1. 1 Challenges for ILC and CLIC polarized electron sources Feng Zhou SLAC Major contributors: Brachmann, Maruyama, Sheppard, and Zhou SLAC Accelerator Seminar 08/27/09

2. 2 Outline Major parameters ILC and CLIC electron source designs ILC and CLIC electron source R&D at SLAC B006 GTF –ILC and CLIC lasers –Cathodes –Gun (@JLAB, led by Matt Poelker) –Full ILC and CLIC beam demonstrations at B006 GTF Summary

3. 3 2 2 2 ILC and CLIC vs SLC e- sources [1] ILC RDR, 2007. [2] L. Rinolfi, CLIC workshop, CERN, 2007. ParametersILC [1]CLIC [2]CLIC ( by SLAC) SLC Electrons/micropulse (@cath)5nC0.96nC12 nC Number of micropulses (@cath)26253122 Width of micropulse (@cath)1.3 ns100 ps2 ns Micropulse repetition rate (@cath) Microbunch repetition rate (@inj) 3 MHz 2 GHz 17 MHz Width of macropulse1 ms156 ns~64 ns Macropulse repetition rate5 Hz50 Hz120 Hz Charge per macropulse13125 nC300 nC24 nC Average current from gun66  A15  A2.9 uA Peak current of microbunch4.0 A9.6 A6 A Current intensity (@1 cm radius)1.25 A/cm3.0 A/cm1.9A/cm Polarization>80% 2 22

4. 4 Complete ILC electron source optics Cathode, HV DC-gun and bunching system NC pre-acceleration system to 76 MeV 5-GeV SC booster linac LTR: spin rotations and energy compression LTR dc-gun, bunching + pre-acceleration

5. 5 ILC high efficiency bunching system DC-gun: 140 kV, 1.3 ns Two SHBs with 216.7 and 433 MHz: bunch compressed down to 200 ps FWHM. One 5-cell tapered-  TW L-band buncher with 5.5 MV/m: bunch compressed down to 20 ps FWHM. Two 50-cell NC TW structures with 8.5 MV/m gradient accelerate beam to 76 MeV. DC gun 140 kV 5nC/1.3ns 1.3 GHz 5-cell  =0.75-0.93 1.3 GHz two 50-cell  =1.0 216 MHz 433 MHz ……

6. 6 ILC electron injector modeling results Gun voltage Injector energy 140 kV 76 MeV Initial charge at the gun Capture efficiency from gun to 76 MeV injector exit within a window: 45ps x 1.0MeV 5 nC >90% Initial bunch length on cathode Final Bunch length FWHM (FW) 1.3 ns 20 ps (45 ps) Energy spread FWHM (FW)100 keV (1.5 MeV) Norm. rms emittance at 76MeV40  m

7. 7 Spin rotations to preserve polarization in the DR: –bends: 1 st from longitudinal to horizontal plane  bend =n  7.929  at 5 GeV, n=7 to get reasonable R56. –solenoid: 2 nd from horiz. to vert., parallel or anti-parallel to the magnetic field in the DR: B z L sole = 26.23 T.m at 5 GeV. Energy compression: R56 and an RF section LTR: spin rotations/energy compression solenoid Energy compression 7X7.929  Emitt. station

8. 8 S2E tracking from gun to DR injection: with and without energy compression w/o energy compression with energy compression  6cm 50MeV 1.2cm z (cm) 88% of e- from the gun are captured in DR longitudinal acceptance 94% of e- from the gun are captured in DR longitudinal acceptance

9. 9 Choices of CLIC electron injector Injector needs to provide: 312 15-ps microbunches with 2 GHz microbunch repetition rate: –Laser time structure on cathode: 312 100ps-1nc micropulses (original scheme by CERN) Space charge high, 3 A/cm Surface charge unclear @ 100-ps/1-nC Bunching system simplified but still can’t be eliminated Laser requirements high: 2 GHz mode-locked laser –DC beam (156ns/300nC) on cathode: surface charge limit is in less problem and space charge much lower and laser requirements are also relaxed (Sheppard, Brachmann, and Zhou, proposal to demonstrate CLIC beam, 2009). 2

10. 10 CLIC bunching system 140kV 156ns 156ns DC beam = 312 2-GHz RF periods  312 2-GHz microbunches Four RF periods (2-GHz) of DC beam on cathode Four 2-GHz microbunches at injector exit 2 GHz

11. 11 Longitudinal phase spaces @ injector 10  ~14ps

12. 12 CLIC electron injector modeling results Gun voltage Injector energy 140 kV 19 MeV Charge required/microbunch @inj Efficiency from gun to injector exit Achieved charge/microbunch within a window (  z  E = 30 ps  0.45 MeV) ~1 nC 88% 1.3 nC Initial DC pulse length on cathode Final phase extension FWHM (FW) # of generated microbunches @ inj 156 ns 14 ps (30 ps) 312 Final energy spread FWHM (FW)100 keV (1 MeV) Norm. rms emittance at injector exit22  m

13. 13 ILC laser schematic (Brachmann) With Brachmann and Sheppard

14. 14 Pulse stretcher M1 M2 D L = f M1 Retro reflector 700 ps With Brachmann and Sheppard

15. 15 3 MHz pulse switching 3 MHz With Brachmann and Sheppard

16. 16 E158/CLIC laser To generate CLIC beam - 300 nC in 156 ns, existing E158 Flash-Ti: laser at B006 needs to be upgraded with Q-switch. Q-switch principle: –Once Q-switch installed inside the cavity is functioning, light which leaves the medium can’t return, and lasing can’t begin. –Laser medium is pumped while Q-switch is set to prevent of light into gain medium. This produces a population inversion but laser operation can’t occur since feedback loop is blocked. –The rate of stimulated emission is dependent on the light entering the medium, the energy stored in the medium increases up to saturation as the medium is pumped. –Change Q-switch device from low-Q to high-Q, large amount of energy is released quickly. Q-switch at B006 is achieved by supplying a /4 voltage to Pockels cell. The hold-off period took place during applied HV and the release of stored energy occurred after HV pulse. The energy can be increased by a factor of ~10.

17. 17 With Brachmann and Sheppard BRT slicer amplitude Q-switch: PC Laser head cavity

18. 18 Preliminary CLIC beam test results With Q-switch, ~0.5 mJ laser beam is transported to gun entrance. On last Friday, we generated 3x10^12 electrons/pulse at bandgap, 50% more than CLIC goals. Will resume the measurements in next week and study the details.

19. 19 Cathode requirements for ILC and CLIC electron sources Less charge limit (surface charge and space charge) High polarization High QE, and Long QE lifetime SLAC has a worldwide unique polarized Gun Test Facility (GTF) at B006: dedicated to study the above four things.

20. 20 Fast Faraday cup to measure time evolution of electron bunch Nanoammeter to measure photocurrent Mott polarimeter Bend: spin rotator laser 120 kV dc gun

21. 21 Surface photovoltaic effect: surface charge at NEA cathode Photon absorption excites electrons to conduction band Electrons can be trapped near the surface Electrostatic potential from trapped electrons raised affinity. Increased affinity decreases emission probability.

22. 22 Good vs bad surface charge limit: observed at B006 GTF

23. 23 The cathode is driven into saturation, electrons photoexcited into CB can still escape if they diffuse to a non-saturated region. But, these electrons spent long time inside structure so it is likely that they suffer spin relaxation. 1 st observation of polarization dependence on charge limit

24. 24 Charge limit at ILC and CLIC sources Charge limit @ILC: –Individual 1.3ns microbunch’s surface charge limit probably ok but it may be accumulated along 1ms of 2625 microbunches. 1ms surface charge limit is not concluded yet until ILC beam is generated and characterized at SLAC GTF. –Space charge (Child law):  V /d, 14 A/cm at 140kV and d=3cm. –1.25 A/cm @1cm laser radius Charge limit @CLIC (only for SLAC scheme): –Surface charge is well understood. –Space charge 0.64 A/cm @1cm laser radius 2 3/2 2 2 2

25. 25 Can we get higher polarization and QE? Optimum Polarization Large valence band splitting E hh-lh > 60 meV High strain splitting and offsets in valence band Effective electronic transport along SL axes High quality SL, uniform layer composition and thickness Low doping in SL Optimum QE High NEA value Thick working layer Heavy doped BBR layer

26. 26 To enhance baseline cathode GaAs/GaAsP Gradient doping in active layer 5x10 cm to 1x10 cm (next to surface) instead of constant 5x10 cm Electrons can be accelerated when getting through band- bending regions. Higher QE is expected SVT samples already delivered to be installed and tested at GTF in next months. 17 -3 17 -3 17

27. 27 To enhance baseline cathode GaAs/GaAsP (con’t) Optimize doping in the active layer and surface to increase polarization: –Surface doping study: fixed doping in active layer to 1x10 cm, measure QE, polarization and surface charge limit as surface doping level. –Active layer constant doping study: fixed doping in the surface, to determine doping level in the active layer w/o depolarization spin. With gradient doping technique and optimization of doping in surface and active layer, it is expected to increase QE and polarization without surface charge limit. All samples to be delivered by SVT in the next a few months (SBIR phase II). 17 -3

28. 28 To explore alternate cathodes Strained-well InAlGaAs/AlGaAs manufactured in Russia: –Large VB splitting (~60 meV) due to combination of deformation and quantum confinement effects in QW. –BBR engineering –Reasonable-thick working layer without strain relaxation

29. 29 Polarization of AlInGaAs/AlGaAs in Russia and B006 GTF

30. 30 Its QE drops by a factor of 5 in B006 CTS/GTF compared with Russian data, 0.8%. Only difference of the cathode: the one used in Russian is fresh while our one was stored for a few months.

31. 31 Its QE recovered by AHC – Atomic hydrogen cleaning QE drop appears after typical storage times of months. It is probably due to insufficient As on the surface - oxide may transfer As  Ga. Some oxides (GaO 3 -like) and carbides can’t be removed by conventional heating T but removed by AHC (see plot). AHC source at B006 CTS: RF dissociating molecular hydrogen in a tube and atomic hydrogen passed through a 1mm hole and travelled 25 cm to cathode. We will process the Russian cathode using AHC at CTS, and expect to recover its QE. Mainz data in PESP2008

32. 32 QE lifetime ILC average current is 25 times more than SLC. Beam lifetime probably becomes concerned. Studied dependence of QE lifetime on parameters: –Beam current: larger beam current naturally has more ions –Laser size: # of ions/area constant or change? –Wavelength: electrons at short wavelength have higher energy and may be less sensitive to contamination? –DC vs pulse beam: QE lifetime same?

33. 33 QE lifetime vs beam current  ~150h  ~80h

34. 34 QE lifetime vs laser size Residual gases are ionized by electrons and then ions accelerated back to cathode, which results in ion damage to cathode or sputter away Cs. If the laser size on cathode is larger, the ion damage is distributed over a wider area and damage to specific location occurs more slowly (here assume # of ions is not changed for smaller vs larger size if without any charge limit). Ion productions dynamics is actually complicated: –Ion trajectory depends on where ions are generated –Ion stopping depth varies for gas species, and ionization cross section varies with electron energy

35. 35 QE lifetime vs laser size  ~150h  ~85h

36. 36 QE lifetime vs wavelength

37. 37 QE lifetime: CW vs pulse

38. 38 QE lifetime: CW vs pulse If only ion back-bombardment affects, it seems QE lifetime for CW and pulse beam should be same. Can we attributed the phenomena to structure surface, which may be also sensitive to microbunch charge? Detail mechanics needs to be further understood. Current confidence of ILC beam (100uA-level) lifetime mostly relies on CEBAF CW data (200uA). Our measurements show that pulse beam lifetime is worse than CW beam even if their average current is same: under same average current for CW vs pulse beam, beam lifetime seems to depend on microbunch charge (peak current?). ILC (CLIC) microbunch charge is 40 (15) times more than CEBAF. To further study the phenomena, we are planning to run different repetition rates/different pulse length at GTF.

39. 39 How to improve QE lifetime: new activation Cs+Li Recently a new activation technique Cs+Li initiated by Mulhollan at SAXET, provides good opportunity to significantly extend beam lifetime. Regular activation consists of the deposition of a low work function metal, followed or interleaved with an oxidizer NF 3 onto clean surface. The lowest affinities can be obtained using Cs, and either oxygen or fluorine as oxidizer. At the coverage Cs, the initial oxidizer absorption sites are between Cs atoms. If access to underlying oxidizer and GaAs surface could be blocked following activation process, then absorbed gas induced decay could be inhibited. The covalent radius of Li is smaller than Cs. So, in principle, a 2 nd smaller covalent radius alkali can be used to block atoms, then enhance beam lifetime.

40. 40 To implement Cs+Li technique into GTF Beam lifetime comparisons (SAXET) Polarization is already verified at B006 CTS (Maruyama and SAXET) But, still lots of work to do before applying it into GTF: –Need to establish mature activation recipes since it seems system dependence. We will play SAXET chamber with Cs+Li to further study the recipes. –Integrate the technique into GTF (it is not trivial) –Check surface charge with Cs+Li –Measure beam lifetime Mulhollan and Bierman, J. Vac. Sci. Tech. A, 2008 1.5x10 Torr CO 2 -10 5x10 Torr CO 2 -10

41. 41 Gun development (JLAB leads) Present Ceramic Exposed to field emission Large area Expensive (~$50k) New design: New Ceramic Compact ~$5k NEG modules inverted gun

42. 42 Full ILC and CLIC beam demonstrations at B006 GTF Existing 3mm-lead shielding was designed for 20uA beam. Our analysis using FLUKA shows it seems ‘ok’ to run 70uA with 3mm lead shielding. Radiation physicist is double checking our calculations. Once ILC laser comes online, we expect lots of interesting work to do: –Polarization in individual bunch –Surface charge and space charge –QE lifetime for particular beam – 70uA with high peak and combined Cs+Li –Mate JLAB 200-300kV gun to B006 GTF?

43. 43 Summary Polarized ILC and CLIC electron sources are designed and modeled. High capture efficiency in both electron sources is obtained in simulations. Parameters for CLIC source is updated (see table). Extensive R&D on ILC and CLIC electron source is ongoing at SLAC B006: –ILC laser is making progress and expected to come online in next March. Existing E158 laser at B006 is being modified to generate full CLIC beam. –Aggressive work to improve baseline cathode GaAs/GaAsP performance and explore alternate cathode AlInGaAs/AlGaAs is moving ahead.

44. 44 E-source parametersILCCLIC (original) CLIC (SLAC proposed) Number of microbunches (@cath)26253121 DC beam Electrons/(micro)bunch (@cath)4.8nC0.96nC300nC Capture efficiency (w/energy comp)88%(94%)-88% Number of microbunches (@inj)2625312 Width of (micro)bunch (@cath)1.3 ns~100 ps156 ns DC Width of microbunch (@inj)20 ps-14 ps Micropulse repetition rate (@cath)~3 MHz2 GHz- Microbunch repetition rate (@inj)~3MHz2 GHz Width of Macropulse1 ms156 ns Macropulse repetition rate5 Hz50 Hz Charge per macropulse13125 C300 nC Average current from gun66  A15  A Peak current of (micro)bunch@cath4.0 A9.6 A1.9 A Current intensity (@1cm radius)1.25 A/cm3.0 A/cm0.64 A/cm Polarization>80% 2 2 2

45. 45 Summary (con’t) Extensive R&D at B006 (con’t): –QE lifetime dependences on parameters are studied: different QE lifetime of CW and pulse beam is observed. And implementing new activation Cs+Li at GTF to significantly extend beam lifetime is planned. –Full ILC and CLIC beam demonstrations at GTF To study charge limit and QE To study individual bunch polarization QE lifetime at 70uA of average current Mate JLAB 200-300kV gun to GTF

46. 46 Backup slides

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