On a day when SWEPCO repair crews were working feverishly to restore power to thousands of Ark-La-Tex residents, the utility's newest power plant began commercial operation.  The 600-megawatt John W. Turk, Jr, power plant near Fulton, Arkansas, in Hempstead County, went on-line just after noon today.  But this is no ordinary coal-fired plant.  SWEPCO President Nicholas Akins says this is the first ultra-supercritical generating plant to go into operation in the United States.   What that means is that it generates electricity more efficiently at higher temperatures, requires less coal, and produces fewer emissions to generate the same amount of power as existing coal-fired units.  SWEPCO owns 73-percent of the Turk plant -- the other 27-percent belongs to the Arkansas Electric Cooperative Corporation, East Texas Electric Cooperative, and Oklahoma Municipal Power Authority.