LCLS Injector Prototyping at at the GTF John John Schmerge, SLAC SLAC November 3, 3, 23 23 GTF GTF Description Summary of of Previous Measurements Longitudinal Emittance Transverse Emittance Active LCLS Prototyping Mg Mg Cathode Electro-Optic pulse pulse length length measurement RF RF pulse pulse shaping Other Technical Issues Solenoid design design Load Load Lock Lock Wakefields/RF kicks kicks Laser Laser Prototyping Summary
Gun Test Facility at at SSRL Linac Coherent Light Source DIAGNOSTICS 3M LINAC RF GUN DIAGNOSTIC ROOM MOD 3 SSRL Injector Vault KLY-3 MOD 2 KLY-2 MOD KLY- 8 m Laser Transport System GTF LASER ROOM GTF Control Room
GTF Linac and Diagnostics Analyzing Magnet Faraday Cup Phosphor Screen & Energy Filter YAG Screen & OTR Screen Phosphor Screen Toroids Linac Coherent Light Source Quadrupole Doublet 3m S-Band SLAC Linac Phosphor Screens & Faraday Cups Solenoid PCRF Gun
Longitudinal Emittance Measurement Technique analagous to quadrupole scan of transverse emittance Gun 3 m linac (vary φ linac ) Quad Doublet Spectrometer Energy Screen Determine Longitudinal φ-space at Linac Entrance Measure Energy Spectra vs linac phase Linear analysis allows only linear time energy correlations Nonlinear analysis allows quadratic and cubic correlations
Measurements E (kev) 5-5 - 5 pc upstream linac downstream linac -.4 -.2..2.4 t (ps) aveq=6.49pc t=3.4424e-5 t2=-.2986 t22=497.2666 kev&rad 9 Linac Coherent Light Source E (kev) 2 5 5-5 - -5-2 29 pc upstream linac downstream linac -2. -... 2. t (ps) aveq=289.7483pc t=.6633 t2=-3.8645 t22=22589.879 kev&rad 25 8 7 2 rms Energy Spread (kev) 6 5 4 3 rms Energy Spread (kev) 5 2 5-4 -3-2 - 2 3 Linac Phase (degrees) -3-25 -2-5 - -5 5 Linac Phase (degrees)
Linear Analysis Fit Fit results after linac Linac Coherent Light Source Parameter 5 pc 29 pc Units τ.7 2.59 ps 2 τ 2-27.4-286 kev ps τ 22 757 39687 kev 2 ε l (kev ps).868.5 kev ps σ E 84. 99.22 kev σ E uncorr 2.65 8.2 kev σ t.33.43 ps slope -257-39 kev/ps
Quad Scan Slice Emittance Measurement Gun Booster Quad Doublet Spectrometer Energy Screen Set Booster Phase for optimum linear chirp Determine Transverse φ-space at Quad Entrance Measure Transverse Beam Size vs quad strength for different energy/time slices (typically slices)
Measurements at at Quad entrance with Q = 29 pc ε n (µm) 6. 5. 4. 3. 2... Emittance -3. -.. 3. Time (ps) Projected values displayed at t= I (A). 8. 6. 4. 2.. Current -3. -.. 3. Time (ps) Projected values displayed at t= α, β (m) 3. 2... -. -2. β x Twiss -3. -.. 3. Time (ps) Projected values displayed at t= x (µm), x' (µrad) 4. 2.. -2. -4. -6. -8. Offsets α x x x -3. -.. 3. Time (ps) Projected values displayed at t=
Phase Space for Each Slice Linac Coherent Light Source x' (µrad) 6 4 2-2 -4-6 tail head projected - -5 5 x (µm) - 4 5-2 6 7 8-3 9-4 -4-3 -2-2 3 4 29 pc 5 pc X' (microradians) 4 3 2 Projected X (microns) 3 2
Measured and Reconstructed Projected Emittance 4. εn (µm) 3.5 3. 2.5 2..5. Measured Projected Emittance Weighted Average Slice Emittance Reconstructed Projected Emittance with offsets Reconstructed Projected Emittance zero offsets Q = 5 pc Q = 3 pc 33% projected emittance growth due to offsets at high charge.5. 25% projected emittance growth due to offsets at low charge.5.6.6.7.7.8.8.9.9 2. 2. B sol (kg)
Slice Emittance ( ') ' x = xg, n n x x dxdx Qn x = x G x x dxdx σ σ σ n (, ) ' ' ' ' n Qn n 2n 22n ( ) 2 ( ') ' = x x G, n n x x dxdx Qn = ( x x )( x x ) G (, ) n n n x x dxdx Qn ( ) 2 ' ' ( ') ' = x x G, n n x x dxdx Q n ' ' ' '
Reconstructed Projected Emittance N ' ' (, ) = n (, ) G x x G x x n= N ' ' Q= G( x, x ) dx dx = Qn n= N ' ' x = xg ( x, x ) dxdx Qn x Q = Q n= N ' ' ' ' ' x = x G ( x, x ) dxdx Qn x Q = Q n= σ σ σ n n Linac Coherent Light Source = x x G x x dxdx = Q + x x 2 N ( ) (, ) n( σ n) ' ' 2 2 n Q Q n= = = N ' ' ' ' ' ( x x )( x x ) G ( x, x ) dxdx Q n ( σ x ) 2n nx n ' 2 Q Q n= 2 N ' ' ' ' '2 '2 22 = ( x x ) G ( ) ( ) x, x dxdx Qn x 22n n x Q = σ + Q n= + x x
Mg Cathode Linac Coherent Light Source Installed on on GTF gun this summer Friction welded insert.9 cm Φ Measured QE before cleaning 8-5 -5 electrons/photon Expected QE after cleaning 5-4 -4 electrons/photon Maximum rf rffield at at the cathode of of 5 MV/m Total dark current over macropulse nc
First Beam with Mg cathode Linac Coherent Light Source 2 mm Electron Beam Laser Beam
Dark Current and Maximum Field Mg Cathode Linac Coherent Light Source Single Crystal Cu Cathode Polycrystaline Cu Cathode QE ( - 5 ) (before cleaning) 8 5 5 Total Emitted Dark Current (nc( nc).5 Maximum Field at the Cathode (MV/m) 5 25 3 Thermal Emittance (µm/mm) m/mm)? <.7?
Planned Measurements with Mg cathode Thermal Emittance QE spatial uniformity Laser Cleaning Procedures Slice and Projected Emittance Longitudinal Emittance Emittance vs vspulse shape Gaussian Laser Pulse Flat-Top Laser pulse Time Time Domain Domain pulse pulse shaping shaping Frequency Frequency Domain Domain pulse pulse shaping shaping
Prototype Bunch Length Monitor Linac Coherent Light Source Electric field from electron beam interacts with photon beam via Electro-Optic effect in in LiNbO 3 or or LiTaO 3 Crystal Time Dependent Polarization rotation proportional to to the bunch shape Crystal 5 mm from 3 3 MeV beam Photon beam pulse length can be be diagnosed with standard optical techniques Autocorrelation Cross-correlation FROG Chirped Optical Pulse can also be be used to to convert the temporal profile to to wavelength modulation Expect Sub ps psresolution
Resolution Determined by by Geometry and E-beam energy Linearly Polarized Laser Beam b EO Crystal Polarization Rotated Proportional to Electric Field Electron Bunch E = γmc 2 Resolution ~b/γc For b = 5mm b/γc =.3 ps Electric field lines are compressed into a disk with opening angle ~/γ
Status Wakefield Mitigation included Use available Green laser at at GTF Photon-Electron beam interaction chamber constructed and ready for for crystal installation Chamber to to be be installed on on GTF beamline later this fall Transverse Electric Field (V/m) 3.5 x 7 3 2.5 2.5.5 b=4 mm b=5 mm b=6 mm Transverse Electric Field from 3 MeV nc Point Charge - -.8 -.6 -.4 -.2.2.4.6.8 Time (ps)
Bunch Length Monitor Diagnostic Chamber E-beam Laser Windows Actuator
Prototype EO Device with Wakefield Plug Mirror Electron Beam EO Crystal Laser Beam Removed Position Measurement Position
RF Pulse Shaping Linac Coherent Light Source Modulate low low level level rf rfamplifier input to to produce desired klystron output pulse shape Measure rf rfamplifier and and klystron amplitude and and phase output Measure gun gun transient response Measure power dissipated in in cavity with with and and without pulse shaping Arbitrary Waveform Generator GTF RF Gun kw S-band Linear Amplifier Klystron XK5 Klystron or Cavity Power (MW) 6 Klystron no pulse shaping 4 Cavity no pulse shaping 2 Klystron with pulse shaping Cavity with pulse shaping 8 6 4 2-2.. 2. 3. 4. Time (µs)
Other Technical Issues: Solenoid Dipole and Quadrupole terms Emittance Compensating Solenoid 4 wire quadrupole/dipole corrector included Integrated Magnetic Gradient (T) Gradient vs Current and Harmonic number.4e-3.2e-3.e-3 8.E-4 6.E-4 4.E-4 2.E-4.E+ k 2 L eff E/c = 4E-6*I(A) + 7E-5 R 2 =.9998 5 5 2 25 Current (A) Dipole Quadrupole Sextupole Octopole Linear (Quadrupole) 26 m quad focal length with no 4 wire corrector 5 m quad focal length with 4 wire corrector limited by accuracy of angle positioning
Solenoid Linac Coherent Light Source Solenoid Field vs Current.35.3.25 Field (T).2.5..5 5 5 2 25 Current (A) Solenoid Field Bz (T).4.3.2. I = 2 A I = 5 A I = A I = 5 A I = 2 A I = 22 A -.3 -.2 -...2.3 Position (m)
Other Technical Issues: Load Lock High Q RF Seal Cooling water connection for for cathode RF tuning with cathode plate deflection
Other Technical Issues: Time Dependent Kicks Measured at at GTF causing increased projected emittance Possibly due to to RF coupler in in gun or or first linac section Investigating source of of kicks at at GTF x offset (µm) 2 5 5-5 - -5-2 Transverse offset vs Longitudinal Position for beam slices Linac Gun Exit. 2. 4. 6. 8. z position (m) Slice Slice 2 Slice 3 Slice 4 Slice 5 Slice 6 Slice 7 Slice 8 Slice 9 Slice Components Q = 5 pc
Other Technical Issues: Drive Laser Transverse Profile Flattening (Aspheric Optic) IR&UV FROG Diagnostics Gaussian Input Flattened Output t Test of IBM optic flattener λ IR FROG Image
Summary Previous Measurements Linac Coherent Light Source σ Euncorrelated σ 2-8 Euncorrelated 2-8 kev kev Large Large correlated energy energy spread spread at at the the gun gun exit exit ε n ε n slice slice.5.5 mm-mrad mrad at at 8 8 A Large Large time time dependent offset offset in in position and and angle angle observed LCLS Prototyping Mg Mg Cathodes tests tests begun begun at at GTF GTF Sub Sub ps pselectron bunch bunch length length monitor monitor to to be be installed later later this this year year RF RF pulse pulse shaping tests tests to to observe mode mode beating beating and and reduced power power dissipation in in the the gun gun planned for for FY4 FY4 Other Issues Solenoid dipole dipole and and quadrupole corrector design design begun begun Load Load loack loackconceptual design design complete Study Study of of time time dependent kicks kicks observed at at GTF GTF underway and and will will include include measurement of of rf rfcoupler amplitude and and phase phase asymmetry Transverse laser laser pulse pulse shaping and and FROG FROG tests tests begun begun at at GTF GTF