Electron Cooling

1
Electron Cooling

• Plans for future electron cooling needs
• PS BD/AC

2
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.

3
Analogy with the mixing of gases
Two gases of different temperatures T1 an T2 tend
to an equilibrium temperature T3
As the electron beam is continuously renewed,
the ion beam temperature tends to the electron
beam temperature. The velocity spread is
reduced by a factor (m/M)1/2
4
Electron cooling setup

• Electron gun thermocathode, Pierce shield,
  accelerating anodes
• final current given by Childs 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

5
Cooling time

• Electron cooling theory gives
• 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.

6
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 (lt 310
  MeV/c)
• In the future LHC requests a variety of ions
• the proposed injection scheme requires fast
  cooling times and stacking

7
Results of Pb54 cooling and stacking in 1997

• 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

8
How to decrease the cooling time?

• Make the cooler longer
• Change the lattice parameters at the cooler
• Ensure a perfect alignment of electron and ion
  beams
• Increase the electron current

9
Cooling Time Vs. L and Ie
Compare measurements made with the standard
machine lattice in 1996 (Lecool 1.5m) and in
1997 (Lecool 3m ).
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.
10
Cooling Time Vs. Lattice Parameters
Cooling times for 300 MeV/c protons Vs.
different b values at the cooler
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
11
Obtaining higher electron beam currents
q
45
0
d
1.5 cm
2a 1 cm
r
0.707 cm
C
P
0.82 microperv
P
6.1 microperv
I
II
To have P
6.1 microperv
I
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 (gt1000 G)

12
ExampleGun with convex spherical cathode of
half-angle ? 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
13
(No Transcript)
14
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.

15
Requested ions for LHC
16
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)

17
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/Bdrift8
• variable B field will give an expansion factor of
  2.83
• ebeam diameter 56 mm, transverse energy 12.5
  meV

18
Parameters for a state of the art cooler
(contd.)

• Efficient collection (DI/Ilt10-4) of the electron
  beam
• Cooling length of 3m
• Closed orbit and coupling compensation
• Associated diagnostics

19
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

20
Thats All For Now