Needle Cathodes for RF Guns

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

• 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
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Design Details

• Recessed needle holder
• 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?
1st-cut Needle Gun
Conventional Gun
Q 20 pC tb 2.8 ps en 0.14 mm
Q 1 nC tb 10 ps en 1 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