A few RF generator questions?

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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steve_rb
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A few RF generator questions?

Post by steve_rb » Sun Nov 01, 2009 10:45 am

I have a 6cm diameter by 12 cm long cylindrical chamber and I want to fill it with H2 gas with 1-5 mTorr pressure. I want to put a two or three turn coil inside and connect it to Rf generator via matching network in order to convert the gas into plasma. I have a few questions:

1. If I choose 13.56 MHZ RF generator what power do I need?
2. What should be the RF generator voltage (should I say rms voltage or P to P voltage)?

Steve

John Futter
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Re: A few RF generator questions?

Post by John Futter » Sun Nov 01, 2009 6:00 pm

Rf gen power depends on what sort of plasma density you want.

I would think that 50 -500 watts of generator power would be sufficient. So a power controllable 1kW unit would be ideal

You mention a coil inside so this is an inductive coupled plasma with plasma proportional to the circulating current in the coil

steve_rb
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Re: A few RF generator questions?

Post by steve_rb » Mon Nov 02, 2009 4:33 am

I want to creat maximum plasma density. Let say I change the pressure from 50mTorr down to 1 mTorr. I want to be able to reach plasma density as high as possible in this range.

I am not good in calculations but if someone could present some calculations indicating what maximum plasma density is achivable between 1-50 mTorr and calculate required Rf Generator powere that would be great.

Steve

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Re: A few RF generator questions?

Post by John Futter » Mon Nov 02, 2009 6:33 am

Steve
The lower the pressure the lower the RF needed as there are fewer atoms to ionise.
also hydrogen has a relatively low work function as compared to argon.
are you planning of using magnetic confinement as well as this has a huge influence.

Are you planning on extracting the the ionised atoms? - this again will play in the equation.

Sounds like you need to do some experimentation unless you can define all variables.

Our accelerator @ work uses H2, He, D2 in its ion source and although the acceleration tube runs @ 2 by ten to the minus six millibar the ion source runs @ several magnitudes higher.
This runs with around 100w of RF day in day out and dead steady in ion current.

I would think by the very small cylinders in the terminal that the gas of the day is being ionised to a very high percentage (neutrals are of no use to us). I.e. a couple of litres of gas @ STP last up to six months of continuous running.

steve_rb
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Re: A few RF generator questions?

Post by steve_rb » Mon Nov 02, 2009 12:08 pm

John Futter wrote:
> Steve
> The lower the pressure the lower the RF needed as there are fewer atoms to ionise.

Problem is the impedance will change and injecting RF power into an empty space needs higher voltage. This is what I can't figure out and don't know how to do calculations. That is why I asked what maxmimum output voltage my Rf generator must generate. watt=voltageXcurrent. When we talk about 1KW for example this could be 1KvX1mA. This means matching capacitors should stand for 1KV and my 1kw RF generator should be able to generate at least 1kv at 1 mA. For generating plasma in 1mTorr (more empty space) I need highest RF geberator output voltage (or may be it is more correct to say matching network poutput voltage) and lower output current. What this voltage should be for 1mTorr and how to calculate?

I need to ionise as more as possible ions at 1 mTorr. For this I need maximum current. What this maximum current should be and how to calculate?

Now that maximu current and voltage is specified I can find maximum power by multiplying voltage by current.

> also hydrogen has a relatively low work function as compared to argon.
I now ionization potential max at 1 Torr for H2 is 15.6 volts and for He is 24.5 volts but don't know about Argon. Also don't know these values at 1 mTorr.

> are you planning of using magnetic confinement as well as this has a huge influence.

Yes I have seen this has huge effect ans I am planning to use it.

> Are you planning on extracting the the ionised atoms? - this again will play in the equation.

Yes I am planing to extract ionised atoms from single and multiple apertures.

>
> Sounds like you need to do some experimentation unless you can define all variables.

yes as you said a lot of experimental works need to be done but I am interested in procedures for determining variables too.

>
> Our accelerator @ work uses H2, He, D2 in its ion source and although the acceleration tube runs @ 2 by ten to the minus six millibar the ion source runs @ several magnitudes higher.

Yes Ion sources for accelerators usually are at lot higher pressure in order for creating higher milliAmps (stronger beams).

There shouldn't be any differences between H2 and D2 from the ion source point of view am I right?

> This runs with around 100w of RF day in day out and dead steady in ion current.

Is you ion source inductively coupled plasma ion source or different type?
>
> I would think by the very small cylinders in the terminal that the gas of the day is being ionised to a very high percentage (neutrals are of no use to us). I.e. a couple of litres of gas @ STP last up to six months of continuous running.

Do you know what percentage of gas is ionised in your ion source?

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Chris Bradley
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Re: A few RF generator questions?

Post by Chris Bradley » Mon Nov 02, 2009 1:10 pm

Are you planning on making a generator or buying one? I'm not entirely convinced your local laws would permit you to make one unless you've got appropriate radio licencing (which would mean you probably wouldn't need to ask this question), and if you buy one then it will/should have suitable impedance matching circuitry that this is of no concern to you.

For the volumes you are likely thinking of, this would be similar to the type of plasma cleaners you might use in labs and such kit would probably be suitable and 2nd-hand-available. These seem to work well, independent of the volume of the cleaning chamber and the volume of crockery and cleaning solvents that you're blasting off in there.

John F's word is the one you should regard as the authority in this regard, though I wouldn't go so far as to imagine that there is actually much ionisation % at any given moment, I would rather think it is quite low (start by looking up Saha's ionisation equation, if you want to figure this out more). Unless you have some sort of magnetic confinement (like anode-source ion generators) to avoid the plasma coming into contact with a solid and forming a sheath in which ion energies are transferred out of the plasma volume, then it will be a 'cold' plasma. A 1-100mTorr inductive RF plasma will have an ion count of around 10^17 to 10^18/m^3.

However, it is an irrelevance for ion generation because only the ions will get accelerated out of it and ever-more ions will be produced essentially instantly to feed the extraction process.

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Chris Bradley
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Re: A few RF generator questions?

Post by Chris Bradley » Mon Nov 02, 2009 1:31 pm

Thought I'd attach some results to represent *typically* what might be seen, as measured from argon in a large industrial vessel.
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Doug Coulter
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Re: A few RF generator questions?

Post by Doug Coulter » Mon Nov 02, 2009 2:21 pm

Well, here's one data point. My very successful ion source uses 2.45 ghz (oven magnetron) with about 50w CW DC input to the magnetron. I use a 980 or so gauss field tuned for ECR at 2.45 ghz, and with no adjustments whatever runs from 1e-5 mbar up to atmospheric. I added DC "pushme-pullyou" electrodes to move the ions out of the discharge tubing into the main tank, and this is used to control how many ions are injected. Of course, when there is less pressure, there are less ions created whcih can be partially compensated via the DC extraction potentials (note I say partially here). By running the maggie at a small fraction of it's nominal power, most considerations of standing wave ratios (reflected power) and impedance matching are rendered moot, and like 13.56 mhz you are allowed to "accidentally" radiate a bit of power in this band, though my source barely tickles a microwave oven leakage detector (shows less than my brand new 1.2kw oven does).

To run 13.56 mhz, the magnetic field for ECR scales as frequency, so you need a smaller field by quite a lot (2450/13.56 * 980 gauss, roughly) -- so low that stray fields from other sources may be a problem getting electron resonance -- even the RF will disrupt such a small field for better or worse.

And this is fairly important -- we found we got a couple extra orders of magnitude better performance with the right field, in almost any parameter you'd want to name -- proportion of atomic vs molecular ions, number of ions per amount of gas, power needed to start the process, lowest pressure usable.

I would have to believe that most amateur (ham radio) linear amplifiers with the old tune and load caps in the 100w+ range would couple fine into your version, using the more or less normal tuning procedures. You may have to tweak the tuning/loading capacitors some, or could probably get into a good range via changing the number of turns and size of your ionizing coil, the equations for which are in every radio engineer's handbook I've ever seen, and other places besides.

Sounds like you were planning to put the coil right into the gas. Theres a reason no one does this and you'll find it out quick. No cooling -- it will melt. Out in the air outside a (preferably quartz) discharge tube this is solved -- convection cooling is your friend. Further, when exposed to hot ions, it will sputter badly, and deposit metal from the coil winding on everything around, shorting out any insulators that may be near. You'll be lucky to get an hour run out of it in that mode before something needs replacing or cleaning up.

Here's a not very good picture of our simple source in operation. It's inside the screen we use to protect ourselves from the HV supply, as we use it to inject the ions right by the fusor grid so it shares the same flange with that feedthrough. The 1" copper pipe pointing towards the camera is the microwave cavity, and the rectangular stuff wrapped around the quartz tube is the ECR magnet and yoke. We let the gas in via a capillary tube at one end of the quartz discharge tube for better performance at lower pressures, and put some charge on that end to repel the ions into the tank. There is another electrode at the in(tank) side of the tube charged negatively to pull the ions into the tank area. Both are powered by a single CCFL inverter and very little current is drawn by them. The negatively charged tank electrode has to be made of a slow sputtering metal or it makes a mess on the quartz and elsewhere fairly quickly -- don't use copper. We use either magnesium or aluminum, metals that don't sputter well with light ions, for that part.

(for those few who don't already know, sputtering is the ejection of atoms or molecules from a target via the impact of hot ions and has nothing to do with evaporation or melting points -- se the Kurt Lesker catalog for details)

Our source is roughly a copy of the one from Rev Sci Ins March 1965 with some improvements we added. I'd post the paper, but the copyright cops might get onto us for doing that -- it's a pay-for thing.
We added the ECR magnetic field to their idea, and the push-pull DC electrodes to control the ion output, among other things.
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Why guess when you can know? Measure!

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