Page 1 of 2 12 LastLast
Results 1 to 10 of 13

Thread: Big Discharge Capacitors

  1. #1
    Join Date
    Feb 2011
    Location
    New Hampshire
    Posts
    3,513

    Default Big Discharge Capacitors

    I have a couple of flash lamp pumped dye lasers. One home built (S. American), the other a harvested medical unit. I have been reading about the history of the development of these and they are pretty amazing. Very efficient and the potential power output is astronomical. The highest single pulse energy I can find anywhere in the literature is 400 J. This is not as daunting as it might seem; 16kJ into 4 big ablating wall, air based flash lamps, close coupled to a meter long dye cell will probably do this. I'm game to try to beat this record. I'm not even going to hope to source xenon flash lamps and in this high energy regime the advantages of xenon shrink. Not to mention, I may blow up a few tubes! The capacitors are another barrier. Large, low inductance capacitors would be easy if I owned LLNL or like Swissrocketman had an in with CERN, but alas...

    Any suggestions? There isn't much on eBay.

  2. #2
    Join Date
    Apr 2014
    Location
    Oakland, CA
    Posts
    78

    Default

    Look up pulse discharge capacitors, they appear on ebay from time to time and I've seen a few at surplus shops.

  3. #3
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
    Infinitus Excellentia Ion Laser Dominatus
    Join Date
    May 2007
    Location
    A lab with some dripping water on the floor.
    Posts
    9,349

    Default

    Ask me what I have in my basement some time. Then ask me to connect you to my medical friend who has/had the cap chargers and a few caps from Candela Dyes, Erbium, and Ruby systems. Last time I was at his place, I tore apart a BIG yellow dye system for him. That means he probably still has the optics, too. I also have the paper on the "additives" some place.

    This big stuff needs to go. I feel the need to stop caring for a acre of land in the middle of nowhere, and so some of this stuff has to go.

    Argon is cheap, and adding it to the air flash might just get you some more radiance. Noble gasses have a low "Specific Heat Capacity" and this makes them easier to drive to incandescence. It will also lower your firing voltage, which may be a "Good Thing".


    Steve
    Last edited by mixedgas; 07-26-2014 at 19:42.
    Qui habet Christos, habet Vitam!
    I should have rented the space under my name for advertising.
    When I still could have...

  4. #4
    Join Date
    Feb 2011
    Location
    New Hampshire
    Posts
    3,513

    Default

    I did look up these capacitors and the selection is pretty sparse at this time. I figure if I can get at least one in the several thousand joule range and the 3-15kV range this would allow me to drive a singe lamp and test the layout. Like the medical unit the diffuse reflector around the lamps and the cell means that additional tubes can be added as they become available. 10usec is a good target for the pulse width and although simmer has no protective effects in the range above 20% of the explosion energy, it might be a clever way to switch the lamp. Thyratrons and even spark gap switches for these currents will be expensive. Alternatively the evacuation of the lamp until breakdown is kind of elegant. It is low cost and tailors the filling pressure to the hold off voltage (by definition) and so maximizes the gas density for a given energy. I've done this before and sub-usec is no problem and largely a function of the capacitor and lamp-lead inductance. I have argon and this will be easy to try.

    Steve,
    Put me in touch with your medical friend. These medical lasers are really pretty well engineered. My Cynosure is nearly 2% efficient and this is the state of the art for efficiency, so I plan to model this beast on this laser. I may even use some of the circulation pumps etc. so your friend may be just the place for some of the sub-systems. OK, I'll ask, "what's in your basement" (he cringes, fearing the answer, but unable to look away). Anyway, I know what I plan to use for the dye mix; around 10^-5 molar rhodamine 590 in 50:50 water:methanol. Start low and titrate in concentrate until the pulse energy fails to rise further then drip in COT until it fails to rise any more. There may be some exotic additives like ammonyx etc that will help, so if you find that list that would be nice. I plan to cool the lamps with a wavelength converting dye mix as opposed to straight water and this appears to have a number of benefits including a marked improvement in efficiency.

  5. #5
    Join Date
    May 2007
    Location
    1 hr from everything in SoCal
    Posts
    2,550

    Default

    I'm a little late to the party but I know Maxwell makes some great HV caps with extremely low ESL. You can try ebay for those. They are heavily used in the tesla coil community due to the low ESL.

    Some that I found:
    http://www.ebay.com/itm/High-Voltage...item3f3316f3db

    and

    http://www.ebay.com/itm/MAXWELL-HIGH...item3cd9db0a52

    and

    http://www.ebay.com/itm/MAXWELL-CAPA...item2592808cc1


    Not sure if that is what you're looking for....

    EDIT: After re-reading, I am not sure these caps are even going to come close to the bang energy you need. Oh well.....
    Last edited by absolom7691; 07-27-2014 at 03:12.
    Those who fail to grasp art are the ones who criticize it.

  6. #6
    Join Date
    Feb 2011
    Location
    New Hampshire
    Posts
    3,513

    Default

    Ya, that's what I mean. It's a little hit or miss. The closest I've found so far is:
    http://www.ebay.com/itm/Maxwell-Ener...item4d2121a731

    The price is pretty good, but the inductance is a little high and the voltage is a little low. Another advantage here is quantity. It would be nice to build the laser and add in caps and tubes later for more power if the target seams achievable.

    It's an intriguing area of laser engineering. The efficiency, not just the power output, rises with PEAK power. The early papers/development of these lasers obsessed with short pulse widths and clever coaxial capacitors and Marx bank circuits were used to keep the pulse widths in the sub usec range. However, as the lasers increased in energies beyond the multiple joule range then multi-msec pulses were being generated with similar efficiency. Triplet absorption has been the nemesis of these lasers and the justification for super-short pulse widths. However, with the addition of triplet quenchers like COT and I believe some other competing processes (maybe photo bleaching of the triplet state?), it is looking like the two overwhelming drivers of efficiency are dye concentration and peak power.

    The maximum dye concentration is dictated by the penetration depth of the pump light through the diameter of the dye cell. The minimum practical diameter of the dye cell (tube) is dictated somewhat by the flow requirements through this tube which needs to be pretty long, like a meter for these very high energy ranges, but more so by the need to present enough surface area so that the non imaging/diffuse pump light will be absorbed here rather than in the multiple reflections from the white walls of the containment vessel. Diffuse reflection is necessary because this allows flash lamps to have diameters substantially larger than the dye tube and this in turn permits higher peak powers.

    Peak power is ultimately limited by the explosion of the flash tube. The larger the diameter and length the better and within a given diameter an increased wall thickness is nearly balanced against the reduced bore, so not much help there. Shortening the pulse width by a factor of 4 decreases the explosion energy by a factor of two and this is why it helps to shorten the pulse length, but of course the increased efficiency is working against the decrease in the absolute pumping energy. This an interesting little article.
    http://link.springer.com/article/10....-0282-4#page-1

  7. #7
    Join Date
    Feb 2011
    Location
    New Hampshire
    Posts
    3,513

    Default

    OK, I think I'm on to something here:
    http://www.surplussales.com/capacito...llCap32634.pdf

    This single capacitor might drive the whole laser and allowing for the leads and lamps, I suspect 25usec would be possible. The outside the envelope question is, what really is the explosion energy of a flash tube? The only paper/ graphs I can find:
    http://laser-caltech.web.cern.ch/las...amp%20Eg&G.pdf

    Page 20 describes this and I am guessing that if the envelope of a 1.3cm ID tube is increased to 3mm thickness and the tube is surrounded by a water jacket and an additional containment tube then I might get an explosion energy somewhere in excess of 20kJ, for a single tube! 4 tubes would allow a life somewhere around 50,000 shots. This is really getting me interested.
    Now, I may have missed something and more powerful dye lasers may have been built and certainly rep rates have allowed average powers to exceed 1kW, but if 400J is the bar, then a world record might be within reach. Even if I fail to raise this bar, what an adventure! Oh, and for what it's worth orange certainly is an under-represented color in many projectors.

  8. #8
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
    Infinitus Excellentia Ion Laser Dominatus
    Join Date
    May 2007
    Location
    A lab with some dripping water on the floor.
    Posts
    9,349

    Default

    I'm sorry, my lawyer "burned" my day, the opposition finally agreed to admit fault and settle, suddenly.


    I have Edgerton's book. The formulas are in it.

    Harold Edgerton: "Electronic Flash, Strobe" 1970, McGraw-Hill, 1970

    Also look for papers by J. Goncz on Xenon, its not hard to stretch his work to other gasses.

    F. P. Schäfer (Ed.), Dye Lasers 2nd Edition (Springer-Verlag, Berlin, 1990).
    F. J. Duarte and L. W. Hillman (Eds.), Dye Laser Principles (Academic, New York, 1990).


    I need another day to work on scanner amps, then I can do a flashlamp/dye data dump.
    I have a ton of PDfs on the PDF drive.

    The Robber Barrons of NE are Expensive, there are other sources. One prefers caps that can "self heal" at reduced capacity if possible.

    I PMed you with the new cell #....

    The Whomping-Willows of dye lasers use coaxial lamps. The lamp is built around the dye chamber.

    Steve
    Last edited by mixedgas; 07-28-2014 at 18:51.
    Qui habet Christos, habet Vitam!
    I should have rented the space under my name for advertising.
    When I still could have...

  9. #9
    Join Date
    Feb 2011
    Location
    New Hampshire
    Posts
    3,513

    Default

    The Robber Barrons of Nebraska are Expensive, there are other sources. One prefers caps that can "self heal" at reduced capacity if possible.
    I'm all ears.

    The Whomping-Willows of dye lasers use coaxial lamps. The lamp is built around the dye chamber.
    That's what I'm seeing, along with 6,8 and even 12 linear lamps. My memory is that these coaxial lamps were a little problematic with achieving an even, circumferential discharge and as a consequence...boom. But if this is not really too significant then seeing as I'm building it I can build it the way I want. The coupling has to be better and it allows me to construct a single lamp vs 4 lamps. One challenge I see is the electrodes. I had hoped to fabricate a tungsten rod inserted into a copper mount /gas port for each end. A 1-2 inch diameter tungsten doughnut? Gasp. Maybe small tungsten pins protruding into the annular gap from a copper doughnut? Also, it is interesting that the flash lamp shock travels at up to 1km/sec or 1mm/usec and so over a few tens of usec, the shock will certainly transverse the dye cell. This can lead to early termination. So many things to think about. The best route is probably to build on the success of those that went before. Those papers will be worth their weight in... rhodamine.

    Thanks

  10. #10
    Join Date
    Dec 2007
    Location
    Nottingham, UK
    Posts
    2,834

    Default

    Eric,
    While this doesn't help you at all...

    I was here again last Thursday:

    ~3MJ of capacitorage.


    - There is no such word as "can't" -
    - 60% of the time it works every time -

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •