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Advanced beam dynamics experiments at SPARC Alberto Bacci on be half the SPARC group

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Advanced beam dynamics experiments at SPARC

Alberto Bacci

on behalf the SPARC group

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

SPARC layout

FLAME laser input line (300TW)

New beam lines under installation : Thoson –PWFA – LWFA

VB cavity for low energy bunch compression and solenoids to emittance

compensation

seeding laser room

seeding line

6 onadulators

photocathode

laser room

Gun 1.6 SW

130MV/m

linac -TW S-band

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

C_Band injector

FEL Single Spike

Velocity Bunching

THz Radiation

2 pulses FEL

4 pulses Narrow THz Rad

LASER COMB

PWFA

Thomson

LWFA_ext

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 abreakdown 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

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 10-10torr 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

Laser Comb: beam echo generation of a train bunches

- - 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)

Movie ext-link

A train of laser pulses at the cathode by birefringent crystal

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)

Giancarlo Gatti

Systematic analysis by simulations (two bunches Train)

GIOTTO (Genetic Interface for OpTimizing Tracking with Optics)

Free parameters (Knobs):

Gun ijection phase

VB ijection phase

Bz field Gun Solenoid

Bz field Twcavity N. 1

φgun=11.45 deg

px

Initial parameters:

Tseparation at chathode = 4.27 ps

Q = 80 pC + 80 pC

σx = σy = 400 μm

Twcavity II–III on crest

x

φgun=36.66 deg

px

Final Condiction:

Tseparation≈ 1 ps

current I = current II

Minimum rms ε I - II

x

The minimum total projected emittance (measurable) corresponds

to a similar behaviour of both sub-bunches (emittance and current)

GIOTTO – Stat. Analysis

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

on crest

remarkable agreement

Tsep. = 4.27 ps

σt-pulses ≈150fs

σx = σy = 400 μm

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

maximum compression VB phase -90.4

σt=140 fs

εx,y(100%) = 4.5,3.3 mm-rad

εx,y (90%) = 3.6,2.6 mm-rad

Espread 0.4% and 0.25% (110 MeV)

Energy separation ≈ 1.5 MeV

350 [A]

Two bunches train caracterization

Over-compression VB phase -95.6

σt I =140 fs, σt II =270 fs

Tseparation≈0.8 ps

εx,y(100%) = 6.2,4.4 mm-rad

εx,y (90%) = 5.8,4.0 mm-rad

Espread 0.16% and 0.4%

Energy separation ≈ 1.2 MeV

FEL Comb at SPARC (two bunches train)

From the spectrum dt ≈ 0.615 ps; comparable with data

<dt> = 0.8 ps

4-pulses-time-structure

time

orizontal dim

Four pulses COMB structure (200 pC)

Laser pulse @ gun cathode

whole train length ≈ 9 ps

σt(per spike) ≈ 200 fs

4-levels-energy-spectrum

longitudinalphasespace

vertical dim

time

energy

energy

4 comb pulses and long. phase space rotation

Over-compression region:

The sub-bunches are well separated; their distance

can be controlled by VB phase injection

φVB=115.7 deg

4 comb pulses last simulations by Tstep

(S1)= -89.5 deg

(S1)= -91.5 deg

C. Ronsivalle

Dog-leg effects are under study

The SPARC THz source – narrow band

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

Martin-Puplet interferometer

Silicon Aluminated screen (40 nm coating)

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

by Fourier

trasforming

Interferogram

CTR

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

Measured

Expected

Interferogram

Spectrum

Analysis by

Chiadroni

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

RF Gun feeding schema

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.

Jitter performance achieved

- 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%

VB

Longitudinal compression >> no CSR >> avoid emittance degradation high improving in the

bunch’s brightness

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

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

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

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

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

A short X-band (11424 MHz)accelerating structure

solution

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

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

Initial debunching:

Non-negligible for

A=(R/L) >> 1

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

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

Deep over-compression

>180 deg

Over-compression

180 deg<<90 deg

Compression

<90 deg

0.0 Deg

-105.8 Deg

-90.0 Deg

=beam rotation in the longitudinal phase space

Operational principle

TSTEP simulations

FEL Comb at SPARC - 2

- Along the shift 200 shots have been acquired

Simulations agree with

the day jitters

65+65 pC, Єx=2.8, γ=232

IV typology of spectrums

have been found

Only RF def. ; VB phase - 92 Deg

Two bunches train