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Laser and Optical Issues in Gatling Gun Development Brian Sheehy June 28, 2012. I. Laser description for Phase I experiments II. Scaling Issues for multiple cathodes synchronization transport III. Other long term optical issues XHV windows with minimal birefringence
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Laser and Optical Issues in Gatling Gun Development Brian Sheehy June 28, 2012 • I. Laser description for Phase I experiments • II. Scaling Issues for multiple cathodes • synchronization • transport • III. Other long term optical issues • XHV windows with minimal birefringence • minimizing stray light & beam halo • homogeneity of bunch charge across 20 cathodes
Phase I Laser System Pulser with Phase-locked loop Periodically – poled LiNbO3 Accelerator RF ref 4W 780 nm Electro-optic modulator 10 W 1560 nm 4 stage EDFA CW DFB laser • 10 W Erbium doped fiber amplifier (EDFA) system at 1560 nm, frequency doubled in periodically-poled LiNBO3 • Continuous Wave distributed feedback laser (CW DFB) + electro-optic modulation for pulse source • control of pulse shape, low jitter • Frequency double to 780 nm in periodically poled material (40% efficiency) • Design allows flexibility in pulse parameters
Laser Requirements • 14 uJ energy per pulse in the 1560 nm fundamental (9 kW peak, 10W avg power) • we will frequency-double to 780 nm in periodically-poled LiNbO3 (PPLN) • expect 40% conversion => 5.6 uJ at 780 nm • for 3.5 nC charge at 0.2% QE, 2.8 uJ is needed • 1.5 nsecFWHM Gaussian pulses • EO modulated CW DFB laser for front end • 704 kHz (14.07 MHz/20) • i.e average power is 9.8 W @1560 nm, 3.9 W @ 780 nm • Contrast -30 dB in the fundamental, -60 dB at 780 nm • Synchronization jitter with respect to RF reference: 10 psecrms • beam dynamics requirement not determined, but probably between 10-100 psec • Amplitude stability • will need 10-3 to 10-4 in the photocathode pulse for eRHIC. Expect maybe 10-2 from EDFA amplifier and polarization extinction ratio, and use noise-eater before the photocathode
Optilab EDFA laser 1560 nm Laser schematic. Abbreviations: MZI, Mach-Zender Interferometer, ER extinction ratio, EDFA erbium-doped fiber amplifier, ABC automatic bias control.
Optilab EDFA test results continued Using 2.8nsec pulse @352 kHz
EDFA module has been tested on site at Vendors and will ship in July • Vendor progress on the doubling module has been very slow. We will implement that ourselves at BNL Frequency doubling module
Scaling to multiple Cathodes: Synchronization • The EO-modulated fiber laser design is extremely stable against timing jitter: no cavity lengths to stabilize, very little is introduced in the pulser electronics. We have tested this with open loop measurements of jitter in a green laser of similar design (Aculight), using a phase detector method (mix reference RF with filtered photodiode signal). • can add fast feedback through the RF driving the pulser, no mechanical components • detectors placed near gun entrance Reference = pulser RF σ = 1.3mV = 700 fsec Reference = Pulser + δf (calibration)
Phase Stability Measurement Layout signal generator 2 (for calibration) signal generator ref Low-pass filter 2 MHz Splitter Mixer Picosecond pulser Monitor signal low noise preamp Aculight Laser Digital Scope or DAQ system 703.5 MHz bandpass filter Fast Photdiode • Extract RF from laser pulse train using fast photodiode + bandpass filter • Mix with reference RF, output • to calibrate (red), drive reference & signal arms with slightly different frequencies • introduces constantly varying phase which yields sinusoidally varying output, the amplitude of which gives the calibration.
Problems in Scaling to multiple Cathodes: Transport • How to manage 20 transport lines to Gun Platform • use large mode area fibers • 15 um core photonic crystal fibers commercially available now • peak intensity at our pulse specs ~ 2 GW/cm2 • larger cores possible • may need less energy than current specs
Problems in Scaling to multiple Cathodes: Transport • Space limitations on Gun Platform table • minimize optics on the table • refractive shaper • relay lenses • pickoff for sampling • l/4 plate • dump • difficult but not impossible
Other long term optical issues • XHV windows with minimal birefringence • using zero-degree sapphire for Phase I • will test depolarization • with wedge/tilt for stray light reduction • pursuing other materials with vendors • stray light reduction • AR coatings capable of withstanding bakeout temperature can be made with ion beam deposition (MPF Products Inc) • working on tilted entry design and dumping window-reflected beam in vacuum • primary reflected beam can be coupled out of chamber • Homogeneity of bunch charge across 20 cathodes • adjustment is easy: laser intensity • need some method of non-destructive charge measurement in the electron beam • use signals from BPM’s, FCT? • inter-cathode variation less problematic than fluctuations from one cathode • each ion bunch “talks” to only one cathode • QE decay is slow
Summary • Phase I laser is under development, 1560 nm section near completion • custom commercial EDFA + in house doubling module • Addressing problems with extrapolation to full 20 cathode gun • Phase I system will be a useful testbed (eg fiber transport, synchronization, noise-eater) • problems are daunting, but not insurmountable.