LCLS XTOD Gas Detector

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					                                                                         Lawrence Livermore National Laboratory




                        LCLS XTOD Gas Detector
                               DOE Review of the LCLS Project
                           Breakout Session SC4 – Photon Systems

                                              October 25, 2006


                                              Stefan Hau-Riege

                                 Richard Bionta, Pat Duffy, Keith
                               Kishiyama, Paul Pax, Marty Roeben,
                                Dimitri Ryutov and Stewart Shen



    This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence
  Livermore National Laboratory under Contract W-7405-Eng-48. Work supported in part by the DOE Contract DE-AC02-
                      76SF00515. This work was performed in support of the LCLS project at SLAC.


10/25/06                                               1
                                                                            Stefan P. Hau-Riege
LCLS XTOD Gas Detector                                                       hauriege1@llnl.gov
                                              Lawrence Livermore National Laboratory



                             Overview
 • To measure the FEL pulse energy, we propose to measure the near-UV
   fluorescence radiation of an area where the LCLS beam intersects a N2
   gas
 • The amount of near-UV radiation correlates to the intensity of the LCLS
   beam
 • The gas detectors provide a non-intrusive measure of the FEL pulse-
   energy
     • in real-time,
     • pulse-by-pulse
     • window-less (differentially pumped)
 • Use N2 because it is cheap and safe
 • Two gas detectors (upstream and downstream of the attenuators)


10/25/06                          2
                                                 Stefan P. Hau-Riege
LCLS XTOD Gas Detector                            hauriege1@llnl.gov
                                                               Lawrence Livermore National Laboratory



             Schematics of the gas detectors
                                      PMT               PD
                                   Electronics      Electronics                Photodiode

           Photo Multiplier Tube                                          Bandpass and ND Filters

                         Coating                                              3 mm apertures
      Differential Pumping                                                    along beam path
             Section
                                                                            Magnet Coils

                      Beam / Gas                                      Cylindrical Vessel
                      Interaction                                Avalanche
   Magnet               Region                      APD          Photodiode
    Power          (~0.1 – 2 Torr N2)            Electronics
  Supply and
  Controller                          Gas Feed
                                         And
                                   Pressure Control

10/25/06                                    3
                                                                  Stefan P. Hau-Riege
LCLS XTOD Gas Detector                                             hauriege1@llnl.gov
                                                     Lawrence Livermore National Laboratory



               Overview of physical processes
                Auger e-   photo e-

                                                                                       H.K. Tseng et al.,
                                                                                       Phys. Rev. A 17,
                                                                                       1061 (1978)
    x-rays


               N2 gas
           r
           B                                                       q


       • N2 molecules absorb a fraction of the x-rays by K-shell photoionization,
         emitting photoelectrons of energy (Ex-ray−0.4 keV)
       • Ionized nitrogen relaxes by Auger decay, emitting
         Auger electrons of energy ~ 0.4 keV
       • High-energy electrons deposit their energy into the N2 gas until they are
         thermalized or reach the detector walls
       • Excited gas relaxes under the emission of near-UV photons
10/25/06                                4
                                                        Stefan P. Hau-Riege
LCLS XTOD Gas Detector                                   hauriege1@llnl.gov
                                                         Lawrence Livermore National Laboratory


             X-ray attenuation in N2 over 30 cm:
                Maximum operating pressure
              10
                             30%      10%           5%               1%
                                                                              0.1%

                                                    pump-limited
                1
 pressure
                              attenuation-limited                           0.01%
   (Torr)

              0.1

                                                                          0.001%


             0.01
                         1                          3                                 8
                                        x-ray energy (keV)
10/25/06                                    5
                                                            Stefan P. Hau-Riege
LCLS XTOD Gas Detector                                       hauriege1@llnl.gov
                                                                Lawrence Livermore National Laboratory



      Electron trajectories and energy deposition
                     in N2 at 8.3 keV
                  without magnetic field                                     B = 250 Gauss
           3.0                                                3.0

           2.5                                                2.5

           2.0                                                2.0
     r (cm) 1.5                                         r (cm) 1.5

           1.0                                                1.0

           0.5                                                0.5

           0.0                                                0.0
                  0   5    10      15    20   25   30                0      5    10      15  20          25   30
                                z (cm)                                                z (cm)




                          Simulations neglect confinement by space charges

10/25/06                                            6
                                                                     Stefan P. Hau-Riege
LCLS XTOD Gas Detector                                                   hauriege1@llnl.gov
                                                                 Lawrence Livermore National Laboratory



      Electron trajectories and energy deposition
                    in N2 at 0.83 keV

                   without magnetic field                                       B = 60 Gauss
            3.0                                               3.0

            2.5                                               2.5

            2.0                                               2.0
      r (cm) 1.5                                        r (cm) 1.5

            1.0                                               1.0

            0.5                                               0.5

            0.0                                               0.0
                   0   5   10      15    20   25   30                0      5    10      15    20    25   30
                                z (cm)                                                z (cm)




10/25/06                                           7
                                                                         Stefan P. Hau-Riege
LCLS XTOD Gas Detector                                                   hauriege1@llnl.gov
                                                                     Lawrence Livermore National Laboratory




                      Magnetic field requirements
                               8.3 keV                                                0.83 keV
           100                                                    100

                            chamber walls                                                              N2
            80                                                     80
   Fraction                                                Fraction
      of    60                                                of    60
   Energy                                                  Energy
     (%) 40                                                  (%) 40                    chamber walls
                                          N2
                     chamber ends
            20                                                     20
                                                                                                  chamber ends

             0                                                      0
                 0       100        200        300   400                 0       20     40        60          80
                        Magnetic Field (Gauss)                                  Magnetic Field (Gauss)




10/25/06                                               8
                                                                             Stefan P. Hau-Riege
LCLS XTOD Gas Detector                                                       hauriege1@llnl.gov
                                                   Lawrence Livermore National Laboratory



                   Photoluminescence of N2
                                                            A.N. Brunner
                                                       Cornell University (1967).




       • N2 has strongest lines in the near UV (between 300 and 430 nm)

       • We have used existing experimental data to estimate the near UV signal

       • We will measure the near-UV signal in photoluminescence experiment
                 at SSRL mimicking the actual detector geometry


10/25/06                              9
                                                      Stefan P. Hau-Riege
LCLS XTOD Gas Detector                                 hauriege1@llnl.gov
                                                                                                     Lawrence Livermore National Laboratory



       Estimate of N2 near-UV photoluminescence
       signal based on existing experimental data
• Air fluorescence is used to detect ultra-high energy cosmic rays (UHECR)
• UHECR has led to detailed investigations of photon yields due to excitations by
  electrons in air and N2 for various temperatures and pressures
                                                                                                                  -3
                                                                                                        1.5x10




                                                                                                                       Fluorescence Yield (316-428nm)
                                Fluorescence Yield (316-428nm)
                                per 0.85 MeV electron per meter   5




                                                                                                                          per energy deposited (eV)
                                                                  4                                               -3
    • Fluorescence yield: M.                                                                            1.0x10
    Nagano et al., Astropart.
    Phys. 22, 235 (2004)                                          3
    • Electron stopping
    power: F. Kakimoto et
    al., Nucl. Instr. Meth.                                       2                                     5.0x10
                                                                                                                  -4
    Phys. Res. A 372, 527
    (1996)
                                                                           Electron stopping power
                                                                  1        in N2 @ 0.85 MeV is
                                                                           dE/dx = 1.7 MeV/g/cm^2
                                                                  0                                   0.0
                                                                   0   5   10 15            20       25
                                                                           p (Torr)


10/25/06                                                                   10
                                                                                                        Stefan P. Hau-Riege
LCLS XTOD Gas Detector                                                                                   hauriege1@llnl.gov
                                                    Lawrence Livermore National Laboratory



  Dependence of the photoluminescence yield on
       the energy of the incoming electron
                           1.5 Torr
                                 15 Torr
                                    150 Torr
                                         760 Torr
   Fluorescence Yield
      per deposited
         energy                                                                 (F. Arqueros et al.,
                                            0.85 MeV
          (a.u.)                                                                 submitted)

                         400 eV     8 keV


                                   Electron energy (eV)


           The fluorescence yield per deposited energy depends only weakly
                         on the energy of the exciting electron

10/25/06                             11
                                                       Stefan P. Hau-Riege
LCLS XTOD Gas Detector                                  hauriege1@llnl.gov
                                                         Lawrence Livermore National Laboratory



                Configuration of the gas detector
                                                 1 cm recess




           x-rays                                                                        2.5”




                                         30 cm

• Options:          • Near-UV reflective coatings (specular or non-specular, e.g. Al)
                    • Near-UV absorbing coatings

 • Concerns:        • Longevity of coating materials (N-radicals, energetic electrons)
                    • Interaction of ions and electrons with chamber walls not very
                      well understood

• Periodic re-calibration of detector to ensure integrity of coating
• Recess detectors to eliminate line-of-sight to detector ends

10/25/06                                12
                                                               Stefan P. Hau-Riege
LCLS XTOD Gas Detector                                         hauriege1@llnl.gov
                                                                                 Lawrence Livermore National Laboratory




         Expected near-UV signal at FEL saturation

                         0.83 keV, 0.1 Torr,                                         8.3 keV, 2 Torr,
                         2.3 mJ, 60 Gauss                                          2.3 mJ, 250 Gauss
                                             Lambertian                                                  Lambertian
                                              reflector                      8                            reflector
                                                                    1.2x10
                  8
           8x10
  Number                                                  Number
                                                                             7
    of                                                         of   8.0x10
 Photons                         specular                 Photons                            specular
                  8              reflector                                                   reflector
           4x10
                                                                             7
                      absorber                                      4.0x10       absorber




              0                                                        0.0




                           (1 cm2 detector; PD ~ 2 cm2, PMT ~ 16 cm2)

10/25/06                                                  13
                                                                                    Stefan P. Hau-Riege
LCLS XTOD Gas Detector                                                               hauriege1@llnl.gov
                                                                                Lawrence Livermore National Laboratory




         Expected near-UV signal for low intensities

                           0.83 keV, 0.1 Torr,                                      8.3 keV, 2 Torr,
                            0.1 mJ, 60 Gauss                                        1 mJ, 250 Gauss
                                                                            3
                                                                       6.0x10                           Lambertian
                                                Lambertian
                                                 reflector                                               reflector
                     4
            4.0x10
                                                                            3
  Number                                                     Number 4.0x10
    of                                                            of
                                    specular                                                specular
  Photons            4                                       Photons
            2.0x10                  reflector                               3
                                                                                            reflector
                                                                       2.0x10   absorber
                         absorber




               0.0                                                        0.0



                         Photo multiplier tube is required to detect these low signals

                             (1 cm2 detector; PD ~ 2 cm2, PMT ~ 16 cm2)

10/25/06                                                     14
                                                                                   Stefan P. Hau-Riege
LCLS XTOD Gas Detector                                                              hauriege1@llnl.gov
                                                          Lawrence Livermore National Laboratory



           Noise from spontaneous radiation
                                              high E,   high E,
                                 low E        no slit   with slit
                 photon energy    0.83          8.3       8.3             keV
                 N2 pressure       0.1           2         2              Torr

   Spontaneous radiation
                 energy           0.2          11.5       0.8             mJ
                 absorbed         2.5          40.5       4.3             nJ
                 energy          0.138         106         11             mJ         FEL fundamental
                 fluctuations      1           460        145             nJ         equivalent

   For comparison: FEL fundamental for low pulse energies
                     energy       100          1000      1000              nJ
                    absorbed      2.4           0.6       0.6              nJ


                         Noise due to statistical fluctuations
                    of spontaneous radiation background is low.

10/25/06                                 15
                                                             Stefan P. Hau-Riege
LCLS XTOD Gas Detector                                        hauriege1@llnl.gov
                                                            Lawrence Livermore National Laboratory



                 Planned experiments:
      Calibration of gas detector at 8 keV at SSRL
                                                       Photo multiplier tube
                             ND filter                 (and other detectors)
                         band pass filter
                          glass window                1 cm recess


         x-rays
        (8 keV,
       1012 ph/s)                                N2                                       Be window




 Measure PMT, APD, and PD signals as a function of pressure and magnetic field



10/25/06                                    16
                                                               Stefan P. Hau-Riege
LCLS XTOD Gas Detector                                          hauriege1@llnl.gov
                                            Lawrence Livermore National Laboratory


                              Timeline
                Vacuum system and magnet are designed,
               now working on electronics and configuration

           Milestone                               Completion Date
    1.     System Concept Review (SCR)                               06/06 
    2.     Integrated Prototype Testing                              7/06 ongoing
    3.     Engineering Specifications Doc. (ESD)                     10/06
    4.     Preliminary Design Review (PDR)                           11/06
   [5.     Calibration Test @ SSRL                                   2/07 ]
    6.     Final Design Review (FDR)                                 1/07
    7.     Fabrication/Test                                          9/07
    8.     Readiness Review                                          10/07
    9.     Installation at SLAC                                      2/08


10/25/06                         17
                                               Stefan P. Hau-Riege
LCLS XTOD Gas Detector                          hauriege1@llnl.gov
                                                    Lawrence Livermore National Laboratory



                 Summary and conclusions
    • We described the physical principles underlying the LCLS N2 gas detector
    • We estimated the magnitude of x-ray attenuation and the detector signal
      in near the near-UV
    • Based on our calculations, we suggest the following design:
        • Gas cell length ~ 30 cm, inner diameter ~ 2.5”

      x-ray energy N2 pressure magnetic field FEL energy # photons at detector
          (keV)       (Torr)     (Gauss)                     (1 cm2 detector)

                                                0.1 mJ          1.2 x 104
           0.83        0.1         60
                                                2.3 mJ          2.8 x 108
                                                 1 mJ           1.5 x 103
           8.3          2          250
                                                2.3 mJ          3.5 x 107




10/25/06                              18
                                                       Stefan P. Hau-Riege
LCLS XTOD Gas Detector                                  hauriege1@llnl.gov