Jim,
We don't have a spec on the beam spot size stability at the radiator.
It is a good question, but I don't think we need one. I would argue
that as follows. Our max. emittance spec (together with the virtual
spot size at the radiator) gives an upper bound on the radiator spot
size of 1.6 mm rms, and we hear that the machine is capable of
something like a factor 2 better than that in x and a factor 5 in y.
We get that as a result of our existing specs and fundamental machine
constraints. That gives a range 5 hours - 50 hours per spot move. If
the spot size varies within that range, it will not have direct impact
on our ability to run GlueX.
- We will monitor the coherent edge shape vs time.
- We will keep a radiation dose map of the crystal, in Coulombs
seen at each spot.
- When the spectrum degrades, we will mark the spot on our crystal
map and move.
- There will be a keep-out zone around each used spot that varies
inversely with the dose at that spot.
This algorithm works regardless of the spot size (within the range
above) even if it varies all over that range during a run. I doubt
that will happen, but we need an answer to the question of how much it
can vary without affecting us. I claim that we only care about the
range stated above, and do not need an additional spec on the spot size
stability at the crystal. That said, it is hard to think of something
that would modulate the spot size at the crystal without throwing
something else that is more important to us, eg. the virtual spot size
at the collimator. That is where our sensitivity lies.
-Richard Jones
Jim Stewart wrote:
48ED2E64.4010308@jlab.org">Thanks for
the quick reply Richard.
I think I wishfully thought it was 14 days per beam move.
Do you know if anyone talked to Jay and Yves about how stable the beam
spot size will be?
Under these conditions we will need a good control of the spot size on
the diamond. We may
also want to shape the beam to fit good areas on the crystal.
Jim
|