Online Photon Diagnostics

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On-line Photon Diagnostics
Kai Tiedtke, S.V. Bobashev, Ch. Gerth, J.
Feldhaus, A. Gottwald, F. Jastrow, U. Hahn, A.A.
Sorokin, M. Richter
ICFA Workshop Zeuthen, 20th April 2005
Kai.Tiedtke_at_desy.de
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Which photon beam parameters are required for the
commissioning?

• Spectrum spikes ? modes, pulse
  length wavelength ? electron
  energy tuning for experimental needs

• Intensity SASE optimisation,
  statistics, saturation, normalisation

• Position Profile stabilisation and focusing

• Timing pump-probe experiments

All these parameters are also required for user
experiments, furthermore, the users need these
information on-line, non-destructive, and
time-resolved !
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Outline

• Introduction
• Gas-Monitor Detector (GMD)
• -Detector Principle (Prototype used
  in TTF 1)
• -Four TTF2 GMD for On-line Intensity
  and Beam Position Measurements
• -Extended GMD Version for Ultra-Short X-Ray
  Pulses
• Gas filled attenuator
• Summary and Outlook

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Requirements for Intensity Detectors

• cover full dynamic range 7 orders of
  magnitude from spontaneous emission to SASE
  in saturation
• on-line detectors (non-destructive) for
  single-pulse measurements (response lt 100ns)
  
• low degradation under radiant exposure
• ultra-high vacuum compatibility

No commercial, calibrated detectors available!
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MCP Diagnostics Unit
Intensity Detector for FEL commissioning
MCP 1
MCP 2
large dynamic range (7 orders of magnitude)
can be scanned to measure beam position and
profile _ will not survive long pulse trains _
the wire produces unwanted diffraction
O.Brovko, A. Fateev, M.Yurkov et al., JINR Dubna
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Gas-Monitor Detector
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Gas-Monitor Detector for Monitoring VUV and EUV
Free-Electron-Laser Radiation
Low particle density gt Transparent,
indestructible
Single photoionisation N Nph x n x s
x l N number of electrons or ions Nph
number of photons n target density s
photoionisation cross section l length of
interaction volume
Reference number at the German Patent Office 102
44 303
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Gas-Monitor Detector
Number of detected charged particles
Nparticle Nphoton s(l) z h natom
Nphoton Q.E.
z effective length h particle
detection efficiency Q.E. quantum efficiency
Gas density is controlled by a spinning rotor
gauge and a thermocouple
 
 
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Quantum Efficiency of the TTF 1 Gas-Monitor
Detector Ion-Current Signal of Xenon
measured data
extrapolated data
Xe 1
Xe 2
Relative standard uncertainty 4
M. Richter, A. Gottwald, U. Kroth, A.A. Sorokin,
S.V. Bobashev, L.A. Shmaenok, J. Feldhaus, Ch.
Gerth, B. Steeg, K. Tiedtke, R. Treusch, Appl.
Phys. Lett. 83, 2970 (2003) M. Richter, G. Ulm,
Chr. Gerth, K. Tiedtke, J. Feldhaus, A.A.
Sorokin, L.A. Shmaenok, S.V. Bobashev, AIP
Conference Proceedings 652, 165 (2003)
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Time Resolved Gas-Monitor Detector Signalfrom
Free-Electron-Laser Radiation at 97 nm (TTF 1)
10-5 hPa Xenon l 97 nm
Ion Time-Of-Flight (TOF) Spectroscopy
Absence of higher-charged ions gt negligible
higher-order effects
M. Richter, A. Gottwald, U. Kroth, A.A. Sorokin,
S.V. Bobashev, L.A. Shmaenok, J. Feldhaus, Ch.
Gerth, B. Steeg, K. Tiedtke, R.Treusch, Appl.
Phys. Lett. 83, 2970 (2003)
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Time Resolved Gas-Monitor Detector Signalfrom
Free-Electron-Laser Radiation at 87 nm (TTF 1)
6 x 1012 VUV photons at 87 nm (hw 14.3
eV) within a photon pulse of 100 fs ? pulse
energy 14 mJ peak power 140 MW
10-5 hPa Xenon l 87 nm
M. Richter, A. Gottwald, U. Kroth, A.A. Sorokin,
S.V. Bobashev, L.A. Shmaenok, J. Feldhaus, Ch.
Gerth, B. Steeg, K. Tiedtke, R.Treusch, Appl.
Phys. Lett. 83, 2970 (2003)
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Four TTF 2 Gas-Monitor Detectors
for Online Intensity and Beam
Position Measurements
10-5 hPa
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Test and Calibration Measurements at BESSY
with the TTF 2 Gas-Monitor Detectors
Beam Position
measured
Accuracy for on-line measurements of relative
beam positions 20 mm
calculated
Two gas-monitor detector sets, which are 20 m
apart, allow on-line monitoring of the angle
1 mrad
beam position / mm
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VUV-FEL
Two gas-monitor detector sets
as permanent parts of the
online photon diagnostics for the measurement of
absolute pulse intensities, beam positions, (and
photon energies) within the framework
of a DESY-PTB cooperation
Gas filled attenuator
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Calculated transmission for the 15m long gas cell
Gas filled attenuator

• 15m long gas-filled attenuator with differential
  pumping stages
• Controlled attenuation of FEL beam for 6-120 nm
• Attenuation of 10-6 (depends on gas)
• Preserves beam attributes (coherence,
  statistics, spectrum, etc.)

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Gas filled attenuator (100m) for the XFEL
Calculated transmission of gas filled attenuator
(length 100 m, P 0.2 mbar)

• To minimize the gas consumption of the
  absorber, a recirculation scheme will be used.

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Gas-Monitor Detector for Ultra-short X-ray Pulses
(e.g., at the SPPS, XFEL)
Example for SPPS 1-10 pulses per second 107
photons/pulse hn 9.4 keV gt number of
photons is 6 orders of magnitude and the
photoionization cross section is at least 3
orders of magnitude lower compared to the
VUV-FEL.
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Extended Gas-Monitor Detector Version for
Ultra-short X-ray Pulses
hw
10-4 hPa
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Test Measurements in the VUV at BESSY
with the X-Ray Gas-Monitor
Detector Prototype
Single bunch operation of BESSY Photon energy
16.5 eV Xenon pressure 4 x 10-4 hPa Max. number
of detected ions per bunch 30
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Summary and Outlook
In order to overcome the difficulties of solid
state detectors in the quantitative detection of
highly intense VUV radiation as emitted by FELs
like TTF, an almost transparent monitor detector
has been developed based on photoionization of
rare gases.
The device has been succesfully used at TTF 1 for
the quantitative measurement of radiation pulses
with peak powers up to 140 MW.
Based on this prototype, four gas-monitor
detectors have been developed and successfully
tested in the PTB Laboratory at BESSY which will
be used at the VUV-FEL as permanent parts of the
photon-beam diagnostics for online measurements
of pulse intensities and beam positions.
The principle of the gas-monitor detector has
been extended for the quantitative detection of
ultra-short X-ray pulses as emitted, e.g., at the
SPPS.
The developed detection systems serve as a
background for photoionization experiments on
free atoms and molecules at VUV-FEL