Coherent Transition Radiation
Laser- and particle wakefield accelerators are capable of producing high-charge electron bunches with few-fs duration. In order to determine their applicability for a range of light source applications including seeding of Free Electron Lasers (FELs), it is necessary to experimentally access the full 3-dimensional structure of such bunches. While their transverse properties as well as their charge and energy can be characterized comparatively easily using knife-edge methods, dipole spectrometers and calibrated detectors, the measurement of the temporal duration and a possible substructure remains challenging.
This requires a temporal characterization technique that is sensitive a temporal range between 0.5 fs and several 10’s of fs in a single shot. In the absence of a short enough probe to map out this temporal range in the time domain, an established solution is the detection of the Coherent Transition Radiation (CTR) spectrum emitted by the electron bunch in the frequency domain. This circumvents the temporal resolution problem in an elegant way, at the expense of having to include additional assumptions when reconstructing the time structure from the measured spectra. In most cases a comparison with simulation results and physics constraints can help to pin down some of the remaining ambiguities.
Transition Radiation (TR) is produced as charged particles traverse the boundary of two different media such as a metal-vacuum or interface. If these particles arrive in a short bunch such as an LWFA electron bunch, then these particles radiate coherently (by emitting CTR) for wavelengths longer than the bunch duration and incoherently for shorter ones. This leads to a short-wavelength cutoff in the detected radiation spectrum at a frequency that is characteristic of the bunch duration. Bunch sub-structures or multiple bunches likewise leave their trace in the detected radiation spectrum according to the Fourier theorem. By experimentally collecting a multi-octave CTR spectrum (see Fig.1) and by using advanced phase retrieval algorithms a reconstruction of the particle bunch’s temporal properties is possible (see Fig. 2) and [1]. CTR spectrometry presents a single shot, non-destructive online bunch characterization technique that is compatible with all LWFA bunch parameters.
Fig.1: Schematic of experimental setup
Fig.2: Example of temporal bunch profile evolution in a length-variable gas cell.
For further reading please see:
- M. Heigoldt, et.al, Temporal evolution of longitudinal bunch profile in a laser wakefield accelerator Physical Review Special Topics - Accelerators and Beams 18, 121302 (2015)
- M. Heigoldt, Temporal dynamics of the longitudinal bunch profile in a laser wakefield accelerator, PhD thesis, LMU Munich (2018)
Research coordinator:
- David J. Campbell (LMU & University of Strathclyde, Glasgow)