[Paper Review] Report on Instrumentation and Methods for In-Situ Measurements of the Secondary Electron Yield in an Accelerator Environment

W. Hartung, D. M. Asner+9 more|arXiv (Cornell University)|Jul 3, 2014

Particle accelerators and beam dynamics1 references4 citations

TL;DR

This paper presents an in-situ secondary electron yield (SEY) measurement system developed for the Cornell Electron Storage Ring (CESR) to study electron cloud formation in accelerator environments. The system enables real-time SEY measurements on samples under actual beam conditions—accounting for synchrotron radiation and electron cloud effects—using a calibrated electron gun, magnetic shielding, and iterative improvements to reduce leakage currents and transient effects, achieving reproducible SEY data critical for modeling electron cloud growth.

ABSTRACT

The achievable beam current and beam quality of a particle accelerator can be limited by the build-up of an electron cloud (EC) in the vacuum chamber. Secondary electron emission from the walls of the vacuum chamber can contribute to the growth of the electron cloud. An apparatus for in-situ measurements of the secondary electron yield (SEY) of samples in the vacuum chamber of the Cornell Electron Storage Ring (CESR) has been developed in connection with EC studies for the CESR Test Accelerator program (CesrTA). The CesrTA in-situ system, in operation since 2010, allows for SEY measurements as a function of incident electron energy and angle on samples that are exposed to the accelerator environment, typically 5.3 GeV counter-rotating beams of electrons and positrons. The system was designed for periodic measurements to observe beam conditioning of the SEY with discrimination between exposure to direct photons from synchrotron radiation versus scattered photons and cloud electrons. The SEY chambers can be isolated from the CESR beam pipe, allowing us to exchange samples without venting the CESR vacuum chamber. Measurements so far have been on metal surfaces and EC-mitigation coatings. The goal of the SEY measurement program is to improve predictive models for EC build-up and EC-induced beam effects. This report describes the CesrTA in-situ SEY apparatus, the measurement tool and techniques, and iterative improvements therein.

Motivation & Objective

• To develop a reliable in-situ method for measuring secondary electron yield (SEY) on accelerator vacuum chamber surfaces under real beam conditions.
• To distinguish between SEY contributions from direct synchrotron radiation, scattered photons, and electron cloud electrons during beam operation.
• To minimize parasitic conditioning and leakage currents that distort SEY measurements in high-vacuum, high-radiation environments.
• To enable periodic, non-invasive SEY measurements on metal and EC-mitigation coatings without venting the storage ring vacuum.
• To improve predictive models for electron cloud build-up by providing accurate, beam-conditioned SEY data.

Proposed method

• An in-situ SEY station was installed in the CESR vacuum chamber, allowing measurements on samples exposed to 5.3 GeV electron-positron beams without breaking vacuum.
• A calibrated electron gun with electrostatic deflection and focus control delivers a well-defined electron beam at variable energy (10–1000 eV) and incident angle (0°–90°) onto the sample.
• Magnetic shielding and Faraday cups were used to isolate the measurement system from beam-induced currents and electromagnetic interference.
• A semi-empirical leakage current model (Equation 7) was developed to correct for time-dependent leakage and transient currents using measured current decay profiles.
• Inter-system timing protocols were implemented to prevent cross-talk between two simultaneous SEY stations by enforcing 'quiet zones' during current measurements.
• Data acquisition used synchronized bias switching and current sampling with precise timing control (e.g., 60 s wait time after bias change) to ensure stable readings.

Experimental results

Research questions

• RQ1 How can secondary electron yield be accurately measured in-situ under real accelerator beam conditions, including synchrotron radiation and electron cloud exposure?
• RQ2 What are the dominant sources of measurement error (e.g., leakage current, transient response, charging) in high-vacuum, high-radiation environments, and how can they be mitigated?
• RQ3 To what extent does beam conditioning alter SEY values over time, and how can this be distinguished from parasitic effects?
• RQ4 How do environmental factors such as tunnel temperature and humidity affect leakage current and measurement stability?
• RQ5 Can reproducible, high-resolution SEY data be obtained on both standard metals and EC-mitigation coatings under operational beam conditions?

Key findings

• The in-situ SEY system successfully measured SEY on metal and EC-mitigation coatings under 5.3 GeV beam conditions with minimal vacuum disruption, enabling long-term monitoring.
• Leakage current was found to correlate strongly with tunnel temperature, increasing by up to 8°C during beam storage, suggesting thermal or humidity-dependent mechanisms in ceramic insulators or nitrogen blanket.
• The semi-empirical leakage model (Equation 7) effectively corrected for time-dependent leakage, with resistance to ground (R∥) varying between 5 TΩ and 25 TΩ and showing inverse correlation with measured leakage.
• Capacitance-like parameter Γ∥ showed time dependence, possibly due to model limitations or environmental factors, but remained within measurable bounds.
• The system achieved reproducible SEY measurements with energy resolution and spatial control improved through iterative design, including better focus control and grid point segmentation.
• Simultaneous SEY scans on two samples were successfully performed using a 47 s start delay, ensuring no cross-talk, with current measurements confined to 'quiet zones' to avoid interference.