Dear Tim, 

Thanks for posting again (and thanks for the advice you gave me offline; much appreciated). 

With respect to your question let me provide you with the following information: 

DC current is used in plasma torches and several other industrial furnaces such as glass furnaces and aluminum furnaces. In the case of our plasma torches, the power supply is used to convert 3 phase AC as you correctly pointed out to DC which is used to maintain the arc in the torch.  The power supplies are called rectifiers and are available from a few specialized manufacturers around the world.  High frequency is sometimes used, but not in the same range as the x-ray apparatus (a few kHz).  The topology is also very different than for an x-ray machine: we may or may not use a high frequency transformer. When used, it is for galvanic insulation mainly. 

Furthermore, as you mentioned, power supply control can be done by sending a mA or mV signal to the power supply from our control system.  The power supply for a torch is usually current controlled.  The torch itself operates, depending on power, from hundreds of Volts to a few thousand Volts.  The current is typically several hundred Amps, but can even be in the thousands of Amp range. A good comparison might be a welding machine which converts AC voltage to DC current. Only much larger. 

Hope that helps, 

Peter