i am not sure if you will find a larger pump. The answer is very probably to design a multi-stage system combining the high-vacuum-pump and a roughing pump(s).
Planning on doing that, but I'm needing somewhere in the neighborhood of 70,000 l/s pumping speed for He. This will take a number of the Varian pumps. Since I can't use a diffusion pump, due to possibility of backstreaming, I'm stuck with turbos.
My suggestion would be to try the diffusion pump route, but add a large liquid nitrogen or Polycold cryo-trap between the pump and chamber to catch any backstreaming oil, and run the pump on Santovac.
I proposed that, we have 14 36 inch DP's without traps and 4 with and they do not want to take the chance that they might deposit oil from the trapped DP's so they want turbo pumps backed by roots blowers, we have the backing system already. I was just looking at a better way to pump the 70,000 l/s of helium we expect to need.
One word of caution: Many years ago I worked on the neutral beam systems for one of the large fusion projects (DIII). We were pumping hydrogen, so depended on liquid helium cryopanels, but we also had large turbos backed by blowers and mechanical pumps with liquid nitrogen trap between pumps and chamber. When the beamlines were dismantled for upgrade to longer-pulse operation, my technician took some of the innards out for analysis, and they were soaked in oil. Turns out the oil from the mechancial pump worked it's way along the pipe walls through the pumps into the chamber. So, if you go to turbos, I would suggest you consider using oil-free mechanical pumps or, as alternative, a ten-foot or so PVC vertical pipe between turbo and blower -- oil doesn't adhere well to PVC, and won't be able to climb that high along the pipe walls.
We've had our problems with backstreaming of oil in the past, it usually happens when the backing pumps are turned off and the system is left under vacuum. The oil vapor will move upstream and condense in cooler places. We have a series of roots blower backed by two stokes 412s with about 40 feet of vertical pipe that has been painted on the inside. I've had the blowers apart and never seen any oil in them. When we shut down the system an automatic purge valve opens to break the vacuum and prevent the backstreaming. I have a similar system but the oil I had in the blowers came from the blower case when the seals failed in the blower. I just need some big big turbo pumps.
