Diagnostics for intense ecooled ion beams

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Diagnostics for intense e-cooled ion beams
by Vsevolod Kamerdzhiev Forschungszentrum Jülich,
IKP, COSY
ICFA-HB2004, Bensheim, October 19, 2004
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Content

• Objects of diagnostics
• What is an electron-cooled ion beam from the
  diagnostics point of view?
• Parameters to be measured and corresponding
  diagnostic methods.
• Diagnostics for electron-cooled beams,
  difficulties and advantages.

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Objects of diagnostics

• Electron beam
• High beam power
• Beam pipe is inside the solenoid
• Electron-cooled ion beam
• Intensities differ in orders of magnitude
• High beam density
• Small transverse dimensions
• Small momentum spread

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Measured parameters (e-beam)

• E-beam position
• Space charge field of the e-beam
• Current
• Temperature
• Neutralization

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Measured parameters (ions)

• Beam current
• Position along the orbit
• Momentum
• Momentum spread
• Profile
• Emittance
• Tune
• BTF

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Interceptive methods or not?

• Interceptive methods
• Not suitable for a circulating beam (operation)
• Any probe will melt when inserted in the dc
  electron beam
• Not interceptive methods
• Often indirect measurements
• Suitable for (high current) rings

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Cooler Synchrotron COSY

• COSY accelerates (polarized) protons and
  deuterons between 300 and 3700 MeV/c for p 535 to
  3700 MeV/c for d
• Kicker extraction, stochastic extraction
• 4 internal and 3 external experimental areas
• Electron cooling at low energy
• Stochastic cooling at high energies

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COSY-Cooler
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COSY-Cooler
Electron energy Up to 100 kV Electron current
0.2 - 3 A Operating at 24,5kV 100-250 mA
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LEPTA
Septum
e trap
Collector
e-gun
e source
Quadrupole
Cooling section
Detector
B?
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LEPTA
Electron gun
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Parameters of e-cooled ion beam
Longitudinal Schottky spectra, uncooled and
cooled proton beam

• Small transverse size/emittance
• High density
• Small momentum spread
• During e-cooling the ion beam is dc
• Often unstable

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Diagnostics in the cooler section

• Pick-ups (at least two) are needed inside the
  cooling section to measure the position of both
  beams.
• To measure the position of the e-beam
  longitudinal modulation must be applied
• Large dynamic range of preamplifiers (variable
  gain)
• Difficulties in mechanical design, bad service
  possibilities (COSY, LEPTA)

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COSY BPMs
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Diagnostics in the cooler section

• Count rate of the particles recombinating in the
  cooler section can be used to find optimum
  alignment of the electron and ion beams and for
  fine tuning the energy of the electron beam.
• Measurement of the profile of recombination
  particles (e.g. MWPC) is the easiest way to
  determine the ion beam profile (only during
  cooling process)

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Example of H0-profile measurement at COSY
Emittance ?m rad
Calculated from the measured H0- Profiles
Beam radius mm
Proton beam current
horizontal vertical
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Diagnostics in the cooler section

• Looking at the ? signals of the pick-up located
  in the cooling section in frequency domain gives
  useful information about residual gas ions
  oscillating in the cooler section.
• Such a pick-up can be used also as a clearing
  electrode (experience at COSY, I.Meshkov,
  A.Sidorin). Applying ac-voltage to the clearing
  electrodes makes it possible to kick out the
  trapped ions, provided the frequency corresponds
  to resonant the frequency of a particular ion
  species.

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Space charge field

• To measure the space charge parabola of the
  electron beam a low intensity cold ion beam can
  be used.
• In this case the ion beam is used as a probe
  which scans the e-beam.
• Procedure
• Inject ion beam in the machine, cool it, measure
  the revolution frequency of the ion beam, make a
  parallel shift of the e-beam using the cooler
  magnetic system, measure the frev again, repeat
  the procedure several times shifting the e-beam
  in both directions from the initial position.

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Temperature of the e-beam

• Longitudinal temperature can be derived from the
  Schottky spectrum of the cooled ion beam
• Beam heating effects should be taken into
  account
• Transverse temperature can be measured by the
  pepper pot method
• Only in the pulsed mode
• Requires complex mechanical design

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The idea of T?-measurement
The electrons move in the longitudinal magnetic
field. Method based on the measurement of
transverse Larmor radius Pulse duration 20-50
?s
The optical analysis of the electron beam
temperature, V. Golubev et all., Proceedings of
the Workshop on Beam Cooling and Related Topics,
1993.
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Profile of the ion beam

• Can be based on
• Ionization of residual gas
• Laser induced luminescence
• Laser induced photo-neutralization
• Light radiation of residual gas, exited by the
  beam particles
• Wire scanner

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Ionization profile monitor
If collecting the electrons additional magnetic
field is required. Position sensitive detectors
are usually based on the MCPs. For dense beams
MCP life time is a crucial issue.
IPMs are installed in TSR, SPS, COSY,RHIC
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IPM at COSY
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Beam Profiles measured in COSY
Profile measurement
Electron cooled proton beam
The proton beam is not cooled
1,3109 particles in the ring, 45 MeV.
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Experience with IPM at COSY

• For the WS anode high amplification factor is
  necessary
• Use of two MCPs in chevron geometry
• High electron density in the second MCP
• Short life time of the MCPs
• Limitations on beam current
• Protection screen is installed
• Triggering of the MCP power supply is applied
• Using an MCP with a phosphor screen is probably
  the best way to build a position sensitive
  detector for IPM

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Laser profile monitor

• Laser induced luminescence (for ions) in
  connection with laser cooling (ASTRID)
• Watching the light using a camera
• Photo-neutralization for H- beam (LANL, BNL,
  ORNL)
• A tightly focused laser beam is directed
  transversely through the beam, causing
  photo-neutralization.
• Scanning the ion beam with the laser and
  simultaneously measure the beam current

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PM based on light radiation of residual gas,
exited by the beam particles
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Spectral analysis of the beam signals

• ?-signals of a pick-up in frequency domain give a
  lot of information
• Exiting the beam und measuring the betatron
  frequencies gives the tune
• Stability information can be obtained using the
  Beam Transfer Function (BTF) method
• Electron cooling improves S/N ratio

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Example of the beam spectrum at COSY
Vertical delta signal
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Vertical BTF
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Transverse stability diagram
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Longitudinal BTF at COSY
For different proton beam currents 0,8 mA 2,7
mA 4,5 mA
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Transverse BTF at COSY
Beam current 3,2 mA 2,5 mA
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Summary

• Electron cooling gives much better S/N ratios
• Schottky diagnostics is a very powerful method
• Schottky spectra of an e-cooled ion beam might
  be strongly distorted
• BTF, longitudinal and transverse
• Online BTF measurement should be further
  developed
• Profiles of an e-cooled ion beam are difficult to
  measure
• Better resolution is needed
• Life time of MCPs
• For new machines diagnostic must be planed
  together with the machine design

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• Thank you