October 3, 2006 E. Steffens – Spin 20061
The HERMES Polarized H&D Gas Target: 10 Years
of Operation
Erhard Steffens
University of Erlangen-Nürnbergand
HERMES Collaboration (DESY-Hamburg)
• Introduction and history
• Polarized gas targets in a high energy storage ring
• Overview of HERMES H&D target
• Summary of runs 1996 to 2005 (H║, D║ H┴)
• Conclusions
October 3, 2006 E. Steffens – Spin 20062
The Clue to High Density: The Clue to High Density: Storage CellsStorage Cells
Polarized atoms from source
Target areal density given by
t = L o
with o = It / Ctot and Ctot = S Ci
Note: Conductance of tube proportional to d3/L
• Ballistic flow from Atomic Beam Source (H, D)• Flow driven by pressure gradient Laser Driven Sources (H, D, 3He)
• Ballistic flow from Atomic Beam Source (H, D)• Flow driven by pressure gradient Laser Driven Sources (H, D, 3He)
Storage Cell proposed by W. Haeberli•Proc. Karlsruhe 1965, p. 64•Proc. Workshop IUCF 1984, AIP Conf. Proc.#128, p.251
Density gain compared to Jet of
same intensity can be up to several hundred!
T-shaped storage cell
October 3, 2006 E. Steffens – Spin 20063
1st Test of a Storage Cell in a 2 GeV Electron Storage Ring: VEPP-3 at
BINP
• Storage cell surface is coated to inhibit recombination and depolarization
• Coatings like Teflon or Drifilm are sensitive to Synchrotron Radiation (= hard X-rays)
• First test of storage cell (uncooled, Drifilm coating) performed by BINP-ANL collaboration in 1988: R.A. Gilman et al, PRL 65 (1990) 1733
• Results show that the coating is stable on the time scale of ½ year!
Aymmetry for electro-disintegration of the deuteron as function of running time for a polarized deuterium storage cell target in the VEPP-3 storage ring at BINP
Found later: In the cooled HERMES storage cell (T = 100 K) a water surface is formed with superior quality
October 3, 2006 E. Steffens – Spin 20064
Expectations Paris 1990
Talk on:
‚Experiments with Dense Polarized Internal Targets‘
I
October 3, 2006 E. Steffens – Spin 20065
Expectations Paris 1990
II
October 3, 2006 E. Steffens – Spin 20066
Past History: FILTEX
• Goal: spin filtering of antiprotons (proposal 1985)
• Tool: dense polarized H target in p_bar storage ring LEAR (CERN)
• Test experiment with protons in the Heidelberg test storage ring TSR 1992
October 3, 2006 E. Steffens – Spin 20067
Past History: FILTEX
October 3, 2006 E. Steffens – Spin 20068
Past History: FILTEX
View into the FILTEX target chamber
- rear: last 6-pole (SmCo)
- center: T-shaped storage cell
- right: cryopump with hole for beam
1014/cm2
demon-
strated!
High
polari-
zation
in weak
field!
W. Korsch, F. Rathmann, K. Zapfe, P. Schiemenz †
October 3, 2006 E. Steffens – Spin 20069
The HERMES Experiment
• HERMES experiment proposed for the HERA 30 GeV polarized electron ring
- Letter of Intent 1988
- Proposal 1990
- Technical Design Report 1993
- First Operation in April 1995 (with pol. 3He target)
Physics: see talk by E. Aschenauer (Wedn.)
• Requirements on the target:- Areal density t = 1014/cm2 in two substates OK- Sampling polarimeter with P/P = 3% ?- Coating stable wrt synchrotron radiation (OK?)- Suppression of Wake Fields in the target Simulations OK- Suppression of beam-induced (rf) depolarization „ „ „
Several open questions left...
October 3, 2006 E. Steffens – Spin 200610
HERMES Target- Overview
Target chamber
October 3, 2006 E. Steffens – Spin 200611
HERMES Target 1996H-Target installed in early 1996
Erlangen
Liverpool
Madison
Marburg
Munich
Yerevan
F. Stock
B. Braun
G. Graw
G. Court
T. Wise et al
October 3, 2006 E. Steffens – Spin 200612
Atomic Beam Source (ABS)
Atomic Beam Source (ABS)
• Collimated cold atomic beams produced by disso-ciator with cold nozzle (100K) and differential pumping system (Si ~104 l/s)
• Spin-dependent focussing of H and D atomic beams by 6-pole magnets: mJ= +1/2
• Nuclear polarization Pz, Pzz
produced by means of rf transitions with ~100% effi-cieny; rapid switching enabled
Bc = 50.7 mT
Heidelberg-Marburg-Munich-Madison
October 3, 2006 E. Steffens – Spin 200613
Design of Target Chamber
H&D
e±
Collimator C2
SC Coils
Wake Field Suppr.
G. Court – Liverpool
T. Wise – Madison
Target Cell 400mm
October 3, 2006 E. Steffens – Spin 200614
Storage Cell Design Storage Cell Design
• Cell optimized for operation in an electron storage ring
• Conducting surface with smooth variation of cross section excitation of wake fields!
• System of W collimators for protection against beam and SR
• Cooled via cooling rails by cold He gas to 60-100K
• 75m Al walls with Drifilm coating - Radiation damage visible!
• But: Very effective wall coating due to ice layer maintained by small fraction of water in the atomic beam !
Liverpool-Madison-Ferrara
October 3, 2006 E. Steffens – Spin 200615
Diagnostics
• Target gas analyzer (TGA)
- measures degree of dissociation to 1% in few minutes
- molecules (with polarization fraction ) dilute nuclear polarization
- enters the final values of target polarization
Sketch of the TGA:
B. Braun, Ch. Baumgarten, P. Lenisa, et al
October 3, 2006 E. Steffens – Spin 200616
Diagnostics
• Target gas analyzer (TGA)
- measures degree of dissociation to 1% in few minutes
- molecules (with polarization fraction ) dilute nuclear polarization
- enters the final values of target polarization
Temperature scan on new cell (hydrogen 1997)
October 3, 2006 E. Steffens – Spin 200617
Diagnostics
• Target gas analyzer (TGA)
- measures degree of dissociation to 1% in few minutes
Temperature scan on used cell (deuterium 2000) after formation of water layer
no T-dependence visible
no recombination in the cell!
October 3, 2006 E. Steffens – Spin 200619
Diagnostics
• Sampling polarimeter (BRP)
- measures substate population ni of sample beam to n/n = 1% in few minutes
- BRP polarizations Pe (electrons) and Pz , Pzz (nuclei) calculated
- Sampling corrections applied to calculate PTarget seen by the beam!
Sample Beam
Detector
Schematic view of Sampling Polarimeter
Erlangen-Heidelberg-Munich-Ferrara-Yerevan
D. Braun,
Ch. Baumgarten,
G. Graw,
D. Reggiani,
P. Lenisa et al
October 3, 2006 E. Steffens – Spin 200620
Diagnostics
• Sampling polarimeter (BRP)
- measures substate population ni of sample beam to n/n = 1% in few minutes
Sample Beam
Detector
Hydrogen hfs-population as function of holding field
B. Braun
Thesis, Munich 1995
October 3, 2006 E. Steffens – Spin 200621
Diagnostics
• Sampling polarimeter (BRP)- measures substate population ni of sample beam to n/n = 1% in few minutes
- BRP polarizations Pe (electrons) and Pz , Pzz (nuclei) calculated
BRP polarization for the deuterium run 2000top: vector pol. Pz
bottom: tensor pol. Pzz
October 3, 2006 E. Steffens – Spin 200622
Beam-induced Depolarization
HERA-e:
• Bunch frequency Bunch = 10.4 MHz
• Bunch length t = 38 ps
Harmonics n = n·B up to very high harmonics! (nmax ≈ 103 corr. to 10 GHz)
Resonance condition:
E(Bn) = h·n
→ resonant guide fields Bn exist!
h·n
Bn
October 3, 2006 E. Steffens – Spin 200623
Beam-induced Depolarization
24
- blue: resonances (mF = ± 1)
-red: resonances (mF = 0) narrow-spaced!
Transverse target only
34
October 3, 2006 E. Steffens – Spin 200624
Beam-induced Depolarization
HERA-e:
• Bunch frequency Bunch = 10.4 MHz
• Bunch length t = 38 ps
Harmonics n = n·B up to very high harmonics! (nmax ≈ 103 corr. to 10 GHz)
Resonance condition:
E(Bn) = h·n
→ resonant guide field Bn exists!
B-scan of -resonances
3-4 1-2
BRP used to detect the resonances induced by harmonics of the bunch field!
Shown: the widely spaced resonances
Ferrara-Erlangen-Peking
October 3, 2006 E. Steffens – Spin 200625
Beam-induced Depolarization
HERA-e:
• Bunch frequency Bunch = 10.4 MHz
• Bunch length t = 38 ps
Harmonics n = n·B up to very high harmonics! (nmax ≈ 103 corr. to 10 GHz)
B-scan of -resonances
BRP used to detect the resonances induced by harmonics of the bunch field!
Shown: the narrow resonances
No beam
Correction coil
For optimum setting of correction current:
Small change of BRP detector current by the bunch field
→ Beam-induced depolarization is low!
Ferrara-Erlangen-Peking
October 3, 2006 E. Steffens – Spin 200626
Target Performance
Target/year H║ (1997) D║ (2000) H┴ (2003)
r 0.055 0.003 0.004
PSE 0.035 ≤0.001 0.055
PWD 0.02 ≤0.01 0.055
PBI absent absent 0.015
PT 0.851±0.033 0.845±0.028 0.795±0.033
t (1014 nucl./cm2)
0.7 2.1 1.1
FOM
(PT2·t)
0.5 1.5 0.7
recombination
spin exchange
wall depol.
beam induced
calculated target polarization →
target areal density →
Figure Of Figure Of MerritMerrit
October 3, 2006 E. Steffens – Spin 200627
Conclusions
• Polarized H&D target successfully operated over 10 years in a HE electron storage ring more or less continuesly
• Several problems solved during commissioning phase(s) thanks to many enthousiastic collaborators – impossible to name them all!
• Nature was kind to us (no show-stopper)
• Technology ready to be used for other projects!
www.fz-juelich.de/ikp/pax