investigation of pulsed spin polarized electron beams at the s-dalinac

SFB 634

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Investigation of pulsedspin polarized electron beams

at the S-DALINAC

PSTP 2013

PSTP 2013 - Martin Espig – 13.09.2013

M. ESPIG, J. ENDERS, Y. FRITZSCHE, M. WAGNERInstitut für Kernphysik, Technische Universität Darmstadt

SFB 634

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Source of Polarized Electronsat the S-DALINAC

Bulk/Superlattice-GaAs 100 keV electron energy Vacuum lifetime ~ 1000 h Charge lifetime 10 (1) C Polarization 86 (4)% Emittance ~ 0.15 mm mrad


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Pulsed Electron Beam in thenormal conduction Injector

90% beam loss

spin- polarized source

Thermionic gun

To the S-DALINAC

Differential pumping stage


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Cathode Lifetime

4

CW-Laser

Pulsed Laser

6 daysFactor 25

Beam Current (mA)

Bea

m T

ime

(day

s)


Max. LaserPower 1 W

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Pulsed Laser Systems

Mode locked 3 GHz modulated Ti:Sa-Laser MIRA HP-D External Cavity Diode Laser

→ repetition rate 75 MHz → repetition rate 3 GHz → (300-500) fs pulse length → 50 ps pulse length

High Polarization at 780 nmHigh Quantum Efficiency at 400 nm


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Reached Electron Bunch Length withthe Ti:Sa-Laser at 780 nm

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Pulse Compressor: Distance Prism-Grating (cm)

Lase

r Pul

se L

engt

h (p

s)

4 ps


optical fiber

pulse compressor

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Reached Electron Bunch Length withpulsed Laser Diodes at 780 nm

7PSTP 2013 - Martin Espig – 13.09.2013

Ele

ctro

n B

unch

Len

gth

(ps)

AC

/DC

Laser Power (mW)

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Investigation of the Electron Bunches

• Electron bunch profile in the time domain• Intensity vs polarization• Bulk GaAs vs Superlattice GaAs• Laser pulse length vs electron bunch length• Space charge effects


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Measurement of Electron Bunches

spin- polarized source

Thermionic gun

To the S-DALINAC

Differential pumping stage


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Measurement of Electron Bunches


160 ps

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ComparisonElectron Beam Length at 100 keV

11

Time (ps)

Ele

ctro

n B

eam

Cur

rent

(a.u

.)


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ComparisonPolarization

12

Bulk-GaAs Superlattice GaAs

Time (ps) Time (ps)Ele

ctro

n B

eam

Cur

rent

(nA

)

Ele

ctro

n B

eam

Cur

rent

(nA

)

Mot

t Sca

tterin

g A

sym

met

ry

Mot

t Sca

tterin

g A

sym

met

ry


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Space Charge Effects

13

longitudinal transversal


Bunch Charge (fC) Bunch Charge (fC)

Bun

ch L

engt

h (p

s)

Bea

m W

idth

(mm

)

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Intensity model by Spicer

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Why?B

eam

Cur

rent

(nA

)

Time (ps)

Diffusion• Production• Electron transport• Emission


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Particular boundary conditions

15

• Activation problems because of too much oxygen in preparation chamber

• poor activation of cathodes• Poor quantum efficiency

• Low emission probability • Additional terms may become important

• Ansatz for new model • Include optical recombination


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Advanced Model

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AB C

direct SRH Auger


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Advanced Model

17

Bea

m C

urre

nt (n

A)

Time (ps)


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Model for Polarization

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BULK GaAs

Superlattice GaAs

SRH-Recombination Spin-Flip

opt. Recombination Diffusion

Laser

Preliminary


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Polarization of the Bunch at 100 keV

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Bulk-GaAs Photocathode

Time (ps)

t = 4,2 ps1/A = 100 ps1/A2 = 61,1 psB = 1E-10 cm³/sD = 200 cm²/s

Time (ps)

Bea

m C

urre

nt (n

A)

Asy

mm

etry


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Polarized Electron Beam at 6.2 MeV

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Ag(e,e)Bulk GaAs

topbottom

→ PolarizationP ≈ 28%

Channel

Cou

nts/

Cha

nnel


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Outlook

• Repeat measurement of intensity and polarization • with improved quantum efficiency• compare with proposed model

• Resolution improvement


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Thank you for your attention

investigation of pulsed spin polarized electron beams at the s-dalinac

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