Fermilab / UChicago Electron-Lens Meeting

Wednesday 6 Sep 2023, 11:00 → 12:00 US/Central

11:00 → 11:20 General Updates

• The Fermilab e-lens test stand is operational and Giulio is finishing up tests with the Gaussian electron gun after which he will test a hollow electron gun for CERN.
• We discussed funding for the McMillan source purchase for which we already have quotes thanks to John. The total cost will be approximately $100k for all the materials. This purchase can be funded either by Fermilab funds or by Young-Kee from her university research account. An important thing to note is that the Fermilab fiscal year ends on September 31. However, this doesn't matter since the funds automatically carry over to the next year, and a big purchase will likely take more than a month to go through the Fermilab purchasing process. Young-Kee will discuss with Giulio offline about funding from UChicago.
• Young-Kee will explore whether any of the graduate students of the incoming cohort at UChicago is interested in accelerator physics. She wants some help in advertising possible thesis and 335 project topics related to the IOTA e-lens project.
• MaryKate is finishing up the documentation regarding the electron sources for cooling. She will provide the actual source parameters and the beam profile once everything is finalized. She'll also participate in the NIOLD experiment which will happen at IOTA in the coming weeks.
• Sergei K. has been working on Noise in Electron Bunches project full time and did not have any updates for this group.
• It's important to note that both MaryKate and Sergei K. has moved on to other projects. Only Giulio, John and Nilanjan are actively working on e-lens related topics. Should we reconsider the frequency of the e-lens meeting? Also MaryKate has a conflict with the usual Wednesday 11 am CT meeting time.

11:20 → 12:00 Influence of Solenoid Field Quality on Electron Cooling

Speaker: Nilanjan Banerjee (Fermilab)

2023_09_05_SolenoidQuality.pptx

• The space-charge voltage depression of a few volts on top of a beam voltage of 1.3 kV is quite a large number in terms of effective velocity in the beam frame. In high-energy coolers the relative energy deviation is usually one order of magnitude smaller.
• The energy at the center of the electron beam is matched to the center-of-mass energy of the ions for all simulations shown here.
• The field quality i.e the ratio of the transverse and the axial magnetic field is measured at the periphery of the good-field region and hence the maximum ratio is quoted. However, the effective temperature which goes into the Parkhomchuk model is actually a measure of typical field quality as seen by the ensemble of electrons encompassing a large volume inside the good-field region. Consequently, simply estimating the effective temperature contribution from the maximum field deviation is an overestimate of the effect of field quality on the cooling rate.
• One interesting question, not addressed in this work is whether magnetic field variations affect the effective temperature in the Parkhomchuk model? Or does it predominantly affect the scattering impact factors and hence the Coulomb logarithm? This was the subject of microscopic N-particle simulations started by D. Bruhwiler et al. for a historical electron cooler design for RHIC. The historical work was never concluded. If only the Coulomb log is affected then the implication is that the cooling force is relatively insensitive to magnetic field variations. However, the present work assumes that the Coulomb logarithm remains unaffected while the effective velocity changes.
• Apart from assuming a constant Coulomb log, the present work also uses the deterministic electron velocities induced by space-charge and magnetic field variations to define local frames of reference to calculate the cooling force. This is a "misuse" of the Parkhomchuk formula which was developed more as a simple fit to the general shape of the cooling force of an ensemble of ions observed in real experiments rather than a model for the actual microscopic dynamics of electron-ion interaction.
• Based on the above principles, a simulation suggests a factor of 3 reduction in cooling rates for the existing superconducting solenoid design. This is not an acceptable amount of degradation.
• This work makes predictions of the cooling rate reduction in the presence of a specific magnetic field model such as for the IOTA solenoid. A more general result addressing the question of how much magnetic field variation can we tolerate in practice would be another useful result.