[PEPPo] Ce+BAF meeting on Wednesday, October 1 @ 11am EDT

[Andriy Ushakov]

Dear e+ Colleagues,

The minutes for the last Ce+BAF meeting have been added to the meeting page<https://jeffersonlab.sharepoint.com/:u:/r/sites/PositronSource/SitePages/2025-10-01-Ce+BAF.aspx?csf=1&web=1&e=SsrdTb>.

Best regards,
Andriy

---

Minutes
Andriy has presented simulation results for the positron yield versus tungsten target thickness for the different transverse sizes of electron beam on target (0.25 mm, 0.5 mm, 0.75 mm, 1 mm) and e- energy of 130 MeV, 250 MeV and 370 MeV.

Joe: How was the positron yield defined and why yield depends on electron beam size?

Andriy: Yield was calculated at the exit of target, and the normalized emittance of positrons was limited by 100 mm mrad (CEBAF limit). The size of electron beam has a strong impact on the positron beam emittance, and the emittance cut was applied at target exit.

Salim: Why was a 4 mm thick tungsten target used in the past for the 130 MeV electrons?

Andriy: 4 mm tungsten target is an optimal thickness for the Figure-of-Merit (defined as the product of positron yield and polarization squared) without applying emittance cut. The optimal thickness for FoM is greater than the optimal thickness for yield.

Joe: We need to know that are the limits for the targets are for each of electron beam energy. This is the focus of the studies.

Volker: Calculations of local heat deposition and temperature rise are needed to determine when the target melts.

Andriy: The energy deposited in the target and peak local power density were calculated and are includes in my next slides.

The deposited power was calculated for pure tungsten, tungsten alloy (85%W+15%Cu) and gold and heat data are shared with Silviu for the thermal analysis.

  *
Total deposited power in W15Cu target is ~10% higher than in W and the deposited power in Au is a few percent less than in W.
  *
Peak energy deposition density PEDD are the same for W and Au, and ~15% less for W15Cu.


Joe: Were normal or flat distributions used for the spatial distribution of the electron beam?

Andriy: The normal distributions have been used.

Joe: Would W15Cu be a better choice than the other options?

Andriy: The lower PEDD of W15Cu is due to lower density. If we use W/g instead of W/m³ as the units of PEDD, then the PEDDs for the considered material are approximately the same.

Salim: The limits for tungsten target are known (~70 J/g, SLC positron source).

Joe: There is a methodology for defining material limits. Silviu's studies will determine those limits and the most suitable material.

Dave: Pure tungsten usually is not the best material.

Joe: This would be a good topic for a future meeting, at which Silviu could explain the target design and the choice of target materials.

Volker: If the beam spot is too small, thermal expansion from the heat could cause the target fracture.

Jay: Why is a tungsten alloy with 15 percent copper being considered? The Qweak collimator was made from W with 10% Cu.

Silviu: W15Cu has a better thermal conductivity.

Dave: The higher electron energies are good for the positron yield. For the fixed current of 1 mA @ 370 MeV the total "dumped" power is 370 kW.

Joe: No, the current is not fixed. We will reduce the current at higher energies.

Eric: The electron beam power was not fixed in Andriy's calculations. Might be it would be better to normalize the positron yield per units of power.

Andriy: Currently, the yield is normalized per primary electron. This allows easy scaling for different beam currents or power.

Joe: We are OK with the normalization per electron. Such normalization allows us to disentangle the parametric studies, and we will come to the currents and power later when we know the (target) limits.

Andriy will continue the energy deposition calculations at 250 and 370 MeV and provide the heat data to Silviu. Additionally, Andriy will perform radiation damage calculations.


________________________________
From: Andriy Ushakov
Sent: Tuesday, September 30, 2025 10:22
To: peppo <peppo at jlab.org>
Subject: Ce+BAF meeting on Wednesday, October 1 @ 11am EDT


Dear e+ Colleagues,

A quick reminder: we will have the Ce+BAF meeting tomorrow, Oct 1 at 11am EDT in TL-1227 and via Zoom.

Agenda

  *   Andriy - Update on calculations of positron yield for 130, 250 and 370 MeV e- and energy deposited in target

Meeting page: Ce+BAF Meeting - October 1, 2025<https://jeffersonlab.sharepoint.com/:u:/r/sites/PositronSource/SitePages/2025-10-01-Ce+BAF.aspx?csf=1&web=1&e=SsrdTb>

Zoom link: https://jlab-org.zoomgov.com/j/1607642988?pwd=K2VMVlkwLzFwR0l0VjNWMFUvSGtrUT09

Best regards,
Andriy

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mailman.jlab.org/pipermail/peppo/attachments/20251003/e8b8b21d/attachment-0001.htm>