I’m always reporting the progress of the Large Hadron Collider so it is good to have a look at the Tevatron for a change. There is an excellent facebook page for the Fermilab collider where they report on the data collected during each store. Over the last week they had a particularly good run and posted this plot.
They collected 78/pb in one week with typical fills of 15 to 20 hours providing as much as 11/pb each time. Given the peak luminosity of 430/ub/s (which by the way was a new peak luminosity record for the Tevatron), the Hübner Factor is about H = 78000/430*3.6*24*7 = 0.3. This is an optimal value for the factor given excellent running conditions with minimal stoppages. The longer term value is presumably a bit less than this.
The turnround times on a good day last week appear to be about 5 hours between fills. In fact they can refill in as little as one hour when all goes well. This is very good compared with the Large Hadron Collider whose best turnround times have been about 2.5 hours.
Given that a Hübner Factor of 0.3 is exceptionally good at the Tevatron you may wonder how the Large Hadron Collider with its longer turnround times could do better. The answer is that it has much better luminosity halflife times. At the Tevatron half the luminosity is lost every 6 to 7 hours (this is my estimate from above plot). The LHC has been producing halflives of about three times as long. During a run of 19 hours at the Tevatron the Luminosity could drop to as little as 15% of its peak while the LHC would still be at 50%. This is enough to double the Hübner Factor for the LHC.
What are the reasons for the longer lifetime at the LHC? Good question! I can only speculate that it is a combination of factors including the fact that the LHC uses two beam pipes compared with the single beam pipe design of the Tevatron. Keeping the bunches further apart outside the collision points should help stability. The LHC may also have better damping systems to keep the beam focused. It’s larger size gives more scope for correcting the blow-up of emittance. The higher energy also helps I think. Perhaps there are other reasons. The LHC is just a much more up-to-date machine.
For now though, it is the Tevatron that is steadily collecting data to add to its already impressive store. It’s a glorious run to the end of its long and fruitful life in September.