1. Progress of Electron Cooling at the Recycler January 26 th, 2006 L. Prost, Recycler Dpt. personnel f Fermi National Accelerator Laboratory

2. 2 Outline  Context of electron cooling at FNAL  Electron cooling  Electron beam  Cooling of antiprotons  Conclusion

3. 3 Antiprotons and Luminosity strategy for increasing luminosity in the Tevatron is mostly to increase the number of antiprotons  The strategy for increasing luminosity in the Tevatron is mostly to increase the number of antiprotons  Provide a third stage of antiproton cooling with the Recycler

4. 4 Antiprotons flow (Recycler only shot) Accumulator Recycler Tevatron Transfer from Accumulator to Recycler Shot to TeV Keep Accumulator stack <100 mA  Increase stacking rate 2600e9 400 e10 200 mA 100 mA

5. 5 Beam Cooling in the Recycler The missions for cooling systems in the Recycler are:  The multiple Coulomb scattering (IBS and residual gas) needs to be neutralized.  The emittances of stacked antiprotons need to be reduced between transfers from the Accumulator to the Recycler.  The effects of heating because of the Main Injector ramping (stray magnetic fields) need to be neutralized.

6. 6 Performance goal for the long. equilibrium emittance: 54 eV-s GOAL MAX Stochastic cooling limit 20% lower 36 bunches at 2 eVs per bunch 36 bunches at 1.5 eVs per bunch

7. 7 Recycler Electron Cooling  The maximum antiproton stack size in the Recycler is limited by  Stacking rate in the Debuncher-Accumulator at large stacks  Longitudinal cooling in the Recycler Stochastic cooling only –~140e10 for 1.5 eVs bunches (36) –~180e10 for 2eVs bunches (36) Longitudinal stochastic cooling has been complemented by Electron cooling

8. 8 How does electron cooling work? “hot” antiprotons “cold” electrons  In the moving frame we have a mixture of gases of “hot” antiprotons and “cold” electrons.  Transfer of energy through Coulomb interactions

9. 9 How does electron cooling work? (cont’) Storage ring Electron Gun Electron Collector 1-5% of the ring circumference Electron beam Ion beam  At electron energies up to ~300 keV:  Direct electrostatic acceleration of electrons with energy recovery.  A strong longitudinal magnetic field accompanies electrons from the cathode to the exit of the cooling section. Magnetic field assures the transport of the electron beam while retaining low temperature of the electrons.  Typical parameters of all existing low-energy electron coolers:  electron kinetic energy: 2-300 keV  electron beam current: up to 5 A  Cooler length: 1-3 m  Magnetic field: 0.1- 0.5 T

10. 10 What makes the Fermilab system unique?  It requires a 4.36 MV DC power supply. We have chosen a commercially available electrostatic accelerator. As a consequence we had to develop several truly new beamline, cooling, and solenoid technologies:  Interrupted solenoidal field: there is a magnetic field at the gun cathode and in the cooling section, but no field in between. It is an angular-momentum-dominated transport line;  Low magnetic field in the cooling section: 50-150 G. Unlike low-energy coolers, this will result in non- magnetized cooling – something that had never been tested;  A 20-m long, 100-G solenoid with high field quality

11. 11 Electron beam design parameters  Electron kinetic energy 4.34 MeV  Uncertainty in electron beam energy  0.3 %  Energy ripple500 V rms  Beam current 0.5 A DC  Duty factor (averaged over 8 h)95 %  Electron angles in the cooling section (averaged over time, beam cross section, and cooling section length), rms  0.2 mrad All design parameters have been met

12. 12 Schematic Layout of Fermilab’s Electron Cooler

13. 13 Electron cooling system setup at MI-30/31 Pelletron (MI-31 building) Cooling section solenoids (MI-30 straight section)

14. 14 Simplified electrical schematic of the electron beam recirculation system  Beam power 2.15 MW  Current loss power 21.5 W  Power dissipated in collector 2.5 kW For I= 0.5 A,  I= 5  A: The beam power of 2 MW requires the energy recovery (recirculation) scheme

15. 15 Current losses Current losses have to be low Gun closes (1)Losses to the tube electrodes should not exceed few µA to avoid overvoltage (2)Losses at the ground should not exceed few tens of µA to avoid damaging the vacuum chamber Collector ion pump current Anode current, 2µA/div Pelletron current, 10µA/div Tube current, 2µA/div Beam current, 0.1A/div Typical plot of losses as functions of the beam current. dI/I =(1.2-1.5)·10 -5.

16. 16 Recirculation stability: Interruptions vs Full discharges Two types of events when the protection system is activated: (1) Recirculation interruption without large terminal voltage drops (2) Full discharges. Accel. side pressure Decel. side pressure Pelletron voltage Beam current Full discharge 4 hours Not typical of ‘normal’ operation Accel. side pressure Decel. side pressure Pelletron voltage Beam current Full discharge 4 hours Not typical of ‘normal’ operation Pelletron voltage Full discharge Beam current 12 hours Decel. side pressure Accel. side pressure A “history plot” while cooling (mostly) 2 nTorr 0.4 MV 0.2 A Access

17. 17 Recirculation Stability: High duty factor has been met (>95%)  Running at high current (> 0.2 A) induces full discharges (~1-2 per week) until the Pelletron needs to be reconditioned. 24 hours No full discharges 5 recirculation interruptions 2 nTorr 0.4 MV 0.2 A Beam current Decel. side pressure Accel. side pressure Pelletron voltage

18. 18 Beam quality  Cooling force depends on rms electron angle in the cooling section (averaged over time, beam cross section, and cooling section length)  Contributions come from  Temperature  Aberrations  Beam motion (vibrations in the Pelletron, MI ramps)  Drift velocity  Dipole motions caused by magnetic field imperfections  Envelope scalloping Cooling section

19. 19 Neighborhood with the Main Injector  Magnetic fields of busses and MI magnets in the time of ramping causes an extensive motion of the electron beam (up to 0.2 mm in the cooling section and up to 2 mm in the return line)  MI radiation losses sometimes result in false trips of the ECool protection system Electron beam motion and MI losses at R04 location in the time of MI ramping. 0.55 Hz oscillation is due to 250 V (rms) energy ripple. 2 sec MI bus current MI loss X Y 1mm

20. 20  Cooling is not magnetized  The role of the magnetic field in the cooling section is to preserve low electron angles, Low magnetic field in the cooling section Transverse magnetic field map after compensation. Bz = 105 G. Simulated angle of an 4.34 MeV electron in this field give r.m.s angle of 50  rad.  A typical length of B  perturbation, ~20 cm, is much shorter than the electron Larmor length, 10 m. Electron angles are sensitive to, not to B .

21. 21 Beam size measurements in the Cooling Section  11 movable orifices (not in phase) in the cooling section The scrapers are diaphragms of 15 mm diameter, located every 2 m. While only one of them is in place, the beam is shifted in some direction until it touches the scraper. The bpm data for the beam center is taken at this point. The beam is shifted in other direction, and the center coordinates at touch are detected again; usually 8 directions are used. Then, the entire procedure is repeated for other scrapers. From these data, the beam ellipse and the scraper offsets are found for every scraper involved. Initial conditions for the beam envelope are fitted for these ellipses. A cylindrical boundary might not guarantee low angles in the middle of the beam because of aberrations radial angle tangential angle density

22. 22 Scraper Measurements Dec 1 (nominal settings, 500 mA) SCC00 SCC70 SCC60 SCC50 SCC40 SCC30 SCC20 SCC10 SCQ01 SCC90 SCC80 Beam radius ~ 4.5 mm Averaged rms angle <0.2 mrad

23. 23 Comparison of two focusing settings Envelope (fit) along the scrapers 0-5 One lens changed by 2 A Average rms envelope angle is 0.5 mrad Nominal Average rms envelope angle is 0.2 mrad

24. 24 Electron angles in the cooling section *Angles are added in quadrature

25. 25 Electron beam status  The electron beam stability and duty factor are adequate for providing cooling when needed  Days of operation without trips for beam current <200 mA ‘Natural’ interruptions are taken care of automatically (Java application)  Duty factor >95% for beam current up to 500 mA  Conditioning takes a shift and can be done when Recycler has low stash Needed >1 month at low current Needed ~2-4 weeks at high current Actually depends on number of full discharges  Beam angles are low enough  Confirmed by the fact that we successfully cool pbars !

26. 26 July 05 electron beam status  Goal for the rms angular spread (<0.2 mrad) had not been met  0.5 A DC beam was not stable  200 mA only  Reliability, reproducibility were still a problemBUT… … (first) cooling attempt was successful !

27. 27 First e-cooling demonstration – 07/15/05

28. 28 Electron cooling drag rate - Theory  For an antiproton with zero transverse velocity, electron beam: 500 mA, 3.5-mm radius, 200 eV rms energy spread and 200 μrad rms angular spread Non-magnetized cooling force model Linear approx.

29. 29 Cooling force – Experimental measurements  Two experimental techniques, both requiring small amount of pbars, coasting (i.e. no RF) with narrow momentum distribution and small transverse emittances  ‘Diffusion’ measurement For small deviation cooling force (linear part) Reach equilibrium with ecool Turn off ecool and measure diffusion rate  Voltage jump measurement For momentum deviation > 2 MeV/c Reach equilibrium with ecool Instantaneously change electron beam energy Follow pbar momentum distribution evolution  Both methods characterize the effectiveness of electron cooling (hence, the electron beam quality) quite locally and not necessarily the cooling efficiency/rate for large stashes

30. 30 Cooling rate for small amplitudes  For small momentum deviations (< 1 MeV) the cooling force is linear: F ≈ - λp. The distribution function in momentum is close to being Gaussian. rms spread 0.17 MeV/c 5x10 10 pbars, 2 mm mrad (n, 95%) Cooled by 500 mA electron beam on axis

31. 31 Cooling OFF-ON  By turning the electron cooling OFF and ON again one can determine both the diffusion and cooling rates Electron beam current, Amps Cooling OFF: Cooling ON: D ≈ 2.5 MeV 2 /hr λ ≈ 43 hr -1 Fit 5x10 10 pbars, 2 mm mrad (n, 95%)

32. 32 Drag force measurements: electron energy jump by +2 keV Momentum distribution (log scale) Beam emittance was measured by Schottky: 1.5 μm (n, 95%). In the cooling section this corresponds to a 0.9 mm radius (rms), electron current 500 mA 1.2 MeV per division 21 MeV/hr Evolution of the weighted average of the pbar momentum distribution function Cooling force is in reasonable agreement with predictions

33. 33 Electron cooling in operation  In the present scheme, electron cooling is typically not used for stashes < 200e10  Over 200e10 stored  Electron cooling used to ‘help’ stochastic cooling maintain a certain longitudinal emittance (i.e. low cooling from electron beam) between transfers or shot to the TeV  ~1 hour before setup for incoming transfer or shot to the TeV, electron beam adjusted to provide strong cooling (progressively) This procedure is intended to maximize lifetime

34. 34 Electron cooling in operation (cont’) Electron cooling prior to extraction Stochastic cooling only Electron cooling between transfers Transverse emittance 3  mm mrad/div Momentum spread 1.25 MeV/c /div Longitudinal emittance 50 eVs/div Pbar intensity 75e10/div

35. 35 Electron cooling between transfers/extraction Electron beam is moved ‘in’ Stochastic cooling after injection Electron beam ‘out’ (5 mm offset) Electron beam current 0.1 A/div Transverse emittance 1.5  mm mrad/div Electron beam position 1 mm/div Longitudinal emittance (circle) 25 eVs/div Pbar intensity (circle) 16e10/div ~1 hour 100 mA 195e10 ~60 eVs

36. 36 Regulating the cooling rate  Two ‘knobs”  Electron beam current Beam stays on axis Dynamics of the gun varies between low and high currents Hence, changing the beam current also changes the beam size and envelope in the cooling section  Electron beam position ‘Regulation’ is obtained by bringing the pbar bunch in an area of the beam where the angles are low 5 mm offset 2 mm offset Area of good cooling

37. 37 Issues related to electron cooling and large stashes  Since started to use the electron beam for cooling, we have dealt with two main problems  Transverse emittance growth At high ‘phase density’ ( i.e. N pbar /  T  L ) and mining (with electron beam on) When the beam got too cold (fixed with dampers) When the electron beam is turned on and/or moved towards the axis (rare occurrences recently)  Lifetime degradation When the beam is turned on and/or moved towards the axis (i.e. strong cooling)

38. 38 Emittance growth during mining (1) [11/14 measurements] e-beam: 500 mA, +3.5 mm offset pbars: 180e10 e-beam: 500 mA, +3 mm offset pbars: 180e10 Stochastic cooling system was turned off when mining, e-beam (when used) remained on Dampers are on for all measurements pbars: 114e10 Initial rate: 17  mm mrad/hour Instrumentation problem

39. 39 Emittance growth during mining (2)  Trial: Changed working point for the tunes (in order to split them more), from 0.414/0.418 (H/V) to 0.453/0.473 (H/V) Electron beam current Horizontal emittance Electron beam position Longitudinal emittance (circle) Vertical emittance (circle) Pbar intensity (circle) Stochastic cooling system is turned off before mining ~2  mm mrad/hour Phase density when mining = 0.9 Mining 227e10 100 mA

40. 40 Emittance growth due to presence of electron beam and/or strong cooling  There are some indications that in some conditions the electron beam might increase the transverse emittance

41. 41 Lifetime degradation throughout a stash Pbars intensity Lifetime (1 hour running average) 500 hours

42. 42 Correlation with presence of electron beam cooling ? 40 minutes of electron cooling on axis at 300 mA Lifetime drops and recovers ~1 hour later But phase density has increased by ~2 !

43. 43 How to improve the lifetime ?  Since we do not know what phenomena cause the lifetime to suffer during electron cooling, we need to continue investigating. However, there are things that could be tried…  Optimization of the cooling procedure  Has been worked since first time we saw electron cooling Optimization of the electron beam current and offset used for cooling Minimizing the time spent at high pbar density Further optimization of the stochastic cooling while cooling with the electron beam  Different working point in tune space  Started on that  Change the electron beam size  Larger ? Smaller ? Arguments both ways… ‘Easier’ to get larger  Increase magnetic field in cooling section

44. 44 Present Recycler performance with electron cooling

45. 45 Conclusion  Fermilab now has a world-record operational electron cooling system  Since the end of August 2005, electron cooling is being used on (almost) every Tevatron shot  Increases of stash sizes are a direct consequence of the ability to cool the beam efficiently (current record 370e10)  Electron cooling allowed for the latest advances in the TeV peak luminosity (current record 177e30)  Lifetime issue now limits the maximum number of anti-protons that the Recycler can stored  Primary focus