TEA laser

Additional recommended knowledge Essential Laboratory Skills Guide Weighing the right way Guide to balance cleaning: 8 simple steps Contents 1 Application 2 Electrical network theory 2.1 High voltage source 2.2 Capacitors 2.3 Spark gap 2.4 Main discharge 2.5 Time evolution 3 Optics 4 Microwave theory 4.1 Spark gap 4.2 Waveguide 4.3 Main discharge 5 The laser medium and preionisation

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.

Application

Nitrogen, hydrogen, neon, excimer lasers, and carbon dioxide are known to lase when pumped by a brief (typically a few ns long) gas discharge. These gases are often (though not by any means always) buffered by helium, and are sometimes doped with some small amounts of impurities for easy ionization, but even air can be lased.

Electrical network theory

The subsections follow the energy-flow.

High voltage source

See Power supply. The capacitors are charged by the high voltage source. After each cycle the discharged capacitors are then charged again.capacitors is very important for charge discharge frequency.

It is possible to use electrostatic generators to power small TEA lasers, though this is not often done.

Capacitors

To provide low inductance and high capacitance, the dielectric must be thin; if a high quality material is not used, the discharge may take place inside the capacitor, ruining it.

The air between the dielectric and the metal is removed by a vacuum pump; then it is replaced by epoxy, which also serves as a dielectric; alternatively, the dielectric itself is made soft by heating, and the whole array is pressed into it, to let the dielectric fill any irregularies in the (polished) metal plates with their rounded edges. The vacuum is then released.

A simple version of this design can be constructed from thin brass or aluminum sheets for the electrodes, and thin plastic sheeting as the dielectric, without any necessity for vacuum or heating. The electrical field imposed during charging causes the opposing plates to attract each other quite strongly, which rapidly forces out most of the air between them. Construction takes only a few hours, and can be accomplished by students who do not have access to a vacuum-forming setup or a precisely-controlled oven. This often makes up for somewhat lower performance.

An array of commercial laser-grade doorknob capacitors can be used instead; but the inductance of such an array is higher, which tends to degrade performance to some extent. Such arrays are very common in excimer and CO₂ lasers, but they may not be suitable for use with neon, or with nitrogen at room pressure.

Spark gap

With a free-running spark gap, the potential difference across the capacitors becomes high enough during charging to ionise the air in the spark gap, which closes the circuit. This effect was used by Bose in 1894. Electron tube or semiconductor switches can also be used, and pressurized triggered spark gaps, which allow more control than free-running open ones, are available commercially.

Main discharge

Two long metal bars act as the electrodes. The bars are typically between 0.9 and 4 mm thick, and are usually round, to avoid areas of high electric field strength and early break down, though a combination of one round bar and one flat bar is also viable, and it is sometimes possible to obtain operation with other shapes and combinations. The distance between the bars is typically between 0.9 and 5 mm, and they should be parallel to each other to within a few dozen micrometres, to avoid sparking at one end. The bars are in series with the spark gap. When the spark gap breaks down, a process that typically takes a few dozen nsec, the voltage across the gap between the metal bars increases rapidly, starting a second discharge. When the bars are not part of the capacitor they must be pressed firmly (by weights or a line of screws) onto them to get a good contact on the whole length. An arc between the capacitor plate and the electrode is undesirable, as the heat of the spark creates small holes in the metal, which leads to further degradation.

Time evolution

To start the cycle, both capacitors are charged. The spark gap then begins to close, discharging the capacitor to which it is connected. Because the spark gap and capacitor have some inductance, they form a resonant circuit, and if the laser does not fire it is possible to achieve nearly twice the initial charge voltage across the laser channel during the first half-cycle of the resulting oscillation. Normally, however, the laser channel does conduct, limiting the voltage that is actually achieved.

When the voltage across the laser electrodes is high enough and a discharge has started between them, the spark gap, the laser channel, and the capacitors form a current circuit. For the current to rise as fast as possible the Inductance of this loop must be low. To achieve this, the loop is kept small (spark gap near the laser electrodes), has a small area (it is, if possible, squeezed flat to form the plates of the capacitors as in the diagram above), and the loop is stretched along its axis, reducing current density and therefore magnetic flux in the loop. (The capacitor plates are effectively two-dimensional.)

It has been demonstrated by various researchers that optimal operation is obtained when the two capacitors have the same value. It has also been demonstrated, repeatedly, that a single spark gap cannot generate a discharge wave in a laser of any reasonable size, so the arrangement shown in the diagram to the right (one rectangular plate and one curved plate) is pointless. It is easier, in any case, to construct two rectangular capacitors of equal size. (Some transmission-line behavior has been observed in a few designs, but it does not appear to be a major contributor to performance.)

At least in the case of neon and nitrogen, the laser starts and terminates while the current is still increasing. Because of this, a fast discharge is mandatory for these lasers. Excimer and CO₂ lasers, on the other hand, can continue to lase for longer periods, and are more efficient.

Optics

One end of the laser is often closed by a flat or concave mirror. In the case of a neon or nitrogen TEA laser, the mirror is positioned as close as the HV allows to the end of the laser channel, because the laser pulse is extremely brief and excess spacing wastes time as the light travels to and from the mirror. The mirror is often (though not by any means invariably) concave (radius = 300 mm) to reduce losses by diffraction. Diffraction also determines the shape of the laser active region, as do the shapes of the electrodes and the spacing between them. As a rule of thumb for light with a wavelength 1/1000 mm and a channel with a length of 1000 mm (which is, as it happens, far too long for neon or room-pressure nitrogen) the channel needs a cross section of 1 mm x 1 mm to produce a reasonably well-defined beam.

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. (This is not entirely accurate. Many TEA nitrogen lasers are not larger than the distance light travels in 1 ns.)

The subsections follow the energy-flow.

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 radioactive 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-extinguishing 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.

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.

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 too 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. (This description fails to match actual experience of TEA nitrogen lasers, which do not spark at all if properly constructed and correctly preionized. In fact, the discharge in the channel of a TEA nitrogen laser is a dim purple glow that is barely visible to the naked eye, except for tiny bright spots on the surface of the cathode.)

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.

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.

To avoid different parts of the discharge stealing each other the current, but instead reinforce the current, the electrodes can be parted and connected with each other, with the initial spark gap, and the pre ionisation electrodes in the fashion of the Marx generator, with the ends shorted so that all energy is available inside.