Ultra-stable flashlamp-pumped laser A.Brachmann, J.Clendenin, T.Galetto, T.Maruyama, J.Sodja, J.Turner, M.Woods

Outline Introduction Laser System Setup Recent Modifications Experimental Results Conclusions and Summary

Introduction SLAC built Flashlamp-pumped Ti:Sapphire laser system Installation in 1993 at the SLAC PES Generation of polarized electrons in combination with SLAC’s Polarized Electron Gun Recent Modifications result in increased stability and output power

Benefit of low jitter Statistics of experiments Reduce Beam loading Reduction of residual dispersion and wakefields Facilitates beam tuning and minimizes losses Time needed to achieve 100 ppb for E-158 assymmetry statistics (for 120 Hz rep. Rate) (assumption that laser is only source of jitter)

Laser System Setup Cavity CCD HBS Spectrometer ‘SLICE’ Photodiode F=750mmF=500mm ‘SLICE’ -PC‘TOPS’ -PC PL /2 flashlamps /2 BrewsterTi:Sapphire PL: Polarizer PC: Pockels cell ‘LONGPULSE’ Photodiode

Laser system periphery SLAC built pulsed power supply SLAC built cooling water system (closed loop > 16 M  ) Commercial Pockels cell driver SLAC built HV power supply and control of TOPS Pockels cell Variety of Controls & Diagnostics integrated into control system (Power supply, Pockels cell HV, Photodiodes, Spectrometer, CCD)

Parameters of operation Modestructure Multimodal (higher order modes dominate) WavelengthTunable (805 nm, 850 nm) Bandwidth0.7 nm Repetition rate120 Hz Peak power of cavity (15  s pulse) 45 mJ ‘Used’ power (50 – 370 ns pulse) 60  J (~ 600  J possible) Stability0.5 %

Temporal pulse profile and timing setup

Recent modifications Cavity optimization according to thermal lensing included in resonator modelling results Elimination of cavity halfwave plate reduces element sensitive to optical damage Wavelength change to 805 nm required by new photocathode yields higher output power  Operation near gain maximum for Ti:Sapphire material

Thermal lensing

Cavity simulations Thermal lens L1L2 flat 2 mcc 5 mcc w0w0 RxRx wxwx x

Spotsize within gain medium as a function of thermal lens and mirror spacing

Wavefront radius of curvature as a function of thermal lens and mirror spacing

Laser stability and e - beam stability near target are highly correlated Jitter [%] 3.95E MEAN TORO 488 TMIT Slice  J (Photodiode) (500 data points)

Optical damage on cavity halfwave plate surfaces (damaged coating)

Controlled crystallographic orientation of laser rod

Power supply stability as a function of high voltage level Jitter [%]HV [kV]

Conclusions and Summary Stable operation of laser systems required for polarized e-beams is achieved ‘Home built’ systems preferred over commercial systems – greater flexibility – better support – straightforward integration into existing control system Development laboratory with duplicate system is essential if continuous production is required

References Humensky et al.; SLAC’s Polarized Electron Source Laser System and Minimization of Electron Beam Helicity Correlations for the E-158 Parity Violation Experiment; NIM; to be published Brachmann et al.; SLAC’s Polarized Electron Source Laser System for the E-158 parity violation experiment; Proceedings of SPIE, Volume 4632, , 2002