FF-2B Recovers from Minor AccidentMarch 17, 2020
While we would love to cheer our readers with some exciting news form the lab, this past month we have been held up by a minor accident and our (mostly successful) efforts to recover from it. On Jan. 20, at 4:00 AM, on a holiday, our lab suffered a brief power outage. Although all our computers have back-up power, our vacuum pumps did not (our mistake, which is now remedied). When power came back on, the main scroll pump did not restart, needing a manual reset. Air started leaking slowly back through the stopped scroll pump into the vacuum chamber, even though some other valves were closed due to the power outage.
By the time we arrived at the lab the following morning, there was nearly one third of an atmosphere of pressure inside the chamber. While we pumped the gas out rapidly, the water vapor in the gas adsorbs (clings) onto the surface of all the metal parts. We needed to get rid of the water because water is mostly composed of oxygen. If we fired a shot with oxygen in the chamber, our beryllium electrodes would oxidize. That would be bad, as beryllium oxide is an electrical insulator. In the second shot, the oxide would be vaporized by the current, eroding our electrodes and creating lots of dust.
So, we started a bake-out, heating the chamber and the vacuum system up to 100 ℃ while maintaining a vacuum to pump out the water. Unfortunately, we then encountered more problems created by the initial accident. First, we found that after a few days, we could no longer reach our goal of several-microtorr pressures (around a ten millionth of an atmosphere). We suspected a leak in our turbo-pump, which pumps down to low pressures. Indeed, we found one, caused by the reverse flow of air during the accident. However, once the pump was repaired, the pressure still did not fall below a few millitorr.
After further tests and consultations with manufacturers of our equipment, we found that both our pressure gauges had failed and could not report pressures below a few millitorr. This had misled us, since the two gauges were agreeing with each other. But both had contamination from water vapor. We then repaired one of the gauges.
But with gauges and pumps working we discovered a small but significant leak in the flange which attached our new upper vacuum chamber to the anode plate on top of the device. Mechanical stress during the bake-out evidently caused the seal to fail. We now see that it needs re-design to be more robust. We’re doing that re-design now and hope to fix the leak and be back to firing by the end of March.