Microbunching Studies using IR Spectroscopy andTDS

Stephan Wesch, C. Behrens, R. Ischebeck, M. Röhrs,H. Schlarb, B. Schmidt, R. Tarkeshian

FLASH seminar – 25th November 2008

Overview

1 Motivation

2 Instrumentation

3 Measurements

4 Conclusion

IR CTR

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3 5 10 20 30 40 50 60 700

1

2

3

4

5

Λ HÐmL

dU×d

Λ-1

HÐJ�

Ðm

L

E0 = 700 MeV, Q = 0.5 nC

Oncrest spectra:• coherent radiation in

spectral range ofλ = [3.3, 20] µm

• intensity level insame order as fromcompressedbunches

• >1% of the electronsinside the bunchradiate coherently

• strong fluctuations ofthe oncrest spectrum

→ nearly always present!

NIR CTRSingle shot (SASE conditions)

Average

1.0 1.2 1.4 1.60

200

400

600

800

Λ [Ð m]

inte

nsity

[a.u

.]

InGaAs spectrometer:• extension of IR spectrum

(λ = [0.9, 1.7] µm)• average intensity decreases down to

optical wavelengths• single shot spectrum consists of

extreme narrow and highly fluctuatingspikes

→ OTR measurements should confirmcoherent signals!

COTR (?)

Observation in June ’08 showed firstevidence of coherent signals in the opticalregime

−15 −10 −5 0 5 10 15 200

5

10

15

20

25

30

35Kly. Ampl. = 0.000;Pulse = 205; 2008−06−05T121514−meas−profile

time [ps]

long

. dis

trib

utio

n

datarms = 4.797pscal = −44.954fs/pixel

courtesy H. Schlarb

time [ps]

X [p

ixel

]

2008−06−05T121514−meas−profile

5 10 15 20 25

0

50

100

150

200

250

300

350

400

450

Questions:• reproduceable ?• special FLASH setting ?• location (longitudinal) in electron

bunch ?

Microbunching

Substructure inside the electron bunch, which is not related through the correletedenergy spread and can radiates coherently via CTR, CRD or CSR.

Simple LSC model:

?

noise

LSC LSCR56BC2 R56BC3

energy mod. energy mod.

density mod. density mod.

→ transformation from energy to density modulation is given by R56

→ substructure length should be varied by changing R56

→ spectrum shape and intensity should be influenced

Note:both R56 are mandatory for detectable IR radiationsee FLASH talk (08-04-08)

IR spectrometer

see H. Delsim-Hashemi FLASH talk (08-03-04)

Characteristics:• staging of 2 dispersive

reflective gratings• radiation detection by 60

pyroelectric senors (onlycoherent signals)

• single shot capability in 3wavelength ranges

i. 3.3 µm - 9 µmii. 9 µm - 22 µmiii. 25 µm - 65 µm

• CTR off-axis screen servedby TDS kicker

• port about 4 m upstream ofTDS screen

• beam transport with THzbeamline

TDS

TDS port + filter wheel + camera system

Port characteristic:• deflection of single bunch by

fast TDS kicker on screen• imaging radiation by Basler

monochrome camera• 2 color filters (Schott glas

BG39 / RG780) in filter wheelto resolve spectral content

TDS

Spectral response of camera × incoherent OTRintensity

400 500 600 700 800 900 10000.0

0.2

0.4

0.6

0.8

Λ HnmL

spec

tral

repo

nse

Ha.u

.L

~ BG39

~ RG780

-- no filter

Intensity variation in respect to camera gain

courtesy H. Schlarb

Plan

FLASH settings:• E0 = 780 MeV• φACC1 = φACC23 = φACC456 = 0◦

• Q = 0.75 nC (± 1.3%)

Procedure:

0. set BC currents

1. readjust phases

2. focus on TDS screen

3. take TDS images

4. take IR spectra

IR spectrometer I

IBC2 = 64 A

3 5 10 200.0

0.5

1.0

1.5

2.0

2.5

Λ HÐmL

dU×dΛ

-1

HÐJ�

Ðm

L

~ IBC3 = 30 A-- IBC3 = 50 A~ IBC3 = 60 A-- IBC3 = 70 A

IBC3 - variation:

→ increase of spectral intensity with R56

→ no shift of center wavelength

IBC2 - variation:

→ no significant intensity variation exceptfor IBC2 =64 A

→ no drift of center wavelength

IBC3 = 60 A

3 5 10 200.0

0.5

1.0

1.5

2.0

2.5

Λ HÐmL

dU×dΛ

-1

HÐJ�

Ðm

L

~ IBC2 = 57 A-- IBC2 = 60 A~ IBC2 = 64 A-- IBC2 = 70 A

IR spectrometer II

IBC3 - variation:

´

´

´

´ ´

í

í

í

í

30 40 50 60 700

5

10

15

20

25

30

IBC3 HAL

XÈF

long

È\H´

10-

3L

Λ Î @3.4, 13.9D Ðm

´

í

IBC2 = 62.5 A

IBC2 = 64 A

IBC2 - variation:

í

í

í

í

55 60 65 70 750

5

10

15

20

25

30

IBC2 HAL

XÈF

long

È\H´

10-

3L

Λ Î @3.4, 13.9D Ðm

í IBC3 = 60 A

Formfactor:

|F |2 = N−2 ·“

dUdλ

˛̨̨incoh.

”−1· dU

dλ

→ averaging F overλ = [3.4, 13.9] µm

Results:• increasing BC3 current

< F > increases up tosaturation (factor of 4!)

• increasing BC2 current< F > no significant effect

OTR images I

Videos

OTR images II

20 40 60 800

10

20

30

40

50

IBC3

inte

nsity

(a.

u.)

BG39 filter

20 40 60 800

2

4

6

8

10RG780 filter

IBC3

inte

nsity

(a.

u.)

20 40 60 800

20

40

60

80no filter

IBC3

inte

nsity

(a.

u.)

20 40 60 800

2

4

6

8

10RG780 filter, no streak

IBC3

inte

nsity

(a.

u.)

IBC2

=64A

IBC2

=57A

IBC2

=0AΣ ROI intensity:

→ intensity increaseswith decreasing BC3current for all filters!

→ low BC2 current �offers highestintensity in redregime

Note:problems withsaturation at oncrest

phases!

OTR spectra III

20 40 60 800

2

4

6

8

10

IBC3

rel.

fluct

uatio

n (%

)BG39 filter

IBC2

=64A

IBC2

=57A

IBC2

=0A

20 40 60 800

2

4

6

8

10

RG780 filter

IBC3

rel.

fluct

uatio

n (%

)

Σ ROI fluctuation:

→ rel. fluctuations 1% to 2% forBG39

→ about 3% up to 4% forRG780 filter

→ low current � setting showsmaximum fluctuations

Note:charge is stable around 1.3%

RG780 filter images at highcurrent show weak intensity

OTR spectra V

20 40 60 800

0.2

0.4

0.6

0.8

1

IBC3

rel.

ratio

BG39 − RG780

20 40 60 800

0.5

1

1.5RG780 − LOLA off

IBC3

rel.

ratio

IBC2

=64A

IBC2

=57A

IBC2

=0A

Intensity ratios:

→ for decreasing BC3 currentpronunciation in the redregime is significant

→ comparsion betweenTDS on - off reasonable

Conclusions

I. Coherent transition radiation signals reaches form the infrared into the opticalwavelength regime!

II. Spectra and TDS images show rapid fluctuating structure!

III. R56 dependencyIR: increasing intensity for increasing R56BC3

no obvious R56BC3 dependency

O: increasing intensity for decreasing R56BC3both R56 at minimum value create maximum intensitymore intensified signal in the red spectral part

IV. Problem of the determination of incoherent OTR!

V. Problem with camera saturation occurs!