electron gun
TRANSCRIPT
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Introduction to Electron Guns for Accelerators
Bruce Dunham
February 25, 2008
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Outline•The Basics
•Cathodes
•Details of Different Accelerator Guns
•RF Guns
•SRF Guns
•DC Guns
•Polarized Electron Guns
•Photoemission Guns for an ERL
•High Voltage
•Vacuum
•Laser System
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Components of an electron gun
• A cathode of some type for generating electrons
• A focusing structure
• An anode (with or without a hole)• Vacuum
• Accelerating voltage
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DC - pulsed
DC Type Guns
DC – always on
DC – amplitude, frequency modulated
time
curr
ent
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RF Type Guns
CW – bunch of e- in every RF bucket, typically from 100’s of MHz to GHz, up to 100’s of pC per bunch
pulsed – not every RF bucket is filled, RF frequencies of 100’s of MHz to GHz, up to ~nC per bunch, with bunch rep rates of Hz to 1 MHz
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Other properties we care about
• average current
•Peak current
•Pulse length
•Emittance (beam quality)
•Reliability (lifetime)
•Physical size
•Cost
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“I Want...”• Lower emittance
– higher gradient
– lower QE
– faster cathode– lower bunch charge
– smaller emission radius
• Larger duty factor
– lower gradient
– higher QE– lower bunch charge
• Higher charge per bunch
– higher gradient
– higher QE– larger emission radius
No matter what – people always want more!
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Electron Gun Examples
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Microwave Oven Power Source
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Cutaway rendering of a color CRT: 1. Electron guns 2. Electron beams 3. Focusing coils 4. Deflection coils 5. Anode connection 6. Mask for separating beams for red, green, and blue part of displayed image 7. Phosphor layer with red, green, and blue zones 8. Close-up of the phosphor-coated inner side of the screen
Cathode Ray Tube Electron Gun
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Comet Corp. industrial x-ray tubes
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SLA
C K
lyst
ron
Ele
ctro
n G
un
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Cathodes
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Types of Electron Emission
•Give the conduction band electrons extra energy
•Thermionic emission
•Photoemission
•Secondary emission
•Change the potential barrier
•Field emission
•Plasma emission
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R.B Miller SureBeam Corp
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R.B Miller SureBeam Corp
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Tungsten is one of the most common thermionic emitters. A good figure of merit is how many electrons can be produced per mass of evaporated cathode surface that evaporates –before it break!
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Tungsten can operate at high temperatures, but still has a high work function –what else can be used?
-oxide coated tungsten (W=1.6eV), but they are sensitive to vacuum and brittle
-dispenser cathodes
porous tungsten impregnated with
BaO, CaO
maybe coated with Ir, Os, Rh
scandate - 5% by wt of Sc2O3, can generate 10s of A/cm2 for 1000’s of hours
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EIMAC (CPI)
Dispenser Cathodes
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OFE Copper
Porous Tungsten
317L Stainless Steel
304L VAR Stainless Steel
304L Stainless Steel
Cupronickel
Moly-Manganese Ceramic Metalization
Aluminum Oxide
SLA
C K
lyst
ron
Ele
ctro
n G
un
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R.B Miller SureBeam Corp
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Photoelectric EffectEinstein won his Nobleprize for work on thephotoelectric effect
• Electrons produced by shining light on surface of metal– Below threshold energy (wavelength) no
electrons are emitted
– Above threshold, electron energy is the same at any color (wavelength) of light independent of intensity
• Einstein proposed that this is due to the particle nature of light, predicted energy dependence of electrons on incident light wavelength
• Electrons emitted from metals are not polarized
Band structure
Surface of metal
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Courtesy Dave Dowell (SLAC)
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Types of Photocathodes
Metals – low efficiency, good time response (prompt), resistant to contamination, need UV laser (copper, Mg)
Semi-conductors – high efficiency, slower time response, sensitive to contamination, visible/IR lasers (GaAs, Cs2Te, K2CsSb, GaN)
QE = # electrons emitted/#incident photons
= S(mA/W) hν/e = 1.24 S(mA/W)/λ(nm)Quantum Efficiency is the figure of merit for photocathodes
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Photonis.com – cathodes used in photomultiplier tubes
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http://en.wikipedia.org/wiki/Photocathode
Night Vision Goggles
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http://www.ireap.umd.edu/FEL/Research/Photocathode.htm#thermionic
Combined Dispenser cathode with photoemission
laser
Heat the cathode up to just below the threshold for emission, then use the laser energy to emit a pulse of electrons
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Diamond Amplifier
http://www.bnl.gov/cad/ecooling/DAP_principles.asp
0
100
200
300
400
500
600
0 5 10
Gradient (MV/m)
Ele
ctro
n tr
ansm
issi
on g
ain
5keV
4keV
3keV
2keV
www.agsrhichome.bnl.gov/AP/BNLapSeminar/2005_sept16.ppt
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Ecath = 120 MV/mτlaser = 2.7 ps rmsσlaser = 0.5 mm rms
Ecath = 43 MV/mτlaser = 5.8 ps rmsσlaser = 0.85 mm rms
Ecath = 8 MV/mτlaser = 13 ps rmsσlaser = 2 mm rms
pulsed!
Electron Guns for Accelerators
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31/2 cell SRF gun Rossendorf
He-vessel
power coupler
HOM filter
gun half-cell
choke filtercathode
cathode cooler
LN2 reservoir
Cathode transfer rod
solenoid
cryostat
SRF gun
Bmax = 110 mTFriedrich Staufenbiel
Forschungszentrum Dresden-RossendorfZentralabteilung Strahlungsquelle ELBEPF 510119, 01314 [email protected]
ERL 2007, May 21 – 25
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Cathodes for Superconducting RF cavities
Jlab Half-Cell
DESY Half-Cell
Courtesy John Smedley (BNL)
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BNL SRF Gun Design
Photoinjector designed to deliver 2 MeV beam, 5 nC/bunch at 9.38 MHzAndrew Burrill, ERL 2007 Workshop
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BNL Gun Cryomodule
Andrew Burrill, ERL 2007 Workshop
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Photocathode choice and challenges
• CsK2Sb is cathode of choice, with a diamond amplified photocathode as the next generation cathode
• Lots of experience with CsK2Sb photocathode deposition, extensive R&D on diamond amplified photocathode Proper engineering
and designParticulate and interface to gun
Actively cooled cathode stalk
Thermal isolation
No vacuum degradation
Cathode lifetime
SolutionsChallenges
Andrew Burrill, ERL 2007 Workshop
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BNL Photocathode deposition system
Designed by AES
Andrew Burrill, ERL 2007 Workshop
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PITZ NC RF gun, 1 nC, 10 kHz
Normal Conducting RF GunM
uch
simpl
er th
an a
n SC R
F G
un!
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Boeing RF Gun
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Los Alamos High Avg Current RF Gun
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Cathode insertion system
External beamline components
Cathode position
Coaxial center conductor
LBNL VHF quarter-wave coaxial cavity gun
65 MHz CWNormal conducting
Cu-plated Al
Large outer diameter provides good accessibility for ion- and cryo-pumps.
All metal structure suitable for bake-out.
RF power sources can be based on commercially available broadcast tubes.
Steve Lidia, LBNL, ERL 2007 Workshop
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Frequency 65 MHzPulse rate CWGap Voltage 0.6-1.0 MVUnloaded Q 3.5x104
Effective Gap Length 4 cmRange of field in planar gap 15-25 MV/mCavity length 1 meterCavity diameter 1.4 mInner conductor diameter 0.3 mRF power for 0.75 MV on gap 65 kWPeak wall power density 7 W/cm2
Vacuum 10-11 TorrRequired pumping speed 25000 liter/secStored energy 5-8 J
Example VHF gun performance
Temperature
Displacement
~1mm
21 °C
75 °C
Steve Lidia, LBNL, ERL 2007 Workshop