Feng Zhou SLAC Major contributors: Brachmann, Maruyama, Sheppard, and Zhou SLAC Accelerator Seminar 08/27/09 1

• 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 2

Parameters Electrons/micropulse (@cath) Number of micropulses (@cath) Width of micropulse (@cath) Micropulse repetition rate (@cath) Microbunch repetition rate (@inj) Width of macropulse Macropulse repetition rate Charge per macropulse Average current from gun Peak current of microbunch Current intensity (@1 cm radius) Polarization ILC [1] 5nC 2625 1.3 ns 3 MHz 3 MHz 1 ms 5 Hz 13125 nC 66  A 4.0 A >80% CLIC [2] CLIC ( by
SLAC)
0.96nC
312 100 ps 2 GHz 2 GHz 156 ns 50 Hz 300 nC 15  A 9.6 A
2
>80%
2 2
[1] ILC RDR, 2007. [2] L. Rinolfi, CLIC workshop, CERN, 2007.
SLC
12 nC 2 2 ns 17 MHz 17 MHz ~64 ns 120 Hz 24 nC 2.9 uA 6 A
2
1.9A/cm
2
>80% 3

• 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 4

DC gun 140 kV 5nC/1.3ns
216 MHz … … 433 MHz 1.3 GHz 5-cell  =0.75-0.93
1.3 GHz two 50-cell  =1.0
• 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.
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Gun voltage Injector energy Initial charge at the gun Capture efficiency from gun to 76 MeV injector exit within a window: 45ps x 1.0MeV Initial bunch length on cathode Final Bunch length FWHM (FW) 140 kV 76 MeV 5 nC >90% 1.3 ns 20 ps (45 ps) Norm. rms emittance at 76MeV 40  m 6

• Spin rotations to preserve polarization in the DR: – bends: 1 st from longitudinal to horizontal plane  bend =n  7.929
– solenoid: 2 nd  at 5 GeV, n=7 to get reasonable R56. 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 Emitt. station 7X7.929
 solenoid Energy compression 7

9800 9800 9740 w/o energy compression 1.2cm
9680 data i 5 9620 9748 longi vv 1 9727 9560 9500 9500 9.874
 10  7  7 z (cm) data 7 9.8747
 10 7 
88% of e- from the gun are captured in DR longitudinal acceptance
7 9790 9769 9706 with energy compression 6cm 9685 1.51192
 10 6 z (cm) 6 1.51212
 10 longi vv 0
94% of e- from the gun are captured in DR longitudinal acceptance
6 8

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

10  ~14ps 11

Gun voltage Injector energy Charge required/microbunch @inj Efficiency from gun to injector exit Achieved charge/microbunch within a window (  z  E = 30 ps  0.45 MeV) Initial DC pulse length on cathode Final phase extension FWHM (FW) # of generated microbunches @ inj Final energy spread FWHM (FW) Norm. rms emittance at injector exit 140 kV 19 MeV ~1 nC 88% 1.3 nC 156 ns 14 ps (30 ps) 312 100 keV (1 MeV) 22  m 12

With Brachmann and Sheppard 13

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

1.1
0.9
0.7
0.5
0.3
0.1
-0.1
0.0E+00
3 MHz 5.0E-07 1.0E-06
t [s]
With Brachmann and Sheppard 1.5E-06 2.0E-06 15

• To generate CLIC beam - 300 nC in 156 ns, existing 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 16

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

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

• 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.
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22

st
• 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.
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• 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 24

Optimum Polarization
• Large valence band splitting E thickness 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 • Low doping in SL
Optimum QE
• High NEA value • Thick working layer • Heavy doped BBR layer 25

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

• 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).
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• 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
QE Polarization 100 10 1 80 10 0 10 -1 60 10 -2 10 -3 40 20 10 -4 10 -5
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0 900 550 600 650 700

, nm 750 800 850

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• 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.
10 SLAC B006 CTS Russian data 1 0.1
0.01
650 675 700 725 750
Wavelength in nm
775 800 825 850 30

• 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.
10 1 0,1 0,01 1E-3 700 725 Fresh Long term storage Hydrogen cleaned 750 775 800 Wavelength, [nm] Mainz data in PESP2008 825 31 850

• 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?
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
~150h

~80h
33

• 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 34


~150h

~85h
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• 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.

• Recently a new activation technique Cs+Li initiated by 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.
nd smaller covalent radius alkali can be used to block atoms, then enhance beam lifetime. 39

• 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 1.5x10 Torr CO 2 5x10 Torr CO 2 Mulhollan and Bierman, J. Vac. Sci. Tech. A , 2008 40

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

• Existing 3mm-lead shielding was designed for 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?
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• 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.
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E-source parameters ILC Number of microbunches (@cath) Electrons/(micro)bunch (@cath) 2625 4.8nC
Capture efficiency (w/energy comp) 88%(94%) Number of microbunches (@inj) 2625 Width of (micro)bunch (@cath) Width of microbunch (@inj) Micropulse repetition rate (@cath) 1.3 ns 20 ps ~3 MHz Microbunch repetition rate (@inj) Width of Macropulse Macropulse repetition rate Charge per macropulse Average current from gun Peak current of (micro)bunch@cath Current intensity (@1cm radius) Polarization ~3MHz 1 ms 5 Hz 13125 C 66  A 4.0 A 1.25 A/cm >80%
2
CLIC (original) 312 0.96nC
312 ~100 ps 2 GHz 2 GHz 156 ns 50 Hz 300 nC 15  A 9.6 A 3.0 A/cm
2
>80% CLIC (SLAC proposed) 1 DC beam 300nC 88% 312 156 ns DC 14 ps 2 GHz 156 ns 50 Hz 300 nC 15  A 1.9 A 0.64 A/cm
2
>80% 44

• 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 45

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