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High Intensity Polarized Electron Sources

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					             High Intensity Polarized
                Electron Sources




                    Evgeni Tsentalovich
07/18/2006                 MIT
Progress over past two decades

       15 years ago                Now
• Unreliable guns at   • Routinely operated productive
  development stage      quality guns (SLAC, JLAB, Mainz,
• Dreams to exceed       Bates…)
  40% polarization     • Strained, superlattice crystals with
                         polarization approaching 90%
                       • New photocathode materials
                       • New gun concepts


07/18/2006
             New requirements
 New generation of accelerators (eRHIC, ILC)
 demand polarized injectors with extreme
 parameters
             • Very high current
             • Very high polarization
             • Low emittance
  Another application: Energy Recovery Linac (ERL)
             • Very high current
             • No polarization
07/18/2006   • Very low emittance
             GaAs photocathodes
Requirements: high QE and polarization

     • Remains the only material for polarized electron
     guns
     • Very high QE
     • Very high polarization
     • But ! Very demanding technology ( Ultra-high
     vacuum requirements)


07/18/2006
     Semiconductor band structure
                                E
                                                         Conducting
                                                           band
                                    Band gap
                                                              Valence
                                                               band



                                         1017 cm 3   - low
Doping (Z, Be) is used to control
                                         1018 cm 3   - medium
the concentration of carriers:
                                         1019 cm 3   - high
 07/18/2006
                 Band structure of GaAs
                                                                   mj
                 Conducting
             E                                                  -1/2       1/2
                   band                          S1 / 2
                                                            1          3

                       1.6 eV               P3 / 2
                                                     -3/2       -1/2       1/2   3/2

                      0.3 eV                   P1 / 2           -1/2       1/2


                                             light  m j  1

                                k
                                             3 1
                                    Pmax          50%
                                             3 1
07/18/2006
                   Strained crystal
                                                            mj
             E                                       -1/2       1/2
                                      S1 / 2
                                                 1          3

                 1.6 eV          P3 / 2
                                          -3/2       -1/2       1/2   3/2

                 0.3 eV             P1 / 2           -1/2       1/2


                                  light  m j  1

                          k
                              Pmax  100 %

07/18/2006
                  GaAs-based photocathodes
                              Strained GaAs:      Superlattice GaAs:
    Bulk GaAs                 GaAs on GaAsP    Layers of GaAs on GaAsP




                                                                    14 pairs
                     100 nm




                                               100 nm
High QE ~ 1-10%                QE ~ 0.15%               QE ~ 0.8%
  Pol ~ 35-45%                  Pol ~ 75%               Pol ~ 85%


07/18/2006
                 Negative electron affinity
                 Most (but not all!) electrons reaching the surface are thermolized
             E


                         Conductive band                              Vacuum
                                                                       level

                  Band gap (forbidden zone)
                                                                 Cs, O(F)
                                                                deposition


                         Valence band

                                                   surface

                                                                 x
07/18/2006
         Photocathodes degradation

Poisoning by residual                      Ion bombardment
       gases
• Oxygen- and carbon-containing    • Most harmful
species are more harmful
                                   • Only high-temperature (~600C)
• Hydrogen and noble gases are     heat cleaning restores QE, and only
more tolerable                     partially
• This degradation can be healed   • Effect is proportional to pressure in
by heat-cleaning at moderate       the chamber and to average current
temperatures (<550 C)



07/18/2006
                 Charge saturation
             E



                                         Vacuum
                                          level




                           surface

                                     x
07/18/2006
                 Charge saturation
                        (SLAC data)




                         5  10 18 cm 3   2  10 19 cm 3


             High doping →low polarization !
07/18/2006
                High gradient doping

High ( ~ 5  10 19 )doped layer ~ 5 nm

                                         • Works very well
                  Superlattice           • The high-doped layer is thin enough
                                         to preserve high polarization
                  Buffer                 • Charge saturation is highly
                                         suppressed (at least for fresh crystals)
                                         • The top layer can survive only few
                  Substrate              high-temperature (~600 C) activations
                                         • Might be problematic for high-current
                                         guns
07/18/2006
                        DC gun design
                           Cylindrical symmetry


             Cathode    Anode
                                                 r'

                                                           r      Emittance:

                                                                
                                                                      drdr
         Normalized emittance  n     doesn’t change with acceleration




07/18/2006
                       DC gun design
                Infinitely small beam spot, no space charge,
                        no nonlinear transverse forces
                                          r'

                                                 r        Emittance:



             Cathode
                                                     
                                                           drdr  0



07/18/2006
                       DC gun design
                   Finite beam spot, no space charge, no
                         nonlinear transverse forces
                                           r'

                                                   r    Emittance:



             Cathode
                                                  
                                                        drdr  
                                                                  0   thermal

                                                    th  C th  R
                  With perfectly linear transverse forces only
                         thermal emittance remains


07/18/2006
      Neglecting thermal emittance
                                        r'
                                       r'

                                              r        Emittance:



             Cathode
                                                  
                                                        drdr  0
                                                                



               Nonlinearity in the gun optics may introduce
                          the emittance growth.




07/18/2006
                     Space charge
  Cathode        Anode
                                 Space charge may change the beam
                                    profile and increase the beam
                                              emittance




             J           J               J            J


                 r           r               r             r



Emittance growth might be suppressed by shaping the laser profile
07/18/2006
             Space charge
• Space charge effects are strongest when
  electrons have low energy (no space
  charge effects for relativistic beam)
• Accelerate as fast as possible – high
  gradient in the gun
• Accelerate as high as possible – high gun
  voltage, to reduce space charge effects
  between the gun and the accelerator
07/18/2006
                                 Space charge
             Worst case
    Child’s law: I max (A)  P  U (V)
                                    3/ 2




  P 
         scenario: large
            d
             2.3  Scathode
                    2
                     - microperveance; d – distance between cathode and

         Space charge influence:
                                            anode


       emitting spot AND
                     I ~ I max      I ~ I max / 10     I ~ I max / 100
        Very strong Strong Weak
            high current
                    Space charge effects could be reduced by


                  density
                  • Increasing gun voltage
                  • Reducing cathode – anode gap
                                                           Limited (breakdowns)


07/18/2006
                  • Increasing the emitting spot     Non-linear transverse forces
                        Emittance:
•    Thermal GaAs cathode (room temperature) ~0.2 mm·mrad ·R(mm)
•    Thermal Cu, Cs2Te cathodes                ~1.2 mm·mrad ·R(mm)
•    Real gun with small emitting spot (JLAB) ~ 5 mm·mrad
•    Real gun with large emitting spot (Bates) ~15 mm·mrad
•    Beam after RF chopping/bunching           ~ 20-100 mm·mrad
•    Estimations for RF (SRF) gun              ~ 1-5 mm·mrad

• ILC requirements                           ~ .05 mm·mrad




07/18/2006
              Polarized electron guns:
              DC                                    RF
  Approved technology (at least
                                          No working GaAs-based RF gun yet
  for ~ 100 kV)
  Require RF chopping/bunching            Beam from the gun is bunched
  RF bunching could be avoided
                                          High acceleration rate, high
  with appropriate laser system
                                          electron energy from the gun
  Low energy beam (space charge! )
  Better suited for large emitting spot

BEST FOR CONVENTIONAL                     BEST FOR APPLICATIONS WITH
APPLICATIONS OR WHEN VERY                 VERY HIGH BRIGHTNESS AND
HIGH CURRENT IS NEEDED                    LOW EMITTANCE
 07/18/2006
             DC Guns: Mainz
                              V = 100 kV
                              Active spot .25 mm


                              I ~ 50 A




07/18/2006
               DC Guns: JLAB




             V = 100 kV           I  120 A
             Active spot 0.2 mm
07/18/2006
             DC Guns: Bates
                              V = 60 kV
                              Active spot 12 mm

                              I peak ~ 30 mA



                              I average ~ 120 A




07/18/2006
             DC Guns: SLAC
                             V = 120 kV
                             Active spot 15 mm

                             I peak  10 A

                             I average ~ 5 A




07/18/2006
             DC Guns: Nagoya




             V = 200 kV
                                 I peak  3 A
             Active spot 18 mm
07/18/2006
             DC Guns: Cornell




                 V = 500 kV (800 ? )


07/18/2006
                  RF guns
• The only practical experience: BINP
  (Novosibirsk)
• Good vacuum conditions with RF on and
  unactivated GaAs crystal installed
• Activated GaAs crystal survived just several RF
  cycles
• Severe back-bombardment resulted in a very
  short life time


07/18/2006
                         RF guns (SLAC)




         1.6 cell pill box                Higher Order Mode (HOM) single cell

             • More open structure
             • No internal irises
             • More effective vacuum pumping


07/18/2006
             RF guns (BNL & AES)




07/18/2006
                             RF guns:
             Warm                            SRF
 Significant practical experience    Very expensive and untested
                                     technology

Unclear if GaAs-based cathode
                                     Best vacuum possible
will survive RF gun conditions

 New, more robust cathode            Wide open apertures (eliminates
 materials may appear (GaN)          back bombardment)


   Much easier to do                Better chances of success


07/18/2006
                 Laser development
     Fiber lasers:
    • Very   short pulses ~ 10 13 sec
    • Mode – locked, but rep. rate limited to 10  10 2 MHz
    • Wavelength 1030 – 1500 nm, but could be frequency-
    doubled
    • Reliable
    • Relatively expensive


07/18/2006
                   Laser development
     Elliptical beams (SLAC)




             • Suppression of non-linear space charge effects
             • Maximizing brightness
             • Might be very useful for RF guns
             • Very challenging task
07/18/2006
                      ILC gun
• DC or RF gun could be used
• ILC emittance requirements are so high that even RF
  gun is unlikely to meet them without dumping ring
• Although dumping ring is still required for RF gun, it
  might be of much simpler design, saving millions
• Conclusion: RF gun would be a better option, but it
  requires significant R&D and the success is not
  guaranteed




07/18/2006
             eRHIC gun (ring-ring)
• Modest intensity and emittance requirements
• Regular DC gun is well suited for the task
• Two options: mode-locked laser or RF chopper/buncher
                    Polarized electron gun for
                   ring-ring eRHIC version is
                  based on
      Mode-locked laser: proven technology
                                      RF chopper/buncher:
                     and doesn’t require any
 • Simplifies injector            • Complicates injector
                         significant R&D
 • No emittance growth in chopper   • Emittance growth in chopper
                                    • Beam compression reduces peak
                                    current demand from the gun
07/18/2006
             eRHIC gun (linac-ring)
                  Extremely high current demand !!!

     I(average) ~ 500 mA
     I(peak) ~ 200 A
     High polarization → strained GaAs → QE ~ 0.1%

     I(mA )  (nm)  Plaser ( W)  QE (%) / 124

   Average laser power ~ 800 W

   Such lasers do not exist. Possible solutions:
   a) array of diode lasers
   b) dedicated FEL – almost unlimited laser power, tunable
07/18/2006
     Problems without known solution
             Heat load (800 W on the cathode)
                                                     HEAT

                                          GaAs                        t=1 mm




                                                     ACTIVE
                                                    COOLING
                                                                             W
  With a conventional cathode stalk             Pt             k  .75
  system, the cathode would heat         T          35o                cm  C o
                                                k S
  up to stellar temperatures, but,
  fortunately, melt first.
                                                                  S  3 cm2

                                  New problem: dynamic cooling (gun off !)
07/18/2006
     Problems without known solution
                           Peak current (~200 A)
                                               2.3  Scathode  U 3 / 2 (V)
        For DC gun :            I max (A) 
                                                           d2
             Scathode  3 cm2
                                 U  1 MV
                d  6 cm


         Larger cathodes? Ring-like cathodes ?

             Emitting spot :


                  What about emittance ???
07/18/2006
Can we relax the requirements?
• With I(average) ~ 40-50 mA the luminosity
  is the same as in ring-ring version
• 40-50 mA gun is still a very difficult task,
  but it is a LOT easier than 500 mA
• Heat load and perveance problems go
  away
• Life time of the cathode is still a major
  problem
07/18/2006

				
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