Objectives for the today's meeting

• Brief review of the proof of principle experiment at PSI. Where we are and what the next steps are. 
• New organisation in WP3: confirmation of deliverables proposed by Marco, decision on priorities and tasks for SY/STI;
• Impact on the Injector of the collider parameter changes - Salim
• In the CHART budget we have an amount to build the target system for the experiment. Now in Marco's deliverables no hardware is planned, could we still use the money for the experiment? i.e. for the AMD or the NC/SC solenoids around capture RF structure. 
• Next meetings: once every two weeks? When?Attendance

Participants:

Paolo Craievich(PSI), Braun Hans-Heinrich (PSI), Iryna Chaikovska (IJCLAB), Frank Zimmermann (CERN), Marco Calviani (STI: section leader Target Integration), Jean-Louis Grenard (CERN, Integration Target FCC), Antonio Perillo Marcone (CERN, Design Target mechanical system), Anton Lechner (CERN Radiation computation)
Simone Gilardoni (CERN Group leader, Radiation computation, Target, ), Salim Ogur (IJCLAB Beam dynamic simulation), Besana Maria Ilaria (PSI, Radiation aspects), Zennaro Riccardo (PSI),  Auchmann Bernhard (PSI) 

Summary:

• The discussion focused on the relevance of the experiment at PSI versus the FCC baseline parameters and concepts (Flux concentrator vs. SC solenoid), in particular with respect to the power dissipation in the target.
• Technically, the dissipated power in the PSI experiment will be 0.3 W mainly due to radiation limitation in the surrounding area. So a cooled target is not strictly necessary if we target a proof of principle of the capture yield and diagnose methods.
• The main concerns from Simone Gilardoni and Marco Calviani are the space availability for a cooled target within the SC solenoid topology under development at PSI. PSI presented two design examples with different bore aperture and cost estimations.
• Even if the integration of a fix cooled target seems in a first rough analysis feasible (the large yield of the approach allows for a reduction of the dissipation <1kW), a detailed analysis/pre-design has not yet been done. Consequently no definitive considerations can be made about the space required. The design of the Solenoid allow for some flexibility and more space could be provided but with  additional costs due to the large SC coils.
• The time window for the realization of the project at PSI is limited. Regarding the solenoid we need to place an order for the HTS tape as soon as possible. The current bore diameter was defined taking into account costs in order to avoid the WTO call for tender which would be very difficult to start at PSI at the moment because the administrative offices are busy with the SLS2 tenders. The WTO process is a complicated and long procedure we are obliged to follow when purchasing parts exceeding given amounts.

At the meeting it was agreed:

• To start re-iterating between CERN and PSI on the source parameters and topology to allow the section of Marco making the first consideration about the feasibility and requirements of a FCC-like fixed cooled target to be used for the design of the solenoid at PSI. Unfortunately the time left is quite limited because we would like to place the order as soon as possible
• To continue the analysis of the flux concentrator in parallel to the SC approach and experiment.

Detailed Minutes: 

Riccardo presentation

• Overview of Pcubed project => WP6 FCC-ee
• Location in SwissFEL with 6 GeV beam on dedicated beam line
• Beam parameters
  • Limitation on the dissipated power on the target is due to the radiation losses, SwissFEL capability is 2 bunches at 100 Hz, but not usable due to the radiation losses
  • Dissipated power on target 0.3 W versus 720-880 in FCC
• Key device to reach highest capture yield is the SC solenoid
  • Advantage is the large aperture: No technical limitation for the aperture, the aperture is presently optimized for the cost and time.
•  Comments
  • Marco P.: optic design completed, order quads placed => we are in the integration phase
  • Antonio: The yield depends as well from material, will the tungsten be also used for FCC?
    • in the experiment we can use the same material as FCC, it will be an amorphous target based on tungsten 
    • Big difference: we don't need cooling for the prototype to be installed in SwissFEL
  • Iryna
    • Other (tungsten) alloys are eventually usable, positron yield is comparable but different mechanical properties
    • Cooling has no effect on the yield
  • Simone
    • Bore aperture has an effect on the yield because the design of the magnet may be significantly different, the aperture is an open point to be addressed now
  • Riccardo
    • SC solenoid can be redesigned in future 
    • Now cost and time are the boundary conditions in this moment  
  • Hans
    • Cooling capability <1 kW, is there a fundamental difficulties with a possible cooling concept?
  • Simone
    • Peak power density increases if reducing the target & beam size
    • Would like to understand how much the solenoid must be adapted for the FCC case
  • Still motivation to go for rotating target?
    • Riccardo: keep in mind that a larger yield will reduce dissipated power on target
    • Original FCC design had 4 kW on target, now around 1 kW (with safety margin of 2)
  • Goal experiment:
    • Develop SC solenoid based on new technology
    • Develop the diagnose methods
    • Prove positron yield with SC solenoid topology
  • Important
    • Hans: the FCC project has to confirm the interest and importance of this experiment for FCC positron source
    • We (PSI and CERN) got money to develop and perform this experiment at PSI 
  • Marco C.
    • Do not see clear superposition of this approach to FCC approach!
  • Bernhard
    • Decision on SC bore side has to be taken now because we have to order the material
  • Hans
    • Experiment and funding was agreed with CERN management
    • Now CERN expert saying if we should not do this experiment!
  • Simone
    • Michael asked the target section to design the final source
    • If this has to be tested at PSI they have to know (define?) the mechanical constraints
  • Hans: key point is to verify and to dimostrate the positron yield. 
  • Iryna: KEK is now starting investigating moving target (beam on target 400 W?)
  • Marco: it is important to look into the FCC design
  • Simone: Start on fixed target and explore how much power can be handled
  • Anton:
    • Flux concentrator is still the main approach for FCC? Where we should focus
  • Paolo: idea is to keep the two approaches with a comparison between them

Status WP3 activities

• Slides from CERN (Jean-Louis Grenard)
  • Slide 3: what they need to start the study
    • Calviani: try to optimize the outcomes based on the time available on the project
      • Reduce parameter space
      • Not all boxes needed at the beginning
      • Slides of Riccardo are an initial point
• Iryna: Design Flux concentrator is available (since last year)
• Hans: scope of PSI activities is to increase the yield and reduce power on target
  • Why looking at two approaches with completely different beam parameters
  • Flux concentrator as a benchmark to compare
• CHART report (will be used for CDR +)
  • We can compare between studies
• Hans: if we want to do the experiment at PSI we need to start the experiment now, if not it will not be done
• Simone: design the FCC target to be integrated in a SC solenoid topology and verify what is the overlap with the PSI experiment
• Shall PSI wait with the order of the solenoid? We need to fix a timeline for that. Simone => next week the team will have a look a give a first feedback
• Deadline end of April => PSI is preparing the purchasing document for the SC wire to be completed by End of April
  • Antonio and Jean-Louis will contact Paolo and Bernhard => Meeting next week to check the parameters
  • CERN will contact PSI for more specific questions
• Iryna: shall we revise the safety factor? Do we need more realistic values
  • Important not to add too many safety factors 

Beam Dynamics (Salim)

The main change in the collider parameters is the lifetime which is an order of magnitude lower than the one presented in CDR0. Currently it is in the order of 1100s while before it was >200min. This means that we should be faster when filling from scratch. With the current bunch population of 2.13e10 it takes about 1000s to fill the collider rings and this is comparable to the current lifetime. Using a population of 3 or 3.5e10 the filling time goes from 1000s to 500s.

Salim's answers:

At CDR0, the first fill (reaching 100%) while beams colliding was happening at 1035 seconds in the operation. There the loss of the charge was small and we can pre-compensate it with ~1.2% of additiional charge during the first fill from the scratch. As you emphasized the luminosity lifetime was 70 mins, and the loss was 1%, which I was thinking to charge 11% per injections during the first fill.  
 
With the current collider table, I tried to make the Booster faster and carry higher charge(~3E10/bunch) so that the loss of charge in the collider is less than what we can inject. Just to give an example in 50 seconds the new collider will loss 5%, it used to be 1.2% for the CDR0. If I loss 5% in one Booster cycle, it means 2*BR cycle (one cycle for e- other for e+), I need to recharge 10% (which is +-5% assymetry limit), if I continue like this the amount I inject 10% losing ~5%, then the collider will be full not at 10 injections of 10% but almost 20 injections of 10% because I am losing 5% due to collisions, this is why regular 10 Booster cycles of ~10% injection will not be enough for the new collider. If I pre-compansate let’s say inject 10% plus 5%loss then I need ~4.5E10/bunch  from the injector, then yes I reach 100% in the collider at 10 injections, or we will wait 20BR cycles to reach 100% if I inject 10% which refers to ~3E10/bunch.
 
In short, CDR0 loses 1.2 % in ~50 seconds (1 booster cycle)
New collider loses ~5% in 50 seconds, therefore it is booster cycle needs to be faster than 25 seconds so that we can fill it while collisions occur by injecting 2 x 5%. Of course, injecting 10% during the top-up leaves no safety margin (due to +-5% imbalance limit), so it means that the booster cycle needs to be even faster, which means it needs higher flux from the injectors, which I am trying to optimize further.
 
For now, 3 or 3.5E10 seems to be adequate for the allowed +-5% imbalance limit and collider will reach 100% charge of e- and e+ at 2 species x 20 injections x 25 s booster cycle.

Comments on the target in SuperKeKB

Chat copy:
From Frank Zimmermann to Everyone 10:50 AM
For SuperKEKB incident beam power on the target is 3-4 kW (not 400 W).
Deposited power seems to be 600 W (from a talk of Iryna).
 
From Hans Braun to Everyone 10:51 AM
Thanks for this key information. So, is it correct to assume that we don't need to move the target?
 
From Frank Zimmermann to Everyone 10:52 AM
I suppose yes, with adequate design and cooling.
 
From Iryna Chaikovska to Everyone 10:59 AM
0.6 kW @50 Hz but they produce e+ @25Hz, so  (0.3 kW)
I just checked
 
From Frank Zimmermann to Everyone 11:01 AM
your slide says 50 Hz, and I think this is the design
https://indico.cern.ch/event/886491/contributions/3797764/attachments/2013939/3365822/positron_sources_muoncol2020.pdf slide 9, source Enomoto san
 
From Iryna Chaikovska to Everyone 11:03 AM
Yes, KEK colleagues always put 50 Hz but in reality they operate at 25 Hz
I clarified this with them because for me at was not clear at the certain moment
More probably the cooling is designed for 0.6 kW
 
From Frank Zimmermann to Everyone 11:09 AM
They claim 50 Hz achieved already since 2019
or at least in Phase 3, since summer '21
 
From Iryna Chaikovska to Everyone 11:13 AM
Could it be that the target is hit @25 Hz but after the DR it is restored to 50 Hz. I could clarify this point.
 
From Frank Zimmermann to Everyone 11:17 AM
OK. Thanks. It would be good to fully clarify their parameters.. I suspect the target must be hit at the same rate as the e+ injection rate.

Further information on SuperKEKB: