Posts Tagged ‘ vacuum tube ’

Wine bottle CRT

So some time ago I was browsing youtube when I cam across the most wonderful video I had seen up to that point. Someone had constructed an old school cold cathode electron accelerator (called CRTs or discharge tubes) out of a wine bottle and some stuff from a local hardware store. I made a mental note that I wanted to try it, but when I saw the cost of the necessary vacuum pump I kind of lost interest. a few months later or so I was looking at instructables, when I saw that Daniel Kramnik had posted instructions on how to make it. This rekindled my interest, but I still didn’t have the money to buy a pump. That summer I saved a little money and at last I had one. Also that summer I had gone to a local neon sign shop and picked up a small neon sign transformer for $20! For the vacuum port on the bottle (also acts as the cathode) both of my sources for making this recommended using a concave surface which helps focus the beam. The original video said that they had used a drawer handle and the instructable said to use a mini doorknob. I couldn’t find either at my local hardware store, so I made my own design for the vacuum port. I had previously tried to setup some kind of system for connecting my vacuum pump that could act as the standard for all of my future projects. When I bought the pump it was claimed to have both a 1/4″ and 1/2″ port on the pump. When I got it, it looked more like a 1/4″ and 1/8″ ports. This was not much of a setback however. More frustrating was that I could not find a single flared hose barb fitting at any of my local hardware stores. I have found some that will work on belljar.net, but they don’t come cheap. It’s about $10 dollars for one of them. After a while I resolved to just tighten the hose I was going to use directly onto one of the ports threads. I had a length of vinyl tubing sitting in my garage that was the right diameter, but its walls were too thin and it collapsed under vacuum. To combat this I found some 1/4″ fuel line that was strong enough to withstand the vacuum, and used a small piece of the vinyl tubing to connect the vacuum pumps port to the fuel line. Hose clamps were used on both connections and lots of teflon tape on the pumps port. For most of my experiments I simply connected the bottle to that one length of tube directly to the pump and it worked surprisingly well. I am currently working on trying to connect a thermocouple gauge tube to get vacuum readings, but am having some problems with leaks  on new connections. For most purposes the single hose connection will be fine. Image This shows how I made the connection with the hose. A flared hose barb will be used in the future. Based on my system for connecting to the pump (using the 1/4″ fuel line) I decided to just buy a whole bunch of hose barbs that fit the fuel line to connect the chamber to the hose. The hose barb size ended up being 1/8″ pipe thread to 1/4″ barb I think. To make the vacuum port on the wine bottle I used one of the hose barbs described above, a 1/8″ pipe nipple, two washers with holes that roughly fit the nipple, and some epoxy (I have tried both five minute epoxy and JB weld successfully). I started by gluing together the two washers. Once that dried I tightly threaded the hose barb onto the nipple (teflon tape is not needed because epoxy will seal it). I then put the nipple through the washers, and glued the bottom of the hose barb down taking care to leave no gaps in the bead of epoxy. I then aded more epoxy around the edges of the hose barb to ensure a good seal. once the assembly was complete I used a rather large amount of epoxy to secure the washers to the top of the bottle once again put more epoxy on the sides to fill in any gaps and get a good seal.ImageImage The vacuum port also acts as the cathode so we will not have to worry about making at least one of the electrodes. The anode however does need to be added. I found a 3/16 (I think) glass/tile spade bit at my local hardware store which was perfect for drilling a hole in the side of the bottle to allow for a wire to be put through to act as the anode. Drilling the hole is pretty strait forward. You just put the bit in your drill apply a little pressure and start drilling slowly and carefully. I found the hole can be a little difficult to start, but putting a piece of masking tape on the spot you want to drill helps keep the bit from wondering away from where you want to drill. You should probably do this before you attach the cathode/ vacuum port, because if the bottle cracks when you are drilling it is trashed and you can’t use it. Once the hole is drilled You just need to slip the wire for your anode in the hole (the shape doesn’t really matter. I’ve put it in a loop and just bent it downwards and it doesn’t really affect performance), and epoxy it in place. I used a thick layer of epoxy on top of the initial seal to help keep the anode wire from breaking from being bent around a lot (my original CRT had to be decommissioned because the anode wire broke off). Image It’s really messy, but it provides a good seal and good strain relief for the wire. The next step is to make a power supply. As a rule of thumb you want a DC power supply that can provide around 10kv. You can use higher but beyond 30kv x-ray production starts to become a concern, so you don’t want to exceed it. You can also use a lower voltage (as I did). I’m not sure what the minimum voltage required to get it to run is, but I would bet you need at least 2kv. I made my power supply by using a 6kv 30ma neon sign transformer with a microwave oven diode to rectify the AC. To find the DC voltage that is output by rectified AC you use the folling formula: Vin (1.414) = Vout. To wire up the CRT I connected one HV terminal of the NST to the cathode of the CRT, and connected the other terminal to the anode of the microwave oven diode. The cathode end of the diode connects the anode of the CRT. The low voltage terminals of the NST get connected to the hot and neutral or mains. Pay attention to the stripe on the diode which indicates the diode’s cathode when wiring up your CRT and always use wire rated for the voltages you are working with. ImageImage All that’s left to do now is fire up your pump, turn on the high voltage and watch the plasma!ImageImageImageImage Nothing happens at atmosphereic pressures because the free mean path is too small, but once your pump has been running for a while plasma begins to form as the accelerated electrons excite the gas (nitrogen) inside the tube. You will notice several features that occur with in the tube which deserve explaining. One of the first features of the plama you will experience is what’s known as a Faraday’s space, and it is the gap in plasma that starts to form at relatively low vacuum, where the collum of plama begin to separate from the cathode. This occurs because there is a higher concentration of electrons near the anode (which electrons are attracted to) than the cathode resulting in more plasma. After pump runs a little longer you will notice the plasma forming stripes. These are called striations and they occur because there are going to differences in the glass (and any other material in the tube) from one part of the tube to another resulting in a higher concentration of electrons, and therefore plasma, in some parts of the tube. These descriptions are probably over simplified, but they get the idea across. If your pump is good enough and your bottle sealed well enough, then after the striations form, the faraday’s space and striations disappear and the bottle gets filled with a uniform light blue plasma. After this happens you may notice another very interesting feature. Starting from the cathode the plasma will disappear, and be replaced with emptiness within the tube. The glass around the emptiness however will fluoresce under the bombardment of electrons, and as the vacuum increases so does this empty space until the amount of gas inside the tube is too low to facilitate cold cathode emission any longer and the tube goes dark (it is unlikely you will reach this low of vacuum with your pump). This is called the Crookes dark space and to me it is one of the most fascinating features that occurs in discharge tubes. This project was a lot of fun, and now I can tell my friends that I have built a particle accelerator! I am far from finished with CRT projects though. In the future I intend to make a CRT oscilloscope from scratch using an electron gun assembly. I also want to try an experiment that uses an old 2 inch CRT along with some other equipment to determine the charge mass ratio of the electron, just like Thomson’s third major CRT experiment. Also for those who are interested I am currently working with some professors from DU to make a more serious and more powerful particle accelerator (atom smasher) to perform experiments I hope to enter in the ISEF. Well that just about does it for this project. I encourage you to try this and have fun with it, but be very careful when dealing with high voltage and vacuum, and make sure you are not producing x-rays! Also You should probably limit the amount of time you leave this running to a minute or so to decrease the chances of something bad happening.