This copper vapor laser project of mine has dragged on a bit but I’m pressing forward. I have wanted to build a copper vapor laser for a while but never seemed to get the drive to work on laser projects. I’ve been feeling interested in this project again so I’ve resolved to put my nose to the grindstone and get the copper laser up and running this Summer.
There is a lot of information out there on copper vapor lasers and their variants, theory and build. I’m mostly going to discuss the build of my copper vapor laser and try not to repeat to much of the available information out there.
My design is a copper chloride laser. Its bore needs to reach a temperature of 750 degrees F / 400 degrees C in order to operate properly. Once at temperature the bore is then zapped by 2 high voltage pulses from capacitors and should lase. One pulse dissociates the copper from the chlorine, the second pulse about 160uS later energizes the copper atoms in the laser bore. If you were to run pure copper you’d need to get the bore up to around 2700 degrees F / 1500 degrees C and that has more expensive material requirements, but a simpler single pulse power supply.
In order to get the 15mm quartz laser bore up to temperature I needed a tube furnace. My tube furnace is of a home built design and was pretty cheap to put together. I used a 24″ long piece of 6″ stove pipe, I capped the ends with insboard, a ceramic refractory material and then coated the inside a refractory ceramic cement. I then installed Inswool, a refractory ceramic blanket, inside of that and coated it all with more refractory ceramic cement. Inside I mounted the heating element from a toaster oven, ran the power leads through the end cap and connected it to an SSR which is driven by a Watlow 36 process controller. The process controller reads the temperature through a thermocouple and maintains the temperature of the bore at the required 750F/400C temps. You can see a couple of photos of testing the heating controls below.
There are some large gaps between the upper and lower halves of the tube furnace. This is due to my inexperience in casting these refractory cements. I may rebuild the tube at a future date, but for now I inserted thin strips of Inswool between the halves which, when crushed a little bit, seal up the gaps and keep the heat inside. You’ll see the strips in some of the later photos.
The tube furnace came up to temp slowly but it did reach the required temperatures. With more thermal insulation in the right spots it will heat much faster.
In the above photo the tube furnace for my copper vapor laser is sitting on a couple of pieces of Insboard. While the temperatures inside are around 750F, the outside can reach close to 200F after some time. I built a base plate and support stand for the copper laser. To do so I cut a large piece of MDF (medium density fiberboard) and coated both sides in red tinted West Systems epoxy resin. I bent up some brackets out of 1″ x 1/4″ aluminum stock, I used a torch to heat it up and then bent it in a vice. I may CNC machine some better looking supports in the future but I may rebuild the tube furnace to be more compact so I don’t want to waste expensive aluminum plates right now. As-is there is plenty of space to install the copper chloride laser tube, power supplies and electronics.
For the laser bore I’ve used quartz, it can easily withstand the temperature required for the laser and it’s easy to cut. With a file you scribe a mark, wet it and then (wearing gloves and eye protection) snap the quartz tube in half. This makes it easy to re-size as needed. The photo below shows the general assembly of the electrodes/bore holders. The design changed slightly while working on this project. The quartz bore is sealed into the copper electrodes with Permatex Copper Ultra RTV gasket maker (not shown in the photo) which will/should seal well and handle high temps and the holders will be mounted to nylon blocks which allow them to be adjustable up and down.
Power supplies for the copper chloride lasers are more complex than those used for a pure copper vapor laser. The power supply needs to be able to supply a high voltage repeating pulse. There is a dissociation pulse and then the lasing pulse about 160uS later. This can repeat as many times per second as you like as long as your high voltage supply can charge the capacitors fast enough and your switch can operate fast enough and handle enough current. The simplest method is a rotating spark gap which connects the two capacitors to the bore of the laser in sequence, repeatedly, which is very loud. Another method is to use high voltage switching devices such as an ignitrons or a thyratrons to switch the power from the capacitors in to the copper laser’s bore. While not as simple as the rotating spark gap, this method certainly has it’s advantage because it’s much quieter. I’ve read that if you run the discharges at high enough repetition rates you can get away with using one capacitor and one thyratron, I am going to give this an attempt and see how it works. The thyratron below is borrowed out of one of my nitrogen lasers.
Before I manufactured the optics mounts and attached them to the ends of the tube I wanted to see how airtight all of my solder joint and silicone seals were so I capped off the optics mounts and gas feed ports and hooked up my vacuum pump. My current pump is only single stage and gets down to about .7 Torr. In order for proper processing to get this laser to work I needed at least a 2 stage pump that can get to below 0.1 Torr. At any rate I attached a 15kV/30mA neon sign transformer to the copper ends of the bore and fired it up. There seemed to be a small leak to find. The tube did come up to vacuum as my pump moves a fair amount of volume, but without the pump running the bore slowly came back up to ambient pressure.
Current Status: Right now I’m looking for a surplus 2 stage vacuum pump and some materials to make the end mirror mounts. I expect to have those done and attached over the weekend if all goes well, no status on a vacuum pump though. If you know of anyone selling a 2 stage or better vac pump in the Ulster County, Orange County (NY State) or surrounding areas drop me a line using the contact box on the left.
I picked up a 2-stage vacuum pump from the local Craiglist listings and it seems to pull deeper. I still need to change out the oil in the pump since it is pretty well used.
I mounted the end optics to the tube and tried some tests under vacuum to see how things were looking. You can see the discharge in the tube in various pictures as it sits inside the DIY tube furnace I made. You can see in the second photo below that some of the high temp silicone I was using had oozed out into the bore.
I’ve also built an adjustable rotary spark gap for the time being until I can get the Thyratron electronics assembled. The rotary spark gap was made inside of an acrylic housing which I machined on my CNC router. The blocks are 6″x6″ and about 2 inches thick each. I routed a large hole in the middle as well as pockets for top and bottom ball bearings.
The first revision had 2 conductors placed statically and I was planning on using a brushless motor with variable speed to change the timing. I was able to get the laser to lase with that setup, but it wasn’t reliable so I rebuilt the rotary spark gap. You can see the first version below. The spindle is a lathed piece of the aluminum. The drive shaft is an Align 450 helicopter main rotor shaft and the electrode on the spindle itself is a tungsten welding electrode counter-weighted with a bolt with quite a few nuts on it on the opposite side.
While testing with the above rotary spark gap I manged to blow up my commercial 30kV .015F capacitor, I had to replace it with something but didn’t have anything suitable. I instead made one out of a thin piece of bare FR4 and some aluminum and copper tape. I blew the first one up, so I made a second one which has been holding up much better although the connections between strips of tape are not real great, there’s lots of corona. The curved corners keep corona down on the edges and reduces the possibility of burn through that you’d get with square corners.
After experiencing some intermittent and scarce lasing success I decided to make the spark gap adjustable. I wanted to be able to tune the timing on the gap without adjusting the speed of the motor. This required some modification to the original spark gap design but I was able to re-use the acrylic housing. The new gap has a fixed electrode on the left hand side and an adjustable electrode on the right hand side. The electrode is pushed/pulled along the copper plate by turning a screw. I added an additional tungsten electrode to the spindle.
I also replaced the motor with a brushed DC motor and now run that at a fixed speed. It drives the rotary spark gap via a timing belt setup.
This setup is working better though I need install taller electrodes and raise the spindle as it is discharging against the lower aluminum plate occasionally which I don’t want and wastes power. However after all of this messing around the lasing is much more visible getting several pulses per second. I think fixing the spacing on the rotary spark gap and getting some good commercial capacitors will improve things a lot. I’ll post more updates as I get things done but for now, here’s a video.
I shortened the bore by 6″ yesterday (after shooting the below video) to try and increase the power density in the bore and remove some of the innactive (unheated) area of the bore which is another waste of energy and things did improve. I ran the laser today and it was much improved over the below video, however I there was a pop and the lasing stopped, so I must have blown another capacitor. I’m waiting for the bore to cool down before I start going over it.
This post about my DIY copper vapor laser will be updated as I get further along with the project (Last Updated 8/11/2012 – 4:35 pm, Originally Posted 7/19/2012)