1. RF-Gun beam based alignment at PITZ/FLASH
M.Krasilnikov, DESY Zeuthen
LCLS Injector Commissioning Workshop (ICW)
October 9-11, 2006

2. M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”
Outline
Beam-Based Alignment (BBA) of RF gun:
Fields and geometry
BBA motivation
Cathode laser BBA:
PITZ
FLASH
Limitations of laser BBA
Main solenoid BBA:
Solenoid micromover system
Main problems and possible solutions
Limitations of solenoid BBA
Conclusions
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

3. RF-Gun: fields and geometry (PITZ and FLASH)
sol.mech.axis ≠mag.axis
sol.mech.axis ≠cavity el.axis
solenoid tilt angles
fields overlapping
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

4. RF-Gun Alignment Motivation
ASTRA Simulations of FLASH (VUV-FEL) Injector (150MeV)
Emittance growth
due to RF -gun misalignment
Solenoid tilt effect
For Ecath=25MV/m solenoid tilt angle of ~3mrad is equivalent to 1 mm transverse offset
5% emittance growth (lower limit estimations):
laser DR<800um or solenoid DR<500um or solenoid Dangle<1.5mrad (0.086deg)
No effect in the matching section has been considered!
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

5. Cathode Laser Alignment (FLASH)
Laser spot positioning at the photo cathode: +
Transverse displacement and angle of the laser beam can be independently changed -
Iris position has to be adjusted for every mirror movement
No VC and e-beam simultaneously
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

6. Cathode Laser Alignment (PITZ)
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

7. RF gun launch phase (SP Phase)
BBA of Laser on Cathode
z=0
z=0.276m
z=0.935m
z=0.778m
Basic measurement:
Mean position of electron beam at LOW.Screen1 (DoubleDiagCross at z=0.778m)
vs.
RF gun launch phase (SP Phase)
Conditions:
Main and bucking solenoids off
All steerers off
Dipole (even it is ~0.2m after the screen) degaussed and off
Bunch charge ~10pC, pulse train laser pulses
Moderate RF power in the gun: MW (exclude dark current)→Pz~ MeV/c
Preliminay (rough) laser alignment:
Using scintillating cathode
Centering in dark current images
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

8. BBA of Laser on Cathode Simulations Measurements&Simulations
Beam charge, Transverse rms Size,
Mean Momentum Vs. RF Phase
Measured and Simulated beam charge at z=0.78m Vs. RF Phase
Beam spot at Diag.Cross screen
(z=0.78m)
“Normal” phases
“Low” phases,
rf focused beam
Measured and Simulated beam rms size at z=0.78m Vs. RF Phase
Beam Offset at z=0.78m Vs. RF Phase
(0.5 mm vertical laser offset on the cathode has been assumed)
As a theoretical background correspondent simulations have been done.
An RF gradient has been fitted to the measured longitudinal momentum of the beam.
A Schottky constant has been applied to fit a phase scan – beam charge (measured with a Faraday Cup) vs. SP Phase.
Alignment feature:
Within a wide range of RF phase the energy varies not too much -> the beam position remains almost the same.
But for the low energies phase (low energies) the beam focusing from RF field takes place resulting in well-focused beam, which is very sensitive to the misalignment of the laser spot on the cathode.
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

9. Cathode Laser BBA. Test Movement
Measurement: Beam offset vs. rf phase
Step 1: Laser test movement
Measurement: Beam offset vs. rf phase
Step 2: Laser alignment
One of results: (X0,Y0) – preliminary coordinates of the center at the screen
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

10. Cathode Laser BBA. Difficulties
Laser intensity non-homogeneity
Laser position jitter
Possible damage of YAG screen homogeneity
Earth magnetic field (x,y,z)~(0.02mT,-0.03mT,-0.009mT)
stdev<X>=13um
stdev<Y>=11um
Extreme example of a damaged screen
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

11. Main Solenoid BBA. Micromover system
beam axis
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

12. M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”
Main Solenoid BBA
z=0
z=0.276m
Basic measurement:
Mean position of electron beam at LOW.Screen1 (DoubleDiagCross at z=0.778m)
vs.
Main solenoid current
z=0.935m
z=0.778m
Conditions:
Bucking solenoid off
All steerers off (or consider in simulations)
Dipole (even it is ~0.2m after the screen) degaussed and off
Bunch charge ~10pC, pulse train laser pulses – to be tuned for Imain
Moderate RF power in the gun: MW (exclude dark current )→Pz~ MeV/c
RF launch phase*
Laser spot size possibly small
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

13. Main Solenoid BBA
Beam position (<X>,<Y>) at LOW.Scr1(z=0.778m) as a function Imain
E-beam displacement with a main solenoid sweep. Possible reasons:
Solenoid transverse offset (Xsol,Ysol)
Solenoid tilt angles (AngleX, AngleY)
Laser (small) offset (Xlas,Ylas) from the center
From the laser BBA: X0=13.0mm Y0=14.8mm
Imain=0A
Other factors to be considered:
RF gun launch phase and gradient
Small offset (from X0,Y0 obtained after laser BBA)
Solenoid calibration
Imain=320A
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

14. Main Solenoid BBA. RF-gun launch phase and gradient
streak-camera measurement
F0=-18deg
(21.4MV/m;22deg)
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

15. M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”
Simulation Tool
Alignment Utility of the V-code - fast tracking code based on the method of moments of particle distribution function
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

16. Main Solenoid BBA. Test movement
Simultaneous simulations of beam position before and after test movement (DXsol=0.4mm)
Advantages:
Same RF gun launch phase and gradient
Same offset X0*,Y0*
Same solenoid calibration
Laser_Beam_CenterX
0.20 mm
Laser_Beam_CenterY
-0.26 mm
XSolMainCenter
0.85 mm
YSolMainCenter
1.12 mm
AngleXSolMain
deg
AngleYSolMain
deg
Ez_Field_At_Cathode
22.2 MV/m
Initial_Phase
deg
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

17. M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”
Main Solenoid BBA
Before BBA
After BBA
Laser_Beam_CenterX
0.20 mm
0.11 mm
Laser_Beam_CenterY
-0.26 mm
-0.27 mm
XSolMainCenter
0.85 mm
0.041 mm
YSolMainCenter
1.12mm
0.063 mm
AngleXSolMain
deg
deg
AngleYSolMain
deg
deg
Ez_Field_At_Cathode
22.2 MV/m
22.4 MV/m
Initial_Phase
-147.2deg
deg
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

18. Main Solenoid BBA. Difficulties
Laser BBA difficulties:
Laser intensity non-homogeneity
Laser position jitter
Damaged YAG screen +
RF phase and gradient jitter
Some uncertainty in the solenoid micromover (especially angles), z-position
Outlook
BPM
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

19. RF-Gun BBA. Conclusions
BBA of Laser on Cathode:
routine procedure at PITZ (FLASH)
based on a measurement of beam position vs. gun launch phase
test movement of the laser beam allows to determine a displacement vector for the laser beam centering
Main solenoid BBA:
multi-parameter task
based on beam position simulation vs. main solenoid current
test movements of the main solenoid and/or cathode laser allows to reduce uncertainty in main solenoid misalignment
Possible improvements:
implement earth magnetic field in BBA procedures
use BPM (LOW.BPM1) for solenoid BBA
more details on RF field profile
solenoid relative displacement online measurement
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

20. M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

21. Reference RF Phase. Beam Size measurements
Transverse Beam Size at Screen 3 as a Function of RF Launch Phase
for Various Main Solenoid Currents
Fmax
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

22. PITZ1 Benchmark Problem: Fields
Field balance in the rf gun cavity
Solenoid calibration
MF compensation
Ibuck= *Imain
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

23. BBA: Step 0 – Rough Laser Alignment on the Cathode
Rough laser alignment using dark current symmetry. TTF
Dark current at TTF at screen Gun3 (z = 1.27m)
Without beam
With beam
Dark current rings originate from the edge of the Cs2Te coating and plug spring region
A laser spot being aligned on the cathode center results in an electron beam centered with dark current rings
First row of pictures -TTF logbook
Second row - TTF logbook
Info (for any case)
One can conclude that the inner dark current ring is generated at the edge of the Cs2Te photocathode. The dark current ring on the screen has also a diameter of ~5mm, so that a 1:1 image of the cathode region is seen. Typical magnet settings for these observations are: Imain=290A, Ibuck=50A (logbook , 7:17).
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

24. BBA: Step 0 – Rough Laser Alignment on the Cathode
Rough laser alignment using dark current symmetry. PITZ
Without beam
With beam
Screen
Diag. Cross (z = 0.87m)
The same pictures obtained at PITZ at different screens.
Screen_PP
(z = 2.62m)
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

25. RF Gun Alignment: Satellites observation
@Diag.Cross
@PP Screen
@Dispersive Arm x y x y x Pz
Vacuum
mirror
@Diag.Cross, “low phase”

26. M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”
BBA of laser on cathode
SP phase = -70
SP phase = -90
SP phase = -50
SP phase = -30
SP phase = -20
SP phase = -100
SP phase = -16
SP phase = -106
SP phase = -125
SP phase = -130
SP phase = -120
SP phase = -116
SP phase = -110
SP phase = -112
SP phase = -114
precision of mirror adjustment: better than 20 µm
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

27. RF-Gun Alignment Motivation
ASTRA Simulations of FLASH (VUV-FEL) Injector (150MeV)
trajectory
Emittance degradation in the matching section is not included
emittance
M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”