Transcript ppt - PAVI 14

Patricia Aguar Bartolomé, Kurt Aulenbacher, Valery Tioukin, Jürgen Diefenbach
Institut für Kernphysik, Universität Mainz
PAVI’14, Syracuse, NY
17th July 2014
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 Physics Motivation
 Polarized Atomic Hydrogen Target
 Status of the Mainz Hydro-Møller Target
 Beam Stabilization Test
 Summary
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Goal: Low energy PV electron scattering experiments at MESA with
systematic accuracy < 0.5% for beam polarization measurements
Hydro-Møller
PV
Detector
MESA (Mainz Energy recovering Superconducting Accelerator)
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Polarimetry Methods
• Compton Scattering: Accurate enough at Ebeam > 4GeV, but accuracy
around 1% at low energies
Not enough for PV-experiments
• Møller Scattering with ferromagnetic target
Advantages
Disadvantages
 Beam energy independent
 Low electron polarization (~ 8 %)
 High analyzing power (~ 80%)
 Target heating
limited to 2-3 mA
 2 particles with final state high
energies
eliminates background
 Levchuk effect ~ 1%
Beam current
 Systematic errors on target
polarization ~ 2%
 Low Pt
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Dead time
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Polarimetry Methods
• Møller Scattering with polarized atomic hydrogen gas, stored in
a ultra-cold magnetic trap
E.Chudakov and V.Luppov IEEE Trans. on Nucl. Sc., 51, 1533 (2004)
Advantages
Disadvantages
 100% electron polarization
 Technical complexity of the target
R&D needed
 High beam currents allowed
Continuous measurement
 Contamination and depolarization effects
of the target gas w/o beam
 Very small error on polarization
 No Levchuk effect
 No dead time
 Expected DPB/PB ≤ 0.5%
for PV-experiments
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Suitable
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Magnetic field B splits H1 ground state
Mixing angle tan2q ≈ 0.05/B(T)
Mixture ~ 53% of
and ~ 47% of
At B = 8T, sinq ≈ 0.3%
, Pe ~ 1-d, d ~ 10-5
Storage Cell
• In a field gradient a force
 Pulls
 Repels
,
,
into the strong field
out of the strong field
H2 recombination (releasing ~ 4.5 eV)
• H+H
higher at low T
cell walls coated with ~50nm
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superfluid He
• Gas density:
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3 1015 cm-3
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T=300mK of the atomic trap can be reached using a Dilution Refrigerator
and the requiered B=8T using a superconducting solenoid
Dilution refrigerator and magnet
shipped from UVA to Mainz
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New Dilution Refrigerator needs
to be designed and produced!!
Test superconducting solenoid
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UVA Superconducting Solenoid Test
Central Field
8T @ 4.2K
Current
Homogeneity
Inductance
Voltage
Clear Bore
76.4 A
1.10-5/10mm DSV
20.3H
0.995V
762 mm
Overall Length
304.8mm
Outer Diameter
167.64mm
•
8 thermo sensors (4 Pt-100, Pt-1000, Si-Diode, 2 Cernox) placed in different
points of the solenoid
•
Several tests with Nitrogen (T~77K) were successfully performed
•
Infeasible Helium (T~4K) test due to the appearance of a big leakrate
•
New cooling set up for the solenoid needs to be designed and produced
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New cooling system set up design
Vacuum
Vessel
•
Most of the new cooling system
components currently under
construction
•
Estimated time to assemble
the new set up ~ August
•
Cooling down of the magnet
with Helium ~ September
Courtesy of J.Bibo and D. Rodriguez
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New cooling system set up design
Copper Shields
(T ~77K)
•
Most of the new cooling system
components currently under
construction
•
Estimated time to assemble
the new set up ~ August
•
Cooling down of the magnet
with Helium ~ September
Courtesy of J.Bibo and D. Rodriguez
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New cooling system set up design
Solenoid
(T~4K)
•
Most of the new cooling system
components currently under
construction
•
Estimated time to assemble
the new set up ~ August
•
Cooling down of the magnet
with Helium ~ September
Courtesy of J.Bibo and D. Rodriguez
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New cooling system set up design
• Most of the new cooling system
components currently under
construction
• Estimated time to assemble
the new set up ~ August
• Cooling down of the magnet
with Helium ~ September
Courtesy of J.Bibo and D. Rodriguez
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Preliminary design of the new Dilution Refrigerator
General considerations
• Low temperature (T=300mK) and high cooling power (Q=75-100mW)
• Optimization by a careful calculation:
-
Heat exchangers
Pressure drop in the pumping lines
Condensation of the mixture
Amount of 3He and 4He gas needed
Volumes of all parts inside the DR (separator, evaporator,
still) and also pumps and lines
- Produce new mixing chamber
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Preliminary design of the new Dilution Refrigerator
Heat Exchangers (HE)

Design of the HE is of major importance. The important parameters
are:
1. Small volume to reach the equilibrium temperature very fast
2. Small thermal resistance between the streams to get good
temperature equilibrium between them

Imperfections and impurities can influence the transport of heat

Thermal boundary resistance between helium and the HE material
at T<1K
Kapitza resistance ~ T3
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Preliminary design of the new Dilution Refrigerator
Module
Ready
Status
Remarks & Problems
Cryostat housing
End 2014
R&D
Construction
Cons. using Super-MLI
Accurate positioning of
solenoid
Stage 1.10 K
End 2014
Development
Construction
HT-HE
Pre-HE
LT-HE
Valves
Stage 0.25 K
End 2015
R&D
(Technologies not yet
under control)
Final-HE
Mixing Chamber
Film Burners
Hydrogen feed system
End 2016
R&D
Literature references
Transition unit not ready
Superconducting
solenoid
End 2014
Test
Detection system
Pumping system
R&D
Summer 2016
Not funded yet
Collaboration?
3He
Still
Evaporator
4He Separator
4He Pre-HE
4He
3He-Filling
End 2016
Target Test
End 2017
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Not funded yet
Volume = 200 l STP
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1.1K stage HE currently under
construction in our Mechanical
Workshop
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Requirements for the PV experiment at MESA
• P2 expected physics asymmetry < 50 ppb
• Beam energy ~ 150 MeV (external beam)
• DPB/PB ≤ 0.5%
• Beam quality:
• Beam parameters are correlated with helicity
Ai
• Noise on beam parameters (helicity un-correlated)
• Beam must be stabilized (DAi
0)
• Helicity correlations must be suppressed (Ai
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DAi
0)
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Beam stabilization and solenoid test set up
Reliable 3T solenoid
for first tests
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Principle of beam stabilization
•
Cavity monitors measure beam position (XYMOs)
•
Steering magnets correct beam direction (WEDLs)
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Beam tests with solenoid
•
•
•
Use an available 3T superconducting solenoid
Gain experience steering <200 MeV beam through a superconducting solenoid
Operate beam position/angle stabilization across the solenoid
•
Most realistic test of polarimetry+beam stabilization for P2 possible before
MESA is in operation
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• PV electron scattering experiments at MESA are planned
systematic accuracy of < 0.5% for the beam polaization measurements
•
Atomic Hydrogen gas, stored in a ultra-cold magnetic trap can provide
this accuracy
•
A solenoid and a dilution refrigerator were shipped from the University
of Virginia to Mainz
•
New cooling down setup of the solenoid and new DR design and
production is in progress
•
Production of a new mixing chamber and a atomic hydrogen dissociator
is also required
•
Beam stabilization test is planned within the next year
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BACKUP
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Planned Beam test setup
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Gas Lifetime in the Cell
Loss of hydrogen atoms from the cell due to:
• Thermal escape through the magnetic field gradient
•
•
Recombination in the gas volume
Recombination in the cell surface
hydrogen
dominates at T > 0.55 K
negligible up to densities of ~1017 cm-3
constant feeding the cell with atomic
E.Chudakov and V.Luppov IEEE Trans. on Nucl. Sc., 51, 1533 (2004)
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Contamination and Depolarization of the Target Gas
No Beam
 Hydrogen molecules ~ 10-5
 High energy atomic states
and
 Excited atomic states < 10-5
 Helium and residual gas < 0.1%
with the beam
< 10-16
empty target measurement
Beam Impact
 Depolarization by beam generated RF field
 Gas heating by beam ionization losses < 10-10
 Depolarized ions and electrons contamination < 10-5
 Contamination by excited atoms < 10-5
Expected depolarization
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Dynamic Equilibrium and Proton Polarization
As a result, the cell contains predominantly
In a dynamic equilibrium, P ~ 80 % in about 10 min.
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Cooling power:
Below 0.3K the dilution refrigerator has much higher cooling power
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