[FFA_CEBAF_Collab] Hall A collaboration meeting discussion re FFA

[Jay Benesch]

Colleagues, 

I got some interesting comments and inquiries after my FFA talk to Hall A.
Unfortunately I was not the last speaker as scheduled as the person who
was supposed to precede me had technical issues and we swapped.  My
statement that the positron and energy upgrade were likely exclusive due
to real estate engendered three questions.  Someone asked for a cost
WAG.  I said $250M for the positrons and at least $500M for the energy
upgrade, since every magnet and stand in the machine have to come out.
Not all replaced, but all moved.  Asked about time scale, I said 2030s.
Patrizia had more questions than I had time to answer and will contact
me separately.  Doug said that positrons should have priority since
there are two conditionally approved experiments.  I said that I thought
that those needed identical beams, either electrons or positrons
collected from the pair production, for systematic reasons and that the
switch would have to take less than a day for the same reasons, perhaps
every two weeks.  I also pointed out that the present plan would produce
positrons only for Hall B.  Gordon Cates spoke up and said that while
the case for positrons was farther along, he wasn't sure which option
had a stronger physics justification.  Stuart emailed me afterwards
asking if I knew how big the emittance was on the last pass.  I was
chagrined to answer that I didn't and forwarded his question.  

I forwarded the question to Alex, Dejan and Stephen.  Alex referred me to 
his JLAAC talk.  I sent it to Stuart.  Stuart is concerned about the large 
normalized emittances at the end of the sequence.  Even reducing the 
number of passes in the final FFA to three, X beam sigma just due to 4 m
dispersion in the Hall arcs will be ~7 mm.  

Someone competent, aka not me, should compute best estimate of geometric
emittances at the end of each arc and at the peak beta in the linacs.   Using these,
beam envelopes including dispersion term where appropriate.   I'd like to see 
such a table for two cases: 

1. present plan, 1100 MeV/linac, three EM passes, two FFAs (4+3)

2. four EM passes at 1200 MeV/linac, 32 one m magnets in arc 1, 32 two m magnets
in arcs 2-4, 32 three m magnets in arcs 5-7 and 32 4m magnets in arc 8 aka move 
all the existing magnets up one pass to decrease B.  Followed by one FFA with five
passes.  Total still ~22 GeV but I suspect beam size will reduce that number, albeit less 
than case (1). 

Dave Douglas chose the radius of the CEBAF arcs to allow 16 GeV with tolerable emittance growth.  

I will ask the same questions at the end of my Hall C Accelerator Status talk. 

Jay