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Designing an Auto Match Unit for RF supply

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Ash Small

Wed Nov 17 2010, 10:29PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Interesting point Steve. We are going off-topic here, but it's my thread so WTF.

Now that you put it like that, I'd suggest that it's all about electrons and protons (nuclei, positive ions).

In any gas, you have the 'average' temperature, energy or whatever you want to call it.

Some of the gas molecules will have less energy, some more.

If a few collisions between gas molecules (remember this is in a magnetic field) result in electon dissociation (I can't think of a better word here), the EM field will prevent re-combination, and lead to 'electron avalanche'. (not sure if my terminology is correct here,) Does this address your point?

The accepted theory is that the electrons are 'torn' from the atoms by the EM field, but I'd argue that in anmy gas above 0 Kelvin, some collisions will be energetic enough to knock electrons off, leading to an avalanche in an EM field.

iJim

Wed Nov 17 2010, 11:28PM

Registered Member #2662 Joined: Fri Jan 29 2010, 10:14AM
Location:
Posts: 36

I had a quick look in to ICP breakdown and found the following:

'Review of heating mechanism in inductively coupled plasma'
Surface and Coatings Technology 131 (2000). 1-11
Sang-Hun Seo, ChinWook Chung, Hong-Young Chang

"At low rf power, a discharge with faint emission and low plasma density is produced and maintained by the dominant Capacitive coupling between the powered electrode and ground. This discharge is called the low-power mode (the E-mode). Transition from the E-mode to the H-mode can be achieved by increasing the rf power. The H-mode exhibits intense plasma emission and high density."

This agrees well with my own observations, first a CCP discharge is struck with the E field (from my trigger system). Then, when the magnetic field is sufficiently strong, then discharge becomes self-sustaining and I could remove the trigger. The plasma changes totally at the transition point... the CCP is cold, discharge current of <10mA. The ICP mode is hot, think blow torch!

Note, when I did this work I was working at atmospheric pressure, hence I needed a few kV to strike a CCP (this isn't easy across a coil consisting of just 2 turns) so I added an ignition system. At low pressure you may only need 100-200V, this is very achievable, you probably don't need an ignition system.

By the way, ionisation energy of Argon is 15.75eV, there's no atoms in the gas phase with sufficient energy to liberate an electron. You need an external energy source to generate a free electron... gamma ray/photon, or of course thermionic from hot electrodes.

Back to your ICP system, what power level will you be operating at? Are you planning to water cool the coil? Also, don't use a long wire between the matching network and coil... the coil needs mounting as close as possible to the match network, preferably no wire at all. I found out the hard way! At 300W my 30cm of urm67 coax melted and became a dead short within 2 minutes (I should have known really, 300W forward power, no reflected, yet no plasma...)

Cheers,

Jim

Ash Small

Thu Nov 18 2010, 12:48AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Jim, I was just speculating on what could initiate an electron avalanche.

The RF generator I have is a 600W unit. It still has the tag on it from when it was repaired by the manufacturer, so I assume it's a good one. It was manufactured by Wessex Electronics, which is now Amplifier Technology in Bristol. I'm assuming I'll have to contact them and try and get a manual for it as it has a 15 pin D socket for a control lead to be connected to. I don't have the lead and I don't have any software etc for it. I'm trying to do as much homework first to increase my chances of being taken seriously by them. It went in for repair on 3-3-98 so it's been knocking around for a while since.

I'm aware that the coil should be connected directly to the AMU, I took a job assembling plasma processing systems for a bit in order to learn more about this stuff. I understand all the 'mechanical' side of things and most of the general theory but I wasn't able to find out a great deal about the workings of AMU's, apart from their function.

My coil will be water cooled.

At the moment I'm puzzling over the 'modified 'L' circuit in the link you supplied. (The 'T' circuit is only good for up to 50 Ohms coil reactance).

While I'm on top of the rest of the project, electronics is not my forte, so any assistance regarding exactly how this works will be greatly appreciated. (re calculating reactances, etc.)

klugesmith

Thu Nov 18 2010, 02:15AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Steve McConner wrote ...

What tangential electric field, where did it come from? If there is an electric field, the field lines must terminate on some conductors with suitable voltages that agree with the integral of the field. So, I argue that the electric field is caused by, indeed is, the voltage on the coil conductors, hence it is capacitive coupling.

Refer to Maxwell's equations, i think traditionally the third:
if you will excuse rough typography "del cross E = - dB/dt".
The electric field everywhere has a curl equal to the rate of change of magnetic flux density.
(Another law says that the electric field everywhere also has a divergence equal to the charge density).

Consider a region with no charges, and a cylindrical column of magnetic flux of area 1 m^2.
Its shape is fixed, but its intensity changes with time at 1 tesla per second,
so the total flux in the bundle changes at 1 weber per second.
If you integrate the electric field vector around any hoop that encircles the whole column,
when you get back to the starting point you will have reached 1 volt.
The smallest such hoop has circumference 2*sqrt(pi), thus a tangential E-field strength of 282 mV/meter.
That determines the current that would flow if a conductor were present -- the electric field is there anyway.
In this case "electric field lines" make closed loops that don't terminate at charges.
Just like we are taught that "magnetic field lines" make only closed loops, because magnetic "charges" don't exist.

I have not worked with inductive ion generators, but expect the following experiment would be telling.
Energize a solenoidal coil with RF, as Ash intends. Inside the coil put a thin cylindrical electrostatic shield (with a longitudinal slot) tied to ground. I claim that at frequencies substantially below self-resonance, the presence of the shield will not much change the RF load presented to power source, and will not interfere with gas ionization.

[edit] I'm a firm believer in experiment over theory, if the experiment is easy to do.
Just ran some numbers re. proposed "can crusher" experiment using a jar of, say, argon under partial vacuum.
To prove a point, jar could be surrounded with slotted electrostatic shield (made from, e.g., a soda can).
6000 volts on 4 turn work coil is 1500 volts/turn, or about 75 volts/cm peak e-field right inside the shield.
E-field could be increased by reducing the coil's turn count or circumference (and thus, inductance), at the risk of exceeding the capacitor's current rating. Would anyone better than me with Paschen's law and ionization of argon care to predict the result?

Steve Conner

Thu Nov 18 2010, 11:38AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Apologies for the off-topic...

Klugesmith: OK, so in my previous post I was wrong by a factor of the square of the speed of light.

So, can we agree that the E-field created by magnetic induction, and available for ionizing things, is equivalent to the voltage across the ends of the coil generating the magnetic field, if the coil had one turn? Or, the voltage across the slot in your proposed electrostatic shield.

On the basis of this I argue that any coil with more than one turn will generate an extra E-field over and above this. In the axial direction, for a solenoidal coil. So, a multiturn coil should be better at igniting, and the electrostatic shield should remove that advantage. I think this agrees with the paper that iJim linked to.

Anyway, apologies for the digression. If we want to discuss this further I suppose it should be split into a new thread...

Ash Small

Thu Nov 18 2010, 12:13PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Klugesmith, some of the plasma sources I worked on had multi-slotted electrostatic shields. some did not. (I have a few in the garage.) I never really understood why they were there. I can supply photos, etc if you are interested.

I'd like to know more about this stuff (E fields, H fields) Can anyone provede any relevant links?. I've not read the paper that iJim reffered to, but I'll look for it later.

While this is going a bit off-topic, it is still relevant to my project. (Should it have it's own thread or not?)

As far as the calculations regarding Paschen's law and ionisation of argon go, I understand enough to know that the pressure of the argon is very important here. What pressures are you proposing to use in your experiment?

klugesmith

Thu Nov 18 2010, 05:43PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Steve McConner wrote ...
On the basis of this I argue that any coil with more than one turn will generate an extra E-field over and above this. In the axial direction, for a solenoidal coil. So, a multiturn coil should be better at igniting, and the electrostatic shield should remove that advantage. I think this agrees with the paper that iJim linked to

Good point, Steve, as always.
In my can crusher example, a very stubby solenoid, the e-field from magnetic induction is
about 1500 V around a 20 cm circumference (7500 V/m)
while the end-to-end voltage drop is 6000 V in 1.2 cm (500000 V/m).
If I do the argon jar experiment, better start -without- an electrostatic shield.

Ash Small

Thu Nov 18 2010, 08:47PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Do I take it that you are saying there is a potential difference between one end of the coil and the other, the shorter this difference, the greater the voltage gradient, and that it is this that initiates the plasma in an ICP device? (ie this is similar to a CCP device)

Steve Conner

Fri Nov 19 2010, 11:05AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yeah, I think that is what we're saying.

Imagine two coils, same shape and size, one with 1 turn of wide, flat strip, the other with 10 turns of thinner conductor. The 1 turn one gets 100 amps of RF, the 10 turn one gets 10 amps.

Both coils have 100 amp-turns, so they produce the same magnetic field. Therefore, they induce the same EMF for ignition of the plasma-to-be, and once the plasma is alight, they will couple the same power into it.

But, the 10 turn one has 100 times the inductance, so it takes 10 times the voltage to force 1/10th of the current through it. (Note that this is the same kVA of RF power for both coils, only the details of the matching network will differ.) Therefore, the 10 turn coil has 10 times the voltage between its ends and this voltage implies an axial E-field that's 10 times stronger than any E-field the single turn coil can generate.

If the plasma can see this voltage, say through the walls of a chamber made of insulating material, then it could make ignition much easier. (Maybe 10 times easier, in this case.)

This is all electromagnetic fundamentals, really. Many of us here were forced to learn it at university, and it all seemed dry and boring, until we made our first Tesla coil and the subject suddenly came violently to life. The hyperphysics site has some good stuff.

Proud Mary

Fri Nov 19 2010, 12:11PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Here is the RF output and matching circuit of a 13.56MHz 1kW RF generator

Isn't 13.56MHz the RFID frequency for contactless smart cards?

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