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Improved negative glow lamp design
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The instability in the operation of the air glow lamp presented earlier is caused primarily by plasma-electrode interactions. So, the solution to that problem consists in sheathing the electrodes with a thin glass tube so as to prevent cathodic sputtering and chemical reactions with active species produced in the discharge. The resulting lamp assembly is shown above, before pumping and sealing. To protect the electrodes I used thin glass capillaries which I sealed individually while keeping them at around 400-500 C. The resulting electrodes thus consist of a central metallic conductor surrounded by a closed air volume at a few hundred millibars of pressure.
Such design prevents the capillary electrodes from "blowing up" when I seal them in the main glass tube. Moreover, the low-pressure air volume contained inside the capillaries become ionized when high voltage is applied across the lamp terminals. This helps the electrode's operation as this ionized gas transfers the central conductor's potential right to the inner capillary surface. This thus increases the electrode capacitance, which ensures an effective power coupling to the discharge in the main lamp volume.
It is obvious that the glow lamp is now a capacitively-coupled discharge tube. As a result it cannot be run on DC power, a potential of alternative polarity is needed for a current to flow between the electrodes. In the next pictures I drove a plasma in air (see there) and water vapor (see there) using a small battery-operated HV-AC inverter circuit. This new lamp concept is found to work well with reactive molecular fills, but it certainly cannot be run from the mains like standard neon glow lamps.
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The instability in the operation of the air glow lamp presented [url=https://trad-lighting.net/gallery/displayimage.php?pid=1145]earlier[/url] is caused primarily by plasma-electrode interactions. So, the solution to that problem consists in sheathing the electrodes with a thin glass tube so as to prevent cathodic sputtering and chemical reactions with active species produced in the discharge. The resulting lamp assembly is shown above, before pumping and sealing. To protect the electrodes I used thin glass capillaries which I sealed individually while keeping them at around 400-500 C. The resulting electrodes thus consist of a central metallic conductor surrounded by a closed air volume at a few hundred millibars of pressure.
Such design prevents the capillary electrodes from "blowing up" when I seal them in the main glass tube. Moreover, the low-pressure air volume contained inside the capillaries become ionized when high voltage is applied across the lamp terminals. This helps the electrode's operation as this ionized gas transfers the central conductor's potential right to the inner capillary surface. This thus increases the electrode capacitance, which ensures an effective power coupling to the discharge in the main lamp volume.
It is obvious that the glow lamp is now a capacitively-coupled discharge tube. As a result it cannot be run on DC power, a potential of alternative polarity is needed for a current to flow between the electrodes. In the next pictures I drove a plasma in air (see [url=https://trad-lighting.net/gallery/displayimage.php?pid=1143]there[/url]) and water vapor (see [url=https://trad-lighting.net/gallery/displayimage.php?pid=1142]there[/url]) using a small battery-operated HV-AC inverter circuit. This new lamp concept is found to work well with reactive molecular fills, but it certainly cannot be run from the mains like standard neon glow lamps.
Keywords: Lamps](albums/userpics/10005/10/normal_DSCF0228m.jpg "Click to view full size image
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Improved negative glow lamp design
The instability in the operation of the air glow lamp presented [url=https://trad-lighting.net/gallery/displayimage.php?pid=1145]earlier[/url] is caused primarily by plasma-electrode interactions. So, the solution to that problem consists in sheathing the electrodes with a thin glass tube so as to prevent cathodic sputtering and chemical reactions with active species produced in the discharge. The resulting lamp assembly is shown above, before pumping and sealing. To protect the electrodes I used thin glass capillaries which I sealed individually while keeping them at around 400-500 C. The resulting electrodes thus consist of a central metallic conductor surrounded by a closed air volume at a few hundred millibars of pressure.
Such design prevents the capillary electrodes from \"blowing up\" when I seal them in the main glass tube. Moreover, the low-pressure air volume contained inside the capillaries become ionized when high voltage is applied across the lamp terminals. This helps the electrode's operation as this ionized gas transfers the central conductor's potential right to the inner capillary surface. This thus increases the electrode capacitance, which ensures an effective power coupling to the discharge in the main lamp volume.
It is obvious that the glow lamp is now a capacitively-coupled discharge tube. As a result it cannot be run on DC power, a potential of alternative polarity is needed for a current to flow between the electrodes. In the next pictures I drove a plasma in air (see [url=https://trad-lighting.net/gallery/displayimage.php?pid=1143]there[/url]) and water vapor (see [url=https://trad-lighting.net/gallery/displayimage.php?pid=1142]there[/url]) using a small battery-operated HV-AC inverter circuit. This new lamp concept is found to work well with reactive molecular fills, but it certainly cannot be run from the mains like standard neon glow lamps.
Keywords: Lamps")
I accidently drove it at way too high current (new batteries in the HF-AC inverter circuit) to the point that one of the electrodes developed a hot spot and a copious amount of sodium was released from that point (way more than shown there). The released sodium cleaned up the lamp's gas fill entirely, no discharge can be struck between the electrodes anymore. The hot spot at the electrode (the blackened one) was so bright that I was sure the discharge had punched a hole through the thin glass sheath. However, a closer inspection reveals that the capillary's mechanical integrity is surprisingly unaffected (see below).
There is some heavy blackening inside the thin glass tube, which I think originates mostly from metals coming from the inner conductor. Otherwise there is no sign of thermal damage to the capillary, not a single crack, deformation, or puncture. This is truly remarkable given how intense that sodium hotspot was. All in all I think it's safe to say that the design is quite robust, but the sodium release from the electrodes must be kept in check as this can definitely getter a low-pressure molecular gas fill. Maybe that issue can be resolved with the use of quartz glass instead of soda lime silicate, which would certainly enable a brighter discharge operation at a higher current (and provide a better access to the discharge's UV emission too). In any case I can't wait to test this lamp concept with an argon fill, hopefully the electrode sheath E-field is lower and the issue of sodium release from ion bombardment mitigated...