Need BRIGHT photoflash mixes

[styropyro]

Hi everybody! I don't post here often but I have been browsing this forum for a decade, and have learned a lot from the posts here. In fact, it was this hobby that mainly inspired me to get a degree in chemistry! (If this is the wrong place for this thread, mods feel free to move/delete)

I am currently on a quest to produce extremely bright flash powder in order to optically pump a synthetic ruby crystal, with the intention of producing short pulses of very intense laser light. To complicate things I am not igniting this powder with a fuse, but instead I am filling a long, thin tube with the powder, running a tiny wire down the length, and pulsing it with a small capacitor bank in order to ignite the powder quickly and uniformly. I need the majority of the powder to ignite in roughly 3 milliseconds or less (to beat out the fluorescence decay lifetime of the excited crystal) so that's why I am using a setup like this.

So my first question is more based on the mechanisms of the exploding flash... is the light from a typical Al/KCLO4 flash powder explosion mainly thermal (blackbody) emission?? If so, attempting to get the mixture to burn hotter will not help much as the emission peak will be shifted to UV, and the light I need for my ruby is blue and green (absorption peaks centered at 404nm and 550nm).

Second, what are some of the better formulas for blue and green colored flash powders? I tried some Ba(ClO3)2 based flash in a couple of my fireworks on the 4th, and found the results to be disappointing. The green color was pale and the powders were slow burning. That being said, in the cap bank pulse setup, I may be able to get a colored mix to go quickly and possibly alter their emission spectrum.

Finally, what have you guys found to be the brightest flash powders under "normal" conditions? I really like the Kirk-Otthmer mix 40/30/30 Al/KCLO4/Ba(NO3)2 as it burns incredibly bright, quite a bit brighter than standard flash IMO. Are there any other mixes that can burn even brighter (even exotic mixes)? I've noticed barium in a lot of the photoflash compositions, and wonder if the green light it produces is what makes it appear so intense...

Thanks in advance for any advice on the matter! I'd go outside and test about every flash mix that I could find, but I have only a two week window to attempt this experiment starting in early August. Attached below is the ruby crystal that I'll be pumping (scored for $40 on eBay )

[WSM]

I've found that magnesium flash makes about the brightest I've seen (and less white Al₂O₃ smoke problems, absorbing radiant energy). For faster burn, use finer particle sized magnesium. Treating the magnesium with dichromate would probably help due to the catalytic effect on the oxidizers, but sieve the dry, treated Mg to break up agglomerated particles and regain the fine particle size (for the optimal mixing).

 

I suspect the atomic spectra from Ba and K oxidizers will balance the overall emissions for the best effect (but test variations to confirm this).

 

Do you plan to silver one end and semi-silver the other end of the ruby crystal? Are you also planning to surround the ruby with a highly polished (reflective) tube to concentrate the radiant energy from the flash?

 

It sounds like an interesting, but tricky experiment. Good luck.

 

WSM

[styropyro]

Thanks for the advice! I didn't know that light absorption from Al2O3 smoke was even a problem! I don't have anything finer than 325 mesh Mg at the moment, but I guess I could just mill some down. I've read that zirconium based flash powders are bright, but haven't tried any myself yet.

I really need an oscilloscope/photodiode setup for this project, but don't want to spend the money.

As for the actual laser design, I'll use external mirrors so I don't have to silver the actual rod. I'm using an elliptical reflector and putting the crystal at one of the foci, and the photoflash charge in the other, which should send most of the light output into the crystal. I'm also going to try to build a "Q-switch" to shorten the pulse time and increase peak power, but I'll need optics than can stand stupidly high power densities of 100MW/cm² for short amounts of time.

[WSM]

Thanks for the advice! I didn't know that light absorption from Al2O3 smoke was even a problem! I don't have anything finer than 325 mesh Mg at the moment, but I guess I could just mill some down. I've read that zirconium based flash powders are bright, but haven't tried any myself yet.

I really need an oscilloscope/photodiode setup for this project, but don't want to spend the money.

As for the actual laser design, I'll use external mirrors so I don't have to silver the actual rod. I'm using an elliptical reflector and putting the crystal at one of the foci, and the photoflash charge in the other, which should send most of the light output into the crystal. I'm also going to try to build a "Q-switch" to shorten the pulse time and increase peak power, but I'll need optics than can stand stupidly high power densities of 100MW/cm² for short amounts of time.

 

Try the 325 mesh but treat it. If you need finer, I recommend buying it rather than milling some.

 

Be aware of the static hazards of finely divided, flammable metal powders, and be very careful. If you try finely divided Zr, be especially careful. It's been reported to ignite and flash while just being poured from a container, from static electricity when dry (it's usually shipped with 25% water, for safety). Zirconium (and hafnium) adsorb hydrogen on the surface of the metals, greatly increasing their ignitability and static sensitivity. This is especially hazardous with very fine powders (much more surface area)!

 

WSM

[Arthur]

Are you certain that flash powder is the best material for this job? It's likely that splatter from the burning metal and the shock wave from the combustion front will damage your crystal's surface.

 

While I like pyro, I'd seriously consider a xenon strobe for the purpose. A tube fed with paper capacitors (low ESR) should have some great pulses and no damage.

[Arthur]

The best shape will be a thin flat one. only the light from the outside of the comp gets out, the rest is partly absorbed by particles above it.

[styropyro]

Are you certain that flash powder is the best material for this job? It's likely that splatter from the burning metal and the shock wave from the combustion front will damage your crystal's surface.

 

While I like pyro, I'd seriously consider a xenon strobe for the purpose. A tube fed with paper capacitors (low ESR) should have some great pulses and no damage.

For this kind of laser, flash powder is not the best solution, but I want to try it anyway. The crystal and charge will be separated by a few alternating layers of polycarbonate/air, which will hopefully protect the crystal.

 

I actually have a massive 12kJ capacitor bank and a few huge Xenon flashlamps, but these are being used for a separate Ruby laser that I'm building. My other ruby is smaller but in practically perfect optical shape, so I'll treat that one better than the one getting hit by flash powder.

 

The real reason why I am interested in this is because the flash powder method seems scalable. If I want to pump MASSIVE slabs of laser media, flashlamps/capacitors will get expensive quickly. Flash powder is cheap, and with electronic igniting it can go uniformly and quickly. Nowadays you can find massive slabs of laser media from old inertial confinement fusion experiments on the surplus market for very cheap relative to their initial cost, but pumping these with lamps would cost way more than I could afford.

 

Thanks again everyone!

[Arthur]

The problem will reveal itself that the light comes from the outer perimeter of the (expanding) powder volume but the shock wave comes from the reacting volume. so the brightness grows the the ^2 power of the diameter of the flash but the damage to the ruby increases to the ^3 power. Rapidly the damage devalues the brightness.

 

The brighter flash bonbs as used by the military for WW2 era night air photography used a dust of metal powder expanded by a HE blast then (quickly!) lit by a flash fire charge so that the light from the inner metal particles passed through the gaps in the particles outside them. But this is very slow.

[3LEGGEDSQUIRREL]

Are you gonna be using Neon transformers, Rotory Spark gap for power to capacitors or what. what is the whole setup look like?

[Arthur]

If you can make discharge tubes then the noble gasses will produce different colour spectra. You are only going to use a millilitre or two! but it will be time consuming The "usual suspects in terms of size can be found with xenon gas in surplus photo flash units from the film era.

 

Fast flashes usually come from high voltages and paper capacitors, photo flash units tend to have a pulse duration in the order of 100ms

[styropyro]

The problem will reveal itself that the light comes from the outer perimeter of the (expanding) powder volume but the shock wave comes from the reacting volume. so the brightness grows the the ^2 power of the diameter of the flash but the damage to the ruby increases to the ^3 power. Rapidly the damage devalues the brightness.

 

The brighter flash bonbs as used by the military for WW2 era night air photography used a dust of metal powder expanded by a HE blast then (quickly!) lit by a flash fire charge so that the light from the inner metal particles passed through the gaps in the particles outside them. But this is very slow.

For larger area pieces of material I'd need a grid type setup for the flash powder to try and cheat out the square cube law. That's cool about the WWII area charges, I bet those things were incredibly bright!! For my setup it's too slow like you say, but it would be cool to witness the illumination from one of those.

 

If you can make discharge tubes then the noble gasses will produce different colour spectra. You are only going to use a millilitre or two! but it will be time consuming The "usual suspects in terms of size can be found with xenon gas in surplus photo flash units from the film era.

 

Fast flashes usually come from high voltages and paper capacitors, photo flash units tend to have a pulse duration in the order of 100ms

At the peak currents I'd have to run the tubes, the spectra becomes fairly close to blackbody so the particular noble gas doesn't matter much any more. Even if I had the lamps I'd still need capacitors, and to get the kind of energy storage I'd need there would have to be a lot of capacitors...which is expensive and takes up tons of space.

 

 

Are you gonna be using Neon transformers, Rotory Spark gap for power to capacitors or what. what is the whole setup look like?

 

My plan is to use an air spark gap...dump the charge from a disposable camera flash circuit into an auto ignition coil and let the cap bank's charge flow through the air plasma. A little wasteful with the stored energy, but that shouldn't matter too much.

[WSM]

Back in the day (>25 years ago) I've worked on analog photo copier equipment that had two roughly 48 cm long xenon flash tubes in series that flashed with a refresh rate of under a second. The powering circuitry included two large capacitors which had to be discharged for five minutes and shorted before servicing (RC time constant). One of those supplies, modified and using appropriate flash tubes, might well have served for a light source for your ruby rod (properly prepared).

 

The power supply was heavy enough it would have been wise to mount it to a small dolly for convenient transportation. As I recall, it required a 170 Vac source, but stepping up from 120 Vac (or down from 240 Vac) is easily accomplished with a step-up transformer wired especially for the purpose.

 

WSM

Edited July 20, 2016 by WSM

[3LEGGEDSQUIRREL]

Not sure how bright this is compared to what you've tried, but here is a flash I made the other day with a 70/30 mix

[3LEGGEDSQUIRREL]

Sound like your having fun styropyro. I haven't made anything like that yet, but I do have 2 15Kv 60mA NST wired parallel running a 6' tall Jacobs Ladder with a 8" plasma arc making ozone.