[FFA_CEBAF_Collab] [EXTERNAL] RE: FFA energy range

[Brooks, Stephen]

Trying to add extra energy range to what we have would push the magnet in a bad direction and it's already basically at the limit of what I'd call sensible.  Only a 15mm total vertical aperture and the 1.5xT field level seems about where to stop for this one.

I'd recommend we don't do any more "designs" until we find out for sure whether the users want energy tunability or not.  The current design is pretty good if you do not want it.

I'm also worried some people don't understand what the energy tunability issue actually is, so here's a refresher:
One of the FFA arcs operated at the nominal maximum running linac energy would have some discrete energies E_1, E_2 ... E_N for example 12, 14, ... 22GeV.
If the users want another energy, there is the option of reducing the linac energy, so (ignoring synchrotron radiation for now), energy E_n becomes f*E_n where f is the fraction of maximum linac energy we use.  That means the accessible energy range for physics becomes a set of bands [f*E_n,E_n].
f=100% means no tunability.  If we set f=90%, the 16GeV energy would become a band [14.4,16]GeV.  However, you might notice that there is a gap between the top of the 14GeV energy's band and the 14.4GeV lower energy of the next turn.  There is also still a gap between the maximum of the electromagnetic arcs, now around 10GeV because we removed one, and the bottom of the 12GeV energy's band, which is 10.8GeV.
When I mention "full tunability", I mean all these bands join together, and presumably the bottom FFA band joins to the top of the electromagnetic range.  So in this example we'd need f<=10/12~0.833.

Anyway, this makes the FFA more difficult because instead of covering the range [E_1,E_N] we are now having to cover the range [f*E_1,E_N], which is larger by a non-trivial amount: about equivalent to a whole extra turn!  That increases the maximum field required in the magnets because there is more quad component relative to the dipole.

     -Stephen

________________________________________
From: FFA_CEBAF_Collab <ffa_cebaf_collab-bounces at jlab.org> on behalf of Berg, J Scott via FFA_CEBAF_Collab <ffa_cebaf_collab at jlab.org>
Sent: 25 May 2023 11:59
To: FFA_CEBAF_Collab
Subject: [FFA_CEBAF_Collab] [EXTERNAL] RE:  FFA energy range

Presumably this could also be addressed with a redesign of the FFA? I would think we could accommodate the additional energy range and flexibility at the cost of larger magnets and maybe some other modest penalties like radiation? Or is there something that stops this from working? I'm not suggesting it's the best choice, just an option.

-Scott

> -----Original Message-----
> From: FFA_CEBAF_Collab <ffa_cebaf_collab-bounces at jlab.org> On Behalf Of Jay
> Benesch via FFA_CEBAF_Collab
> Sent: Thursday, May 25, 2023 11:53 AM
> To: FFA_CEBAF_Collab <ffa_cebaf_collab at jlab.org>
> Subject: [FFA_CEBAF_Collab] FFA energy range
>
> Colleagues,
>
> I did not understand this limitation when we changed to one FFA.  I
> suggest that being able to vary the beam energy is more important to
> physics than 22 GeV.  It would make the splitters easier to design too;
> for that matter it's not clear that six splitters fit in the tunnel at
> all.
>
> Jay
>
>
> -------- Forwarded Message --------
> Subject: Re: [EXTERNAL] Re: FFA energy range
> Date: Thu, 25 May 2023 15:41:43 +0000
> From: Brooks, Stephen <sbrooks at bnl.gov>
> To: Jay Benesch <benesch at jlab.org>
> CC: Katheryne Price <kprice at jlab.org>
>
> Yes, essentially having the tunability costs you a turn because it
> requires the FFA to accommodate lower energy beams at the low energy end
> of the range, which would increase the ratio unless the highest energy
> is also lowered.
>
> You have three options:
> ~22GeV with no tunability, just discrete energies (14, 16, 18, 20, 22)
> ~22GeV with a percent or two of tunability, so a small range around each
> energy above
> ~20GeV with close to full tunability (I haven't checked if it can 100%
> cover with no gaps)
>
>       -Stephen
>
> ________________________________________
> From: Jay Benesch <benesch at jlab.org>
> Sent: 25 May 2023 11:36
> To: Brooks, Stephen
> Cc: Katheryne Price
> Subject: Re: [EXTERNAL] Re: FFA energy range
>
> Stephen,
>
> I did not understand that.  If we had only five FFA passes, 20 GeV top,
> could the good field region and tune accommodate some energy span?  The
> users might prefer that to fixed energies, especially given the
> unreliability of our SRF.
>
> Jay
>
> On 5/25/23 11:27, Brooks, Stephen wrote:
> > There isn't any adjustable linac energy range in the 1-FFA solution.  This
> is one of the requirements that ended up being dropped when we changed to a
> single FFA.  Or to put it another way, accommodating linac tunability to get
> a fully continuous energy range was one of the things that pushed me towards
> the 2-FFA solution.
> >
> > Of course we could accommodate a couple of percent by running at slightly
> dubious tunes.
> >
> >       -Stephen
> >
> > ________________________________________
> > From: Jay Benesch <benesch at jlab.org>
> > Sent: 25 May 2023 11:22
> > To: Brooks, Stephen
> > Subject: FFA energy range
> >
> > Stephen,
> >
> > I've forgotten the allowed energy range as it's been a year since it's
> > been discussed.  I remember linac energy range 1000-1100 MeV.  Is that
> > correct?
> >
> > Jay
>
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