1. Magneto-Optic Kerr Effect in a Magnetized Electron Gun
Benjamin Hardy, Bowling Green State University
Joseph Grames, Thomas Jefferson National Accelerator Facility
Motivation
Photocathode/ Vacuum Chamber
Solenoid Magnet
Beamline
Abstract
A magnetized electron beam is desired to “cool” ion beam in electron-ion collider (EIC)
Electrons produced in a longitudinally magnetized field are magnetized
Need to measure magnetic field at photocathode however Photocathode is in Vacuum
Magnetized electron sources have the potential to improve ion beam cooling efficiency. At the Gun Test Stand at Jefferson Lab, a solenoid magnet will be installed adjacent to the photogun to magnetize the electron beam. Due to the photocathode operating in a vacuum chamber, measuring and monitoring the magnetic field at the beam source location with conventional probes is impractical. The Magneto-Optical Kerr effect (MOKE) describes the change on polarized light by reflection from a magnetized surface. The reflection from the surface may alter the polarization direction, ellipticity, or intensity, and depends linearly upon the surface magnetization of the sample. By replacing the photocathode with a magnetized sample and reflecting polarized light from the sample surface, the magnetic field at the beam source is inferred. A controlled MOKE system has been assembled to test the magnetic field. Calibration of the solenoid magnet is performed by comparing the MOKE signal with magnetic field measurement. The “Kerr-mometer ” will provide an adequate description of the field at the electron beam source. The report summarizes the method and results of controlled tests and calibration of the MOKE sample with the solenoid magnet field measurements.
Photocathode surface
2 types of photocathode geometries investigated:
Steel and Molybdenum
Modulated Polarization Method
Results
Ellipticity
Rotation
𝜀 𝑘𝑒𝑟𝑟 = √2 4 𝐽 1 𝑉 1𝑓 𝑉 𝐷𝐶
𝜃 𝑘𝑒𝑟𝑟 = √2 4 𝐽 2 𝑉 2𝑓 𝑉 𝐷𝐶
VDC
Conclusions
Permanent Magnet tests show setup works outside of large B fields
Stability of Puck is incredibly important during experiment
Kerr-mometer will be used at electron beam source
Step 3
(PEM) modulates the polarization of light by applying stress at resonant frequency (≈42. kHz)
The photo-elastic effect is the induced circular birefringence of a material (silica crystal) due to applied stress
V2f
V1f
Step 4
Set up propagates the Kerr effect (ε and Ө) at harmonics of the PEM frequency
Lock-In Amplifiers detect the 1f and 2f amplitudes of the analyzer light to extract the Kerr signal
PEM
Step 2
Laser reflects off magnetized iron foil on puck
Circular birefringence result: rotation (Ө) and change in ellipticity (ε)
Biased Photodiode w/ Preamp
Analyzer 45⁰
Lens
Solenoid
Step 1
Green laser polarized at 0⁰
Acknowledgements
Steel Puck
Joe Grames, for being a friend and a mentor that I can count on to push me further as a physicist. Hari Areti for his care and giving me the chance to work at Jlab for the summer. The Center for Injectors and Sources for all of their support. Riad Suleiman for his willingness to help. Shukui Zhang for always being available to assist in our experimentation by loaning equipment and advice. Jay Benesch for the magnetic model graph. Matt Poelker for his encouragement. Bubba Bullard for polishing the foils. Mike Beck and Joe Meyers for their help and enthusiasm in the MMF. Lisa Surles-Law for her energy and encouragement.
References
Pure Fe foil (polished)
Polished iron foil on steel and molybdenum pucks
The Center for Injectors and sources website
Laser
Molybdenum Puck
Polarizer 0⁰
* This work is supported by the National Science Foundation, Research Experience for Undergraduates award and the Department of Energy, Laboratory Directed Research and Development contract DE-AC05-06OR23177.