Fermilab Electron Cooling System

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Fermilab Electron Cooling System

• Nov 13, 2006
• Sergei Nagaitsev

f
Fermi National Accelerator Laboratory
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Fermilab Complex

• The Fermilab Collider is a Proton-Antiproton
  Collider operating at 980 GeV

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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

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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)
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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

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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

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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
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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 lt100 e10 ? Increase
  stacking rate

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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.

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

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Gerard K. ONeill (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.

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First Cooling Demonstration

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

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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
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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.

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Fred Mills, one of the Fermilab physicists
working on the electron cooling tests in 1980,
writes
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Fermilabs legacy Cool Before Drinking...
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Ee300 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.
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Schematic of Fermilabs Electron Cooler
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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

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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

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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

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The RD program

• 20-Mar-01- Fist time HV on both tubes
• 28-Dec-01 - 0.6 A in the short beam line
• 18-Nov-02 - Imax1.7 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

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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

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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

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

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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
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Current losses
Current losses have to be low

• 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.
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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.
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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
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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.
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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.
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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
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Electron angles in the cooling section
Angles are added in quadrature
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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.
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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 (lt 0.2
  MeV/c) and small transverse emittances (lt 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

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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.

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

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Electron cooling in operation

• In the present scheme, electron cooling is
  typically not used for stacks lt 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

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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
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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
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Typical longitudinal cooling time (100 mA,
on-axis)
e-folding cooling time 20 minutes
1111010 pbars 5.2 ms bunch
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Strong transverse cooling is now routinely
observed
100 mA, on axis Stochastic cooling off
1351010 pbars 6.5 ms bunch
e-folding cooling time (FW) 25 minutes
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Transverse (horizontal) profile evolution under
electron cooling
Flying wire data
100 mA, on axis for 60 min
Deviation from Gaussian
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Evolution of the number of antiprotons available
from the Recycler
Recycler only shots
Ecool implementation
Mixed mode operation
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Present Recycler performance with electron cooling
MAX
GOAL
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Luminosity density by source
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Tevatron collider Run II Latest achievements and
targets
Horizontal emittance only (vertical flying wire
broken)
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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 Recyclers 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,
  EPAC06)