Glass Blowing for Vacuum Devices

Panel Neon

This is not strictly a panel neon, because at the time of writing, I have no way of filling it with neon gas. It can only be air filled.

I start with a length of 8mm diameter thin wall tube and an 80mm length of 0.5mm diameter nickel/iron wire (Ni48/Fe52). The wire is folded double. I bend the wire until the lengths are parallel and 1 to 2 mm apart.

I then introduce the cut end of the tube to the flame slowly in a horizontal position. I rotate the tube with just the last 10mm in the flame. When it reaches the consistency of Plasticine, I remove it from the flame and squash it slightly between two pieces of graphite. The end should not be completely closed. It is best to proceeded immediately to the next step without allowing the end of the tube to cool down. If it is allowed to cool, great caution must be exercised when re-heating it to avoid cracking.

This is how the end of the tube looks after squashing it. The purpose of this step is to keep the lead-in wires centred while sealing them in.

I then hold the tube horizontally with the squashed end horizontal. The free ends of the Ni/Fe wire are inserted so that they pass about 22mm into the tube. I hold the wire with a pair of pliers and heat the end of the tube until the wires stick to the glass. When this happens, I turn the gas up high and keep turning the tube over in the flame until the end is red hot. I then squash the end of the tube flat between two flat pieces of graphite. I then immediately return it to the flame and heat it red hot again.

I try to ensure that at least the last 2mm of tube fuses together completely. When using Ni/Fe wire, the formation of bubbles is a good indication that the glass is hot enough. No bubbles indicates a leaky seal. I then anneal the seal carefully and then insulate it with ceramic wool. I allow it to cool slowly for at least ten minutes.

For the next step, something heat proof is required, to hold the device during the sealing operation.

The tube is then attached to the pump. Only a rotary pump is required. The pressure needs to be reduced to about 10mBar. The tube can be powered on the pump to assess the appearance of the discharge at different pressures. When the tube is at the desired pressure, I valve it off and stop the pump.

I used the blow torch to seal off the tube. The tube must first be heated gently by holding the flame near it. I then heat a short section just beyond the wires as evenly as possible until it starts to collapse. The tube collapses much more suddenly than usual because of the pressure acting on it. I try to get it to collapse evenly, but it is very difficult. When it has fully collapsed and fused, I draw the tube away until it detaches. I then immediately anneal it in the sooty flame and then wrap the whole device in ceramic wool to cool for 15 minutes. Failure to anneal the seals at this stage has been the source of many failures.

Unfortunately, because it is filled with low pressure air rather than neon gas, it requires around 1000V to strike. This means that it can not be operated directly from the mains supply as a panel neon can. If gas filling equipment were available it would be fun to try different gas fillings. Flammable gas must never be used though.

This tube was filled with air at 10mBar. It is 40mm in length. It is being powered by my trusty 0 to 10kV high frequency power supply.