1. Beam-Based Feedback in LCLS-II
P. Emma
March 30, 2017

2. Beam-Based Feedback Loops
Beam-Based Feedback is Separate from RF-Based Feedback
Transverse Feedback (LCLSII-2.7-PR-0075)
Read 5-10 BPMs (X & Y) with ~p/2 net b-phase separation
Control 2 XCOR’s and 2 YCOR’s upstream of the BPMs (also ~p/2)
Maintains ref. beam position (DX & DY) at BPMs & collimators
Longitudinal Feedback (LCLSII-2.7-PR-0076)
Read 4 bend system BPMs and 2 relative bunch length monitors
Control RF voltage and phase of each upstream main linac section
Maintains e- energy and peak current at each location
Timing Feedback (LCLSII-2.7-PR-0076, no cavities in baseline)
Fast vs. Slow Feedback
At commissioning time we need mainly slow drift-feedback (~1 Hz)
Upgrade to fast-feedback may be justified later (operations exp.)
Slow feedback is very useful at re-start, or during tuning
Some concern of cross-talk over various loops (diff. time scales?)

3. LCLS-II Transverse Feedback Stations (DRAFT)
“28NOV16”
m-wall
BC2
LTUS
SXR L2 L3
Sec Linac
Sec Linac
L3
BSYDMP 
Bypass
LTUH
HXR 3
HTR
CM02
CM03
BC1 L1
CM01
3.9
Gun Launch 
L1
L2 
heater
 IR
 UV
DIAG0

4. Beam-Based Trajectory Feedback
LCLSII-2.7-PR-0075

5. Gun-Launch Angle (X & Y position only)
F-Cup
BPM02
SOL01
YAG01
SOL02
BPM
X or YCOR
Gun-Launch keeps e- centered at CM01-entrance

6. Heater-Launch Loop
CM01
HTR
BPM
X or YCOR
Don’t use cold magnets in feedback loops due to hysteresis?
Heater-Launch keeps e- aligned with heater IR beam

7. DIAG0-Launch Loop (low rate, off-axis beamline)
Kicker
BPM
X or YCOR
DIAG0-Launch keeps e- centered on diagnostics

8. L1-Launch Loop Keeps beam centered on 4 collimators
Kicker
STC0
BPM
X or YCOR
Collimator
c=45
Keeps beam centered on 4 collimators
Still works when OTRC006 or STC0 stopper are inserted
Should correctors be downstream, of DIAG0 kicker?

9. L2-Launch Loop BC1 L2 Keeps beam centered on 4 collimators BPM
X or YCOR
Collimator

10. Maintains launch into long L3-linac
L3-Launch Loop L3
BC2
BPM
X or YCOR
Maintains launch into long L3-linac

11. Bypass-Launch Loop c=45 Keeps beam centered on 4 collimators BPM
X or YCOR
Collimator

12. LTUS-Launch Loop BY2 c=45 Keeps beam centered on 4 wire-scanners
BPM
X or YCOR
Collimator

13. SXR-Launch Loop (fast loop?)
TDUND
BPM
X or YCOR
Keeps beam launched into SXR undulators
Last loop before und. so might convert to fast loop later?

14. BSY-Dump Loop Keeps beam on BSY dump Need both position and angle?
new XCOR & YCOR needed
BSY
Dump
BPM
X or YCOR
Keeps beam on BSY dump
Need both position and angle?
If yes, needs 2 new correctors
Fast raster-coils are separate

15. Beam-Based Longitudinal Feedback
Slow (drift) feedback initially (Matlab)
Read energy BPMs & bunch length monitors
Adjust RF phases and amplitudes (global)
Also holds bunch charge constant
Maybe upgrade to high rate later
Similar system works well at LCLS-I E V0 j1 V1 j2 V2 j3
CM01
CM02,3
3.9GHz
CM04
CM15
CM16
CM35
Gun
750keV L0
12A LH
100MeV L1
12A HL
BC1
250MeV L2
80A
BC2
1.6GeV
L3 (10)
1.0kA
BYP/LTU
4.0 GeV
BPM
Bunch Length
LCLSII-2.7-PR-0076