Electron Cooling Electron Cooling Plans for future electron cooling - - PowerPoint PPT Presentation

25th January 2001 PS Days 2001 Slide 1

Electron Cooling Electron Cooling

Plans for future electron cooling needs PS BD/AC

25th January 2001 PS Days 2001 Slide 2

What is electron cooling? 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.

25th January 2001 PS Days 2001 Slide 3 gas A gas B T 1 T 2 T 3 electron beam ion beam kT = 1/2m v kT = 1/2M V 2 2 V = v(m /M ) 1/2

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 tempera The velocity spread is reduce by a factor (m/M)1/2

25th January 2001 PS Days 2001 Slide 4

Electron cooling setup Electron cooling setup

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

• Electron gun: thermocathode, Pierce shield, accelerating anodes

– final current given by Child’s Law: I = ρV3/2 – the parameter ρ is the perveance and is given by 7.3μP (r/d)2

• Interaction section
• Collector
• The whole system is immersed in a longitudinal field

25th January 2001 PS Days 2001 Slide 5

Cooling time Cooling time

• Electron cooling theory gives :

– where θ is the relative difference in angle between the ions and electrons (θi - θe), [θi=√(ε/β)] – the parameter η = Lcooler/Lmachine – and Ie is the electron current.

4 5 2 3

β γ η θ τ Z A I e ∝

25th January 2001 PS Days 2001 Slide 6

Electron cooling at CERN 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

– the proposed injection scheme requires fast cooling times and stacking

25th January 2001 PS Days 2001 Slide 7

Results of Pb Results of Pb54+

54+ cooling and

cooling and stacking in 1997 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

A v e r a g e a c c u m u la te d in te n s ity : 6 E 8 io n s P e a k in te n s ity : 7 .1 E 8 io n s

25th January 2001 PS Days 2001 Slide 8

How to decrease the cooling How to decrease the cooling time? 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

e

I ⋅

∝ η

θ

τ

3

25th January 2001 PS Days 2001 Slide 9

Compare measurements made with the standard machine lattice in 1996 (Lecool = 1.5m) and in 1997 (Lecool = 3m ).

machine 1 ecool 3m,1.5m

m achine1-97:1/th[m s] m achine1-96:1/th[m s]

Inverse transverse cooling time

• f 88.86 MeV/c/u Pb54+ ions as

a function of electron beam intensity for 1.5m and 3m setup.

Cooling Time Vs. L and Cooling Time Vs. L and Ie Ie

25th January 2001 PS Days 2001 Slide 10

Cooling Time Vs. Lattice Cooling Time Vs. Lattice Parameters Parameters

• 25
• 20
• 15
• 10
• 5

Cooling times for 300 MeV/c protons Vs. different β 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

25th January 2001 PS Days 2001 Slide 11

Obtaining higher electron beam Obtaining higher electron beam currents currents

• 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)

θ = 45 0 δ = 1.5 cm 2a = 1 cm rC = 0.707 cm P I = 0.82 microperv P II = 6.1 microperv 5 . 7 =

P P To have P

δ = 0.54 cm would be necessary EC EA Cathode φ = 0 Anode φ = ΔU rC 2a δ EA EC Cathode φ = 0 Anode φ = ΔU 2a δ θ

I II

25th January 2001 PS Days 2001 Slide 12 Ua = 3kV , I = 0.92A , P = 5.6 microperv B = 2000 Gauss Beam diameter = 1 cm Example Example Gun with convex spherical cathode of half Gun with convex spherical cathode of half-

• angle

angle θ θ = 45 = 450

0.

. Simulation with the program SSAM/CERN. Simulation with the program SSAM/CERN. Gun geometry provided by A.Shemyakin/FNAL. Gun geometry provided by A.Shemyakin/FNAL.

450 a= 0.5 cm

25th January 2001 PS Days 2001 Slide 13

250 500 750 1000 1250 1500 1750 2000 2250 50 100 150 200 250 300 350 400 450 500 550 600

Gun perveance measurements at Fermilab. (Experimental error bars are not given.) Child's law fit on data: Icoll= Pgun Ucath

Pgun = 6.1 μperv

Collector current Icoll , mA Cathode Voltage Ucath , V

25th January 2001 PS Days 2001 Slide 14

Electron beam expansion 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 r r const r B = ⇒ =

2 //

• t

B B E E const B E = ⇒ =

//

25th January 2001 PS Days 2001 Slide 15

Requested ions for LHC Requested ions for LHC

Pb In Kr Ar O He A 208 115 84 40 16 4 Z 54 37 29 16 4 1 βinj c .095 .095 .095 .095 .095 .095 Vecool kV 2.32 2.32 2.32 2.32 2.32 2.32 I mA 200 220 200 400 400 400 ρ μP 1.79 1.97 1.79 3.58 3.58 3.58 βej c .25 .42 .35 .185 .3 .35 Vecool kV 16.8 52.1 34.5 8.98 24.67 34.5 I mA 600 1000 1300 800 3000 2000

25th January 2001 PS Days 2001 Slide 16

Required performance of a new Required performance of a new ecooler ecooler

• Electron energy range: 2 keV to 55 keV
• Electron currents between 100 mA and 3 A (maximum

perveance of 4 μP)

• 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)

25th January 2001 PS Days 2001 Slide 17

Parameters for a Parameters for a “ “state of the art state of the art” ” cooler cooler

• Electron beam:

– convex cathode, diameter approx. 20 mm – high perveance (4μP), 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

25th January 2001 PS Days 2001 Slide 18

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

• Efficient collection (ΔI/I<10-4) of the electron beam
• Cooling length of 3m
• Closed orbit and coupling compensation
• Associated diagnostics

25th January 2001 PS Days 2001 Slide 19

Where are we now? 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

25th January 2001 PS Days 2001 Slide 20

That That’ ’s All For Now s All For Now