6D Characterization of Witness Beam before Injection in LWFA B. Marchetti, D. Marx 1st EuPRAXIA Collaboration Week DESY, 19-23 June 2017

Outline This talk will present ongoing numerical studies for Beam Characterization at the ARES-linac at SINBAD relevant for EuPRAXIA How does the typical witness e-beam look like? How does the lattice for diagnostics look like? What do we mean with 6D Characterization of the Witness Beam? Bunch Length Longitudinal Phase Space Slice emittance on different transverse planes (not covered by this talk) 3D Reconstruction of e-Bunch Charge

Overview of e-bunch Parameters Witness beam for lwfa

Characteristics of Witness Beams From EuPRAXIA parameter table LWFA with external injection: At SINBAD we are investigating the range Q=0.5pC-30pC, especially the limit in the characterization of very low charge and ultra-short e-bunches. Higher charges (up to 50 pC) and longer bunch lengths (up to 30fs) working points will allow to relax the requirements on the longitudinal and transverse resolution of the measurement.

Example of ultra-short probe for LWFA Beam shape at the exit of the bunch compressor Q=2.7 pC Energy = 100 MeV Transverse Normalized Emittance = 0.2 mm mrad σt=0.461 fs

longitudinal charge profile reconstruction Guidelines for Lattice Design, Resolution Calculation longitudinal charge profile reconstruction

SINBAD-ARES linac SINBAD overview – Cfr: U. Dorda et al. “SINBAD – The accelerator R&D facility under construction at DESY”, NIM A 829, p. 233-236 (2016). Talk by J. Zhu describes examples of achievable working points suitable for LWFA Dogleg to second experimental area TDS + diagnostics beamline and experimental area ARES linac E=100MeV Magnetic compressor Temporary experimental area Space for future possible energy upgrade

How does the considered diagnostics beamline look like? RF cavity: Frequency f = ω/(2π) Peak deflection voltage Vy Linac Not to scale Magnet & screen positions not yet fixed For the moment we assume no matching lattice between the bunch compressor and the TDS

How does the considered diagnostics beamline look like? RF cavity: Frequency f = ω/(2π) Peak deflection voltage Vy Linac Fig. Credits: D. Malyutin PhD thesis Give bunch time-dependent kick => convert longitudinal to transverse Variable polarization (see later in the talk) allows deflection at any angle

How does the considered diagnostics beamline look like? RF cavity: Frequency f = ω/(2π) Peak deflection voltage Vy Linac Longitudinal Resolution: e-bunch: Energy E Vertical emittance εy Fig. Credits: D. Malyutin PhD thesis High frequency ~12 GHz (X-band) allows 3-4 times higher resolution than S-band RF cavity: Frequency f = ω/(2π) Peak deflection voltage Vy

Resolution Range EuPRAXIA case εn=0.3mm*mrad Δφ = π/2 f=12GHz εn=1mm*mrad Δφ = π/2 f=12GHz Typical β function at ARES after magnetic bunch compression <30m. If we want to avoid adding a matching section between BC and TDS we need to work with higher deflection voltages.

Longitudinal Beam Profile Characterization Beam line & e-Bunch Parameters V = 40MV β(s0) = 6m (as from bunch compression) Δφ = π/2 Q=2.7 pC Energy = 100 MeV εn= 0.2 mm mrad σt=0.461 fs Simulated Measured Streaked Beam @ Screen Longitudinal Resolution = 0.42 fs Code used: Elegant Space Charge OFF (implementation ongoing) t [fs] Screen resolution: 10μm x [mm]

Technical Issues limiting Operation at High Voltage RF Phase jitter: RF Phase jitter causes transverse jitter of the beam on the screen in the direction of the streaking of the TDS. ~0.1deg RMS phase jitter can be tolerated during the calibration procedure by using a screen directly at the cavity exit. The bunch profile measurement is single shot. The phase jitter will anyway have strong impact on the measurement time. For 10Hz operation we estimated a waiting time of about 30 seconds to get one shot on the high resolution measurement screen at 40MV. For high charges and high rep. rates radiation issues might arise (many e-bunches lost along the diagnostics line). Arrival time beam jitter effect is negligible if <10fs RMS Temperature stability of the cavity.

3D charge distribution reconstruction Introduction of X-Band TDS Design with Variable Streaking Direction 3D charge distribution reconstruction

Novel X-Band TDS design with Variable Polarization Variable Polarization Circular TE11 Mode Launcher This new design allows for changing the streaking direction of the TDS. A. Grudiev, CLIC-note-1067 (2016) Collaboration working on first prototype See “X-Band TDS Project”, MOPAB044, Proc. IPAC 2017 At DESY SINBAD, FLASH2, FLASHForward and potentially XFEL involved

Reconstruction of 3D Charge Distribution of the witness beam Relies on streaking at multiple angles Completely new measurement technique Reference: MOPAB045, Proc. IPAC 2017 Principle: Streak beam at different angles and measure intensity at screen Identify the longitudinal slices for each streaking direction Combine 1D transverse slice profiles from different streaking directions to form a 2D transverse slice profile Stack longitudinal slices together to form complete 3D charge profile reconstruction

Tomographic Reconstruction – Input Beam Input beam production simulated by J. Zhu Input beam used for test simulations at the TDS Example of correlation x-z that we wish to detect

Tomographic Reconstruction - Analysis 4 xy screen profiles [2D] out of 16. Convert y to t and divide into slices (0.85 fs) [2D] For each slice, take projection on x axis [1D] and combine using tomographic reconstruction (SART algorithm) [2D] Stack slices [3D] Screen resolution 20μm.

Tomographic Reconstruction – Final Result Actual 3D e-beam distribution at the screen location Reconstructed 3D distribution Sixteen streaking angles (only 1 calibration needed) Quadrupoles in the lattice OFF Simulations in elegant No space charge, no jitter, no misalignment effects so far included

Tomographic Reconstruction – Artefacts Bunch length decreases with TDS off Bunch length increases with TDS on Slices don’t match up Beam longitudinal phase space @ TDS entrance Induced Energy spread in the TDS (Panofsky-Wenzel Theorem) E=84MeV  velocity bunching effect! This effect is frozen for highly relativistic beams

Longitudinal phase space measurement Extension of the Lattice design for Energy Spread measurement, low Voltage measurement Longitudinal phase space measurement

Longitudinal Phase Space Measurement Reference: MOPAB046, Proc. IPAC 2017 Theoretical resolution achievable for dipole spectrometer: Voltage of only 2MV per cavity (4MV in total) used to limit TDS induced energy spread Dipole bending angle: 0.93 rad (53deg). Simulations done with Elegant (CSR included)

Longitudinal Phase Space Measurement Original (left) and reconstructed (right) slice energy spread There is significant induced energy spread from the TDS visible in the reconstruction The correlation is well reproduced

Conclusions Longitudinal Characterization for low charge ultra-short Bunches has been discussed Attosecond resolution is in principle possible by using high gradient X-band TDS Limits for high TDS voltage operation are set by stability limits (RF jitter, temperature stability) Space charge force introduces artefacts in the measurement and might play an important role especially at low energies and will be soon included in the study A novel X-band TDS Design with Variable Polarization opens new opportunities for beam Characterization Allows streaking of bunch at all angles We have discussed 3D charge profile reconstruction using tomographic techniques, that would allow for characterization of the transverse vs longitudinal correlation of the charge profile, important both for LWFA and PWFA Slice emittance measurements (e.g. via quadrupole scan) were not covered by this talk but complete the aimed characterization of the witness beam. Studies are foreseen.

Thank you for the attention ! Acknowledgements The numerical studies presented in this talk are done by D. Marx. Many thanks to: R. Aßmann, F. Christie, R. D‘Arcy, U. Dorda, M. Huening, J. Osterhoff, S. Schreiber, M. Vogt, J. Zhu, A. Grudiev (CERN), P. Craievich (PSI) for profitable discussions. Thank you for the attention !