Nitrogen TEA laser

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Contents

Description

A nitrogen transverse electrical discharge in gas at atmospheric pressure (TEA) laser is a laser in which the lasing medium is the nitrogen in air at atmospheric pressure. The output of such a laser is at 337.1 nm, which is invisible ultraviolet light. The dyes used in ordinary paper, clothing, and other materials cause the beam to appear blue. A TEA nitrogen laser is a pulsed laser, capable of megawatt pulses, but low average power.

The four basic components of a TEA laser are the electrodes, the capacitors, the spark gap, and the inductor. The laser presented here will be constructed on top of its own capacitor. The lasing action will occur between two central rails, or electrodes, each attached to half of the capacitor. The inductor is a critical part of the circuit that makes this laser function. The spark gap is simply a high voltage high-speed switch for firing the laser.

This is by far the cheapest laser to build at home, but not necessarily the easiest. It will likely take substantial adjustment to get working, so be patient.

TEA-laser-diagram.png

Function

The inductor will be a low resistance path for the direct current of the power supply and both sides of the capacitor will charge simultaneously. When the voltage reaches a level that makes the air in the spark gap break down the left capacitor will discharge very quickly. The inductor will pose a high resistance to the rapidly changing voltage, keeping the other half of the capacitor charged. This causes a large voltage difference to rapidly build up across the electrodes. The rapid discharge between the electrodes causes the nitrogen to lase.

A more advanced design might use a Blumlein transmission line to generate the pulse required.


Materials

  • Plexiglas, or acrylic, 18" x 24" for the base and top clamping plates
  • Two 12" x 1/8" aluminum rods or tubes for main electrodes (rounded helps avoiding corona discharge)
  • Aluminum foil for condensator plates
  • 4-8 mil polyethylene sheet for dielectric
  • Two acorn nuts for the spark gap electrodes
  • Nylon nuts and bolts or other suitable type to clamp the Plexiglas plates
  • Misc. wire

Equipment

Procedure

Fig. 1   The bottom of the capacitor.
Fig. 2   The dielectric of the capacitor.
Fig. 3   The placement on the top capacitor and plastic plates. Note the gap between the upper capacitor plates, as well as the larger gap between the plastic plates.
  1. Cut, from Plexiglas, a 12" x 12" base for your laser. Then cut two 9" x 5.75" upper plates. Drill holes in the base and upper plates such that the outer edges of the upper plates may be bolted to the base. See Fig.3. The best way to cut Plexiglas is to score it by running a straight edge back and forth many times along a straight edge. When you've dug a good line into the plastic, simply bend the sheet over the edge of a desk, away from the side you made the score on. It does take some practice.
  2. Cut a piece of aluminum foil, with as few wrinkles as possible (Fig. 1), leaving approximately the clearances shown in Fig 2. It is important to leave .5" of space on all sides. It is also important to make sure that the sheet protrudes from the top capacitor plates when installed. See Fig. 3. This will form the bottom capacitor plate. It is important to leave plenty of space around the edges, or arc will be able to form around the outside of the dielectric to the upper plate.
  3. Cut a piece of your plastic sheet to cover all but the lowest .5" of the capacitor plate. Make sure this dielectric extends all the way to the edges of the base plate. A dielectric layer between the capacitor plates prevents arcing between the plates under high voltage, as well as increasing capacitance. Extending it beyond the edge of the plate ensures no arcs can 'sneak' around it. See Fig. 2.
  4. Now cut two more plates of aluminum foil. These should extend to .5" from the dielectric on all sides, and be spaced 1/8" from each other in the center.
  5. When everything is positioned, bolt the top plates to the base plate through the holes in the corners. In order for this laser to work, the pulse of current through the electrodes needs to be extremely fast. The principle factor that will slow the pulse down is inductance. Inductance is increased by wrinkles in the capacitors, air between the plates and the dielectric, etc. By clamping the plates between the acrylic, these factors are minimized.
  6. Place the acorn nuts on the left hand side of the laser, such that the rounded ends face each other, and the hexagonal part of one rest on the top plate, and the other rests on the bottom plate. Sparks will form between these nuts. When a spark forms, the laser will fire.
  7. Wind ten turns or so of wire around one of your fingers to form the inductor. Attach one end of the inductor to each electrode rail by inserting it into the base of each tube. Then place the electrodes in the center channel between the plates, making sure that about 1 mm of space is maintained between them, and that each one touches aluminum foil along its length. I secured the rails by putting pressure on them with a rubber band and piece of wood. Electrode spacing is critical, however it’s impossible to adjust until the laser is firing. For now just set them in place.

Adjustment

The complete TEA laser.

Connect the positive lead of your high voltage supply to the left hand electrode rail, and the negative lead to the bottom capacitor plate. Start by adjusting the spark gap as follows:

  1. Turn on your high voltage power supply.
  2. Observe the frequency at which sparks form between the acorn nuts. If no sparks form, then your spark gap is too far apart. If continuous, or more than ~3 sparks per second form, then your gap is too small.
  3. Turn off the power supply, and discharge the capacitor by placing a screwdriver (held by the plastic handle) across the spark gap.
  4. Adjust the spark gap and try again.

If you cannot get the gap to fire slow enough, try placing 100 MΩ of resistance in series with the power supply. If you cannot get your gap to fire fast enough, ensure that your supply produces the proper voltage. Once your spark gap works reliably, turn off the room lights and observe the space between the electrode rails. Each time the spark gap fires, you should see a uniform purple discharge between the entire length of the aluminum tubes. If there are hot arcs forming, then the rails need to be moved further apart at that area. If there is no discharge at all, then the rails need to come closer together. The easiest way to adjust might to walk each end of the electrodes closer or further apart until the desired result is achieved. If you cannot achieve any purple discharge then your inductor might be too small, or your rails too short. 12" rails are certainly long enough. Try adding turns to your inductor.

Results

The ultraviolet light from the laser makes the T-shirt fluorescence
The laser operating in the dark.
The beam dot at the top, the spark gap at the left.

When you have everything working, you'll notice a beam that makes standard white computer paper or clothing glow bright blue. Don't confuse the beam with light from the discharge or spark gap. When it works, you'll know, it’s striking. Try firing the laser at a piece of paper with highlighter on it, you'll notice strong blue green fluorescence.


Safety

  • Laser radiation
The TEA nitrogen laser is a class IIIb laser. The beam present an eye and skin hazard. This includes both intrabeam viewing and specular reflections. Always wear UV-blocking eye protection if you experiment with these types of laser.
  • High Voltage
The power supply may be dangerous, see Electrical safety.


Acknowledgement

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