Beam Tests of Beampipe Coatings for Electron Cloud Mitigation in Fermilab Main Injector

TL;DR

This study evaluates the effectiveness of various beampipe coatings in reducing electron cloud formation in the Fermilab Main Injector through extensive testing over five years, providing insights into material performance and environmental sensitivities.

Contribution

It provides a comprehensive comparison of titanium nitride, amorphous carbon, and diamond-like carbon coatings for electron cloud mitigation in a high-energy accelerator.

Findings

Titanium nitride and amorphous carbon coatings significantly reduce electron cloud density.

Contamination from vacuum leaks can compromise coating effectiveness.

Stray magnetic fields and bunch length influence electron cloud signals.

Abstract

Electron cloud beam instabilities are an important consideration in virtually all high-energy particle accelerators and could pose a formidable challenge to forthcoming high-intensity accelerator upgrades. Dedicated tests have shown beampipe coatings dramatically reduce the density of electron cloud in particle accelerators. In this work, we evaluate the performance of titanium nitride, amorphous carbon, and diamond-like carbon as beampipe coatings for the mitigation of electron cloud in the Fermilab Main Injector. Altogether our tests represent 2700 ampere-hours of proton operation spanning five years. Three electron cloud detectors, retarding field analyzers, are installed in a straight section and allow a direct comparison between the electron flux in the coated and uncoated stainless steel beampipe. We characterize the electron flux as a function of intensity up to a maximum of 50 trillion protons per cycle. Each beampipe material conditions in response to electron bombardment from the electron cloud and we track the changes in these materials as a function of time and the number of absorbed electrons. Contamination from an unexpected vacuum leak revealed a potential vulnerability in the amorphous carbon beampipe coating. We measure the energy spectrum of electrons incident on the stainless steel, titanium nitride and amorphous carbon beampipes. We find the electron cloud signal is highly sensitive to stray magnetic fields and bunch-length over the Main Injector ramp cycle. We conduct a complete survey of the stray magnetic fields at the test station and compare the electron cloud signal to that in a field-free region.