4/13/23 -- Target power limits -- Max, Silviu, Andriy, Victor

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To determine a reasonable set of goals for the LDRD, we need to know what limitations we are working under. Emittance and dp/p cuts imposed by merger and injector acceptance cause e+ yield to be low; on the other hand, merging beam into injector only makes sense if available diagnostics can realistically see it.

  • Numbers...
    • Raw e+ yield peaks at target thickness of 875 µm. Thermal power deposition favors thinner targets, the e+ yield as a function of power deposition peaking at a thickness of about 400 µm.
    • Optimum momentum and fractional yield as a function of dp/p cut are independent of target thickness.
    • Combining raw yield with emittance and dp/p cuts suggests total yield is only weakly dependent on target thickness above 600 µm.
    • Silviu and Andriy think 750 µm is a good compromise. Power deposition is about 50% of primary beam power. Estimated power limit for fixed target at 2 mm rms beam radius is about 1 kW, so primary beam current < 280 µA. Cutting to dp/p = 5% and geometric emittance 100 mm mrad in x and y gives a yield of 3e-7, i.e., 90 pA. In case the cuts needs to be more conservative (very likely) or other real-life constraints reduce the yield further, we should expect at least a 10-fold reduction, so 10 pA or less.
  • Considerations...
    • We have 2 weeks left to finish the pre-proposal, so very soon we'll need to know what we're proposing, and it shouldn't sound completely grotesque.
    • Pending discussion with injector acceptance folks, it increasingly seems like the plan to merge the e+ beam into the injector will make this project too difficult within the scope of one LDRD.
    • Joe suggested yesterday that if the power limit of the target turned out to make the project unfeasible, we could pivot to a proposal focused on making and testing a rotating target.
      • Reality seems to be somewhere in between. The target can handle 1 kW but not 2, enough to make some "beam" (albeit hard to detect and work with), but the total complexity of the setup may be unmanageable if we tried to do everything at once.
      • A rotating target could be made but would need about twice as many resources as a fixed one.
      • A more reasonable proposal could be to build a fixed target, build the first half of the chicane to shine a high-current e- beam on the target ("easy", i.e., very low risk), and systematically explore all relevant dependencies for model validation (variable target thickness etc.). This would lend credibility to our models for future positron source developments. Detection system could be similar to the original PEPPo, reusing many components, but it would include more collimation etc. to study the phase space and manage the high-power electron and gamma background. While difficult enough in itself, this approach would remove the complication of merging and accepting the beam for now. Considering the anticipated time scale of serious positron production for physics, the LDRD could be followed up by another separately funded project aiming at performing the merge, possibly with RF capture (nice but totally out of scope for now).
  • Path forward
    • Andriy and Silviu will devise some reliable numbers for power deposition on the one hand and dissipation limits on the other hand, make sure everything is consistent.
    • Silviu will work out a preliminary budget estimate for the target design and fabrication.
    • Max will meet with injector folks tomorrow to understand how hard our lives will be
    • Max will work with Liz on the budget
    • Converge on a reasonable set of objectives within one week, then write up LOI