June 28, 2012

Brian Sheehy

Laser and Optical Issues in Gatling Gun Development

Brian Sheehy June 28, 2012

I. Laser description for Phase I experimentsII. 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

parameter unit spec comment

wavelength nm 780

repetition rate kHz 70414.07 MHz / 20 cathodes

pulse energy at photocathode uJ 2.8

assuming QE=0.2% & 3.5 nC bunch chg

average laser power needed at cathode W 2

assuming QE=0.2%

avg laser power output W 4

pulse width nsec 1.5 Gaussian FWHM

jitter psec 10 rms

amplitude stability 1.00E-03requiresnoise-eater

contrast 1.00E-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 EDFA10 W

1560 nm

Periodically – poled LiNbO3

4W780 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 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 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 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 pulse @352 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

-0.05 0 0.05-6

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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 MHzSplitter

703.5 MHz bandpass filter

low noise preamp

Fast Photdiode

Aculight Laser

Monitor

Mixer

Digital Scope or DAQ system

Phase Stability Measurement Layout

ref

sign

al

• 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.

)cos(

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/cm2

• 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• l/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.