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

A laser-cooled electron source for single-shot femtosecond

X-ray and electron diffraction

Citation for published version (APA):

Luiten, O. J. (2010). A laser-cooled electron source for single-shot femtosecond X-ray and electron diffraction. In Proceedings of the Paul Scherrer Instituut (PSI), 29 november 2010, Villingen, Switzerland

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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.

A laser-cooled electron source for

single-shot femtosecond

X-ray and electron diffraction

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

resolve atomic length and time scales:

Structural dynamics...

CeO

catalyst nanoparticle

Myoglobin

X-ray or electron pulse

X-ray or electron pulse

radiation damage, repeatability → single-shot!

Linac Coherent Light Source at SLAC

X-FEL based on last 1-km of existing linac

1.5-15 Å Free Electron Laser

5 c m





undulator

1 0 G e V

₂

1 0

1 0

m

2

u

r a d













Single-pass X-ray FEL

~ k A

p e a k

I

Electron beam emittance:

Why ultracold?

x

p

n

x

m c















 







~

4

n

r a d

x



















Single-pass gain →

Electron beam emittance:

Why ultracold?

x

p

n

x

m c















 







~

4

n

r a d

x



















Single-pass gain →

Electron beam emittance:

Why ultracold?

x

p

n

x

m c















 







~

4

n

r a d

x



















Single-pass gain →

s o u r c e

₂

e

n

k T

m c







Electron beam emittance:

Why ultracold?

x

p

n

x

m c















 







~

4

n

r a d

x



















s o u r c e

₂

e

n

k T

m c







Single-pass gain →

cannot be reduced

very much

(bunch charge)

s o u rc e

₂

e

n

k T

m c







Electron beam emittance:

Why ultracold?

x

p

n

x

m c















 







~

4

n

r a d

x



















500× lower!!

Single-pass gain →

cannot be reduced

very much

(bunch charge)

s o u rc e

₂

e

n

k T

m c







Electron beam emittance:

Why ultracold?

x

p

n

x

m c















 







~

4

n

r a d

x



















500× lower!!

Single-pass gain →

Cold source → compact X-FEL!

cannot be reduced

very much

I

I

Magneto-Optical Trap (MOT)

N ≤ 10

10

_{Rb atoms,}

R = 1 mm, n ≤ 10

18

_m

-3

T

_e

Ultracold Plasma

I

I

_{Electron temperature}

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

e

k T





1 p s

T

_e

1 0 K









V

Rb

+

_e

-V

I

I

Ultracold beams!

T

_e

≈ 5000 K (0.5 eV) →

10 K

conventional

photo & field

emission sources

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

Emittance

_~

n

T

e



~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

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);

Ultracold beam experiments

2 cm

cathode

UHV

UHV

laser beams

(trapping, ionization)

-30 kV

300 ps

e

-

= 50

m

m

Accelerator

V

_A

≤ 30 kV

y

z

1 m

MCP

Ultracold beam experiments

Solenoidal

lens



_y

= 50

m

m

Phosphor

screen

σ

= 30 μm

U = 2.1 keV

F = 1.13 kV/cm

λ = 474 nm

T = 405 ± 43 K

Solenoid Current (A)

RM

S

b

e

a

m

siz

e

(

m

m)

Photo-ionization experiment:

beam waist scans @ fixed energy

0 0

1

1

4

F

E

h c

R y

F











_



_







Excess energy

1

1

h c



















4

R y

F

F

0









₀



₀

0

F



0

F



5 P

5 S

R b



→ Stark shift

0

F



Implications for X-FEL: GPT simulations

RF cavity

• 2-cell

• S-band

• 50 MV/m

rf

-i

n

c

o

u

p

le

r

RF cavity

• 2-cell

• S-band

• 50 MV/m

• with laser ports

rf

-i

n

c

o

u

p

le

r

MOT coils

RF cavity

• 2-cell

• S-band

• 50 MV/m

• with laser ports

rf

-i

n

c

o

u

p

le

r

MOT coils

Lasers:

• Excitation

• Ionization

RF cavity

• 2-cell

• S-band

• 50 MV/m

• with laser ports

Initial conditions

Charge

1-100 pC

MOT Density

10

18

_/m

3

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

Implications for X-FEL: GPT simulations

1 p C

5 8

m

Q





R



m

1 0 0 p C

2 7 0

m

-20 0 20 40 60 80 100 GPT

z [mm]

c

e

n

te

r

1

0

%

s

li

c

e

z [mm]

rm

s

e

m

it

ta

n

c

e

[

m

ic

ro

n

]

rms-emittance

center 10%

Acceleration of 100 pC to 2 MeV

?

Implications for X-FEL: GPT simulations

E

_r

, color coded on z

electrons

ions









4



















2

1

2

K







L







3

4

1



I

eK

mc

L





m







4

2

3

1





I

e

mc

P







e

mc

2









Q

I





/



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

ρ

_FEL

0.005

0.0016

λ

_U

4

0.8

mm

Gain Length

37

23

mm

Power (1D)

6

0.2

GW

Repetition rate (10

11

_/s)

_0.1

₁₀

_kHz

Single-shot femtosecond

electron diffraction

SINGLE SHOT

SINGLE SHOT

0.1 pC (10

6

_{e) , 100 keV}

(200)

(311)

(111)

(220)

9 nm Au foil

U = 95 keV

Q = 0.2 pC

Van Oudheusden et al.,

PRL (2010)

X-rays:

Thomson scattering

Electrons:

Rutherford scattering

Complementary information!

high density, bulk

gas phase, surfaces

2 9

2

6 .6

1 0

m

T









2 4

2

1 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

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

10

5

-10

6

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

10

5

-10

6

Electrons – why not?

femtosecond laser photoemission...

100 μm

...electron bunch acceleration...

E = 10 MV/m

100 μm



...electron bunch acceleration...

100 μm

...electron bunch acceleration...

100 μm

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

...electron bunch acceleration...

100 μm

100 μm

x

y

Laser

intensity

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

...evolution into uniform ellipsoid.

100 μm

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

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

y

hard-edged, uniform ellipsoids

Thijs van Oudheusden, TU/e:

Phosphor screen image integrated

over y-direction

Parabola

phosphor screen image

y

→

Bunch compression with 3 GHz RF cavity in TM

₀₁₀

mode

E

_z

-field

B

_F

-field

E

_z

-field

x

y

TM

₀₁₀

: E

_z

-field

TM

₀₁₀

: B

_F

-field

x

y

E

_z

RF cavity

Bunch compression with 3 GHz RF cavity in TM

₀₁₀

mode

F

 

e E

x

y

RF cavity

Bunch compression with 3 GHz RF cavity in TM

₀₁₀

mode

F

 

e E

x

y

RF cavity

Bunch compression with 3 GHz RF cavity in TM

₀₁₀

mode

F

 

e E

x

y

RF cavity

Bunch compression with 3 GHz RF cavity in TM

₀₁₀

mode

F

 

e E

x

y

RF cavity

Bunch compression with 3 GHz RF cavity in TM

₀₁₀

mode

F

 

e E

x

y

RF cavity

Bunch compression with 3 GHz RF cavity in TM

₀₁₀

mode

F

 

e E

x

y

3 GHz RF cavity

longitudinal E-field

50 fs

100 kV

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

TM

₁₁₀

: E

_z

-field

TM

₁₁₀

: B

_x

,B

_y

-field

x

y

E

_z

RF cavity TM

₁₁₀

mode

v



B



Bunch length measurement with RF streak cavity

x,t

y

z

RF cavity in TM

₁₁₀

mode

Cavity off

x,t

y

z

RF cavity in TM

₁₁₀

mode

Limitation temporal resolution:

Cavity off

x,t

y

z

RF cavity in TM

₁₁₀

mode

10 µm slit to improve temporal resolution

10 µm slit to improve temporal resolution

Cavity on

x,t

y

z

B



F



e v



B

RF cavity in TM

₁₁₀

mode

v



Streak image on screen

RF bunch compression

Van Oudheusden et al.,

PRL (2010)

RF bunch compression

67 fs!

Van Oudheusden et al.,

PRL (2010)

10 μm slit

100 fs fit

67 fs fit

streak

Van Oudheusden et al.,

PRL (2010)

10 μm slit

100 fs fit

67 fs fit

streak

Van Oudheusden et al.,

PRL (2010)

coherence length ≤ 1nm sufficient

→ conventional photocathode source OK!

Crystals of biomolecules…

a = 5-10 nm

coherence length ≥ 10 nm required

Summary

• Ultracold laser-cooled electron source:T

_e

≈ 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,