science verification (CSV). Here we present a studyof the temporal phase stability of the entire ALMA system from antennas to thecorrelator. We verified the temporal phase stability of ALMA using data, takenduring the last two years of CSV activities. The data consist of integrationson strong point sources (i.e., bright quasars) at various frequency bands, andat various baseline lengths (up to 600 m). From the observations of strongquasars for a long time (from a few tens of minutes, up to an hour), we derivedthe 2-point Allan Standard Deviation after the atmospheric phase correctionusing the 183 GHz Water Vapor Radiometer (WVR) installed in each 12 m antenna,and confirmed that the phase stability of all the baselines reached the ALMAspecification. Since we applied the WVR phase correction to all the datamentioned above, we also studied the effectiveness of the WVR phase correctionat various frequencies, baseline lengths, and weather conditions. The phasestability often improves a factor of 2 - 3 after the correction, and sometimesa factor of 7 improvement can be obtained. However, the corrected data stilldisplays an increasing phase fluctuation as a function of baseline length,suggesting that the dry component (e.g., N2 and O2) in the atmosphere alsocontributes the phase fluctuation in the data, although the imperfection of theWVR phase correction cannot be ruled out at this moment.

ALMA Temporal Phase Stability and the Effectiveness of Water Vapor Radiometer