High-Current Polarized Source Developments

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High-Current Polarized Source Developments

• Evgeni Tsentalovich
• MIT

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OUTLINE

• Introduction, motivation
• EIC requirements
• Developments at MIT-Bates
• RF gun developments
• Conclusion

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eRHIC (Linac-ring version)

• Polarized electron source with an extremely
  high current crucial element of the project

Average laser power 80 W (fresh
crystal) Hundreds Watts might be needed as
crystal loses QE
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Main challenges
High average current cathode damage by ion
bombardment
High heat load on the cathode tens of Watts of
laser power
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High Average Current

• Average current of tens or even hundreds of mA is
  required
• Modern state-of-the-art guns produce 100-200 ?A
• Average current of 1 mA achieved in tests at
  JLab and Mainz lifetime 20 h
• Average current of up to 10 mA achieved at Mainz
  with very short lifetime (needs active cathode
  cooling)

Main problem ion backbombardment
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Ion Damage
Ion damage is inversely proportional to emitting
area
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Damage location
Electrons and ions follow different trajectories.
Usually, ions tend to damage central area of the
cathode.
Ring-like cathodes ?
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Ion Trapping in CW Beam
Cathode
Beam line
Anode
Ions produced below the anode are trapped in the
electron beam. Half of them will drift toward the
gun and get accelerated in the cathode-anode gap
toward the crystal.
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JLAB results for anode biasing
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High Intensity Gun Studies at MIT/Bates

• The project investigates the feasibility of
  extracting very high (tens, perhaps hundreds of
  mA) current from the gun.
• The project addresses issues of high average
  current and high heat load on the cathode.
• Phase I studies of ion damage, design and
  construction of the cathode cooler, gun
  simulations.
• Phase II design and construction of the gun and
  the beam line, beam tests.

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Ion Damage Studies - Apparatus

• Existing gun.
• New diode array laser (?808 nm, P up to 45 W).
• Existing test beam line. This beam line was not
  designed for high current and beam losses of
  5-10 are typical. These losses produce
  out-gassing, and reduce the lifetime by both
  poisoning the cathode and ion bombardment.
  Relatively low lifetime and significant ion
  damage allowed to conduct the measurements fast.
• CW current one can expect ion trapping.

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Ring-shaped Laser Beam
Fiber
L1
L2
Axicon
Cathode
Axicon (conical lens) in combination with a
converging lens (L2) produces ring-shaped beam in
the focal plane of L2. Lens L1 reduces the laser
beam divergence (25? from the fiber). Without
axicon, a very small beam spot will be produced.
QE could be mapped by moving the L2
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Axicon-based System Simulations
L1
L2
Axicon
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Axicon-based System Simulations
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Beam Profile (no axicon)
FWHMlt.5 mm
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Axicon Beam Profile
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Axicon Beam Profile
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Axicon Beam Profile
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QE map of the Fresh Crystal
QE,
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QE change (small spot in the center)
Run 12.32 C
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QE change (run with axicon)
Run 17.35 C
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QE change (axicon, anode biased 1 kV)
Run 17.62 C
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QE change (large spot in the center)
Run 17.46 C
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QE change (small spot in the corner)
Run 16.84 C
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Radial distribution
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Lifetime
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High Intensity Run (1 mA)

• Achieved .5 mA with laser power of .25 W (QE.34
  )
• Achieved 1 mA with laser power of 1.16 W
  (QE.15)
• Gun vacuum pressure rise (factor of 10)
• Current dropped to 132 ?A in 1 hour
• At laser power of 1.16 W, QE degrades even
  without HV ! Overheating.
• Thermal estimate (thermal conductivity through
  the stalk only
• .01-.025 W/degree

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Conclusion

• Ion damage is concentrated near the center of the
  cathode in every configuration.
• Ring-shaped beam allows to improve the lifetime
  significantly.
• Biasing the anode improves the lifetime of the CW
  beam.
• Active cooling is a must for laser powers
  exceeding 1 W.

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New optics
Old optics Small spot Axicon New optics
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Gun Simulation

• Large emitting area produces large emittance
• Although emittance is less important for eRHIC,
  large beam could result in beam losses near the
  gun.
• The main purpose of the simulations is to
  minimize the beam losses in the gun and beam
  line.
• The second goal ion distribution optimization

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Gun Simulations - Ions
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Gun Simulations - Ions
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Gun Simulations - Ions
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Cathode Cooling

• The conceptual design of the test chamber is
  completed.
• The test chamber will validate the adequacy of
  the cooling power, HV and high vacuum
  compatibility and vacuum cathode handling with
  manipulators.

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Cathode Cooling
HV
Water in
Water out
Manipulator
Crystal
Cathode
Laser
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DBR Equipped Crystal
Normal cathode
Cathode with Distributed Bragg Reflector (DBR)
In normal cathode, only 30 of light is
reflected. In DBR-equipped cathode 99 of light
is reflected.
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Polarized RF guns

• No positive results as yet, and very few
  attempts.
• Very high average current is not expected.
• Main advantages high brightness, low emittance,
  high electron energy. High peak current could be
  achieved.
• Several normal-conducting RF gun projects are
  under way (SLAC, JLAB). High Order Mode (HOM)
  and Plane Wave Transformer (PWT) concepts are
  used. These concepts allow improved vacuum
  conditions.
• BNL develops Superconducting RF gun. Two versions
  are under way.

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BNL_at_AES SRF gun
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Conclusion
Very significant results are expected in the next
couple of years !