November 3, 2004 John Schmerge, SLAC Injector RF Design Review [email protected]and 0 Mode Interaction in RF Gun John Schmerge, SLAC November 3, 2004 LCLS 1.6 Cell Gun 2 resonant frequencies 2 resonant frequencies Bead drop results Bead drop results Full and half cell field probes Full and half cell field probes Measurements Time Domain Time Domain Frequency Domain Frequency Domain Calculations No pulse shaping No pulse shaping With pulse shaping With pulse shaping Summary
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Injector RF Design Review [email protected] November 3, 2004 John Schmerge, SLAC and 0 Mode Interaction in RF Gun John Schmerge, SLAC November.
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November 3, 2004 John Schmerge, SLACInjector RF Design Review [email protected]
and 0 Mode Interaction in RF Gun John Schmerge, SLAC
November 3, 2004
LCLS 1.6 Cell Gun 2 resonant frequencies2 resonant frequencies Bead drop resultsBead drop results Full and half cell field probesFull and half cell field probes
Measurements Time Domain Time Domain Frequency Domain Frequency Domain
Calculations No pulse shaping No pulse shaping With pulse shaping With pulse shaping
Summary
November 3, 2004 John Schmerge, SLACInjector RF Design Review [email protected]
Multiple Modes2 modes with different resonant frequencies and different longitudinal field profiles.
ff – f – f00 = 3.5 MHz on GTF gun = 3.5 MHz on GTF gun
Beating observed during gun filling time (transient effect)Beating observed during gun filling time (transient effect)
Presence of 0 mode affects e-beamTime dependent ratio of full cell to half cell field due to the 180˚ Time dependent ratio of full cell to half cell field due to the 180˚ phase shift between the two modes affects longitudinal phase phase shift between the two modes affects longitudinal phase space.space.
Additional transverse fields in the cell-cell aperture affect Additional transverse fields in the cell-cell aperture affect transverse phase space.transverse phase space.
Measure each modeMeasure in time or frequency at a particular longitudinal position Measure in time or frequency at a particular longitudinal position (RF probes).(RF probes).
Measure as a function of longitudinal position averaged over time Measure as a function of longitudinal position averaged over time (bead drop).(bead drop).
November 3, 2004 John Schmerge, SLACInjector RF Design Review [email protected]
Bead Drop Measurement
-1000
-900
-800
-700
-600
-500
-400
-300
-200
-100
0
0 20 40 60 80 100
Bead End Position (mm)
Fre
quen
cy S
hift
(kH
z)
SF sim, pi-modepi-mode measSF sim, 0-mode0-mode meas
0-mode
-mode
November 3, 2004 John Schmerge, SLACInjector RF Design Review [email protected]
Scale2" 3"0 1"
Laser Port
RF Probe Location on GTF Gun
Scale2" 3"0 1"
Half Cell Probe
Full Cell Probe
Waveguide Feed
November 3, 2004 John Schmerge, SLACInjector RF Design Review [email protected]
November 3, 2004 John Schmerge, SLACInjector RF Design Review [email protected]
Frequency Domain
0.0E+00
2.0E-03
4.0E-03
6.0E-03
8.0E-03
1.0E-02
2850 2852 2854 2856 2858
Frequency (MHz)
Mag
nit
ud
e
Input
Gun
0.0E+00
2.0E-03
4.0E-03
6.0E-03
8.0E-03
1.0E-02
2850 2852 2854 2856 2858
Frequency (MHz)M
agn
itu
de
Input
Gun
Flat klystron pulse (3 ms) With pulse shaping (0.56 ms)
November 3, 2004 John Schmerge, SLACInjector RF Design Review [email protected]
Summary0 mode produces a measurable 3.5 MHz beating on the gun field
Effect more significant with short pulses used for rf pulse shapingEffect more significant with short pulses used for rf pulse shaping
Estimated time dependent full cell to half cell field ratio varies by Estimated time dependent full cell to half cell field ratio varies by less than 10% and for special cases less than 2%.less than 10% and for special cases less than 2%.
Effect depends on the filling time (coupling coefficient), mode Effect depends on the filling time (coupling coefficient), mode separation and exact input waveform. Timing can be adjusted to separation and exact input waveform. Timing can be adjusted to minimize beating.minimize beating.
Mode beating can be advantageous as it allows for variable field balance depending on laser timingShot to shot jitter on the beating will be dominated by fluctuations on the klystron input pulse.Measure beating at GTF with rf pulse shaping.