Field Emission Cathode
Gating for RF Electron Guns
J.W. Lewellen and J. Noonan
Accelerator Systems Division
Advanced Photon Source
Argonne National Laboratory
Argonne National Laboratory
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A U.S. Department of Energy
Office of Science Laboratory
Operated by The University of Chicago
Thanks and Acknowledgements
Sandra Biedron (ANL/ES)
Court Bohn (NIU)
Dave Dowell (SLAC)
Katherine Harkay (ANL/ASD)
Terry Maynard (ANL/IP)
Todd Smith (Stanford)
Various drive lasers of the
authors’ acquaintance, for
motivation
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Outline
• Cathode Options
• Field Emission Cathode Gating Scheme
• Conclusions & Wrap-Up
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Cathode Overview
“Ideal” Cathode Checklist:
9 Long lifetime
9 Rapidly switchable on/off
9 Damage resistant
9 High charge density
9 Cryogenic compatible
9 Simple operation
Photocathodes
Emission mechanism: Use laser pulse to
excite electrons off the cathode surface
Advantages:
9 Rapidly
switchable
9 High charge
density
Disadvantages:
8 Efficiency-lifetime
tradeoffs
8 External drive
laser required
Thermionic cathodes
Field-emission cathodes
Emission mechanism: heat the cathode to
“boil” the electron sea in the metal
Emission mechanism: Electric field pulls
electrons from cathode surface
Advantages:
9 Long lifetime
9 Robust
9 Simple to
operate
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Disadvantages:
8 Not rapidly
switchable
8 High temperature
required
Advantages:
9 Very simple
9 High charge
density
9 Rapid turnon/off
Disadvantages:
8 Problematic gating
(for rf app.)
8 Damage questions
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Beam Emission Timing
Normalized electric field
ZeroCrossing
1
1
3
0
4
1
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2
50
0
50
100
150
200
Field phase [deg]
250
300
350
400
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Field Emission Cathode Gating Scheme
FE cathodes emit electrons when the field is high enough
• operate at low temperature (unlike thermionic cathodes)
• emit only under “internal” influences (unlike photocathodes)
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Will it work?
Simulation:
- Incorporated the Shottky
emission model directly
- Also includes space-charge
effects
4
3
Current density [arb. units]
•
3
j = α⋅
(β E m )
⋅e
Φ
2
−
BΦ 2
(β E m ) 2
2
1
0
0
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50
100
Phase [deg]
150
200
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FE Cathode Gating: Other Comments
Electromagnetic field data from file AUTOCATH.SF
9
9
8
8
7
7
6
6
5
5
4
4
3
3
2
2
1
1
Electromagnetic field data from file AUTOCATH.SF
0
Problem title line 1: Design of a cathode cell for a dual-frequency, srf, electon microscope gun
autoca01.tbl
10
-2
5-17-2004 14:31:48
14
Z:\gun\design\MICROS~1\beamsim\parmela\CathPos\AUTOCATH.SF
autoca03.tbl 5-17-2004
5-17-200414:08:22
14:30:02
1.5
2.5
0
Problem
srf, 10
electon microscope
gun
0 title line 1:
2 Design of a4 cathode cell 6for a dual-frequency,
8
12
1
2
1.5
Ez
1
.5
0
-.5
.5
Ez
•
10
Ez (MV/m)
•
Proper field addition in both space
and time Æ special gun geometry
Scales well to ~ 50 mA beam
current without special focusing
Superconducting version highly
desirable for efficient use of rf
power, true CW operation potential
Ez (MV/m)
•
-1
0
0
2
4
6
8
Z (cm)
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10
12
14
0
2
4
6
8
10
12
Z (cm)
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14
Scaled Performance
Beam current
(mA)
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Comments
•
Basic idea
- Strong 3rd harmonic at the cathode to launch the beam
- Fundamental dominates in body of gun for more conventional
dynamics
- Details of emitter tbd; there are possibilties
•
Initial studies aimed at electron microscopy
- Low beam currents
- Low space charge
•
Extensions to higher currents?
- Theoretically possible
- Cathode-region focusing needed
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Conclusions & Wrap-Up
•
Field-emitter cathodes are now a possible choice for rf gun use
- first- and third-harmonic field combination to gate emission
- proper selection of recess depth and other factors to ensure
good beam propagation through the gun
•
Moderate (1 – 50 mA) average beam currents appear possible
•
Beam energies of 1.5 – 2 MeV appear possible
•
This gun design, in combination with standard phase-space
manipulations to reduce the energy spread etc., can drive a
number of relatively high-volume applications
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