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Upgrades and Plans for UED Facility at BNL

This document provides an overview of the upgrades and plans for the Ultrafast Electron Diffraction (UED) Facility at Brookhaven National Laboratory (BNL), including refurbishment of the photoinjector, laser room improvements, environmental control upgrades, low-level RF stability improvements, and more.

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Upgrades and Plans for UED Facility at BNL

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  1. UED Capability & Status Marcus Babzien 21st ATF Program Advisory Committee (APAC) and User’s Meeting Brookhaven National Laboratory November 14-16, 2018

  2. UED in BNL Building 912 • First and only operational element of ATF-2 in Buiding 912 is the UED Facility • ATF focus is developing this accelerator-based capability to its fullest extent • Experimental station treated as a co-developed “beam line” for material science users • Required element to validate the accelerator capabilities UED and Laser Room

  3. Overview of UED Upgrades & Plans • Gun refurbishment • Cathode re-polishing • Laser window improvement • Vacuum improvement • Environmental control upgrades • Low-level RF stability improvements • Drive laser pulse shortening • Support tunable pump wavelength • Digital low-level RF with diagnostics & lower drift • Bunch length diagnostic • Repetition rate increase study • Study optimal operating point

  4. Photoinjector Refurbishment Before After UV images of cathode surface Work: Cathode re-polishing process updated & validated Gun opened and cathode backplate re-polished Laser delivery improved by removal of secondary entrance/exit windows Vacuum level improved by replacement of pumps/gauges/seals Results QE only ~50% than before polishing without laser Much improved ratio of photocurrent to dark current No indication of multiple bunches

  5. Environmental Control Upgrades • 2 redundant high-capacity outdoor chiller units installed • Chilled water cooling distribution system in place • UED Cleanroom operating on new high capacity chilled water supply • Previous dedicated chiller in place as backup unit • Facility operation now possible independent of weather conditions

  6. LLRF stability improvements Original Calibration During Optimization f Power • Added slow phase control loop with independent phase measurement and software feedback to overcome drift that degraded long-integration time experiments • Beam setup and tuning enhanced by improving IQ modulator calibration: now provide a much larger range of usable & orthogonal phase and amplitude adjustment to operators

  7. Drive Laser Pulse Shortening • Order placed mid-September for custom 100 fs oscillator as drop-in replacement, expect installation Spring 2019 Drive laser currently produces 160-180 fs pulses for UED sample pumping. (e- probe is typically shortened via compression) The drive laser amplifier stretcher/compressor/optics are designed for bandwidth supporting down to 50 fs pulses, and the current 160 fs oscillator does not produce a chirped pulse long enough to completely optical avoid damage at nominal output energy Shorter oscillator seed pulse would lead to improved temporal resolution closer to that available at other UED facilities

  8. Digital Low-Level RF Diagnostic Will replace existing analog chain Although phase noise floor is ~3 dB higher, internal mixers and diagnostic elements will provide more comprehensive monitoring, jitter & drift measurement, and feedback Modified version of chassis used extensively at NSLS-II Unit in place & ready for installation & interfacing

  9. Support Tunable Pump Wavelength Early career award for mid-IR pump-probe materials science requires an Optical Parametric Amplifier to access the full range of wavelengths To reach the needed OPA output energy levels of ~100 mJ at l=10-11 mm, a more energetic pump pulse from a new Ti:sapphire booster amplifier is needed to pump the OPA with 25 mJ “Topas HE” OPA on order, awaiting final input beam specifications to proceed to manufacturing All amplifier parts, pump laser, and CPA compressor ready for integration & testing

  10. Bunch Length Diagnostic • CTR Interferometer & Detector commissioned by LDRD experiment Currently installed, but must be removed to pass large diffracted UED beam More compact version to be designed and tested downstream at UED screen, then moved upstream to UED sample location for routine use

  11. Repetition Rate Increase BNL UED has higher charge than many other UED facilities, but would still benefit from increased repetition rate Long integration times limit the rate of parameter scans and increase sensitivity to drift The present limiting systems were identified and the effort required for increasing the repetition rate were surveyed Klystron is capable of 50 Hz, radiation study may impose additional shielding requirements No hardware roadblocks to 50(48) Hz operation were identified

  12. Optimal Operating Point Study Probe pulse charge should be maximized for single-shot sensitivity, but spot size increases and reduces spatial resolution A systematic study of optimized spot size versus charge will enable selection of optimal tradeoff for different experimental needs Needs dedicated running time in schedule

  13. Conclusions Significant upgrades to the UED facility have been completed this year Both instrument operating parameters and operational efficiency have been improved Near term upgrades underway UED will continue to evolve as a productive facility

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