Operational Experience with the Cornell ERL Prototype DC Electron Gun

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Operational Experience with the Cornell ERL
Prototype DC Electron Gun

• I. Bazarov, B. Dunham, Heng Li, Yulin Li, X. Liu,
  D. Ouzounov, C. Sinclair, K. Smolenski
• Cornell University
• Laboratory for Elementary-Particle Physics

Supported by National Science Foundation grant
PHY 0131508
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Outline

• Experience gained from 2 years of operations with
  prototype DC gun
• Problems and lessons learned
• Remaining challenges
• Related talks
• High Voltage Power Supply Uwe Uhmeyer, Kaiser
  Systems (today)
• Laser System Dimitre Ouzounov / Heng Li,
  Cornell (Friday)
• Vacuum Charles Sinclair, Cornell (Friday)
• NEG Pumping Paolo Manini, SAES Getters (Friday)

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Towards a high brightness x-ray source using an
energy recovery linac
Without Energy Recovery 5 GeV 100 mA 500
MW of power!
With Energy Recovery 15 MeV 100 mA 1.5 MW
of power
A superconducting LINAC is required for high
energy recovery efficiency
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The Energy Recovery Linac Project at Cornell

• Injector
• Linac 1
• 2.5 GeV turn-around
• Linac 2
• 5 GeV transfer line
• 5 GeV turn-around (CESR)
• 5 GeV transfer line
• Beam dump

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Injector Requirements
Current status Maximum achieved voltage 440kV
/ Typical operation 250-300kV Maximum achieved
current 20mA (with DC laser) Emittance 1.8
µm rms normalized
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The ERL Phase 1A Project Cornell ERL Injector
e2V 100 kW 1.3 GHz klystrons
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ERL DC Photoemission Gun - High Level Requirements
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Photocathode Gun Geometry
16.5 inch flange
-750 kV
Insulator
Basic design has been used in GaAs polarized
electron sources for decades ( _at_ 100 kV).
Electron beam
GaAs Cathode
Laser input
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Photocathode Gun Geometry

• Cathode cooling via conduction link to SF6
  space.
• Optical alignment of electrodes for long
  transfers.
• 30 minute cathode swap.

Cathode Swap
Electron beam
GaAs Cathode
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GaAs Photocathodes

• GaAs is still our cathode of choice . . .
• - good quantum efficiency (QE)
• low thermal emittance (cold)
• fast time response (_at_ 520 nm)
• But . . .
• need extreme UHV
• limited lifetime (200 hours)
• minimum thermal emittance near bandgap (where
  the QE is lowest)
• thermal emittance degrades at higher QE
• . . . We are willing to try other cathodes

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Cathode Preparation System

• Series of chambers to load, clean and prepare
  cathodes
• Long (1m) orthogonal translators to move cathode
  puck through preparation system and then install
  in gun
• Designed for flexibility in choice of cathode
  system, cleaning, and preparation

Load Lock
Heater / Hydrogen cleaning
Surface Preparation, QE mapping, Storage
Translation into Gun
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Preparation System Improvements

• Easy storage of multiple cathode pucks
• Faster / easier motions (magnetic translations
  vs. mechanical / bellows)
• Better vacuum isolation between chambers
• Modular design to allow addition of processing
  chambers for K2CsSb development
• Horizontal cathode surface in heat cleaning stage
  (to prevent Indium solder flow)

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Vacuum System

• 400 l/s Perkin-Elmer ion pump
• Massive NEG pumping for H2 - using 20 modules of
  WP1650 ST 707. (740 l/s x 20) 15,000 l/s
• 400C air firing of vessel and internal
  components, followed by 160C bakeout
• 5 x 10-12 Torr typical static pressure

Reduction in hydrogen outgassing from stainless
steels by a medium-temperature heat treatment J.
Vac. Sci. Technol. A 26 (5), Sep/Oct 2008
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Insulator Design
-750 kV

• Large size to keep field gradients low
• Field emitted electrons can build up on the
  insulator and punch through (2 events on each
  ceramic)
• Some correlation between segmented stalk and
  punch through sites.
• External SF6
• High mechanical stresses due to SF6 pressure and
  bakeouts
• Difficult to find suppliers
• Braze difficulties due to large size

600mm
e-
e-
12 MV/m at 750kV
Manufactured by CPI, Beverly, MA
Exploring alternate designs
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Insulator Failures
Flange
Vacuum failure during cool down from extended
bakeout 250C
Ceramic
Braze
Abutment face Flange to Ceramic
Copper ring
TIG weld (3)
Kovar ring B
Kovar ring A
Braze (2)
Ceramic
Braze (1)
Abutment face Flange to Ceramic
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Resistive Insulators
Resistive surface coatings
Resistive alumina

• Thick enough to stop gt500 kV electrons and drain
  them away (need at least 1 mm thick coating)
• Not so thick as to draw too much current (thermal
  run away)
• Resistance drops non-linearly with voltage
• Dust from coating has limited us so far
  processing events ablate coating and it coats
  electrodes leading to increased field emission
• Good up to 440 kV

• This material from Morgan (Wesgo) has worked well
  for Daresbury currently fabricating a new
  insulator using it.
• Expected test Spring 2009.

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Bulk Resistivity Insulators

• Currently fabricating 2 bulk resistivity
  ceramics. Raw ceramic produced by Morgan /
  Wesgo. Final brazing by CPI Beverly.
• One in the Cornell size 16.5 CF x 24 length, a
  second identical to the Daresbury unit on 14 CF
• Expect finished parts early 2009

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Segmented Insulators
Toshiya Muto, KEK / JAEA ERL Project. Segmented
insulator to be built by Hitachi For 500kV DC Gun
Kyocera for JAEA Ø435mm x 515mm
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Segmented Insulators

• Currently working towards the design of a
  segmented insulator Opera calculations, still
  requires vacuum / braze joint design
• Trying to identify possible vendors Kyocera,
  Hitachi, National Electrostatics Corp., Haimson
  Research

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750 kV HV power supply, SF6 tank
Custom floating ammeter to measure field emission
current during processing
Gun and HVPS inside a high pressure SF6 vessel
750 kV, 100 mA supply Kaiser Systems, Beverly, MA
Good for operations, not ideal for processing
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Processing Lessons

• Diagnostics available during processing RGA,
  Vacuum pressure (lt1x10-9T) , Floating ammeter (lt
  50µA), Radiation monitor probes (PMT) surrounding
  gun to determine direction.
• Typical process rate of 1 kV/hr.
• He processing very effective, allows for quick
  removal of stubborn field emitters, (1x10-4T He)
  but requires care.
• Leaks at ceramic bottom flange solved by
  extending protection ring to cover gasket joint.

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Electrode Testing and Preparation
0 to -125 kV
Test
Electrode
3-4 mm
anode
Pico-ammeter
150 mm
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Large Area Electrode Testing and Preparation
High pressure water rinsing is the key to
removing particles that cause field emission!
Hand polished SS (pink)
Hand polished 316LN SS, high pressure water rinse
(blue)
Electropolished 316LN SS, high pressure water
rinse (yellow)
High pressure (1000 psi) water rinser
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High Pressure Rinsing

• 3-4 hours of exposure to sweeping 1000 psi jets
  of filtered DI water.
• HPR of electrodes done in class 100 cleanroom.
• Larger pieces stalk and gun vacuum chamber in
  Class 1000 cleanroom.
• Final assembly of gun in class 100 cleanroom,
  sealed and transported to experimental floor.

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Future work

• Continue tests of novel materials and coatings
  Niobium electrodes, TiN coatings
• Continue development of electropolishing for SS,
  Ti, and Nb electrodes
• Further photocathode materials development
  K2CsSb
• Robust ceramics can survive field emission and
  multiple vacuum bake cycles

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Towards a MV gun

• What are the beam quality benefits of a MV gun?
• Where is the point of diminishing returns?
• What are the technical hurdles that need to be
  resolved?
• Inverted and novel geometries
• What collaborations can be formed to achieve
  these goals and meet the requirements?
• Lunch and afternoon discussion sessions

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ERL 2009 Workshop
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