velocity bunching experiment @ sparc daniele filippetto on behalf of sparc team

Velocity bunching experiment @ SPARC

Daniele Filippettoon behalf of SPARC team

D. Filippetto HBEB-MAUI_09

Outline

• The velocity bunching concept

• SPARC hardware overview

• VB experiment @ SPARC

• Emittance degradation by solenoid

misalignment

• Conclusions


• The velocity bunching concept:

• the beam is injected in a long accelerating structure at the 0 crossing field phase

•Injection at low energies where The beam is slower than the phase velocity of the RF wave (typically the first LINAC after the gun)

• it will slip back to phases where the field is accelerating, but at the same time it will be chirped and compressed.

•Compression and acceleration take place at the same time within the same linac section

At SPARC the beam is acceleratedfrom 4-5 MeV up to 20-25 MeV(instead of 60-65)


100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

-95 -90 -85 -80 -75 -70 -65 -60

RF compressor phase (deg)

Av

era

ge c

urr

en

t (A

)

LOW COMPRESSION

OVER-C

MEDIUM COMPRESSION

HIGH-C

Peak current vs RF compression phase

SPARC nominal case

Initial parameters:

1 nC beam

10 ps long


If the transverse emittance can be preserved during the longitudinal focusing, the beam brightness can be increased

L. Serafini, M. Ferrario, “Velocity Bunching in PhotoInjectors” , AIP CP 581, 2001, pag.87

may avoid the phase space degrading effects observed in magnetic compression experiments on photoinjector-derived beams

SPARC

... What happens to the transverse plane?


S-band Gun

Velocity Bunching

Long Solenoids

Diagnostic and Matching

Seeding

THz Source

150 MeV S-band linac

10m

Undulators

u = 2.8 cm

Kmax = 2.2

r = 500 nm

15m

SPARC overview:


• Iron joke (blue) for field lines guiding• 1 single and 4 triplet coils surrounding two LINAC section, indipendently powered


Diagnostic hardware:

Dipole magnet RFdeflector

Quadrupoletriplet

screens

Spectrometer system:Θ=14 deg;Lm=26.7cm;Ld=2.13m;Pixel size=30um;

Energy resolution:about 15keV @ 150MeV;

Overall resolution (RMS): 10-4≤ DE/E ≤ 10-2

Time measurement resolution:

SPARC typical parameters:

mL

GHzf

MeVE

MVV

m

MeVpixelfs

MeVpixelfs

RF

DEFL

yB

4

856.2

150

5.1

70

100@/33

150@/50

MeVfs

MeVfs

LeE

VRF

DEFL

yRESt

B

B 100@60

150@90

/

_


VB run @ SPARC:

0 5 10 15 200

0.5

1

1.5

2

2.5x 10

-5

time (ps)

inte

nsity (A

rb. U

nits)

7.3 ps FWHMLaser parameters:

Xrms =358um

Yrms =350um

TFWHM=7.3ps

Energy @ cathode = 170uJ

Gun parameters:

Gun Input Power=7.5MW

Gun Peak Field=105MV/m

e-Energy out of the gun=4.7MeV

Working inj.phase=30 deg.

e-beam charge @30deg=280pC


C-Factor Vs RF compressor phase:

Maximum energy

First linac section used as compressor

C=15

240 fs rms

C=3

1000 fs rms

C=3 chosen for characterization measurements:

Useful in a hybrid scheme with magnetic compressor (SPARX case) Less sensitive to relative phase jitter


-5 0 50

1

2

3

4x 10

-4 SLICE ENERGY SPREAD

Time (ps)

DE/E

rm

s

E-beam parameters @ LINAC exit, C=1:

Max energy on crest 147.5MeVTotal DErms 0.16MeV DE/Erms 0.11%Charge 280pCBunch Length RMS 3.01psSlice Peak Current30AmpsLongitudinal emittance 159.6 keV*mm

-2000 0 2000 4000 60000

5

10

15

20

25

30

35

Z (um)

Slice c

urr

ent (A

mps)

Beam slice current profile

LONGITUDINAL TRACE SPACE

Energy [MeV]

Tim

e [ps]

146 147 148 149 150

-3

-2.5

-2

-1.5

-1

-0.5

x 1013


Effect of solenoid:TW solenoids OFF Vs ON (660Gauss) C=1

156 158 160 1620

2

4

6

Gun solenoid current (A)

(mm

mra

d)

Hor. (red) and Ver. (blu) emittance

0 10 20

400

600

800

1000

Distance from the cathode (m)

x (re

d)

y (b

lue)

( m

)

Beam envelope @ 161 A with TWsol ON

Isol=161 A

Best emittance after solenoid scan with TW-SOL off:εx=1.4umεy=1.5um

TW-SOL on:εx=1.85umεy=1.65um

TW sol on


TW solenoids Off VS ON, slice emittances:

•The solenoid misalignment leads to an increase of the projected emittance, which is not found looking at the slice emittances;

•the mismatch parameter is similar in the two cases;

•The difference is due to slice centroid misalignement (will be treated more in detail further on in the presentation);

•A beam based alignment is mandatory to reach lower projected emittances;

0 1000 2000 3000 40000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0

10

20

30

40

50

60

70

TW Solenoids ON

Hori

zonta

l Em

itta

nce

(um

)

Z position along the bunch (um)

TW Solenoids Off

Curr

ent (A

mps)

0 1000 2000 3000 40000.0

0.5

1.0

1.5

2.0

2.5

Mis

mat

ch

Z position along the bunch (um)

TW Sol OFF TW Sol ON


Beam after compression @C3

No compression Compression @C3

Bunch charge 280 pC 280pC

Injection phase (S1)

0 deg (on crest)

-87deg

Beam Energy 147.5 MeV 101 MeV

Total energy spread

0.11% 1.1%

Bunch length 3.01ps RMS 0.97ps RMS

TW Solenoid field 0 450 Gauss

0 800 1600 2400 3200 4000 48000

20

40

60

80

100

120

140

Slice C

urr

ent (A

mps)

Z (um)

Slice Current compressed Slice current uncompressed


Beam after compression @C3

156 158 1601

1.5

2

2.5

3


Em

ittan

ce (m

m m

rad)

H emittanceV emittance

Bsl=1.1x1014 A/m2

Emittance without TW solenoids

(Gun solenoid current=157Amps):

Ex=6.2 mm mrad

Ey=4.0 mm mrad

For a compression factor C=3:

Gain of a factor 3.7 on the maximum slice current (30 Vs 110)

Loss of a factor 1.15 on the minimum slice emittance (1.2 Vs 1.4)

Gain of a factor 2.7 on the slice Max Brigthness (0.41 Vs 1.1x1014)

ΔB/C=0.9


• Low charge/max Compression Case:Bunch Charge= 60pCBunch length rms= 1.95 psLongitudinal emittance= 54.2 keV*mmLaser spot size rms= 250um

-95 -90 -85 -800

5

10

15

20

Phase (deg)

Com

pre

ssio

n fa

cto

r

Extreme compression WP


TW solenoids OFFGun sol Current(151Amps):

Ex=4.1 mm mrad Ey=3.4 mm mrad

Beam @ C-17 (TW sol 45Amps):

Energy=97.6

MeV

DE/Erms =1%

Ipeak=217.5 Amps

Ex=1.52 mm

mrad

Ey=1.62 mm

mrad

Proj. emittance

B ≈ 2x1014 Amps/m2

Prel

imin

ary


• Critical point: Proj. emittance degradation due to solenoids misalignment•The solenoid force is energy dependent:

KL=qB0/2m0cβγ

• strong energy-time correlation in VB conditions• different focusing forces for different time slices• if the beam is propagating off axis respect to the magnetic field, the slice centroids will experience time dependent kicks

Induced longitudinal-transverse correlation, proj. emittance increase

Lower Energies

higher Energies


Example: 1mm solenoid misalignment (H)

Out linac2

Out linac1

On crest

VB conditions

VB conditions

Effect on transverse beam shape along the Linac:

PARMELA runs simulatingthe two TW solenoids 1mm off axis respect to the rf cavity, on crest and in the VB conditions

measurements


X-phi Y-phi

Simulated X e Y vs phi at linac output

QS for slice emittanceRFD on

QS for projected emittanceRFD off

same quadcurrents

-60 -40 -20 0 20 40 60

0

0.5

1

1.5

2

2.5

x 104

Quad strengthBeam

dim

en

sio

ns

higher emittance value

Effect on emittance measurement:

tim

e

X

YX


Slice centroid spread exclusion:Projected emittance from slice

αn, βn, γn, εn twiss parameters of slice n

156 158 1601

1.5

2

2.5

3


Em

ittan

ce (m

m m

rad)

H. proj. emitt.V. proj. emitt.H. emitt from slice


M.Ferrario, V.Fusco, M.Migliorati, L. Palumbo,Int. Journal of Modern Physics A ,Vol 22, No. 3 (2007) 4214-4234

222

',,

'2'2,

2',

2

2',,

2',

2,

2'2'2

2

crossn

centn

envn

Totaln

scentscentsssscentscentscrossn

scentscentscentscentcentn

ssssenvn

;

;[]1

[]1

[]

1

1 11

s

s

s sp

s

s

p

N

spp

N

s

N

pp

p

N

np

p

NN

NN 2'2'2, xxxxtotxn

uses the slice centroid

different from 0 only if slice centroids do not lie on the same axis

correlation between slice centroid spread and single slice dimension in ph.sp.

εxenv=0 εx

cent=0 εxcross≠0

;ss xxx

Slice centroid contribution to the emittance:


•From the slice emittance with the quad scan, the values of alpha beta and emittance for each slice are calculated at

one precise position

•From the QS measurements also the system for slice centroids (both in X and X’) can be written and solved (first

order system)

•All the 3 emittance terms can be calculated

156 158 1601

1.5

2

2.5

3


Em

ittan

ce (m

m m

rad)

H. proj. emitt.V. proj. emitt.H. emitt from slice

Measured H. Projected emittance @157A (red dot)= 2.3um

εxenv=1.5 um

εxcent=0.52 um

εxcross=1.72 um

εxtot=√(εx

env)2+ (εxenv)2 +(εx

env)2=2.34um

0 200 400 600 800 1000 1200 1400 1600-6

-4

-2

0

2

4

6x 10

-4

Z position (um)

X c

entr

oid

+ s

igm

a (m

)

0 200 400 600 800 1000 1200 1400 1600-6

-4

-2

0

2

4

6

8

10

12x 10

-5

Z position (um)

Mea

n +

RM

S d

iver

gen

ce (

rad

)

Slice centroids Vs Z Slice mean divergence Vs Z

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14div

erg

ence (m

rad)

X (mm)

Transverse phase space distorsiondue to beam misalignment


Conclusions:

Next steps• BBA on TW solenoids

• emittance study as function of TW solenoid fields (field

shaping)

• Longitudinal phase space detailed studies (slice DE)

• THZ production, ICS experiments, FEL single spike, laser

comb

• Demonstrated transverse emittance preservation in the

VB regime for medium compression factors;

• Preliminary studies on high CF show an emittance

decrease, but still work to do to fully compensate.

• Higher total energy spreads make the beam emittance

sensitive to magnetic components misalignment (quads,

sol., etc...)

• The slice centroid spread contribution to the projected

emittance can be isolated and measured

velocity bunching experiment @ sparc daniele filippetto on behalf of sparc team

Documents

filippetto hbebmaui

sparc daniele filippetto

pc slide

phase velocity

sparc overview

fs rms c

behalf of sparc team

ps long slide