# Plasma Impedance Matching Network

Dear all,

is there any one with plasma background?

I am developing an algorithm for a plasma impedance L type matching network.

Initially, I set the Load and Tune units to some specific positions. The algorithm is simply tuning a possible minimum SWR starting from the initial position towards the minimum or maximum position of the units and set back to a position where minimum SWR occurred.

This works for non- plasma loads. But, while testing with plasma, after completing the tuning and setting back to the exact position where minimum SWR happened, it results in largely different SWR and high reflected power reading. I couldn't find out why this happens.

Thank you a lot

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Welcome to PF.
It appears the path you take in the minimum SWR search is changing the result. Maybe you must approach from the same starting point or run the algorithm twice.
What is the RF excitation frequency?
What are the approximate dimensions of the plasma chamber?
How do you measure the SWR?
Are there a number of different modes available within the chamber?
Is variation in adjustment changing the mode?

Dear all,

is there any one with plasma background?

I am developing an algorithm for a plasma impedance L type matching network.

Initially, I set the Load and Tune units to some specific positions. The algorithm is simply tuning a possible minimum SWR starting from the initial position towards the minimum or maximum position of the units and set back to a position where minimum SWR occurred.

This works for non- plasma loads. But, while testing with plasma, after completing the tuning and setting back to the exact position where minimum SWR happened, it results in largely different SWR and high reflected power reading. I couldn't find out why this happens.

Thank you a lot

The plasma changes the impedance of your system. It can act as a capacitor or an inductor.

What kind of antenna are you using to generate the plasma? Inductive, capacitive?

Some inductive type antennas have three plasma modes which can exist - E,L,H - depending on the power used and how well the antenna is coupling. Transistioning of these modes cause abrupt changes to the impedance. When entering the H mode from the L mode - reflectance can drop significantly.

The autotuner that we have needs to be tuned by hand while the RF source is on. This is for calibration.

At what pressure is the plasma being generated? How well regulated is the vacuum system? If you are operating in the mTorr range, and your pressures are fluctuating, then the plasma density will fluctuate as well, causing the impedance to change, causing the auto tuner to constantly compensate.

Welcome to PF.
It appears the path you take in the minimum SWR search is changing the result. Maybe you must approach from the same starting point or run the algorithm twice.
What is the RF excitation frequency?
What are the approximate dimensions of the plasma chamber?
How do you measure the SWR?
Are there a number of different modes available within the chamber?
Is variation in adjustment changing the mode?
Dear, Thank you very much for your help.

1. The RF frequency is 13.56MHz.
2. The dimension of the chamber ≈ 50 cm x 60cm x 30 cm( Width x Length x Height )
3. The RF Voltage , Current and phase difference are captured through VI sensor. From these, Forward power, reflected power and then SWR is computed.
4. There is no mode option with in the Chamber.

Thank you once again.

The plasma changes the impedance of your system. It can act as a capacitor or an inductor.

What kind of antenna are you using to generate the plasma? Inductive, capacitive?

Some inductive type antennas have three plasma modes which can exist - E,L,H - depending on the power used and how well the antenna is coupling. Transistioning of these modes cause abrupt changes to the impedance. When entering the H mode from the L mode - reflectance can drop significantly.

The autotuner that we have needs to be tuned by hand while the RF source is on. This is for calibration.

At what pressure is the plasma being generated? How well regulated is the vacuum system? If you are operating in the mTorr range, and your pressures are fluctuating, then the plasma density will fluctuate as well, causing the impedance to change, causing the auto tuner to constantly compensate.
Dear Helmholtzerton,

Depend on power frequency , the Chamber operates as inductive or capacitive. Plasma is being generated at a pressure of 1.3 torr. But some times it fluctuates up and down. might this be a reason?

Thank you a lot

Dear Helmholtzerton,

Depend on power frequency , the Chamber operates as inductive or capacitive. Plasma is being generated at a pressure of 1.3 torr. But some times it fluctuates up and down. might this be a reason?

Thank you a lot
At around 2 Torr is when I'm able to start to break down air into plasma. - but with that high of a pressure, fluctuations haven't been a problem with a simple hand value to control pressure.

When I am in the mili Torr region, say 10mTorr, it is much easier to see swings in pressure without proper equipment. If the pressure were to fluctuate 5-10%, the reflected reading will jump around as plasma generation is effected by how many neutrals are in the chamber. Pressure is associated with neutrals. Once we opted to use a mass flow controller, these pressure swings were no longer an issue.

I don't see fluctuations being much of a problem at 1.3 Torr. Are you seeing wide pressure swings? By how much?

If your algorithm simply seeks to find the minimum SWR from some information being feed to the device - how well shielded is this system? 13.56MHz could be effecting this signal. Just plug a simple loop antenna into an oscilloscope and hold it up in the room where the plasma is produced. You might be surprised by how much RF you pick up.

Douglas Sunday