Nixie Sputtering at Pulsed Currents vs Constant Current

[Jeff Walton]

I'm curious if there is any definitive data which shows the relationship between the amount of sputtering of a pulsed current vs a constant current in a nixie.  Example:  A tube runs at 3mA constant current vs a tube run at 50% duty cycle using a 6mA current pulse.  In theory, depending on the tube response time, the apparent brightness might be similar.  If things were linear, there would be no difference in the amount of sputtering that would be generated by equal time periods.  If things were not linear and maybe the effect is exponential, maybe in the example above, the sputtering could be many times greater for the pulsed example in the same time period.  Obviously, this impacts the tube life, cathode poisoning and possibly other factors.

Is there any factual data that can shed some light (no pun) on this?  It would be useful in discussions about the pros and cons of drive method.  I'm guessing that there must have been some study during the production years of Nixies, although it may have pertained more to the density of sputtering from a fixed area at different current rates with different materials as opposed to a discussion of pulsed drive (which had not yet come of age).

[Dekatron42]

I only know that Burroughs mentions the B5750/B5755 and B5850/5755 in the Application note "Multiplexed operation of NIXIE tubes" #N101 at the end of page 3 where they talk about somewhat higher gas pressure to reduce sputtering and to achieve other things to get these Nixies to work better while being multiplexed. You can find that application note here: http://worldpowersystems.com/archives/Burroughs/

/Martin

[marta_kson]

The Weston book Cold Cathode Glow Discharge Tubes mentions some tests of this. The findings are that sputtering is exponential. At constant current increases with current raised to the power of 2.5 and 1.5 at 50% duty if I remember correctly.
As the perceived brightness also drops slower than duty indicates there would be a great benefit to use pulsed current. Muxed drive should then be an advantage if the peak current is kept down.

Maybe Dalibor could do some testing? That might have been of great interest for all of us in this group.

[gregebert]

One of my 6091's had darkened in the front, presumably from sputtering. After running for several hours in a different position, and at 3X rated current, the substance that was deposited on the inside front of the glass has dispersed. 

I dont know if it's cathode material, or mercury condensate

This particular tube was displaying the same numeral for almost 4 years (no depoisoning routine). Other numerals showed slight degradation but appear to be clearing-up.