An Examination of the Amateur Scientist Circuitboard Nitrogen Laser

First of all, Small describes his laser as a Blumlein circuit, and speaks of “the Blumlein phenomenon”.

The real Blumlein phenomenon is the fact that Alan Dower Blumlein essentially invented stereophonic sound. He even got a patent on it. Among audio engineers, he is rightly famous. There are Web pages about this, and someone has written a biography of him. Among electrical engineers, however, he is also known for his work on transmission lines. He came up with something called a “Blumlein line” or “Blumlein circuit”, or sometimes just “Blumlein”.

In his article, Small states that “at the instant the switch closes”, a discharge wave is initiated in the circuitboard capacitor that presumably forms one of the transmission lines of the device. Let’s think about that for a moment.

First off, the word “instant” is not defined in physics, electronics, or engineering, except when people are discussing mathematical entities (“...the instantaneous value of the second derivative...”). It’s not appropriate here, and in plain point of fact, it’s meaningless. (This should be a significant warning about any description of nitrogen laser circuit behavior that employs this word.)

Second, even if we were to pretend that “instant” had a meaning, that it meant, say, “appreciably less than 1 nanosecond,” there wouldn’t and couldn’t be any such instant in any case. The switch in question is an untriggered spark gap, designed and constructed so that it includes a nice big one-turn inductor. Even excellent spark gaps, well designed and carefully triggered, take several nanoseconds to initiate; and the free-running spark gap in this laser is slower than even a reasonably good triggered one.

(Note, added 03 May, 2007)

I hope to measure the switch closure time of a small TEA nitrogen laser of very similar design, and I will publish the oscilloscope trace here if and when I succeed.

Light travels at a finite velocity, which is very roughly 300,000,000 meters per second in a vacuum or in air. In materials with higher density (and higher refractive index), it is correspondingly slower. As Small points out, a discharge wave in a transmission line travels at the speed of “light” too, but that speed turns out to be related to the impedance of the line — the electrical equivalent, if you will, of the refractive index.

Something Small never addresses (possibly because it had not yet been examined or measured when he wrote his article) is the fact that even after the discharge starts, lasing does not begin immediately. It takes time to pump enough nitrogen molecules into the upper laser level to establish a population inversion; a typical nitrogen laser starts lasing about 10 nanoseconds after current begins to flow in the laser channel. Here, for example, is a diagram that I have adapted from one that appears in a published paper:

Another problem with Small’s explanation is that he claims to have produced a travelling optical wave. Let’s think about this, too, for a moment.

By Small’s own admission, the output pulse from his laser is about as long as a broomstick, or a bit longer; let’s say 6 feet, which is about 6 nanoseconds. If we think about a time during which the entire laser channel is above threshold, and is lasing, light that starts at either end will be amplified by the discharge in the channel, and will reach the other end just over 1 nanosecond later, because the laser channel is a little over 1 foot long.

If we assume that one end of the channel reaches threshold first, and then a hypothetical discharge wave “walks along the channel” as Small proposes, to create a travelling optical wave, what do we see from the two ends of the laser? The “back” end, where lasing starts first, should show a small amount of output, which will increase as the leading edge of the electrical discharge wave gets farther away. That is, the back end will lase first, but not very strongly, and the output from that end will increase during the first nanosecond or so until the entire channel is above threshold and lasing from that end has reached full power.

After that first nanosecond, the discharge wave (and the initial laser light from the back end) simultaneously reach the front end, and lasing begins there. Thus, the pulse from the front end should have a much sharper leading edge than the pulse from the back end.

After that point, however, the entire channel is above threshold, so for the rest of the pulse, which is to say the next 5 or 6 nanoseconds, output from both ends will be identical, or nearly so. Needless to say, this fails to match Small’s description of the action; but Small’s description fails to match his own statements about the laser and what it does.

There is only one way in which such a laser, which is only 1/6 as long as the pulse it emits, can produce dramatically higher output from one end than the other, and that is if an arc or spark interferes with the output at one end. (It is possible for an arc to form after lasing has ceased, so the arc itself is not a reliable indicator, but if the laser puts out a large pulse at one end and little or no pulse at the other, you can fairly well take it for granted that something is interfering, and the only something that usually occurs in a nitrogen laser head is an arc or a spark.)

If anybody can show me such a laser that puts out a large pulse from one end but not the other without any arc or spark formation, I would very much like to see it.

The travelling optical wave phenomenon described by Small has, indeed, been observed in a discharge laser: see the John D. Shipman reference. Alternatively, it is sometimes possible to produce a travelling wave by angling the electrodes slightly, so that one end of the channel begins to arc before the other, which has a similar effect; but the length of the laser channel has to be a substantial fraction of the length of the output pulse for this effect to produce a dramatic asymmetry of power from the two ends.

Of course, the discharge wave has to be relatively straight; and it has to be angled correctly with respect to the channel, so that the region of intersection advances down the channel at the speed of light. Shipman was obliged to use a series of solid dielectric switches driven by cables of precisely graduated lengths in order to create this effect. His laser had to be rebuilt every time it was fired. It is extremely unlikely that a circular discharge wave would be very effective at producing a travelling optical wave even if the Scientific American laser were capable of producing a discharge wave, which it clearly is not.

If I may once again quote Schwab & Hollinger, writing about discharge waves, “For low-impedance Blumlein generators (as used with N₂ lasers), propagation time on the transmission lines is on the order of 5 ns/m. Considering rise times of about 25 ns, which are inherent to many reported N₂ lasers, the traveling-wave concept becomes obsolete.... If the advantages of true traveling-wave excitation shall be utilized, either multiple spark gaps, solid-dielectric spark gaps, or lines with high characteristic impedance (e.g., 20 ohms) must be employed.”

The problem with high impedance is that such lines deliver only a small fraction of the current that is delivered by low-impedance lines, and must operate at much higher voltage to produce the same power (alternatively, you can use lots of them in parallel, as in the lasers reported in the Woodward, Ehlers, and Lineberger paper). The Schwab & Hollinger laser, btw, which does not even attempt to provide travelling-wave excitation, produces over 1 MW peak output power at 12 kV charging voltage. It’s a very decent design, though not as easy to construct as some.

It is possible that Small inadvertently introduced some angle between his electrodes; but as we have already seen, that is not sufficient to produce a travelling optical wave in his laser.

Small claims, in his discussion of scaling, that although a discharge path one meter long can develop an output pulse of almost a million watts, “...there is a trick to it. Because the laser turns itself off so quickly, radiation does not have time to travel the full length of the column before the gain automatically drops to zero.” This is just plain stupid: he himself states elsewhere in the article that the length of the pulse from a low-pressure nitrogen laser is “usually less than 10 nanoseconds”; 10 nanoseconds is more than three meters, so he’s contradicting himself. (As it happens, a discharge path one meter long can develop more than three million watts, and can do so without any “tricks”, though Small didn’t know that when he was writing because it hadn’t been accomplished yet.)

I should perhaps point out once again the fact that the pulse can be terminated either by accumulation of nitrogen molecules in the lower laser state, or by a decrease in the electron temperature in the discharge, so that the nitrogen is no longer being pumped effectively. Most Do-It-Yourselfers are unaware of this, even though it is often what is actually happening in their lasers. It appears that at pressures on the order of 30-60 Torr, a nitrogen laser that is pumped hard enough can run for 15 nanoseconds or so, depending on its design. (For further information, see the “high power” references in the list at the end of this page.)

Small claims that the Scientific American laser puts out 50 to 100 kilowatts. I suspect that someone must have measured the output of one of these machines, but just at the moment I don’t recall ever seeing any actual reported numbers. There are, fortunately, some informal ways to estimate power output. I believe that it takes only a few dozen kW to threshold a small dye laser, and the Scientific American laser can certainly do that.

There are other, somewhat informal ways to estimate peak power, but they tend to require that the laser be focused to a point, which is very difficult with most nitrogen lasers because the beam is more or less a wide ribbon shape. If you can focus down to a point, you will probably find that it takes about 200 kW peak power to produce a spark when the beam is focused onto a metal surface (be very careful about possible reflections!); see the Bergmann & Eberhardt reference, below. It probably takes considerably more than a megawatt to produce a spark in open air at 337 nm.

I have to confess that crap like Small’s explanation drives me right up the wall, especially in the pages of Scientific American (which used to be a real science magazine), and double-especially when it comes out of MIT.

Anyone who bothers to perform any kind of decent testing on a laser built to Small’s design can’t fail to detect the huge bogosity quotient, and I can’t believe that he didn’t have access to decent equipment, MIT being, after all, one of the premier institutions for this sort of thing in the entire world.

Moreover, just reading the Shipman article carefully is enough to tell you that Small’s laser can’t do what he claims it does, and that article was published two years before Small’s Scientific American piece, so presumably he had access to it. It’s even moderately likely that he read it, though apparently not carefully enough. Small certainly should have known that there’s no such thing as an instant in physics, and that even a one-turn coil (such as the one he builds into his spark gap) has substantial inductance.

Argh.

In Small’s defense, I have to point out that there was relatively little literature available to him; almost all of the nitrogen laser articles I’ve got were published after the Scientific American nitrogen laser appeared. On the other hand, Shipman’s article is quite clear about some of the conditions that must be achieved in order to create a travelling discharge wave. (Shipman even suggests the technique of using multiple coaxial cables that later became the basis of the Woodward, Ehlers, & Lineberger lasers; and he suggests graduated lengths to create travelling-wave excitation, which those lasers didn’t use, though their article mentions the possibility.)

Unfortunately, the explanation given in the Scientific American article is an example of the worst effects of bad scholarship (and accidental memetic engineering): its claims have been believed and repeated by a great many people who failed to check or fully understand them, and they have polluted a large sector of amateur and even professional nitrogen laser work. I have seen the words “At the instant the switch closes” in articles from major journals, for example. Also, although it isn’t just Small’s doing, the term “Blumlein” has become hopelessly polluted. Too many articles and Web pages refer to LC inversion circuits as “Blumleins”. It is likely that Shipman’s laser could, with some justice, be described as a Blumlein circuit, and there are one or two others, very similar to his, that were reported; but those few are about it, as far as I’m aware. Nothing else comes close. Not your laser, not my laser, not anybody’s laser. Throughout much of the professional research literature, however, any LC-inversion laser is referred to as a Blumlein circuit.

H. G. Heard
“Ultra-violet Gas Laser at Room Temperature”
Nature, v. 200 (1963), p. 667

This article presents the discovery of the nitrogen laser.

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A. W. Ali
“A Study of the Nitrogen Laser Power Density and Some Design Considerations”
Applied Optics, v8n5, May 1969, pp 993-996