Transcript Electron Cooling

Electron Cooling
Plans for future electron cooling needs
PS BD/AC
25th January 2001
PS Days 2001
Slide 1
What is electron cooling?
• Means to increase the phase space density of a stored ion
beam.
• Mono-energetic cold electron beam is merged with ion beam
which is cooled through Coulomb interaction.
• Electron beam is renewed and the velocity spread of the ion
beam is reduced in all three planes.
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Slide 2
Analogy with the
mixing of gases
gas B
gas A
T2
T1
T3
Two gases of different
temperatures T1 an T2
tend to an equilibrium
temperature T3
electron
beam
2
kT=1/2mv
1/2
V=v(m/M)
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ion
beam
kT=1/2MV
2
As the electron beam is
continuously renewed, the
ion beam temperature tends
to the electron beam temperatu
The velocity spread is reduced
by a factor (m/M)1/2
Slide 3
Electron cooling setup
25 26 27 28 29
1 2 3 4
20 21 22 23 24
5 6 7 8
9 10 11 12
13 14 15
16 17 18 19
• Electron gun: thermocathode, Pierce shield, accelerating anodes
– final current given by Child’s Law: I = rV3/2
– the parameter r is the perveance and is given by 7.3mP (r/d)2
• Interaction section
• Collector
• The whole system is immersed in a longitudinal field
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Slide 4
Cooling time
• Electron cooling theory gives :
q A 5 4

 
2
hI e Z
3
– where q is the relative difference in angle between
the ions and electrons (qi - qe), [qi=(/)]
– the parameter h = Lcooler/Lmachine
– and Ie is the electron current.
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Slide 5
Electron cooling at CERN
• Improve the quality of low energy ion beams
– many experiments on LEAR and AD not possible
without electron cooling
– used to cool (anti)protons, H-, oxygen, and lead
ions
– first electron cooling device to be used routinely on
a storage ring
• Increase of the duty cycle of the machine
– cooling time much less than what can be obtained
with stochastic cooling at low energies (< 310
MeV/c)
• In the future LHC requests a variety of ions
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PS Days 2001
Slide 6
– the proposed injection
scheme requires fast
th
Results of Pb54+ cooling and
stacking in 1997
Beam Intensity [E8 ions]
8
6
3
4.8
3
5.1
9
5.3
5
5.5
8
5.7
3
5.9
5
6.0
3
6.1
6
6.1
9
6.1
2
6.2
7
4.3
Linac III rep rate : 2.5 Hz
Ion beam energy : 4.2 MeV/u
Electron energy : 2.35 keV
Electron current : 105 mA
beam lifetime : 6.5s
8
3.8
4
3
3.2
1
2.4
2
7
1.3
Average accumulated intensity : 6E8 ions
Peak intensity : 7.1E8 ions
0
2
4
6
8
10
12
Time [ s]
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• Stacking at the 2.5 Hz Linac
repetition rate
• Saturation effect on the
accumulated intensity due to
vacuum degradation and beam
loss
• Cooling times of 200 ms
obtained with an electron current
of 120 mA
• missing a factor of 2 in cooling
time and in accumulated
intensity
PS Days 2001
Slide 7
How to decrease the cooling
time?

•
•
•
•
q
hI e
3
Make the cooler longer
Change the lattice parameters at the cooler
Ensure a perfect alignment of electron and ion beams
Increase the electron current
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Slide 8
Cooling Time Vs. L and Ie
Compare measurements made with the standard machine
lattice in 1996 (Lecool = 1.5m) and in 1997 (Lecool = 3m ).
machine 1 ecool 3m,1.5m
10
9
8
Inverse transverse cooling time
of 88.86 MeV/c/u Pb54+ ions as
a function of electron beam
intensity for 1.5m and 3m setup.
7
1/th[s-1]
6
5
4
machine1-97:1/th[ms]
3
machine1-96:1/th[ms]
2
1
Ie[mA]
0
0
50
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100
150
200
250
300
350
400
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Slide 9
Cooling Time Vs. Lattice
Parameters
Cooling times for 300 MeV/c protons Vs. different
 values at the cooler
40000
Mach1
35000
Mach97
Mach97-1
cooling-down time [ms]
30000
Mach97-2
25000
20000
15000
10000
5000
0
-25
-20
-15
-10
-5
position [m m ]
0
5
10
15
Cooling times for 300 MeV/c protons
Vs. different values of D at the cooler
Comparison of inverse cooling times for 88.86 MeV/c/u
Pb54+ ions Vs. Ie for all tested machine optical settings
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Slide 10
Obtaining higher electron beam
currents
EC
EA
EA
EC
rC
d
2a
Anode
f =DU
q
Cathode
f =0
2a
d
Anode
f =DU
q = 45 0
d = 1.5 cm
2a = 1 cm
r C = 0.707 cm
Cathode
f =0
I
P I = 0.82 microperv
II
P II
= 7 .5
PI
P II = 6.1 microperv
To have P I = 6.1 microperv
d = 0.54 cm would be necessary
• Analytical studies of diodes in space charge regime have shown that a
convex spherical diode has a higher perveance than a planar diode
• However the beam emitted from a convex cathode is divergent and hence
has large transverse velocities.
• STRONG SOLENOIDAL AXIAL FIELD is needed (>1000 G)
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Slide 11
Example
Gun with convex spherical cathode of half-angle q = 450.
Simulation with the program SSAM/CERN.
Gun geometry provided by A.Shemyakin/FNAL.
Ua = 3kV , I = 0.92A , P = 5.6 microperv
B = 2000 Gauss
Beam diameter = 1 cm
a= 0.5 cm
450
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Slide 12
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Slide 13
Electron beam expansion
• Convex cathodes are generally small in size and need a
strong axial magnetic field
– electron beam is smaller than the injected ion beam
– not compatible with insertion in the storage ring
• Cure: decrease the axial field adiabatically such that the
electron beam size is larger than the ion beam and also the
field in the toroids and cooling section does not perturb the
machine.
B//
B
= const  r = ro
2
r
Bo
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Et
B
= const  E = Eo
B//
Bo
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Slide 14
Requested ions for LHC
A
Z
inj
Vecool
I
c
kV
mA
r
ej
Vecool
I
mP
c
kV
mA
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Pb
208
54
.095
2.32
200
1.79
.25
16.8
600
In
115
37
.095
2.32
220
1.97
.42
52.1
1000
Kr
84
29
.095
2.32
200
1.79
.35
34.5
1300
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Ar
40
16
.095
2.32
400
3.58
.185
8.98
800
O
16
4
.095
2.32
400
3.58
.3
24.67
3000
He
4
1
.095
2.32
400
3.58
.35
34.5
2000
Slide 15
Required performance of a new
ecooler
• Electron energy range: 2 keV to 55 keV
• Electron currents between 100 mA and 3 A (maximum
perveance of 4 mP)
• Electron beam diameter of 3.5 cm in the interaction region
(variable?)
• Transverse energies less than 100 meV
• Good beam alignment
• Minimum perturbation to the machine (closed orbit,
coupling, vacuum)
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Slide 16
Parameters for a “state of the art”
cooler
• Electron beam:
– convex cathode, diameter approx. 20 mm
– high perveance (4mP), variable intensity (multiple
anodes) and variable energy
• Magnetic field:
– Bgun = 0.6 T, Bdrift = 0.075 T [Bgun/Bdrift=8]
– variable B field will give an expansion factor of 2.83
• ebeam diameter = 56 mm, transverse energy = 12.5 meV
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Slide 17
Parameters for a “state of the art”
cooler (contd.)
•
•
•
•
Efficient collection (DI/I<10-4) of the electron beam
Cooling length of 3m
Closed orbit and coupling compensation
Associated diagnostics
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Slide 18
Where are we now?
• Theoretical work for the design of a new gun and collector
• Linear testbench is being commissioned
– spare gun and collector for AD
– test of the high perveance electron gun
• Tests at other laboratories (MSL, MPI Heidelberg, GSI)
– beam expansion, optimum lattice parameters
• New ideas
– hollow gun
– open collector
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Slide 19
That’s All For Now
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Slide 20