A cnc plasma machine with glycol pool and argon curtain will produce zero slag at the kerf(setup for the material of course). (see crude photobelow)
Blue = tank
Black = sacrificial grid
Green = glycol/ fluid medium
Red = material
Yellow = argon curtain(shielding)
Stainless/inconel/monel/etc also requires argon jet gas instead of regular compressed air. Torch height control is the biggest thing to getting super clean cuts with plasma machines, but is typically an expensive add on.