Injector RF Design Review November 3, 2004 John Schmerge, SLAC  and 0 Mode Interaction in RF Gun John Schmerge, SLAC November 3, 2004 LCLS 1.6 Cell Gun 2 resonant frequencies Bead drop results Full and half cell field probes Measurements Time Domain Frequency Domain Calculations No pulse shaping With pulse shaping Summary

Injector RF Design Review November 3, 2004 John Schmerge, SLAC Multiple Modes 2 modes with different resonant frequencies and different longitudinal field profiles. f  – f 0 = 3.5 MHz on GTF gun Beating observed during gun filling time (transient effect) Presence of 0 mode affects e-beam Time dependent ratio of full cell to half cell field due to the 180˚ phase shift between the two modes affects longitudinal phase space. Additional transverse fields in the cell-cell aperture affect transverse phase space. Measure each mode Measure in time or frequency at a particular longitudinal position (RF probes). Measure as a function of longitudinal position averaged over time (bead drop).

Injector RF Design Review November 3, 2004 John Schmerge, SLAC Bead Drop Measurement

Injector RF Design Review November 3, 2004 John Schmerge, SLAC Laser Port RF Probe Location on GTF Gun Half Cell Probe Full Cell Probe Waveguide Feed

Injector RF Design Review November 3, 2004 John Schmerge, SLAC Time Domain Measurement Temporal Response with ≈ 6 MW incident power E cathode ≈ 90 MV/m

Injector RF Design Review November 3, 2004 John Schmerge, SLAC Time Domain Measurement Temporal Response with ≈ 6 MW incident power E cathode ≈ 90 MV/m

Injector RF Design Review November 3, 2004 John Schmerge, SLAC Frequency Domain Measurements Full Cell ProbeHalf Cell Probe

Injector RF Design Review November 3, 2004 John Schmerge, SLAC Transfer Function V out /V in Full CellProbeHalf Cell Probe  mode Sum (full cell) or difference (half cell) of two second order band pass filters

Injector RF Design Review November 3, 2004 John Schmerge, SLAC Output Response Equations  mode only  and 0 mode

Injector RF Design Review November 3, 2004 John Schmerge, SLAC GTF Measurements and Fits from 1998 Fit Gun Field WaveformFit Reflected Power Waveform

Injector RF Design Review November 3, 2004 John Schmerge, SLAC Step Function Response Steady state response at applied frequency Transient response at resonant frequencies

Injector RF Design Review November 3, 2004 John Schmerge, SLAC Transient Response Including 0 Mode f = MHz f  = MHz f 0 = MHz Q 0  = Q 0 0 =   = 1.3  0 = 0.7   = 576 ns  0 = 779 ns f = MHz f  = MHz f 0 = MHz Q 0  = Q 0 0 =   = 2.0  0 = 1.1   = 440 ns  0 = 637 ns GTF LCLS

Injector RF Design Review November 3, 2004 John Schmerge, SLAC Transient Response With Pulse Shaping f = MHz f  = MHz f 0 = MHz Q 0  = Q 0 0 =   = 1.3  0 = 0.7   = 576 ns  0 = 779 ns f = MHz f  = MHz f 0 = MHz Q 0  = Q 0 0 =   = 2.0  0 = 1.1   = 440 ns  0 = 637 ns GTFLCLS

Injector RF Design Review November 3, 2004 John Schmerge, SLAC Transient Response With Pulse Shaping f = MHz f  = MHz f 0 = MHz Q 0  = Q 0 0 =   = 2.0  0 = 1.1   = 440 ns  0 = 637 ns f = MHz f  = MHz f 0 = MHz Q 0  = Q 0 0 =   = 2.0  0 = 1.1   = 440 ns  0 = 638 ns LCLS Adjusted timing LCLS Increased  f

Injector RF Design Review November 3, 2004 John Schmerge, SLAC Frequency Domain Flat klystron pulse (3  s)With pulse shaping (0.56  s)

Injector RF Design Review November 3, 2004 John Schmerge, SLAC Summary 0 mode produces a measurable 3.5 MHz beating on the gun field Effect more significant with short pulses used for rf pulse shaping Estimated time dependent full cell to half cell field ratio varies by less than 10% and for special cases less than 2%. Effect depends on the filling time (coupling coefficient), mode separation and exact input waveform. Timing can be adjusted to minimize beating. Mode beating can be advantageous as it allows for variable field balance depending on laser timing Shot to shot jitter on the beating will be dominated by fluctuations on the klystron input pulse. Measure beating at GTF with rf pulse shaping.