p and 0 Mode Interaction in RF Gun John Schmerge, SLAC November 3, 2004

1. p and 0 Mode Interaction in RF Gun John Schmerge, SLACNovember 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 John Schmerge, SLAC

2. Multiple Modes • 2 modes with different resonant frequencies and different longitudinal field profiles. • fp – f0 = 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). John Schmerge, SLAC

3. Bead Drop Measurement John Schmerge, SLAC

4. RF Probe Location on GTF Gun Waveguide Feed Laser Port Full Cell Probe Half Cell Probe John Schmerge, SLAC

5. Time Domain Measurement Temporal Response with ≈ 6 MW incident power Ecathode≈ 90 MV/m John Schmerge, SLAC

6. Time Domain Measurement Temporal Response with ≈ 6 MW incident power Ecathode≈ 90 MV/m John Schmerge, SLAC

7. Frequency Domain Measurements Full Cell Probe Half Cell Probe John Schmerge, SLAC

8. Transfer Function Vout/Vin Sum (full cell) or difference (half cell) of two second order band pass filters p mode Full Cell Probe Half Cell Probe John Schmerge, SLAC

9. Output Response Equations p mode only p and 0 mode John Schmerge, SLAC

10. GTF Measurements and Fits from 1998 Fit Gun Field Waveform Fit Reflected Power Waveform John Schmerge, SLAC

11. Step Function Response Steady state response at applied frequency Transient response at resonant frequencies John Schmerge, SLAC

12. Transient Response Including 0 Mode f = 2856.00 MHz fp = 2856.03 MHz f0 = 2852.53 MHz Q0 p = 12000 Q0 0 = 12000 bp = 1.3 b0 = 0.7 tp = 576 ns t0 = 779 ns f = 2856.00 MHz fp = 2856.03 MHz f0 = 2852.53 MHz Q0 p = 12000 Q0 0 = 12000 bp = 2.0 b0 = 1.1 tp = 440 ns t0 = 637 ns LCLS GTF John Schmerge, SLAC

13. Transient Response With Pulse Shaping f = 2856.00 MHz fp = 2856.03 MHz f0 = 2852.53 MHz Q0 p = 12000 Q0 0 = 12000 bp = 1.3 b0 = 0.7 tp = 576 ns t0 = 779 ns f = 2856.00 MHz fp = 2856.03 MHz f0 = 2852.53 MHz Q0 p = 12000 Q0 0 = 12000 bp = 2.0 b0 = 1.1 tp = 440 ns t0 = 637 ns GTF LCLS John Schmerge, SLAC

14. Transient Response With Pulse Shaping f = 2856.00 MHz fp = 2856.03 MHz f0 = 2852.53 MHz Q0 p = 12000 Q0 0 = 12000 bp = 2.0 b0 = 1.1 tp = 440 ns t0 = 637 ns f = 2856.00 MHz fp = 2856.03 MHz f0 = 2848.00 MHz Q0 p = 12000 Q0 0 = 12000 bp = 2.0 b0 = 1.1 tp = 440 ns t0 = 638 ns LCLS Adjusted timing LCLS Increased Df John Schmerge, SLAC

15. Frequency Domain Flat klystron pulse (3 ms) With pulse shaping (0.56 ms) John Schmerge, SLAC

16. 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. John Schmerge, SLAC