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					                                    Computation of space charge
                                   effect in Allison scanner and its
                                   application to the measurement
                                             of emittance



                                        T. Gorlov, C. D. Long, M. P. Stockli,
                                              T. R. Pennisi, S. Assadi



                                                  HB 2008, Nashville, TN
                                                     August 26, 2008




1   Managed by UT-Battelle
    for the Department of Energy
Outline

     SNS LEBT emittance measurements
     Demonstration of the problem
     Method of solution of the problem
     Conclusions and future problems




2   Managed by UT-Battelle
    for the Department of Energy
SNS Accelerator Complex

                                                                                                             Accumulator Ring
                                                                                             Collimators


                                          1 GeV          Accumulator Ring:
                                          LINAC                                  Injection                   Extraction




                                                                                                   RF



                                                                                                           RTBT Liquid         Hg
65 keV          2.5 MeV            87 MeV 186 MeV      387 MeV        1000 MeV                                            Target

             MEBT           DTL         CCL    SCL, b=0.61   SCL, b=0.81


                                                                                                                           Target
Ion Source
+
LEBT
                                           Emittance
                                          measurements




3   Managed by UT-Battelle
    for the Department of Energy
SNS LEBT and parameters of the beam

                                             Parameters of the beam




                                                12 cm          Distortion
                                         -
                                     H         Space charge
                                               development


                                             Allison scanner




                                    Emittance
                                   measurements
4   Managed by UT-Battelle
    for the Department of Energy
                                   Allison emittance scanner
                                                                                                    Distribution of current in
                                                                                                     phase space obtained
                                                                                                         by the scanner

                                                                                         I(x,x’)




                                     Parameters for   Name                          Value
                                     simulation
                                    L                Length between two slits      120.95 mm            Measurements of
                                                                                                       rms emittance and
Scanner parameters                  Δa               Entrance slit width           0.5 mm
                                                                                                         Twiss parameters
                    
                                    Δb               Exit slit width               0.5 mm
                                    I                Current of the beam           50 mA
                                    T                Energy of the beam            65 keV
                    
                    
Ion beam parameters                 Rt               rms radius of the beam        2 mm
                                    αt               Twiss parameter of the beam   -1
                    
                    
                                    εt               Normalized rms emittance      0.2 π mm∙mrad


5   Managed by UT-Battelle
    for the Department of Energy
Front slit                                                                                               Exit slit




                                     h slit=0.5 mm
                                   h slit=0.3 mm


                                                     Parameters of the beam




       The distribution of the current                                   The distribution of transverse
       density just before the front slit                               emittance just before the front slit
6   Managed by UT-Battelle
    for the Department of Energy
Front slit                                                                                      Exit slit




                                   h slit=0.5 mm




                                                   The distribution of the
                                                   beam density between
                                                   the front slit and the
                                                   exit slit




           Computation without                                               Computation with
           space charge                                                      space charge
7   Managed by UT-Battelle
    for the Department of Energy
      Simulation of emittance distribution measurements

             Original Gaussian       Computation without        Computation with
            emittance distribution     space charge              space charge




                                     error=2.5 %                        error=17.5 %


               True emittance                      Measured emittance


8   Managed by UT-Battelle
    for the Department of Energy
Calculated dependence of emittance change


                                                                                            m  t
                                                                         f ( t )  100            , (%)
                                                                                              t
                                                              h=0.5 mm
                                                       h=0.4 mm
                                                h=0.3 mm
                                         h=0.2 mm                        h-width of the front slit
                                   h=0.1 mm
                                                                         and exit slit


                                                                         The dashed dependences
                                                                         can be written analytically




9   Managed by UT-Battelle
    for the Department of Energy
         Algorithm of reconstruction of the true emittance
                 Parameters, required for
              calculation of the dependences
                      Parameters       Name                          εm=f(εt)
                      for simulation
                                                                  Measured emittance
                      L                Length between two slits
                      Δa               Entrance slit width
                      Δb               Exit slit width
                      T                Energy of the beam
                      I                Current of the beam

Measured  Rt                          rms radius of the beam
           
parameters  tg(α)                     Slope of ellipce

                                                                                       Reconstructed
                                                                                         emittance




                                          α




 10 Managed by UT-Battelle
    for the Department of Energy
Conclusions
  -It is shown that space charge effect can yield error up to
   few dozens percents in transverse emittance
   measurements of low energy H- beam by Allison Scanner
  -We developed a new method for analysis of emittance
   data that allows to take into account space charge and
   estimate the error of measurements

  Future problems
  -It is necessary to develop or extend the method of
   analysis of emittance data of any distribution
  -It is also necessary to confirm the effect of space charge
   experimentally by measuring the emittance as the function
   of the slit width



11 Managed by UT-Battelle
   for the Department of Energy

				
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posted:10/18/2012
language:English
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