1. Ampere-class, High Quality RF Electron Source
Xiangyun Chang
AAC 2016, Gaylord National Convention Center, National Harbor, MD August 2, 2016

2. INTRODUCTION
High peak and average current, high quality bunched electron beams are of high interest for modern accelerator based applications:
E-cooling of ion beams
High average power Free Electron Lasers (FELs), Terahertz light sources.
Industrial accelerator applications: treatments on water, biosolids, medical waste, etc.
Existing main electron source technologies are very difficult to meet the requirements.
Photocathode DC/RF guns: expensive, hard to generate high average current beam. State-of-the-art: 75 mA. Lifetime: short.
Thermionic RF guns: have typically 10-5 duty cycles due to the back-bombardment. Poor beam quality, α-magnet must be used.
Other electron source technologies. Not yet demonstrated high average capability.
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3. Thermionic RF Gun Issue: Back-bombardment
Cathode
A typical RF gun (BNL/SLAC/UCLA 1.5 cell gun). f=2856 MHz, Epeak=100 MV/m
Electron energy at gun exit or back-bombardment (BB) energy vs. cathode launching phase (initial phase).
Electrons from section I and section II can escape from the cavity.
Electrons from section III strike back on cathode (back-bombardment) under decelerating field.
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4. Thermionic RF Gun Issue: Back-bombardment
Back-bombardment power density is large (a few - tens kW/mm2 ) and is directly on cathode surface.
Rapid temperature increase of the cathode during the RF macro pulse causes the current to increase and the beam energy to decrease
Typical macro-pulse width is a few μs with 10-5 duty cycle.
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5. Thermionic RF Gun Issue: Emittance Growth
Phase space of section I beam
Phase space of section II beam
Section I beam (from 0° to somewhere after ϕPeak) has small emittance
Section II beam has large emittance. α-magnet needed.
To achieve high-average, high-quality beam:
Eliminate back-bombardment to allow CW operation
Suppress section II beam to obtain high quality beam even without α-magnet
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6. FAR-TECH’s approach: 2 core Steps
Step I, a short accelerating gap RF cavity
Obtain high peak field on cathode even for a normal conducting cavity.
Pushes ϕPeak close to 90° (75°) and ϕBB close to 180° (152°).
Still has considerable back-bombardment power.
A short accelerating gap 476 MHz RF gun
Energy at gun exit vs. initial phase
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7. FAR-TECH’s approach: Step II
Step II, use a floating grid cathode.
A thin metal film is perpendicular to RF field
ETot= ERF + EDC
QNet (therefore EDC) is adjustable if we have both tunable charging/discharging channels.
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8. Emission Window of A Floating Grid Cathode
Small ICH (IDS)
Large ICH (IDS)
Consider a “floating grid” with negative bias is placed in front of the thermionic cathode
At sufficient high EDC, ϕEnd<ϕBB, the back-bombardment beam is eliminated and section II beam is suppressed.
Allows CW operation, every RF bucket filled, high average current.
Small emittance.
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9. How to provide the grid charging/discharging channels?
Grid charging can be simply the interception of the main beam by the grid
How about grid discharging?
FAR-TECH’s solution:
Attach an emitter on the “floating grid”.
Thermionic, Field Emitter Array (FEA), etc.
Control the discharging anode bias.
As long as the discharging emitter has enough emission:
Adjusting the DC bias through an external power supply >>> control EDC >>> control emission phase window >>> eliminate back-bombardment for CW operation to obtain high average current; suppress section II beam for high quality beam.
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10. Emittance of Multiple Beamlets
Emittance degrade due to the multiple beamlets effects can be improved by the double-grid technique demonstrated in an DC double-grid FEA.
Single-gate emittance: 1 mm.mrad / 1 mm size FEA*.
Double-gate emittance: 0.1 mm.mrad / 1 mm size FEA*.
*: P. Helfenstein et al. J. Appl. Phys. 113, (2013).
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11. Double-grid thermionic RF gun
Double grid thermionic RF gun cathode
“Floating grid” allows double or more grids be used in thermionic RF gun. A good quality beam from the double-grid thermionic RF gun is expected.
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12. Double-grid FEA RF gun
Double-gate FEA could be directly applied to an RF gun provided the tips have enough field gradient ~ GV/m. Based on the demonstrated low emittance of the DC field FEA, it is reasonable to expect a low emittance beam from this “floating grid” FEA RF gun.
A lower work function material can lower the required tip field gradient dramatically. High tip length over tip diameter ratio + reentrant cathode stalk could meet the field gradient requirement.
FEA tip current density vs. RF phase. EPeak = 1.4 GV/m, work function 2.65 eV [LaB6].
Double grid FEA RF gun cathode
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13. Wide emission bunch length issue
From results of a similar technique, Inductive Output Tube (IOT), the emission phase window (bunch length) from a “floating grid” RF gun could be 30° to 60°. Too large for low energy spread and small emittance.
One of the possible solutions:
Choose half gun frequency for the rest cavities.
Let the beam bunches be on peak accelerating and peak decelerating phase alternately in the first half frequency cavity.
Use a dipole magnet to separate the bunches, dump the lower energy bunches and let the higher energy bunches enter the downstream cavities.
A higher frequency cavity may be used to further improve the beam quality.
Results: bunch length becomes 15° to 30° for the booster cavities, while it sacrifices the average current by half. Fortunately our source could be designed fairly big to allow this sacrifice.
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14. A few points in the design
Use a mask on main emitter of the thermionic “floating grid” RF gun to reduce the charging/discharging currents.
Discharging anode DC bias power supply for negatively biased “floating grid” RF gun is simply a variable resistor. System is simple and therefore reliable. It also has an intrinsic stable mechanism:
Arcing will be suppressed automatically.
Arcing falling in back-bombardment phase is less likely to happen.
It is preferred to have larger size emitter to achieve higher average current, a larger size emitter requires a larger aperture and larger gap accelerator, therefore a lower frequency cavity is preferred. For an a few hundred mA or more average current gun, it is recommended that the RF frequency be less than 1 GHz.
Neither a thermionic nor a FEA requires an ultra-high vacuum. Their lifetimes are expected to be very long and are easy to be replaced.
FAR-TECH, Inc. has done many pioneer work on this study. The preliminary results supports our model
“Floating grid”
Main emitter mask
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15. An example
Assume we need a thermionic RF gun with 1 A average current, assume the effective emission area is 1/3 of the main emitter area after adding the main emitter mask, the emission window of the gun is controlled to be 45°, and the main emitter diameter is 15 mm, then the emission current density of the main emitter needs to be 13.6 A/cm2, which is reasonable for many types of thermionic cathodes, such as a LaB6/CeB6.
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16. Another example
Consider a single hole “floating grid” RF gun, cathode is a thermionic-field-emission type cathode with a mm tip, the cathode induced emittance of < 0.1 µm and average current of up to ~ 100 mA is expected. Emittance will be dominated by the RF induced emittance and bunch compression induced emittance.
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17. Summary
The “floating grid” thermionic RF gun and FEA RF gun technique is based on mature or demonstrated techniques such as the IOT technique and double-grid FEA technique.
This technique provides a new option for high average-current, low emittance bunched electron source, with many attractive characteristics:
achieve very high average current, up to more than 1 A possible,
low emittance, better than that of a photocathode RF gun possible with the double-grid technique,
stable,
long lifetime, (> 10,000 hrs),
easy to fabricate, operate, replace, and maintain,
cheap.
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18. Thank you!
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19. 300 MHz cavity Frequency: 300 MHz Acc. gap: 3 cm Aperture: 2 cm
ECathode: 12 MV/m
EPeak: 21 MV/m
r/Q: 180 Ω
PTotal: 10 kW
Peak power density:
18 W/cm2
Beam energy: 235 keV
ϕPeak = 82°
ϕBB = 159°

20. 75 MHz cavity Frequency: 75 MHz Acc. gap: 4 cm Aperture: 2 cm
ECathode: 11 MV/m
EPeak: 20 MV/m
r/Q: 200 Ω
PTotal: 8 kW
Peak power density:
1.5 W/cm2
Beam energy: 330 keV
ϕPeak = 87°
ϕBB = 168°

21. Proprietary Current density vs. temperature
Current density enhancement by field
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