[BTeam] higher linac current capability

[Michael Tiefenback]

"Too stringent" is a term I consider inapplicable to this.  Lower energy makes it all easier to accomplish, and provides a better measure of headroom.  One will never be able to touch all transfer function combinations, which are critical in BBU interactions.  We had to touch the linac focusing only very slightly to eliminate BBU when we encountered it long ago.  We had every opportunity not to see it, given that it was driven by so few (a single?) cavities.  It seems much wiser to make the lower energy test with greater likelihood of driving marginally vulnerable modes.

Or so it seems to me.  One issue still open: the previous low-energy experience was w/o C100s in use.  This means they were far off-resonance.  The BBU participant modes do not require any particular fundamental cavity mode frequency, but the test would seem more representative of our accelerator were all cavity gradients scaled down proportionally from how we expect them to be driven for Moller and contemporary experiments.

Michael Tiefenback

________________________________
From: BTeam <bteam-bounces at jlab.org> on behalf of Jay Benesch <benesch at jlab.org>
Sent: Friday, January 31, 2020 16:55
To: BTeam <bteam at jlab.org>
Cc: Jian-Ping Chen <jpchen at jlab.org>; Paschke, Kent Dieter (kdp2c) <paschke at virginia.edu>; Xiaochao Zheng <xiaochao at jlab.org>; Brad Sawatzky <brads at jlab.org>; Gordon D. Cates <cates at virginia.edu>; Garth Huber <Garth.Huber at uregina.ca>; Cynthia (Thia) Keppel <keppel at jlab.org>
Subject: [BTeam] higher linac current capability

Colleagues,

It is proposed to test whether the C100s can support 800 uA so CEBAF can
complete the approved physics program (sans SoLID) this decade.  MOLLER
has requested 80 uA fifth pass, leaving little or no beam current for
Hall C with 450 uA limit.  lem suggests that the linacs can _now_
support 800 uA at 1000 MeV/linac.  BBU instability is a concern because
the HOM damping in the C100s is poor.  A test is warranted.  If BBU is
not an issue, relatively inexpensive changes (H wall stub tuners) should
be able to better match C100 and C75 cavities to klystrons at the higher
current.  If BBU is an issue, linac optics changes might help.

CREX contingency runs Friday 3/27 to Saturday 4/4.  It is proposed that
Ops "banks" beam studies time for most of February and March, totalling
96 hours.  The test could then be scheduled Wednesday April 1 to
Saturday April 4.  This would interrupt Halls B/C/D but has long term
benefit to C.  a1n/d2n could schedule their last target change during
CEBAF setup.  45 uA five pass to C (carbon target) and 115 uA fifth pass
to A (carbon) would then put 800 uA in the linacs. Wien assumed left as
is so little polarization to B and C during the run but easy restore.

Two energy options present:

990 MeV, 100 MeV below design, provides a stringent BBU test at an
energy within expected range for high current experiments.

930 MeV setup is available in allsaves from December 2018.  Fifth pass
to A, second pass to C, 40 uA max during allsave.  Perhaps too stringent
a test, given BBU scaling inversely with energy.

I attach a spreadsheet which has beam property measurements made during
the Nov. 2018 930 MeV setup.  These may be compared to the ones made a
year later with improved procedures.

Comments?  Shall we discuss it Tuesday February 4?

Jay

DOE documents allowing 2000 kW beam in CEBAF

Environmental Assessment Determination (8 pages)
https://jlabdoc.jlab.org/docushare/dsweb/Get/Document-50496/EA%20Determination%20-%20Proposed%20Upgrade%20&%20Operation%20of%20CEBAF%20&%20FEL%20Accelerators%202004%2010-Year%20Plan.pdf

Environmental Assessment (395 pages)
https://jlabdoc.jlab.org/docushare/dsweb/Get/Document-15071/EA-1534!.pdf
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