Laser Kids
RUS

. : The Mountain that Spawned a Mice. (Bealieau-Lamberton-Pearson. Anti-guide) : .

 

The epic fails can often teach more than bright success can. Exactly due that fact I place here this description of a laser, meaning that it is an example of "how not to do"

1. The Concept.

The idea was to develop a laser being simple and powerfull simultaneously.
The one could be made by a lazy man and being capable to easily spark up the air. and fed by wire (not by low inductance wide strips)
What was unsuitable in the previous habitual TEA laser with a barrier discharge
preionizer?

  1. It is hard to obtain a suitable aluminium strip for the electrodes. It is even more hard to saw aluminium angle stock along in order to obtain the strips.
  2. Plain dielectric plates for electrode holders are also scarce, especially for larger samples of the laser.
  3. It bugs to polish the electrodes.
  4. In case of the dielectric foil breakdown one has to reassemble the laser completely. And it is desirable tha laser was 'perpetual', and it should be tolerant to such a commonplace as overvoltage.
  5. TEA laser with a barrier discharge preionizer is hardly scalable towards enlargement. And it is desirable to have a surplus active volume.

A schematics with sectioned electrodes and resistors in series with each esection (Bealieau scheme) is simple to undestand its principles. One should however keep in mind that it is not operational at athmospheric pressure when used without helium (and especially in presence of oxygen). If only one electrode is sectioned (e.g. the cathode) and if the distance between electodes is much more than the gap between the sections (and this situation is very common) then the discharge tends to slide along many sections and to focus ino one thick bright arc instead of forming a chain of weak arcs along the electrodes. One can make this process harder to take place by sectioning both electrodes, by adding a preionization and by enshortening the feeding electric pulse. Similar to the common TEA but softer.

The main drawback of the Bealieau lasers is the huge number of the resistors, pin electrodes, and efforts to install and tightly seal all these things. But if we expect that laser in any case will require the evacuating, then we can assume that at slightly lower pressure the discharge will be more stable and we can reduce number of pins. And all efforts on reducing the pressure are on the shoulders of the vacuum pump. And it probably won't swear if we ask it to drop the pressure slightly more.
This was the logic of this laser design.
And further there is what we got.

2. The Construct.

Laser is presented by a plastic sewer tube hawing the outer diameter 50 mm. The walls of the tube contain pair of straight chains of holes. 48 pairs of M5 screws are put into those holes (heads towards inside the tube). The chains of screw heads form basically the sectioned electrodes. Diameter of the screw heads is ~10 mm and the (discharge) gap between them is ~30 mm. Active length is ~750 mm, that gives 3x1x75=225 cubic centimeters of active volume. Preionizers are formed by wires stretches parallel to the electrodes. Distance between wires and electrodes is about 10 mm. Small capacitors connect the wires to the opposite polarity.
gora

Wire type preionizer makes this scheme similar to Lamberton-Pearson one. The resistive ballasting makes it similar to Bealieau one. Since that it was called "Bealieau-Lamberton-Pearson". But the similarity ends here. Further begin differences.

  • Bealieu one has cathode in form of chain of pins. They sthongly tense the field and provide pre-discharge corona to ignite and make necessary preionization. Here the sections are large and have round profile. They provide wide discharge column, while at Bealieau's the discharge width depends only to how the streamers diffuse in the inter-electrode gap.
  • Bealieau one has comparatively high ohm resistors, and they stabilize the discharge and provide the necessary delay of its develop relatively to the preionization time. Here the value of the resistors is choosen relatively low, the discharge time of the capacitor loaded onto the total resistance of all resistors is less than the discharge time of the same capacitor loaded only by its own inductance and by inductance of the conductor lines. It means that resistors do not make the discharge alower (but also they do not make it more stable). Their influence onto the laser efficiency is also neglible. Their only task is to distribute current uniformly along the electrodes and to help the preionizer to form many nice sparks and not the single big one.
  • Preionizers here utilize sparks. Lamberton scheme (where corona discharge is used) works better with smaller diameter of wires. (Cases of usage of 50 mcm wires are known) When the stake is made on sparks hitting the wires from the sections of the electodes, the diameter of wires can be large. 0.5 mm naked copper wires work well.
  • Placement of the wires does also differ from Lamberton's one:
    inside_tube

    discharge_inside_tube discharge_inside_tube2
    Ballast resistors have 100 Ohm value. (In place of resistors there were also used 3 uH choke coils and nichrome wires. In both cases there was good lasing, but with chokes the top operational pressure and energy were lower. Nichrome wires tend to burn. However the results with them (33 Ohms each) were even better than with the resistors.)

Front and rear ends of the tube are sealed with DIY mirror mounts. The rear mirror is as usual the curved (rear view) car mirror, having been washed out from paint. (Yes it is placed by its aluminized side towards the discharge.) Its focal length is about 1 meter. At the front one can use an uncoated ZnSe or Ge plate. Best results (higher working pressure without considerable energy loss) are at 50% reflectivity. Tube length is also 1 meter. I'm sorry but it occasionally formed a confocal resonator.
I didnt want it but it did.
Another tube with another 48 pairs of screws forms a suitable terminal board.
Refer to the picture.

bealeau01

bealeau02 bealeau03

All this husbandry is fed from a (comparatively) low ESL capacitor of 40 nF through a spark gap (of strengthened construction). Connections are made by 50mm wide aluminium foil strips up to 1 meter long. It means not 'by wire' but the 'low inductive' is also hard to say. Charging voltage is 30 kV.
gora_scheme

3. The Result.

The gained results are shown in the table.

Mixture air:
:CO2
1:2
combustor
:CO2
1:1
combustor
:CO2:He
1:1:2
combustor
:CO2:He
1:1:6
combustor *
:CO2:He
1:2:6
Optimal
pressure torr
40 40 50 100 100
Output mJ
at opt.pressure
120 90 120 120 100 **
Max working
pressure, torr
50 50 70 240 240
Output, mJ
at max.pressure
40 70 100 120 60

* "Combustor" - air, where the oxygen was eliminated by combustion of organic fuels. For details look at the guide on longtitudal pulsed CO2 laser.
** The energy was measured by average power at 0.3..0.5Hz rep.rate using simple DIY Peltier calorimeter.

As one can see the energy does not impress. Peltier readings show only 120 mJ.
Taking into account that at those repetition rates the calorimeter can lie, and orienting to the size and appearance of the spots on a carbon paper (comparing them with ones of known neodimium laser), it may be estimated that it was 0.25..0.5 Joules of output at optimal pressure on the best gas mixtures.
Maximum pressure also does not impress. With helium and practically without the oxygen it barely reaches 200 torr.
Outcoming beam has a form of 12x24 mm sized strip. When unfocused it can't be seen without a calorimeter. When focused slightly it makes glow on things. The spots on carbon paper are deep and well shaped when their diameter is less than 10 mm.
When focused to 1 mm the beam burns things (see video). It takes two to three pulses to destroy a match head (to show the naked wood of the match) and still it can not burn it.

Here are the photos of carbon paper, sandpaper and matchbox marked by the beam.

damaged_target1

damaged_target2 damaged_target3

Due to the fact that common mirrors have too much aberrations when large sized
beam is being focused, it appeared to be impossible to focuse the beam sharper
than to 1 mm. Maybe this fact is responsible for absence of sparks in the air.
Or (more probably) the peak power is still too low.

4. THE CONCLUSIONS.

If trusting scientific papers with these discharge sizes and energy depositions they were able to build lasers having 2..10 J of output. Here we have 20..100 times less. It is common for a DIY-er to get efficiences 10 times lower than scientists do. Careful inspection by exclusion method shows that it is due to gas mixture impurities. But It is doubtfull that You will take course to the nearest chemicals shop to buy a bottle of chemical grade nitrogen and helium. More positive point of view - is to get used to 0.5%..1% efficiences and to think how to adopt those results.

The other things are worse.

  1. Without helium the area of pressures where the laser works rather well didn't hit the range of pressures obtainable by reversed fridge compressor. It means one need a "real" vacuum pump. Maybe a rather cheap one, but real.
  2. The design of the laser appeared to be too complex and disgusting when being assembled. At the idea stage it seemed that the most difficult part would be to install the screws from the inside the tube. However usage of a stick with a magnet on its end had allowed to make all the operation in less than 20 minutes. Sealing appeared to be much harder. Time and efforts consuming procedure of sealing with a glue gun even with a help of gas torch havent eliminated all leaks. Laser leaks at a rate of 2..3 torr per minute. Mounting of the resistors also was not the most happy deed. One positive moment - the resistors appeared to be stable under the high voltage pulses. (Usually Bealieau schemes suffer from resistor failures. In this one the resistors never failed all over the time.)
  3. The sizes appeared to be huge. Naturally the gain is evident. In comparison with the previous small TEA laser this one has 3 times larger sizes and 6-10 times greater energy. But it is evident only to mind. The eye sees the freaking huge laser and expects it to punch through metal plates in one pulse. And when it manages only with paper or matches it feels disappointing.

 

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