Electron cooling at the Recycler Status report

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Electron cooling at the RecyclerStatus report

• Lionel Prost
• DOE Tevatron Operations Review
• March 21, 2006

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Electron cooling status - Summary

• The electron cooling system installation was
  finished on February 25th, 2005, at which date
  commissioning began.
• Cooling of 8 GeV antiprotons with the electron
  beam was first demonstrated on July 15th, 2005
  and was reported at the COOL05 workshop held in
  Galena, IL.
• Electron cooling was used for almost all collider
  shots since
• On October 5th, 2005, Recycler-only shots were
  implemented.
• Availability and reliability of the electron
  cooling system is important for this mode of
  operation to be profitable
• It effectively passed the burden of cooling from
  the Accumulator onto the Recycler
• Fermilab now has a world-record operational
  electron cooling system
• 4.3 MeV, up to 1.5 A DC (U-Bend mode)
• Beam power 6.5 MW (New world record)

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Electron cooling system setup at MI-30/31
Added section
(includes additional pumping)
Beam line accommodates both U-bend and vertical
bend magnets
Fast acting valves
Magnetically shielded to protect e beam from
fields imposed by the MI bus
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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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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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Recirculation Stability High duty factor has
been met (gt95)

• Running at high current (gt 0.2 A) induces full
  discharges (1-2 per week) until the Pelletron
  needs to be reconditioned.

2 nTorr 2 nTorr 0.4 MV 0.2 A
Beam current
Pelletron voltage
No full discharges 5 recirculation interruptions
Decel. side pressure
Accel. side pressure
24 hours
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Electron angles in the cooling section
Angles are added in quadrature
Part of shutdown activities
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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
  lt200 mA
• Natural interruptions are taken care of
  automatically (Java application)
• Duty factor gt95 for beam current up to 500 mA
• Conditioning of the Pelletron takes a shift (8 h)
  and can be done when the Recycler has a low stack
• Less than once per month at low current (i.e. lt
  200 mA)
• Once per 2-4 weeks at high current (i.e. gt 200
  mA)
• Actually depends on number of full discharges
• Beam angles are low enough
• Confirmed by the fact that we successfully cool
  antiprotons !

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First e-cooling demonstration 07/15/05
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Run II project milestones

• Plan Actual
• Commissioning begins 02/01/05 03/01/05
• U-bend commissioned 03/14/05 04/15/05
• Full beam line commissioned 04/04/05 05/04/05
• A 0.5-A DC beam 07/08/05 07/26/05
• Cooling of antiprotons 09/08/05 07/15/05
• Electron cooling operational 01/02/06 10/03/05
• (contribute to HEP)

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Electron cooling in operation

• In the present scheme, electron cooling is
  typically not used for stacks lt 200e10
• Over 200e10 stored
• Electron cooling along with stochastic cooling is
  used to 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)
• The electron beam current is set to 100 mA
• Adequate for present number of stored antiprotons
• Longitudinal emittance of the bunch before mining
  and extraction is typically lt 70 eVs
• Some indirect interaction may exist between
  electron cooling and stochastic cooling

This procedure is intended to maximize lifetime
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Adjusting 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
• Adjustments are obtained by bringing the
  antiprotons bunch in an area of the beam where
  the angles are low

Area of good cooling
Schematic transverse profiles Pink pbars Blue
e-beam
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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Electron cooling drag rate - Theory

• For an antiproton beam with zero transverse
  velocity, electron beam 500 mA, 3.5-mm radius,
  300 eV rms energy spread and 200 µrad rms angular
  spread

Non-magnetized cooling force model
Linear approx.
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Cooling force Experimental measurements

• Two experimental techniques, both requiring small
  amount of antiprotons, coasting (i.e. no RF) with
  narrow momentum distribution and small transverse
  emittances
• Diffusion measurements
• For small deviation cooling force (linear part)
• Reach equilibrium with electron cooling
• Turn off electron cooling and measure diffusion
  rate
• Adding external source of noise (constant) can
  lead to the determination of the entire curve
• Equilibrium distribution gives the shape of the
  curve
• Diffusion rate (with the added noise) gives the
  amplitude
• Electron energy jump method
• Reach equilibrium with electron cooling
• Instantaneously change electron beam energy
• Follow antiprotons 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 stacks

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Drag Force as a function of the antiprotons
momentum deviation100 mA, on axis, nominal
cooling settings
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Improvements/upgrades to be implemented during
the shutdown

• The control system of the bending magnets power
  supplies has posed various reliability problems
• It has been entirely replaced by an in-house
  design using an Internet Rack Monitor (IRM)
• New system was tested on the bench and showed
  excellent stability
• New system was also tested on two (out of 10)
  magnets before the start of the shutdown
• Installation of a ground bypass around the
  cooling section
• Uncompensated bus currents from MI ramping induce
  currents to the beam pipe, which in turn induce
  an erroneous reading of the bend fields by the
  NMR probes
• Noisy field readings
• Connect low resistance cable to the Recycler beam
  pipe
• Should reduce induced currents by a factor of 10
  (from 100 mA to 10 mA)
• Improve bending fields regulation

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Improvements/upgrades to be implemented during
the shutdown (cont)

• Shielding for cameras in the MI tunnel
• CCD cameras to be used with Optical Transition
  Radiation (OTR) detectors could not survive more
  than a few hours the level of radiation coming
  from MI losses
• Affects several diagnostics stations at the end
  of the cooling section and the return line
• Cameras will now be installed away from the MI
  beam line and shielded from direct radiation
• Upgrade of the SF6 gas recirculation system
• Present system is undersized for operational
  conditions
• Partly because of the larger volume
• New specifically designed all-in-one skid
• Operation at a lower gas temperature
• Reduction of down-time for maintenance and
  service
• Improved diagnostics to monitor the system

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Conclusion

• Although commissioning of the electron cooler
  started 1 month behind schedule, cooling
  demonstration of 8 GeV antiprotons was achieved
  2 months ahead of schedule
• 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
• Recycler not only stores antiprotons but cools
  them to the required emittances
• Increases of stack sizes are a direct consequence
  of the ability to cool the beam efficiently
  (current record 437e10)
• Electron cooling allowed for the latest advances
  in the TeV peak luminosity (current record 172e30
  cm-2 s-1)