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That is a very hot and wide bead for a "superalloy" Gas coverage at the back of the pool becomes more critical, especially with oscellation added to the operation.
A custom gas shielding trail fixture (possibly built into the torch head) may provide superior gas coverage on the trailing edge of the weld as it solidifies...
The best exotic GTAW consultants on the face of the earth are at PWT
http://www.pwt-online.com/These guys have made alot of people alot of money over the years. They give advice and also sell custom made trailing gas shields for GTAW, PAW and GMAW. They also do EB and Laser process control services.
Give them a contact.. Ask for Gregg.. and Tell him that Larry B from United sent you...
Your solution will be forthcomming.