Inductively Coupled Plasmas

After a bit of fuddling I’ve gotten some inductively coupled plasmas started. These pictures are each around 1E-1 torr. The two coils are 0.5uF and 5uF respectively. They are driven with a signal generator (@13.56 or 27.12MHz)->100W shortwave amplifier->MFJ 941B tuner->BNC aligator clips. The plasmas were started with ~500V 20kHz AC on different feedthroughs.

Tuning was difficult but was made easier by monitoring radiated power with a SDR. It helped to add a 220pF capacitor in series to move the impedance closer to something that a tuner would normally be used for. A 275pF capacitor would have yielded a resonant circuit at 13.56MHz and 5uF but the real part of the impedance would still have been mismatched. Using the balun tuner output failed, presumably because the 1:9 turns ratio moved the impedance further out of range. I also tried using a homewrapped ~10:1 inductor around a ferrite core (because the real impedance here is tiny compared to 50 ohms) and got some less impressive plasmas.

At higher pressures (not shown) the plasma is much more concentrated around the coil and dimmer.
Antenna analyzer fed through the tuner when tuned to achieve a glow.