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Conundrum

Wed Apr 11 2012, 07:34AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

Hi..
Have just been examining some interesting work on N2 lasers.

It seems that Kapton PCBs might have worked, but the thickness is absolutely critical as is the dielectric integrity.
This could explain why they were never able to duplicate that working PCB laser.

It occurs to me that one way to make it work would be to take a sheet of standard single sided PCB stock, then polish it so that it is perfectly flat with no burrs or dings and further clean with:- acetone, IPA etc and then handle by the edges ONLY!!!
Then obtain some barium titanate powder, and further grind down in mortar & pestle to ensure that the smallest particle size is less than 10um.
This is vital as the next stage is to mix the BaTiO3 with sodium silicate aka water glass.
Use a mechanical mixing method such as a magnetic stirrer; this can be improvised from a PCB fan.

As this is an experiment, several small PCBs should be made first before coating the large board due to the cost of board stock.
Once done there is no way to unmuck it so its a one shot deal.

The next stage is to coat your PCB evenly.
Build a "pot" to contain the silicate/titanate mix, and then carefully pour into the pot leaving a thickness of approximately 0.1mm of liquid.
At this stage it should be obvious if you haven't cleaned the board enough as the liquid will ball up.

Put the completed assembly on a hot plate (I use one for keeping plates hot in kitchens!) and carefully dry out.
To ensure this is done correctly I usually paint a dot of silver on the unused section and then meter to ensure that it is fully insulating.

Now comes the really nasty part.
You need to coat the whole upper surface apart from your discharge channel interconnects with a conductor.
To do this, you will need silver paint or a similar conductor.
Silver works the best but is very expensive and can sometimes eat through the dielectric.
See note earlier about silver dot.

Dry out again, but do so carefully so as not to fracture the dielectric.
It will resemble a grey powder like the coating on small piezo speakers, in fact I believe that they use something very
much like this but using a ceramic.

Once done, do a simple meter test. Beware as they can build up a LOT of voltage due to differential heating and cooling.

If all seems fine, install your resistors, electrodes and spark gap then test with white paper or Highlighter pen.
If the discharge appears at one end then adjust gap until the discharge is approximately the same along the tube.
If it still won't lase, adjust the spark gap until the gap is correct according to

You may have to experiment as the thickness of said dielectric is ridiculously critical.
I have heard that it can work with many dielectrics but the capacitance is only part of the issue.
It has to be correct capacitance for the area of the electrode, and discharge within a nanosecond or so.
So if the dielectric is too thick it will discharge too slowly and nothing will happen.
If too thin it will likely work well but rapidly degrade and fail due to punch through.

Post your results here!!
-A

Mattski

Wed Apr 11 2012, 03:46PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

What is the purpose of the barium titanate and sodium silicate mixture? Are you combining two dry powders, making a slurry with water, melting them, or combining them with kind of binder when you combine them?

Wouldn't it just be easier to use a sheet of copper clad polyimide and etch/cut out the channel?

Pinky's Brain

Wed Apr 11 2012, 06:00PM

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

PCB is used because of convenience more than anything else AFAICS.

If you want to use something else why DIY a ceramic capacitor? Energy density for ceramic capacitors is in the same order of magnitude as with BOPET/BOPP ... replacing the PCB with those seems a hell of a lot easier to me.

Conundrum

Thu Apr 12 2012, 02:53AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

Mattski wrote ...

What is the purpose of the barium titanate and sodium silicate mixture? Are you combining two dry powders, making a slurry with water, melting them, or combining them with kind of binder when you combine them?

Bariam titanate increases the capacitance per unit area, sodium silicate is both dielectric and insulator.
No, I plan to use dry titanate and liquid silicate, then mix afterwards.
Might need to functionalise the surface to make them mix though..

Its the thickness which is the problem.

If you had access to a PCB house, and impedance testing of every board then it would work, otherwise it wouldn't.

Also, polystyrene capacitors might be viable but again you'd need to get the formula right and the breakdown strength etc.

Pinky's Brain

Thu Apr 12 2012, 04:00PM

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

What's the point in polystyrene? Mylar and BOPP are easy enough to get and for a tea laser you don't need an awful lot either, the mylar page separators some people used might be more expensive than a polystyrene plate ... but is that still relevant when you're saving 90 cents on a dollar? There is an order of magnitude difference in dielectric strength between mylar and polystyrene.

Forty

Thu Apr 12 2012, 08:01PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

Always wondered if this stuff would work nicely for a TEA laser. I have a few sheets of it. Suppose I could try it sometime.

two conductors with an insulator in between, an almost instant planar capacitor. I would probably submerge one edge at a time into a shallow solution to etch the edges cleanly and without risking damage to the rest of the conductor sheet. Then the only trouble would be etching the channel on the top face.

I've also wondered why simple ceramic hv capacitors aren't used instead of the plates setup. Is it a discharge time thing?

Conundrum

Sat Apr 14 2012, 06:44AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

Yes, it is.
the discharge is in the nanoseconds, that visible flash you see in the channel is the N2 afterglow.

That said, ceramic caps can be used to build the HV supply as this isn't so critical.

-A

plazmatron

Sun Apr 15 2012, 01:48PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

The capacitance of these types of lasers must be very low, in order to attain the fast discharge required.
I have heard of acetate sheets being used for this particular application successfully on the amateur front. Failure of the dielectric in this situation is normally caused by air gaps between the foil plates and the dielectric, producing Ozone and heat, and eventually punching through.

In the Laboratory, water has been used as a suitable dielectric in ultra-fast discharges like this.

It is possible to run an ordinary nitrogen laser, as described in the Scientific American article, with ceramic capacitors, resulting in much higher powers. They are much less likely to fail in service, and if you wish, you can run them at atmospheric pressure, simply by adding Helium to the chamber.

Conundrum

Sat Apr 21 2012, 08:45AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

Yeah, I read about the use of He as an electron path enhancer.

What I'd like to see someone try is a metallised helium "plasma balloon" filled with He and Ar or Ne to get those nice long discharges.
I think this could work, but would be heinously expensive and hard to drive unless they used out of phase piezo transformers driven from a base unit.

-A

Pinky's Brain

Sat Apr 21 2012, 02:15PM

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

plazmatron wrote ...
It is possible to run an ordinary nitrogen laser, as described in the Scientific American article, with ceramic capacitors, resulting in much higher powers. They are much less likely to fail in service, and if you wish, you can run them at atmospheric pressure, simply by adding Helium to the chamber.

Isn't the lower (partial) pressure of the N2 in fact a necessity to use slower capacitors?

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