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Lecture 5 Electron Cooling

Lecture 5 Electron Cooling. History, Concept, Demonstrations at Novosibirsk and Fermilab and Emerging Developments Swapan Chattopadhyay, Cockcroft Institute in collaboration with and based on major contributions from Sergei Nagaitsev, Fermilab. Electron cooling.

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Lecture 5 Electron Cooling

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  1. Lecture 5Electron Cooling History, Concept, Demonstrations at Novosibirsk and Fermilab and Emerging Developments Swapan Chattopadhyay, Cockcroft Institute in collaboration with and based on major contributions from Sergei Nagaitsev, Fermilab

  2. 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”

  3. Gerard K. O’Neill (1927-1992) • Was a professor of physics at Princeton University (1965-1985). 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. • CONTRIBUTED SIGNIFICANTLY to THE DEVELOPMENT OF PARTICLE COLLIDERS where the concept of LUMINOUS COLLISIONS gave impetus to various COOLING concepts, including ELECTRON COOLING

  4. 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 1-5% of the ring circumference Ion beam

  5. 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 vp p * represents rest frame

  6. Electron cooling: long. drag rate • For an antiproton with zero transversevelocity, 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

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

  8. 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.

  9. 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;

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

  11. Russian Legacy…..”cool before drinking”

  12. Schematic of Fermilab’s Electron Cooler

  13. Electron beam design parameters • Electron kinetic energy 4.34 MeV • Uncertainty in electron beam energy  0.3 % • Energy ripple 500 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

  14. 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

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

  16. 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

  17. Bcz = 90 G Rcath = 3.8 mm Bcz = 105 G Rbeam = 3.5 mm Bz = 0 R = 2 -10 mm (normalized) (normalized) (normalized) Beam line Cathode Cooling section Effective emittance Figure of merit: magnetic flux inside the beam in the cooling section = effective emittance outside the longitudinal magnetic field 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.

  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. 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.

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

  21. 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

  22. 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 • Reach equilibrium with ecool • Instantaneously change electron beam energy • Follow pbar momentum distribution evolution

  23. Traces (from left to right) are taken 0, 2, 5, 18, 96 and 202 minutes after the energy jump. Example: 500 mA, nominal settings, +2 kV jump (i.e. 3.67 MeV/c momentum offset), on axis ~3.7 MeV/c 2.8 × 1010 pbar 3-6 p mm mrad

  24. 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

  25. 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

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

  27. 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

  28. Electron cooling between transfers/extraction Electron beam ‘out’ (5 mm offset) Electron beam current0.1 A/div Transverse emittance1.5 p mm mrad/div Electron beam position1 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

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

  30. 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

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

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

  33. 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

  34. 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)

  35. 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

  36. Antiproton Production • 1x108 8-GeV pbars are collected every 2-4 seconds by striking 7x1012 120-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

  37. Recycler – Main Injector Recycler 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 1.6-3 seconds it delivers 120 GeV protons to a pbar production target. It also delivers beam to a number of fixed target experiments. Main Injector

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

  39. 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.

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

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

  42. 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

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

  44. Present Recycler performance with electron cooling MAX GOAL

  45. Luminosity density by source

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