Why does a layer-like structure occur in a cold plasma?

[Indula]

I have observed a layer like structure (made of plasma) in a cold plasma and wonders what causes it. Following image shows the observation.

 

[mfb]

Was there an electric or magnetic field in the plasma volume?
The image reminds me of the Franck-Hertz experiment.

 

[Indula]

Yes there is a high electric field. About 15kV of sinosoidal voltage is applied. Frequency was about 23 kHz.

 

[zoki85]

Yes there is a high electric field. About 15kV of sinosoidal voltage is applied. Frequency was about 23 kHz.

What's the pressure and kind of gas you are using? At lower pressures current contribution due to high drift velocities of charge carriers may be important. Change the frequency and observe what happens

 

[Vanadium 50]

wonders what causes it.

Why don't you describe what you are doing in some detail? If you let information out slowly, you will only get your answer slowly.

 

[Indula]

Why don't you describe what you are doing in some detail? If you let information out slowly, you will only get your answer slowly.

Yes. What I'm trying to do is to build a sensor array that would measure the plasma density of the plasma inside. So I have implemented a circular disk like plates inside the chamber and I'm measuring the current sensed by each plate.

What's the pressure and kind of gas you are using? At lower pressures current contribution due to high drift velocities of charge carriers may be important. Change the frequency and observe what happens

Here I'm using Argon gas and the gas pressure is about 500 milliTorrs. Actually the gas pressure changes since I'm keeping the vacuum pump working the whole time.

 

[Vanadium 50]

Yes. What I'm trying to do is to build a sensor array that would measure the plasma density of the plasma inside. So I have implemented a circular disk like plates inside the chamber and I'm measuring the current sensed by each plate.

More slow oozing of information. "in some detail" means "more than two sentences". Could anyone build the experiment based only on what you posted? I am guessing (!) you have only a roughing pump. Could it be vibrations from that? If the answer is "that's stupid", I would claim you will get fewer stupid answers if you describe your setup.

 

[Fred Wright]

I have the following observations due to my experience with thin film coating using a magnetron sputtering machine. If an inert gas such as argon is included in a cathode gas tube, the free ions and electrons are attracted to opposite electrodes and a small current is produced. As voltage is increased some ionisation is produced by collision of electrons with gas atoms, named the "Townsend" discharge. When the voltage across the tube exceeds the breakdown potential, a self sustaining glow discharge occurs - characterised by a luminous glow. The current density and voltage drop remains relatively constant, the increase in total current being satisfied by the area of the glow increasing. Increasing the supply voltage increases current density and voltage drop, this is called the "abnormal glow region". Further increase in supply voltage concentrates the glow into a cathode spot and arc discharge is apparent.
Once the condition for a sustained discharge is met, the tube exhibits the characteristic glow discharge, so called because of the associated luminous glow. It has been established that free ions and electrons are attracted to opposite electrodes producing a discharge - however for a discharge to be self-sustaining requires regeneration of the electrons by the positive ion bombardment of the cathode. This produces secondary electrons and enhances ionization. The resulting positive ion excess creates a positive space charge near the cathode. The voltage drop experienced is termed the "cathode fall". If the discharge is established in a long narrow tube it exhibits a glow pattern in five regions of the tube. They are, proceeding from cathode to anode:
1. "Cathode glow" (intense and narrowly distributed).
2. "Crookes dark space" (no glow) .
3. "Negative glow" ( intense and broadly distributed).
4. "Faraday dark space" (no glow).
5. "Anode glow" (not so intense and broadly distributed).
The positive ion density in the "Crookes dark space" is very high; as a result a significant voltage drop is experienced between it and the cathode. The resulting electric field accelerates the positive ions which produce secondary electron emission from the cathode. These electrons accelerated in the direction of the anode cause ionization, generating positive ions to sustain the discharge. Subsequently, excitation of the gas results in intense illumination in the negative glow region. From this stage the electrons have insufficient exciting or ionising energy, resulting in the "Faraday dark space". Towards the anode a small accelerating field can produce ionization and excitation, the gas again becoming luminous.
These observations may have nothing to do with your experiment, but your pressure and applied voltage roughly correspond to the parameters of a magnetron sputtering machine.

 

[zoki85]

Here I'm using Argon gas and the gas pressure is about 500 milliTorrs. Actually the gas pressure changes since I'm keeping the vacuum pump working the whole time.

Then these are the striations stimulated/supported in low pressure argon by RF fields.
W. Crookes and N. Tesla have studied them first in noble gases filled long tubes.
Beautiful and interesting effects. Many aspects still poorly understood.
Here's the study concentrating on CF₄ gas.

 

[Indula]

Okay. I'll try to describe it in more detail.

The plasma that I'm trying to emulate here is Capacitively Coupled Plasma. This type of plasma is heavily used in microchip manufacturing to process the silicon wafers to deposit or etch the wafer to make intricate structures of electronics.

The Vacuum system: The chamber is made up of a quartz glass tube and with two aluminium plates on both sides, that has been machined with groves and inside each grove there is a rubber layer. The arrangement locks itself when the air is pumped out and the vacuum builds up.
The bottom electrode has all the tube connections and the feedthrough that bears the wires. There are 8 copper cables that comes out of the chamber.
To measure the vacuum pressure I'm using a digital vacuum gauge capable of measuring 0-20,000 microns.
The pump is just a normal vacuum pump that refrigerator repair people use in the industry.

There is also another aluminum cup like structure (Where the wires comes out in a circular array, it makes easy to attach the sensors that way, and also avoids the sharp edges on the lower electrode due to gas input holes and feedthrough maintaining a smooth plasma sheath on the lower electrode)

You can see the plasma sheath on the lower electrode and the bulk plasma clearly in the above picture.

The Electrical system: The high voltage is created using a flyback transformer and the transformer is driven using a ZVS power supply. The transformer I used is shown below.

Those A,B,C connections are connected to the ZVS power supply which can be shown as a schematic below. Note: exact circuit could be different, since I'm using already made circuit bought from a supplier. But the functionality is similar. The output creates a sinusoidal wave once the primary of the transformer is connected, since the circuit capacitance forms a tank circuit that oscillates. The output voltage of the transformer can vary from 10kV to 31kV @ about 23kHz. The transformer ratio is 15:2400 (Primary:secondary).

The bottom electrode is grounded and the top electrode is powered with the transformer output.

The Argon gas is supplied through a gas regulator that reduces the tank pressure to a usable pressure. The Argon here is not pure argon, it contains some amount of carbon dioxide.

The measurement system: Since my primary focus is to measure the current that flows in each wire connected to each electrode, I'm using a non-invasive current probe made by tektronix. The probe is capable of measuring high frequency current as low as 1mA. Usually the observed current for the chamber above is usually 60-70 mA.

Another observation I made was the above layer like structures are more frequently formed when the supplied voltage to the chamber is low. And when the voltage is increased and more ions build up the plasma becomes very stable and doesn't show any layer like structures inside.

 

[Indula]

I have the following observations due to my experience with thin film coating using a magnetron sputtering machine. If an inert gas such as argon is included in a cathode gas tube, the free ions and electrons are attracted to opposite electrodes and a small current is produced. As voltage is increased some ionization is produced by collision of electrons with gas atoms, named the "Townsend" discharge. When the voltage across the tube exceeds the breakdown potential, a self sustaining glow discharge occurs - characterized by a luminous glow. The current density and voltage drop remains relatively constant, the increase in total current being satisfied by the area of the glow increasing. Increasing the supply voltage increases current density and voltage drop, this is called the "abnormal glow region". Further increase in supply voltage concentrates the glow into a cathode spot and arc discharge is apparent.
Once the condition for a sustained discharge is met, the tube exhibits the characteristic glow discharge, so called because of the associated luminous glow. It has been established that free ions and electrons are attracted to opposite electrodes producing a discharge - however for a discharge to be self-sustaining requires regeneration of the electrons by the positive ion bombardment of the cathode. This produces secondary electrons and enhances ionization. The resulting positive ion excess creates a positive space charge near the cathode. The voltage drop experienced is termed the "cathode fall". If the discharge is established in a long narrow tube it exhibits a glow pattern in five regions of the tube. They are, proceeding from cathode to anode:
1. "Cathode glow" (intense and narrowly distributed).
2. "Crookes dark space" (no glow) .
3. "Negative glow" ( intense and broadly distributed).
4. "Faraday dark space" (no glow).
5. "Anode glow" (not so intense and broadly distributed).
The positive ion density in the "Crookes dark space" is very high; as a result a significant voltage drop is experienced between it and the cathode. The resulting electric field accelerates the positive ions which produce secondary electron emission from the cathode. These electrons accelerated in the direction of the anode cause ionization, generating positive ions to sustain the discharge. Subsequently, excitation of the gas results in intense illumination in the negative glow region. From this stage the electrons have insufficient exciting or ionising energy, resulting in the "Faraday dark space". Towards the anode a small accelerating field can produce ionization and excitation, the gas again becoming luminous.
These observations may have nothing to do with your experiment, but your pressure and applied voltage roughly correspond to the parameters of a magnetron sputtering machine.

Thank you for your detailed explanation.

You have noted that when the glow discharge occurs the voltage remains constant. I observed that when I tried to measure the voltage applied to the chamber using the following high voltage probe, the measured voltage seemed to be staying constant although I changed the input voltage to the transformer. (Please read the above post for details)

Following image shows what I've observed. I'm using an oscilloscope to observe the high voltage probe measurement.

Is it what you meant through your discussion? Do you know of a reason why this happens ?

 

[Indula]

More slow oozing of information. "in some detail" means "more than two sentences". Could anyone build the experiment based only on what you posted? I am guessing (!) you have only a roughing pump. Could it be vibrations from that? If the answer is "that's stupid", I would claim you will get fewer stupid answers if you describe your setup.

I have given more details above about this experiment I did.