1. Needle Cathodes for RF Guns
John Lewellen
Argonne National Laboratory
Charles Brau
Vanderbilt University

2. Applications of Interest
SASE-FEL devices and variants thereof
various beam energies
various operating wavelengths
Energy Recovery Linacs
Linear Colliders
…and so forth…

3. Beam Properties Bunch Charge Peak Current Bunch Length Longitudinal
Emittance
Beam
Brightness
Energy Spread
Spot Size
Transverse
Emittance
Spot Divergence

4. Measures of Brightness*
Normalized Brightness:
Pierce parameter:
SASE-FEL gain is proportional to r
* Martin Reiser, Theory and Design of Charged Particle Beams

5. To Increase Brightness…
Increase the Bunch Charge
Decrease the Bunch Length
Decrease the Emittance

6. Other Considerations Beam Average Power Beam Halo Power
Aperture Clearance Ratio
Longitudinal Wakes
Transverse Wakes
CSR “Wakes”

7. Thus… Decreasing the emittance is the preferred route
Increasing the charge increases many deleterious effects
Decreasing the emittance reduces many deleterious effects
Decreasing the emittance is the preferred route
So long as emittance goes down faster than charge, you win
Even if emittance and charge go down together, you may still win

8. Why a Needle Cathode?
Gradient Enhancement
Lower Emittance

9. Gradient Enhancement Eo a b

10. p-Mode Gun Design

11. Design Details Recessed needle holder Bulk cavity differences
reduced rf fields around the base
needle tip even with normal backplane
Bulk cavity differences
outer cell radii changed to maintain field balance, frequency
slightly reduced shunt impedance

12. Simulation Details Rapid changes in length scales
Model gun using three separate “cells”
Verify field continuity
Consider particle “size” (for GPT 3-d space-charge)
Use as many auto-scaling features as the sim. code permits

13. On-Axis Field Comparison
200mm flat top radius, 300mm needle radius
“Effective” needle height is ~ 1.3mm

14. How Good Must It Get? “Conventional” Gun “1st-cut” Needle Gun Q ~ 1 nC
tb ~ 10 ps
en ~ 1 mm
Q ~ 20 pC
tb ~ 2.8 ps
en ~ 0.14 mm

15. What Next? Try alternate gun geometries (e.g. higher-order-mode)
Radius the cathode for higher field
Check results with other simulation codes (e.g. GPT)
Improved optimization routines
Optimization including field maps