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Registered Member #1667
Joined: Sat Aug 30 2008, 09:57PM
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Posts: 374
radhoo wrote ...
So you're saying I can simply use one of these: and forget about the vacuum pump at all?
Well, see for yourself:
The higher the pressure, the narrower the pumping window, we're talking about ~1ns for nitrogen at atmospheric pressure. There exists a relation for the lifetime of the excited level as a function of the operating pressure: A peaking capacitor is introduced to compensate for the "slow" marx bank. In my opinion, it is worth reducing the pressure to buy some time to pump the laser medium. Cut 6 doorknob capacitors in the design and you'll have the money to buy a single stage vacuum pump off ebay.
The cool thing with peaker caps is that you can put a coaxial cable in between the laser head (tube and peaker cap) and the pulse generator itself.
ps. just read the wiki article that now covers some quite interesting topics. I haven't thought of diffraction limits yet. I like the thought of having graded index refocusing lenses inside my laser made entirely out of plasma! (a parabolic radial fall-off of the index of refraction is ideal for lenses and behaves much like an aspheric lens.
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
hboy007 wrote ...
Well, see for yourself:
Very inspiring, the topic in this thread becomes more exciting with each day. I would also want to try this, given the fact that I also have some 30KV doorknob capacitors, and maybe also try to reduce the pressure by using a fridge compressor . I am tempted in trying to seal the lasing chamber, so it would not require the pump . Any comments on this?
It would be great if you could put more pics of your setup, including some close-ups. A simplified circuit diagram would also be appreciated.
Registered Member #1667
Joined: Sat Aug 30 2008, 09:57PM
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Posts: 374
I also planned to seal the laser but the first reservoir I built was kinda lame so I left it out.
A few words to the schematic drawing. One of the builders claims that the spark gap in parallel to one of the caps forms a resonant circuit and produces a negative half-wave pulse that adds to the voltage drop across the tube. This may be a contributing factor. A quick estimation (nano Farad caps, nano Henry parasitic inductance, T/2 = pi*sqrt(LC)) suggests that the negative pulse occurs on the nanosecond scale.
However, I think that for low pressure laser tubes, the breakdown voltage is reached before inversion can occur. A 30cm long tube can be ignited with around 10-15kV and when using the resonant circuit as a voltage source in series to the "main" capacitor C2, it should be C1 >= C2 (don't forget the energy dumped into the spark gap plasma). Still there arises the question what time scale we are talking about. A typical discharge may last 100-200ns with the laser terminating at 20ns after the pumping process begins. Assuming the negative half-wave contributes to the active pump phase, inversion would have to occur within 10ns after the SG fires. Even SGs themselves aren't that fast, they have a rise time of a few to a few tens of ns.
Effective usage of an inversion effect may rely on small capacitors and a high resonant frequency of the series peak voltage source. Assuming the laser operates at higher pressures, higher voltages are needed to ignite the tube and reach inversion. Then a parasitic resonant circuit may be a promoting factor for laser operation. As for my design (C2 > C1), I assume that the energy from C1 is burnt up in the SG flash to some extent, leaving C2 connected to the tube terminals with a low resistance plasma path. A fraction of the energy stored in C1 is stored in and radiated from the magnetic field around the conductors.
Operation modes could be investigated by changing the C2/C1 ratio and their values in respect to the operating pressure and the time window implied by the pressure.
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
Thank you for the well documented explanations, as well for the quality circuit diagram. Just a suggestion, you could edit the first thread to make it contain all the vital info: working principle, formulas, schematics, etc. Some readers appreciate to have everything concentrated in one post, instead of having the info scattered through all the thread.
Off-topic: how to do cut the plexiglass to get clear, clean edges? Is there any good transparent adhesive for plexi?
Can you provide more details on heating the epoxy as well, I see that you have a special technique for making the mixture better. Since epoxy is one of the key ingredients of this design, I believe it might prove useful.
Registered Member #1667
Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374
Looking back at the documentation I wrote for myself there is definitively a need to write it all up again. That would also require to work through some literature again and aggregate the references. Please be patient, I'm currently finishing a (commercial) software project that will help me finance my study. Apart from that, I'm working on a motorized tripod head (I'm a passionate photographer and love 360° panoramas). There is a lot more to do for my homepage, two other projects need documenting. When I pick this up, I shall take the time and rewrite some articles in English, too.
Getting plexiglass clear is a bit of a pain. It can be sanded in multiple steps and the finishing is done by flame polishing. This in turn leaves strong tensile forces in the surface layer of the plexiglass sheet that lead to cracking when gluing, fusing or cleaning the material with solvents.
The gold standard for plexiglass processing is diamond polishing. I have yet to find a company that accepts prototype and low volume jobs. The guys I found via youtube, e.g. were not cooperative.
As for the epoxy, I have both worked with "Uhu Endfest 300" which is a 2K glue and a resin flooring mixture. It may be quite instructive to get a pack of epoxy compounds for electronics potting. Compared to the glue, it is affordable to play around with. I've seen epoxy harden within a few minutes after a runaway chemical reaction. Larger volumes can reach temperatures in excess of 100°C.
The glue liquefies when being heated, then starts to cure. I'd have to try that again to tell you exactly what it looks like when it's done but at least you can stop heating when the curing becomes noticeable.
ps. there are fusing agents that glue plexiglass faces without leaving marks. You can also try Methacrylate glue that polymerizes.
Registered Member #2941
Joined: Fri Jun 25 2010, 08:08AM
Location:
Posts: 143
radhoo wrote ...
Cedric, I have a few questions regarding your test as well.
cedric wrote ...
I get it to laze, with the fridge pump !!!
Can you elaborate on this part, and maybe provide more pictures, videos, details? How good is a fridge compressor for this purpose? I have one at hand, and it would be my first option in trying this kind of laser. Also can poor vacuum be compensated by higher voltages?
cedric wrote ...
I am not there yet,the lazing is very hieratic ,the beam is not so bright and the beam profile is not very nice ,but all together ,now that I know how to do it ,I am planing to build one bigger (10 cm tube with internal diameter of 6 mm),same capacitor ,and steadier construction, of course I will need better vacuum . I also think this type of setup is perfect for pulse multigas laser (given enough power and a resonator it's possible to laze about anything
Can you try adding a little NaHCO3 inside the tube, the plasma should heat it up and release CO2. I am curious if it changes anything having CO2 inside (2NaHCO3 → Na2CO3 + H2O + CO2 → Na2O + 2CO2 + H2O ) Or adding a little mercury for a mercury vapor atmosphere inside the tube. Just a few ideas, to see if it would still laze.
sorry it took me a bit of time to answer, I don't have any picture or video of the working laser with the pump and since I disassemble it to make a new better one I can not take picture anymore. the fridge compressor is just a regular fridge compressor ,to my understanding it work in this case because the channel in narrow (2.5 mm) and short (7 cm),I think that if I had a better pump it would work better. I notice that while running it ,lasing would take place only if I archive a discharge through the tube and not just ionization,in tea laser ,it's not a requirement .as hboy007 mention it ,nitrogen is a really high gain medium ,a spark in between two mirror might be enough to get lazing ,the big difference in between atmospheric pressure and low pressure laser is about the rising time of the electrical pulse .with a conventional spark gap the rise time is about 10 ns,and the time for population inversion with low pressure nitrogen in about 20 to 30 ns ,witch mean that for low pressure laser ,much more of the electric pulse can be use to pump the medium. I join some picture of the pump and the new laser...
I am building a new prototype ,much better done ,using ceramic tube and a vacuum reservoir ,I am looking forward to test other gas ,like co2 ,argon ,krypton ,the difficulty is to find suitable optic ,I won't try mercury since the electrode are in aluminum and that would lead to there destruction.
Registered Member #1667
Joined: Sat Aug 30 2008, 09:57PM
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Posts: 374
I was wondering, too. I hope the aluminium tape / foil is just a means of protection during construction.
As for the window, I've tested and approved of BK7 microscope slides:
My power supply is a homebrew offline flyback converter base on a TV flyback transformer and a BU2508AF transistor with signal generator and driver circuitry.
Registered Member #2941
Joined: Fri Jun 25 2010, 08:08AM
Location:
Posts: 143
well of course the aluminum tape is not part of the optic it was there to protect the laser channel from dust, for the tube I use ceramic because I had one tube of the right length and because my previous glass tube broke du to arcing (at list this one is a good dielectric )it's a bargain, I agree with hboy007 that a narrow channel is better for lazing ,but if the vacuum is deep enough large bore might work just as well. also and it might sound trivial ,with my previous one I get lazing photon bouncing from the wall cavity giving this weird half circle near the dot, I south at list I won't have this problem, + the optic tuning should be more easy with a large tube. I also integrated a vacuum container and a vacuum valve in the aluminum block at the end ,so fare the vacuum since to hold ,but I will know only once I get a decent pump... the power supply is fly-back base ,with a pwm switching a laptop power supply ...
the last picture is a (very bad) x-ray of the valve, the vacuum tank is visible under it, close by a steel ball..
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longitudinal excitation nitrogen laser
