Intensity Feedback System for Laser Beam Control
The intensity feedback system aims to maintain stable beam current and investigate DC gun performance. Using motor-controlled half wave plates and electro-optical modulators, the system achieved 2% rms stability. Initial measurements with photodiodes revealed challenges, but signals from the gun table were successfully used for feedback. Low-frequency disturbances were identified as the main source of variations in beam current.
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Laser and Beam Intensity Feedback K. Mernick 11/8/2018
Outline The goal of the intensity feedback is to have stable beam current. It became a high priority at the end of the run in order to study DC gun performance and separate beam loading effects from power supply ripple. The RF group got involved because we have significant experience with feedback design and implementation, existing hardware, firmware, and software used in many different systems, and could get something running quickly. After deciding on this approach on 8/21, we were testing feedback on the laser (without beam) by 8/30, with beam by 9/4, nested loops for laser and beam current tested on 9/7, and had a system usable without expert intervention by 9/13. Performance of the feedback system meets the requirement of 2% rms stability with significant margin. Some details to improve operational robustness and accommodate changes in the laser setup will be required before the next run.
Laser Intensity Control There are two devices that implement the intensity control of the laser A motor-controlled half wave plate provides a large dynamic range with slow response An electro-optical modulator provides a fast response, but has limited range Initial attempts at feedback using photodiodes in the laser trailer were not successful the intensity of the laser pulses delivered to the cathode did not correlate well with the measurements from the trailer EOM (Fast Intensity Control) HWP (Slow Intensity Control) Temp. Shaping Aperture Transport Laser source Gun Table
Laser Intensity Measurements Initial measurements for intensity feedback were done with slow photodiodes, measuring at baseband. We looked at the 704 MHz component from 3 faster diodes to determine if we could find a good signal for the input to the feedback system. The measurements after the shutter in the laser trailer and on the relay table in the laser transport did not correlate well with beam current. The measurement on the gun table did, so we went ahead with that signal as the input to the feedback. Laser trailer EOM (Fast Intensity Control) HWP (Slow Intensity Control) Temp. Shaping Aperture Transport Laser source Gun Table
Measured Disturbances Low frequency disturbances contributed the majority of the variations in the beam current 5 10 beam pickup photodiode 4 10 3 10 2 10 1 10 Power density 0 10 -1 10 -2 10 -3 10 -4 10 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Frequency (Hz)
Measured Disturbances 5 10 beam pickup photodiode Same as last spectrum, zoomed in x-axis 4 10 3 10 2 10 1 10 Power density 0 10 -1 10 -2 10 -3 10 -4 10 0 50 100 150 200 250 300 350 400 450 500 Frequency (Hz)
Feedback Design DC Gun EOM (Fast Intensity Control) HWP (Slow Intensity Control) Laser source Aperture Transport Gun Table * * ??(?) FCT + mode Button Pickup Laser reference ?? Beam current reference ??(?) ?? Acceleration, Transport, Cooling + ? ??,?? * *
Feedback Implementation The EOM control block is basically identical to the cavity amplitude control for the 200 MeV H- LINAC (DC voltage control of a modulator on PA anode HV). The nested loop structure for beam current and laser intensity is similar to an amplifier linearization loop inside a cavity feedback loop.
Feedback Performance Laser Intensity / Beam Current 0.15 Laser feedback without beam current feedback 0.1 0.05 dI/I 0 -0.05 -0.1 laser, fb on, rms = 0.003 beam, fb on, rms = 0.007 laser, fb off, rms = 0.035 beam, fb off, rms = 0.038 0 5 10 15 20 25 30 Time (s)
Feedback Performance 0.04 approx rms amplitude of frequency components > f laser, fb off beam, fb off laser, fb on beam, fb on 0.035 0.03 0.025 Laser feedback without beam current feedback 0.02 0.015 0.01 0.005 0 0 20 40 Frequency (Hz) 60 80 100
Feedback Performance Laser Intensity / Beam Current 0.02 laser, fb on (10 mA), rms = 2.70e-03 beam, fb on (10 mA), rms = 2.40e-03 laser, fb on (20 mA), rms = 4.27e-03 beam, fb on (20 mA), rms = 3.01e-03 Beam current & laser feedback 0.015 0.01 0.005 dI/I 0 -0.005 -0.01 -0.015 -0.02 0 5 10 15 20 25 30 Time (s)
Feedback Performance -3 x 10 approx rms amplitude of frequency components > f laser, fb on (20 mA) beam, fb on (20 mA) laser, fb on (10 mA) beam, fb on (10 mA) laser, fb on (20 mA) beam, fb on (20 mA) laser, fb on (10 mA) beam, fb on (10 mA) -3 Beam current & laser feedback 10 4 -4 10 3.5 -5 10 3 Power density 2.5 -6 10 2 -7 10 1.5 -8 10 1 -9 10 0.5 -10 0 10 0 20 40 Frequency (Hz) 60 80 100 0 1 2 3 10 10 Frequency (Hz) 10 10
To Do Going Forward The EOM provides a (fast) voltage controlled attenuator. The HWP needs to be set so that there is enough headroom in the laser power for the EOM to regulate intensity. A script automatically triggered by setpoint changes was used to send commands to the motor controller for the HWP based on an open-loop estimate of its required position. We plan to implement feedback to set the HWP position to keep the EOM control voltage around some nominal level. This is conceptually similar to algorithms we have used for coarse and fine cavity tuners. The technical hurdle for this is just making the pieces talk through the Control System. We need to coordinate this with the Controls and Instrumentation groups. (We have done this before for motors on various cavity tuners, but, on new systems, have taken full control of the motor with a controller integrated into the LLRF platform.) The planned changes to the components on the laser table should not have a major impact on the feedback system. EOM (Fast Intensity Control) HWP (Slow Intensity Control) Temp. Shaping Laser source AOM
Conclusion Intensity feedback achieved 0.3% rms beam current stability. Minor changes are necessary for the next run, but the overall design has proven successful.
