Status Report of the Darmstadt Polarized Electron...

Status Report of the Darmstadt

Polarized Electron Injector*

Yuliya Poltoratska

PST 2009 Ferrara 07.09.2009

*Supported by DFG through SFB 634

SFB 634

07.09.2009 | PST 09 Ferrara | Yuliya Poltoratska | 2

Outline

Introduction

Polarized electron source

• Requirements

• Test stand of the electron gun

• Characterization of the electron beam

Implementation at the S-DALINAC


S-DALINAC

Design values: Max. energy: Energy spread: Max. beam current: Duty cycle:

130 MeV±10-4

60 µA3 GHz cw


S-DALINAC

1

3

1

2

3

Experiment sites:

Injector: Photon scattering

Tagger: Photon scattering

QClam spectrometer

2

4

4 High-resolution spectrometer

Experiment to determinenucleon polarizability

Photon Scattering:

e-

Target

γ

Radiator

Ge-Detector

γ

SFB 634

2

1

3

Thermionic Gun: 250 keV

Injector Linac: 10 MeV

Main Linac: 130 MeV (after 3 passes)

12

3

Nucleon Polarizability

e-

H2

γ

Radiator

γ

p

NaI

(e,e‘x)-Coincidence experiments

ep, n, α

Target

e‘Spectrometer

SemiconductorScintillators


• Low-q (< 1 fm-1) experiments with polarized beams

→ Completely unexplored field

Polarized electron and photon scattering

• Parity-violation effects

→ Polarized bremsstrahlung

• Measurement of additional structure functions

→ Polarized electron scattering

Planned Experiments


Polarized Source: Requierements

Polarization: ≈ 80%

Average current: > 60 µA

Repetition frequency: 3 GHz cw

Long lifetime

Good availability, robustness of operation

Compact form:

→max. height including safety distance 2.3 m


Test Stand

1 m

ElectronGun

PreparationSystem

LaserSystem

AlphaMagnet

Wien Filter

MottPolarimeter

DifferentialPumping

Stage

• Cathodes: Bulk GaAs, Strained Layer GaAs

• Laser wavelength: 830 nm and 780 nm

• Gun height (with corona shielding): 1.8 m

from beam axis


Vacuum System

• Improvement of vacuum pressure in cathode and preparation chambers via uniform bake out procedure: Mass spectrometer values one order better than before

• Exchange of the Cs dispencer and tungsten filament reduced pressure rise during the activation process

• Preheating of the cathode puck in the Load lock chamber


80 mm

Position ofphotocathode

Electrode

Fabrication:

• Material 1.4429-ESU (316 LN)

• Highly polished surface

EM – Simulations*

• Cathode surface: E = 0.85 MV/m

• Edges: E = 4 MV/m

*(CST EM StudioTM).


1 m

Electrongun

Lasersystem

Alphamagnet

Differentialpumping

stage

Wien fliterMott

polarimeter

Beam Characteristics

Fluorescent screenWire scanner

Bulk GaAs (λ=830 nm)εn,x = (0.146 ± 0.037) mm mrad

εn,y = (0.197 ± 0.089) mm mrad

Preparationsystem

AlphamagnetAlpha

magnetAlpha

magnet

Strained GaAs ( λ=830 nm)εn,x = (0.125 ± 0.025) mm mradεn,y = (0.176 ± 0.071) mm mrad


Spin Manipulation and Polarimetry

1 m100 keV

Mott polarimeter*

Wien filter

Chopper

Electrongun

Preparationsystem

Lasersystem

Alphamagnet

Prebuncher

Differentialpumping

stage

Beam

B

E

5 cm

1

2 3

4

* R. Barday T 81.2


Polarimetry: Mott Polarimeter

• Elimination of the instrumental asymmetry by helicity switching• Foil thickness extrapolation – self supported gold foils 40 - 500 nm

Bulk GaAs: P = (35.5 ± 1.4)%Strained superlattice: P = (86 ± 3)% (830 nm)

P = (76 ± 2)% (808 nm)


Achievements at the Teststand

Second emittance measuring point after Wien Filter

Experiments with 90 keV and 100 keV polarized beam:

December 2008 and August 2009 : Measurement of bremsstrahlung polarization correlations

New pulsed Ti:Sapphire laser system (MHz repetition rate)

• – time-of-flight measurements: background reduction• – microsecond isomers


100 keV Polarized Electron Gun

5 m

10 MeV Injector

To Experimental Hall40 MeV Linac

1st Recirculation

2nd Recirculation

Experimental Area

Optical Transfer

From Laser Lab

S-DALINAC Polarized Injector („SPIN“)

250 keV Thermionic

Electron Gun

200 keV Thermionic

Electron Gun


Polarized Injector at S-DALINAC

Preparation system

100 keV Polarized electron gun

Differential pumping stages

Wien filter

1 m

Same transport for 100 keV polarized and

200 keV unpolarized beams

200 keV unpolarized beam

Injector cryostat

Prebuncher system

Mott polarimeter

Chopper

Optical transfer from laser lab2 options: Pulsed laser beam from Ti:Sa laser system

Cw laser beam from diode laser system


Polarimeters

A

B D

ii

AB

C

C

D

Polarimeters*

100 keV Mott Polarimeter

10 MeV Mott Polarimeter

Møller Polarimeter

Compton Transmission Polarimeter

D

Source

i

i

ii

Source of Polarized Electrons

Laser Systems

E

E

E

E

Experiments with polarized electrons/photons


Chopper- Prebuncher System

• nvBunch length at superconducting part of S-DALINAC: ~ 5 ps

→2-stage Buncher1

• nvCutting electron bunches from thermionic gun at S-DALINAC:

• nvVarying bunch lengths depending on laser settings

3 cm

6 GHz Buncher

[1] V.I. Shvedunov, Proc. EPAC 96, p. 1556

• nvApplicable for 100 keV polarized and 200 keV unpolarized beams

• nvRF control system for 3 GHz and 6 GHz cavities2 [2] A. Araz T 79.4


Chopper and Prebuncher System

• nv Bunch compression for acceleration at the superconducting part of the S-DALINAC

• nv Varying bunch lengths depending on laser beam and photo cathode

• nv Possibility to cut electron bunches from the thermionic gun at the S-DALINAC


Bunch Length

2 stage harmonic buncher


Electron energy

V-Code:M. Krassilnikov, A. Novokhatski, T. Weiland,W. Koch, P. Castro, ICAP2000, Darmstadt (2000)

• 100 keV injection energy to the superconducting Linac

• 2-cell capture structure necessary V-Code simulations

Injection energy:

Thermionic gun:

200 keV

Polarized gun:

100 keV


SummaryTest stand set up, put into operation

Performence of 2 experiments

Beam parameters:• Energy: 100 keV

• Duty cycle: 3 GHz cw

• Normalized Transverse Emittance (x): (0.146 ± 0.037) mm mrad

(y): (0.197 ± 0.089) mm mrad

Degree of polarization: Bulk : (35.5 ± 1.4)%

Strained-Superlattice: (86 ± 3)%


Outlook

• Installation at the S-DALINAC – starting autumn 2009

• Installation & comissioning of polarimeters for experiments

• Experiments – begin 2010


Collaboration Institut für Kernphysik, TU Darmstadt

R. Barday, U. Bonnes, M. Brunken, C. Eckardt, R. Eichhorn,

J. Enders, C. Heßler, M. Platz, M. Roth, M. Wagner,

Institut für Theorie Elektromagnetischer Felder, TU Darmstadt

W. Ackermann, S. Franke, W. F. O. Müller, T. Weiland

Institut für Kernphysik, Universität Mainz

K. Aulenbacher


550 600 650 700 750 800 850 900 950

10-5

10-4

10-3

10-2

10-1

100

101

QE

(%

)

λ (nm)

0

10

20

30

40

50

60

70

80

90

P (

%)

Messung St. Petersburg Messung Mainz Messung Mainz (optimiert) Messung St. Petersburg Messung Mainz

Quantum Efficiency and Polarisation


Polarized Bremsstrahlung

theory: H.K. Tseng and R.H. Pratt, Phys. Rev. 7 (1973) 1502exp.: No exp. results for long. pol. electrons


Photofission Experiments

-θ

Eγ

Nγ

Pγ ≥ 75%

e-

238U Target

γ

Bremstarget

θ

Unpolarized test experiment

Sensitivity A ≈ 10-3

Expected A ≈ 10-4

→ Active Target

Role of weak interaction in nuclei

Bremsstrahlungspectrum


(e,e´x) Experiments

( )TTTLTLTTTTLTLTTTLLMott

WvWvWvWvWvWvd

d

d

d′′′′ +++++

Ω=

Ωσσ

Inclusive electron scattering

- two structure functions: longitudinal/transversal

Exclusive electron scattering

- interference terms: L, T, LT, TT

Polarized electron scattering

- parity violation, final state interaction

Polarized electrons and polarized targets

- polarization transfer

e

e’

γ

A

A*

xA


„Fifth Structure Function“

• Only two data sets: D(e,e’p)12C(e,e’p)

• S-DALINACLow momentum transfer

S. Dolfini et al., PRC 60 (1999) 064622

D(e,e‘p)


3He Break-Up Reaction

Estimated statistical uncertainty after two weeks of beam time

Three-Body-Force investigation

No data at low momentum transfer

Calculations byJ. Golak, Crakow

Status Report of the Darmstadt Polarized Electron...

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