During AC welding using GTAW the arc becomes unstable due to low or no current flow during a period of time during the cycle of current. The high frequency basically keeps the arc initiated and stable during that period of time.

Some power sources are available with Square Wave Forms in which this is not required as curernt flow always.

There is a document that may mention this at http://www.millerwelds.com/pdf/gtawbook.pdf

Hope this helps and have a good day

Gerald Austin