Slice Parameter Measurements at the SwissFEL Injector Test Facility Eduard Prat Costa PSI/DESY/KIT Mini-Workshop on Longitudinal Diagnostics for FELs 11 March 2013 eduard.prat@psi.ch

Contents Introduction Longitudinal phase-space measurements Slice emittance measurements High-level applications Summary and next steps eduard.prat@psi.ch 2

SwissFEL Injector Test Facility (SITF) Phase 1: Electron source and diagnostics (03/2010 – 07/2010) Phase 2: Phase 1 + (some) S-band acceleration (08/2010 – summer 2011) Phase 3: The full machine Summer 2011: installation of bunch compressor. All S-band rf available from April 2012 X-band: power expected in April 2013 quadrupole BPM+screen screen Gun S-band X-band Bunch compressor FODO section Transverse deflector High-energy spectrometer E= 7 MeV E= 250 MeV Phase 4: Installation of new PSI gun + undulator experiment (08-09/2013) eduard.prat@psi.ch

Design optics for phase 3 CSR effects βx < 10m between 3rd and 4th dipole of BC Longitudinal resolution At the TD: βy = 40m Between TD and profile monitor: sin(Δy) = 1 resolution ~ 4μm for VMAX=5MV (assuming εy=0.5μm, E=250MeV) Energy spread resolution At the spectrometer: βx = 0.214m Dx = 0.267m resolution ~ 14keV (assuming εx=0.5μm, E=250MeV) Optics for symmetric single quad scan βx = βy = 15m αx = αy = 1.385 TD FINXB entrance BC eduard.prat@psi.ch 4

Transverse Deflector (TD) A Transverse Deflector (TD) is used to measure the longitudinal bunch profile and the slice parameters of the beam (energy, energy spread, transverse position, transverse size and emittance) TD gives a transverse (vertical) kick which depends on the longitudinal position of the beam. Calibration: the phase of the deflector is changed to obtain the calibration between vertical position and bunch arrival time. The centroid shift gives an idea of the equivalent time jitter of the beam. y at observation point y’ at TD position Calibration: S = 1.43mm/ps equivalent jitter = 0.11ps At the TD: βy = 40m Between TDC and monitor: sin(Δy) = 1 resolution ~ 4μm for VMAX=5MV (assuming εy=0.5μm, E=250MeV) eduard.prat@psi.ch 5

Longitudinal phase-space measurements Combining the TDC with an energy spectrometer the longitudinal phase-space can be measured At the SIFT these measurements are done at the spectrometer at the end of the lattice The bunch length is the beam size in the vertical plane normalized by the TD calibration Beam centroid position and beam size give information about energy and energy spread: σ2 = ε•β + D2•δ2 (δ = Δp/p) Optics resolution ~14keV (βx = 0.21m, Dx = 0.27m, εx=0.5μm, E=250MeV) Present resolution limited at around 80keV due to screen resolution More studies will be shown in Gian Luca’s talk, and more to be done soon Measurement example (230 MeV, on-crest) Slice energy spread and current profile Beam image Bunch length 2.73 ± 0.21 ps Energy spread at the core 76.4 ± 0.32 KeV eduard.prat@psi.ch 6

Slice emittance measurements Streaked beam Transverse deflection (streaking) Optics. The optics are scanned using 5 quads between transverse deflector and observation point (end of FODO section). Generate regular phase-advance in x Keep beam size under control Keep longitudinal resolution constant Transverse deflector calibration. At each optics the phase of the deflector is changed to obtain individual mm-ps calibration for each optics. Slice analysis. The beam is split into slices, using the centroid from Gauss fit as a reference. Per each slice the beam size from Gauss fit is obtained. Emittance/mismatch determination. From the beam sizes per each optics the emittance and optics are obtained per each slice. Projected parameters are also obtained Reconstructed emittance and optics eduard.prat@psi.ch 7

Optics for slice emittance 5 quads (F10D1.MQUA.60/70/80/90/100) are used to: Scan phase-advance in x Keep βx under control (35≤βx≤40) Keep βy small (βy<10) sin(μy) ~ 1 Beta functions at the screen k-values Phase-advances eduard.prat@psi.ch 8

Resolution, errors and matching Beam image close to screen resolution limit SwissFEL profile monitor (YAG + OTR) is used for emittance measurements. Beam size resolution is ~15μm, equivalent to an emittance resolution of ~ 3nm (E=250MeV, β=35m) Signal to noise ratio is good enough to measure slice emittance down to ~1pC level Errors Statistical errors from beam size variations (this is what is shown in the error bars of the measurements) Systematic errors: optics mismatch, energy error, quad field errors, screen calibration and resolution, errors associated to Gauss fit, etc. Matching Beam core is always matched to exclude errors due to optics mismatch Matching of the core works normally in 1-2 iterations Successful matching gives us confidence in the obtained emittance values eduard.prat@psi.ch 9

Optimum Emittances 200pC 10pC By doing a full optimization we have achieved the following emittances (uncompressed beam) These emittance values fulfill the SwissFEL requirements for uncompressed beams Emittance values are stable in short-term and optimum settings are reproducible 200 pC 10 pC Projected emittance ~0.30μm ~0.15μm Slice emittance ~0.20μm ~0.10μm 200pC 10pC eduard.prat@psi.ch 10

Example application: “thermal emittance” measurements Goal: compare thermal emittance for different wavelengths (267.5nm is our standard value). Method: scan the laser beam size and measure the slice emittance at low charge, keeping the charge density constant (10pC with 1.8mm aperture, bunch length of 10 ps). Measured values (~600nm/mm) are much better than simulation assumptions (910nm/mm). Degradation when going from 267.5nm to 260nm is only 5-10%. High order contributions possibly due to space charge or rf fields (not seen in simulations with ASTRA) We need further investigations (investigate space-charge effects, gun gradient contribution, etc.) λ=260.0nm λ=267.5nm Thermal emittance ~630nm/mm ~585nm/mm eduard.prat@psi.ch

High-level applications High-level applications written in Matlab (they use the generic toolbox ADAQA) One emittance tool for all type of measurements (projected and slice) Automatic analysis, data saving, plotting of results and logbook entry Routinely used in operation User needs to do some preparation and checks: Measure the beam energy Switch on TD and check the zero-crossing phase Be sure that the used screen is in and that the ROI is properly chosen For emittance measurements, check that the quads do not steer out of the ROI Plot for slice emittance measurement eduard.prat@psi.ch 12

GUI for longitudinal phase-space measurements eduard.prat@psi.ch 13

GUI for emittance measurements eduard.prat@psi.ch 14

GUI for optics matching eduard.prat@psi.ch 15

Summary and next steps Summary Methods to measure slice beam parameters well developed and tested at the SwissFEL Injector Test Facility Longitudinal phase-space Longitudinal resolution = 4μm (for VMAX = 5 MV) Energy spread resolution = 14keV (now limited to ~80keV due to screen resolution) Slice emittance Emittance resolution around 3nm Successful optics matching Great results achieved for uncompressed bunches High-level applications developed for these measurements, routinely used in operation Next steps Emittance optimization for compressed bunches with X-band cavity Long. phase-space measurements for compressed bunches with X-band cavity Improve profile monitor at energy spectrometer eduard.prat@psi.ch 16