Laser Wire Scanner test on CTF2 Laser Wire Scanner test on CTF2 - - PowerPoint PPT Presentation

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Laser Wire Scanner test on CTF2 Laser Wire Scanner test on CTF2 - - PowerPoint PPT Presentation

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Laser Wire Scanner test on CTF2 Laser Wire Scanner test on CTF2 Motivation Experimental set-up Time and space overlap X-ray detection Result of the scan Future improvements and perspectives CERN Royal Holloway University

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SLIDE 1

Laser Wire Scanner test on CTF2 Laser Wire Scanner test on CTF2

Laser Wire Scanner workshop Nanobeam 2002 Royal Holloway University of London G.A. Blair

  • T. Kamps
  • Motivation
  • Experimental set-up
  • Time and space overlap
  • X-ray detection
  • Result of the scan
  • Future improvements and perspectives

CERN

  • J. Bosser

H.H. Braun

  • E. Bravin
  • E. D’Amico
  • S. Döbert
  • S. Hutchins
  • T. Lefèvre
  • R. Maccaferri
  • G. Penn
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SLIDE 2

LWS : Motivations LWS : Motivations

CLIC Project : Main beam

For measuring for very small beam size at high energy Beam size : 40-0.4 µm Beam energy : 9 - 1500 GeV Using the spatial performances of a laser (very small spot size : a few λ)

CTF 3 and CLIC Drive beams

For measuring beam profile on a high average current beam Beam size : 50-500 µm Beam current : 3.5- 35 A Beam energy : 50 MeV- 2 GeV Non - degradable detector compared to classic wire scanners or optical diagnostic (OTR and Cherenkov)

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SLIDE 3

3 GHz Photo-injector 3 GHz Accelerating cavity Nd:YLF laser

1047nm, 3mJ, 4ps 262nm, 10µJ, 4ps

Focusing triplet Current and Position Monitor IR to UV doubling crystals

LWS : Experimental Set LWS : Experimental Set-

  • up

up

Beam dump Laser shutter Laser focusing & scanning systems (1µm resolution) IR filter Remotely controlled delay line Laser Photodiode (2.5mJ) Laser virtual focus (30µm over 1cm)

  • Total scattered photons

Scattered photons on the detector

1 X P E H U

  • R

I

  • S

K R W R Q V

  • D

Y H U D J H G

  • 3KRWRQHQHUJ\NH9

X-ray detector

  • 600 photons

with 17keV averaged energy

  • Detection angle

26mrad

(Can tolerate 5mrad misalignment)

Spectrometer 100µm thick Al Window Electron beam size of 160 µm rms 50MeV, 1nC

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SLIDE 4

3 GHz Photo-injector 3 GHz Accelerating cavity Nd:YLF laser Focusing triplet Current and Position Monitor IR to UV doubling crystals

LWS : Experimental Set LWS : Experimental Set-

  • up

up

Beam dump Laser shutter IR filter Remotely controlled delay line Laser Photodiode Laser virtual focus X-ray detector Spectrometer

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SLIDE 5

LWS : Overlap technique LWS : Overlap technique

3 GHz Photo-injector 3 GHz Accelerating cavity Nd:YLF laser

1047nm, 3mJ, 4ps 262nm, 10µJ, 4ps

Remotely controlled delay line Focusing triplet Laser shutter Spectrometer Current and Position monitors IR to UV doubling crystals Beam dump IR Focusing & scanning systems IR virtual focus imaging CCD Laser Photodiode Scintillator & CCD camera

~3mm offset (~2mrad)

Doubling Crystal In

Laser

IR filter Streak camera

time

OTR Light OTR screen

Electron

Electron energy 50MeV

(minimize energy dispersion)

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SLIDE 6

Streak camera images

Focus mode (2D) Laser Electron

x y

Streak mode Sweep speed 10ps/mm Electron Laser

time y

LWS : Overlap performances LWS : Overlap performances

Delay introduced by the presence of the doubling crystal

45ps

Estimated accuracy: ±3ps and ±300µm

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SLIDE 7

3 GHz Photo-injector 3 GHz Accelerating cavity Nd:YLF laser

1047nm, 3mJ, 4ps

Focusing triplet Current and Position Monitor IR to UV doubling crystals Oscilloscope X-ray detector

LWS : X LWS : X-

  • ray detection

ray detection

600 photons with 17keV averaged energy Expected signal 3.8 mV Laser focusing & scanning systems IR filter Remotely controlled delay line Doubling Crystal out Laser shutter Beam dump Laser Photodiode (2.5mJ)

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SLIDE 8

LWS : X LWS : X-

  • ray detector

ray detector

Photo-multiplier tube Lead loaded plastic Scintillator Mu metal Thin aluminized Mylar foil

  • (
  • 0.006mV

6 L Q J O H

  • S

K R W R Q

  • V

L J Q D W X U H

  • Y

R O W D J H

  • P

9

  • 307+LJK9ROWDJHN9

Calibration curve done at ESRF on the Swiss Norwegian beam line

Source: 8000 photons of 20 keV within 150ps

600 photons with 17keV averaged energy Expected signal of 3.8mV (PMT high voltage : 1.65kV)

Detector assembly

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SLIDE 9
  • Not correlated with a significant change in the bunch charge
  • Very sensitive to a steerer located along the accelerating cavity
  • Change in the position of the laser on the photo-cathode or Drift in the RF

phase or in a power supply

LWS : Raw signals LWS : Raw signals

  • Slow variation (30%) due to

temperature changes in the laser room

  • Fast variations (20%) due to the

shot to shot reproducibility in the UV laser pulse energy

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SLIDE 10

LWS : Background subtraction LWS : Background subtraction

  • +LJKEDFNJURXQGOHYHO
  • $FFHSWDEOHFRQGLWLRQV
  • Compensated signal (mV)

/DUJH)OXFWXDWLRQV

σ: RMS error of the background subtraction technique

Laser off values are used to evaluate the background signal

Give an estimate

  • f background level
slide-11
SLIDE 11
  • Signal to noise ratio changes between 1/8 and 1/30
  • 11 scans are under the average value

LWS : Statistics on the scans LWS : Statistics on the scans

  • 8000 photons of 20keV
  • 2000 photons of 1MeV
  • 1000 photons of 20MeV

Background level ~

Expected signal to noise ratio

  • average

( [ S H F W H G

  • V

L J Q D O

  • W

R

  • Q

R L V H

  • U

D W L R

  • 1XPEHURIVFDQV
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SLIDE 12

Statistical noise : r.m.s value of the histograms of the compensated data

  • Statistical noise changes from 0.3 to 3.5 mV
  • 9 scans are above the average value

LWS : Statistics on the scans LWS : Statistics on the scans

  • average

6 W D W L V W L F D O

  • Q

R L V H

  • P

9

  • 1XPEHURIVFDQV
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SLIDE 13

Total of 9 scans with a S/N ratio better than 1/10 and a RMS error smaller than 1mV

LWS : Scattered photons measurements LWS : Scattered photons measurements

Overlap position Offset position 1mm

No scan : Acquiring data at fixed position

Averaged value 1.04mV Averaged value

  • 0.06mV
  • 6

6 12

Laser on values Laser off values Laser on - Laser off values Detector signal (mV)

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SLIDE 14
  • &RPSHQVDWHGVLJQDO

&RPSHQVDWHGDQGDYHUDJHGVLJQDO

  • LWS : Profile measurements

LWS : Profile measurements

Longitudinal profile : Scan ±18ps

1.5ps offset compared to the overlap values (2ps offset maximum)

σHOHFWURQ σODVHU SV

  • ([SHFWHGVLJQDO
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SLIDE 15
  • &RPSHQVDWHGVLJQDO

&RPSHQVDWHGDQGDYHUDJHGVLJQDO

  • µ

LWS : Profile measurements LWS : Profile measurements

Vertical profile : Scan ± 250µm

σHOHFWURQ µP σODVHU µP

([SHFWHGVLJQDO

25 µm offset compared to the overlap values (150 µm offset maximum)

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SLIDE 16

LWS : Conclusion and Perspectives LWS : Conclusion and Perspectives

  • Thomson photons have been detected
  • LWS profiles are in accordance with the beam dimension measured by optical means
  • Small offsets of maximum 2ps and 150µm have been observed which corresponds to the

accuracy of the overlap technique.

  • The signal to noise ratio is still too low to allow an accurate measurement

Possible improvements using a second detector in parallel for direct background measurement (gain a factor 2)

  • Background consideration is a key issue in the use of LWS. The main source of background

comes from the accelerating cavity (beam halo losses)

  • With our background subtraction technique we can tolerate a signal to noise ratio of 1/10.
  • Concerning the CTF3 machine, a much higher laser power would be required to the obtain an

adequate signal to noise ratio. Q-switched lasers do not deliver enough power Ti:Sapphire lasers must be foreseen (but very expensive)