Updated 20-XI-2011
Sodium Vapour
Spectral Properties
Lamp Technology
Vapour Pressure
Current Density
Gas Filling
Sodium Migration
Failure Mechanisms
Lamp Designs
Low Voltage Style
     Compton's Lamp
     Philora DC
     GE NA-9
High Voltage Style
     Philora AC
     SO/H U-Tube
     SOI/H Integral
     SOX/H Coated
     SLI/H Linear
Self-Starting Style
     Double Ended
     Single Ended
Control Gear
Series Operation
Autoleak Reactance
Ballast-Ignitor System
High Frequency Electronic

The Rare Gas Filling

Penning Gases
At room temperature, sodium is solid and there is insufficient vapour present for the lamp to be started at a practical voltage. Filling the discharge tube with a rare gas appreciably lowers this starting voltage - and in case a mixture of two or more rare gases is employed, the striking voltage can be reduced further. For many rare gas mixtures, there is a particular ratio where the electrical ionisation potential of the mixture is lower than that of either of the individual gases. Such compositions are known as Penning mixtures, named after the Dutch Philips engineer who made this discovery. Figure S11 reveals how the striking voltage of a 180W SOX lamp varies with the percentage of argon in the neon fill gas.
Figure S11 - The Ne:Ar Penning Mixture

Argon Clean-Up
The diagram reveals that for the lowest striking voltage, lamps should be made with around 0.25% argon in neon. Unfortunately, in practice it is not possible to work in this range because the special borate glass used for the discharge tube has a certain affinity for argon gas. During lamp life it adsorbs or 'cleans up' a certain amount of argon. This must be compensated for by filling the lamps with a small excess of argon, to ensure that striking voltages are maintained sufficiently low during the entire lamp life. It is customary to fill most lamps with 1% argon, and 1.5% for the 18W and 10W versions, where ignition is rather more critical because they are designed for operation on ordinary choke-type ballasts without any ignition aid.

Incidentally, a knowledge of this argon clean-up phenomenon is important when attempting to light very old sodium lamps that have not been used for many years. During the period when the lamp is warming up, the rate of argon clean-up is relatively fast because it is during this period that the gas filling is being most heavily ionised. High velocity argon ions impinge the glass wall with greater force and the chance of their adsorption is rather high. If such a lamp is switched off soon after its first ignition, there is a horrible possibility that sufficient argon will have been adsorbed that the lamp's striking voltage becomes so high that it can never be started again. However the glass will liberate the majority of the adsorbed argon over a period of several hours if maintained at its typical high operating temperature. This phenomenon only affects older generations of lamps made before the mid 1960s when different glass compositions were used. Furthermore, lamps which have been in storage for a very long period of time are particularly prone to show accelerated argon adsorption the first time they are switched on. It is therefore advisable when lighting such lamps to allow them to run up to full operating temperature for several hours, before switching off. Once the glass has been conditioned by this treatment, the lamp will be safe to use again with shorter switching cycles on future occasions provided it is not left without lighting for many years again.

Lamp Efficacy Considerations
Lamp efficacy is optimum when a buffer gas having a low molecular weight is employed. Helium is the lowest molecular weight gas that is compatible with such a lamp, however it cannot be used because it diffuses out through the glass relatively quickly. Neon is the next best choice, but it must be combined with a small amount of the heavier argon to achieve the aforementioned Penning mixture and enable the discharge to ignite at a suitably low voltage.

Efficacy is also best when the gas filling pressure is low. This is because the fewer atoms of rare gas that are in the tube, the less likely it is that light-generating sodium ions will knock into these and lose their energy in an atomic collision. Such kind of energy losses create heat and result in lower lamp efficacy. The lowest possible pressure of neon-argon filling might at once seem the most desirable choice - but as always in lamp technology, it is not quite so simple because gas pressure has a strong impact on life.

Lamp Life Considerations
The electrical current is supplied to the discharge tube via a pair electrodes, coated with a trimetallic oxide mixture of electron-emissive materials. This emitter coating enables the current to be injected into the discharge space with the minimum energy loss, again to yield good lamp efficacy. The electrodes necessarily run hot so as to achieve enhanced thermionic emission, and during life the emitter coating is gradually consumed by evaporation and sputtering effects. After it has gone, the lamp will begin to rectify, drawing large currents that will either cause failure of the built-in fuses inside the cap, or destruction of the electrode assembly in case no fuses are fitted.

It is fairly simple to prolong lamp life by slowing the rate of emitter loss from the electrodes simply by increasing the pressure of the rare gas filling. The increased concentration of gas atoms in the vicinity of the electrodes helps to prevent atoms of emitter material from leaving the electrode surface. Gases having a high molecular weight are the most effective for this task. Converseley as explained in the previous section, for reasons of high lamp efficacy it is more desirable to employ a fill gas having a low molecular weight.

So in practice we remain confined to the low molecular weight gases, and must consider increases in their fill pressure as a mechanism of extending lamp life. This however is also counter-productive to the achievement of a high lamp efficacy, because the increased concentration of rare gas atoms in the discharge space also leads to more frequent elastic and inelastic collisions with sodium ions. These collisions take away their energy that would otherwise have been available for conversion into light. A compromise must therefore be reached between efficacy and lifetime. Bearing in mind the fact that other failure mechanisms are different for the high wattage and low wattage lamp types, which result in differing lives for the two variants, different filling pressures have been established as 4-8 torr for the high wattage lamps, and 8-12 torr for the low watts range.

Advanced Gas Mixtures
In the late 1970s a novel Neon-Argon-Xenon mixture was introduced by Thorn Lighting in its Linear Sodium range. Xenon, being a heavier atom, naturally retards the evaporation rate of the electrodes and their emitter coating. This allowed gas fill pressures to be reduced slightly for a given lamp life - and as explained earlier, reduced pressure is desirable for increasing lamp efficacy. However it is necessary to keep the concentration of xenon as low as possible to avoid it becoming involved in a collision with a sodium ion, leading to some energy loss in the discharge.

In practice it was found that there exists an optimum concentration of 99% Neon, 0.8% Argon, 0.2% Xenon which allows a small reduction in fill pressure and delivers a small but still significant improvement in both lamp efficacy and lifetime. It was employed by Thorn in the 140W and 200W High Output Linear Sodium lamps only, where the performance boost it delivered was slightly more significant than in the circular cross-section discharge tubes employed in the normal SOX range. The difference in the characteristic rare-gas discharge colour before the lamp has run up is depicted in Figure S12.
Fig. S12 - Ne:Ar (above), Ne:Ar:Xe (below)