1. Feng Zhou LCLS-II AP meeting 02/23/2017
Tuning Procedures for Early Injector Commissioning (EIC)
Feng Zhou
LCLS-II AP meeting
02/23/2017

2. Technical scope for EIC
Pre-beam checkout
Baking for gun/buncher and CW RF conditioning
RF baking C for the gun (80kW peak, 3-4% duty cycle: ~3kW average) ~3 days
Tape baking ~150C for the buncher ~3 days
CW RF conditioning for gun/buncher
Study for ‘quick’ gun power-turn on
Major parameters to be characterized:
Bunch charge, QE, and QE lifetime for different repetition rate
Dark current vs. different gun energy/repetition rate
Thermal emittance
Beam energy
Phase and amplitude calibration for gun/buncher
BBA for the components

3. ~1 MeV LCLS-II injector source in EIC
RF gun, buncher, 2 solenoids, 5 pairs of x/y correctors
Diagnostics: 1 ICT, 2 BPMs, 1 YAG screen, and one 300-W temporary dump w/ a Faraday cup

4. Pre-beam checklist for EIC
First cut for the checklist, 10 pages in total for EIC.
May add more details for the gun including pumps/gauges/RTDs in the gun body, two couplers and loadlock system

5. ‘Quick’ gun power-turn on
~half an hour for every gun-turn on, which may become an issue for user operations (availability)
New frequency push-pull tuner may help (x2 tuning range, about 250kHz) but …
How to shorten RF gun turn-on time (~ a few minutes?):
Block the gun water flow?
Increase the heater power?
How to slow down the temperature drop due to trips caused by vacuum or temperature or reflected power
Block the gun water flow after the trip?
Developing a plan for the study in the EIC
To discuss with LBL experts

6. Dark current, bunch charge, QE & QE lifetime, and thermal emittance
Dark current can be characterized with the Faraday cup for different gun energy
Will borrow FC electronics from Diag0
Bunch charge can be measured with the ICT/faraday cup and the calibrated two BPMs TMIT
BPM electronics ready for EIC
QE and, QE lifetime for different repetition rate
Thermal emittance can be measured by scanning the solenoid strength to record the beam size at the YAG screen (simple matrix: solenoid + drift)
The method used at the LCLS1 and Apex

7. Assume gun phase is fixed, similar to LCLS1
Laser launch phase
Assume gun phase is fixed, similar to LCLS1
Scan the charge (w/ ICT) vs. laser phase to determine zero- crossing laser phase w.r.t. RF reference
Set laser phase to on- crest
Zero-crossing

8. Buncher phase Buncher’s zero-crossing phase:
Beam energy with buncher’s zero-crossing phase  gun energy
On-crest phase at -3 due to phase slippage at <1MeV energy
The -3 accuracy is not important for the optimized performance
Bunching vs. debunching at zero-crossing :
Beam size at YAG screen much larger beam size for bunching than debunching
Debunching
Bunching
Zero-crossing
Bunching
Debunching

9. Buncher phase calibration steps
Measure gun energy
Scan the energy with the gun and buncher vs. buncher phase to determine zero-crossing phases
Determine the zero-crossing phase for bunching through comparison of the beam size at the YAG screen
Set buncher to desired phase w.r.t. zero-crossing at bunching side

10. Gun/buncher RF amplitude calibration (beam energy measurement)
Assume the gun/buncher RF power has been pre- calibrated by RF experts.
We are to discuss gun/buncher RF calibration with beam energy measurement:
w/ a corrector: to measure the beam displacement at a viewing screen or BPM vs. corrector strength (slope)
d: the distance between corrector and screen/BPM
x/BL: the slope

11. Gun amplitude calibration
Calibration steps:
Turn off buncher and adjust SOL1 strength to focus the beam at YAG screen
Scan the XC01 corrector and measure corresponding beam’s x-displacement (slope)
Extract the gun beam energy with the measured slope
Calibrate gun energy for different gun RF amplitude
May resolve ~10 keV beam energy
SOL1= kG.m
10 keV energy difference

12. Buncher amplitude calibration
Measure gun energy
Turn on the buncher, set its phase on-crest with nominal RF power, and focus the beam at the YAG screen with adjustment of SOL1 strength
Scan XC03 corrector and measure corresponding beam’s x- displacement at YAG (slope)
Obtain net energy gain from the buncher by subtracting the gun energy from the total
Calibrate different buncher RF amplitude
@nominal RF power

13. Requirement for transverse alignment
Misalignment
-growth
Cathode
100 µm
<2%
Solenoid
0.5mm or
2mrad
<2.5%
Buncher
1mm or
<3%
CM01
0.5 or
0.5mrad
<1%
And also alignment is crucial for subsequent emittance tuning with optimization of the solenoid strength

14. BBA for cathode APEX experience: Steps for the cathode alignment
Cathode plug is aligned well with the gun (~10 m level)
Only central 5mm of the cathode is doped with Cs
Steps for the cathode alignment
Map QE with scanning the laser mirror in x-plane and record laser mirror setting when QE starts dropping to zero on both sides of the central area, x1 and x2
Set laser mirror in x-plane = (x1+x2)/2 and repeat same step as above but for y-plane
Cathode center =[(x1+x2)/2, (y1+y2)/2]
Could align ~100m level according to APEX experience

15. BBA for solenoids
Method: given the initial offset x0, x0’, y0, y0’ for solenoid, the beam’s x- and y-displacement at downstream BPM can be calculated with the matrix (a solenoid + a drift):
where C=cos(KL)
S=sin(KL)
KL=BL/(2B)
K=B/(2B)
B: solenoid strength
L: solenoid length
d: distance between solenoid exit and BPM

16. BBA procedures for SOL1/2
Alignment steps:
Record beam’s x- and y-displacement at BPM1 for >3 different SOL1 strength
Directly solve the SOL1 misalignments (x0, x0’, y0, y0’) according to the equation
Correct the misalignments through moving the solenoid actuators (x, y, x’, and y’),
Check beam alignment again after the correction with different SOL1 strength; more iterations if still not fully aligned
Replace SOL1/BPM1 with SOL2/BPM2 then can align for SOL2

17. BBA for buncher Set buncher to debunching zero-crossing
Eliminated the effect of dispersion due to the stray fields
Smaller beam size (extra focusing from debunching phase)
Turn on SOL1 and adjust its strength to focus the beam at YAG screen
Correct the offset using pairs of correctors xc01/02 through measuring difference of centroid beam at YAG for buncher on/off; may align <100 m.

18. HLAs requested
Gun and buncher RF turn-on process (apex scripts available)
Laser phase scan (migrated from LCLS1)
Buncher phase calibration (new)
Beam energy (apex script available)
Thermal emittance (apex script available)
Cathode alignment (new)
Solenoid alignment (new)
Buncher alignment (new)

19. Beam performance with one cavity failure
Nominal
(grad/phase)
Cav1 failure
Cav4 failure
Cav5 failure
Cav1
Cav2
Cav3
Cav4
Cav5
Cav6
Cav7
Cav8
7.9/-4
14/0
16/0
16/1
16/6
7.1/-8
15/-4
16/13
15/0
 (µm)
I (A)
E (MeV)
@100pC
0.29 12 94
0.54
7.5
0.4 96
0.39 95