Electro-optic longitudinal diagnostics for EMMA extraction

1. Electro-optic longitudinal diagnostics for EMMA extraction Steven Jamison ASTeC STFC Daresbury Laboratory

2. Electro-optic capabilities • fundamental (time resolution/time window, ...) • measurability Demonstrated results on other machines • scaling to EMMA proposed EMMA scheme (s) • beam requirements for operation • options for chosen beam parameters

3. electro-optics in other machines... • high g bunches (100 MeV -> multi GeV) • femtosecond bunches (fastest possible time resolution) • high charge 100pC – few nC • motivation for non-destructive ultrafast diagnostic electro-optics in EMMA • picosecond bunches.. • low g bunches • motivation for compactness, low cost Measurability: requires extrapolation to EMMA parameters

4. Measurement of Coulomb field through off axis laser probe Temporal profile field dependent on radial position:dt ~ 2R/cg 20 MeV at 2mm offset  dt ~ 300 fs High time resolution needs close proximity to bunch (equally true of CDR, Smith-Purcell, Electro-optic etc)

5. single shot methods... Spectral decoding • time resolution limits coupled to time window • experimentally simple Temporal decoding • time resolution independent of time window. Highest res. • experimentally complex Spatial encoding • time profile coupled to Coulomb transverse profile • experimentally moderately complex

6. spectral decoding works well for “slow” modulations • Measure probe intensity I() • known (initial)(t) •  infer I(t) fast modulation broad bandwidth very fast modulations destroy initial frequency-time map

7. spectral decoding limits... Can observe oscillating features on shorter time scale than usual “time resolution”

8. Spectral decoding limits... measure change in probe spectrum… input probe time res.Fnc. Coulombfield Defines a temporal limit on allowed modulations Gaussian probe Tc = 10 ps (20 ps window) To = 30 fs tlim ~1.4ps

9. Temporal decoding Temporal profile of probe pulse Spatial image of 2nd harmonic Time resolution set by gate pulse duration + SHG phase matching (typically ~ 50 fs resolution)

10. Encoding Time Resolution... material response probe laser bunch • velocity mismatch of Coulomb field and probe laser • Electro-optic frequency mixing efficiency; c(2)(w)

11. Material response, R(w) Must consider spectral components of Coulomb field Ideal: uniform response across all frequencies (normalised) ZnTe GaP Below cut-off efficiency  L Cut-off decrease with thickness

12. Spectral content of EMMA longitudinal bunch profile (?) Frequency domain Time domain slow temporal variations can enhance signal strength with thicker crystals

13. EO Capabilities and Scaling transverse effects tres = 2R/cg (300 fs for 20 MeV @ R=2mm) Coulomb field ~ R-1 (R-2 for large R) measurement technique Spectral decoding • experimentally simple • time window ~ [ tlim ]2 20 ps => 1.4 ps Temporal decoding • experimentally complex • time resolution ~ 100 fs signal ~ L, tres. ~ L-1 material properties charge (density) signal magnitude: • ~ Q { rz(t) } ; balanced detection • ~ Q2 { rz2(t) } ; zero background detection

14. Temporal Decoding expts. at FLASH..

15. “Time resolution” szactual ~ 30fs  szmeasured ~55 fs szactual ~ 90fs  szmeasured ~90 fs sz ~ 90fs (rms) EO crystal measurement positionR ~6mm 65mm thick GaP E ~ 450MeV, Q ~ 1nC.... ~100pC in 100fs

16. Electro-optic experiments at FELIX bunch profile fromTemporal Decoding Comparison of Temporal & Spectral decoding measurement positionR ~1-2 mm 500mm thick ZnTe E ~ 30 – 50 MeV, Q ~ 200 pC in 700fs

18. Signal strength extrapolation • E ~ 20 MeV, • Q ~ 30pC in 5ps • 1 mm ZnTe • R ~ ??? EMMA FLASH FELIX • E ~ 450MeV, • Q ~1nC.... ~100pC in 100fs • 65 micron GaP • R ~ 6 mm • E ~ 30 – 50 MeV, • Q ~ 200 – 300 pC in 700fs • 0.5 mm ZnTe • R ~ 1-2 mm ... x 1/160 ... x 40 ... ??? .... x 1/10.... x 2.... ??? • expect ~ 25% magnitide of EO effect with EMMA parameters • expect ~ 5-10% signal in zero background detection { prop. to rz2(t) } • 32 pC measurement should be OK • 16 pC probably not sufficient

19. Proposed scheme for EMMA additional laser transport to EMMA diagnostic existing EO expt on ERLP

20. ERLP Electro-optic beam-pipe section.... Laser entrance from behind chicane dipole encoding (bunch profile into optical pulse) probe laser bunch to laser diagnostic Generic for Temporal or spectral decoding decoding(optical pulse into profile measurement)

21. EMMA Electro-optic beam-pipe section.... ~ 1 metre 3 cm laser output to spectrometer... ...or temporal decoding cross-correlator laser input

22. (temporal decoding) setup for FELIX expt. CCD camera short pulse chirped probe

23. Temporal decoding at FLASH - DESY (original) Spectral decoding at DESY

24. Proposed Electro-optic Diagnostic on EMMA... • use existing Ti:S system from ERLP EO diagnostic expts. • additional vacuum beam pipe for laser transport • use spectral decoding (if lower time res. acceptable). • ~ 1 ps resolution limit for 20ps window • ~ 500 fs for 5ps window Ti:S on ERLP.... T0 ~ 30 fs immediately “upgradable” to Temporal decoding if high time resolution required. Signal magnitude requires highest charge compatible with other constraints • 16 pC not likely to be sufficient • 32 pC sufficient (extrapolation from previous experiments)