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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