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 ID: 276160
Download Presentation The PPT/PDF document "Laser and Optical Issues in" is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
Slide1
Laser and Optical Issues in Gatling Gun DevelopmentBrian 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 cathodesSlide2
parameter
unit
spec
commentwavelengthnm780repetition ratekHz70414.07 MHz / 20 cathodes pulse energy at photocathodeuJ2.8assuming QE=0.2% & 3.5 nC bunch chgaverage laser power needed at cathodeW2assuming QE=0.2%avg laser power outputW4pulse widthnsec1.5Gaussian FWHMjitterpsec10rmsamplitude stability1.00E-03requiresnoise-eatercontrast1.00E-06
Phase I Laser System
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
Electro-opticmodulator
Pulser with Phase-locked loop
4
stage EDFA
10 W
1560 nm
Periodically – poled LiNbO
3
4W
780 nm
CW DFB laser
Accelerator RF refSlide3
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 needed1.5 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 W @1560 nm, 3.9 W @ 780 nmContrast -30 dB in the fundamental, -60 dB at 780 nmSynchronization jitter with respect to RF reference: 10 psec rms 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 photocathodeSlide4
1560 nm Laser schematic. Abbreviations: MZI, Mach-Zender Interferometer, ER extinction ratio, EDFA erbium-doped fiber amplifier, ABC automatic bias control.
Optilab
EDFA laserSlide5Slide6
Optilab
EDFA test results continued
Using 2.8nsec pulse @352 kHz Slide7
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 Slide8
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 entranceReference = pulser RFσ = 1.3mV = 700 fsecReference = Pulser + δf (calibration)Slide9
signal generator 2 (for calibration)
signal generator
Picosecond pulser
Low-pass filter 2 MHzSplitter703.5 MHz bandpass filterlow noise preampFast PhotdiodeAculight LaserMonitorMixer Digital Scope or DAQ systemPhase Stability Measurement Layoutrefsignal 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.Slide10
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 specsSlide11
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 Slide12
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 slowSlide13
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.