Ultra Cold Electron Beams

for the Heidelberg TSR and CSR

Dmitry A. Orlov#

M. Lestinsky #, F. Sprenger #, D. Schwalm #, A. S. Terekhov* and A. Wolf #

� Cold electron target for TSR

� Electron beam formation

� Cold electrons from cryogenic photocathodes

� Experimental: target, photocathode setup

� First results with photocathode-driven target

� Electron cooler with cryogenic photocathodes

for low-energy electrostatic storage rings

Photocathode source at 90 K : kTC ≈ 10 meV

kT┴ = 0.5 meV ; kT║ = 0.02 meV

#Max-Planck-Institut für Kernphysik, Heidelberg* Institute of Semiconductor Physics, Novosibirsk

vrel≠0

vrel=0

cooler

target

TSR

TSR electron target - concept

Electron cooling force

10-3

2

1

ionν∝

ionν∝

Enhanced experimental resolution:

� separation of cooling and target operation:

- continuously cooled ion beam,

- no change of ion velocity

� adiabatic acceleration: lower kT║

� magnetic adiabatic expansion: lower k T┴kBT┴ = 3.5 meV ; kBT║ = 0.02 meV

� cold source of electrons: photocathode at 90 K

kBT┴ = 0.5 meV ; kBT║ = 0.02 meV

vrel≠0

~0.2 ... 8 MeV/uDetectors

(ions and neutrals)

e-target

vrel=0e-cooler

Electron beam formation

3/1

0

222

|| 42 eCB

B ne

CeU

TkTk

πε+≅kT║ reduction:

Phase-space conservation

v║

E

∆E

∆E

∆v ∆v'

U0

Acceleration Magnetic adiabatic expansion

C = 1.9 - fast accelerationC < 1.9 - slow (adiabatic) acceleration

adiabatic invariant: E┴/B=const.

B0Bguide

αα C

guide

kTkT

B

B== ⊥

0

kT║ = 0.1 meV

Thermocathode kTC = 110-120 meV α = 20 kT⊥ = 5-6 meV α = 90 kT⊥= 2 meV (CRYRING)

Photocathode kTC = 10 meV α = 20 kT⊥ = 0.5 meV

kT║≪ kT⊥

Electron-ion collision resolution

rEv∝

V║

V⊥

Recombination velocity

Flattened electron distributionkT║≪ kT⊥

DR rate coefficient

Real resonance position

kT⊥kT║

Thermocathode kTC= 110-120 meV

Photocathode kTC= 10 meV

||2 2ln16)2ln( kTEkTE r+= ⊥δeVkT µ28|| =

2ln⊥kT

4(ln2E rk

T || )1/2

meVkT 5=⊥

meVkT 45.0=⊥

EF

E vac

ThermocathodevacuumT=1300-1500 K

kT=110-120 meV

Electron beam from photocathode

Photocathode principle

Fully activated cathode: QY= 20-30%

Strong energy and impulse relaxationsenlarge electron energy spreads in vacuum

E vac

E F

E c

(CsO)

E v

GaAs vacuum

T= 80 K

Thermocathode principle

kT=7 meV

300 K 90 K

Energy distributions of photoelectrons“2D”-measurement

(E|| , E⊥ )

Electron beam from photocathode

Photocathode principle

QYeff=1 %

Laser power (800 nm) for 1 mA: < 1 W Efficient heat transfer from the cathode

D. A. Orlov et al., Appl. Phys. Lett. 78 (2001) 2721

Suppres

sion

Suppression

Energy spreads about kT

E vac

E F

E c

(CsO)

E v

GaAs vacuum

T= 80 K

Suppres

sion

Suppres

sion

kT=7 meV

Fully activated cathode: QY= 20-30%

Strong energy and impulse relaxationsenlarge electron energy spreads in vacuum

TSR electron target section - overview

~0.2 ... 8 MeV/uDetectors

(ions and neutrals)

e-target

Interaction section 1.5m

Electron gun withmagnetic expansion

α≈10...90

Adiabaticacceleration

CollectorTSR dipole

Movable ion detectorNeutrals detector

Ion beam

e-

Photocathode setup

Vacuum conditions:UHV (10-12 mbar)H2O + O2 + CO2 <10-14 mbar

High requirements for surface preparation

Photocathodeat 90 K

Atomic hydrogen cleaning:Closed cycle of operation

Photocathode gun

Laser illumination up to 1 W

Temperature rise 15-20 K/W at 90 K

U. Weigel et al., NIM A 536 (2005) 323

Photocathode performance at the TSR target

� Lifetimes 7-15 hours at 0.15-0.2 mA

� Non-stop operation

� Closed cycle of operation

� kBT┴ = 0.5 meV kBT║ = 0.02 meV

Beam profileLifetime data

12 16t [hours]840

0.2

0.1

Photocurrent [mA]

Electron collision energy (milli-eV)

Recom

bina

tion rate coe

fficien

t (10

-9 cm

3 s-

1 )

kT⊥⊥⊥⊥ ~ 2 meV

(CRYRING data)

Model cross section withkT⊥⊥⊥⊥

~ 0.5 meV, kT|| ~ 0.02 meV

TSR e-target with cryo-photocathode

HD+ (1sσ, v = 0, J ) + e → HD** (1sσ nℓλ , v'J' ) → H(n) + D(n' )

H. Buhr et al., work in progressD. A. Orlov et al., J. Phys.: Conf. Ser. 4 (2005) 290.

kBT║

kBT┴

Photocathode performance

at the target

Photocathode performance at the target

Sc18+ (2s1/2) + e- → Sc17+ (2p3/210d3/2)3

Fit of resonance strengths and energieskT⊥⊥⊥⊥ = 0.88 meV and kT║= 27.5 µeV

61 states of type (2p3/2 10l, j)

Hyperfine structuresplitting: 5.59 meV

Radiative correctionsof Sc18+ 2s-2p splitting:

44.3107 eV

bio-molecule source

250 keV protons from

high current injector

ECR molecular ion source

negative ion source

YAG laser

reaction microscope

Electroncooler/target

detection corner

U

detection corner

CSR

circumference 35m

6° deflectors

39° deflectorsquadrupoles

Ultra-low energy electron cooler for the Heidelberg CSR

ii

torME

eZBRy 2∝∆

Low energy ions

Strong ion deflection in toroid

Toroid optimization

1.6

5.1

54

165

5.5

Electron energy[eV]

>300

>300

100

60

15

Bmax[Gauss]

300100

30032

3003

3001

101

Ion energy[keV]

Ion mass[amu]

Low magnetic field

Low energy electrons

Adiabaticity ☺!

Toroid – simple solution ☺!

Rtor ↓, B ↑

505050

20

H.Fadil [poster]

Ultra-low energy electron cooling

Cold cathode

Photocathode at 90 K kTC = 10 meV

Strong TLR

Very low B (10-50 G)

α/|| CkTkTkT ≅≅ ⊥

e

Ccool

n

T 2/3

∝τ

n

cooln

TT2/1

|||| ⊥∝τe

cooln

T2/3

⊥⊥ ∝τ 3/1

0

222

|| 42 eCB

fB ne

CeU

TkTk

πε+≅

e

cooln

T2/3

||∝τ

High B

TLR suppressed

e

Ccool

n

T 3

∝τ

meVTk CB 10≅2/3/2 UAP µ=

Ultra-low energy electron cooling

5050.50.160.005

500500.5

0.05

0.20.0041.63001000.60.025.1300326.40.85430030.70.025.5101204.21653001

Electron density

[10 6 cm -3]

Electron current[mA]

Electron energy[eV]

Ionenergy[keV]

Ionmass[amu]

Coolingtime

(hot beam)[s]

Cooling time (cold beam)

[s]

1.6

kBTe = 0.5 meV

LC = 3.3

ηηηηC = 0.028

Sel = 0.1 Sion ( hot beam )

2/3

220

⋅

⋅⋅=

cM

Tk

LZn

MC

e

eB

Ce

ioncoolτ

� Cold photocathode source beneficialfor low-energy electron cooling (CSR, low-energy ion beams)

Summary and outlook

� Cold photocathode source works close to design parameters

� Electron target + photocathode: Strong gain in energy resolution and accuracy at meV collision energy

� Work to improve currents, lifetimes and energy spreads in progress

Atomic hydrogen treatment

Hot capillary source

It is basically consists of a W-capillary heatedby electron bombardment to 1800-2000 K

EfficientNarrow angular distribution of H-atoms

Low capillary temperature (no w-contamination)

H2

samplefilament

oven

palladium tube

W-capillary

Closed cycle

of the photocathode operation

Superconducting solenoid

Preparation chamber

Loading chamber Hydrogen chamber

Gun chamber

Manipulator

Photocathode section - overview

Photocathode setup at the TSR