June 28, 2012 Brian Sheehy 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

June 28, 2012 Brian Sheehy parameterunitspeccomment wavelengthnm780 repetition ratekHz MHz / 20 cathodes pulse energy at photocathodeuJ2.8 assuming QE=0.2% & 3.5 nC bunch chg average laser power needed at cathodeW2 assuming QE=0.2% avg laser power outputW4 pulse widthnsec1.5Gaussian FWHM jitterpsec10rms amplitude stability1.00E-03 requires noise-eater contrast1.00E-06 Phase I Laser System 10 W Erbium doped fiber amplifier (EDFA) system at 1560 nm, frequency doubled in periodically-poled LiNBO 3 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 Electro-optic modulator Pulser with Phase- locked loop 4 stage EDFA 10 W 1560 nm Periodically – poled LiNbO 3 4W 780 nm CW DFB laser Accelerator RF ref

June 28, 2012 Brian Sheehy 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 nsec FWHM Gaussian pulses EO modulated CW DFB laser for front end 704 kHz (14.07 MHz/20) i.e average power is 9.8 nm, nm Contrast -30 dB in the fundamental, -60 dB at 780 nm Synchronization jitter with respect to RF reference: 10 psec rms beam dynamics requirement not determined, but probably between psec Amplitude stability will need to in the photocathode pulse for eRHIC. Expect maybe from EDFA amplifier and polarization extinction ratio, and use noise-eater before the photocathode

1560 nm Laser schematic. Abbreviations: MZI, Mach-Zender Interferometer, ER extinction ratio, EDFA erbium-doped fiber amplifier, ABC automatic bias control. Optilab EDFA laser

June 28, 2012 Brian Sheehy

June 28, 2012 Brian Sheehy Optilab EDFA test results continued Using 2.8nsec kHz

Frequency doubling module 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

June 28, 2012 Brian Sheehy 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)

June 28, 2012 Brian Sheehy signal generator 2 (for calibration) signal generator Picosecond pulser Low-pass filter 2 MHz Splitter MHz bandpass filter low noise preamp Fast Photdiode Aculight Laser Monitor Mixer Digital Scope or DAQ system Phase Stability Measurement Layout ref signal 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.

June 28, 2012 Brian Sheehy 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/cm 2 larger cores possible may need less energy than current specs

June 28, 2012 Brian Sheehy 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 /4 plate dump difficult but not impossible

June 28, 2012 Brian Sheehy 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

June 28, 2012 Brian Sheehy 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.