1. 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

2. 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 , 2016

3. CEBAF Overview Thanks to Arne Freyberger for slide
Ops StayTreat, June , 2016

4. 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 , 2016

5. 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

6. 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….

7. 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

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

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

10. 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 , 2016

11. Accelerator Downtime FY06Q4 – FY07Q3
Fiber-based lasers reduce downtime

12. …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

13. 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 , 2016

14. 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 , 2016

15. 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 , 2016

16. 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

17. 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 , 2016

18. 12 GeV Bleedthrough
Same as 6 GeV bleedthrough – DC beam bleeding through neighboring chopper slits…
…and now with MHz lasers, bleedthrough can pass through the same slit (what Joe calls self-leakage) producing a bleedthrough bunch 2ns behind “good” bunch.
Halls expecting 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 , 2016

19. 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 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 , 2016

20. 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 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 , 2016

21. 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 , 2016

22. 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 , 2016

23. 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 , 2016

24. Questions? Objections?
Thank You
Ops StayTreat, June , 2016

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

26. 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 , 2016

27. 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 , 2016

28. 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