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					A Modular Path Corrector for 4GLS

                     Peter Williams
     ASTeC - Daresbury Laboratory &
                  Cockcroft Institute
     ERL07 Workshop, 23rd May 2007
4GLS Beamlines Schematic

   Dual High Peak Current XUV-FELs
   High Average Current ID Loop & High Peak Current VUV-FEL
                Share main linac (three beams!)
   Dual High Peak Current IR-FELs
4GLS Beamlines: High Average Current Loop
  HACL Bunch Parameter                            100 mA Operation VUV-FEL Operation
  Energy at VUV-FEL, E                                    550 MeV           550 MeV
  Normalised Emittance at 600 MeV,                     2 mm mrad         2 mm mrad
  RMS Projected Energy Spread                                 0.1 %            0.1 %
  RMS Bunch Length at Device             100-450 fs in six straights           100 fs
  Bunch Charge                                                77 pC            77 pC
  Bunch Repetition Rate                                    1.3 GHz     n 4.33 MHz
  Electron Beam Average Power at 600 MeV                    55 MW       n 200 kW
  Injector Energy                                          10 MeV            10 MeV
  Dump Energy                                            ~ 10 MeV          ~ 10 MeV
Path Length Adjustment for ERLs

   Figure shows bunch placement on main linac
    RF waveform                                1 nC             77 pC


   We must energy recover the 77pC HACL                            180°
    bunches
                                                          ~9°

   Ensure π out of phase for deceleration

   Reliably introduce path length retardation of up to 1 wavelength ~23cm

   Must do achromatically, ie without introducing transverse dispersion (ηx),
    desirable to do this independently from longitudinal dispersion (R56),
    possible to achieve isochronously (R56=0)
Path Length Adjustment in Daresbury ERL Prototype

   In ERLP this will be done by physically moving first arc – a triple bend
    achromat. Also used in JAEA ERL, SDALINAC + others
       Impractical for 4GLS – mechanical tolerances preclude accurate control when arc is
        wider than ~10m


                                                                 11.5 cm trombone




                                                 Nominal Gun Energy        350 keV
                                                 Injector Energy           8.35 MeV
                                                 Circulating Beam Energy   35 MeV
                                                 Linac RF Frequency        1.3 GHz
                                                 Bunch Repetition Rate     81.25MHz
                                                 Max Bunch Charge          80 pC
                                                 Bunch train               100 ms
                                                 Max Average Current       13 µA
Other Approaches for Path Length Adjustment in ERLs

   JLab ERL uses Bates arcs / kickers
       Two doglegs and a π-bend dipole arranged for isochronicity
       Symmetric kicker dipoles at π entrance increase path length
       Narrow width not practical for 4GLS configuration
       π-bend dipole would need to be 4.2m for 1.5T field


   CTF3 use variable field, one-period wiggler
       Only achieves small path length difference
       Difficult to compensate for proportionally large R56 variation – for δL=23cm, ΔR56=17cm

   Cornell propose varying radius of path around CESR ring
       Cross talk between path correction and fixed ID arc lattice in 4GLS

   Douglas proposed magnetic mirror
       Unusual shape and intricate edge structure make
        manufacture impractical
Progressive Bunch Compression in 4GLS HACL


                     Final Decompression / Path Correction
                     System




                      VUV-FEL




   Shortest bunch                                            Beam Propagation
   at VUV-FEL

  • An additional requirement – decompress the bunch for linac re-entry!
          Otherwise we cannot energy recover
Path Length Adjustment for 4GLS HACL

   Two non-dispersive doglegs girder mounted such that they move
    transverse to the beamline introducing extra path, coupled by set of
    bellows that expand accordingly. Our moving doglegs
   These introduce negative longitudinal dispersion (arc-like R56) (higher
    energy particles retarded with respect to bunch centre)
   Compensate for this with classical dispersive chicane, introduces
    positive longitudinal dispersion (chicane-like R56) - overcompensate to
    decompress the bunches. Our final decompressor
   Chicane need not physically move, dispersion between centre dipoles
    ensures only small angle deviation compensates → achieve by ramping
    magnets




              Minimal displacement                Maximal displacement
Engineering Layout of Doglegs




                                Thanks to Simon Appleton, DL
Path Correction System Placement in 4GLS HACL




   Final Decompression / Path Correction System
Path Correction System Placement in 4GLS HACL

XUV-FEL Injector

                                                                                         Main Linac




                                                                    HACL Injector




     Final Decompression / Path Correction System

                                Note final decompressor not quite correct in this drawing – transverse
                                displacement of ~0.5m to doglegs ~1.0m
Characteristics of the 4GLS HACL Moving Doglegs



                             Dispersion in Doglegs – Maximal Displacement

                               Note zero dispersion in expanding section!




                             R56 in Doglegs – Maximal Displacement
Characteristics of the 4GLS HACL Moving Doglegs



                            β functions in Doglegs – Maximal Displacement




                              Quadrupole Strength in Doglegs as a
                            function of horizontal displacement angle

                             We need to ramp these to cancel edge
                              effects introduced by the movement
Characteristics of the 4GLS HACL Re-Entry Section



                            Dispersion through final compressor and moving
                                    doglegs – minimal displacement




                            Dispersion through final compressor and moving
                                   doglegs – maximal displacement
Characteristics of the 4GLS HACL Re-Entry Section




                             R56 through final compressor and moving
                                  doglegs – minimal displacement




                             R56 through final compressor and moving
                                 doglegs – maximal displacement
R56 Through The 4GLS HACL


                                                                       Final Decompressor
                                                           156° arcs
                                                                            Moving doglegs




      XUV / HACL                     48° arc
      spreader


       Have you spotted the deliberate mistake?
       Yes – the R56 at the end is not back to zero –
       I have not introduced enough in the final decompressor here!
Characteristics of the 4GLS HACL Re-Entry Section




                             β functions through final compressor and
                             moving doglegs – minimal displacement




                             β functions through final compressor and
                             moving doglegs – maximal displacement
A Modular Path Corrector For 4GLS HACL: Conclusion

   We have presented a novel system to introduce a continuously variable
    path length difference without variation of longitudinal dispersion for
    ERLs

   Modular design ensures independence from rest of machine

   Length of section (12 m) fits into 4GLS design

   Need to evaluate beam disruptive effects (e.g. CSR) on energy recovery
    process in start-to-end simulations

   Thanks to Hywel Owen (DL) and Sergey Miginsky (BINP)
A Modular Path Corrector For 4GLS: Summary
      On re-entry to the main linac of 4GLS for deceleration we require:
    1.     The bunches to be π out of phase with respect to the accelerating bunches – introduce arbitrary
           path length of up to one RF wavelength
    2.     Any path length introduced to be independent of longitudinal dispersion
    3.     Decompression of short bunches to ensure small energy spread for energy recovery
    4.     The system to achieve this must fit in the existing 4GLS HACL design

    We propose dedicated section with two parts, a final de-compressor chicane and girder-mounted
     moveable doglegs connected by expanding bellows




    The dipoles in the chicane ramp slightly to compensate for arc-like R56 generated in the doglegs

    The system is 12m long and would be placed just before re-entry to the 4GLS main linac

				
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