mam@charm.UUCP (Matthew Marcus) (04/05/84)
, Several people have responded to my rash offer to provide references re: the N2 'laser' I described during the discussion on the nature of photons. Rather than mailing to each, and discovering that I can't get to half the sites, I'll spread it over the net. First, some references. Read these, especially those with Von Bergmann as one of the authors, and then the details I give below will make some sense. I'm relying on the references to provide the pictorial material I can't in this medium. 1) H.M. Von Bergmann, A.J. Penderis, JPE 10,602(1977) 2) R.S.H. Boulding, "Principles and Practice of Radar", Van Nostrand (p. 286 begins explanation of Blumlein circuit which puts the ZAP! into the laser) 3) H.M. Von Bergmann, JPE 10,1210 (1977) 4) V. Hasson, H.M. Von Bergmann, JPE 9,73(1977) (NOTE: 'JPE'='J. Phys. E') 5) V. Hasson, H.M. Von Bergmann, D. Preussler, Appl. Phys. Lett. 28,17(1976) 6) H.E.B. Andersson, Physica Scripta 4,215(1971) 7) J.W. Keto, T.D. Raymond, S.T. Walsh, RSI 51,42(1980) 8) B.S. Patel, RSI 49,1351(1978) (Note: RSI=Rev. Sci. Inst.) 9) Yu.L. Sidorov, A.N. Sukhanov, Sov. J. Quant. Electr. 8,334(1978) 10)J.H. Crouch, W.S. Risk, RSI 43,632(1972) (Special Blumlein) 11)H. Hugenschmidt, J. Wey, Optics Commun. 29,191(1979) 12)J.G. Small, Sci. Am. 1974 (Jun?), Amateur Scientist column (Not the best design, but a start) I started with a PC board about 1'x2' (I'm a little weak on sizes, since I'm typing this from memory). I etched off a border 3/4" wide on top and 1/2" wide on the bottom. Since large voltages will appear between the top and bottom layers of copper, it was important that edges not be facing each other across the thickness of the board. That way lies field concentration resulting in spark-holes in the board and a short life. Guess how I found out. On the top, I etched a stripe down the middle (the short way). This stripe is about 1/2" wide. Now the top has two copper plates, each about 11x10". At one corner, I etched circular holes of 7/8" (top) and 1-1/8" (bottom) diameter (concentric). Concentric with these, I drilled a 1/2" hole. This part of the board is where the low-inductance spark gap went. The bottom half of the spark gap was a brass cup soldered to the board. Soldered to the bottom of the cup was a steel ball-bearing (1/8") which serves as the bottom electrode. A copper tube soldered into the cup carried compressed air (shop air, 70psi) which was used to pressurise the gap and increase the breakdown voltage. The top half was another brass cup, with a tapped hole in the center. Through this hole protruded a steel screw with the bottom rounded off. This screw is the top electrode. The top of the top cup had a cicular recess machined into it into which a plastic tube, 1/2" OD x 2" long was glued. The top of this plastic "tower" held a Cajon compression fitting through which a 1/8" dia. brass rod fitted. The "tower" assembly sealed in the compressed air which would have otherwise leaked out through the threads in the tapped hole for the top electrode, and also provided a way of adjusting the gap. The brass rod has a screwdriver-like blade formed in the end, so by rotating the rod I could change the gap, hence the rep. rate and power of the laser. To the end of the rod, I glued a plastic rod as a handle. I discovered that the phenolic which I used for the purpose was slightly conductive, so I would slowly get charged up to the extent of drawing an unpleasant spark when I touched anything grounded after a few minutes of twiddling. I solved this problem by attatching a grounded hose clamp to the handle. (OOPS!, time for a para. ) The top (screw) and bottom (bearing) electrodes of the spark gap faced each other through the 1/2" hole in the board. The discharge electrodes were made of aluminium, 6" long x 1/4" thick. The edges of the electrodes were rounded and polished to reduce sparking. What we are after is a uniform glow discharge. The electrodes were mounted on brass stands so their bottom surfaces were 1/4" off the board, and they faced each other with a 3mm gap. The electrodes were paralell to the stripe on the middle of the board, and symmetrically placed with respect to it: +--------------\ /----------------+ | | | | | | | | +--------------/ \----------------+ | | | | -----------+ +------------------------- ============================================================ ------------------------------------------------------------ where ==== is the insulating board (G10), and the horizontal is much expanded. The + here ^ is the edge of the etched-off stripe. The horizontal ----- lines are either parts of the electrodes or copper on the board. Over the electrode assembly, I put a Lucite box fitted with Brewster windows on the ends and nipples for N2 flow. The N2 was Bell Labs house gas, which is reputed to come from LN2 boiloff. I found that a little moisture helps reduce sparking, so I bubbled the gas through water. The two halves of the top copper were connected with a 5-turn coil of hookup wire. The whole assembly was placed in a metal box with a hole in the top for the spark gap adjuster. High voltage (30kv?) was applied to the top of the spark gap, hence to the whole top copper. The gap was pressurized with shop air and N2 flowed in the electrode area. When the gap breaks down, a large voltage is developed across the gap between the main electrodes. A purple glow discharge forms, and light comes out both ends. For better output, a mirror (coated for N2 light) was placed in an adjustable holder on the side of the box. The gain of N2 is so high and the pulse so short that the mirror only has to be aligned well enough for light to make one or two round trips between the far end of the electrodes and the mirror, so a very crude holder is satisfactory. The HV was supplied by a homebuilt circuit involving an ignition coil and a TV tripler. These HV components were kept inside the box, and fed with a square wave from a push-pull circuit outside. The master oscillator was a 555, and its frequency was varied for best output. I suspect that there is some sort of resonance in the ignition coil. The power for this circuit came from a +-15V power supply built on the "brute force and ignorance" principle, using two filament transformers in parallel for the AC. With the above and the references, I think you should be able to match or better my efforts along this line. Remember that a Blumlein is a very efficient spark-gap transmitter, and will zap anything electronic in the room unless shielded (See subject line). A battery-operated, floating DVM with no test leads goes nuts when near an unsheilded version of the laser. Happy zapping, and don't forget your rubber-soled shoes! {BTL}!charm!mam