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

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

COMB TECHNIQUE: MAIN KNOBS Gun solenoid field TW solenoid field main effect: Emittance Compensation secondary effect: relative amplitudes of bunches Gun S1 S2 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

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

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 5.7MeV Charge 40pC/80pC/50pC/30pC Energy 168-109 MeV

Measurement Simulations S1phase 0 deg -89.97deg -105.76deg

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

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

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 3p 2p 1p In this region modulation in the bunch profile In this region nf pulses train In this region nf=n0

Four pulses from laser COMPRESSION REGION S1 phase selects the number of pulses (S1)= -89.5 deg (S1)= -91.5 deg (S1)= -96.5 deg (S1)= -93.5 deg

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)=-101.5 deg (S1)=-103.5 deg (S1)=-105.5 deg

DEEP OVER-COMPRESSION: BEAM DYNAMICS IN S1

DEEP OVER-COMPRESSION: BEAM DYNAMICS IN S1

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

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

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

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

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

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

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

DOGLEG TRANSFER MATRIX PROPERTIES for R16=R26=0 THz dogleg parameters R56~ -5 mm T566=7.6465 mm  z/=0.16 ps/%

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= - 0.005 mm/mrad=50 um/%

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 %

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

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

LINAC OUTPUT THZ STATION (setting in DGL,R16=R26=0,R56=-5 mm) d12= -0.048 ps d12= 0.84 ps UNDER. COMP. RMS energy spread=0.84% t=0.3 ps t=0.54 ps --------------------------------------------------------------------------------------------------------------------- d12= -0.048 ps d12= 0.24 ps MAX. COMP. RMS energy spread=0.74% t=0.14 ps t=0.29 ps --------------------------------------------------------------------------------------------------------------------- OVER COMP. d12= -0.7745 ps d12= -0.82 ps RMS energy spread=0.69% t=0.48 ps t=0.4 ps

LINAC OUTPUT THZ STATION (non achromatic setting in DGL,R56=0) d12= -0.048 ps d12= 0.84 ps UNDER. COMP. RMS energy spread=0.84% t=0.3 ps t=0.426 ps --------------------------------------------------------------------------------------------------------------------- d12= -0.048 ps d12= -0.007 ps MAX. COMP. RMS energy spread=0.74% t=0.14 ps t=0.25 ps --------------------------------------------------------------------------------------------------------------------- OVER COMP. d12= -0.7745 ps d12= -0.8 ps RMS energy spread=0.69% t=0.47 ps t=0.4 ps

CONTROL ON SPACE CHARGE Length= 0.5359 ps OFF ON Length= 0.5366 ps Negligible effect

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”

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 R56 effective T566 These three terms are functions of the momentum error  respect to the beam centroid

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

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

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

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

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)

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

LINAC OUTPUT THZ STATION (setting in DGL,R16=R26=0,R56=-5 mm) DEEP OVER COMP. t=1.2646 ps t=1.1712 ps RMS energy spread=0.46% t=0.07 0.17 0.274 0.238 ps t=0.0983 0.17 0.28 0.27 ps d12= 1.6796 ps d23= 1.1611 ps d34= 0.99659 ps d12= 1.399 ps d23= 1.13 ps d34= 1.035 ps 4 3 2 1 4 3 2 1

LINAC OUTPUT THZ STATION (off energy DGL + 1 MeV setting as before) DEEP OVER COMP. t=1.0147 ps t=1.1712 ps RMS energy spread=0.46% t=0.07 0.17 0.274 0.238 ps t=0.18 0.2 0.26 0.24 ps d12= 1.399 ps d23= 1.13 ps d34= 1.035 ps d12= 1.3 ps d23= 0.93 ps d34= 0.81 ps 4 3 2 1 4 3 2 1

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

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

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

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

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

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