Ultra Cold Electron Beams for the Heidelberg TSR and...
Transcript of Ultra Cold Electron Beams for the Heidelberg TSR and...
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