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This report presents the experimental results of transverse and longitudinal emittance measurements at the Gun Test Facility (GTF). The goals, facility layout, and future improvements are discussed. The report also compares the GTF injector with the LCLS injector and highlights the major differences. The measurement techniques, improvements, and conclusions are included.
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Injector Experimental Results John Schmerge, SSRL/SLACApril 24, 2002 • Goals • Gun Test Facility Layout • Transverse Emittance • Longitudinal Emittance • Future Improvements/Conclusions John Schmerge, SLAC
Experimental Goals • Transverse Emittance • en < 1.2 mm-mrad (projected) • Q = 0.2-1 nC • Longitudinal Emittance • Dt < 10 ps • sg /g < 0.1% • Laser • sjitter < 1 ps • Temporal shape – flat-top • Elaser > 100 mJ (UV) John Schmerge, SLAC
GTF Linac and Diagnostics Analyzing Magnet Solenoid Phosphor Screen Phosphor Faraday YAG Screen & OTR Screen Screens & Cup Quadrupole PCRF Faraday Phosphor Screen & Energy Filter Doublet Gun Cups Toroids 3m S-Band SLAC Linac John Schmerge, SLAC
GTF Beamline John Schmerge, SLAC
Major Differences between GTF and LCLS Injector • Laser • Nd:glass instead of Ti:saphire • 20% energy fluctuations • Gun • Single RF power monitor • 10 Hz operation • Beamline • 90 cm gun to linac drift distance instead of 140 cm • Single linac section instead of two • No solenoid around the first linac section John Schmerge, SLAC
Published Thermal Emittance for Cu John Schmerge, SLAC
Emittance Measurement Technique at GTF Quadrupole Scan Technique Measure beam size vs quadrupole current or strength. counts Background subtracted image pixels rms spot size calculation using projection cut off at 5% of maximum John Schmerge, SLAC
Emittance vs Charge Parameters Egun = 110MV/m gun = 40 Rcat = 1mm Bsol» 2.0kG Elinac = 8.3 MV/m John Schmerge, SLAC
Other Laboratories Published Results John Schmerge, SLAC
QE vs Position QE = 3.6 ±0.4 10-5 100 MV/m 40° injection John Schmerge, SLAC
Improvements • Data Acquisition and Analysis • Include space charge (20-25% reduction in reported e) • Use OTR screen instead of YAG screen for improved resolution • Beamline • Optimize gun field ratio and phase (minimum e near 30 degrees) • Optimize gun to linac matching • Eliminate 15 m focal length quadrupole in solenoid • Pulse Shaping • Improved transverse profile to reduce slice e • Laser temporal pulse shaping to reduce projected e John Schmerge, SLAC
Longitudinal Emittance Measurement Technique analagous to quadrupole scan of transverse emittance Spectrometer Booster (vary fbooster) Gun Determine Longitudinal f-Space at Linac Entrance Energy Screen Longitudinal : Measure Energy Spectra vs booster phase Transverse: Measure Beam Size vs quad strength John Schmerge, SLAC
Measurements Minimum Energy Spread Maximum Energy Correlated Energy Spread DETotal = -ERF cos(fRF) Df = 400 keV or 8 % FWHM PARMELA predicts 2% correlated energy spread John Schmerge, SLAC
Path to Low Emittance Measurements at the GTF John Schmerge, SLAC
Conclusions • 1.5 mm projected emittance with 100 A beam measured at GTF with Gaussian temporal pulse shape • Measurements agree with PARMELA simulations • Emittance can be further reduced by: • Improved laser spatial uniformity • Improved solenoid with reduced quadrupole field • Optimize gun field, phase and linac matching • Emittance will be reduced using flat-top temporal laser pulse shape • Including space charge in analysis will reduce the reported emittance • Utilize OTR screens with improved resolution John Schmerge, SLAC
