1. Fermilab Electron Cooling System
Nov 13, 2006
Sergei Nagaitsev f
Fermi National Accelerator Laboratory

2. Fermilab Complex
The Fermilab Collider is a Proton-Antiproton Collider operating at 980 GeV
Tevatron
Main Injector\
Recycler
Antiproton
source
Proton source D0
CDF

3. Tevatron Program
Greatest window into new phenomena until LHC is on
1500 collaborators, 600 students + postdocs
Critically dependent on luminosity
Doubling time a major consideration

4. Tevatron: key is luminosity
Luminosity history
for each fiscal year
Integrated luminosity
for different assumptions
Top Line: all run II upgrades work
Bottom line: none work
( pink/white bands show the
doubling times for the top line)

5. Antiprotons and Luminosity
The strategy for increasing luminosity in the Tevatron is to increase the number of antiprotons
Increase the antiproton production rate
Provide a third stage of antiproton cooling with the Recycler
Increase the transfer efficiency of antiprotons to low beta in the Tevatron

6. Antiproton Production
1x108 8-GeV pbars are collected every 2-4 seconds by striking 7x GeV protons on a Inconel target
8 GeV Pbars are focused with a lithium lens operating at a gradient of 760 Tesla/meter
30,000 pulses of 8 GeV Pbars are collected, stored and cooled in the Debuncher, Accumulator and Recycler Rings
The stochastic stacking and cooling increases the 6-D phase space density by a factor of 600x106
8 GeV Pbars are accelerated to 150 GeV in the Main Injector and to 980 GeV in the TEVATRON

7. Recycler – Main Injector
The Recycler is a fixed-momentum (8.9 GeV/c), permanent-magnet antiproton storage ring.
The Main Injector is a rapidly-cycling, proton synchrotron. Every seconds it delivers 120 GeV protons to a pbar production target. It also delivers beam to a number of fixed target experiments.
Main Injector

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

9. 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.

10. Final goal for Recycler cooling: Prepare 9 (6 eV-s each) bunches for extraction

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

12. 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

13. How does electron cooling work?
The velocity of the electrons is made equal to the average velocity of the ions.
The ions undergo Coulomb scattering in the electron “gas” and lose energy, which is transferred from the ions to the co-streaming electrons until some thermal equilibrium is attained.
Electron
Gun
Electron
Collector
Electron beam
Storage ring
Ion beam
1-5% of the ring
circumference

14. Moving foil analogy
Consider electrons as being represented by a foil moving with the average velocity of the ion beam.
Ions moving faster (slower) than the foil (electrons) will penetrate it and will lose energy along the direction of their momentum (dE/dx losses) during each passage until all the momentum components in the moving frame are diminished.
Foil p vp
* represents rest frame

15. Electron cooling: long. drag rate
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.
Lab frame quantities

16. Electron cooling
Was invented by G.I. Budker (INP, Novosibirsk) as a way to increase luminosity of p-p and p-pbar colliders.
First mentioned at Symp. Intern. sur les anneaux de collisions á electrons et positrons, Saclay, 1966: “Status report of works on storage rings at Novosibirsk”
First publication: Soviet Atomic Energy, Vol. 22, May 1967 ”An effective method of damping particle oscillations in proton and antiproton storage rings”

20. Gerard K. O’Neill ( )
Was a professor of physics at Princeton University ( ). He invented and developed the technology of storage rings for the first colliding-beam experiment at Stanford. He served as an adviser to NASA. He also founded the Space Studies Institute.

21. First Cooling Demonstration
Electron cooling was first tested in 1974 with 68 MeV protons at NAP-M storage ring at INP(Novosibirsk).

22. Europe – 1977 – 79, Initial Cooling Experiment at CERN
First cooler rings
Europe – 1977 – 79, Initial Cooling Experiment at CERN
M.Bell, J.Chaney, H.Herr, F.Krienen, S. van der Meer, D.Moehl, G.Petrucci, H.Poth, C.Rubbia– NIM 190 (1981) 237
USA – 1979 – 82, Electron Cooling Experiment at Fermilab
T.Ellison, W.Kells, V.Kerner, P.McIntyre, F.Mills, L.Oleksiuk, A.Ruggiero, IEEE Trans. Nucl. Sci., NS-30 (1983) 2370;

23. Stochastic cooling
Stochastic cooling was invented by Simon van der Meer in 1972 and first tested in 1975 at CERN in ICE (initial cooling experiment) ring with 46 MeV protons. FNAL

26. Fred Mills, one of the Fermilab physicists working on the electron cooling tests in 1980, writes:

27. Fermilab’s legacy: Cool Before Drinking...

28. Ee=300 keV
Budker INP design
1 m
1 - electron gun; 2- main “gun solenoid”; 4 - electrostatic deflectors;
5 - toroidal solenoid; 6 - main solenoid; 7 - collector; 8 - collector solenoid; 11 - main HV rectifier; 12 - collector cooling system.

29. Schematic of Fermilab’s Electron Cooler

30. 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: G. Unlike low-energy coolers, this results in non-magnetized cooling – something that had never been tested before;
A 20-m long, 100-G solenoid with high field quality

31. History of commissioning
March 2005
For the first time, 5 MV in the Pelletron (at the Recycler setup)
July 2005
Imax = 0.4 A; I = 0.2 A is stable enough
First cooling of 8 GeV pbars
September 2005
All shots are e-cooled
Electron beam is used at 50 – 100 mA and is stable

32. High-energy (relativistic) electron cooling
Novosibirsk, 1987: Tested a prototype for a 1-MV,
1-A electron beam system.
Fermilab, 1983: D. Cline et al., “Intermediate energy electron cooling for antiproton sources using a Pelletron accelerator”
For a pulsed electron beam in a Pelletron the beam quality is adequate for electron cooling
Fermilab, UCLA, NEC, 1989: Tested a 2-MV, 0.1-A recirculation system with a Pelletron.
Fermilab, IUCF, NEC, 1995: started to work on a 2-MV Pelletron again.
Fermilab, 1999: Purchased a 5-MV Pelletron
Fermilab, 2004: Installed a 5-MV Pelletron in the Recycler

33. The R&D program 20-Mar-01- Fist time HV on both tubes
28-Dec A in the short beam line
18-Nov Imax=1.7 A; beginning of a shutdown
17-Jul DC beam recirculated through the full-scale line
30-Dec A DC beam
29-May A beam with required beam properties in the cooling section

34. Pelletron Disassembly/Move
Pelletron at Wide Band Lab Before Disassembly (5/04)
1.5 Months to Disassemble
Lower Half of Pelletron Being Transported to MI31
All Components Transported 3-Miles Across the Laboratory

35. Lab Wide Shutdown to install electron cooling
Before and After Pictures of E-Cool Section of MI Tunnel
13-Week Shutdown
Modified MI Utilities, Removed Recycler Section, Installed all Beam Lines
Lab-Wide Effort

36. Electron beam design parameters
Electron kinetic energy MeV
Uncertainty in electron beam energy  0.3 %
Energy ripple V rms
Beam current 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

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

38. Specific features
Energy recovery scheme
Transport of the beam with a large effective emittance
Low magnetic field in the cooling section
Sharing the tunnel with the Main Injector

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

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

41. Effective emittance
Figure of merit: magnetic flux inside the beam in the cooling section = effective emittance outside the longitudinal magnetic field
Cathode
Cooling section
Beam line
Bz = 0
R = mm
(normalized)
Bcz = 90 G
Rcath = 3.8 mm
Bcz = 105 G
Rbeam = 3.5 mm
Low energy portions of the acceleration and deceleration tubes have to be immersed into a longitudinal magnetic field.
A 3D beam line has to provide an axially symmetrical beam transformation.

42. 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

43. 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
MI bus current
1mm X Y
MI loss
2 sec
Electron beam motion and MI losses at R04 location in the time of MI ramping Hz oscillation is due to 250 V (rms) energy ripple.

44. Low magnetic field in the cooling section
Cooling is not magnetized
The role of the magnetic field in the cooling section is to preserve low electron angles,
Transverse magnetic field map after compensation. Bz = 105 G.
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 .
Simulated angle of an 4.34 MeV electron in this field give r.m.s angle of 50 rad.

45. Cooling in barrier buckets
Keep momentum spread constant, compress the bunch length by moving the rf barrier
Simulation (MOCAC) of electron cooling + IBS, 500 mA e-beam, 600x1010 pbars
100 eV-s
50 eV-s
30 minutes

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

47. Example for the longitudinal friction force
For an antiproton with zero transverse velocity, electron beam (uniform): 500 mA, 3.5-mm radius, 200 eV rms energy spread and 200 μrad rms angular spread
Note: Units have been changed to more ‘convenient’ units
Linear approx.

48. Longitudinal cooling force measurements - Methods
Two experimental techniques, both requiring small amount of pbars (1-5 × 1010), coasting (i.e. no RF) with narrow momentum distribution (< 0.2 MeV/c) and small transverse emittances (< 3 p mm mrad, 95%, normalized)
‘Diffusion’ measurement
For small deviation cooling force (linear part)
Reach equilibrium with ecool
Turn off ecool and measure diffusion rate
Voltage jump measurement
Instantaneously change electron beam energy
Follow pbar momentum distribution evolution

49. Example: 500 mA, nominal settings, +2 kV jump (i. e. 3
Example: 500 mA, nominal settings, +2 kV jump (i.e MeV/c momentum offset), on axis
~3.7 MeV/c
2.8 × 1010 pbar
3-6 p mm mrad
Traces (from left to right) are taken 0, 2, 5, 18, 96 and 202 minutes after the energy jump.

50. Extracting the cooling (drag) force
Evolution of the weighted average and RMS momentum spread of the pbar momentum distribution function
15 MeV/c per hour

51. Drag Force as a function of the antiproton momentum deviation
100 mA, nominal cooling settings (both data sets)
Error bars ≡ statistical error from the slope determination

52. Comparison to the non-magnetized model
100 mA, nominal cooling settings (both data sets)

53. Electron cooling status: From installation to operation
Bringing electron cooling into operations consisted of three distinct parts
Commissioning of the electron beam line
Troubleshoot beam line components
Check safety systems
Ensure the integrity of the Recycler beam line at all times
Establish recirculation of an electron beam
Cooling demonstration
Energy alignment
Interaction of the electron beam with anti protons
Reduction of the longitudinal phase space
Cooling optimization (continued focus at this time)
Optimization of the electron beam quality
Stability over long period of times
Minimize electron beam transverse angles
Define best procedure for cooling anti protons
Maximize anti protons lifetime
Understand and model the cooling force

54. Electron cooling in operation
In the present scheme, electron cooling is typically not used for stacks < 200e10
Allows for periods of electron beam/cooling force studies
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
In addition, electron beam intensity is kept at 100 mA
Improves beam stability
Higher currents do not cool faster/deeper
May help lifetime too

55. Adjusting the cooling rate
Change electron beam position (vertical shift)
Adjustments to the cooling rate are obtained by bringing the pbar bunch in an area of the beam where the angles are low and electron beam current density the highest
Area of good cooling
electrons
electrons
pbars
pbars
5 mm offset
2 mm offset

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

57. Typical longitudinal cooling time (100 mA, on-axis)
e-folding cooling time: 20 minutes
111×1010 pbars
5.2 ms bunch

58. Strong transverse cooling is now routinely observed
100 mA, on axis
Stochastic cooling off
135×1010 pbars
6.5 ms bunch
e-folding cooling time (FW): 25 minutes

59. Transverse (horizontal) profile evolution under electron cooling
Flying wire data
100 mA, on axis for 60 min
Deviation from Gaussian

60. Evolution of the number of antiprotons available from the Recycler
Recycler only shots
Ecool implementation
Mixed mode operation

61. Present Recycler performance with electron cooling
MAX
GOAL

62. Luminosity density by source

63. Tevatron collider Run II: Latest achievements and targets
Typical <45 best>
Best PL
Goal
Peak Luminosity (PL)
E31 cm-2 s-1 17
22.9 30
Pbars in Recyler
E10
285
387
600
Pbar bunches 36
Unstacked Pbars
276
374
588
MI 95% long. 150 GeV
eV s
1.9
2.5
Proton 95% norm. trans. collision
mm mrad
16*
16.9 20
Pbar 95% norm. trans. collision 5
3.2
10-12
* Horizontal emittance only (vertical flying wire broken)

64. Conclusion
The electron cooler reliability has been improved and is believed to be adequate for the remaining of Run II
Electron cooling rates are sufficient for the present mode of operation of the accelerator complex
Fast transfer scheme and/or storing and extracting 600e10 may require some changes/improvements
Changing our operating point (tune space) improved the Recycler’s performance
Emittance growth during the mining process has been almost completely eliminated
Lifetime of large number of particles has improved significantly
Delivered longitudinal emittance is smaller
Better coalescing efficiency
Longitudinal cooling force (drag rate) agrees to within a factor of 2 with a non-magnetized model
Not shown in this report (see ICFA-HB2006, EPAC’06)