Bleedthrough (sometimes we call it “leakage”) Ops Staytreat June 28 – 30, 2016 Matt Poelker with help from John Hansknecht, Reza Kazimi, Todd Satogata, Mike Tiefenback and others

Origin of bleedthrough CEBAF drive laser history 6 GeV versus 12 GeV Outline Origin of bleedthrough CEBAF drive laser history 6 GeV versus 12 GeV Plans to reduce bleedthrough Issues and Questions Ops StayTreat, June 28 - 30, 2016

CEBAF Overview Thanks to Arne Freyberger for slide Ops StayTreat, June 28 - 30, 2016

our three CEBAF Injectors DC beam from a thermionic gun operating at 2mA Our chopper system created the rf time structure 2mA polarized beam? Not an option. This is still difficult today Ops StayTreat, June 28 - 30, 2016

Synchronous Photoinjection A B C A B C …. Beam Current DC beam = bleedthrough 111ps 1497 MHz Extracting DC beam, very wasteful, most of the beam dumped at chopper. Need ~ 2mA from gun to provide 100uA to one hall. Gun lifetime not good enough…..yet. RF Chopper Sets Bunchlength

Gain Switched Diode Lasers – 100% reliable 0.5 GHz 1 GHz 1.5 GHz 2 GHz Diode seed laser Diode amplifier Amp makes DC light Always phase locked to accelerator Relatively easy to turn OFF fast (important) Not easily wavelength tunable Not much power, only ~ 100mW and there’s dc light….

Ti-sapphire Lasers at CEBAF Needed more laser power for high current experiments Diode lasers out, Ti-sapphire laser in Less DC light! but… Re-align Ti-sapphire lasers each week Homemade harmonic modelocked Ti-sapphire laser. Seeded with light from gain switched diode. No active cavity length feedback. It was a bit noisy…. Commercial laser with 499 MHz rep rate from Time Bandwidth Products

Accelerator Downtime FY05Q4 – FY06Q3 Realign Ti-sapphire lasers each week

Fiber-Based Drive Laser 780nm Gain-switched seed Frequency-doubler 1560nm ErYb-doped fiber amplifier John Hansknecht to the rescue…. Ops StayTreat, June 28 - 30, 2016

Fiber-Based Drive Laser ~ 30 ps e-bunch autocorrelator trace CEBAF’s last laser! Gain-switching better than modelocking; no phase lock problems, no feedback Very high power Telecom industry spurs growth, ensures availability Useful because of superlattice photocathode (requires 780nm) Ti-sapp Ops StayTreat, June 28 - 30, 2016

Accelerator Downtime FY06Q4 – FY07Q3 Fiber-based lasers reduce downtime

…DC Light = bleedthrough… A B C A B C…. Beam Current DC beam = bleedthrough 111ps 1497 MHz Gain switched laser systems generate some DC light, which bleeds into adjacent chopper slits (and into Hall D bucket). Modelocked lasers generate less bleedthrough, but very reluctant to revisit them…. RF Chopper Sets Bunchlength

RF Separation – Existing (500 MHz) B A C Turn on RF separator Beam to Halls Hall Lasers Hall A: 500 MHz C B A C B A C B A C B A Hall B: 500 MHz C B A C B A Beam Hall C: 500 MHz C B A C B A C B A B A C B C A A B C C B A C B A Beams begin to separate Accelerator Frequency 1500 MHz Separator frequency is 1/3 of the fundamental frequency Recirculated Beams RF Separator Cavity 500 MHz Thanks to Tom Oren and Reza Kazimi for this cool slide B A C Ops StayTreat, June 28 - 30, 2016

6 GeV Bleedthrough B C A A B C C B A Upstream of the chopper…. B C A A at the separator…. DC beam from each laser generates beam that passes through neighboring chopper slits Ops StayTreat, June 28 - 30, 2016

6 GeV Bleedthrough Each laser makes beam for the assigned hall, plus a few nA for neighboring halls (when halls are operating with a fully open chopper slit) In practice, this is only a problem for Hall B: bleedthough can dilute their beam polarization Solution: extract ~ 10 uA for Hall B and “neck down” their slit. Make bleedthough a small contribution to total current, polarization remains high Ops StayTreat, June 28 - 30, 2016

RF Separation – Proposed (750 MHz) B A C Existing Frequency Proposed Frequency A/B/C separator is located downstream C B A Turn on RF separator Beam to Halls B C A C B A 750 MHz 500 MHz C B A C B A C B A C B A Beam C B A C B A C B A C B A D C B A B C A C B A Hall Lasers Hall A: 250 MHz Hall B: 250 MHz Hall C: 250 MHz Accelerator Frequency 1500 MHz C B A Empty Buckets – Cutting the laser pulse rate for each hall in half (from 500 MHz to 250 MHz) leaves “empty buckets”, which are directed to Hall D Four-Hall Operation! (D+3) Every other “bucket” goes to Hall D Separator frequency is 1/2 of the fundamental frequency Hall D Beam (existing hall lasers run at 500 MHz) Hall D: 250 MHz 5th Pass RF Separator Cavity 750 MHz New Hall D laser fills empty buckets at 250 MHz B A C Thanks to Tom Oren and Reza Kazimi for this cool slide

12 GeV Bleedthrough B C A D at the separator….conventional 6 GeV bleedthrough B C A A B C D B A C D D …plus the 12 GeV self-leakage, i.e., bleedthrough bunch 2ns from “good” bunch Self-leakage bleedthrough from A, B and C lasers – all of it goes to Hall D Hall D self-leakage bleedthough will go to the hall with shared chopper slit Ops StayTreat, June 28 - 30, 2016

12 GeV Bleedthrough Same as 6 GeV bleedthrough – DC beam bleeding through neighboring chopper slits… …and now with 249.5 MHz lasers, bleedthrough can pass through the same slit (what Joe calls self-leakage) producing a bleedthrough bunch 2ns behind “good” bunch. Halls expecting 249.5 MHz beam also get beam at 499 MHz New 12 GeV bleedthrough problem: Hall D gets beam when their laser is OFF, problematic when radiator is inserted or retracted, can’t just close slit Solution: turn OFF ALL lasers or insert a beam dump Ops StayTreat, June 28 - 30, 2016

Bleedthrough “Conclusions” Bleedthrough electrons originate at the injector chopper, and are delivered to an unintended hall Bleedthrough electrons are produced at the photocathode by DC laser light Bleedthrough electrons pass through neighboring chopper slits (“old” 6 GeV bleedthrough) Now in 12 GeV era with 249.5 MHz lasers, bleedthrough electrons can also pass through the intended hall slit, producing a bleedthrough bunch delayed by 2ns from “good” bunch Ops StayTreat, June 28 - 30, 2016

Bleedthrough “Conclusions” continued Mostly, bleedthrough is a problem for Hall B – polarization dilution. Solve this problem by extracting ~ 10uA for Hall B and “neck down” their slit For 249.5 MHz lasers, the bleedthrough beam that is produced by A, B and C lasers will be delivered to Hall D. Only a problem when Hall D wants to insert or retract their radiator Hall D’s bleedthrough beam will go to the hall of shared chopper slit., i.e., Hall A or Hall C Ops StayTreat, June 28 - 30, 2016

How to Minimize Bleedthrough? It’s the Gun Group’s job to reduce bleedthrough Great reluctance to revisit Ti-sapphire lasers, trading the bleedthrough problem for poor reliability problem This summer, we will determine if bleedthrough originates from seed laser, the fiber preamp or the fiber amplifier. Optimized seed and preamp settings could reduce bleedthrough, or we could install fast rf-attenuators If dc light originates at the fiber amplifier, not so many options exist. Perhaps a semiconductor saturable absorber mirror (SESAM) We are always going to live with bleedthrough… Ops StayTreat, June 28 - 30, 2016

Issues and Questions So do you agree with me? DC light makes electrons that go to neighboring halls, but aside from polarization dilution at Hall B, and our inability to completely turn OFF beam at Hall D when they insert/retract a radiator, why do we care? We lived with bleedthrough in 6 GeV era, the 12 GeV era looks to be pretty much the same from a bleedthrough perspective… How to explain high background at Hall A compton polarimeter? Separator setup problem? Maybe I’ve oversimplified story by neglecting that sometimes lasers are not coincident at the photocathode, and the chopper is not always perfectly setup. i.e., maybe bleedthrough looks like different beam with different optical properties and transport conditions, and this beam goes where we don’t want it? Ops StayTreat, June 28 - 30, 2016

Issues and Questions How to help Ops determine where the problem originates, bleedthrough at injector chopper, or separator setup issues? Let’s start using the Bleedthrough script again CIS to reduce bleedthrough as much as possible CIS to ensure laser spots are coincident at photocathode and the laser beams are same size Can we improve our chopper setup procedure? How to quickly know the chopper is setup properly, and to fix the setup quickly when required? Like before, troubleshooting will involve a combination of turning OFF lasers and closing slits We will likely benefit from having a 4-slit chopper. Having two beams share a chopper slit will certainly add confusion when troubleshooting bleedthrough Ops StayTreat, June 28 - 30, 2016

Questions? Objections? Thank You Ops StayTreat, June 28 - 30, 2016

Beamline vacuum window Laser Table Now 3-laser configuration Pockels Cell RWP IA IHWP Beamline vacuum window Tune Mode Generator Ops StayTreat, June 28 - 30, 2016

Beamline vacuum window Laser Table Summer 2016 New 4-laser configuration IA Pockels Cell RWP IHWP Tune Mode Generator Beamline vacuum window Ops StayTreat, June 28 - 30, 2016

Halls A,B,C 5th Pass Vertical Separation To Halls A,B,C C C C C Beam C C A C B B B B B B Accelerator Frequency 1500 MHz A A A A A RF Separator Cavity 500 MHz Ops StayTreat, June 28 - 30, 2016

5th Pass RF Separation Hardware Horizontal 5th Pass Separator (kicks out Hall D) Vertical 5th Pass Separator (Halls A,B,C) D+2: 500 MHz D+3: 750 MHz