Electron Cooling

1. Electron Cooling Plans for future electron cooling needs PS BD/AC PS Days 2001

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. PS Days 2001

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 PS Days 2001

4. Electron cooling setup • 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 PS Days 2001

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. PS Days 2001

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 (< 310 MeV/c) • In the future LHC requests a variety of ions • the proposed injection scheme requires fast cooling times and stacking PS Days 2001

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 PS Days 2001

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 PS Days 2001

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. PS Days 2001

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 PS Days 2001

11. EC EA EA EC r C d d 2a 2a Anode Anode q f D = U f D = U Cathode Cathode f f = 0 = 0 I II P = II 7 . 5 P I Obtaining higher electron beam 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) q 0 = 45 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 PS Days 2001

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 PS Days 2001

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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. PS Days 2001

15. Requested ions for LHC PS Days 2001

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) PS Days 2001

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/Bdrift=8] • variable B field will give an expansion factor of 2.83 • ebeam diameter = 56 mm, transverse energy = 12.5 meV PS Days 2001

18. 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 PS Days 2001

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 PS Days 2001

20. That’s All For Now PS Days 2001