Given the remarkable longevity of the Voyagers, what I'm surprised about is that there hasn't been much discussion about their componentry and why they've been so reliable. For example I've seen nothing written about the engineering involved in Voyagers' TWTs and why they have been so reliable.
For instance, what is the cathode material, barium, strontium, thorium oxide, etc. used in these TWTs? Was its selection criteria based on emitters with the lowest work function/highest emission at the lowest temperature with preservation of the heater life foremost in mind, and or was it based on oxides with highest ruggedness—least affected by cathode poisoning, etc. Discussions about Richardson's laws and cathode emitters is something I almost never come across these days let alone how they've played a role in the engineering of Voyagers' longevity.
Whilst component manufacturers consider these matters, terrestrial users generally don't, we just reach for replacement parts when components fail. Perhaps I'm just not reading the right material but given the remarkable performance of these spacecraft, I'm surprised we're not focusing on the science and engineering that's made that all possible.
No doubt those who're involved in space engineering are focused on these issues but it seems to me not much information has filtered down to even people like me who have some limited knowledge of the technology let alone the general science-reading public.
Using the Voyagers' history and notoriety would be an excellent way to interest students in the physics of TWTs not to mention the material science and the engineering used in their design and manufacture.
When one considers it, there's a lot of fascinating science and engineering involved in making this 'relic' from the vacuum tube era function and keeping it so.
I think part of this might be because it is still considered proprietary information. Kind of crazy, but there is really only one company in the US that makes space qualified TWTs which is Stellant systems (formerly L3, formerly Hughes microwave) who made the Voyager TWTs as well.
You're likely right about the proprietary nature of such manufacturing but perhaps I'm reading too much into this. As I mentioned in an earlier post the emission in the CRT of my 43-year-old Sharp TV is still OK—at least as far as the quality of the image is concerned.
The technology used in the Wehnelt cylinder in my TV's CRT is of a similar vintage to Voyager (I'm pretty sure the TV was manufactured around 1979), and given the millions of CRTs made to similar a quality around that time it's likely that manufacturering techniques and reliability figures were widely known throughout the industry by then.
Anecdotally, I've noticed that CRTs made from the early '70s had much better longevities than their earlier '50s counterparts, whether this was because the formulation of cathode emitters had changed or manufacturing techniques had improved or both is an open question.
Of course, such comparisons are tenuous but that's all I've got to go on, for starters, the Wehnelt in the Voyager's TWT would have been deigned to carry more current. Perhaps NASA has some spare TWTs that one day someone will reverse engineer and we'll know for sure.
Nevertheless, it seems to me that knowledge about component reliability is critical to NASA, so it's likely the answer already exists somewhere in the depths of NASA's archives.
Another question I'd like answered, do the Voyagers cycle down or turn off the heater during TX downtime? Turning it off risks metal/cycling fatigue, leaving it on risks reducing the cathode life—or even poisoning it from stray ions.
A final point, presumably the TWT's output has dropped over the decades, how is this monitored and do we know the percentage drop in power output (from the TWT not the Pu power source)?
The fact the TWT is still working seems quite remarkable—but then perhaps I shouldn't be overly surprised, one of my TV sets, a 23" Sharp, is 43 years old and the CRT still works well (and it's switched on for several hours every day).
I suspect that now the heaters are turned off during Tx downtime, but I think this is probably because they Pu power is much lower and so thy want to conserve as much power as possible. Even if they did have more power, Voyager only makes contact once per day so I think the increased thermal cycling is not much compared to reduction in cathode life so they probably go this route anyway. I do know that other missions I have worked on that have much more frequent contacts, we do keep the Tx heaters on so there is probably some number of cycles where one becomes more dominant.
> A final point, presumably the TWT's output has dropped over the decades, how is this monitored and do we know the percentage drop in power output (from the TWT not the Pu power source)?
This is telemetry from the TWTAs on the helix current and the anode voltage. Along with the DC power draw of the TWTAs I think you can interpret some of what the power output is and how it changed over life. Some of this might be curve fitting to ground-based life tests so there could be some estimation. The ground is also certainly measuring the received power so they can estimate power output from the spacecraft from that as well.
> The fact the TWT is still working seems quite remarkable
You're not the only one. From this article [0] a JPL engineer says "Nobody can explain why the Voyager TWT is still working".
[0] https://spinoff.nasa.gov/Traveling-Wave-Tubes-Travel-Far