From: Brendan Fox (fox@drax.Colorado.EDU)
Date: Mon Jun 30 2003 - 18:38:40 EDT
Hi y'all,
Here is my summary of today's (Monday, June 30th) RHIC Spin Collaboration
Meeting.
- brendan
=========================================================================
(1) Did we reach saturation of luminosity and polarization during the pp
running in Run-03 where saturation is being defined relative to what
could be achieved with current understanding and hardware?
- Luminosity
No, the bunch currents were 0.7e11; we could probably have
reached 1.0e11 which, for 50 bunches, reaches the 1e13 limit
on total current (sum of both rings) which arises from the vaccuum
pressure rise and subsequent severe background issues at the
experiments. Two more weeks of running without any concerns of
polarization would have been helpful on this matter.
- Polarization
Yes.
(2) What are critical machine issues for Au-Au?
- bunch merging in the booster to bring up the Au intensity to 1e9
* this involves stacking bunches in the booster and requires
improvements to the rF system. These improvements are underway
during the current shutdown.
* by stacking bunches, we draw more ions from the tandem and there
is a worry about the foils in the source. This is likely not an
issue for Run-4 since you have two tandems, so, if one goes out,
you can use the other while working on the down one.
* since the bunches will be stacked longitudinally, it is important to
control the longitudinal emittance; otherwise, one cannot stack
as many "initial" bunches into a single "final" bunch. This matter
needs to be studied.
- storage rF
* this was improved during the dA run by eliminating some noise in the
the system.
* during the dA running, the three common mode cavities was not used
since the two species do not have the same requirements. So,
these cavities will provide additional power per beam in Run-04.
However, the tuning of the common mode cavities is trickier because
both beams are affected by the same cavities.
* during Run-3, especially near the end of the pp run, the storage rF
tripped off often. These trips were likely a result of having lost
one of the windows in the rF system. All of the other windows
survived so it is likely that this window was just the weakling.
So, this should not be as much of problem in Run-04.
* in the past, the storage rF cavities have also served as Landau
cavities during the ramp. This year, dedicated Landau cavities are
being installed.
* as a goal, they aim to squeeze the longitudinal profiles so that
60 and 65% of the collision vertices are usable at PHENIX.
- stochastic cooling
* this idea addresses issues of growth of the bunch during a store.
* there is a plan to install a kicker & pickup at 12:00 and 4:00 during
this shutdown so that the use of schochastic cooling can be studied
during Run-04.
- the NEG coating and addressing the pressure rise problem
* focusing on the areas most affected by the pressure rise problem, the
plan is to coat 60m of approximately 700m of warm section in RHIC
with a NEG coating during this summer. This coating will serve as a
getter which will sucks up residual gas and holds it better.
* some possible side effects: coating could affect the impedance of the
machine, might affect the pumping (though that should be a positive
not negative effect), and one needs to think about how the hydrogen
jet target and the coating co-exist
- beam lifetime
Unlike in p-p, Au-Au is not as affected by the beam-beam interaction
because its Z/A is 0.4 that of p. The issue for Au+Au is more one of
intrabeam interaction. From simultuations, the lifetime is expected
to be ~1.5 hours for Au-Au with 1e9 ions per bunch.
(3) What are critical machine issues for p-p?
- polarization from the AGS
* for Run-4, the warm helical snake will be in place. With this, they
expect to raise the AGS polarization from ~0.4 to ~0.5 for the same
bunch emittance of ~10pi seen in Run-3. To achieve this emittance
without massively scrapping the bunches in the booster as was needed
during Run-3, thinner stripping foils are being installed in the
booster.
- polarization in RHIC
* going from injection to store, we typically lost ~25% of the
polarization (i.e. 40%->30%).
It is believed that this loss occurred during the beta* squeeze as
opposed to along the energy ramp. This opinion is based on a few
ramps in which there was no beta* squeeze and no loss of
polarization. The data need to be checked to see if the conclusion
is completely true or whether there was also loss on the ramp.
* tune control and, maybe, orbit control
The tune control was not reliably operational until near the end of
the run. When it was, it controlled the tune quite well during the
ramp. Having such control will certainly improve the polarization
retention. It is expected to be used regularly in Run-4.
* tune to a new working point
Putting the working point between the 3/14 (0.21) and 1/4 (0.25) spin
resonances results in a more limited tune space than is available
at ~0.195. So, tuning to a lower working point may benefit
polarization retention. However, such a point where the machine
is stable has to be identified first. Presently, they are working on
simulations to locate this point. Questions which arose were:
o can you simulate the beam-beam tune shift effects accurately?
Absolutely, maybe not. But, relatively, is the goal. Learning
that one working point is better relative to another is sufficient
at this time to make progress on this matter.
o would having different working points per species be an issues?
Probably not.
o would you also tune Au-Au to the new working point if this point
is found?
If there was a big gain, probably yes. However, Au-Au is not
limited by beam-beam interactions, so it is unlikely that there is
a big gain out there for it.
o would a change in the working point affect the tune feedback
control (PLL) system?
Probably yes though, in principle, the system should handle it
gracefully, but we would need to see how things.
(4) In a p+p commissioning run in Run-04, what would be the key items which
would be pursued?
- in AGS:
* tuning for polarization with the weak snake
- in RHIC:
* understanding the beam-beam interaction by:
(A) testing new working point to have more tune space to work in
(B) improving the non-linear corrections to reduce the size of the
resonances
* test of the Ng coating to control the pressure rise when pushing
bunch intensities past 1e11
* 250 GeV ramp
* spin flipper commissioning
As Waldo pointed out, we have been commissioning the spin flipper
on "when convenient" basis. Since the experiments do see
crossing-by-crossing differences, we might consider allocating a
reasonable amount of commissioning time to bring it up to an
operational state.
* grow the emittance during the or end of the ramp so that lower
emittance bunches can be ramped in the AGS (good for polarization)
and then become higher emittance bunches when colliding in
RHIC (good for luminosity)
* the polarized hydrogen jet target needs a shakedown run
(5) What is the amount of time needed to do this commissioning?
Thomas said that they would need 5 weeks (which includes the setup time)
to do the necessary studies. People questioned whether this time was
sufficient and whether collisions at experiments were needed to assure
that what is learned from these studies would be representative of the
future performance. Thomas agreed to think about how much "experiment"
running would be needed on this front.
(6) What is a reasonable scenario to use as a "success" model for developing
the beam-use proposal?
We had a long discussion about this matter and we'll be returning to it
again when we get together in roughly 2 weeks.
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