TJIIRRS: Number 21
A Look At Some Room-Pressure (TEA) Nitrogen Laser Designs
(04 April, 2011, ff)
!! WARNING !!
These lasers use high voltages, and capacitors that can store lethal amounts of energy. They put out invisible ultraviolet light that can damage your eyes and skin. It is important to take adequate safety precautions and use appropriate safety equipment with any laser; but it is crucially important with lasers that involve high voltages and/or produce invisible beams!
If you use an open spark gap, you need to be aware that it will destroy your hearing unless you use adequate ear protection. I strongly suggest a pair of earmuffs of the type used by people at rifle and pistol ranges, and it is a good idea to use earplugs in addition to the muffs. If you aren’t using adequate hearing protection, an open spark gap will also give you a nasty headache if you run it for a while.
The nitrogen laser was discovered in 1963. Originally, this laser was operated at moderate pressure, roughly a dozen to a few dozen Torr of nitrogen. Although it fairly quickly became clear that the addition of helium did not interfere, and that it was possible to lase a mixture of nitrogen and helium that was at room pressure, that mixture still contained only a small partial pressure of nitrogen, and the lasers were still what Jarrod Kinsey refers to as “TERP” Transversely Excited at Reduced [partial] Pressure.
It took at least four years before anybody was able to lase nitrogen at room pressure, but when the dust finally settled it turned out that TEA (Transversely Excited Atmospheric [Pressure]) nitrogen lasers are actually a lot easier to build than TERP lasers.
This page details my effort to produce a design that is easy to construct, use available materials and parts, and provide reasonable performance, and then my effort to produce a design that provides well over 1 MW of output power. As usual with TJIIRRS pages, I detail quite a few failures, because this is a track of the effort; if you just want to build a laser, try this page.
The first design uses a single sheet of overhead projector transparency as its dielectric (an idea pioneered by Jarrod), and is driven by the power supply from a small electrostatic air cleaner, which puts out 5,000 volts. The second is somewhat more sophisticated; it uses a sheet of 96% pure alumina as its dielectric, and operates at somewhat higher voltage.
1: A simple and straightforward TEA nitrogen laser
at relatively low voltage
(04 April, 2011)
The Power Supply
You can certainly build a high-voltage power supply yourself, if you want to. It’s an interesting project, and the experience of constructing such a supply is likely to be very useful if you end up building more lasers. I am not going to do that here, however, because this is a laser page, not a power supply page, and because I don’t have the time. Instead, I have taken the power supply out of an old electronic air cleaner, and I’m going to use that. Here is the air cleaner before I disassembled it:
This is what the right side looked like with the cover off:
Here is the power supply on its new base:
(Please note that this is just a temporary setup. It is not safe to have high voltages exposed as they are here, and I will eventually put this power supply into an insulating box.)
I made a voltage divider by putting a 100-million-ohm high-voltage resistor in series with an ordinary 10,000-ohm resistor, and I used the divider to measure the output voltage from this supply. The schematic diagram on the bottom of the case says that the supply puts out 5500 VDC at 0.3 milliamps, but that turns out to be a description of one polarity, not both: I measured roughly 5980 volts on the positive terminal and roughly 6390 volts on the negative terminal, for a total of over 12 kV. Remember, though: that’s the open-circuit voltage; when the supply is pushing any reasonable amount of current, the voltage will be lower. Still, the open-circuit voltage is what will determine the spark-gap spacing later on, when we get that far along.
Groundplane, Baseplate, Possible Dielectrics…
(2011.0810, early evening)
I have decided to use a piece of single-sided circuit board as the ground plane of the laser, because I have several pieces that are of a reasonable size. They are very thin, though, and that is going to cause some problems. Here is the board, next to a steel chassis that will probably serve as the base; you can see that I have marked the board so I can cut it to an appropriate width.
This circuit board is so thin that it can be cut with a pair of scissors. (I don’t recommend doing that if you have anything better, though, because it isn’t very good for the scissors.) Here is the cut-down board, nestled inside the base:
You can see a few minor dents in the copper. These are not what we want, and you should prevent this sort of thing from happening if you can. (A thicker circuit board won’t flex as much when you are moving or positioning it, and will be more stable.)
The next step is to find a piece of plastic that can serve as a dielectric. I had originally intended to use overhead projection transparencies, but then I went to an office supply store and priced them: $40 for 100 sheets. We’ll have to see what the local thrift stores have to offer, because that just isn’t viable.
Today I went to the hardware store and 4 thrift stores, and got various things for this project. Here are some possible dielectrics (including a nice new sealed box of 50 overhead projection transparency sheets, for which I paid $1.21 plus tax after the usual 25% discount that they offer on Mondays), some possible electrodes, and parts for some spark gap designs:
(The styrene is 15 mils thick, and there are several sheets in the package. The acrylic is 10 mils thick and brightly fluorescent; there are two sheets of it.)
It begins to look like the dielectric strength of styrene plastic is fairly low, 450V/mil, and so is the dielectric constant, 2.85. The dielectric constant of acrylic is probably about 3, but depends on which source you check. (One source I found on the Web states that the dielectric constant is 4.) The dielectric strength is even lower, only 400V/mil at 1/8″ thickness; but it should go up as the thickness decreases, and could be 500V/mil or even more at thicknesses of a few mils.
I do not know what kind of plastic the overhead transparencies are made of, but I may be able to determine the dielectric constant: my digital multimeter can measure capacitance, and it is easy enough to make a temporary capacitor. This may also let us begin to understand the effect of air between the plastic and the conductor[s].
I used the large sheet of circuit board for one of the capacitor plates and a 2″ x 12″ piece of brass (visible in the photo above) as the other, but with an overhang of an inch, so it was effectively 2″ x 11″. (I think we can safely pretend that both plates were 2″ x 11″, because material that is relatively far away is not going to have much influence.) I measured the thickness of a projection transparency sheet to be 4 mils, and the capacitance (when I pressed down fairly hard on the top plate to flatten it and exclude air) was a little over 2500 pf. (It was only about 1500 pf when I was not pressing down. This is, obviously, important; but when a laser of this sort operates, the high voltage tends to pull the capacitor plates toward each other, which helps expel some of the air.)
Going by the usual formula (which is C[in pf] = 0.224KA/d if you have two plates, with the measurements in inches), I get K as just a shade over 2. This could indicate the presence of some air, as the lowest K I’m aware of in any ordinary plastic is about 2, and most are closer to 3 or even 3.5.
As a comparison check, I redid the measurement with a sheet of 96% pure aluminum oxide, 40 mils thick. This material has a dielectric constant of either 9.0 or 10.0, depending on what source you believe. If the correct value is actually 9.0, we would expect the capacitance to be just over 1100 pf, and in fact that is the value I measured, so perhaps there isn’t too much air present here. On the other hand, if the value really is 10.0, then there is at least some air. (2011.0412, evening) It occurred to me that although the dielectric constant and strength of polystyrene are not very good, there was actually a reasonable chance that they’d be good enough for a first try. The material I have is 15 mils thick, and even at 450V/mil that should be just about enough to handle the 5500 Volts that one polarity of the power supply is rated to provide. (I decided to use the negative output rather than the positive, for no particular reason.) The other advantage is that the styrene pieces are long enough to permit me to use the brass sheets without cutting them down, which is definitely a good thing. The less chopping and machining I need to do for this project, the happier I am, especially for something that is just a prototype, and may have different measurements from the next version. I rounded the corners of the brass sheets that serve as capacitor plates and also as the electrodes of the laser, smoothed the edges at the corners and ends (but not the edges that face each other and serve as electrodes for the laser), and threw the prototype together, using old lead-acid batteries as weights to hold the capacitor plates down (and also to hold the electrical attachments to them). Because the brass pieces are formed by stamping them out of larger sheets, one face of each is slightly convex and the other is slightly concave; I put them with the convex sides down. I had two 1.8K power resistors left over from something else, so I used those as a charging resistor. The spark gap is a slightly fancy variant of Jarrod Kinsey’s design; I used carriage bolts as electrodes. One of the bolts is mounted on a sheet of brass shim; the other is weighted down to the ground plane. Once I had all of the parts at the table, I think it took me all of 15 or 20 minutes to put the laser together. Much to my surprise, it then took me only about 10 more minutes to tweak it to the point at which I saw laser light from it. Mind you, this is probably the lousiest laser I have ever built. It lases maybe once every 25 or 30 times it fires, on average. …But it does lase! Here are some photos. First, two overviews:
A Crude Prototype
It occurred to me that although the dielectric constant and strength of polystyrene are not very good, there was actually a reasonable chance that they’d be good enough for a first try. The material I have is 15 mils thick, and even at 450V/mil that should be just about enough to handle the 5500 Volts that one polarity of the power supply is rated to provide. (I decided to use the negative output rather than the positive, for no particular reason.) The other advantage is that the styrene pieces are long enough to permit me to use the brass sheets without cutting them down, which is definitely a good thing. The less chopping and machining I need to do for this project, the happier I am, especially for something that is just a prototype, and may have different measurements from the next version.
I rounded the corners of the brass sheets that serve as capacitor plates and also as the electrodes of the laser, smoothed the edges at the corners and ends (but not the edges that face each other and serve as electrodes for the laser), and threw the prototype together, using old lead-acid batteries as weights to hold the capacitor plates down (and also to hold the electrical attachments to them). Because the brass pieces are formed by stamping them out of larger sheets, one face of each is slightly convex and the other is slightly concave; I put them with the convex sides down. I had two 1.8K power resistors left over from something else, so I used those as a charging resistor. The spark gap is a slightly fancy variant of Jarrod Kinsey’s design; I used carriage bolts as electrodes. One of the bolts is mounted on a sheet of brass shim; the other is weighted down to the ground plane.
Once I had all of the parts at the table, I think it took me all of 15 or 20 minutes to put the laser together. Much to my surprise, it then took me only about 10 more minutes to tweak it to the point at which I saw laser light from it. Mind you, this is probably the lousiest laser I have ever built. It lases maybe once every 25 or 30 times it fires, on average. …But it does lase!
Here are some photos. First, two overviews:
(The black mug at the left edge of the first photo and the lower left corner of the second photo is a weight that is holding the ground lead in contact with the ground plane. The Honeywell mug is maintaining the position of one of the spark gap electrodes, and keeping it in contact with the ground plane. The yellow “Dr. Science” mug [the one that says “I Know More Than You Do”] blocks the light from the spark gap so it doesn’t interfere with my view of the laser light on the paper target.)
Next, a picture of the spark gap with some sparks in it (I didn’t take the time to try to capture it with a single spark), and a picture showing part of the laser channel, with bright sparks in it. These are usually undesirable, because they tend to occur early in the firing cycle, and when that happens they prevent the laser from working correctly; but occasionally they occur after lasing has ceased, and when that’s the case they can be ignored unless they damage the surfaces of the electrodes.
In this particular case, it seems that the power supply ramps up the voltage faster than the 3.6K of resistor can keep up with it, and many of the sparks I see are occurring during charging, long after the discharge has finished. (I can actually hear some of them, which I wouldn’t be able to do if they were occurring sooner.) I will replace the charging resistor with an inductor soon.
Finally, a picture of some output, causing a piece of white paper to fluoresce. Sorry I don’t have more pixels of this, but the spot is small, and I wanted to keep the camera a slight distance away from the HV leads.
Now that you’ve seen how straightforward this can be, let’s do a slightly better job of it…
A Better Laser
First things first: let’s take a look at what we have, so we can decide what to change. Here are two views of the electrode profile:
As you can see, it is ridiculously rough. One thing we are going to have to do is clean it up and make it smoother.
Another item that may change is the dielectric. As Jarrod Kinsey has pointed out, a thinner dielectric is associated with lower inductance, and in a laser of this type that becomes an important issue. If I use 2 thicknesses of overhead transparency the capacitance increases only modestly because the dielectric constant is lower; but the inductance should also decrease, and that should compensate. In fact, with some luck it should more than compensate. In combination with the slight increase in stored energy, that should significantly enhance the performance… if the dielectric can withstand 12,000 Volts.
Another way to decrease the inductance is to change the design of the spark gap. That is one of the first things I want to try.
(2011.0413, early evening)
I have made only minor revisions so far, but the performance has significantly improved. First, I moved the plastic sheet slightly to the side, because it was showing signs of contamination, electrode material deposited on it from all of the sparking. That material is now underneath one of the capacitor plates, where it cannot get in the way.
Second, I removed the charging resistors and installed a charging inductor. Here is a photo:
The performance of the laser immediately improved. It now lases at one end most of the time, and on the other end just about every time it fires. Here are photos of the two ends:
Here is a wretchedly amateurish video, showing the laser in operation. I have not had a chance to do any editing, and it is quite difficult to shoot video in the dark, so I offer it without any apologies despite the fact that everything of interest is in the lower half of the frame.
It opens with a view of the laser aimed at what I am currently referring to as the “A” target. You can see the “I know more than you do” mug, illuminated by the spark gap, to the left of the target. I then walk around to the other side of the table so you can see the “B” target, and finally I walk back to the first side. I will obviously have to reshoot this mess, but I wanted to get at least something up on the Web.
I tried pushing some nitrogen gas into the channel, and was very pleased to find that the output, even at the “A” end of the laser, became visible with the room lights on. I have not photographed that yet, as I do not have 7 hands and have not had a chance to build a little holder for the balloon with the nitrogen supply. This result is quite important, though, as it demonstrates with great clarity the difference between air and reasonably pure nitrogen: nitrogen will give you something like 10 times as much output power and energy as air.
[[More as it transpires...]]
About Nitrogen Lasers
The nitrogen laser was discovered in 1963. As far as I can recall, it was the first ultraviolet gas laser, the first pulsed ultraviolet laser, and possibly the first-ever ultraviolet laser. It puts out short pulses of light at a wavelength of about 337.1 nm, a little shorter than the wavelength of an ordinary “blacklight” but not quite short enough to be described as “midwave UV”. This light is not visible, and it is even more dangerous than the light from small visible lasers. In addition, the laser operates on high voltage, so you should ONLY attempt to built it if you are prepared to exercise appropriate safety precautions. (I will list a number of these as we proceed.)
The nitrogen laser has very high gain. Excited nitrogen gas amplifies so well, in fact, that nitrogen lasers can usually operate without any mirrors at all, though especially with low-pressure ones it is common to use only a single mirror. (Using a mirror at one end of the laser lets you get all of the output at the other end, instead of having it divided between the two ends. With TEA nitrogen lasers there is another way to accomplish this, which I discuss in one of the videos.)
[Note: There are a few people who claim that the lack of mirrors means that a nitrogen isn’t a real laser, because it doesn’t involve a Fabry-Perot resonant cavity. That view is defective in two ways. First, the word LASER is an acronym that stands for “Light Amplification by Stimulated Emission of Radiation”, in this case, optical radiation: photons; there are also masers, which amplify microwaves rather than light. Neither version of the acronym includes any mention of resonant cavities, and any claim that a resonant cavity is necessary (for whatever mysterious reason) is ridiculous on the face of it. It is generally acknowledged, in fact, that there are quite a few naturally-occurring masers in outer space, which obviously do not involve resonant cavities. In addition, it is entirely possible to build a nitrogen laser that operates inside a resonant cavity; it just doesn’t happen to be necessary.]
Every laser must be “pumped”. That is, energy must be provided to it in an appropriate form. Some lasers are optically pumped. This can be accomplished with flashlamps, arc lamps, other lasers, other kinds of lamps including incandescent lamps, or even the sun. Others are driven directly by electricity. There are even a few chemical lasers. It all depends on the active material and its requirements, and on various other criteria that we aren’t going to get into here because there is neither space nor time. (If you are interested, there are various resources on the Web. I will link to some of these at the bottom of the page.)
All or nearly all nitrogen lasers are pumped by electrical discharge, usually in fairly pure nitrogen gas; but a nitrogen laser can even operate with air as its active medium, and that is what we will start with. I will note, however, that air is a lousy laser, and that if you can get or produce nitrogen that contains 0.5% oxygen or less, you will get much better performance. (I demonstrate the difference, using the laser design on this page, in one or more of the videos.)
2: A TEA nitrogen laser with an alumina insulator
(04 April, 2011)
This is a somewhat more sophisticated laser than the one presented in the first section. It is based on a sheet of 96% pure Al2O3, a material that is used as a substrate in the electronics industry. These sheets are regrettably expensive, but seconds have recently become available to artists working in ceramics, and we can happily take advantage of that fact.
The distributor for these sheets is in the process of changing its name as I write, and does not yet have them listed on its Web page; I will endeavor to correct the URL as new information becomes available.
(More as it transpires…)
Last modified: Thu Apr 28 14:49:21 EDT 2011