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$: A laser-cooled electron source for single-shot … GFA and SwissFEL Accelerator Seminar A laser-cooled electron source for single-shot femtosecond X-ray and electron diffraction Monday,$
A laser-cooled electron source for single-shotfemtosecond X-ray and electron diffractionLuiten, O.J.

Published in:Proceedings of the Paul Scherrer Instituut (PSI), 29 november 2010, Villingen, Switzerland

Published: 01/01/2010

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I

GFA and SwissFEL Accelerator Seminar

A laser-cooled electron source for single-shot femtosecond X-ray

and electron diffraction

Monday, 29 November 2010, 16.00 h, WBGB/019 Dr. Jom Luiten Eindhoven University of Technology,

Eindhoven, Netherlands

In 2009 the first hard X-ray Free Electron Laser (XFEL) has become operational – LCLS at Stanford University – which enables recording the full diffraction pattern of a tiny protein crystal in a single, few-femtosecond shot. This is an enormously important development but it requires a large-scale facility and investments at the national, if not the international, level. For reasons of size, costs and accessibility of the setup a small-scale XFEL, affordable by a university laboratory, would be highly desirable. A promising route towards a small-scale XFEL is the development of low-emittance electron sources, which enable lasing at reduced electron energies. We have developed a new, ultracold pulsed electron source, based on near-threshold photo-ionization of a laser-cooled gas. The source is characterized by an effective electron temperature of ~10 K, almost three orders of magnitude lower than conventional sources. This should enable normalized RMS emittances 1-2 orders of magnitude lower than photocathode sources, at comparable bunch charge. I will discuss the properties of this new source and the possible implications for XFELs. Another route we are investigating is to use electrons directly. Electrons and X-rays both enable the study of structural dynamics at atomic length scales, yet the information that can be extracted by probing with either electrons or X-rays is quite different and, in fact, complementary. A pulsed electron source with the XFEL capability of performing single-shot, femtosecond diffraction would therefore be highly desirable as well. The primary obstacle facing the realization of such an electron source is the inevitable Coulomb expansion of the bunch, leading to loss of temporal resolution. We have developed a method, based on radio-frequency (RF) techniques, to invert the Coulomb expansion. We will report on the first experiments demonstrating RF compression of 0.25 pC, 100 keV electron bunches to sub-100 fs bunch lengths. We have used these bunches to produce high-quality, single-shot diffraction patterns of poly-crystalline gold. By combining the laser-cooled, ultracold electron source with RF acceleration and bunch compression techniques, single-shot, femtosecond studies of the structural dynamics of macromolecular crystals will become possible with electrons as well.

Artist’s impression of a laser-cooled RF photogun.

Contact: Romain Ganter (5279)

29 Nov 2010 PSI - SwissFEL 1

A laser-cooled electron source for

single-shot femtosecond

X-ray and electron diffraction

Jom Luiten


Thijs van Oudheusden – PhD student

Peter Pasmans – PhD student

Wouter Engelen – PhD student

Adam Lassise – PhD student

Marloes van der Heijden – Master student

Joris Kanters – Master student

Bas van der Geer, Marieke de Loos – Pulsar Physics (GPT)

Peter Mutsaers

Edgar Vredenbregt

Netherlands Technology

Foundation

NL Foundation for Fundamental

Research on Matter

FEI Company

Technical support

Louis van Moll

Jolanda van de Ven

Eddie Rietman

Ad Kemper

Harry van Doorn


resolve atomic length and time scales:

Structural dynamics...

CeO2 catalyst nanoparticle Myoglobin

1 Å @ 100 fs


X-ray or electron pulse

ultrafast diffraction

X-ray or electron pulse


radiation damage, repeatability → single-shot!

ultrafast diffraction


Linac Coherent Light Source at SLACX-FEL based on last 1-km of existing linac

1.5-15 Å Free Electron Laser

OPERATIONAL


Coherent Diffractive Imaging of Biomolecules

29 Nov 2010 PSI - SwissFEL 9LCLS…


Modern day cathedrals…

X-ray light sources: modern day cathedrals…


…how about small chapels?


5 cmu

undulator

1 0 G e V1 0

21 0 m

2

u

ra d

Single-pass X-ray FEL

~ k Ap ea k

I

→ single-pass gain


Electron beam emittance:

Why ultracold?

xx p

n x

m c

~4

n ra d

x

Single-pass gain →

Overlap between electron and X-ray beam…



Why ultracold?

xx p

n x

m c

~4

n ra d

x


…lower normalized emittance → less acceleration.



Why ultracold?

xx p

n x

m c

~4

n ra d

x


s o u rc e 2

e

n

kT

m c

…lower normalized emittance → less acceleration.



Why ultracold?

xx p

n x

m c

~4

n ra d

x

s o u rc e 2

e

n

kT

m c


cannot be reduced

very much

(bunch charge)


so u rce 2

e

n

kT

m c


Why ultracold?

xx p

n x

m c

~4

n ra d

x

500× lower!!


cannot be reduced

very much

(bunch charge)


so u rce 2

e

n

kT

m c


Why ultracold?

xx p

n x

m c

~4

n ra d

x

500× lower!!


Cold source → compact X-FEL!

cannot be reduced

very much

(bunch charge)


Laser-cooled charged particle source


I I

Magneto-Optical Trap (MOT)

N ≤ 1010 Rb atoms,

R = 1 mm, n ≤ 1018 m-3

T < 0.001 K


Te

Ultracold Plasma

I I Electron temperature

Killian et al., PRL 83, 4776 (1999)

ekT

1 p s 1 0 Ke

T


V

Rb+ e-

V

I I

Ultracold beams!

Te≈ 5000 K (0.5 eV) → 10 K

conventional

photo & field

emission sources

Claessens et al., PRL 95, 164801 (2005)

Emittance ~n e

T

~20× lower than

conventional sources


Moreover...

• Each shot a new source – no cathode problems;

• Up to 10 nA average current: 10 pC @ 1kHz;

• Ionization volume fully controlled by laser beam

overlap;

• ultracold ion bunches → model system for space

charge dynamics.


Ultracold electron beams

• photo-ionization experiments

• implications for compact X-FEL

Outline

Single-shot, femtosecond electron diffraction

• RF bunch compression

• ultracold electron source


Ultracold beam experiments



Claessens et al., PRL 95, 164801 (2005);

Claessens et al., Phys. Plasmas 14, 093101 2007;

Taban et al., PRSTAB 11, 050102 (2008);

Reijnders et al., PRL 102, 034802 (2009);

Taban et al., EPL91, 46004 (2010);

Reijnders et al., PRL 105, 034802, (2010).



2 cm

cathode

UHV

UHV

laser beams

(trapping, ionization)

-30 kV

300 pse-


z = 50 mm

Accelerator

VA ≤ 30 kV

y

z

1 m

MCP


Solenoidal lens

y = 50 mm

Phosphorscreen


2928-01-2010

σxi = 30 μm

U = 2.1 keV

F = 1.13 kV/cm

λ = 474 nm

T = 405 ± 43 K

Solenoid Current (A)

RM

S b

ea

m s

ize

(m

m)

Photo-ionization experiment:

beam waist scans @ fixed energy

Te=35 K


0 0

1 14

e x c

FE h c R y

F

Excess energy

0

1 1h c

0

4F

R yF

0

0

0

0F 0F

3 / 25 P

1 / 25 S

R b

→ Stark shift0F

Electric field strength F


Te≈10 K


Implications for X-FEL: GPT simulations

RF cavity

• 2-cell

• S-band

• 50 MV/m

• with laser ports


rf-i

nco

up

ler

RF cavity

• 2-cell

• S-band

• 50 MV/m




rf-i

nco

up

ler

MOT coils

RF cavity

• 2-cell

• S-band

• 50 MV/m




rf-i

nco

up

ler

MOT coils

Lasers:

• Excitation

• Ionization RF cavity

• 2-cell

• S-band

• 50 MV/m



29 Nov 2010 37PSI - SwissFEL

Initial conditions

Charge 1-100 pC

MOT Density 1018/m3

Ionizaton volume Uniform in r

Parabolic in z

Aspect ratio (R/L) 1:10

Ionization time 1 ps

Initial temperature 10 K

Cavity parameters

Maximum field 50 MV/m

Field-balance 1:1

2 R

1½ L


1 p C 5 8 mQ R m

1 0 0 p C 2 7 0 mQ R m


-20 0 20 40 60 80 100

GPT z [mm]

0.01

0.1

1

cente

r 10%

slice

-20 0 20 40 60 80 100

GPT z [mm]

0.01

0.1

1

rms e

mitta

nce [m

icro

n] rms-emittance

center 10%

Acceleration of 100 pC to 2 MeV

?


Er, color coded on z

electronsions

Emittance compensation schemes?


4

radn

21

2

2

2

Ku

rad

g

u

FEL

L

34

1

rad

u

g

IeK

mcL

m

2

32

2342

3

1

FELI

e

mcP

2

e

mcFEL

W

2

2

Wz

QI

/max

Basic FEL equations:


Charge 100 1 pC

Maximum field 50 20 MV/m

Slice emittance 0.1 0.02 micron

Assumed peak current 1 0.1 kA

Wavelength 0.5 0.1 nm

Energy 1.3 1.3 GeV

ρFEL 0.005 0.0016

λU 4 0.8 mm

Gain Length 37 23 mm

Power (1D) 6 0.2 GW

Repetition rate (1011/s) 0.1 10 kHz

Two scenarios: high and low charge


Single-shot femtosecond

electron diffraction


SINGLE SHOT

polycrystalline Au foil

0.1 pC (106 e) , 100 keV


average of 20 single shot pictures



SINGLE SHOT


0.1 pC (106 e) , 100 keV

(200) (311)

(111) (220)


9 nm Au foil

U = 95 keV

Q = 0.2 pC

Van Oudheusden et al.,

PRL (2010)


Why use electrons?


X-rays:Thomson scattering

Electrons:Rutherford scattering

Complementary information!

high density, bulk gas phase, surfaces

2 9 26 .6 1 0 m

T

2 4 21 0 m

R


Electrons vs X-rays

Property Electrons (100 keV) Hard X-rays (10 keV)

Wavelength / Å 0.04 1.2

Mechanism radiation

damage

Secondary electron

emission

Photoelectric effect

Ratio (inelastic/elastic)

scattering

3 10

Energy deposited per

elastic event

1 >1000

Elastic mean free path 1 105-106


Electrons vs X-rays



Mechanism radiation

damage

Secondary electron

emission



scattering

3 10


elastic event

1 >1000


1000× less radiation damage per count!


Electrons vs X-rays



Mechanism radiation

damage

Secondary electron

emission



scattering

3 10


elastic event

1 >1000


106 electrons sufficient for single-shot diffraction!


Electrons – why not?


Electrons – why not?

Source brightness & Coulomb forces


femtosecond laser photoemission...

100 μm

106 electrons from 100 μm spot


...electron bunch acceleration...

E = 10 MV/m

100 μm

106 electrons from 100 μm spot & 10 MV/m



100 μm

E = 10 MV/m


... and irreversible space-charge blow-up!


100 μm

E = 10 MV/m


100 μmx

y

Laser

intensity

Luiten et al., PRL 93, 094802 (2004)

Shaped fs laser pulse...


...evolution into uniform ellipsoid.

100 μm

Luiten et al., PRL 93, 094802 (2004)

→ linear & reversible Coulomb expansion


U = 95 keV, Q = 0.2 pC

hard-edged, uniform ellipsoids

Thijs van Oudheusden, TU/e:

Phosphor screen image integrated

over y-direction

phosphor screen image

x →

y →


hard-edged, uniform ellipsoids

Thijs van Oudheusden, TU/e:

Phosphor screen image integrated

over y-direction

Parabola

phosphor screen image

x →

y →

U = 95 keV, Q = 0.2 pC


Bunch compression with 3 GHz RF cavity in TM010 mode

Ez-field BF-field Ez-field

x

yz


TM010 : Ez-field TM010 : BF-field

x

y

Ez



RF cavity


F e E x

yz


3 GHz RF cavity

longitudinal E-field

50 fs

100 kV

Van Oudheusden et al., JAP 102, 093501 (2007)

The setup


The setup


TM110 : Ez-field TM110 : Bx,By-field

x

y

Ez

RF cavity TM110 mode

Bunch length measurement with RF streak cavity

29 Nov 2010 PSI - SwissFEL

v

B

Bunch length measurement with RF streak cavity

x,ty

z

RF cavity in TM110 mode

transverse B field

73

F ev B


Cavity off

x,ty

z


transverse B field

Limitation temporal resolution:

v


Cavity off

x,ty

z


transverse B field

10 µm slit to improve temporal resolution

v


10 µm slit to improve temporal resolution

Cavity on

x,ty

zB

F e v B


transverse B field

v


Streak image on screen

Streak cavity on cavity off


RF bunch compression


PRL (2010)


RF bunch compression

67 fs!


PRL (2010)


10 μm slit

100 fs fit

67 fs fit

streak


PRL (2010)


10 μm slit

100 fs fit

67 fs fit

streak


PRL (2010)

Limited by streak cavity!


coherence length ≤ 1nm sufficient

→ conventional photocathode source OK!

Crystals of atoms or small molecules:


Crystals of biomolecules…

a = 5-10 nm

coherence length ≥ 10 nm required

→ ultracold electron source!


Summary

• Ultracold laser-cooled electron source:Te ≈ 10 K;

• Ultracold source interesting for compact X-FEL;

• Single-shot, sub-ps electron diffraction demonstrated;

• RF compression of 100 keV, 0.1 pC bunches: 10 ps →100 fs;

• Ultracold source & RF bunch compression → single-shot,

femtosecond electron diffraction of biomolecules.

a laser-cooled electron source for single-shot … gfa and swissfel accelerator seminar a...

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