High Frequency Start Arc Generator For Pulse Arc Welder
It seems like you want a saturable transformer; one transformer winding in series with the weld current, the other winding is coupled to an RF oscillator.  When the RF oscillator is active, but no weld current is flowing, the transformer core
couples the two.  When the weld current starts to flow, it magnetizes the transformer core (and that decouples the RF winding from the circuit).  You might get by with a simple pulse into the transformer, using a fast (DIAC or SCR?) switch, instead of RF.