RF-Gun beam based alignment at PITZ/FLASH M.Krasilnikov, DESY Zeuthen LCLS Injector Commissioning Workshop (ICW) October 9-11, 2006

M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH” Outline Beam-Based Alignment (BBA) of RF gun: Fields and geometry BBA motivation Cathode laser BBA: PITZ FLASH Limitations of laser BBA Main solenoid BBA: Solenoid micromover system Main problems and possible solutions Limitations of solenoid BBA Conclusions M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

RF-Gun: fields and geometry (PITZ and FLASH) sol.mech.axis ≠mag.axis sol.mech.axis ≠cavity el.axis solenoid tilt angles fields overlapping M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

RF-Gun Alignment Motivation ASTRA Simulations of FLASH (VUV-FEL) Injector (150MeV) Emittance growth (@z=15m) due to RF -gun misalignment Solenoid tilt effect For Ecath=25MV/m solenoid tilt angle of ~3mrad is equivalent to 1 mm transverse offset 5% emittance growth (lower limit estimations): laser DR<800um or solenoid DR<500um or solenoid Dangle<1.5mrad (0.086deg) No effect in the matching section has been considered! M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

Cathode Laser Alignment (FLASH) Laser spot positioning at the photo cathode: + Transverse displacement and angle of the laser beam can be independently changed - Iris position has to be adjusted for every mirror movement No VC and e-beam simultaneously M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

Cathode Laser Alignment (PITZ) M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

RF gun launch phase (SP Phase) BBA of Laser on Cathode z=0 z=0.276m z=0.935m z=0.778m Basic measurement: Mean position of electron beam at LOW.Screen1 (DoubleDiagCross at z=0.778m) vs. RF gun launch phase (SP Phase) Conditions: Main and bucking solenoids off All steerers off Dipole (even it is ~0.2m after the screen) degaussed and off Bunch charge ~10pC, pulse train 10-50 laser pulses Moderate RF power in the gun: 0.8-1.5MW (exclude dark current)→Pz~2.6-3.3MeV/c Preliminay (rough) laser alignment: Using scintillating cathode Centering in dark current images M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

BBA of Laser on Cathode Simulations Measurements&Simulations Beam charge, Transverse rms Size, Mean Momentum Vs. RF Phase Measured and Simulated beam charge at z=0.78m Vs. RF Phase Beam spot at Diag.Cross screen (z=0.78m) “Normal” phases “Low” phases, rf focused beam Measured and Simulated beam rms size at z=0.78m Vs. RF Phase Beam Offset at z=0.78m Vs. RF Phase (0.5 mm vertical laser offset on the cathode has been assumed) As a theoretical background correspondent simulations have been done. An RF gradient has been fitted to the measured longitudinal momentum of the beam. A Schottky constant has been applied to fit a phase scan – beam charge (measured with a Faraday Cup) vs. SP Phase. Alignment feature: Within a wide range of RF phase the energy varies not too much -> the beam position remains almost the same. But for the low energies phase (low energies) the beam focusing from RF field takes place resulting in well-focused beam, which is very sensitive to the misalignment of the laser spot on the cathode. M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

Cathode Laser BBA. Test Movement Measurement: Beam offset vs. rf phase Step 1: Laser test movement Measurement: Beam offset vs. rf phase Step 2: Laser alignment One of results: (X0,Y0) – preliminary coordinates of the center at the screen M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

Cathode Laser BBA. Difficulties Laser intensity non-homogeneity Laser position jitter Possible damage of YAG screen homogeneity Earth magnetic field (x,y,z)~(0.02mT,-0.03mT,-0.009mT) stdev<X>=13um stdev<Y>=11um Extreme example of a damaged screen M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

Main Solenoid BBA. Micromover system beam axis M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH” Main Solenoid BBA z=0 z=0.276m Basic measurement: Mean position of electron beam at LOW.Screen1 (DoubleDiagCross at z=0.778m) vs. Main solenoid current z=0.935m z=0.778m Conditions: Bucking solenoid off All steerers off (or consider in simulations) Dipole (even it is ~0.2m after the screen) degaussed and off Bunch charge ~10pC, pulse train 10-50 laser pulses – to be tuned for Imain Moderate RF power in the gun: 0.8-1.5MW (exclude dark current )→Pz~2.6-3.3MeV/c RF launch phase* Laser spot size possibly small M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

Main Solenoid BBA Beam position (<X>,<Y>) at LOW.Scr1(z=0.778m) as a function Imain E-beam displacement with a main solenoid sweep. Possible reasons: Solenoid transverse offset (Xsol,Ysol) Solenoid tilt angles (AngleX, AngleY) Laser (small) offset (Xlas,Ylas) from the center From the laser BBA: X0=13.0mm Y0=14.8mm Imain=0A Other factors to be considered: RF gun launch phase and gradient Small offset (from X0,Y0 obtained after laser BBA) Solenoid calibration Imain=320A M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

Main Solenoid BBA. RF-gun launch phase and gradient streak-camera measurement F0=-18deg (21.4MV/m;22deg) M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH” Simulation Tool Alignment Utility of the V-code - fast tracking code based on the method of moments of particle distribution function M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

Main Solenoid BBA. Test movement Simultaneous simulations of beam position before and after test movement (DXsol=0.4mm) Advantages: Same RF gun launch phase and gradient Same offset X0*,Y0* Same solenoid calibration Laser_Beam_CenterX 0.20 mm Laser_Beam_CenterY -0.26 mm XSolMainCenter 0.85 mm YSolMainCenter 1.12 mm AngleXSolMain -0.0808 deg AngleYSolMain 0.0580 deg Ez_Field_At_Cathode 22.2 MV/m Initial_Phase -147.2 deg M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH” Main Solenoid BBA Before BBA After BBA Laser_Beam_CenterX 0.20 mm 0.11 mm Laser_Beam_CenterY -0.26 mm -0.27 mm XSolMainCenter 0.85 mm 0.041 mm YSolMainCenter 1.12mm 0.063 mm AngleXSolMain -0.0808 deg -0.0158 deg AngleYSolMain 0.0580 deg 0.0057 deg Ez_Field_At_Cathode 22.2 MV/m 22.4 MV/m Initial_Phase -147.2deg -148.5 deg M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

Main Solenoid BBA. Difficulties Laser BBA difficulties: Laser intensity non-homogeneity Laser position jitter Damaged YAG screen + RF phase and gradient jitter Some uncertainty in the solenoid micromover (especially angles), z-position Outlook BPM M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

RF-Gun BBA. Conclusions BBA of Laser on Cathode: routine procedure at PITZ (FLASH) based on a measurement of beam position vs. gun launch phase test movement of the laser beam allows to determine a displacement vector for the laser beam centering Main solenoid BBA: multi-parameter task based on beam position simulation vs. main solenoid current test movements of the main solenoid and/or cathode laser allows to reduce uncertainty in main solenoid misalignment Possible improvements: implement earth magnetic field in BBA procedures use BPM (LOW.BPM1) for solenoid BBA more details on RF field profile solenoid relative displacement online measurement M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

Reference RF Phase. Beam Size measurements Transverse Beam Size at Screen 3 as a Function of RF Launch Phase for Various Main Solenoid Currents Fmax M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

PITZ1 Benchmark Problem: Fields Field balance in the rf gun cavity Solenoid calibration MF compensation Ibuck=0.074847*Imain M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

BBA: Step 0 – Rough Laser Alignment on the Cathode Rough laser alignment using dark current symmetry. TTF Dark current at TTF at screen Gun3 (z = 1.27m) Without beam With beam Dark current rings originate from the edge of the Cs2Te coating and plug spring region A laser spot being aligned on the cathode center results in an electron beam centered with dark current rings First row of pictures -TTF logbook 29.03.2004 Second row - TTF logbook - 10.04.04 Info (for any case) One can conclude that the inner dark current ring is generated at the edge of the Cs2Te photocathode. The dark current ring on the screen has also a diameter of ~5mm, so that a 1:1 image of the cathode region is seen. Typical magnet settings for these observations are: Imain=290A, Ibuck=50A (logbook 10.04.04, 7:17). M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

BBA: Step 0 – Rough Laser Alignment on the Cathode Rough laser alignment using dark current symmetry. PITZ Without beam With beam Screen Diag. Cross (z = 0.87m) The same pictures obtained at PITZ at different screens. Screen_PP (z = 2.62m) M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

RF Gun Alignment: Satellites observation @Diag.Cross @PP Screen @Dispersive Arm x y x y x Pz Vacuum mirror @Diag.Cross, “low phase”

M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH” BBA of laser on cathode SP phase = -70 SP phase = -90 SP phase = -50 SP phase = -30 SP phase = -20 SP phase = -100 SP phase = -16 SP phase = -106 SP phase = -125 SP phase = -130 SP phase = -120 SP phase = -116 SP phase = -110 SP phase = -112 SP phase = -114 precision of mirror adjustment: better than 20 µm M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

RF-Gun Alignment Motivation ASTRA Simulations of FLASH (VUV-FEL) Injector (150MeV) trajectory Emittance degradation in the matching section is not included emittance M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”