MAGNETOOPTIC KERR EFFECT IN A MAGNETIZED ELECTRON GUN

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MAGNETO-OPTIC KERR EFFECT IN A MAGNETIZED ELECTRON GUN Benjamin Hardy, Bowling Green State University

MAGNETO-OPTIC KERR EFFECT IN A MAGNETIZED ELECTRON GUN Benjamin Hardy, Bowling Green State University Joseph Grames, Thomas Jefferson National Accelerator Facility Abstract Motivation Beamline 3500 A magnetized electron beam is desired to “cool” ion beam in electron-ion collider (EIC) 3000 Electrons produced in a longitudinally magnetized field are magnetized Need to measure magnetic field at photocathode however Photocathode is in Vacuum Photocathode/ Vacuum Chamber Steel 2000 1500 1000 Photocathode surface 500 0 2 types of photocathode geometries investigated: Steel and Molybdenum Solenoid Magnet Molybdenum 2500 B (Gauss) 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. Magnetic Model Moly & Steel Pucks 0 10 20 30 40 50 60 70 Z (cm) Modulated Polarization Method V DC Results Rotation 1 f Ө (rad) V Steel Trial 1 0. 002 0. 001 Set up propagates the Kerr effect (ε and Ө) at harmonics of the PEM frequency Lock-In Amplifiers detect the 1 f and 2 f amplitudes of the analyzer light to extract the Kerr signal 0. 0005 PEM -4 -2 0 0. 00012 2 Kerr-mometer will be used at electron beam source Calibration Mirror 2 E-05 Moly Trial 1 0 4 -4 -2 0 2 Kerr ε with arb offsets 0. 00013 0. 00073 Steel Trial 2 0. 00068 Calibration 1 0. 00011 R 2 = 9. 909 E-01 Calibration 2 2 Calibration 2 Step 1 -3 -2 -1 0. 00058 Steel Trial 1 0. 00053 Side of Foil Trial 0. 00048 Calibration Mirror 0. 00043 Mirror Trial 0. 00038 Moly Trial 1 0 1 B field (k. Gauss) Green laser polarized at 0⁰ Polarizer 0⁰ 3 4 9 E-05 Steel Trial 1 7 E-05 Mirror Trial Calibration Mirror 5 E-05 Moly Trial 1 3 E-05 -4 -3 -2 -1 0 1 2 3 4 B field (k. Gauss) Acknowledgements Steel Puck Polished iron foil on steel and molybdenum pucks 2 ε (arb) Ө (rad) R = 9. 739 E-01 -4 4 B field (k. Gauss) 0. 00063 Solenoid * This Mirror Trial 4 E-05 Kerr Ө with arb offsets Circular birefringence result: rotation (Ө) and change in ellipticity (ε) Len s Laser 8 E-05 6 E-05 Step 2 Biased Photodiode w/ Preamp Analyzer 45⁰ Steel Trial 1 B field (k. Gauss) Laser reflects off magnetized iron foil on puck Calibration 2 0. 0001 Moly Trial 1 0 Calibration 1 0. 00014 Stability of Puck is incredibly important during experiment Side of Foil Trial Calibration Mirror Trial 0. 0015 Step 4 Conclusions Permanent Magnet tests show setup works outside of large B fields Calibration 2 0. 0025 The photo-elastic effect is the induced circular birefringence of a material (silica crystal) due to applied stress 0. 00016 Calibration 1 0. 003 (PEM) modulates the polarization of light by applying stress at resonant frequency (≈42. k. Hz) 2 f Steel Trial 2 0. 0035 Step 3 V Raw Kerr ε RAW Kerr Ө ε (arb) Ellipticity Pure Fe foil (polished) Molybdenum Puck References The Center for Injectors and sources website https: //wiki. jlab. org/ciswiki/index. php/Main _Page Joe Grames, for being a friend 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. work is supported by the National Science Foundation, Research Experience for Undergraduates award 1359026 and the Department of Energy, Laboratory Directed Research and Development contract DE-AC 05 -06 OR 23177.