[Clas12_rgh] beam energy

[Volker Burkert]

That is a nice graph. So, for Axel’s energies of 4 pass and 3 pass, the corresponding magnetic fields would be   0.8 x 5 = 4Tesla, and 0.6x5 = 3 Tesla. According to Chris’ curves a polarization of about 75% at 4 Tesla and at 3 Tesla about 45-50%, still not too bad.
Volker


From: Christopher Keith <ckeith at jlab.org>
Date: Wednesday, August 28, 2024 at 12:48 PM
To: Volker Burkert <burkert at jlab.org>, Axel Schmidt <schmidta at jlab.org>, Eugene Pasyuk <pasyuk at jlab.org>
Cc: Eugene Pasyuk via Clas12_rgh <clas12_rgh at jlab.org>
Subject: Re: [Clas12_rgh] beam energy
I’ve attached a plot that shows a crude estimate of the polarization vs field dependence, assuming two different models of dynamic nuclear polarization: the solid effect (SE) and thermal mixing (TM).  For ammonia, TM is probably the dominate mechanism.

Both cases indicate that we can’t expect better than about 30% at 2.5 T and 1 K.  We can cool to lower temperatures and improve the polarization, but this requires a different type of refrigerator that is significantly more complex to construct and operate.

As Eugene and Wei have already said, there’s another problem.  To provide a larger scattering angle for transverse polarization, the new 5T magnet was designed with a wider separation between the two sets of superconducting coils.  This degrades the uniformity of the central field.  The vendor used iron shims to compensate for this effect, so the uniformity is only sufficient in a narrow band around 5.0 T.

We can replace the iron shims with new ones designed for lower fields.  Or, if we get a new magnet, we can possibly replace the iron shims with a set of shim coils, driven by a separate power supply.

Chris





From: Clas12_rgh <clas12_rgh-bounces at jlab.org> on behalf of Volker Burkert via Clas12_rgh <clas12_rgh at jlab.org>
Date: Wednesday, August 28, 2024 at 12:15 PM
To: Axel Schmidt <schmidta at jlab.org>, Eugene Pasyuk <pasyuk at jlab.org>
Cc: Eugene Pasyuk via Clas12_rgh <clas12_rgh at jlab.org>
Subject: Re: [Clas12_rgh] beam energy
Hi Axel,

For such a large range in energy it might be better to also lower the polarizing target magnetic field with the chicane magnets and use different microwave frequencies. The prize to pay is lower polarization. In the past people have used 2.5 Tesla mag. field with still decent polarizations (50-60% for protons??) . But Chris may want to comment on this option.

Volker


From: Clas12_rgh <clas12_rgh-bounces at jlab.org> on behalf of Axel Schmidt via Clas12_rgh <clas12_rgh at jlab.org>
Date: Wednesday, August 28, 2024 at 11:52 AM
To: Eugene Pasyuk <pasyuk at jlab.org>
Cc: Eugene Pasyuk via Clas12_rgh <clas12_rgh at jlab.org>
Subject: Re: [Clas12_rgh] beam energy
Dear Eugene,
Thank you for raising this issue.

Acknowledging that the approved experiments are all designed for 5th pass beam (and that obviously takes priority), I would be very curious to know how much additional cost and effort would be needed to accommodate 4th pass, or even 3rd pass beam energies. Lower energies would probably be a lot better for measuring two-photon exchange, the physics August and I are investigating.

I realize that this would require a MUCH larger chicane bend. Would this increase the size/cost/complexity by a large factor? At what point does it become completely infeasible?

We are still in the early stages for exploring this. We'll do simulations and report to the group. But this seemed like a good moment to register this point.

Thanks,
Axel

On Aug 28, 2024, at 11:03, Eugene Pasyuk via Clas12_rgh <clas12_rgh at jlab.org> wrote:

Hello RGH enthusiasts,

While thinking of the chicane design, it occurred to me that, so far, in all our simulations, we have considered beam energy of 11 GeV. 11 GeV may not be available. Since the target magnet field is fixed to be 5 T, we will have more bending for lower beam energies. It has two consequences: more background in the forward detector and a larger offset of the middle chicane magnets. The latter means we would need a wider opening of the chicane magnets. The maximum opening for the existing design that produces enough BdL needs to be checked with the company. It may require a substantial redesign of the magnet compared to the existing version they built for someone.
We should define the minimum acceptable beam energy for this experiment and run simulations with this condition.

-Eugene




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