Transcript ELECTRON COOLING STATUS •Why electron cooling? •Optimum parameters for the LEIR cooler.

ELECTRON COOLING STATUS
•Why electron cooling?
–LHC requirements, implications for LEIR, results of 1997
cooling & stacking experiments.
•Optimum parameters for the LEIR cooler.
–Technical considerations, design specification.
•Cost & manpower.
•Schedule.
–Where we are, where we are going.
•Summary.
http://tranquil.home.cern.ch/tranquil/LEIR/
14th January 2004
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Slide 1
Why electron cooling?
• LHC requirements for Pb ions:
– Luminosity L = 1x1027 cm-2 s-1.
– Number of ions per bunch = 7x107.
– Normalised emittance of 1.5 mm.
• Implications for LEIR:
– 1.2x109 ions accumulated and cooled in 1.6 s at 4.2 MeV/u.
– Acceleration to 72 MeV/u.
– At extraction 0.9x108 ions to the PS with an of emittance < 0.7 mm.
Only a storage ring with fast electron cooling can meet these requirements.
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Slide 2
Cooling experiments with Pb ions.
• Cooling and stacking tests made between 1994 and 1997.
– Short periods in 1994 and 1996.
– Dedicated run in 1997 with a specially prepared machine.
• Investigated:
–
–
–
–
Ion beam lifetime.
Cooling time as a function of various parameters.
Stack equilibrium emittance and emittance growth.
Stacking at Linac III repetition rate of 2.5 Hz.
• Results well documented
– “Experimental Investigation of Electron Cooling and Stacking of Lead Ions in a
Low Energy Accumulation Ring”, Particle Accelerators, Vol. 63 pp. 171-210.
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Slide 3
What did we learn from the tests?
• Ion beam lifetime.
– Strong dependence of the lifetime on the charge state and electron current.
– Measured rate coefficients cannot be explained by radiative recombination.
• DIELECTRONIC RECOMBINATION SEEMS TO BE THE DOMINANT EFFECT (STILL A PUZZLE
FOR ATOMIC PHYSICISTS). THREE-BODY RECOMBINATION? VACUUM EFFECTS?
• USE CHARGE STATE 54+.
• IMPROVE VACUUM IN THE MACHINE.
• Cooling times.
– Near linear increase of the cooling rate as a function of electron current.
– Expected gain due to increased cooler length did not show up.
– Strong influence of the lattice parameters on the cooling process.
•
•
•
•
ELECTRON BEAM SPACE-CHARGE INCREASES THE DRIFT VELOCITY.
ELECTRON BEAM UNSTABLE ABOVE 120 mA.
ALIGNEMENT TOLERANCES CRITICAL.
INTERMEDIATE VALUES OF b ARE BETTER & FINITE VALUE OF D INCREASES THE
COOLING RATE.
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Slide 4
What did we learn from the tests?
• Equilibrium emittance fits the LHC requirement and emittance growth is not an issue.
• Stacking principle demonstrated and is compatible with the filling scheme.
• Factor of 3 missing in the total
accumulated intensity in 1.6 s.
Beam Intensity [E8 ions]
8
2
6
19 6.2
3
5 .1 6.1 6.
.0 6
93 6
6
13
5.
5.
39 5
5
.5
5
Linac III rep rate : 2.5 Hz
Ion beam energy : 4.2 MeV/u
Electron energy : 2.35 keV
Electron current : 105 mA
–
8 5.
.7
–
83
4.
37
4.
4
3
beam lifetime : 6.5s
8
.8
COOLING TIME LIMITED BY THE
PERORMANCE OF THE GUN.
INTENSITY LIMITED BY LOSSES
DUE TO CHARGE EXCHANGE AND
ELECTRON-ION RECOMBINATION.
3.5x108
23
3.
41
2.
2
37
1.
Average accumulated intensity : 6E8 ions
Peak intensity : 7.1E8 ions
0
2
4
6
8
10
12
Time [s]
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Slide 5
Parameters for the LEIR cooler
• Choice of parameters based on the results from the 1994-97 experiments, our
experience of operating electron cooler devices (LEAR/LEIR, AD) for more than 12
years and collaborations with other accelerator laboratories (MSL Stockholm, MPI
Heidelberg).
• Electron energy range from 2 keV to 40 keV.
• High perveance gun (6 mP at 2.3 keV => Ie = 600 mA).
• Variable electron beam density.
• Cold electron beam, Et<100 meV, E//<1 meV.
• Adiabatic expansion.
• Maximum cooling length possible. 3m?
• Homogeneous magnetic guiding field (DBt/B//<10-4).
• Efficient collection of the electron beam (DIe/Ie<10-4).
– Electrostatic deflector plates.
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Slide 6
The LEIR electron cooler
High perveance, variable density gun
2.5m cooling section
Adiabatic expansion solenoid
Electron beam collector
90o toroid to bend the electron beam onto the ion beam
Integrated closed orbit distortion correction
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Slide 7
Vacuum system & power supplies
• The vacuum system must follow the stringent criteria applied for the LEIR machine.
– 316LN stainless steel, hydroformed bellows.
– NEG coated vacuum chamber, NEG cartridges close to the gun and collector where there is
a high gas load.
– The whole system will be bakeable at 350oC.
• High voltage power supplies.
– gun (40kV/10mA), control (-/+ 2kV,5mA), grid (6kV,5mA)
– suppressor (6kV,5mA), collector (5kV,5A), electrostatic bends (4x 6kV,5mA)
– Common spares with AD electron cooler, use existing HT infrastructure.
• Cathode heating power supply(20V,5A).
• Magnetic elements
– 3 power supplies needed for the gun/collector solenoids, toroids and cooling solenoids.
– All standard CERN power supplies (1000A,200V & 500A,100V).
– 26 small (10A,70V) power supplies for steering coils.
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Slide 8
Cost & manpower
Item
• Infrastructure and a lot of material from
the LEAR installation will be reused.
Cost (kSFr)
Design
Responsible
160
BINP
1200
BINP
Vacuum system (incl.
Bake out, pumps etc.)
550
CERN
Power supplies (HT,DC)
530
CERN
Electrical installation
100
Gun, collector
Magnets
• Manpower needs (FTE):
Support frame
– 2003
Vacuum chambers
• 1.6 cat 2, 1 cat 3, 0.5 IS
– 2004
• 1.6 cat 2, 1.3 cat 3, 1.5 IS.
– 2005
• 1.6 cat 2, 1.3 cat 3, 1 IS.
– TOTAL :
4.8 cat 2, 3.6 cat 3 & 3 IS over 3 years
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Cooling installation
50
Controls
50
CERN
Related instrumentation
120
CERN
Industrial support
240
TOTAL
3000
Slide 9
Schedule, where we are
• Technical specifications made in 2001/2002.
– LEIR electron cooler conceptual study, PS/BD/Note 2001-17.
– Specifications for the LEIR electron cooler magnetic components, PS/BD/Note 2002-18.
– General mechanical parameters for the LEIR electron cooler, PS/BD/Note 2002-23.
• Design/feasibility study completed by BINP in April 2003.
– Modifications requested at the September meeting.
– Vacuum specifications made by AT/VAC group, waiting for final drawings of vacuum
components.
• Addendum to the CERN-Russian Federation Agreement (“Skrinsky II”) approved in
June 2003.
• Construction of the solenoids (“pancakes”) started at BINP.
• Vacuum material ordered.
• Power supplies ordered (PO group).
• ECEB (bld 233) refurbished.
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Slide 10
Schedule, where we are going
• 1st half of 2004
– Delivery of material to BINP, production of solenoids, vacuum elements, electron gun and
collector.
– Magnet measurements and adjustments.
• July, August 2004
– Tests in Novosibirsk.
• Vacuum leak tests. Ultra-high vacuum not needed at this stage.
• Generation of electron beam with characteristics needed for Pb 54+ ions (2.3 keV, 600mA variable density {x10
less in the centre}electron beam, electron beam collection inefficiency <10 -4).
• Test at higher energy (40 keV, 3A).
• September 2004
– Delivery to CERN.
• October 2004 – March 2005
– Vacuum elements cleaned and prepared, remounting, magnet measurements, bakeout of
complete system to reach ultimate vacuum, commissioning with beam, ready for cooling.
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Slide 11
Summary
• The design of the new cooler is technically sound.
– The different ideas/techniques that we will use have been demonstrated on existing
coolers.
– However the LEIR cooler will be the first to incorporate them all on one device.
• More variables to deal with. Commissioning a little more complicated.
– Cooling with a variable density electron beam has yet to be demonstrated.
• Keep a close eye on the results from IMP Lanzhou, China (2004).
• Backup solution? Use the gun as a “classical” high perveance gun i.e. no variable density.
• Schedule is tight and leaves little room for important delays.
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Slide 12
Some photos of the IMP cooler
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Slide 13
The electron gun
• Convex thermionic cathode at high
voltage. Cathode radius = 14mm.
• Control electrode shapes the electron
beam density.
– Equivalent perveance of 6 mP on the
border and a factor of 10 less in the
centre.
• Grid electrode determines the intensity.
• The gun is immersed in a strong
longitudinal field (2.35 kG).
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Slide 14
Current density, A/cm^2
Variable density profiles
0.075
0.045
0.03
0.015
Current density, A/cm^2
0
15
10
5
0
Radius, mm
5
10
15
15
10
5
0
Radius, mm
5
10
15
0.02
0.016
0.012
0.008
0.004
0
Current density, A/cm^2
Electron beam profiles with control electrode potentials
Uc = 0V, +100V, +200V, +350V, +400V, +600V,
grid potential Ug=500V and cathode potential Ucath = 1000V.
0.06
0.005
Vc = -100 V
0.004
0.003
0.002
0.001
0
15
10
5
Expirement
Calculation
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0
Radius, mm
5
10
15
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Slide 15
Adiabatic expansion
• Needed for:
– Adapting the electron beam size to the injected beam size for optimum cooling.
B// r  const  r  ro
2
Bo
B
Bo=0.235T, B=0.075T, ro=14mm => r=24.8mm
– Reducing the magnetic field in the toroids, thus reducing the closed orbit distortion.
– Reducing the transverse thermal temperature of the electron beam.
Et
B
 const  E  Eo
B//
Bo
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Bo=0.235T, B=0.075T, Eo=100meV => E=32meV
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Slide 16
The electron beam collector
• Suppressor electrode slows down the
primary electrons at the collector
entrance.
• Magnetic field is reduced to spread out
the electrons on the collector surface.
• Surface on which the electrons are
collected is water cooled.
B=750 G
B=560 G
B=375 G
B=300 G
B=225 G
0,01
Jloss/Je
1E-3
1E-4
1E-5
1E-6
1E-7
0,0
0,5
1,0
1,5
2,0
Electron current (A)
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Slide 17
The electrostatic bend
10
500
8
Jlosses (mA)
400
Jlosses (mA)
• Electrons experience a centrifugal force
in the toroid.
• This drift can be compensated by an
additional magnetic field in the opposite
direction.
• Reflected and secondary electrons
however are excited by this field and
can oscillate between the gun and
collector before being lost.
• Complete compensation is obtained by
superimposing an electric field on the
magnetic field
300
200
0,5
1,0
1,5
Electron beam current (A)
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4
2
100
0,0
6
0
0,0
0,5
1,0
1,5
Electron beam current (A)
Slide 18
2,0
Closed orbit perturbation correction
• The vertical magnetic field component
of the toroids induce a horizontal
perturbation on the closed orbit.
Pb + 54 4 MeV/u
40°
• Correction dipole placed in the toroid.
Pb
+ 54
72MeV/u
Verical field distribution.
0.1
By, T
0.05
0
0.05
0.1
4
3
2
1
0
1
2
3
Z, m.
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Slide 19
4