Jim Clark, Matt Poelker, Steve Covert, Phil Adderley, Riad Suleiman, John Hansknecht, Marcy Stutzman, Joe Grames

Transcription

1 CEBAF Polarized Electron Source Jim Clark, Matt Poelker, Steve Covert, Phil Adderley, Riad Suleiman, John Hansknecht, Marcy Stutzman, Joe Grames JLAB Summer Lecture Series, July 17 th 2012

2 How do we see what we measure? M ~ 938 MeV/c 2 M ~ MeV/c 2 mass ~ 1/ ~ energy

3 What to do? Build a 5 mile long electron microscope! Electron Neutron Proton Make electrons energetic enough (E beam ) to peek inside proton or neutron (M nucleon ).

4 How to make the electrons powerful? Use radio(frequency) waves!!! Accelerating Debunching Bunching Decelerating Electrons gain energy on each crest!

5 The C in CEBAF

6 Jefferson Lab Accelerator Site East Arcs 2 SRF Linacs (600 MeV) Polarized Source Lab CEBAF Injector Injector Test Cave West Arcs 3 Experimental Halls

7 Continuous Electron Beam Accelerator Facility A B C A B C 111 ps 2 Linacs 1497 MHz RF Synchronous Lasers 499 MHz, ΔФ = 120 Pockels Cell A B C C B A A B C RF Separators 499 MHz Gun

8 Photo Finish, but at 2 billionths of a second!!! 3 lasers pulsing B DC beam, not so useful A C A B C A B C 60 degrees 1497 MHz

9 What about the probing with spin? Target You measure an experimental asymmetry

10 Electron Bunch Spin & Polarization 0% Polarization v 50% Polarization v Thomas Jefferson National Accelerator Facility J. Grames JLab Summer Detector Series, July 7, 2008

11 Parity Violation Experiments at CEBAF Experiment Energy (GeV) I (µa) Target A pv (ppb) Maximum Charge Asym (ppb) Maximum Position Diff (nm) Maximum Angle Diff (nrad) Maximum Size Diff (δσ/σ) HAPPEx-II (Achieved) H (20 cm) Was not specified HAPPEx-III (Achieved) H (25 cm) ±100 3±3 0.5± PREx Pb (0.5 mm) ±10 2±1 0.3± QWeak H (35 cm) ±10 2±1 30± Møller H (150 cm) ±10 0.5± ± PV experiments motivate polarized e-source R&D

12 What does 234 ppb even mean? A A - B 1,000,000,000 1,000,000,000 Target B A = 500,000,117 B = 499,999,883

13 Polarized Electron Source Musts Good Photocathode Many electrons/photon High Polarization Fast response time Good Laser Lots of Photons CW Pulses High Polarization Good Electron Gun Accelerate Electrons Happy Photocathode Integrate Laser

14 Photoemission from GaAs Bare GaAs surface Large Work Function Positive Electron Affinity (PEA) NEA Surface Activation Dipole layers of Cesium + NF 3 Reduces Work Function Negative Electron Affinity (NEA) PEA=4.07 ev - E gap Conduction Band - χ NEA ~ 0.2 ev Valence Band + Light GaAs Vacuum GaAs Vacuum Quantum Efficiency = N e - / N

15 Bulk GaAs Laser excitation from P 3/2 to S 1/2 : E gap < E < E gap +Δ Electron Polarization: 50% P e S 1/2 E E -1/2 +1/2 J S 1/2 E gap =1.42 ev 3 σ+ σ- 1 3 P 3/2 E F Δ=0.34 ev -3/2-1/2 +1/2 +3/2 P 3/2 P 1/2 k -1/2 +1/2 P 1/2 σ+: Right-handed circularly polarized light σ- : Left-handed circularly polarized light Reverse electron polarization by reversing light polarization m j

16 The First GaAs Photoemission Source P = = 50% Thomas Jefferson National Accelerator Facility J. Grames JLab Summer Detector Series, July 7, 2008

17 First High Voltage GaAs Photogun A to Electrons into the accelerator Dec., 1977 Collaboration announces parity violation June, 1978 Thomas Jefferson National Accelerator Facility J. Grames JLab Summer Detector Series, July 7, 2008

18 Higher P: Breaking GaAs Degeneracy Split degeneracy of P 3/2 : Introduce strain on GaAs crystal by growing it on substrate (GaAsP) with different lattice constant High polarization by laser excitation from P 3/2 to S 1/2 : E gap < E < E gap +δ S 1/2 m j = -1/2 +1/2 σ+ σ- E gap P 3/2 m j = -3/2-1/2 +1/2 +3/2 δ=20 50 mev Higher QE: Alternating layers of GaAs and GaAsP Superlattice GaAs

19 Strained layer GaAs Bandwidth Semiconductor (formerly SPIRE) MOCVD-grown epitaxial spin-polarizer wafer Lattice mismatch split degeneracy of P 3/2 0.1 μm 250 μm 250 μm 600 μm Strained GaAs GaAs 1-x P xx=0.29 GaAs 1-x P x 0<x<0.29 p-type GaAs substrate Thomas Jefferson National Accelerator Facility J. Grames JLab Summer Detector Series, July 7, 2008

20 Strained Layer - Superlattice GaAs -3 Be doping (cm ) GaAs (5 nm) GaAsP (3 nm) GaAs (4 nm) 14 pairs GaAs 0.64 P0.36 (2.5 μm) GaAs 1-x Px, 0<x<0.36 (2.5 μm) p-type GaAs substrate SVT associates, per SLAC specs. Thomas Jefferson National Accelerator Facility J. Grames JLab Summer Detector Series, July 7, 2008

21 And, it really works! Experiment Figure of Merit 2 P I sup. 2 P I str. = 1.38 Thomas Jefferson National Accelerator Facility J. Grames JLab Summer Detector Series, July 7, 2008

22 V-Wien Filter Spin Solenoids PreBuncher H-Wien Filter Polarized Electron Injector Polarized Electron Source (130 kv) Synchronous Photoinjection SRF Acceleration (65 MeV) 5 MeV Mott Polarimeter Synchrotron Light Monitor Gun#2 Apertures Chopper Buncher Capture Cryounit Cryomodules FC#1 Bunchlength Cavity FC#2 Injection Chicane Gun#3 100/500 kev Mott Polarimeter 45 MeV Dump/ Spectrometer Electron Spin Manipulation Bunching & Acceleration 500 kev SRF Acceleration (6 MeV)

23 Laser Room Bird s Eye View Electron Beam Line Two Guns Thomas Jefferson National Accelerator Facility J. Grames JLab Summer Detector Series, July 7, 2008

24 Laser Room for Dust & Climate Control Lasers are cool!!! Thomas Jefferson National Accelerator Facility J. Grames JLab Summer Detector Series, July 7, 2008

25 Attenuator IA Rotatable GaAs Photocathode Gain-switched Diode Laser and Fiber Amplifier LP HWP LP PC WP LP Shutter Hall Delayed Helicity Fiber Helicity Generator T-Settle Fiber Helicity Fiber nhelicity Fiber HV Supply (0 90 V) Vacuum Window RHWP 15 Dipole Charge Feedback (IA) V-Wien Filter Target BCM Parity DAQ Charge Feedback (PITA) Spin Solenoids H-Wien Filter BPMs Position Feedback HV Supply (0 4 kv) Pockels Cell CEBAF 5 MeV Helicity Magnets PZT Mirror IHWP Electron Beam

26 Fiber-based Drive Laser RF Locked Low-Power (1 mw) 1560 nm Fiber Diode 1560 nm 780 nm Frequency-doubler 1560 nm to 780 nm 30% Efficiency (2 W) 499 MHz High Power (6 W) 1560 nm Fiber Amplifier

27 Load-lock Photogun Best vacuum inside HV Chamber, which is never vented except to change electrodes Photocathode Heat and Activation takes place inside Preparation Chamber Use Suitcase to replace photocathodes through a Loading Chamber Activation Laser Loading Chamber Preparation Chamber HV Chamber Storage Manipulators

28 Electron Gun Cut-Away Laser shines on GaAs & frees the electrons -130,000 Volts 0 Volts the -130kV battery accelerates and forms the electron beam.

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30 Who wants polarized electrons? Photocathode slowly dying!!! Thomas Jefferson National Accelerator Facility J. Grames JLab Summer Detector Series, July 7, 2008

31 Better Vacuum = Longer Lifetime ~2.5E-11 Torr ~5.0E-11 Torr >15.0E-11 Torr Better Vacuum Thomas Jefferson National Accelerator Facility J. Grames JLab Summer Detector Series, July 7, 2008

32 Ion Back-Bombardment High energy ions focused to electrostatic center We don t run beam from electrostatic center laser light IN electron beam OUT laser light IN electron beam OUT anode residual gas cathode Ions create QE trough to electrostatic center Thomas Jefferson National Accelerator Facility J. Grames JLab Summer Detector Series, July 7, 2008

33 Bad, bad ions Imperfect vacuum => QE degrades via ion backbombardment Electrostatic center QE[%] Thomas Jefferson National Accelerator Facility J. Grames JLab Summer Detector Series, July 7, 2008

34 Vacuum regimes Air ~ / Torr-cm 3 Low, Medium Vacuum (>10-3 Torr) Viscous flow interactions between particles are significant Mean free path less than 1 mm High, Very High Vacuum (10-3 to 10-9 Torr) Transition region Ultra High Vacuum ( Torr) Molecular flow interactions between particles are negligible interactions primarily with chamber walls Mean free path ,000 km Extreme High (<10-12 Torr) Molecular flow Mean free path 100,000 km or greater Thomas Jefferson National Accelerator Facility J. Grames JLab Summer Detector Series, July 7, 2008

35 Vacuum Conditions at CEBAF Application Pressure Range Location Vacuum Regime Beamline to dumps 10-5 Torr Target to dump line Medium Insulating vacuum for cryogens Targets, Scattering Chambers RF waveguide warm to cold windows 10-4 Torr to 10-7 Torr Cryomodules, transfer lines Medium to high 10-6 to 10-7 Torr Experimental Halls High to very high 10-7 to 10-9 Torr Between warm and cold RF windows Warm beamline vacuum 10-7 to 10-8 Torr or better Arcs, Hall beamline, BSY, some injector Warm region girders 10-9 Torr or better Girders adjacent to cryomodules Differential pumps Below Torr Ends of linacs, injector cryomodules and guns Baked beamline to Torr Y chamber, Wien filter, Pcup High to very high High to very high Very high to ultrahigh Ultrahigh vacuum Ultra high vacuum Polarized guns to Torr Inside Polarized guns Ultra/Extreme high vacuum SRF cavity vacuum < Torr Inside SRF cavities with walls at 2K Thomas Jefferson National Accelerator Facility Extreme high vacuum J. Grames JLab Summer Detector Series, July 7, 2008

36 We understand Alice s worry The woods were dark and foreboding, and Alice sensed that sinister eyes were watching her every step. Worst of all, she knew that Nature abhorred a vacuum Gary Larson Thomas Jefferson National Accelerator Facility J. Grames JLab Summer Detector Series, July 7, 2008

37 Where does the gas come from? Outgassing from the system Metal and non-metal (viton o-rings, ceramics) all outgas Primarily water in unbaked systems Primarily hydrogen from steel in baked systems Leaks Real Gaskets not sealed Cracks in welds, bellows, ceramics, window joints Superleaks that only open at very low temperatures Virtual Small volumes of gas trapped inside system (screw threads, etc.) that pump out slowly over time Gas load caused by the beam Desorption of gases by elevated temperatures, electrons or photons striking surfaces, etc. Engineered Loads (targets, etc.) where gas is added Permeation of gasses through materials Viton gaskets worse than metal seals Hydrogen can permeate through stainless steel! Thomas Jefferson National Accelerator Facility J. Grames JLab Summer Detector Series, July 7, 2008

38 Ultra High Vacuum Pumps Getter Pumps Chemically active surface Titanium sublimed from hot filament Non-Evaporative Getters Molecules stick when they hit Does not work well for inert gasses such as Argon, Helium or for methane Ion Pumps Electric field to ionize gasses Magnetic field to direct gasses into cathodes where they are trapped Has some pumping capability for noble gasses NEG pump array Baking used to get pressures below Torr 250 C for 30 hours removes water vapor bonded to surface that otherwise limits pressure Avoid contamination by oils due to roughing pumps, fingerprints, machining residue. Ion Pump Thomas Jefferson National Accelerator Facility J. Grames JLab Summer Detector Series, July 7, 2008

39 High Polarization from GaAs: Trending toward higher current E(L)IC ? 1 Series1 Series2 Series First polarized beam from GaAs photogun Year First low polarization, then high polarization at CEBAF Thomas Jefferson National Accelerator Facility (Insert your name) breaks world record!!! J. Grames JLab Summer Detector Series, July 7, 2008