Advanced beam dynamics experiments at SPARC

Alberto Bacci on be half the SPARC group

PITZ Collaboration meeting, 27-28 October 2011, Zeuthen (Berlin)

SPARC layout

linac -TW S-band

Gun 1.6 SW130MV/m

VB cavity for low energy bunch compression and solenoids to emittancecompensation

6 onadulators

seeding laser room

photocathodelaser room

New beam lines under installation : Thoson –PWFA – LWFA

FLAME laser input line (300TW)

seeding line

FEL Single Spike

THz Radiation

LWFA_ext

LASER COMB

Velocity Bunching

PWFA

Thomson

C_Band injector

4 pulses Narrow THz Rad

2 pulses FEL

SPARC Velocity Bunching applications

Progress towards high brightness beam:-Gun RF pulse shaping 130MV/m (Marco Bellaveglia)

-Laser Profile Optimization (Giancarlo Gatti)

-Higher thermal Stability

Before (11 MW - 112 MV/m – 4.7 MeV) – 5÷10 discharge per minute Now (14 MW – 130 MV/m – 6.2 MeV) – ~ 1 discharge per hour 10-10 torr vacuum level inside the Gun are maintained

Marco Bellaveglia, M. Ferrario, A. Gallo, RF pulse shaping optimization to drive low emittance RF photoinjector, to be published

New RF pulse shaping for Gun feeding

Goals:• Increase the gun accelerating gradient• Maintain the residual phase noise, respect to the main oscillator, below 100fs• Have a breakdown rate as low as possible

Solution:•In the first 3us the RF level is kept as low as possible to make the PLL (Phase Locked Loop) working•The RF is brought to the maximum level in the last 0.8 us

- P.O.Shea et al., Proc. of 2001 IEEE PAC, Chicago, USA (2001) p.704.- M. Ferrario. M. Boscolo et al., Int. J. of Mod. Phys. B, 2006 (Taipei 05 Workshop)

Laser Comb: beam echo generation of a train bunches

Movie ext-link

The technique used for this purpose relies on a birefringent crystal, where the input pulse is decomposed in two orthogonally polarized pulses (ordinary, extraordinary) with a time separation proportional to the crystal length.

Different crystal thickness are available (10.353 mm in this case).

Putting more crystals, one can generate bunch trains (e.g. 4 bunches).

The intensity along the pulse train can be modulated (e.g. PWFA)

A train of laser pulses at the cathode by birefringent crystal

11/1/ )Lvv(=Δτ gego

Giancarlo Gatti

Experimental results

Systematic analysis by simulations (two bunches Train)GIOTTO (Genetic Interface for OpTimizing Tracking with Optics)

φgun=36.66 deg

φgun=11.45 deg

Initial parameters:Tseparation at chathode = 4.27 psQ = 80 pC + 80 pCσx = σy = 400 μmTwcavity II–III on crest

Final Condiction:Tseparation ≈ 1 pscurrent I = current IIMinimum rms ε I - II

Free parameters (Knobs):Gun ijection phaseVB ijection phaseBz field Gun SolenoidBz field Twcavity N. 1

The minimum total projected emittance (measurable) corresponds to a similar behaviour of both sub-bunches (emittance and current)

x

px

x

px

GIOTTO – Stat. Analysis

Two bunches train caracterization Qt=166 pC (92+78)

remarkable agreement

εx,y(100%) = 0.8,1.1 mm-mrad, Espread for each pulse < 0.1 % (170 MeV)εx,y (90%) = 0.5,0.5 mm-mrad, σt1 ≈ σt2 ≈ 1ps

350 [A] σt=140 fsεx,y(100%) = 4.5,3.3 mm-rad εx,y (90%) = 3.6,2.6 mm-radEspread 0.4% and 0.25% (110 MeV)Energy separation ≈ 1.5 MeV

on crest

maximum compression VB phase -90.4

Tsep. = 4.27 psσt-pulses ≈150fs σx = σy = 400 μm

Over-compression VB phase -95.6

Two bunches train caracterization

σt I =140 fs, σt II =270 fsTseparation≈0.8 psεx,y(100%) = 6.2,4.4 mm-rad εx,y (90%) = 5.8,4.0 mm-radEspread 0.16% and 0.4%Energy separation ≈ 1.2 MeV

FEL Comb at SPARC (two bunches train)

2

dt

From the spectrum dt ≈ 0.615 ps; comparable with data

<dt> = 0.8 ps

4-pulses-time-structure

tim

e

orizontal dim energy

verti

cal d

im

4-levels-energy-spectrum

energy

longitudinal phase space

time

Four pulses COMB structure (200 pC)

Laser pulse @ gun cathode

whole train length ≈ 9 ps σt(per spike) ≈ 200 fs

4 comb pulses and long. phase space rotation

Over-compression region: The sub-bunches are well separated; their distancecan be controlled by VB phase injection

φVB=115.7 deg

(S1)= -91.5 deg

(S1)= -89.5 deg

4 comb pulses last simulations by Tstep

C. Ronsivalle

Dog-leg effects are under study

THz radiation can be easily produced by means of CTR

It is difficult to put high charge in sub-ps bunches

A laser comb structure in the longitudinal laser profile can solve this problem

The SPARC THz source – narrow band

o Operating spectral range: 100 GHz-5 THzo It allows to reconstruct the beam profileo First test with pyroelectric detector; foreseen Golay cell or bolometers

Martin-Puplet interferometerSilicon Aluminated screen (40 nm coating)

The SPARC THz source

InterferogramCTR

spectrum

by Fouriertrasforming

Interferogram

MeasuredExpected

Spectrum

Narrow THz radiation measured Interferogram for bunches train show 2N-1 peaks (inter-distance = sub-bunches distance)

Radiation spectrum is strongly suppressed outside the comb rep. ferquency

Analysis byChiadroni

Conclusion

•The SPARC linac has improved the machine stability and the gun gradient

•We have demonstrated, from experimental point of view, that one can control pulse spacing, length, current and energy separation by properly setting the accelerator.

•A very good agreement with simulations

Thanks for your attention

using an RF switch controlled by a trigger signal coming from the machine trigger distribution, we can decide when the RF level jump takes place. One can decide the power ratio between the two levels combining the DC coupling in the input branch and the value of the fixed attenuation in the low power branch. The phase shifter is used to minimize the phase jump during the switching transition, due to the two different paths of the signal.

RF Gun feeding schema

Marco.bellaveglia@lnf.infn.it

• Time jitter relative to the main RF clock (PLLs ON)

– Linac RF devices phase noise (standard phase detection): 40÷100 fsRMS

– Photo-cathode LAM measured time jitter (resonant Laser Arrival Monitor): <250fsRMS

– e-bunch time jitter

• BAM (resonant Bunch Arrival Monitor): <250fsRMS

• RF deflector centroid jitter (image analysis): <150fsRMS

• Laser amplitude stability (from new timing)

– Laser amplification timing locked to machine trigger

– Amplitude jitter always <5%

Jitter performance achieved

VB

Longitudinal compression >> no CSR >> avoid emittance degradation high improving in the bunch’s brightness

Beam in injected ahead of peak accelerating phase, and compresses as it slips back in phase

Beam executes 1/4 “synchrotron” oscillation in longitudinal phase space.

It permits to reach very high compression (~50 times)

The VB causes a non-linear longitudinal phase space deformation that limits the compression capability of the method

Cause of deformation is the non-linear RF accelerating fied - S-band (2856 MHz) SLAC type structures

solution A short X-band (11424 MHz) accelerating structure

The first section to compress, the following sections to accelerate and to reduce relative energy spread.

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Considering very short bunches the compressing factor can be pushed at higher values;

-Short bunches show a lower longitudinal phase space deformation, which is evident directly from analitical considerations

Initial debunching:

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Non-negligible for

A=(R/L) >> 1

Compression<90 deg

Over-compression180 deg<<90 deg

Deep over-compression>180 deg

=beam rotation in the longitudinal phase space

COMPUTED COMPRESSION CURVES OF THE SINGLE BUNCHES OF THE FOUR-PULSES TRAIN

0.0 Deg-90.0 Deg-105.8 Deg

Operational principle

FEL Comb at SPARC - 2

Two bunches train

•Along the shift 200 shots have been acquired

IV typology of spectrumshave been found

Simulations agree with the day jitters65+65 pC, Єx=2.8, γ=232

Only RF def. ; VB phase - 92 Deg

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