Attending: L. Ahrens, K. Brown, J.
Dicello, J. Feldman, D. Gassner, J.
Geller, L. Heilbronn, A. Kronenberg, A. McNerney, J. Miller, J. Mills,
P. Pile, R. Prigl, N. Tsoupas, M. Vazquez
We discussed beam parameter requirements
for BAF with regard to species,
required intensity range, and beam profile. The following tentative
specifications were agreed on:
1.) Intensity
The upper limits given in the Conceptual
Design Report (CDR) for
some species seem to be adequate. There is an energy falloff towards lower
energies, roughly as 1/sqrt(Mom.), but steeper towards the minimum
extraction energy which is reached when the spiral pitch of the resonant
particles is equal to the thickness of the thin septum at D3. The minimum
energies shown in Table 2.3.2 of the CDR are well above this threshold, but
lower energies would imply significant reductions in the intensities
available.
Lower intensity limits are 10**3 per spill for Heavy Ions and about
10**10 for protons. There does not seem to be any need for low intensity
proton beams.
The repetition rate for a 0.5 second spill length is expected to be
at least 0.33 Hz.
2.) Beam profile
For physics runs, a 1cm diameter beam
(full width at 90% of peak
intensity) is desired, a Gaussian profile is fine.
For sample exposures, the beam area should cover the range from a
small beam up to 20 cm diameter or square. At the large diameters, the
intensity should be uniform to +- 5%. To be more specific, the dose
accumulated during an exposure anywhere in the flat beam area should be
within +- 5% of the dose requested, as determined by the dosimetry
instruments. Assuming that the accuracy of these instruments is 1% or
better, the +- 5% criteria can be directly applied to the beam profile
delivered by the transport.
There was some concern about the octupole
horn at the edge of the
wide beam, a roughly 20% overshoot over the flat center, may turn out to be
lower in reality.
There is also some interest in much wider beams, up to 50cm wide by
50cm high.
3.) Species
Species mentioned other than those in
Table 2.3.2 of the CDR were
Helium, Neon, Argon, Carbon and Nitrogen. The noble gases cannot be
delivered by the Tandem, at least not without a major investment. The R&D
on EBIS sources that is currently going on at BNL may be more promising in
this respect. This has to be looked at. There is a strong interest in
"cocktail beams", i.e. irradiating the same sample with different species
almost simultaneously. The time to switch from one species to the other
should be of the order of 1min or less. It was mentioned that two heavy ion
beams from the Tandem plus a proton beam could be available during polarized proton
running for RHIC, but this would require the BAF users to cover the
operating costs for the Tandem.
Other issues discussed:
Longterm exposures:
There is interest in long term exposures
over days at low
intensities. A rough estimate lead to the conclusion that the required
intensities are roughly equal to the minimum intensities needed for the
physics program.
Spill structure:
WG warned that the BAF spill is likely to
have significantly more ripple
(low order harmonics of 60Hz) than the AGS slow extracted beam. For most
experiments this is not a problem. JD mentioned however, that oxygen
diffusion processes have time constants of microseconds to milliseconds and that intensity
variations over milliseconds may be a concern for some future experiments. Thus some kind
of fast spill control is certainly desirable.
R. Prigl