It is doubtful that your code will be in arcs, and is probably made up of many short, straight lines, since the only arc available in CNC is a circle.
To mitigate that, you can set Mach 3 to continuous velocity, which is what Hood said. This smooths out the short lines, by accelerating into the next line, before the previous lines has finished.G61 in your code means absolute stop after each line. G64 means continuous velocity.
I don't know what you are using to generate your code, or what system you are using to turn the plasma on and off, but check the code to make sure it is not turning the plasma off after every line.