5/11/23 -- Scope updates following pre-proposal submission -- Max, Silviu, Andriy, Victor

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While we have not gotten any reponse to the pre-proposal yet, there have been two dominant pieces of hypothetical criticism:

  1. CEBAF is not a good place for this type of experiment because of the experimental schedule forcing everything to happen during the SAD, and
  2. The anticipated scientific output may not be enough, i.e., what do we learn from a measurement that we wouldn't be able to just simulate?

The project can be subdivided into the following components/stakeholders:

  1. Build the target, plan for heating measurements (Silviu)
    • Thermometry includes temperature sensors on the target ladder as well as a pyrometer on a motion stage to measure the temperature distribution in at least one axis.
    • The wiggling mechanism will be included.
    • Addressing criticism 2: There is a discrepancy between the simulated power deposition and the analytical formula, raising doubts about the validity of the respective underlying models. By JLab standards, the target is unusually thick compared to the radiation length; at the same time, it is the highest-power beam target JLab has employed so far. This makes the predictions hard to compare with existing experience.
    • We can use Silviu's CAD pictures of the target ladder for the proposal. We will have to see if they provide a good sense of scale.
    • There is a pending proposal to test different W/Cu alloys instead of pure W, the potential upsides relating to a better tradeoff of thermal properties and positron yield. Regardless of whether that gets funded or not, including this option in our proposal as well seems valuable and costs very little.
    • While at first glance it seems appealing to do a comparative measurement by heating the target with a high-power laser or some other external means, the comparison would not work because the heat source would not penetrate the bulk of the material in the same way an electron beam will, instead depositing all the heat on the surface. Given the thickness of the target, we expect this difference to affect the thermodynamics significantly. We will forgo these ideas and focus on the electron beam.
  2. Put beam on the target (Max)
    • Addressing criticism 1: The experiment could be done either at CEBAF, LERF, or UITF, with the respective caveats. We do not have to make this decision now, but being flexible and providing some example scenarios will make the proposal more appealing.
  3. Post-irradiation damage assessment (Andriy)
    • Structural defects of irradiated samples are difficult to measure, damage is localized to small area; regardless, to study any damage that scales with integrated charge, we should plan to irradiate multiple samples with different amounts of integrated charge under the same conditions (scale run time, not current). Silviu says the turn-around time for target swaps is on the order of one shift.
    • There may be synchrotron-light-based ways to assess structural defects, but they would be expensive unless we had a collaboration partner that would do it for free.
    • Mechanical damage like cracks will readily show up on an SEM image. SEM images look good and help address criticism 2, so we will find a way to take them. This should be cheap and comparatively easy.
  4. Simulate dynamics of particle shower (Victor)
    • Knowing where the particles go is superficially important for thermal management.
    • Detailed studies of the dynamics of the shower in suitable field configurations also open a pathway toward a small positron beam source.
    • Andriy will simulate an example phase space of the full shower after the target for some set of parameters that makes sense, assuming there are no further components after the target, just so we can write something about how to manage the thermal load downstream a little more precisely.

Max is largely unavailable in June and is planning to write up the full proposal by the end of May.