TEA laser
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A TEA laser (Transverse Electrical discharge in gas at Atmospheric pressure) is one of the easiest and least expensive ways (compared with other models) to generate laser light. The atmospheric pressure requires high voltage which is counteracted by the transverse discharge having lower voltage requirement than longitudinal discharge.
[edit] Application
Nitrogen, hydrogen, neon, excimer lasers, and carbon dioxide are known to lase only when pumped by an approximately 1 ns long gas discharge, especially when buffered by helium and doped by some small amounts of impurities for easy ionization.
[edit] Electrical network theory
The subsections follow the energy-flow.
[edit] High voltage source
See Power supply.
[edit] Capacitor
The capacitor is charged by the high voltage source. To have a low inductivity and a high capacity the dielectric must be thin, and if a high quality material is not used, the discharge may take place in the capacitor. The air between the dielectric and the metal has to be removed by a vacuum pump. Then it is replaced by epoxy or the dielectric is made soft by heating and the whole capcitor is pressed to let the dielectric fill any irregularies in the polished metal plates with their round edges. An array of commercial doorknob capacitors can also be used. After each cyle the discharged capacitor is then charged again.
[edit] Spark gap
When the capacitor's is charged the potential difference between its plates is high enough to ionise the air in the spark gap, which closes the circuit. This was used by Bose in 1894, but electron tubes or semiconductor switches can also be used. Pressurized spark gaps are available commercially.
[edit] Main discharge
Two long metal bars, acting as the electrodes. The distance between them is between 0.9 and 2 mm and the bars must be put parallel to each other within a few micrometres of error. The metal bars are in series with the spark gap. When the voltage across the spark gap breaks down, the whole voltage falls of over gap between the metal bars starting a second discharge. The bars are between 0.9 and 2 mm thick and round to avoid areas of high electric field strength and early break down. When the bars are not part of the capacitor they must be pressed firmly (by a line of screws) onto them to get a good contact on the whole length.
An arc is undesirable, for the heat of the spark evaporates small holes into the metal.
[edit] Time evolution
In the beginning the spark gap is discharged across the waveguide capacitor and the energy delivering capacitor in series (the spark gap does not see that it really discharges one of them and charges the other). Big capacitors lead to high current, but not short pulses. The waveguide capacitor should have a by a factor of two smaller capacitance to get to the following paragraph:
When the voltage across the laser electrodes is high enough and there a discharge has started, the spark gap, the laser channel and the energy delivering capacitor form a current circuit. For the current to rise as fast as possible the Inductance of this loop must be low. That means that the loop is small (spark gap near the laser electrodes), has a small area (it is squeezed flat to form the plates of the waveguide capacitor), contains no practical but small connectors, and the loop is stretched along its axis reducing current density therefore magnetic flux in the loop (the waveguide capacitor plates are two-dimensional).
[edit] Optics
One end of the laser is often closed by a concave mirror. It is as close as the HV allows to the end of the laser channel to let the light stimulate emission as fast as possible. It is concave (radius = 300 mm) to reduce losses by diffraction. Diffraction also determines the shape of the laser active region. As a rule of thumb for light with a wavelength 1/1000 mm and a channel with a length of 1000 mm the channel needs a cross section of 1 mm x 1 mm
[edit] Microwave theory
Since a TEA laser is larger than the distance, which a light pulse travels in 1 ns, one must use a waveguide, in order to lead the pulse from the spark gap to the electrodes around the nitrogen.
The subsections follow the energy-flow.
[edit] Spark gap
Electrical impedance jumps between the thin spark in the spark gap and the wide dielectrically loaded part of the waveguide are minimized as the spark gap dives into the waveguide. A compromise between as low as possible impedance jump between the thin spark and the wide waveguide and as high as possible voltage across the gap leads to:
- (saturation power) = (Electrical breakdown voltage) * (gas pressure in spark gap)
The external voltage supply raises the voltage within 0.1 s. When the ionization threshold is reached, every electron generated by cosmic or radiactive radiation creates an avalanche, which in dense enough gas forms a streamer. This streamer needs about one ns to span a 10 mm spark gap. Then every 5 ns current is doubled through impact ionization. Because the spark starts from a low current, the power supply is for some time delivering more current, than is consumed by the spark. But eventually the voltage will drop below ionization threshold preventing other sparks. The first spark is already hot and grows into an arc until the resistance of the arc gets lower than the impedance of the waveguide and the voltage drops to zero thereby self-estinguishing the arc. A more fundamental limit to speed is the inductivity of the spark, which refuses to grow to a diameter greater than 0.4 mm, and so on a linear voltage scale the spark gap typically generates an about 10 ns long electrical pulse.
[edit] Waveguide
The waveguide is shaped like a concave mirror so that the microwave pulse - which is emitted by the spark gap in a circular fashion - is collimated to a parallel pulse front. This pulse front runs through a part of the waveguide, which is loaded with a dielectric thus slowing the speed. This pulse front is tilted relative to the direction of the laser channel so that the laser pulse runs in it like a surfer on a water wave.
The low frequency high voltage is blocked by a capacitor in one wall of the waveguide close to the spark gap. This looks to ns pulses like an additional waveguide. In order to extinguish the spark gap after t=ns again, its capacity must be smaller than C=10nF (R C=t).
To make the spark gap as fast as possible it has to see a low as possible load. In a lumped element circuit model the spark gap sees the two capacitors (one formed by the waveguide, one blocking the DC) in series. The load of this series is often minimized by giving both capacitors the same capacity. The ends of both capacitor reflect waves, increasing the load. The round-trip-time (spark gap)-(waveguide)-(laser channel)-(waveguide) should be the same as the desired pulse length, so that the reflected wave extinguishes the spark gap after it has generated the pulse.
[edit] Main discharge
The voltage rises within 10 ns. When the ionization threshold is reached, a spark is generated. Current flows through this spark, but is to low to compensate for the high current coming from the waveguide. Therefore many more sparks are generated until the parallel resistance gets lower the impedance of the waveguide and the voltage drops to zero. The lasing action takes place at the fast leading edge of the voltage pulse, the rest of the pulse is more or less wasted in sparks afterwards. Therefore even for the 10 ns long pulses from the spark gap, a waveguide-structure suiteable for 1 ns is preferred.
The resistance is changing constantly. This means that only a small time slice of the electrical pulse is not reflected at all. For Energy efficiency this should be the maximum.
[edit] The laser medium and preionisation
Each ion becomes a spark. Furthermore natural nitrogen has one ion every mm³. In sealed 20 Hz laser 10000 ions per mm³ are left over from the last discharge. More ions can be generated by Vacuum UV-radiation, which is emitted by already existing sparks and has 10 mm mean free path. If the voltage is rising rapidly enough, these ions can alo become sparks.
After each shot the gas is replaced by means of a laminar airflow (convection or active). The new gas has a high gain, because:
- The new gas is cold that means it has low doppler broadening.
- The new gas is not excited in any laser-quenching, metastable state.
- The new gas has a low pressure that means it has low collisional broadening.
The new gas was close to the last discharge and so received a lot of VUV radiation and contains a lot of ions and "good" metastables to start a smooth discharge.
