Series Flashlamp Question

[fluoronium]

Hi everybody! Long time lurker here, just rarely post. About a year ago I got bitten by the flashlamp pumped SS laser bug, and in that time have gotten a few small YAG lasers, as well as various capacitors and lamps to experiment on. I learned a bunch from tearing the lasers down and playing with the L and C values of the flash circuit, as well as aligning the cavity mirrors. Now I'm working on a fairly large ruby laser, and have slowly accumulated parts over the past year in preparation for the project. My ruby is pretty big, 1/2" diameter by 6" long, and from pictures I've compared it to the doping is probably around 0.05% Cr3+. Obviously it will need a lot of light to pump, so I bought three big flashlamps that I'll be using to pump the rod, and have constructed a gold plated pumping cavity to house the lamps and rod.

So my question is related to the electrical properties of these lamps when wired in series. I've been using Koechner's "Solid State Laser Engineering" book as a reference for the lamp calculations, in which the lamps Ko value (dynamic impedance) is a necessary parameter needed to design a circuit to fire them. From my understanding this parameter describes how the impedance of the lamp drops in time due to the increasing ion density on the lamp when being fired, and is apparently a function of the gas fill pressure, length, and bore size. For Xe lamps of fill pressure 450torr, the given expression is simple, Ko=1.27(l/d), where l is lamp length and d is the diameter. It seems that for series lamps, this impedance should be additive, considering that adding the impedance of the three lamps in series is the same as the dynamic impedance of a lamp that is simply three times longer than each single lamp. Even so, I am not confident in this, because I am not familiar with where this equation came from, and do not know the assumptions used to derive it. Could any of the pros here provide some input whether I am overthinking things or about to pop my lamps?? I appreciate any help!!

[mixedgas]

Having worked on 10,20 and 30 Hz systems with series lamps, as well as a 52 Joule system with 3 pairs of series lamps, all you need to do is add the cathode fall voltage as a loss in series with the lamps, and design it electrically as two identical lamps in series. Its additive as you suspect. So you loose 20 to 90 volts of effective charge (lamp type dependent) from the cap and some energy loss from anode heating.. Other then they need to be identical lamps, everything else works like one longer lamp. Make sure you use simmer current on the lamps, around 200-400 mA, this will make you much happier in terms of consistent firing with series lamps. Yes there were some tweeks on the factory lamp spreadsheet we used, but they are minor and I'm under NDA. Just calculate as one long lamp for now. You will be very happy with series lamps, as long as they have a simmer system.
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Find papers by Doug Gonz (cant remember the spelling) for a better lamp model, and there is a Perkin Elmer flashlamp spread sheet floating around on the web some place.
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I'll post a link to the design papers when I get into the lab computer tomorrow.
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Steve

[fluoronium]

Thank you so much for the reply! I wasn't going to use a simmer originally but it looks like I will have to put something together, and I can learn from it in the process. I'm super stoked to get this running, but it looks like there is still some work to be done! Once this thing is running I want to experiment with Q-switching so I can do some air breakdown.

[mixedgas]

See:
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J. H. Goncz, “Resistivity of Xenon Plasma,” J. Appl. Phys. 36, 742–743 (1965).
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Simmer-enhanced flashlamp-pumped dye laser
T. K. Yee, B. Fan, and T. K. Gustafson
15 April 1979 / Vol. 18, No. 8 / APPLIED OPTICS
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Simmer hardware is as simple as it gets in the above publication, which also describes "prepulse simmer" Pre-pulse is either a small cap discharged thru the lamp prior to the main bank, or can be done by ramping the simmer up to say 2 Amps for a few milliseconds before firing. This reduces the risetime of the lamp and you get a sharper leading edge on the lamp current, which boosts laser pulse risetime. If you want to snap air, you need a fast, q-SWITCHED pulse in most cases.
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Steve

[planters]

Another thing to add when setting lamps to operate in series is a high ohm (5Mohm is a good place to start) resistor in parallel with the lamps. This needs to be a HV resistor as well. If you are going to use simmer then this is not necessary as the tubes are already conducting. But, if you over voltage them and use external, or series triggering then the resistors insure that each tube sees the same potential prior to breakdown. This prevents one of your tubes from prematurely discharging and carrying the other(s) along with it, if the floating electrodes do not remain at the expected intermediate voltage.

[fluoronium]

See:
!
!
!
J. H. Goncz, “Resistivity of Xenon Plasma,” J. Appl. Phys. 36, 742–743 (1965).
!
!
!

Simmer-enhanced flashlamp-pumped dye laser
T. K. Yee, B. Fan, and T. K. Gustafson
15 April 1979 / Vol. 18, No. 8 / APPLIED OPTICS
!
!
Simmer hardware is as simple as it gets in the above publication, which also describes "prepulse simmer" Pre-pulse is either a small cap discharged thru the lamp prior to the main bank, or can be done by ramping the simmer up to say 2 Amps for a few milliseconds before firing. This reduces the risetime of the lamp and you get a sharper leading edge on the lamp current, which boosts laser pulse risetime. If you want to snap air, you need a fast, q-SWITCHED pulse in most cases.
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Steve

Thanks for the references! Can't find a source that isn't behind a paywall right now but I start grad school in a few days and will have access to papers there.

Another thing to add when setting lamps to operate in series is a high ohm (5Mohm is a good place to start) resistor in parallel with the lamps. This needs to be a HV resistor as well. If you are going to use simmer then this is not necessary as the tubes are already conducting. But, if you over voltage them and use external, or series triggering then the resistors insure that each tube sees the same potential prior to breakdown. This prevents one of your tubes from prematurely discharging and carrying the other(s) along with it, if the floating electrodes do not remain at the expected intermediate voltage.

I assuming you mean a resistor in parallel with each lamp, correct? That way it's just a voltage divider, similar to balancing series capacitors. I'll put some HV resistors on my shopping list then!


Thanks again for the help you guys!! This is the kinda stuff that I couldn't find on Google or in my SS laser book. I'm moving tomorrow but once I get settled in to my new place I'm gonna go marathon on this project. I just realized that my gold plated pump chamber may not be great for pumping ruby as it probably absorbs a significant amount of blue/green light, so I'm gonna have to whip something else up. Might just nickel plate a metal tube and use that as my reflector. It would also be cool to play with some dyes in the cooling liquid to try to convert some of the otherwise useless longwave UV light from the lamps into blue/green light matching ruby's absorption.

[mixedgas]

When you get to grad school, "Review of Scientific Instruments" is a must read for laser interested folks.. I'm not responsible if you flunk out by getting addicted to binge reading RSI in your first week. I've spent whole Saturdays and Late Nights in RSI Binge mode. You have been warned. Next thing to do is get the departmental masochist/machinist to teach you slowly how to use a Manual Milling Machine over a period of a few years.
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Steve

[fluoronium]

I will put that on my reading list, thanks!! My PhD will be in optics so I am committing to the sickness for the long haul. Just googled RSI and found this as the description...

RSI covers everything from free-electron lasers, plasmas and ion sources to steady-state and time resolved optical studies, atomic force microscopy, and new methods for measuring nuclear relaxation times

Yup...definitely gonna end up on a binge there.

I'm gonna get the metalworking certificate that my school offers so I can have access to their metal shop...it has really sucked making all of my metal parts with a hacksaw and hammer, only to spend hours on producing a crappy product. If anything will get me flunked out it will be that damn metal shop.

[planters]

I assuming you mean a resistor in parallel with each lamp, correct? That way it's just a voltage divider, similar to balancing series capacitors. I'll put some HV resistors on my shopping list then!

That's right. Just make sure that their watt rating is sufficiency that they will not over watt with the full charging voltage of the caps across them.

[mixedgas]

RSI covers stuff all the way back to the early 1940s. From simple to complex, if its instrumentation, or even remotely related, it's in there somewhere.


Steve