compton polarimeter for eic alexandre camsonne. comments from report the requirements for...
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Compton polarimeter for EIC
Alexandre Camsonne
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Comments from report• The requirements for bunch-to-bunch accuracy of the polarization
measurement are essential,but have not been specified. An evaluation of rates and the development of a scheme, which satisfies the requirements for bunch-to-bunch accuracy of the polarization measurement, are essential. A further study of the backgrounds and efforts to find ways how to reduce it, have a high priority.
• The committee considers a high-quality polarization-measurement program essential for EIC and supports the idea of a “Compton polarimeter test bed”. It recommends that the detailed requirements on polarization knowledge be worked out and the resulting detector specifications evaluated, for both EIC machine designs. A close contact between the other groups working on EIC polarization and the machine experts from both EIC machine designs is strongly encouraged.
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ERHIC• 6.6 GeV to 21.2
GeV • 9.4 MHz Repetion
rate• Up to 21
recirculations• 50 mA with
“gatling gun” design
• 80 % min polarization
• Similar to CEBAF
Vadim PtitsyneRHIC Accelerator DesignEIC2014
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MEIC
• Storage ring – Ring ring• 748.5 MHz = 1.33 ns bunch
structure• 3 A at 3 GeV and 180 mA at 11
GeV• 2 macrobunch with one
polarization 2.3 us• Measure polarization average
of the two macrobunch• Every electron bunch crosses
every ion bunch
Warm large booster(up to 25 GeV/c)
Warm 3-12 GeV electron collider ring
Medium-energy IPs withhorizontal beam crossing
Injector
12 GeV CEBAF
Pre-boosterSRF linac Ion
source
Cold 25-100 GeV/cproton collider ring
Three Figure-8 rings stacked vertically
Electron cooling
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Compton asymmetrye + g e’ + g’
s( ) e + g e’ + g’
s( )
)
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Low Q2 chicane and Compton
Compton detectors
Low Q
2 chica
ne
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MEIC beam parameters
F. Lin --- ---
Beam energy Gev 3 5 6 7 9 11
Beam current A 3 3 2.0 1.1 0.4 0.18
Total SR power MW 0.49 3.82 5.28 5.38 5.35 5.37
Linear SR power density kW/m 1.02 7.87 10.88 11.08 11.01 11.06
Energy loss per turn MeV 0.17 1.27 2.64 4.89 13.37 29.84
Energy spread 10-3 0.34 0.56 0.68 0.79 1.01 1.24
Longitudinal damping time ms 83.6 18.1 10.4 6.6 3.1 1.7
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MEIC Bunch Structure In Collider Ring
8
… …… …
Empty buckets1.33ns 748.5MHz
Polarization (Up) Polarization (Down)
bunch train & polarization pattern in the collider ring
Empty buckets
2.3μs, ~1700 bunches 2.3μs, ~1700 bunches
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MEIC Bunch Pattern for Continuous Injection
9
duty factor 6.9e-4
……
100 ms (~4 damping time) 1s, , Iave=6μA
Macro bunch train
Detector duty factor 0.98
duty factor 4.3e-4……
1.33 ns, 748.5 MHz 173.5 pC
2.3μs, ~1700 bunches
173.5 pC1.33 ns, 748.5 MHz
2.3μs, ~1700 bunches
……
duty factor 0.0167
……60s
Iave=100 nA
1 second
At 100nA average injected current, Pequ/P0> 96% for the whole energy range
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Time structure
• Bunch to bunch : 1.33 ns• Polarization state : 2.3 us• Top off : 1 min
• Measure asymmetry for one laser state and polarization
• Can generate more macro bunches for systematics check, time max for measurement is 2.3 us
+3.7 us
-3.7 us
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A. Camsonne EIC Users meeting
11
ERHIC• 5 GeV to 21.2 GeV • 10.8 MHz
Repetition rate• 50 mA with
“Gatling gun” design up to 24 sources
• Need to measure each sources polarization
• 80 % min polarization
• Similar recirculation to CEBAF6/27/2014
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eRHIC beam parameters
• 50 mA with up to 20 sources• 10.8 MHz repetition rate• ERL LINAC allows helicity structure with helicity
flip from the source• Need individual measurement of each source
– 100 ns max for each measurement– Logic signal to flag which source is recorded
• Rates sufficient to measure all sources in a few minutes
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EIC R&D Meeting July 2014 13
Reasons why polarization / Current CAN vary From Bunch to Bunch
E.C. Aschenauer
Polarisation:Hadrons in a storage ring:source instabilitiesBeam-Beam effectsbunch-to-bunch emittance variation, Characteristic scale can be seen from AGSRHIC polarization profile variation for different bunches after accelerationleptons in a storage ring:Beam-Beam effectssource instabilities
leptons in eRHIC What is the expected fluctuation in polarisation from cathode to cathode in the gatling gun
from Jlab experience 3-5% Is there the possibility for a polarization profile for the lepton bunches
if then in the longitudinal direction can be circumvented with 352 MHz RF
Current:Hadrons & leptons in a storage ring:Variations in transfer efficiency from pre-accelerator to main ring beam-beam interaction is important, it affects the bunch lifetime during the storeleptons in eRHIC What fluctuation in bunch current for the electron do we expect
limited by Surface Charge, need to see what we obtain from prototype gun
Requirement:
measure polarisation with enough statistical precision in sufficiently short time
units to monitor polarisation as function of time and parameters influencing
polarisation
hadron and lepton polarimetry are critical
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EIC R&D Meeting July 2014 14E.C. Aschenauer
Technology: Compton Back scattering measure photon and lepton complementary & redundancy
e-Polarimeter location at IP
overlap of bremsstrahlungs and compton photons only possible if we have number of empty p-bunches = # cathods in gatling gun
luminosity loss before/after IP need to measure at location spin is fully longitudinal or transverse
1/6 turn should rotate spin by integer number of π segmented Calorimeter longitudinal polarization Energy asymmetry transverse polarization component position asymmetry
After IP: does collision reduce polarization problem at ILC for eRHIC very small need to measure at location, where bremsstrahlung contribution is small
Before IP: need to find room for photon calorimeter
Introduce dog-leg for polarimeter minimizes bremsstrahlungs photon impact creates synchroton radiation
Other considerations: # of cathods in gatling gun: golden number is 20
This guarantees that a hadron bunch collides always with electrons produced from one particular cathode, avoiding/reducing significantly harmful beam-beam effect of electron beam parameter variations on the hadrons
eRHIC lepton polarimeter
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Compton rates
• Green laser, 1.3 degrees crossing angle• beam 350 um
Energy Rate ( kHz/W/A)
Max current (A)
Rate kHz/W
eRHICcurrent (A)
Rate(kHZ/W)
3 316 3 948 0.05 15.85 298 3 894 0.05 14.96 290 2 580 0.05 14.57 283 1.1 311.3 0.05 14.19 269 0.4 107.6 0.05 13.4
11 258 0.18 46.44 0.05 12.9
•laser of a few watts : 10 KHz to 1 MHz - sufficent statistics in a few seconds•1000 W cavity : rates from tens of KHz to MHz level ( if background high )
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Compton polarimetry requirements
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Compton polarimeter in low-Q2 chicane
Same polarization as at the IP due to zero net bend
Non-invasive continuous polarization monitoring
Polarization measurement accuracy of ~1% expected
No interference with quasi real photon tagging detectors
c
Laser + Fabry Perot cavity
e- beam
Quasi-real high-energy photon tagger
Quasi-real low-energy photon tagger
Electrontracking detector
Photon calorimeter
Electron detector implementation in low Q2
D = -22.4 cmD’ = -0.04 At 10 GeV assume D = 5 mCompton edge : 2.6 GeV = 5.8 cm +0.0104*L = 10.84 cmZero crossing 1.514 GeV = 3.39 cm + 0.006*L = 6.39 cm
D = 10 mCompton edge = 16.2 cmZero crossing = 9.39 cm
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Chicane • Different type of magnet for• dipole 1 and 4 can be C magnets• Dipole 2 and 3 can be C magnets in other direction or
open dipoles• New design going from 15 m to 30 m
c
Laser + Fabry Perot cavity
e- beam
Quasi-real high-energy photon tagger
Quasi-real low-energy photon tagger
Electrontracking detector
Photon calorimeter
Beam size shrinks with distance to interaction point : reduction of halo in detector
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Simulation• Simulation Dave Gaskell ( small aperture )
15 meter setup
Photon
Electron
• Very sensitive to Halo• Simulation for cavity
design
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Simulation• Simulation Dave Gaskell ( large aperture )
15 meters setup
Photon
Electron
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Hall A Compton chicane
Vertical motion of electron detector to move detector close to the beam( up to 5 mm )Photon
detector on movable table
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Roman pot technique evaluation in Compton test stand
• Need to evaluation contribution of window
• Evaluate gain in cabling complexity with full vacuum solution
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Photon detector Synchrotron radiation study
(Mike Sullivan)
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Beam pipe
X-ray reduction
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Sloped beam pipe
Need <10 W / mm
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Synchrotron radiation in detector with different shieldings 5 GeV
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Synchrotron radiation in detector with different shieldings 11 GeV
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Energy deposit in photon detector for different shieldings
Need to evaluate effect on detector performances
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Detector technologies
• Radiation hard ( 3 mA current ! )– Diamond– Silicon ( regular/ high radiation / cryo cooled )
• Radiation hard and fast– Superconducting detector NbN NbTi ( Early Career )– Quartz detector
• All in vacuum• Roman pot option
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Quartz Integrating Detector
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Tracking-Integrating Hybrid System
Quartz detector lacks segmentation, so no “built-in” knowledge of the analyzing power from fitting spectrum Will investigate using microstrip detector in tandem with chunk of quartz Periodic measurements at lower luminosity will allow use of strip detector to
constrain analyzing power
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Compton photon vs electron• Past experience
– HERA photon only longitdudinal and transverse (1.4 % )– SLAC : SLD electron detection 50 GeV best Compton measurement ever
made 0.5 %
• For Compton electron– Larger displacement the better for improved resolution and signal to
background ratio– Opposite requirement to photon detection where minimum is sought to
reduce• Need to evaluate optimum case : photon only, photon and
electron detection, electron detection only• 2 Ips : one optimized for photon and one for electron
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GEMC• EIC standard setup• Easy to add
additionnal detectors
• Need to start looking at background
• Shielding and collimation optimization
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Single Superconducting Nanowire Photon Detectors (SNSPD)
•Thin superconducting stripe of 5 to 10 nm thickness
•Meander geometry to maximize surface, typical width of strip 10 nm and length about 100 nm
•Signal speed depends on material, substrate and geometry
•Mostly developed for astrophysics with IR sensitivity : Nasa Jet Propulsion Laboratory, Lincoln Laboratory ….
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Single Superconducting Nanowire Photon Detectors (SNSPD)
• Review : Chandra M Natarajan et al 2012 Supercond. Sci. Technol. 25 063001 doi:10.1088/0953-2048/25/6/063001
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Superconducting electronics• Detector are fast, need fast electronics to take advantage
• Detector will be in Helium bath, integrated superconducting electronics can reduce
• Need to redevelop standard electronics tools– Amplifier– Discriminator– Logic– Analog to Digital Converter– Time to Digital Converter
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Superconducting strip detector
• Superconducting NbN Microstrip Detectors (1999 ) RD 39 Collaboration R. Wedenig and T.O. Niinikoski CERN et al
• Could see charged alpha•But could not detect minimum ionizing particle at that time because of electronics sensitivity
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Discriminator• Characterization of
superconducting pulse discriminators based on parallel NbN nanostriplines
M Ejrnaes et al 2011 Supercond. Sci. Technol. 24 035018 doi:10.1088/0953-2048/24/3/035018
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Conclusion
• Photon doable but difficult, need more studies• Electron looks reasonable
– Halo contribution would be useful– Simulation in GEMC for background studies– Option to give up on photon and optimize only for
electron