Self-seeding Free Electron Lasers - PowerPoint Presentation

Slide1

Self-seeding Free Electron Lasers

J. Wu

FEL Physics Group

Beam Physics Department

Oct. 26, 2010

Accelerator Research Division Status Meeting

Slide2

October 26, 2010ARD Status meeting

jhwu@slac.stanford.edu

J. Wu, FEL Physics Group

2

Brief description of a Self-Amplified Spontaneous Emission (SASE) Free Electron Laser (FEL) as LCLS

Schemes to improve the longitudinal coherenceSelf-seeding as one of the possibilitiesMonochromatorCrystals for hard x-rayVariable Line Spacing Gratings for soft x-rayIssuesElectron bunch centroid energy jitterElectron bunch energy profile imperfectness

Outline

Slide3

jhwu@slac.stanford.eduJ. Wu,

FEL Physics Group

3

A laser (standing for Light Amplification by Stimulated Emission of Radiation) is a device which produces electromagnetic radiation, often visible light, using the process of optical amplification based on the stimulated emission of photons within a so-called gain medium.

The emitted laser light is notable for its high degree of spatial and temporal

coherence, unattainable using other technologies.Spatial coherence typically is expressed through the output being a narrow beam which is diffraction-limited, often a so-called "pencil beam." Temporal (or longitudinal) coherence implies a polarized wave at a single frequency whose phase is correlated over a relatively large distance (the coherence length) along the beam.What is a laserConceptual physics, Paul Hewitt, 2002

October 26, 2010

ARD Status meeting

Slide4

SASE FEL

Starts from

undulator

Spontaneous Emission

 random startup from shot noise  intrinsically a chaotic polarized

light, e.g., in the linear exponential growth regime, the FEL energy fluctuation distribution falls on a g-distribution functionCollective effectsSelf-Amplified Spontaneous Emission (SASE)Guided mode  mode selection  transverse coherenceSlippage  temporal coherent within slippage distance  coherent spike

SASE FEL

SASE

FEL

Slide5

Gain

guiding—mode selection

for LCLS

courtesy S. Reiche

SASE FEL—Transverse Coherence

Slide6

6

Photon

slips (advances) over electron bunch, the electrons being swept by the same photon

wavepacket

(which is also growing due to bunching) will radiate coherently  coherent length  coherent

spikeSpike duration on order of . For LCLS, less than 1 fs (0.3 mm) at saturationSpeed of light = c

Speed of electron < c

SASE

FEL—Temporal Coherence

Slide7

7

FEL power along the

undulator

LCLS

1.5 Å SASE

FEL Performance

Saturation early with power on order of GW

Instability:

exponential

growth

Instability:

s

aturation

Slide8

8

FEL bandwidth along the

undulator

LCLS

1.5 Å SASE

FEL Performance

Bandwidth on order of 1E-3

Bandwidth decreases as 1/z

1/2

Slide9

9

FEL temporal profile at 60 m

LCLS

1.5 Å SASE

FEL

Performance

Slide10

10

FEL spectrum at 60 m

LCLS

1.5 Å SASE

FEL

Performance

Slide11

11

Reason for wide bandwidth: coherent length shorter than the entire pulse length

Decrease the entire pulse length  low charge, single spike

Increase the coherent length  seeding with coherent length to be about the entire pulse length

Temporal Coherence

LCLS low charge operation mode [Y. Ding et al., PRL, 2009]

Slide12

SASE and seeded FEL

FEL Types: Amplifiers & Oscillators

SASE Amplifier

Laser or HHG

Seeded Amplifier

(external seeding)

Modulator

Buncher

Radiator



in

/

n

Harmonic Generation

EEHG, HGHG, etc. (external seeding)

Oscillator

(self-seeding)

Mirror

Mirror

J.B. Murphy and J. Wu, The Physics of FELs, US Particle Accelerator School, Winter, 2009

Slide13

13

Originally proposed at DESY

[

J.

Feldhaus

, E.L. Saldin, J.R. Schneider, E.A. Schneidmiller, M.V. Yurkov, Optics Communications, V.140, p.341 (1997) .]Chicane & monochromator for electron and photon

Schematics of Self-Seeded FEL

chicane

electron

1

st

undulator

2

nd

undulator

SASE FEL

Seeded FEL

monochromator

electron dump

FEL

Slide14

14

For a transform limited Gaussian photon beam

For flat top

Gaussian

pulse, at 1.5

Å, if Ipk= 3 kA, Q = 250 pC, sz  10 mm, then transform limit is: s

w

/

w

0



10

-6

LCLS normal operation bandwidth on order of 10

-3Improve longitudinal coherence, and reduce the bandwidth improve the spectral

brightness

The coherent seed after the

monochromator

should be longer than the electron bunch; otherwise SASE will mix with Seeded FEL

Transform Limited Pulses

Slide15

15

Reaching a single coherent spike?

Low

charge might reach this,

but bandwidth will be broad

Narrow band, “relatively long” pulse  Self-Seeding.In the following, we focus on 250-pC case with a “relatively” long bunch, and look for “narrower” bandwidth and “good” temporal coherenceFor shorter wavelength (< 1 nm), single spike is not easy to reach, but self-seeding is still possibleSingle Spike vs Self-Seeding

Slide16

16

Seeding the second

undulator

(vs. single

undulator

followed by x-ray optics)Power loss in monochromator is recovered in the second undulator (FEL amplifier)Peak power after first undulator is less than saturation power  damage to optics is reducedTwo-Stage FEL with MonochromatorWith the

same

saturated

peak power

, but with two-orders of magnitude

bandwidth

reduction, the

peak brightness is increased by two-orders of magnitude

Slide17

17

For hard x-ray, crystals working in the Bragg geometry can serve as the

monochromator

Original proposal invokes 4 crystals to form the photon

monochromator

, which introduces a large optical delay  a large chicane has to introduce for the electron to have the same amount of delay  is not favored.Two electron bunch scheme More recent proposal uses single diamond crystal  the monochromatized wake as a coherent seedHard x-ray self-seeding Monochromator

G.

Geloni

et al., 2010

Y. Ding et al., 2010; G.

Geloni

et al., 2010

Slide18

18

LCLS

: Two-bunch HXR Self-seeding

~ 4 m

Si (113)

Si (113)

SASE

Seeded

U1

U2

Y. Ding, Z. Huang, R.

Ruth

, PRSTAB 13,

060703 (

2010)

G.Geloni

et al

.

, DESY

10-033 (2010

),

Before U2

After U2

Spectrum

Slide19

Single diamond crystal proposal

G.

Geloni

et al., 2010

Slide20

Single diamond crystal proposal

G.

Geloni

et al., 2010

Slide21

Power distribution after the SASE undulator (11 cells).

Spectrum after the diamond crystal

Power distribution after diamond crystal

6 GW

10

-5

FWHM

6.7



10

-

5

G.

Geloni

et al., 2010

Slide22

22

Optical components (assuming dispersion in vertical plane)

(horizontal) Cylindrical focusing M

1

: Focusing at re-entrant point

(rotational) Planar pre-mirror M2: Varying incident angle to grating G(rotational) Planar variable-line-spacing grating G: Focusing at exit slitAdjustable/translatable exit slit S(vertical) Spherical collimation mirror M3: Re-collimate at re-entrant pointSoft x-ray self-seeding monochromator

2

nd

undulator

M

1

M

3

G

h

g

M

2

e-beam

source

point

re-entrant

point

1

st

undulator

Y. Feng, J. Hastings, P. Heimann, M. Rowen, J. Krzywinski, J. Wu, FEL2010 Proceedings. (2010)

Slide23

23

Peak current ~3 kA

Undulator period 5 cm,

Betatron function 4 m

For 250 pC case, assuming a step function current profile,

sz  7 mm.Gain length ~ 2.1 mSASE spikes ~ 1606-Å Case: Electron Bunch

Slide24

24

6-

Å

FEL power along the

first

undulator6-Å SASE FEL Parameters

saturation around 32 m with power ~10 GW

LCLS-II uses about 40

meter long

undulators

Slide25

25

6 Å FEL temporal profile at 30 m in the first undulator: challenge

6 Å SASE FEL Properties

Slide26

26

6 Å FEL spectrum at 30 m in the first undulator

Spiky

spectrum: challenge

6 Å SASE FEL Properties

Slide27

27

Effective SASE start up power is 1.3 kW.

Use

small start up seed power 100 kW.

Monochromator

efficiency ~ 0.2 % (at 6 Å)Phase space conservation: bandwidth decreases 1 to 2-orders of magnitude (~ 160 spikes)Take total efficiency 5.010-5 Need 2 GW on monochromator to seed with 0.1 MW in 2nd

und.

6-Å

Case - Requirement on Seed Power

2 GW

0.1 MW

Slide28

28

Temporal profile at ~25 m in the

2

nd

undulator for seed of 100 kW

~12

m

m

6-Å

Self-Seeded

FEL

Performance

Slide29

29

FEL spectrum at ~25 m in the

2

nd

undulator for seed of 100 kW

FWHM 5.2



10

-

5

6-Å

Self-Seeded

FEL

Performance

Slide30

30

Effective pulse duration 12

m

m,

s

z ~ 3.5 mm Transform limited Gaussian pulse  bandwidth is 3.210-5 FWHM.(For uniform pulse  4.410-5 FWHM)

The seeded FEL bandwidth (5.2

10

-

5

FWHM) is

close to

the transform limited bandwidth

6-Å

case — transform limited

Slide31

Parameter

6 nm

6

Å

unit

Emittance

0.6

0.6

m

m

Peak Current

1

3

kA

Pulse length rms

35

12

fs

Bandwidth FWHM

24

5.2

10

-

5

Limited Bandwidth

15

4.4

10

-

5

Seed Power

100

100

kW

Power on Mono

50

2000

MW

Mono Efficiency

10

0.2

%

Over all Efficiency

20

0.5

10

-

4

Sat. Power

5

10

GW

Sat. Length

30

35

m

Brightness Increment

50

150

Self-Seeding Summary at 6 nm and 6

Å

31

J. Wu, P. Emma, Y

. Feng, J. Hastings,

C. Pellegrini,

FEL2010 Proceedings. (2010)

Slide32

October 26, 2010ARD Status meeting

jhwu@slac.stanford.edu

J.

Wu,

FEL Physics Group

32Electron centroid energy jitter can lead to both timing jitter and also a detuning effectTake 6 nm as example, FEL parameter r ~ 1.2 ×10-3 R56 ~ 3 mmTiming jitter 12 fs

Issues

FEL detuning theory

; positive detune



longer

gain length,

higher

saturation power; negative detune 

longer

gain length,

lower

saturation power

X.J. Wang et al.,

Appl

. Phys.

Lett

. 91, 181115 (2007).

Slide33

October 26, 2010ARD Status meeting

jhwu@slac.stanford.edu

J.

Wu,

FEL Physics Group

33The previous slide shows the power fluctuation due to centroid energy jitter, the spectrum bandwidth seems to be less affected. Issues

Slide34

October 26, 2010ARD Status meeting

jhwu@slac.stanford.edu

J.

Wu,

FEL Physics Group

34Electron bunch energy profile imperfectnessIn the second undulator, with the injection of monochromatized coherent seed, the FEL process is essentially a seeded FELStudy a linear energy chirp on the electron bunch first, The FEL bandwidth where and

Issues

J. Wu, P.R. Bolton, J.B. Murphy, K. Wang,

Optics

Express

15

, 12749 (2007);

J. Wu, J.B. Murphy, P.J. Emma et al., J.

Opt

. Soc. Am. A

24

, 484 (2007).

Slide35

October 26, 2010ARD Status meeting

jhwu@slac.stanford.edu

J.

Wu,

FEL Physics Group

35Take 1.5 Å as exampleInitial coherent seed bandwidth 10-5;The electron energy chirp is taken for four cases: over the rms bunch length, the rms correlated relative energy spread is 0.5 r (green), r (

purple

), 2.5

r

(

blue

), and 5

r

(

red

)

Issues

Slide36

36

Start with 10

-6

bandwidth, 10 MW seed, well cover the entire electron bunch the FEL power along the

undulator

LCLS Self-Seeded FEL Performance

Saturation early with power on order of GW

Slide37

37

FEL temporal profile at 40 m

LCLS Self-Seeded FEL Performance

Slide38

38

FEL spectrum at 40 m

The

nonuniform

energy profile affects the bandwidth

LCLS Self-Seeded FEL Performance

FWHM 10

-

5

October 26, 2010

ARD Status meeting

jhwu@slac.stanford.edu

J.

Wu,

FEL Physics Group

Slide39

October 26, 2010ARD Status meeting

jhwu@slac.stanford.edu

J.

Wu,

FEL Physics Group

39Electron bunch energy profile imperfectnessStudy a linear energy chirp together with a second order curvature on the electron bunch, where Issues

A.A. Lutman, G. Penco, P. Craievich, J. Wu, J. Phys. A: Math.

Theor

.

42

, 045202 (2009);

A.A. Lutman, G. Penco, P. Craievich, J. Wu, J. Phys. A: Math.

Theor

.

42, 085405 (2009);

Slide40

October 26, 2010ARD Status meeting

jhwu@slac.stanford.edu

J.

Wu,

FEL Physics Group

40Electron bunch energy profile imperfectnessElectron bunch can have an energy modulation, IssuesJ. Wu, A.W. Chao, J.J. Bisognano, LINAC2008 Proceedings, p. 509 (2008);B. Jia, Y.K. Wu, J.J. Bisognano, A.W. Chao, J. Wu, Phys. Rev. ST Accel. Beams 13, 060701 (2010);

J. Wu, J.J. Welch, R.A. Bosch, B. Jia, A.A. Lutman, FEL2010 proceedings.

(2010).

Slide41

41

LCLS excellent electron beam quality leads to short gain length, early saturation. This makes possible to add more functions

Two-stage FEL with

monochromator

reduces the bandwidth by 2 order of magnitude with similar peak power  increases the brightness by 2 order of magnitudeSome details about electron energy centroid jitter and energy profile imperfectness has been looked intoSummary

Slide42

42

Thanks for your attention

!

Thanks to

Y. Cai

for providing this chance! Special thanks to:P. Emma, Z. Huang, J. Arthur, U. Bergmann, Y. Ding, Y. Feng, J. Galayda, J. Hastings, C.-C. Kao, J. Krzywinski, A.A. Lutman, H.-D. Nuhn, T.O. Raubenheimer, M. Rowen, P. Stefan, J.J. Welch of SLAC, W. Fawley, Ph. Heimann of LBL, B. Kuske of HZB, J.B. Murphy, X.J. Wang of BNL, C. Pellegrini of UCLA, and J. Schneider of DESY for fruitful discussions. ……