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Highlights of the Polarized Electron/Positron Source Meeting at the - - PowerPoint PPT Presentation

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Highlights of the Polarized Electron/Positron Source Meeting at the 17 th International Spin Symposium, Kyoto, Japan & Photocathode Lifetime Measurements to 10 mA using the New CEBAF 100 kV GaAs DC Photogun Joe Grames CASA Seminar

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SLIDE 1

Highlights of the Polarized Electron/Positron Source Meeting at the 17th International Spin Symposium, Kyoto, Japan & Photocathode Lifetime Measurements to 10 mA using the New CEBAF 100 kV GaAs DC Photogun Joe Grames CASA Seminar December 7, 2006

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SLIDE 2

Symposium Topics

  • Fundamental Symmetries and Spin
  • Spin Structure of Nucleons
  • Spin Beyond the Standard Model
  • Spin in Hadronic Reactions
  • Spin Physics with Photons and Leptons
  • Spin Physics in Nuclei
  • Spin Physics with RI Beams
  • Acceleration, Storage, and Polarimetry of Polarized Beams
  • Polarized Ion and Electron Sources and Targets
  • Future Facilities and Experiments
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SLIDE 3

Session 9B : Polarized electron (positron) sources Presentations

  • ral : 15

poster : 6

  • JLAB
  • SLAC
  • University of Mainz
  • University of Bonn
  • CERN
  • DESY
  • St. Petersburg
  • KEK
  • Osaka Electro-Communication University
  • Rikkyo University
  • Nagoya University

http://spin.phys.nagoya-u.ac.jp/~spweb/spin2006.html

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SLIDE 4

Session 9B: Topics

Photocathode Development

strained super-lattice photocathode gridded photocathode, pyramidal shape photocathode

Low Emittance Beam Production Polarized electron source for SPLEEM Pol.e±

  • S
  • urce for ILC

Polarized electron beam injector Polarized positron beam production

Pol.e- source operation

High average current operation High current density test

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SLIDE 5

550 600 650 700 750 800 850 900 10

  • 5

10

  • 4

10

  • 3

10

  • 2

10

  • 1

10 10

1

20 40 60 80 100

QE

QE, %

λ, nm Polarization

Polarization, %

SL In0.155Al 0.2Ga0.645As(5.1nm)/Al0.36Ga0.64As(2.3nm), 4 pairs

(Y. Mamaev, St.Petersburg)

Polarization (max.) = 92%, Quantum Efficiency = 0.6%

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SLIDE 6

High Surface Charge Density Superlattice Photocathodes

(M. Yamamoto, Nagoya University)

Superlattice photocathode:

  • Surface <100 nm is GaAs
  • Similar doping, e.g., Zinc
  • Concern: heat => diffuses dopant
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SLIDE 7

High Surface Charge Density Superlattice Photocathodes

(M. Yamamoto, Nagoya University)

8000 1600 20 18 3.3 25 1.6 240 Bunch Charge (pC) Bunch Width (ps) Laser Spot Size (mm) Peak Current (mA/mm2)

ILC-like 10’s μA JLAB-like 100’s μA

GaAs/GaAsP, surface p-dope density 6x1019/cm3

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SLIDE 8

(M. Kuwahara, Nagoya University)

Polarized e- Extraction from a Pyramid-Shaped Photocathode Extraction of polarized electrons by F.E. Electrons extracted by F.E. have higher polarization than NEA’s. long lifetime compared with NEA surface.

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SLIDE 9

Session 9B: Topics

Photocathode Development

strained super-lattice photocathode gridded photocathode, pyramidal shape photocathode

Low Emittance Beam Production Polarized electron source for SPLEEM Pol.e±

  • S
  • urce for ILC

Polarized electron beam injector Polarized positron beam production

Pol.e- source operation

High average current operation High current density test

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SLIDE 10

Low Emittance Beam from GaAs-GaAsP superlattice photocathode εrms = 0.096±0.015 π.mm.mrad

(N. Yamamoto, Nagoya University)

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SLIDE 11

Session 9B: Topics

Photocathode Development

strained super-lattice photocathode gridded photocathode, pyramidal shape photocathode

Low Emittance Beam Production Polarized electron source for SPLEEM Pol.e±

  • S
  • urce for ILC

Polarized electron beam injector Polarized positron beam production

Pol.e- source operation

High average current operation High current density test

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SLIDE 12

(T. Yasue, Osaka Electro-Commmunications University)

Reflection Diffraction sample Electrons

Low energy electrons: strong interaction with surfaces

  • relatively high reflectivity
  • small penetration depth

SURFACE SENSITIVE

s

  • u

r c e e l e c t r

  • n

a n a l y z e r man i p u l a t

  • r

20 20cm

CCD c ame r a sample

  • bjective

lens beam separator energy filter screen e- source

HV

LEEM: Low Energy Electron Microscopy

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SLIDE 13

φ P

  • Co/W(110)
  • 3.8eV
  • FOV=25mm
  • in-plane

φ=0o φ=45o φ=90o φ=-45o φ=-90o M M M

CONTRAST: P·M

P // M: maximum (minimum) P ⊥ M: 0 SPLEEM: Spin Polarized LEEM

(T. Yasue, Osaka Electro-Commmunications U. & S. Okumi, Nagoya U.)

3 μm spot

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SLIDE 14

Session 9B: Topics

Photocathode Development

strained super-lattice photocathode gridded photocathode, pyramidal shape photocathode

Low Emittance Beam Production Polarized electron source for SPLEEM Pol.e±

  • S
  • urce for ILC

Polarized electron beam injector Polarized positron beam production

Pol.e- source operation

High average current operation High current density test

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SLIDE 15

International Linear Collider

Parameter Units SLC ILC Electrons per bunch nC 16 6.4 Bunches per pulse # 2 2820 Microbunch rep rate MHz 17 3 Pulse rep rate Hz 120 5 Cathode area cm2 3 TBD Cathode bias kV

  • 120

TBD Bunch length ns 2 TBD Gun to SHB1 drift cm 150 TBD en,rms,gun (fm EGUN) 10-6 m 15 20

(Jym Clendenin, SLAC)

500 GeV COM

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SLIDE 16

(Axel Brachmann, SLAC)

Polarized Electron Injector Layout

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SLIDE 17

Laser Development – Laser system beyond state of the art – Challenge is 3 MHz amplification (Pave ~ 100 mW, Pburst ~15 W) Photocathode Development – Demonstrate performance with high Pburst – Combination of high-QE & low-SCL (doping), while high-P Gun Development – Baseline Design: 120 kV SLC Gun – Higher voltage will reduce (SH) bunching timing requirements – Polarized RF gun: R&D effort to explore feasibility

(Axel Brachmann, SLAC)

Polarized Electron Injector Layout

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SLIDE 18

Polarized Positrons for the ILC (1) Helical Undulator (SLAC) (2) Laser Compton (KEK) e- beam E >150 GeV Undulator L > 150 m

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SLIDE 19

(P. Shuler, DESY)

The E166 Experiment at SLAC P=80%

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SLIDE 20

polarized e+

Pb conveter γ-ray E = 40 MeV

(T. Omori, KEK)

Accelerator Test Facility for ILC at KEK

P= 73 ± 15(sta) ± 19(sys) %

  • M. Fukuda et al., PRL 91(2003)164801
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SLIDE 21

(T. Omori, KEK)

Compton Cavity Collaboration – Dedicated e- Ring

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SLIDE 22

Session 9B: Topics

Photocathode Development

strained super-lattice photocathode gridded photocathode, pyramidal shape photocathode

Low Emittance Beam Production Polarized electron source for SPLEEM Pol.e±

  • S
  • urce for ILC

Polarized electron beam injector Polarized positron beam production

Pol.e- source operation

High average current operation High current density test

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SLIDE 23

Goal: Deliver high average current (> 1mA) and high polarization (> 80%) with long photocathode operational lifetime in support of new accelerator initiatives. Enhance our understanding of photocathode decay mechanism. Will undoubtedly allow us to improve existing polarized guns

  • perating at lower average current and unpolarized guns at

milliAmp beam currents (e.g., Lightsources).

Further Measurements of Photocathode Operational Lifetime at Beam Intensity > 1mA with the NEW 100 kV DC GaAs Photogun

  • J. Grames, M. Poelker, P. Adderley, J. Brittian, J. Clark,
  • J. Hansknecht, E. Pozdeyev, M. Stutzman, K. Surles-Law
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SLIDE 24

Photocathode Lifetime limited by ion back-bombardment.

  • 35 weeks/year
  • 100 μA at 85% polarization is fairly routine
  • One photocathode operates for year(s), and three or four activations
  • 2-3 Users simultaneously; one is always Parity Violation experiment

CEBAF => busy, productive NP program

September 2006 Activation (#5) Yesterday, the “tired” photocathode

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SLIDE 25

Ion Back-Bombardment

residual gas cathode electron beam OUT anode Which ions more problematic? laser light IN QE trough to electrostatic center Ions accelerated & focused to electrostatic center We don’t run beam from electrostatic center

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SLIDE 26

Charge Lifetime Steadily Decreasing NEG replacement Summer 2003 improves lifetime

Data compiled by M. Baylac

CEBAF Gun Charge Lifetime (2001-2004)

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SLIDE 27

Present JLab Polarized Electron Gun Cathode (GaAs)

NF3 Laser

Ceramic Insulator

Cs

Non evaporable getter pumps (NEG) 4,000 liter/s pump speed ⇒ 5E-12 Torr Anode

e-

NEG coated beampipe

  • 100 kV
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SLIDE 28

The Wafer…

Wafer from vendor Stalk Mounted

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SLIDE 29

Paradigm Shift (Peggy Style => Load Lock Gun)

Wafer from vendor Stalk Mounted Puck Mounted

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SLIDE 30

3 Chambers

  • Load/Hydrogen/Heat
  • Prepare NEA surface
  • High Voltage, Good Vacuum

Photocathode Lifetime Test Bed

  • Low-P bulk GaAs
  • High QE (15-20%) => mA’s
  • 200 C/day vs. 20 C/day

BTLLPEG Operation (2003-2006)

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SLIDE 31

Improvements limiting the active area

No more hydrogen cleaning Study one sample without removal

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SLIDE 32

Improvements restoring ion damage site

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SLIDE 33

Ion Pump Locations Improvements to monitor gun & beamline pressure

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SLIDE 34

Ion Pump Locations Improvements to monitor gun & beamline pressure

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SLIDE 35

Photocathode Lifetime Studies & Operation (2003-2006) We’ve learned about photocathode lifetime…

  • vs. gun & beamline pressure (leaks, pumping, gauging)
  • vs. laser (spot size, position, reflections, power levels)
  • vs. GaAs preparation (active area, cleaning)
  • vs. beam handling (optics, orbits, beam losses)

We’ve learned about functionality of a Load Lock gun…

  • Round pucks + gravity = rolling
  • Manipulator alignment + bake-outs
  • Activation, heating, cooling
  • Sensitivity of manipulators to bake temperature
  • Multiple photocathodes > 1 photocathode

Work mainly presented at workshops & recorded in proceedings…

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SLIDE 36

NEW Load Lock PhotoGun for CEBAF What’s next (really, now!)…

  • Improve gun vacuum
  • Block ionized gas from the photocathode
  • Load multiple photocathodes with the “suitcase”
  • Design-out the handful of little & big “features”
  • Transfer technology to the CEBAF program
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SLIDE 37

Top View

High Voltage Chamber

Suitcase & Load Lock Chamber

  • Mount wafer on puck in lab
  • Holds 4 pucks (e.g., bulk, SL, SSL)
  • Load Lock: 8 hour bake @ 250 C
  • No H-Cleaning

Beam

Activation Chamber

  • Manipulators 150 C bake
  • New & Used puck storage
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SLIDE 38

The “suitcase”

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SLIDE 39

High Voltage Chamber

  • “Side ceramic” design
  • load chamber at ground potential
  • No moving parts at HV

Side View Activation Chamber

  • Mini-stalk heater
  • Mask selects active area
  • UHV IP supplies gauge activation
  • Keyed & eared pucks
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SLIDE 40

High Voltage Chamber

  • “Side ceramic” design
  • load chamber at ground potential
  • No moving parts at HV

Side View Activation Chamber

  • Mini-stalk heater
  • Mask selects active area
  • UHV IP supplies gauge activation
  • Keyed & eared pucks
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SLIDE 41

Outgassing Rates vs. Bakes

0 E+00 1 E-12 2 E-12 3 E-12 4 E-12 5 E-12 6 E-12 2 4 6 8 10 12 14 Bake number Outgassing Rate

304SS without (blue) and with (red) electroplishing and vacuum firing

NEG coating (Ti/Zr/V) 100 hrs @ 70 C 200 L/sec

Improvements to the High Voltage Chamber

304 SS: Electropolished & Vacuum Fired (AVS: 3 hrs @ 900 C @ 3x10-6 T)

  • Careful electrode alignment
  • Lipped to flatten field profile
  • Bias anode or support
  • Rear windows view “tee”
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SLIDE 42

New Load Lock Gun Assembled & Running Spring ‘06

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SLIDE 43

(Best Solution – Improve Vacuum, but this is not easy)

Benchmarking Photogun with Operational Lifetime

Bigger laser spot, same # electrons, same # ions electron beam OUT residual gas cathode Ionized residual gas strikes photocathode anode laser light IN Ion damage distributed

  • ver larger area
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SLIDE 44

Load lock port (GaAs on puck) NEG pipe Laser (1 W @ 532 nm) & attenuators Faraday Cup (450 C bake) High Voltage (-100 kV) Activation (Cs/NF3, Mask=5 mm)

Experimental Setup

350 μm 1500 μm Spot Size Adjustment 7 Precision Ion Pump Supplies Solenoid Centering

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SLIDE 45
  • Set beam current (1-10 milliAmps) at Faraday Cup
  • Run laser power (<1 Watt) PID to fix beam current
  • Record ion pump current at 7 beam line locations
  • Record laser power/setpoint via “pickoff” detector

Example Run (5 mA)

1/e Charge Lifetime = Charge Extracted ln (QE /QE ) i f

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SLIDE 46

OLD NEW

NEW vs. OLD Load Lock Design (small laser spot)

Damage ~ (a·I + b·I2)

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SLIDE 47

Gun Ion Production ~ Beam Intensity x Gun Pressure ~ (a·I + b·I2)

HV Chamber Pressure vs. Beam Intensity

Leakage Current

Pgun = P0 + 4 pA/mA

Sept July

New UHV

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SLIDE 48

SMALL vs. LARGE Laser Spot (BP vs. LL)

Tough to measure >1000 C lifetimes with 100-200 C runs! 5

15

1500 350 2 ≈ 18 Expectation:

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SLIDE 49

Is Ionized Gas from the Beamline Limiting Charge Lifetime?

Ionization cross section for H2 Ωtot= σ(E)·dE ~ 4·10-18 cm2

∫

Lgap ~ 5 cm Pgun ~ 5·10-12 Torr Ygun ~ 1.5·107 ions/C Ω(100keV) = 4·10-19 cm2 Lbeamline ~ 100·Lgap Pbeamline ~ 20·Pgun Ybeamline ~ 200·Ygun

(Plot taken from talk by F. Dylla)

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SLIDE 50

Repelling Beamline Ions with Biased Anode

Design Shield Design = Trap!!

Beamline Ions

Anode Contributed by E. Pozdeyev Bias Bias

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SLIDE 51

Biased Anode: Null Result ?

5.1 mA (40 C) 2.9 mA (120 C) 3.2 mA (140 C) +1 +2 +2 +2 0 +2.5

  • 2.5

BIAS Conclusion: Not observing improvement.

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SLIDE 52

0,2 0,4 0,6 0,8 1 1,2 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 Time, hr Beam current, mA

U= + 65V U= -65V

VERDI V5

NEG 500l/s

NEG 200l/s

Beam Dump

NEG 200l/s

IGP

150 cm 90 cm

JBL

NEG 200l/s NEG 200l/s

80 cm

aperture d=10 mm

100kV DC gun Insulator

U

(K. Aulenbacher, University of Mainz)

Biased Ion Repeller

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SLIDE 53

Preparation Chamber: Hydrogen Degradation of QE

30 min ~2 L

10-9 10-10 10-11

Preparation chamber dominated by hydrogen Pressure (Torr)

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SLIDE 54

Tantalyzing Discovery: Hydrogen Barrier Enhances Lifetime

1 3 2

Wafer QE improves as the hydrogen barrier is removed. All three spots ~25 C before QE starts to fall. Once the barrier is removed QE falls as usual.

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SLIDE 55

QE Largest at Beam Spot Location

2 weeks 1000x reduction (12% to 0.012%) ~5 Coulombs extracted 10x improvement at spot 4x improvement on surface

~103 L

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SLIDE 56

Preliminary tests using 532 nm

Lifetime test of strained superlattice @ 1 mA

Lifetime ~200 C at 1 mA (532 nm)

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SLIDE 57

We are ready to challenge our 120 kV, 16 mA PS 13 mA!

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SLIDE 58

Spin’06

  • Exciting PES & PPS work
  • n-going, informative

meeting & fun…

  • Call to “younger” PES folks

to think about the future

  • Useful discussions about

ILC PES & JLab involvement Summary & Outlook

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SLIDE 59

NEW gun charge lifetime 2-3x better => likely vacuum, electrode improvements. Larger laser spot improves charge lifetime, consistent with previous experiments. Exceptionally good Charge Lifetime >1000 C at high currents >1mA In fact, difficult to measure when using large laser spot. Anode biasing to +/- 2.5kV yields no measurable improvement; ions created downstream of anode not a problem, at least not in test stand with good vacuum. First demonstration of surface barrier that enhances operating lifetime, albeit at expense of initial QE. Look for other coating material that preserves QE, but does not reduce QE. => Photocathode lifetime measurements at >1mA using GaAs/GaAsP superlattice. => Install load lock in tunnel in July 2007.

Summary & Outlook