1. LCLS-II physics meeting 3/25/2015
Injector layouts
Injector RF coupler correction w/ a weak quad
Feng Zhou
LCLS-II physics meeting 3/25/2015

2. This is a team work … Tor/JohnS – initiated the layout work for me
Dowell – develop theory on the RF coupler correction with quad
Z. Li showed me about his early work on quad correction for RF couplers
Vivoli (FNAL), Christos P (LBL), Adam (Cornell) and me – simulations and crosscheck with different codes
Chad (LBL) – understand the solenoid and benchmark ImpactT etc
Bruce (Cornell) – provided drawings
Henry – made layouts drawings …

3. Concerns for the baseline layout
Some emittance concerns
Aperture concerns: smaller physical apertures at SOL/BPM
Baseline layout
SOL1
SOL2

4. Baseline layout: emittance concerns
Emittance increases from CDR layout to baseline layout due to the increased drift between SOL2 and CM
Have to turn off both Cav2 and Cav3 to get 0.6 um emittance otherwise the emittance is >0.8um (300pC)
Beam size at the Cav1 coupler is large, causing ~30% emittance growth without coupler correction (300 pC)

5. Baseline layout: aperture concerns (300 pC)
z (m)
half aperture
h (mm)
rms beam size
x (mm)
h/x
SOL01
0.306
14.3
5.47
2.6
valve1
0.481
20.0
5.25
3.8
XC01
0.566
15.0
5.15
2.9
YC01
BPM01
0.629
4.7
3.2
BUN01
0.895
23.8
3.97
6.0
XC02
1.029
25.5
3.57
7.1
YC02
7.2
Laser injection
1.179
11.1
3.4
3.3
IM01
1.324
13.5
3.35
4.0
YAG01
1.468
19.1
3.34
5.7
XC03
1.55
3.37
7.0
YC03
SOL02
1.689
3.39
4.2
BPM02
1.931
2.78
5.4
XC04
1.94
2.76
YC04
valve2
2.079
2.48
8.1

6. E-beam size at SOL1
2.5cm close to physical aperture 2.8cm

7. Proposed beamline modifications
New proposal
Baseline layout
Use Cornell SOL/BPM package (7 cm rather than 3cm of physical aperture)
Move SOL2 upstream
Shorten drift between SOL2 and CM
SOL1
(Cornell)
SOL2
(Cornell)
SOL1
(Apex)
SOL2
(Apex)

8. Solenoid: Apex vs. Cornell
Use same layout: only replace the solenoids
It looks not matter using apex or Cornell solenoids in term of emittance, but they have different apertures:
Cornell 7 cm of physical aperture
Apex 2.8 cm of physical aperture
SOL1/SOL2
Emittance (um)
Cornell/Cornell
0.565
Apex/Apex
0.595
300pC

9. Aperture comparison (300 pC)
h/x
w/ proposed layout
w/ baseline layout
w/ Cornell demonstrated injector
SOL1
6.1
2.6
5.4
BPM1
3.2
5.0
x/y correctors
2.9
SOL2 9
4.2
4.5
BPM2
4.7
Laser injection
Easy to make larger
3.3

10. Emittance comparison (300 pC)
Proposed layout
Baseline layout
Best operating mode (both Cav2 and Cav3 off)
0.56 um
(10% reduction from baseline layout)
0.62um
Mode with one cavity failure (turn on cav2 or cav3 due to one of 4th to 8th cavity failure) um um
Emittance growth (due to the cav1 coupler)
10%
30%
With 1D cavity field
With 3D cavity field including coupler
Note all of my simulations in this talk use conservative cathode/laser parameters: 1) 1um/mm of thermal emittance, and 2) spatial uniform laser profile

11. Tracking with RF couplers w/o correction
300pC
Cav1
Baseline layout: emittance growth 30%
New layout: emittance growth 10%

12. RF coupler correction – quad ‘stigmator’
RF coupler of Cav1 has strong effect on the emittance growth (300 pC):
RF coupler corrections
Standard corrections (FNAL)
Simple and inexpensive but effective method – dc quad ‘stigmator’; motivated by the work: Z. Li Ph. D thesis and D. Dowell LCLS-II tech. note

13. Quad stigmator principle
RF coupler kick:
Corrections with skew quad:
Dowell, LCLS-II tech. note

14. Quad correction for RF couplers @ 300 pC (baseline layout)
Integrated strength 3Gs with 0.2 rad of rotation in front of CM
The emittance growth caused by the RF couplers is almost completely corrected with a weak quad

15. Summary New proposed layout has advantages:
RF coupler effect can be almost completely corrected with a weak quad in front of CM
Proposed layout
baseline layout
Best operating mode (both Cav2 and Cav3 off)
0.56 um
0.62um
Mode with one cavity failure (one of 4th to 8th cavity) um um
Emittance growth due to RF couplers (w/o correction)
10%
30%
Half apertures/rms-beam-size 6

16. This is a team work … Tor/JohnS – initiated the layout work for me
Dowell – develop theory on the RF coupler correction with quad
Z. Li showed me about his early work on quad correction for RF couplers
Vivoli (FNAL), Christos P (LBL), Adam (Cornell) and me – simulations and crosscheck with different codes
Chad (LBL) – understand the solenoid and benchmark ImpactT etc
Bruce (Cornell) – provided drawings
Henry – made layouts drawings …