Nitrogen tranverse excitation air LASER


SPECIFICATIONS

Input voltage range: 5000 - 10000 VDC , 10W or more

Output power range (average): 5-20 mW

Output power for each pulse: 100KW up to 1MW

Pulse duration: 5-10 ns

Output Wavelenght: 337.1 nm

Pulse repetition: depends on input power, capacitance and spark gap distance, generally 0.1s - 1s

Laser channel lenght: 28 cm

Resonator: only backmirror used

PART LIST

Part Name
Quantity
50KVDC-20KVDC >10W high voltage power supply
1
1Mohm 2W High voltage resistor
1
Enamel copper wire for inductor
1
Alluminium rods (30cm)
2
Various bolts and nuts
----
Pexiglass or polystirene 4mm sheet
1 m^2
Polyethilene LDPE foil (0.4mm thickness)
2 m^2
Alluminium flashing
0.5 m^2
Brass acornuts and bolts of sime size
2
First surface mirror
1

 

SCHEMATIC

Model

Construction

This LASER is constructed on a polystirene sheet. An alluminium layer foil is placed and acts as a ground plate ( - ), this is important for preionization (i will discuss it later). After that the dielectic is placed on it leaving 1 cm from each border of the foil). The dielectric thickness determinates the max voltage, capacitance, and important , the inductance. Thicker foil means less capacitance, more voltage and more inductance(since the loop area increases). Many reports that the inductance grows faster than voltage so the risetime is reduced (di/dt = V/L). Less capacitance means also less energy for bang. Many peoples reports that thinner foil works better but has a limited lifetime and limited voltage strenght, so the spark gap must be brought closer. After placing the dielectric I proceed to place the cap plates (separate) made with alu foil. The cap plates but be at 0.5 cm from each other, not closer (or will arc-over), not farther (or we will loss preionization, that helps to trigger the lasing). The caps must be clamped with other polystirene sheets and with bolts/nuts. After that the electrodes were placed touching the foil and with adjustable distance. The charging inductor is some turns of enamel wire connecting the two plates, must be optimized for best performance (squeezing or wideing it). The spark gap must be closer as possible to ground plane to minimize inductance (read further to know why). The electrodes distance must be adjusted for the best performance. When it will lase, a strong fluorescence will appear on paper or marker. Having some Fluorescein will visualize the beam. The power supply must be current limited or you will fire some hudred shots in a couple of seconds. The best repetition rate is 1shot/second or more, to put less stress possible on the dielectric.

Description

This LASER uses the nitrogen present in the air (78%) as the active medium but unlike many gas lasers it works at atmosfetic pressure. This feature allows to eliminate the need of the a vacuum pump. There is a drawback though, i will explain it introducing the quantodynamics of this type of laser. The laser works by pumping the electrons in a higher energy unstable state, this state relaxes not-radiatively into another unstable state with time duration of some nanoseconds. After that it may fall into a metastable state (with duration of severan milliseconds) emitting the characteristic wavelenght with a spectral with related to the relaxing time. This metastable state adsorbs UV. The lasing transition is defacto this passage from the unstable state to the metastable. This means that we have to pump the laser very quickly (il the order of nanoseconds) to have the population inversion enough to achieve the lasing threshold (by stimulated emission). While in low pressure nitrogen lasers the lifetime of the upper state is tens of nanoseconds, in the air atmosferic pressure laser, due to molecular collision causing faster disectitation in a non radiative way, the lifetime is 2-3 nanoseconds max. The lifetime [ns] can ba calculated from this formula t = 36/(1+p/58) ([1] Compact high-power TEA N2 laser B.S. Patel Review Of Scientific Instruments, 49(9), Sept 1978) in function of pressure [Torr]. This means that the circuitry must be designed for fast risetime, needing very low inductance (tens of nanohenries ) and higher possible voltages (not too high or the dielectric will punch through). The LASER capacitor plates, spark gap, etc must be designed for small loop area, to ensure lower possible inductance or the laser WILL NOT WORK good or at all. Believe me, i've been there, it is frustrating. Anyway this laser has many positive features also in addition of being cheap. It has a very high gain, eliminating the use of resonator mirrors (though using them gives higher power and less divergence), it is called a superradiant laser. The enemies of this laser are inductance, humidity (stopping the LASING by faster disextitation), oxigen (disextitates by chemical reaction with nitrogen, producing oxides), and oxone production corroding the dielectric causing punch-throughs. It is doomed for self destruction after 100-200 shots, but the repair is easy, though takes some time to align the electrodes again, that can be frustrating.

 

Difficulty/Cost: 70% HIGH (Lots of calibrations needed, but the cost is low)

Danger::: 60% HIGH + 10%

This circuit is dangerous. This laser outputs coherent UV light enough to cause retinal damage and skin diseases. Use safety googles and don't stay into the beam path. The spark gap emits UV light as well, i don't recommnt looking at it. Also it produces some amounts of RF with large spectral bandwitch (since the discharge is very fast). Remember also that you are saying fluorescence of the beam, not the beam itself, that can be many times stronger than the spot, in fact the human eye cannot see UV. Although the average power is low, the peak power is very high, enough to obliterate severals stands of DNA in one pulse, possibly causing cancer. Also, because this circuit uses high voltage there is shock hazard and burns risk. The capacitor cannot store lethal amouts of energy but the peak current is very high (since the circuit is designed for low inductance also), enough to knock you down and lose recall. This circuit also produces some ozone and NOx that are irritating for your respiratory system.

Video Gallery

Pictures of the laser in action. Some spots are missing since the pulses have very short time duration even if they are visualized by fluorescence

Video 1 (880 Kb) - Download

Video 2 (721 Kb) - Download

Note: in the video 1, at the end you will see a very big spot. This spot was emitted during dielectric failture. In fact the spark gap was bypassed and the total inductance was smaller than usual causing a very high power emission, the last before repair.

Image Gallery

These images of the laser shows the floureshence of the beam on the paper. Sadly the laser failed many times and the only pictures i was able to take are these listed down here.

The TEA LASER without the power supply

Lasing at close distance. Notice the cool blue spot on paper (fluorescence) and the plasma in the laser channel.

Lasing at medium distance. Notice the strong divergence of the spot, no focusing lens used

Lasing at far distance. Notice again the strong divergence of the spot.

Very bright spot on paper. Notice the violet plasma in the laser channel and no sparks