Les Sources d’Électrons de Forte Intensité et de Faible Emittance

T. Garvey,

Laboratoire de l’Accélérateur Linéaire – Orsay.

Journées Accélérateurs de la SFP – Roscoff, 10 octobre, 2005.

Motivation for new high brightness injectors – new synchrotron

radiation sources• SASE Free Electron Lasers at VUV and

X-ray wavelengths – High peak brightness

• Energy Recovery Linacs– High average brightness synchrotron radiation source.

Not an exhaustive review of all existing or planned projects!Will evoke major issues in the design of injectors for FEL and ERLProjects.

Single-pass Free Electron Lasers

Energy:

Energy width:

Narrow resonance E/E ≤

 

Gain Length:3/1

2

23

0ˆ

2

3

1

IKe

mcL ur

g

21

2

2

2

Kuem

Beam size:

r small high electron density for

maximum interaction with radiation fieldEmittance ≤

Peak current inside bunch:Î > 1 kA feasible only at ultrarelativistic energies, or may dilute emittance bunch compressor

SASE FEL Projects

• TTF VUV-FEL (6 – 30 nm)

• European X-FEL (0.1 – 6 nm)

• SPARC (500 nm)

• LCLS (~ 0.15 nm)

• SCSS (~ 3.6 nm)

• BESSY (50 – 1 nm)

• DUV-FEL…….

Gun types DC

(or pulsed HV)

RF

Thermionic emission Thermionic emission

Photo-emission Photo-emission

Cathodes – metallic,semi-conductor (high Q.E.)

CeB6, LaB6….

GaAs (polarised beams)

Also – Field emission guns for intense e-beams.

RF gun – electrons are “born” in high field (~ 100 MV/m) region (c.f. DC guns ~ few MV/m) ⇒quickly accelerated to high energy ⇒ reduces detrimental effect of space-charge forces on the beam emittance.

Laser-driven allows generation of beams with ⇒same temporal structure as laser ⇒ rapid ( ~ pico-seconds, c.f. gridded DC guns ~ 1ns).

Advantages of laser triggered RF Gun

CTF-3 gun under designed at LAL/Orsay

RF Photo-injectors use many technologies

• Photo-cathodes– Quantum efficiency, thermal emittance, dark current,

aging……

• Cathode lasers– Long pulse train, intensity stability, synchronisation,

temporal and spatial homogeneity…..

• Gun cavity– Geometric form, symmetric coupling, surface fields….

• Analytic theory and numerical simulations– Longitudinal bunching, transverse emittance

compensation, wake-fields….

• Diagnostics– Ultra-short pulses, low emittances, E.O. techniques, RF

deflectors……

Gun geometry design

Waveguide coupler in exit cell –Coupling from both sides – reduces dipole “kick”

Form of irises – reduce peak surface field for given accelerating field

Bunch compressionNeed high peak currents (~ 1 - 2 kA) for SASE FEL.RF gun limited to < 100 A (1nC, 4 ps for TTF) to mitigate space charge effects ⇨ need to perform longitudinal compression.

• Magnetic compressionnot necessarily part of the injector

• Ballistic compression → “classical” injector approach

• “Velocity” (or RF) bunching → incorporates bunching into the injector.

Magnetic Chicane CompresserMagnetic Chicane Compresser

Powerful radiation generates energy spread in bendsPowerful radiation generates energy spread in bends Powerful radiation generates energy spread in bendsPowerful radiation generates energy spread in bends

Causes bend-plane emittance growth (DESY experience)Causes bend-plane emittance growth (DESY experience) Causes bend-plane emittance growth (DESY experience)Causes bend-plane emittance growth (DESY experience)

x = Rx = R1616((ss))E/EE/E

bend-plane emittance growthbend-plane emittance growth

ee––RR

zz

coherent radiation coherent radiation forforzz

overtaking length:overtaking length: L L00 (24 (24zzRR22))1/31/3

ssxx

LL00

Ballistic bunching

• Usually at low energy, typical in injectors with DC guns.• Buncher cavity imparts energy chirp to give compression in downstream drift space.

Velocity BunchingFor non-rigid bunch, relative movements takeplace within bunch to reduce phase spread.

Technique considered for CTF probe beam linac

Observed on DUV-FEL linac (BNL)

2

w

ˆ 3I0

8

3

ˆ I

2Ioth

' 0

Emittance Compensation: Emittance Compensation:

Controlled Damping of Plasma OscillationControlled Damping of Plasma Oscillation

100 A => 150 MeV

L. Serafini and J.B. Rosenzweig, Phy Rev. E, Vol. 55, 1997.

Space charge effects can dominate to high energies for these intense beams and low emittances. “Injector” can be > 100 MeV

Space-charge effects and non-linear RF fields play a role in the evolution of the RMS emittance. Must determine at what point to accelerate and ‘freeze’ the emittance oscillations.

0

0.5

1

1.5

2

2.5

3

3.5

0 2 4 6 8 10Z_[m]

GunLinac

rms beam size [mm]

rms norm. emittance [um]

-0.04

-0.02

0

0.02

0.04

0 0.001 0.002 0.003 0.004 0.005 0.006

z=0.23891

Pr

R [m]

-0.05

0

0.05

0 0.0008 0.0016 0.0024 0.0032 0.004

z=1.5

Pr

R [m]

-0.04

-0.02

0

0.02

0.04

0 0.0008 0.0016 0.0024 0.0032 0.004

z=10

pr_

[rad]

R_[m]

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

-0.003 -0.002 -0.001 0 0.001 0.002 0.003

z=0.23891

Rs [m

]

Zs-Zb [m]

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

-0.003 -0.002 -0.001 0 0.001 0.002 0.003

Z=10

Rs [m

]

Zs-Zb [m]

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

-0.003 -0.002 -0.001 0 0.001 0.002 0.003

z=1.5

Rs [m

]

Zs-Zb [m]

Final emittance = 0.4 m

Matching onto the Local Emittance Max.,

Example of an optimized matchingExample of an optimized matching

M. Ferrario et al., “HOMDYN Study For The LCLS RF Photo-Injector”, Proc. of the 2nd ICFA Adv. Acc. Workshop on “The Physics of High Brightness Beams”, UCLA, Nov., 1999, also in SLAC-PUB-8400

Movable Emittance-Movable Emittance-MeterMeter

for the SPARC projectfor the SPARC project

0

1

2

3

4

5

6

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0 0.5 1 1.5 2 2.5 3

HBUNCH.OUT

sigma_x_[mm]enx_[um]

Bz_[T]

sig

ma

_x

_[m

m]

Bz

_[T

]

z_[m]

emittance envelope

Free-Electron Laser at the TESLA Test Facility

cath

od

e la

ser

magnetundulator

superconducting cavities

bunch compressor

rf-gunbooster

FELbeam

electron dump

Photon DiagnosticsArea

The TESLA Test Facility at DESY: TTF-FEL I

60 mm-keVLong. emittance σE σl

1.2 - 1.5 kAPeak current

~ 1 psBunch length (rms)

~ 8 mm-mradTransverse projected emittance (rms)

~ 3 nCBunch charge

220-270 MeVBeam energy (during FEL operation)

value electron beam parameter

S-band Photo-injectors – high peak currents

• Waveguide coupling to cavity• Tuners for frequency regulation

CTF-2 Gun - CERN

ELYSE Gun – LAL/Orsay

Waveguide and tuners break circular Symmetry of gun bad for emittance.⇒

1800 pulses

Free-Electron Laser at the TESLA Test Facility

The TTF-1 photo-emission electron source

•UV laser impinging a Cesium Telluride cathode provide electrons via photo-emission•The cathode is located in a 1-1/2 cell rf-gun (f=1.3 GHz) with peak E-field of 40 MV/m•The laser is a 4 ND:YLF laser (262 nm)

TTF injector II

typical parameters for TTF 1-FEL:repetition rate: 1 Hz

bunch frequency: 1 - 2.25 MHz

bunch charge: 1- 3 nC

bunch length (rms): ~3 mm (1 nC, after booster )

norm. emit., x,y: ~ 4 µm ( @ 1nC)

p/p: 0.13 % rms ( @ 16 MeV )

injection energy: 16 MeV

Free-Electron Laser at the TESLA Test Facility

•Coaxial input coupler prevents transverse rf- kicks (and the associated emittance growth)•Laser will be upgraded to generate plateau-like distributions•In its final upgrade the source will generate 7200 bunches stack on a 800 sec rf-pulse

The new electron source

• RF-gun commissioned at the DESY-Zeuthen PITZ facility

•Requirements : < 2 mm-mrad at the undulator, Q=1 nC

BESSY Berlin, CCLRC Daresbury,

DESY (HH + Z), INFN Frascati,

INFN Milano, INR Troitsk,

INRNE Sofia, LAL Orsay,

MBI Berlin, TU Darmstadt,

U Hamburg, YERPHI Yerevan

Collaborating Institutes:

(1.3 GHz)

PITZ Collaboration

VUV-FEL Gun: Longitudinal Phase Space

max. mean momentum:4.72 MeV/c

min. rms momentum spread:33 keV/c

4.4

4.5

4.5

4.6

4.6

4.7

4.7

4.8

4.8

10 20 30 40 50 60 70f / degree

pm

ea

n /

MeV

/c

0

20

40

60

80

100

120

140

160

pR

MS /

keV

/c

bunch length:

Minimum bunch length: FWHM = (21.04 ± 0.45stat ± 4.14syst) ps = (6.31 ± 0.14stat ± 1.24syst) mm

good agreement with simulations !

Q = 1 nC

beamlet size is measuredfor 3 slit positions: 1,0,1

7.0 22

n

nYy screeny

screen

n

Transverse Emittance Measurements

beam spot at screen 2

single slitpositions

beamlets at screen 3

Single Slit Scan Technique222 xxxxnx

0.9

1.2

1.5

1.8

2.1

2.4

2.7

3.0

0 10 20 30 40 50

Ibuck, A

no

rm. e

mit

tan

ce /

mm

mra

d

ExEySQRT(Ex*Ey)ASTRA sim.

Q = 1 nC,

= m – 5°

Imain = 305 A

Start-up requirement of TTF2 is clearly fulfilled !

requirement for VUV-FEL (30 nm) = 3

requirement for VUV-FEL (6 nm) = 2

requirement for XFEL = 0.9

1.3 GHz 1 1/2 cell RF Gun

Main SolenoidBucking Solenoid

VUV-FEL Gun: Transverse Emittance

ATF at BNL•Measurement of impact of transverse non-uniformity on emittance•Used a mask•Q=0.5 nC (kept constant)•Emittance for uniform beam is about 1.5 mm-mrad •Long. Length is 3 ps FWHM

100 %

60 % 50 %

90 %

5060

7080

90100

•As predicted by simulation, uniform beam gives the best emittance •Emittance doubles for the 50 % modulation case

(extracted from ATF News Letter 03/2002)

On 23.06.2003 longitudinal shape changed to flat top

FWHM = 7 ± 1psFWHM 18-23 ps

rise and fall time about 5-8 ps

Until 23.06.2003 - Gaussian longitudinal laser shape:

PITZ: Cathode Laser Pulse Profile

Minimum measured emittance

≥ 3 mm mrad 1.6 mm mrad

012345678

0 1 2 3 4 5 6 7 8 9 10z / m

Xrms (CDS14) / mm

EmX (CDS14) / mm mrad

Xrms (no booster) / mm

EmX (no booster) / mm mrad

Outlook PITZ2

Booster Cavity

study emittance conservation principle

booster cavity (TESLA, CDS - Cut Disc Structure)

further improvement of beam quality

work on laser, cavities, photo cathodes,developments on simulation tools

Towards to the XFEL Photo Injector:• 60 MV/m at the cathode• Cathode laser improvement

Simulations for 60MV/m at the cathode

Free-Electron Laser at the TESLA Test Facility

•Injection directly into a TESLA accelerating module makes the emittance compensation scheme more efficient (~1.3 mm-mad)

•Use of a 3rd harm. RF-section to correct the longitudinal phase space distortions

The new injector configuration

BC

ACC. MOD. ACC. MOD.

Matchingsection

Diagnosticssection

3rd harmrf-structure

dump

rf-gun

BESSY FEL2.2 GeV linac using TESLA cavities1 kHz repetition rate, 40 MV/m cathode field→ 75 kW average dissipation !!c.f. needs of Arc-en-Ciel.

Gun being developed in framework of EUROFEL

A variation of the RF-gun concept: the pulsed photodiode

M. Van der Wiel et al.,

T.U. Eindhoven,

2 MV HV 1 ns pulse on a 2 mm diode gap:100 pC @ 100 fs bunch Bn=1.2.1015

Advanced Laser-Plasma High Energy Accelerators towards X-rays

J. Rodier – LAL/Orsay

University of Strathclyde, Glasgow.

Energy Recovery Linac Injectors• ERL – combines LOW transverse emittance beam properties with HIGH average (high duty cycle / CW) current.• Beam properties at high energy are determined by injector quality and not by dynamic equilibrium properties of a Storage Ring

High brightness light source

4GLS – Daresbury Laboratory

High Current ERL Injector Requirements

• Output energy ~ 7 – 10 MeV

• CW average current ~ 100 – 500 mA

• Transverse emittance < 6 mm-mrad

• Longitudinal emittance ~ 150 keV – ps (rms)

• Bunch length ~ 2 – 7 ps

• Energy spread < 0.5 % @ 7 MeV

• High Power RF feed-through possibilities.

JLab High Brightness Gun

Extremely reliable

Delivered 5.5 kiloCoulombs from one cathode

Same cathode for 2 years

Modest improvements for 10 kW Upgrade

Jefferson Lab Gun Approach• Highest average brightness is produced by lowest charges at

highest frequency (J.B. Rosenzweig PAC95 pp. 957-960) • However FEL needs high peak current, contrary to this scaling

• Compromise: highest frequency for which charge (when later bunched to high peak current) has sufficient small signal gain and linac is stable to BBU, etc.

– Highest brightness is achieved by high gun gradient and fast acceleration

• IR Demo operated 3.8 to 4.2 MV/m limited by cesium on gun ball

• Upgrade should achieve >6 MV/m with cesium only on GaAs

• Get to >10 MeV as quickly as possible using srf high gradient cavities

• Long cathode lifetimes (measure in coulombs/cm2 not time!)

• 5,500 Coulombs from one cathode demonstrated (3 mm spot radius)

• Limited by ion back bombardment from background gas

• Vacuum pumping in DC gun 100 times rate in rf gun

JLab 10 kW Upgrade IR FEL Injector demonstrated

performance•Pulsed operation at 8 mA/pulse (110 pC/bunch) in 16 ms-long pulses at 2 Hz repetition rate

•CW operation at 9.1 mA (75 MHz) with 122 pC/bunch

•Routinely delivers 5 mA CW and pulse current at 135 pC/bunch for FEL operations

•400 A peak current at wiggler

The injector is driven by a350 kV DC GaAs Photocathode Gun

Beam

Hybrid DC gun /SRF cavity photo-injector in design engineering

• SBIR development by AES Corp. partnered with JLab

• 750 MHz operation

JLab concept of a high voltage DC gun married to a low frequency rf cavity (shown here with SNS cavities)

133 pC / RF bucket→ 100 mA @ 10 MeV100 kW of beam power!!!

Twin Coupler Attached to the CORNELL ERL Injector Cavity

Modified version of TTF-IIICoupler.Mechanical changes to Increase average power – 75 kW

Normal (Cs2Te) and superconducting (Nb) photo-cathodes under study.SC cavity prohibits use og magnetic field for Emittance Compensation.

Conclusions• Recent years (~ 10) have seen impressive progress in the

development of photo-injectors for short wave-length SASE FEL’s.– Many subjects need to be developed further – cathode life-time, dark

current levels, shaping of laser pulse (temporally and spatially), gun cavity technology for high duty cycle, longitudinal beam compression techniques which leave transverse emittance un-disturbed.

• Increasingly sophisticated BD simulations are guiding injector design for low emittance.

• DC guns, originally investigated for nuclear physics, are being up-graded for ERL requirements.– marriage of DC gun and SC RF technolgy

• The development of superconducting RF technology has had a major influence on the design of new synchrotron light sources.

Acknowledgements

I thank the following people for providing material :

Ph. Piot (FNAL)K. Flöttmann (DESY Hamburg)F. Stephan (DESY Zeuthen)M. Krasilnikov (DESY Zeuthen)F. Marhauser (BESSY)D. Jaroszynski (Univ. Strathcyde)M. Ferrario (INFN Frascati)G. Neil (JLab)