1. COMB beam: TSTEP simulations up to THz station
C. Ronsivalle,E. Chiadroni,A. Mostacci
"S2E COMB simulations“ meeting, 4-5 October 2011, Frascati,INFN-LNF

2. OUTLINE four pulses train
MORE ABOUT ON COMPRESSION CURVES
TRANSVERSE EMITTANCE CONTROL:
four pulses train
S2E UP TO THz STATION IN DIFFERENT OPERATION CONDITIONS:
Sensitivity to DOGLEG parameters of electron beam (spot and LPS) and THz radiation
CONCLUSIONS

3. COMB TECHNIQUE: MAIN KNOBS
Gun solenoid field TW solenoid field
main effect: Emittance Compensation
secondary effect: relative amplitudes of bunches
Gun S S S3
Gun inj. phase
Bunches distance in energy and phase at the linac entrance
S1 phase
Final bunch spacing
Train length
Relative amplitudes of bunches
S3 phase
Energy separation
Typical operating gradient in accelerating section: 20.9,20.9,12.54 MV/m

4. Experimental procedure:
COMB RUN (MAY-JUNE 2011) STRATEGY:
Goal:
transport of the comb beam with an interdistance of about 1 ps in the THz dogleg line and radiation measurements in the THz station
Experimental procedure:
2 pulses: beam characterized in only some representative points of the compression curve. Selection criteria: S1 phase moved of ~ -90 deg (around max. compression) respect to the on-crest value and then moved again of ± 5-6 deg (compression and over-compression) respect to this point
4 pulses: as two pulses, but extending S1 phase in the deep over-compression region to get a proper separation
2 pulses train
Qtot=200 pC
4 pulses train
Over-compression
180 deg<
<90 deg
Compression
<90 deg
Deep over-compression
>180 deg
=beam rotation angle in the longitudinal phase space after compression

5. MEASURED LONGITUDINAL PHASE SPACES
Compression
<90 deg
Over-compression
180 deg<
<90 deg
Deep over-compression
>180 deg
=beam rotation angle in the longitudinal phase space after compression
Gun energy MeV
Charge
40pC/80pC/50pC/30pC
Energy MeV

6. Measurement Simulations
S1phase
0 deg
-89.97deg
deg

7. Initial pulse train (n0 bunches): S1 phase as a selector of the number of pulses in the final train (n0nf  1)
Over-compression
Deep over-compression
Compression
In this region modulation in the bunch profile
In this region nf pulses train
In this region nf=n0

8. Initial pulse train (n0 bunches): S1 phase as a selector of the number of pulses in the final train (n0nf  1)
Over-compression
Deep over-compression
Compression
In this region modulation in the bunch profile
In this region nf pulses train
In this region nf=n0

9. Initial pulse train (n0 bunches): S1 phase as a selector of the number of pulses in the final train (n0nf  1)
Over-compression
Deep over-compression
Compression
4p p p p
In this region modulation in the bunch profile
In this region nf pulses train
In this region nf=n0

10. Four pulses from laser COMPRESSION REGION
S1 phase selects the number of pulses
(S1)= deg
(S1)= deg
(S1)= deg
(S1)= deg

11. Four pulses train in the deep over-compression region
-low current
-high stability
-possibility to get equal currents in the train
(S1) tunes relative amplitude and separation
(S1)= deg
(S1)= deg
(S1)= deg

12. DEEP OVER-COMPRESSION: BEAM DYNAMICS IN S1

13. DEEP OVER-COMPRESSION: BEAM DYNAMICS IN S1

14. DEEP OVER-COMPRESSION: BEAM DYNAMICS IN S1
Z=244 cm Z=261 cm Z=300 cm

15. DEEP OVER-COMPRESSION: BEAM DYNAMICS IN S1
Z=244 cm Z=261 cm Z=300 cm

16. TRANSVERSE EMITTANCE
emittance under control (expecially on the vertical plane affecting the longitudinal measurements resolution) but not optimized (no crucial parameter for THz)
100% and 90% values have been measured and the Twiss parameters retrieved from the 90% values have been used for the transport in the dogleg line
In order to verify the compatibility between the measured values and the expected values they have been compared with the emittance retrieved by simulations
Model limitations:
Uniform distribution
Round beam
Aligned solenoids

17. 4 pulses train (deep-overcompression,Qtot=200 pC): total projected emittance
Simulated bunch length vs z

18. 4 pulses train (deep-overcompression,Qtot=200 pC): total projected emittance & single bunches emittance
The projected emittance depends on
Single bunches emittance
Mismatching
operation
good emittances
good matching
<2 mm-mrad

19. 4 pulses train (deep-overcompression,Qtot=200 pC): single bunches emittance optimization
Not optimized (in measurements condition)
optimized

20. good emittances operation good matching <2 mm-mrad
=1.3 ps ps ps
Same RMS bunch length, but different current distribution Inside the train

21. DOGLEG TRANSFER MATRIX PROPERTIES
for R16=R26=0
THz dogleg parameters
R56~ -5 mm
T566= mm
 z/=0.16 ps/%

22. TRANSPORT IN THE DOG-LEG LINE UP TO CTR TARGET
TRACE3D output
111 MeV
Dispersion
Dispersion scale=0.8 mm/mrad
R16=0
R26=0
R56= mm/mrad=50 um/%

23. Error on dogleg q-poles R16 R26 R56
DEVIATIONS IN DOGLEG Q-POLES SETTINGS LESS THAN 1% DURING OPERATION
Error on dogleg q-poles
R16
R26
R56 0%
-5 mm
+8%
0.004
-0.052
-8%
-0.004
0.051
-10 mm
4 pulses
in deep-overcomp.
E/E=0.46%
Emittance vs z in THz line
0 %

24. SPOT EVOLUTION IN DOGLEG OF 4 PULSES TRAIN IN DEEP-OVERCOMPRESSION
=0.46%
y vs x plots (scale=6.5 mm)

25. LONGITUDINAL PHASE SPACE EVOLUTION IN DOGLEG UP TO THZ STATION:
TWO-PULSES COMB

26. LINAC OUTPUT THZ STATION
(setting in DGL,R16=R26=0,R56=-5 mm)
d12= ps d12= 0.84 ps
UNDER. COMP.
RMS energy spread=0.84%
t=0.3 ps
t=0.54 ps
d12= ps d12= 0.24 ps
MAX. COMP.
RMS energy spread=0.74%
t=0.14 ps
t=0.29 ps
OVER
COMP.
d12= ps d12= ps
RMS energy spread=0.69%
t=0.48 ps
t=0.4 ps

27. LINAC OUTPUT THZ STATION
(non achromatic setting in DGL,R56=0)
d12= ps d12= 0.84 ps
UNDER. COMP.
RMS energy spread=0.84%
t=0.3 ps
t=0.426 ps
d12= ps d12= ps
MAX. COMP.
RMS energy spread=0.74%
t=0.14 ps
t=0.25 ps
OVER
COMP.
d12= ps d12= -0.8 ps
RMS energy spread=0.69%
t=0.47 ps
t=0.4 ps

28. CONTROL ON SPACE CHARGE
Length= ps
OFF ON
Length= ps
Negligible effect

29. TOTAL BUNCH LENGHTENING INDEPENDENTLY ON THE CHIRP SIGN
SOME LENGHTENING ALSO FOR R56=0
OPPOSITE BEHAVIOUR OF SINGLE BUNCHES LPS ALSO WITH SAME SIGN CHIRP AT THE LINAC OUTPUT
LPS EVOLUTION IN THE DOGLEG IS DOMINATED BY NON-LINEARITIES GIVEN BY HIGH ORDER CHROMATIC TERMS.
EVALUATION BASED ON THE APPROACH (describing an off energy beam in a dogleg) IN
R.J. England, J.B. Rosenzweig et al….
“Sub-picosecond Phase Space Manipulations and Beam Shaping Using Sextupole-Corrected Transport in Dispersionless Translating Sections”

30. OFF ENERGY BEAM IN DOGLEG
Definitions:
=design momentum,
=beam centroid momentum
Centroid error respect to the design trajectory
Momentum error respect to beam centroid
Momentum error respect to design trajectory
The usual relation
can be rewritten as
offset effective R effective T566
These three terms are functions of the momentum error  respect to the beam centroid

31. offset
effective R56
effective T566
For the comb beam we can define these quantities for the whole beam and also for each bunch in our train

32. T566=-95 cm due to the contribution of T166 and T266
Computation of Tijk elements by TRANSPORT code in THz dogleg line with killed dispersion and R56=-5 mm
Exit of dipole 2:
T566=-95 cm due to the contribution of T166 and T266
dispersion
T166
T266
T566

33. EFFECTIVE R56 vs ENERGY ERROR RESPECT TO DOGLEG REFERENCE ENERGY
can change sign

34. p0 p0 p0 LINAC OUTPUT THz STATION
for different dogleg reference energies I i p0
Centroid energy error = -2 MeV
Centroid energy error=0 (dogleg energy= beam average energy)
Centroid energy error=2 MeV p0
Non linear bunching through the dogleg
Effective R56 of the two bunches depends on their energy p0

35. EFFECT ON THz RADIATION: Computed normalized form factor
(whole beam)=0
Error on q_poles=+0.43%
Energy shift==-1.7%
It could justify the experimental observation of unexpected 1 THz radiation
LINAC OUTPUT THz STATION
(matlab script by E. Chiadroni)

36. LONGITUDINAL PHASE SPACE EVOLUTION IN DOGLEG UP TO THZ STATION:
FOUR-PULSES TRAIN

37. LINAC OUTPUT THZ STATION
(setting in DGL,R16=R26=0,R56=-5 mm)
DEEP OVER COMP.
t= ps
t= ps
RMS energy spread=0.46%
t= ps
t= ps
d12= ps
d23= ps
d34= ps
d12= ps
d23= 1.13 ps
d34= ps

38. LINAC OUTPUT THZ STATION
(off energy DGL + 1 MeV setting as before)
DEEP OVER COMP.
t= ps
t= ps
RMS energy spread=0.46%
t= ps
t= ps
d12= ps
d23= 1.13 ps
d34= ps
d12= 1.3 ps
d23= 0.93 ps
d34= 0.81 ps

39. EFFECT ON THz RADIATION:
Computed normalized form factor and interferogram
(matlab script by E. Chiadroni)
LINAC
OUTPUT
DGL
R16=R26=0,R56= -5 mm
Ebeam-Edogleg=0
Ebeam-Edogleg= -1 MeV
COMPATIBLE WITH MEASUREMENTS

40. ENERGY SPREAD COMPENSATION BY S3 PHASE
RESIDUAL EFFECT OF LOCAL CHIRP
OUT LINAC
THz station

41. ENERGY SPREAD COMPENSATION BY S3 PHASE: THz
OUT LINAC
THz STATION

42. CONCLUSIONS ANALYSIS UNDER WAY:
THE LONGITUDINAL AND TRANSVERSAL DYNAMICS OF OVER-COMPRESSION USED IN THE COMB EXPERIMENT OPERATION HAS BEEN CAREFULLY INVESTIGATED
THE EFFECTS OF DOGLEG PARAMETERS IN THE RANGE OF VARIATION DURING THE EXPERIMENT HAVE BEEN EVIDENCED
HIGH ORDER CHROMATICS (T566>>R56) EFFECTS AND EVENTUAL OFF-ENERGY OPERATION (E=1 MeV) AFFECT THE LONGITUDINAL PHASE SPACE TRANSPORT IN THz DOGLEG LINE EXPECIALLY FOR THE TWO-BUNCHES COMB BEAM (HIGHER ENERGY SPREAD)
ANALYSIS UNDER WAY:
Data analysis: RFD measurements on UTL for comparison with DGL and to eventuaòòy evidence Y/x correlation
Qscan data analysis: study of the growth of emittance vs charge. Comparison with simulations

43. TRANSVERSE EMITTANCE VS S1 phase:measurements vs simulations for 4 pulses beam
Qtot=200 pC
Good compatibility except for the max. compression point

44. TRANSVERSE EMITTANCE VS S1 phase:measurements vs simulations for 2 pulses beam
Moving from the compression to the over-compression region the emittance increases both in measurements and simulations.
Measurements and simulations are in good agreement for the low charge beam, whilst the emittance growth seems to be understimated in simulations for the high charge beam