I know I should be helping get our Burning Man proposals in final shape, but I wanted to post some thoughts I’ve had on bubble ignition with lasers. For those of you who don’t know what I’m talking about, we’ve periodically discussed whether our hydrogen plus oxygen bubbles could be ignited remotely with a laser. I think it’s agreed that this would be pretty awesome. Unfortunately it turns out it’s probably quite hard.My initial thought along these lines was to use some kind of photochemical reaction along the lines of a demo I saw in my freshman chemistry class. If you shine blue light on a mixture of hydrogen and chlorine, it explodes, forming hydrochloric acid. You can see this in this video on youtube:This works because blue light photodissociates the molecular chlorine into atomic chlorine. The atomic chlorine is very reactive and reacts with the hydrogen in a chain reaction, forming HCl. Chlorine is photodissociated by UV - blue light in the 300 - 400 nm range.In principle you can do exactly the same thing with either the oxygen or hydrogen in our bubbles.Unfortunately, the wavelengths you need to photodissociate either oxygen or hydrogen are in the deep UV, and can only be generated by very expensive sources like fluorine excimer lasers. Even worse, these deep UV wavelengths are efficiently absorbed by the atmosphere. So direct photodissociation of H2 or O2 is probably out. One potential possibility is to dope our bubbles with something that can be efficiently photodissociated to spark the reaction (chlorine would work, but it’s pretty toxic) but this seems likely to be pretty complicated.There is another possibility: laser spark ignition. It turns out that if you focus enough laser power into a small spot, the resulting electric field is enough to ionize the air and create a spark. Once again, youtube has video of it (it’s amazing what you can find on youtube):This should ignite a bubble just as well as an electric spark (and in fact, there’s a lot of research into using laser spark ignition to replace spark plugs). There’s just a few problems. First, you need a lot of laser power. That video is using 200 mJ pulses that are probably a few nanoseconds long. That means the instantaneous power is on the order of 10 MW. The average power is only 4W, but you can imagine that if you happen to be in the way of one of those pulses you will be unhappy. So laser safety concerns preclude using this in an open space.Second, that laser is probably pretty expensive.Third, you have to have the beam come to a tight focus to get the spark. That means it’s not enough just to point the laser at the bubble, but you need to focus the spot into the bubble. That’s probably pretty hard to do.So, my initial ideas for laser ignition of bubbles seem not to be feasible … but I’ll keep thinking!

We’ve been hard at work in the shop, building the prototype for our new V3 Hydrogen Bubble Machine and testing a voice control interface for Kurt’s candelabra project. We now have 3 fire tubes connected via USB interface to our flame simulator software. When input comes through the microphone, it is analyzed for amplitude averages over short time periods. This is essentially a VUMeter algorithm. The analysis is displayed on the screen and simultaneously sent to a Measurement Computing USB-1024LS Digital I/O board. The pin-outs 0-5V are connected to Kurt’s amplifier board which boosts the voltage range to 0-24V. This then controls a set of solenoids connected to our 1/2″ copper flame tubes.We have been working on a good ignition system and just had some luck using nichrome wire to form a hotwire ignitor on about 8A of current. But for now, in the lab, it doesn’t seem to matter since each of the candelabra’s flame tubes seems to stay lit long enough after the solenoid closes to relight the next time it opens.Here’s some video of our 3-tube voice-controlled Candelabra demonstration:

And here’s a clip showing propane ignition with nichrome wire: