[BTeam] higher linac current capability

Sun Feb 2 16:04:44 EST 2020

Gentle folk:

I fully expect that configuring SRF for higher circulating current is more restrictive an activity than is restoring the low-energy (for example, summer HPS run) configuration.  The current and momentum scale linearly for a BBU test, and there is no reason to believe that (for example again) the HPS setup will differ in any way more significant than the evening-up of the linac gradient profile.  SRF power requirements drop as the product of momentum and current, which is quadratic in the scale factor for momentum when seeking a BBU threshold.

If the goal is truly and principally a BBU test, I see no reason to keep the beam momentum unnecessarily high.  At higher momentum we are more subject to S.R.-driven emittance growth, which will be a much smaller nuisance at 900 MeV per pass than more than 900 MeV/linac.

Respectfully submitted,

Michael Tiefenback
________________________________
From: BTeam <bteam-bounces at jlab.org> on behalf of Jay Benesch <benesch at jlab.org>
Sent: Sunday, February 2, 2020 10:59
To: Xiaochao Zheng <xiaochao at jlab.org>; BTeam <bteam at jlab.org>
Cc: Jian-Ping Chen <jpchen at jlab.org>; Paschke, Kent Dieter (kdp2c) <paschke at virginia.edu>; 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: Re: [BTeam] higher linac current capability

Colleagues,

After reviewing the draft 2021 schedule it seems to me that a test at
the present energy April 3-4 is a more prudent first step.  If we can
send 45 uA five pass to C (carbon) and 75 uA five pass to A (carbon,
setup required) without BBU issues this will provide a step towards
higher luminosity.  Since the 2021 schedule has 980 MeV/linac and 910
MeV/linac runs, subsequent BBU tests at higher linac currents could be
run then.

All three tests will require effort and agreement by JLab management to
revise the 900 kW limit prescribed in many places.

April 3-4 is the end of the CREX contingency period.  CREX removal and
SBS installation is to start April 6.  Running the test at the present
energy implies beam will always be in Hall C.  a1n/d2n on He3 while five
pass to A is set up, then carbon while current is ramped in both halls.
I expect a day of RF stabilization as the current increases.  With this
plan, He3 target replacement will not be possible for the two days even
if necessary.

Again, if approved, beam studies time would be banked in the next two
months so no scheduled beam delivery for Physics is taken.

Jay

On 2/2/20 10:37 AM, Xiaochao Zheng wrote:
> Hi Jay:
>
> I just want to add that the date April 1-4 may not be when we schedule
> to change our cells. The cell typically last a month, and changing it
> too early or too late is not practical for the running of A1n/d2n.  And
> if we don't change the cell then we will lose 3 days of beam time.
>
> Is your proposed date Apr 1-4 flexible to some extent?
>
> Thanks,
>
> Xiaochao
> ------------------------------------------------------------------------
> *From:* Jay Benesch <benesch at jlab.org>
> *Sent:* Friday, January 31, 2020 4:55 PM
> *To:* BTeam <bteam at jlab.org>
> *Cc:* Cynthia (Thia) Keppel <keppel at jlab.org>; Garth Huber
> <Garth.Huber at uregina.ca>; Jian-Ping Chen <jpchen at jlab.org>; Gordon D.
> Cates <cates at virginia.edu>; Brad Sawatzky <brads at jlab.org>; Xiaochao
> Zheng <xiaochao at jlab.org>; Paschke, Kent Dieter (kdp2c)
> <paschke at virginia.edu>
> *Subject:* 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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