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I was looking today through pdg table of collider parameters and I saw that some turn-around time for LHC is $\approx 200$ min. I tried to search on it, but everywhere it is kind of assumed that you already know what it is. In pdg review on accelerator physics, it was barely mentioned in Tevatron description

The route to high integrated luminosity in the Tevatron was governed by the antiproton production rate, the turn-around time to produce another store, and the resulting optimization of store time

Could anyone explain to me what does this turn-around time means?

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From the paper that estimates the mentioned number:

"In the following we define the turnaround time of an accelerator storage ring as the time between the end of one and the start of the next physics run. For an accelerator storage ring the measurement of the turnaround time starts with the beam at top energy of the accelerator and comprises the ramp down of the magnet system to the injection energy settings after the beam
extraction, the time required for setting up the machine for the next injection, the time required for injecting new beams into the machine, the time required for the beam acceleration (ramp), the optics transition for the physics run (squeeze) and the time required for adjusting the beam conditions so that the detectors can start again data taking."

Source: https://cds.cern.ch/record/1133125/files/p34.pdf

Essentially, it's the amount of time you need to stop the beam and start it again. The reason this is necessary is because, as particles collide, the intensity of the beam starts decreasing; at some point, the intensity (and therefore the collision rate) drops to the point where it would be more efficient to just stop, dump the rest, and refill the beam, rather than continue on collecting data at a lower rate. The precise point at which this happens is determined by how much time you lose when you have to stop and restart (i.e. the turn-around time).

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    $\begingroup$ Might be worth saying a few words about why you would bother if there is still beam in the accelerator: because every event observed decreases the luminosity of the beam by removing particles, so over time the event rate drops. The decision about when to dump and re-fill depends on the turn-around time. $\endgroup$ – dmckee Feb 11 '18 at 0:02
  • $\begingroup$ @dmckee Added in edit. $\endgroup$ – probably_someone Feb 11 '18 at 0:10

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