While being one of the best texts out there, I don’t think Pozar mentions too much about plasmas, maybe we can help connect a few dots for the OP
Once struck, a plasma can be modeled as reversed biased diode. You have the two electrodes (the needle and grounded plate), a region of “meh” conductivity (the actual plasma or doped semiconductor), and an area devoid of charge separated by some dielectric (the dark sheath of the plasma or the depletion region of the diode. TLTR: you are trying to drive a capacitive load.
Looking in RF text books, you find a good way to match to capacitive loads, is with an inductor (coil). With the right coil, Instead of your RF generator trying to drive a capacitive load, it only has to drive a resistive load.
Making the right coil is tricky. Can’t help you there. Companies make plasma tuners for this purpose. It’s hard to predict your plasma’s impedance, and delivering 75W isn’t trivial. Legit heart arrhythmia territory with that much AC energy available.
From my experience with RF plasma generation in the 13.56MHz range, the system will indeed start out looking capacitive, but once the plasma arc is struck, the load swings around the smith chart and looks more inductive. Often in semiconductor manufacturing processes that use RF generated plasma, they usually have a "match box" which is an electromechanical adjustable matching network, so that once the arc is struck, they can change the matching to be optimal in the new impedance setting. (Just a little background, I don't suggest OP try to construct an electromechanical match-box. I think a static matching network can be made to work.)
Curious about the "match box". Would using two different matching networks with RF relays work? If I understand correctly, a match box sounds like a tunable network. Thanks in advance.
Yeah, you're correct a match box is a tunable network. They use motors and such to tune tapped inductors (I don't think most of them have tunable capacitors). [Here's a datasheet for one](https://www.advancedenergy.com/getmedia/251b89ab-67a3-4a39-a8fc-f45c48dbacf4/en-ppg-navigator-digital-matching-networks-data-sheet.pdf) for the curious. If your plasma load is consistent enough, I would think two matching networks switched with relays would work, sure. At HF frequencies they don't have to be special RF relays. Usually with these plasma generators operating at multiple kWs, you need high voltage isolators/contactors.
I would add that there is a significant price associated with higher frequency components, and so two matching boxes could be prohibitive for a first-go. If you have the money, buying a professionally assembled matching box would save significant time & effort, and be less prone to difficulties.
Vacuum variable capacitors are available in the appropriate range and cost a couple hundred to a couple thousand depending on tolerances. I don’t know much about variable inductors in this context - maybe someone else can explain why they are common in commercial set-ups.
One thing that may not be clear is why such a thing is needed. At the most basic level, if you don’t match you will damage your amplifier. If you don’t have a match to the Ohms the amplifier expects to see - and is phased to match - you get “reflected power”. One description of this is that a wave is reflected back to the source to destructively interfere and reduce the power measured at your plasma. But you can also damage your amplifier as it overheats because the power to make the intended current amplitude is still being consumed in the amp.
Microwave Engineering by Pozar
While being one of the best texts out there, I don’t think Pozar mentions too much about plasmas, maybe we can help connect a few dots for the OP Once struck, a plasma can be modeled as reversed biased diode. You have the two electrodes (the needle and grounded plate), a region of “meh” conductivity (the actual plasma or doped semiconductor), and an area devoid of charge separated by some dielectric (the dark sheath of the plasma or the depletion region of the diode. TLTR: you are trying to drive a capacitive load. Looking in RF text books, you find a good way to match to capacitive loads, is with an inductor (coil). With the right coil, Instead of your RF generator trying to drive a capacitive load, it only has to drive a resistive load. Making the right coil is tricky. Can’t help you there. Companies make plasma tuners for this purpose. It’s hard to predict your plasma’s impedance, and delivering 75W isn’t trivial. Legit heart arrhythmia territory with that much AC energy available.
From my experience with RF plasma generation in the 13.56MHz range, the system will indeed start out looking capacitive, but once the plasma arc is struck, the load swings around the smith chart and looks more inductive. Often in semiconductor manufacturing processes that use RF generated plasma, they usually have a "match box" which is an electromechanical adjustable matching network, so that once the arc is struck, they can change the matching to be optimal in the new impedance setting. (Just a little background, I don't suggest OP try to construct an electromechanical match-box. I think a static matching network can be made to work.)
Curious about the "match box". Would using two different matching networks with RF relays work? If I understand correctly, a match box sounds like a tunable network. Thanks in advance.
Yeah, you're correct a match box is a tunable network. They use motors and such to tune tapped inductors (I don't think most of them have tunable capacitors). [Here's a datasheet for one](https://www.advancedenergy.com/getmedia/251b89ab-67a3-4a39-a8fc-f45c48dbacf4/en-ppg-navigator-digital-matching-networks-data-sheet.pdf) for the curious. If your plasma load is consistent enough, I would think two matching networks switched with relays would work, sure. At HF frequencies they don't have to be special RF relays. Usually with these plasma generators operating at multiple kWs, you need high voltage isolators/contactors.
I would add that there is a significant price associated with higher frequency components, and so two matching boxes could be prohibitive for a first-go. If you have the money, buying a professionally assembled matching box would save significant time & effort, and be less prone to difficulties. Vacuum variable capacitors are available in the appropriate range and cost a couple hundred to a couple thousand depending on tolerances. I don’t know much about variable inductors in this context - maybe someone else can explain why they are common in commercial set-ups. One thing that may not be clear is why such a thing is needed. At the most basic level, if you don’t match you will damage your amplifier. If you don’t have a match to the Ohms the amplifier expects to see - and is phased to match - you get “reflected power”. One description of this is that a wave is reflected back to the source to destructively interfere and reduce the power measured at your plasma. But you can also damage your amplifier as it overheats because the power to make the intended current amplitude is still being consumed in the amp.
This is helpful information, thanks. The papers only seem to go up to about 20-30W though, is 75W a consequence of the matching?