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Thread: Pulsed Argon?

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    Default Pulsed Argon?

    I have been experimenting with some flashlamp designs. I learned that one of the best ways to sputter electrode material (even tungsten) is to discharge at well over the breakdown voltage and if discharging AT the breakdown voltage there is little sputter even in gas that is often used to enhance sputtering (argon). At these pressures of several torr, when the lamps are driven hard they move into the wall ablation regime and interesting things happen. But, the point that occurred to me is that as the energy increases, prior to the generation of Si lines and the generalized growth of broad spectrum deep blue and UV thermal emission, there are lines at 514nm and 488nm among numerous others. The lamps, even at these more moderate energies are blindingly bright. Could a pair of cavity optics placed on the ends of such a flashlamp, using hollow electrodes and brewster or AR windows, produce a pulsed argon ion laser output?

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    What is the fill pressure? There is a 10-100 micron pulsed argon, and a quasi cw argon at 200-300 millitorr. Argon is not known to like high pressures for lasing.

    Most noble gas pulsed ions have a similar regime unless there is helium around. He-KR will lase at higher pressures at 476 only.


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    The lines are visible from 3 torr down to about 0.5 torr. Voltages above this range deliver enough energy that the thermal emissions begin to swamp the lines and below this, trigger so easily that a continuous discharge occurs. The pressure also peaks during the pulse and this is through restricted diameter attachment lines. I'm not sure what the pressure is during the actual pulse. The current during the pulse is on the order of 1,000-2,000A/cm.

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    Is the plasma smooth or wall guided? Tubing bore size? Plasma Length? Any other gasses available? Your plasma density is "close enough to try" If I were going to try this I'd just install AR coated windows instead of Brewsters. I'd use two HRs in a confocal cavity to ensure maximal chance of lasing. I'd raid Steve Roberts' basement for some optics, God knows he has enough sets...

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    Those are good questions. Typically these ablation-wall lamps fill the bore (in my case 10mm) and as the current increases upwards to beyond 3-4000A the inner surface of the wall vaporizes and there is an annular region that radiates as a grey body mixing thermal and now Si lines as well. One estimate is about 10^17 atoms. Through my welding glasses the plasma is very smooth (pretty actually). I have been working with 6,12 and 24cm electrode spacings. I have helium and nitrogen, but no precise means of measuring them unless I did a premix in a spare tank. The pulse duration can be as little as 12usec or as long as 100usec depending on arc length and capacitance.

    As an interesting aside, you know how everyone makes a big deal about short feed lines and even flat wide copper strips (try insulating those!) to minimize inductance and pulse duration? Forget about it. The lamps, even 6cm long 10mm ID have so much impedance that an extra meter of lead is barely detectable in its effect on pulse length. Same thing with the capacitors. As long as it is a true pulse capacitor with inductance in the nH regime it matters little whether it is 40nH or 200nH, the lamp dominates. It's all about capacitance and lamp dimensions.

    Plano-Plano AR windows would be easier and with such a stout plasma channel, alignment in the confocal setting would hopefully be less challenging than with a typical argon.

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    I can come up with optics for 24 cm.
    I'm curious, as this is a regime that no one has played with professionally in a long time.
    Lets try. I can supply both argon and WL optics. I bought about 75 WL optics at SELEM, so not great loss if I send you a few pairs.

    Do you have hollow or sidearm electrodes to allow lasing?

    Avoid nitrogen.

    If it works, lases, don't tell anyone the details. My new boss is open to letting me publish in actual journals for once. I know that seems selfish, but just once I'd like my name on a paper.

    I just read a paper in REV-SCI-INST based on something I posted on PL about 5 years ago, I'm both pleased and not so pleased. It would have been nice if the Polish authors had put "S. Roberts, Private Communication" in the citation. :-( It had to happen sooner or later.

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    OK,
    Firstly, if a paper is generated I do not want you to refer to a private communication with planters. Thy might consider you some kind of a nut.

    The electrodes will pose a bit of a challenge. Right now, I am using 6mm diameter cerium doped tungsten rods mounted in a ceramic coated end cap. The ceramic material protects the stainless electrode support from discharge induced sputtering on the cathode end. I would go with a hollow stainless tube separating two quartz tubes and live with the sputtering for a first iteration. One quartz tube contains the discharge while its shorter partner would separate the electrode from the window. I have avoided side electrodes or arms because these tubes are being operated at extreme loading and the plan is to pack them into a dye chamber, so they have to be compact and linear.

    I am running pure argon at this point to avoid oxidation. In the ablation regime the gas becomes much less important, it just heats the wall. No one seems to be using helium. I don't know why. But, I haven't tried.

    Do you mean 24cm mirror to mirror?

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    We also have a friend in Phoenix with a quartz lathe for stepping the bore size if needed, ie large tube for electrodes necked down to the 10 mm bore. He also stocks quartz to pyrex and quartz to metal seals.

    I also have a large pile of glass to metal seals.

    Steve
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    Last edited by mixedgas; 04-29-2015 at 15:34.
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    The intriguing thing about this approach is the potential robustness of the discharge. By pulsing, current density way beyond what a CW plasma tube could survive in a large diameter tube might make high gain and many lines available. Alignment might be easier as well. Or it might not work at all. I'm not sure of the mechanism that favors specific transitions in argon, but it might be worth a try.

    I think a 10mm ID stainless tube bored out on each end to 14mm (matching the OD of the quartz) would be pretty good and relatively simple. If that doesn't work then a more elegant and complicated approach probably wouldn't be worth the effort.

    To this point I have been getting away with a simple two stage vacuum pump (an excellent Welch beast). This will get me down below 100um and that is then saturated with pure argon so that it is nearly pure argon at 100 um, yet the system is operated at a couple of mm pressure. I have a diffusion pump, but for the original application which was characterizing ablative flash lamps, I didn't bother to set this thing up.

    Obviously, you know these ion lasers much better than me. What does contamination do? Are there buffer gases that help and what about O2 and N2?

    Time interval...


    Now, I think your excited. You're editing for private consumption. Sorry everyone, we'll let you know what happens.
    Last edited by planters; 04-29-2015 at 18:03.

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    Pure Molybdenum washers are made for high temperature, high vacuum, applications. At ~ 3.00$ each, there is a source of cheap electrodes that no one else has ever thought of. You can braze to them with Harris Safety-Silv or other Cu-AG brazes. It has a work function of 4.2, compared to pure tungsten's 4.5, so the emission from an arc hot spot is pretty good. It does ablate faster then tungsten, by a factor of 2.

    N2 = Bad for Argon, so is H2. H2 will flat out quench things.
    O2 lases bright yellow when pulsed.
    At the energies your using, Neon will go at very low pressures.

    Xenon is notorious for pulsed lasing at pressures 100u and below, used for dye pumps.


    See your PM

    Steve
    Last edited by mixedgas; 04-30-2015 at 06:53.
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