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Needle Cathodes for RF Guns

Needle Cathodes for RF Guns. John Lewellen Argonne National Laboratory Charles Brau Vanderbilt University. Applications of Interest. SASE-FEL devices and variants thereof various beam energies various operating wavelengths Energy Recovery Linacs Linear Colliders …and so forth….

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Needle Cathodes for RF Guns

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  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

  11. 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

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