Joss Research Institute Web Report #16B: Continuing Look into Hollow-Cathode Helium-Metal Laser Designs for the DIYer, part 1


TJIIRRS Number 16B:

Further Examination of Possible Hollow-Cathode
Laser Designs for the Do-It-Yourselfer
Part 1: Quadrupoles






Preliminaries

(01 January, 2011, ff)

Note: A dipole has 2 parts, and it has opposite charges or poles across from each other:

 + - 

A quadrupole has 4 parts, and it has like charges (or poles) across from each other:

 +  - -  + 



This page describes the process of constructing and debugging several hollow-cathode lasers that are operated as quadrupoles. The first is made of copper tubing, and uses sputtered copper vapor in a helium buffer, probably with a small amount of argon added to it to enhance the sputtering. The second is built of stainless-steel tubing; it will use a mixture of helium and iodine or helium and argon as its active medium, and will not require sputtered metal. If I get really ambitious there may be a third and fourth, formalized versions of the first two, possibly with different insulators.

My effort is to make these lasers relatively easy to construct and operate, and to avoid parts that are expensive, difficult to obtain, or require much machining.

Side note: it became clear to me, in the course of working with the first version of this machine, that silicone rubber caulk is not viable as a vacuum seal at pressures below about 1 Torr. That probably should have been obvious in advance, but I missed it. (These things happen; at least it’s clear now.) Granted, these lasers operate at several Torr; but contaminants in the gas mixture are not helpful, so I am avoiding RTV.




I: The Copper Quadrupole

(2011.0226)

For the first version of this laser I built a quadrupole from copper tubing. For various reasons it was not successful, but I did learn quite a bit from it. This time I am using glass (rather than plastic) for the insulators on the sides of the structure, and (as mentioned above) epoxy rather than RTV. It also became clear while I was working with the earlier machine that there were some issues with the structure as it was designed and built; for example, the discharge confined itself to the bore only under fairly restricted circumstances. It should (we hope) be relatively easy to prevent arcs at the ends of the quad; we’ll see how well the obvious method works.

Here is a preliminary photo of one pair of electrodes at an early construction stage:

The small glass tubes that I’m using as insulators are not easy to see in this photo; they are melting-point capillaries that I acquired on eBay. (Hematocrit capillaries would also work.) If you look carefully, though, you can see the ABS spacer between the two copper tubes. It holds them 0.3 mm apart for initial assembly.

I have set up both pairs of tubes with dots of J-B Weld to hold the glass tubes in place, but I have not applied any more epoxy to them yet. I will probably do that before I attempt to assemble them into a quadrupole structure, as it seems much easier that way.

Meanwhile, I am also rebuilding the vacuum manifold on the roughing system that I’m using with these lasers; but that is peripheral to this effort, and is discussed on a different page.

(2011.0227)

I have applied transparent epoxy to one side of each pair. Here is a detail of one end:

(The epoxy is on the right half of each pair. It is not easy to see.)

(2011.0228, afternoon)

Both pairs now have epoxy down both sides of the capillary tubes. I hope to assemble them into a quadrupole structure this evening. I’m a bit concerned about drilling holes into the sides of the tubes, and I think perhaps that will wait until the quadrupole is built and strengthened. (I want to block off the ends of all four tubes, and have vacuum only down the bore of the laser.)

Once I have the quadrupole built, I expect to use TO-220 ceramic insulators as endpieces. I have not yet decided whether to attach end fittings directly to the endpieces or interpose short pieces of glass tubing, but at the moment I am leaning toward direct attach.

(Later, that evening)

I spaced the halves of the quadrupole 0.3 mm apart with a wider (~6 mm) strip of ABS plastic, and I have put the third set of glass tubes on with small dabs of J-B Weld. When the J-B has had time to cure, I will probably add epoxy as I did with the halves of the quadrupole, to strengthen the structure before I turn it upside down and apply the final set of tubes.

(2011.0308, early AM)

Some epoxy got between two of the tubes, and I have ordered a set of very thin circular sawblades, in hopes that I can remove some of it. That should let me operate this quadrupole. In the worst case I can still separate the two pairs of tubes, after which either I should be able to remove most of the epoxy that is in the wrong location and put the two pairs back together again, or just build another pair and reconstruct the quadrupole using it.

In the meanwhile, I have acquired a pair of end fittings, and I continue to think about relevant issues. One problem with this design, just as with the string-of-beads design in the first section on this page, is differential thermal expansion. I cannot permit either head to get too hot during operation.

(2011.0326, afternoon)

Here is a preliminary look at the ends of this quadrupole and the next one, with a rule so you can get a good sense of the overall sizes and the bore sizes:

I still have to fill the ends of all the tubes, and then cut small holes in the copper ones. (The stainless steel tubes already have laser-cut holes in them.)

(2011.0304, afternoon)

I have been interrupted by a conference, but I did manage to fill the ends of the tubes with epoxy. I still need to drill a hole in each of the copper tubes.

[[More as it transpires...]]




II: Stainless Steel (He-I2 and He-Ar)

My rationale here is that He-I2 operates at relatively high pressures (several Torr), emits several visible lines, and lases well in hollow-cathode structures that are a few mm across. It may even be possible to use mirrors from a HeNe laser, as the best line in the visible spectrum seems to be at 612.7 nm. That’s a bit more orange than the common 632.8 nm HeNe line, but still fairly close, and I hope that the mirrors will have good reflectivity (though it is possible that I will get beams from both ends of the device, assuming it lases at all).

If I can build a structure that is decently leakproof, I should be able to lase a helium-argon mixture in it; this version of the argon laser emits at 476.5 nm, a pleasant blue color. This, however, involves only a few milliTorr of argon, and adjustment is likely to be rather tweaky.

.-=^=-.-=^=-.-=^=-.-=^=-.-=^=-.-=^=-.-=^=-.-=^=-.-=^=-.-=^=-.-=^=-.

(Earlier note: 09 January, 2011, morning)

Jarrod Kinsey has pointed out one of Dr. Mark Csele’s pages, on which he describes running a small CW argon laser in pulsed mode. For some reason, he finds that he must apply remarkably long electrical pulses (more than 10 μsec) in order to get any lasing. I was using much shorter pulses with an earlier version of this laser, and did not succeed; this may help explain why.

.-=^=-.-=^=-.-=^=-.-=^=-.-=^=-.-=^=-.-=^=-.-=^=-.-=^=-.-=^=-.-=^=-.

I will be introducing the iodine (or argon) along with the helium, so I don’t want the wall material to sputter into the discharge or to react with iodine. Stainless steel seems to be a reasonable material in both regards, and it is readily available, so I have used it here. (See the photo at the end of the previous section.) I bought the tubes from The Electronic Goldmine; I think they cost all of 99 cents each, plus shipping. I got 6 of them, knowing that I would probably have trouble with at least one of the pairs (which, in fact, I did — I didn’t align the tubes well enough when I glued the insulators to them).

The Paschen relationship for helium suggests that the highest viable pressure for a structure that amounts to a tube with inside diameter of ~2.5 mm is a few Torr. That is somewhat low for helium-iodine, which has been found in at least one study to operate best at about 13 or 13.5 Torr; but this design may be viable anyway — at least one published study has used a comparable design at pressures of more than 10 Torr.

(2011.0326, afternoon)

See the end of the previous section for a view of the end of the stainless steel quadrupole. This quad has a problem: in one area there is epoxy between the tubes. It does not obstruct any of the bore, and if I am very lucky it will not have any effect. If I am not so lucky, a discharge will form across the epoxy surface instead of inside the bore; but we’ll cross that bridge if and when it burns under us. (If such a discharge did form, it would probably carbonize the epoxy and short-circuit the tube. It’s a good thing that these tubes are inexpensive.)

2011.0404, afternoon)

I have filled the ends of the tubes with epoxy. The next step is to put end plates on the structure. Then I need to add gas ports and either mirror mounts or Brewster-angle windows, or possibly plain AR-coated windows.

[More as it transpires...]




III: A More Formal Quadrupole Design

My initial feeling about the active structure of this laser is that I want to build something that has a more-or-less-circular bore. Obviously, the structures I built above, from round tubes, fail to meet this criterion. There are two obvious ways to achieve a circular bore, the first of which is to use a “string of beads” structure like the one on the next page, made of copper and brass for a copper vapor laser, or of stainless steel for a He-I2 or He-Ar laser. The other is tweakier and more difficult, but should have significantly better performance. It is achieved by clamping 4 rods together and drilling a 2.5-mm, 3-mm, or even 3.125-mm hole down the line where they join. I would have to do this in sections that are quite short, because I don’t have any special drills, but that isn’t too terrible. I debate whether to space the rods before or after I drill them, and I think I would most likely clamp them with the spacers in place. This effectively provides a centerpunch at the point where they come together, and gives me a bore that really is more or less a circle. (If I were to drill first and then space, the bore would be slightly off-round.)

Here is a rough diagram, not to scale. The yellow-green areas with green borders are stainless steel; the pale pinkish-orange circle is the bore; the blue lines are spacer/insulators. (I don’t have anything fancy, so unless I can find some nice glass or ceramic ribbon or maybe circuitboard that is about 0.3 mm thick, I will probably use ABS plastic from the hobby shop, and try to keep it well away from the discharge.) In addition to providing insulation, these are somewhat structural; but I strongly suspect that other external structural members will be required, as the spacings between the pieces must be maintained firmly. I have shown these in gray with black borders; they should be regarded as schematic rather than directly representative, as I don’t know what they would (or will) look like if I actually build a design of this type. Note that the only areas that are under vacuum (other than the bore, which is the salmon-colored circle in the middle) are the thin (0.3 mm) gaps that extend out from the bore to the spacers. They are indicated in mauve.

(02 January, 2011, early afternoon)

Frankly, this would be seriously nontrivial to construct, and I have diverged in order to build string-of-beads copper ion laser (see the next page) and the quadrupoles described earlier on this page. Despite being difficult, though, this design is attractive specifically because the bore is circular. That apparently enhances electron oscillation in the discharge (which gives the laser its desirable characteristics), and I suspect that it also has a greater tendency to produce a TEM00 mode structure.

[[More as it transpires.]]




References:

(02 January, 2011, afternoon, ff)

Here are some useful abstracts. In some cases I have found and read the article at the University library or online; in others I am probably going by what I have found in the abstract, as that was enough information to get me started.



Longitudinal hollow cathode copper ion laser: optimization of excitation and geometry (Proceedings Paper)

Diana B. Mihailova; Margarita G. Grozeva; Annemie Bogaerts; R. H. Gijbels; Nikola V. Sabotinov

Proceedings Vol. 5226, 12th International School on Quantum Electronics: Laser Physics and Applications, Peter A. Atanasov; Alexander A. Serafetinides; Ivan N. Kolev, Editors. (4 November 2003) pp 49-53

                Abstract

It is demonstrated experimentally that for copper ion lines laser excitation in a longitudinal hollow cathode discharge (HCD) an optimum current density (approximately 1 A/cm2) exists. Above this value a saturation and even decrease of the laser power is observed. Due to the axial inhomogeneity of the longitudinal discharge the possibility to increase the laser power by increasing the cathode length is also limited. To determine the proper cathode length for a sputtering copper ion laser, the axial current and spectral lines intensity distribution at conditions typical for laser oscillation are measured, showing a maximum at the anode ends of the cathode. Numerical modeling for exactly the same discharge conditions and tube design is also performed. The results are compared with the measured data and reasonable agreement is reached. Based on the results of the experiments and calculations we have demonstrated that the most efficient laser oscillation is achieved when the laser active volume comprises a series of anodes and cathodes, each cathode with a length of approximately 1 x 2 cm.

[Note: the “x” between 1 and 2 in that last sentence is actually a “divided by” sign. When I find the correct HTML code, I will restore it. Unfortunately, I am not entirely sure what they meant by it.]

The conclusions here put a severe limit on the performance I can expect to get from my “string of beads” sectional tube, with alternating anodes and cathodes; but at least it appears that such a tube can be a viable laser.



Comparison of Cu-II 781 nm Lasers Using High-Voltage Hollow-Cathode and Hollow-Anode-Cathode Discharges

K. A. Peard, Z. Donko, K. Rozsa, L. Szalai, and R. C. Tobin

IEEE Journal of Quantum Electronics, V30N9 (Sep 1994) pp 2157-2165

                Abstract

Voltage-current characteristics and the Cu-II 780.8 nm laser performances are described for a novel segmented hollow cathode and for three- and four-slot hollow-anode-cathode (HAC) tubes. Each of these operate[s] at a higher voltage and with higher slope resistance than a conventional hollow cathode, and [they] produce improved laser performance. The best laser performance is obtained with the segmented tube. The application of a longitudinal magnetic field raises the discharge voltage and enhances the laser performance for the segmented tube, and raises the voltage for the four-slot HAC tube. The magnetic field lowers the voltage and reduces the laser performance with the three-slot HAC tube. The voltage effects are attributed to the deflection of the fast electrons by the magnetic field strength, and represent experimental evidence for the oscillation of electrons in a hollow-cathode discharge.

Note: I have performed minor editing on this, adding a few commas and the items enclosed in square brackets. This is a crucially important paper, and is probably available on the Web in PDF format.



Dependence of the infrared output power of a hollow cathode Cu+ laser on He and Ar pressures

Eichler, H. J. Wittwer, W.

[Optisches Institut, Technische Universitä, Berlin, Germany]

Journal of Applied Physics, V51N1 (Jan 1980) pp 80-83

                Abstract

The dependence of the infrared laser power produced by a hollow cathode copper ion laser on the He and Ar pressure is reported. An argon partial pressure of 1 mbar is found to be optimum and nearly independent of the helium partial pressure and discharge current. The optimum He pressure is about 25 mbar, dependent on the discharge current. A simple rate equation model is given and compared to the experimental results.



Parametric study of a high-voltage hollow-cathode infrared copper-ion laser

K A Peard, K Rozsa and R C Tobin

Journal of Physics D: Applied Physics, V27N2 (?Feb? 1994) p 219

                Abstract

A new, segmented-electrode, high-voltage, hollow-cathode, Cu II laser, operating on the 780.8 and 782.5 nm transitions, is studied. Parametric measurements are presented for the discharge voltage, the spatially averaged copper-atom concentration and, for the two transitions, the small-signal gain coefficient, the laser threshold current and the laser output power. The dependence of the laser output power on the active length, the buffer-gas pressure and the transmission of the output coupler is modelled using equations for a low-loss, homogeneously broadened, standing-wave laser. For the 780.8 nm line, the lowest threshold current is 0.19 A for a 5 cm active length and a 0.1% transmission output coupler, and the maximum output power is 58 mW for a 3 A discharge current, a 7.5 cm active length and a 2% transmission output coupler. This performance exceeds that of conventional hollow-cathode lasers on the same transition, the improvement being attributed to the increased operating voltage, which gives a more efficient pumping discharge by simultaneously raising the ground-state copper-atom concentration (through the increased sputtering yield of ions bombarding the cathode) and increasing the efficiency of ionization.



Dependence of Gain and Laser Power for Cu-II 780.8-nm Transition on the Diameter of a Segmented Hollow Cathode Discharge

Szalai, L. Donko, Z. Rozsa, K. Tobin, R.C.

IEEE Journal of Quantum Electronics, V31N8 (Aug 1995) pp 1549 – 1553

                Abstract

The dependence of laser performance and discharge characteristics on the diameter of a segmented hollow cathode discharge for the Cu-II 780.8 nm transition is presented. This transition has a special importance since its upper level is common to potential CW VUV laser transitions (150-170 nm). Laser tubes with internal diameters of 2, 3, 4, and 5 mm were investigated. Decreasing the diameter resulted in an increased gain for a given current (up to 100 %/m in the 2-mm diameter, 5-cm-long tube at 1-A current). The highest output power was obtained from the large-diameter tubes (20 mW from a 5-cm-long, 5-mm-diameter tube at 2-A current, without optimizing the output coupler). This work is a part of a series of investigations aimed at the optimization of the segmented hollow cathode discharge, which has already been found to be the most efficient type of discharge for cathode sputtered metal ion lasers.

[Note: This is a very helpful paper. It may be available on the Web, and is well worth reading.]








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This work is supported by
the Joss Research Institute
19 Main Street
Laurel MD 20707-4303 USA





Contact Information:

My email address is a@b.com, where a is my first name (just jon, only 3 letters, no “h”), and b is joss.

My phone number is +1 240 604 4495.

Last modified: Thu Jan 20 13:06:36 EST 2011

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I am a Researcher of the Joss Research Institute. I work primarily on lasers and ceramics, with occasional excursions into other areas.

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