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Battery powered ion trap

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zhenre

Fri Apr 09 2010, 04:19AM

Registered Member #2792 Joined: Fri Apr 09 2010, 03:57AM
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Posts: 3

I am making a battery powered ion trap to use as a demo for science classes, outreach, etc.

I've already succeeded in making a prototype which sort of works but is a bit too finnicky.

Basically to make an ion trap all you need 4 rods where you put an AC high voltage between one pair and the other. Pretty much any voltage can work for the appropriate sized ions, but in this case we're trying to catch relatively large particles (flour, cornstarch, etc.). Empirically it seems that 3-5kV, at 50-100 Hz with a few mm separation between the rods makes for very stable traps. A laser pointer then allows you to see the ions.

In the first iteration we used a dual MOT (and a variac to vary the voltage) plugged into the wall to generate the signal. This worked decently well but was quite dangerous (HV and high current) and not at all portable.

In the second iteration we made a solid state HV supply powered by a 9V battery, which we used both to power an H-bridge and as the voltage used to drive the a CCFL transformer which produced around 1kV at 10kHz. We then used a few stages of CW multiplication to get up to around 3 kV DC. This part works great but we need ac voltage not dc.

We thought about how to turn the dc into ac but have not been able to think of a good way to do it solid state yet. What we've done which worked to some extent is just to take a CPU and make a brushed commutator using a some wires, copper tape, and a cpu fan. This amazingly works but is fairly power hungry and more important difficult to mount in a stable way. Essentially every time you use the trap you have to readjust the brushes and everything.

One possibility would be to try and use the same method but just make it more mechanically robust. However, it'd be very nice to have an all solid-state version (that is fairly cheap) of the driver circuit because then we could just have a bunch of boards made and let people stuff them.

We thought about using IGBT's but it seems they are only cheap below 1.2kV and I've only seen N-type (meaning you couldn't do full wave and get 2.4kV) . We also thought about having two power supplies and just turning them on and off but if we used resistors to drain the current then the load would be too high for sufficiently fast draining, and we couldn't figure out how to change the polarity of the power supply to actively drain the circuit with out some sort of switch (which brings us back to the needing HV transistors). It might be possible to modify our geometry such that 1.2kV works ok but we are pretty happy with the geometry if there is a way to get to higher voltage. These are just some of the things we've considered but would be happy to entertain any other ideas.

Any help would be appreciated.

Patrick

Fri Apr 09 2010, 05:48AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

is it AC Sine wave or AC square wave you need for this device?

And yes you will not find 1kv IGBTs that are usful or cheap...but i fail to see why you need seconday side switching? just use 400 or 600 volt switches like IGBTs or MOSFETS on the primary side.

can i see this on you tube?

ScotchTapeLord

Fri Apr 09 2010, 07:23AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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Posts: 635

If you need high voltage at mains line frequency, the solution is simple- a transformer.
It's just a matter of finding the appropriate one. From the description, it sounds like your output is effectively an open circuit. In this case, output current is not important, so Microwave Oven Transformers are impractical, as they are meant for large currents.
You have a variac, so you can get almost any voltage between 0 and your transformer's rated secondary output.

My suggestions would be to find the lowest voltage Neon Sign Transformer (it will be small-ish) that you can (I have a 9kv/20mA one, and it's not bulky) and use it with your variac. (NOTE: It cannot be a solid state device if you need low frequency! It needs to be "old style" - it would be a good idea to ask the forum to confirm the suitability of one before investing in it!)

Perhaps you can use a higher voltage with larger separation and do away with the variac? Otherwise, if I was correct in assuming that the output it an open circuit, then you would only need a very small variac, as compared to one used with a Microwave Oven Transformer.

Since you want such a low frequency, you can only make things so small. As frequency goes up, amounts of copper and iron go down, and it requires a more complex solid state solution. If you can use a higher frequency than 100 Hz, like in the 5-50kHz range, then you can make it a lot smaller (probably pocket-size!), but like I said, it'll be far from 50-100Hz.

Sulaiman

Fri Apr 09 2010, 05:22PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Since 5kV rms is 7 kV peak you will not find any solid stste switch/transistor that is reasonably priced, the obvious alternative is a line frequency transformer.
The 'problem' with transformers is core saturation which for a fixed frequency corresponds to volts-per-turn. So you will need many many turns and/or a very large core cross-sectional area, this applies to an inductor in a resonant circuit also.

The simplest/cheapest/safest solution that I can think of is an OBIT (oil burner ignition transformer) of the simple transformer (not 'electronic' type).
Almost impossible to kill someone with the output of an OBIT as they are limited to a few tens of mA (like a neon sign transformer only lower power / smaller)
e.g.

etc.

Bored Chemist

Sat Apr 10 2010, 01:42PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

A Neon sign transformer would do a nice job of getting to the right sort of voltages. I think the problem would be that you can't change the frequency of the mains.
However you could get the transformer from an inverter and run that from an audio amplifier driven by a signal generator. That would give you roughly mains voltage at a variable frequency to feed to the neon transformer. It would be messy and inefficient, but since you don't need any real power it might work.

radiotech

Sat Apr 10 2010, 07:46PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

For years we used ion traps on TV CRT's, just a magnet on the neck of the tube. And there were some that used electromagnets, instead of permanrnt magnets.

The magnets, could not influence the ions since they too heavy. The tubes just directed the beams of ions and electrons in a harmless direction, so the ions couldn't destroy the phosphor screen and the magnet bent only the electrons back to where they were needed.

How are you going to prove in your demo that what is beeng seen are ions?

(there also is way, using a common radio tube with a phosphor screen and an electromagnet to calculate the mass of the electron and could be done in a school physics lab by every student)

Patrick

Sun Apr 11 2010, 04:50PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

wait... when you said 50-100 Hz was needed, did you mean it had to be variable between that, or that 60 Hz would work?

zhenre

Tue Jun 29 2010, 10:07PM

Registered Member #2792 Joined: Fri Apr 09 2010, 03:57AM
Location:
Posts: 3

Thank you for your responses. I've also included some pictures where you can get a sense of scale of the device and even catch an ion or two if you look carefully.

Patrick: Ideal would be variable frequency but you can compensate for non-ideal frequency by changing the voltage...so fixed 60 Hz could be ok in principle but we'd like it to be battery powered. Sinewave is best, but square wave is fine for this scale of trap. I haven't posted to youtube or anything yet because I'd like to improve the design a bit first.

radiotech: Here the ions are actually flour or really any fine powder, and you see them by using a laser pointer and looking at the scattered light.

Bored Chemist: NST would probably work and are probably a better solution than the MOT (except for the fact that MOT are more readily available for salvage) but I think (please tell me if I'm wrong) that they require a fair bit of power. A small one that I looked at claimed .5A@120V ~ 60W which is fine from the mains or maybe even a car battery but I was really hoping to make it portable so I could pass it around a classroom or something like that.

Sulaiman: I hadn't known about the OBIT before. I looked it up and it seems like it might be in the same category as the NST only a little smaller perhaps...Do you have any idea what the power dissipation is like with no load?

The hope is to run the whole trap for lets say 30min off of one or two 9V batteries. Do you think that an NST or OBIT could do that? Otherwise can you think of a way we could either do the switching or make a low dissipation low frequency transformer. I don't care too much about weight but it would be nice if the size is small as the trap is only about 6x2x2 in^3 or so.

Pinky's Brain

Wed Jun 30 2010, 12:44AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

The NST is perfect apart from size.

The problem with simply putting a sine through a high winding ratio transformer is that no matter how small the capacitance of the trap you are going to be pushing a fair bit of current because of transfer losses.

A quick simulation with numbers from the web near that of a car ignition coil (10 mH:100H, 1 Ohm:1 kOhm, 0.75 coupling factor), 50 V 100 Hz AC input and a 25 nF ballast capacitor (with the load in parallel, ignored for the simulation) showed about 0.75 ampere AC current through the primary ... so around 25 Watt ... don't think the 9 volt battery will hack it.

PS. with a mismatched ballast the current shoots up fast.

IntraWinding

Wed Jun 30 2010, 01:14AM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

If the MOT works and size/weight isn't a problem, could you just box it up with suitable strings of resistors on the secondary outputs to limit the current to some safe value. You could add resistors to the primary too for extra safety.

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