[FFA_CEBAF_Collab] [EXTERNAL] Re: FFA at CEBAF WG mtg. this Friday, March 7

Mon Mar 17 09:34:12 EDT 2025

Dear Scott,

sorry for my delayed response to your email.

The defocusing quad closest to the splitting dipole does help to
separate the different-energy beams. The second, focusing, quad
serves a dual purpose. 1) it makes the different-energy beams
roughly parallel, but, being a dfocusing quad, it also focuses
horizontally (around the orbits for the different energies).
I had this in mind but neglected to mention it in my writeup.
It will hopefully help to bridge the long quad-less distance
between the splitter and the FFA-arc proper.

With best regards,
Volker

On 3/11/25 08:36, Berg, J Scott wrote:
> I don't think there's anything particularly "heretic" about this idea. We have two basic issues we need to address: first, that we need the beams to separate as rapidly as possible so we can get them into separate focusing channels, and second, that need a sensible focusing pattern, i.e., if we focus the beam in one plane, we can't go on forever without focusing in the other plane.
> 
> I at least think of the two ends of the splitter differently in this regard. On the linac end, the beams with different energies are following the same trajectory, so they need to get separated with a dipole. If that dipole is followed by a horizontally defocusing quadrupole, that separation can be increased. Now, since the different energies follow the same orbit on the linac side of the dipole, there's no harm to their trajectories to put a horizontally focusing quadrupole on the linac side of the dipole; thus we get focusing in both planes, and get the improved separation of the horizontally focusing quadrupole. It sounds like a good story but when I tried it, things did not improve as much as I would have hoped, but I didn't put much time into it.
> 
> On the FFA end, we have the advantage of the fact that we can choose the exit point of the FFA cell to be the point with the highest orbit angular divergence, and that this divergence is likely higher than what we could generate easily in the splitter lines. The trick is getting focusing in the horizontal plane. What I would propose here would be to allow the beam to be overfocused horizontally, so you end up with negative alpha in both planes. As soon as you can get it into separate lines you need to catch the beam with a double to start the beam focusing again. Effectively I've started with the defocusing quadrupole you've mentioned, I'm just pointing out that it's already there for the taking.
> 
> -Scott
> 
>> -----Original Message-----
>> From: FFA_CEBAF_Collab <ffa_cebaf_collab-bounces at jlab.org> On Behalf Of
>> Volker Ziemann via FFA_CEBAF_Collab
>> Sent: Friday, March 7, 2025 7:15 PM
>> To: Ryan Bodenstein via FFA_CEBAF_Collab <ffa_cebaf_collab at jlab.org>
>> Subject: Re: [FFA_CEBAF_Collab] [EXTERNAL] Re: FFA at CEBAF WG mtg. this
>> Friday, March 7
>>
>> Dear colleagues,
>>
>> I played around with my 'heretic' suggestion for a beam separation
>> system where a
>> defocusing quad helps the dipole to increase the separation and a
>> focusing quad QF
>> makes the beam parallel again. I only did a thin-lens version to see
>> whether it
>> works and how the solution scales with energy. Unfortunately the beam only
>> comes out parallel for one energy, but the variation is moderate,
>> because the
>> focal length of the QF is only wrong by at most 30% if the energy varies
>> by a factor
>> of four. I attach a brief writeup for your criticism. Please be aware
>> that I just
>> sat down and doodle with  a pencil. The idea must be vetted, so feel
>> free to shoot
>> from your hip.
>>
>> Cheers,
>> Volker
>>
>>
>> On 3/7/25 14:04, Brooks, Stephen via FFA_CEBAF_Collab wrote:
>>> If you want to investigate having "more gradient", try filling the whole line, at
>> least the FFA end, with alternating quad cells with perhaps 10T/m of field.
>> (How to do this might be my problem i.e. magnets).
>>> This should keep the betas and the dispersions down.
>>> Bulk path length can be done with overall line geometry as you can add
>> dipole to those quads.
>>> Beta matching should be easier as it will require just a tweak to magnet
>> lengths, generating beta phase advance is easier with shorter cells.
>>> The difficult one will be R56, as we now have smaller dispersion.  You want
>> dispersion in dipoles for R56 adjustment, but dispersion in dipoles is exactly
>> what you don't want for I_5 emittance growth!  I suggest trying to solve this by
>> reducing R56 in the FFA cell.  I've already got cells with sextupole like option
>> "C" in the attached spreadsheet that can make R56 about 3x smaller than the
>> baseline.  Dejan has one that does similarly.  Maybe we can flatten the TOF
>> entirely with higher order fields, I'll need to try that later.
>>>
>>> As for HOW we make those gradients in crowded spaces, have a look at Figure
>> 5 in my IPAC'21 paper:
>>> https://urldefense.proofpoint.com/v2/url?u=https-
>> 3A__accelconf.web.cern.ch_ipac2021_papers_tuxc07.pdf&d=DwIGaQ&c=CJqEz
>> B1piLOyyvZjb8YUQw&r=Ogg4WFNBwvADBq3fkmCLiJ7SaRDPYtawHzJElJMB0jE
>> &m=cH7WU-M_-M030-
>> k_S17mRkr75wnVCT0EW6v_YxWQ87S8SVaj7pRPPxmlw4KCKDJv&s=fGqlkxTts3
>> 0s-Qa_wVTcxPAJqnArLrUiNBFMJfHc4-w&e=
>>>
>>> It's too tiny for our beam pipes, so consider doubling the size:
>>> gradient 50T/m -> 25T/m
>>> bore diameters 20mm -> 40mm
>>> beam separation 30mm -> 60mm
>>> midplane slot full height 12mm -> 24mm
>>>
>>> That would fit around some crowded splitter pipes I think?
>>>
>>> In the end, I'll prepare a tool to do the 3D optimisation for Halbach cuboids
>> (no iron).  You may want electromagnets for energy tunability, which will also
>> be complicated, but possible in principle.
>>>
>>>        -Stephen
>>>
>>> ________________________________________
>>> From: FFA_CEBAF_Collab <ffa_cebaf_collab-bounces at jlab.org> on behalf of
>> Ryan Bodenstein via FFA_CEBAF_Collab <ffa_cebaf_collab at jlab.org>
>>> Sent: 07 March 2025 12:39
>>> To: Ryan Bodenstein via FFA_CEBAF_Collab
>>> Subject: Re: [FFA_CEBAF_Collab] FFA at CEBAF WG mtg. this Friday, March 7
>>>
>>> Dear Colleagues,
>>>
>>> Attached (and uploaded), you’ll find our minutes from today’s meeting.
>>>
>>> Have a great weekend!
>>> Ryan
>>>
>>> From: FFA_CEBAF_Collab <ffa_cebaf_collab-bounces at jlab.org> on behalf of
>> Alex Bogacz via FFA_CEBAF_Collab <ffa_cebaf_collab at jlab.org>
>>> Date: Monday, March 3, 2025 at 6:35 PM
>>> To: Alex Bogacz via FFA_CEBAF_Collab <ffa_cebaf_collab at jlab.org>
>>> Subject: [FFA_CEBAF_Collab] FFA at CEBAF WG mtg. this Friday, March 7
>>>
>>>
>>> Dear Colleagues,
>>> We will continue our design efforts this Friday, March  7, at 11:00 am.
>>>
>>>     *   Splitter design update - Donish
>>>     *   AOB - ALL
>>> Please, follow a ZOOM invitation below.
>>> Cheers,
>>> Alex
>>>    ___________________________________
>>> S. Alex Bogacz,
>>> Accelerator Physics Group Leader
>>> Center for Advanced Studies of Accelerators
>>> Jefferson Lab
>>>
>>> ZOOM connection:
>>>
>>> The link is here: https://urldefense.proofpoint.com/v2/url?u=https-3A__jlab-
>> 2Dorg.zoomgov.com_j_1614898082-3Fpwd-
>> 3DTnUzMS81M2sxbDZIbERJU01tYkJCQT09&d=DwIGaQ&c=CJqEzB1piLOyyvZjb8
>> YUQw&r=Ogg4WFNBwvADBq3fkmCLiJ7SaRDPYtawHzJElJMB0jE&m=cH7WU-
>> M_-M030-
>> k_S17mRkr75wnVCT0EW6v_YxWQ87S8SVaj7pRPPxmlw4KCKDJv&s=ijdBv8crAld
>> FAkjSX-4kepOd1GAw5Be7qFewetnsKPI&e=
>> <https://urldefense.com/v3/__https://jlab-
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>> __;!!P4SdNyxKAPE!F5zCpFGgJS3KO_S1sHB3NQfeCP1upRTf7FX7VbSgd2ijtDp9gn
>> 3MM_23mVZ6yj0kD07KWnprWuDrbn_qigKUvGGb1d4$>
>>>
>>> In case of problems, the zoom room is below:
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>>> Passcode: 123456
>>> ________________________
>>>
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>>
>> --
>> Volker Ziemann, Director for Accelerator Education
>> Jefferson Laboratory, 12000 Jefferson Avenue, Newport News, VA 23606, USA
>> ziemann at jlab.org, +1 757 269-7845, https://urldefense.proofpoint.com/v2/url?u=https-3A__cern.ch_ziemann&d=DwIGaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=NlDON_OO93ezSqWc6gEJPjLSAuj2GoGN9JQWHpMEsyQ&m=R72QW6Cox3HZZcXNbhHfehdUqrCiAX_Q6stG4gkqIUNvtHvd5TdWBy3_s_GKovtQ&s=D4nsIojHnL_A30cU8gIlQ5lZ96ffWTraX796EMkUeqk&e=
>> My books: https://urldefense.proofpoint.com/v2/url?u=https-3A__cern.ch_ziemann_mybooks&d=DwIGaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=NlDON_OO93ezSqWc6gEJPjLSAuj2GoGN9JQWHpMEsyQ&m=R72QW6Cox3HZZcXNbhHfehdUqrCiAX_Q6stG4gkqIUNvtHvd5TdWBy3_s_GKovtQ&s=3BYUaV-mbreFuMdvl-zUSUGfzUYoZpZFGEcXM2bmKb0&e=

-- 
Volker Ziemann, Director for Accelerator Education
Jefferson Laboratory, 12000 Jefferson Avenue, Newport News, VA 23606, USA
ziemann at jlab.org, +1 757 269-7845, https://cern.ch/ziemann
My books: https://cern.ch/ziemann/mybooks