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

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

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

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)

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

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

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 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

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

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.

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

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

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

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

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

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

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”

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.

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

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;

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 - 1980.

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

Fermilab’s legacy: Cool Before Drinking...

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.

Schematic of Fermilab’s Electron Cooler

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 results in non-magnetized cooling – something that had never been tested before; A 20-m long, 100-G solenoid with high field quality

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

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

The R&D program 20-Mar-01- Fist time HV on both tubes 28-Dec-01 - 0.6 A in the short beam line 18-Nov-02 - Imax=1.7 A; beginning of a shutdown 17-Jul- 03 - DC beam recirculated through the full-scale line 30-Dec-03- 0.5 A DC beam 29-May-04- 0.1 A beam with required beam properties in the cooling section

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

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

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 All design parameters have been met

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

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

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

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 =(1.2-1.5)·10-5.

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 = 2 -10 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.

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

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. 0.55 Hz oscillation is due to 250 V (rms) energy ripple.

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.

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

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

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.

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

Example: 500 mA, nominal settings, +2 kV jump (i. e. 3 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 Traces (from left to right) are taken 0, 2, 5, 18, 96 and 202 minutes after the energy jump.

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

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

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

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

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

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

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

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

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

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

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

Present Recycler performance with electron cooling MAX GOAL

Luminosity density by source

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. emit. @ 150 GeV eV s 1.9 2.5 Proton 95% norm. trans. emit. @ collision mm mrad 16* 16.9 20 Pbar 95% norm. trans. emit. @ collision 5 3.2 10-12 * Horizontal emittance only (vertical flying wire broken)

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)