Coherent Optical Transition Radiation (CTR/OTR) is emitted when relativistic electron beams, accelerated by an intense laser pulse focused on a solid target, exit from the solid target into vacuum (1). CTR is a commonly used diagnostic to characterize ultrashort electron bunches in conventional accelerators, e.g., it is used at the European X Free Electron Laser (XFEL), where it recorded single-shot 3D imaging of the electron bunch, measuring the absolute position of the electron bunch with microscale resolution (2). Used in the context of high-intensity laser-matter interactions, CTR could demonstrate that electrons are produced in the form of micro-bunches. This was revealed through interferometry patterns observed in the angular distribution patterns of the CTR light, as recorded by a CCD camera in the near field. Micro-bunching has also been confirmed through the spectral measurements of CTR (3), (4), (5). One of the exciting aspects of CTR is that it allows measuring the transverse size of the electron bunches, in the few microns regime, by imaging (in the far field) the source target (6). Hence, it allows direct measuring of the high-intensity laser pattern from which the electrons are born, and this at full power.

In conclusion, CTR is the only diagnostic that can directly image the laser interaction at full power on target, hence solely able to measure the real on-target intensity and energy deposition, which no other diagnostic can directly achieve.

CETAL team constructed a simplified optical setup for imaging and spectral recordings of the CTR emission from thick solid targets when interacting with high intensity laser pulses. This new setup was used to record CTR images and spectra resulting from interaction of high intensity laser pulses with Al 70 µm and Ti 25 µm foils.

The laser pulses were focused onto the targets with a dielectric coated, 10° off-axis parabola with an f/# of 30. In the optical design shown in Fig. 1, the focal spot was imaged with a microscope system composed by an objective L2 of 1 inch diameter with f = 75 mm and the lens L3 with f = 200 mm, which create the image onto a CCD camera. An IR filter was mounted in front of the entire imaging system in order to cut down 800 nm by 2-3 orders of magnitude.

The spectral system collects the CTR with a Thorlabs collimator which is transported to the spectrometer by an optical fibre with a broadband transmission (200 ÷ 1000 nm). The spectra recorded with the iDUS camera connected to the KYMERA spectrometer will be analysed using specialized ANDOR software that was provided with the spectrometer. In order to make possible observation of the CTR spots without saturation at high laser energy levels (~5 J), a variable optical attenuator was added.

CTR images recorded at high energy laser regime, up to 3 J per pulse, reveal an intensity profile very similar with the focus profile classically recorded with the microscope at very low energy (~20 pJ), for both types of foil targets (Al and Ti). At higher energy, up to 5.1 J, besides the CTR fingerprint of the focus, a large area of light occurs. That could be explained by laser beam aberration which becomes important at high energy levels.



References:

1. Theoretical study of transition radiation from hot electrons generated in the laser–solid interaction Physics of Plasmas. Jian Zheng, et al. 2003, Physics of Plasmas, Vol. 10, p. 2994-3003.
2. Coherent optical transition radiation as a tool for ultrashort electron bunch diagnostics. Gianluca Geloni, et al., Proceedings of DIPAC09, Basel Switzerland, p. 251 -253.
3. Observation of coherent transition radiation. Uwe Happek, Albert John Sievert and E. Blum, 1991, Physical Review Letters, p. 2962-2965.
4. Fast Electron Transport in Ultraintense Laser Pulse Interaction with Solid Targets by Rear-Side Self-Radiation Diagnostics. J. J. Santos, et al.. 2002, Phys. Rev. Lett., Vol. 89, p. 025001.
5. Evidence of Ultrashort Electron Bunches in Laser-Plasma Interactions at Relativistic Intensities. S. D. Baton, et al. 2003, Phys. Rev. Lett., Vol. 91, p. 105001.
6. Observation of Coherent Optical Transition Radiation Interference Fringes Generated by Laser Plasma Accelerator Electron Beamlets, Alex Lumpkin et al., 2018, IEEE Advanced Accelerator Concepts Workshop (AAC), p. 1-5, doi: 10.1109/AAC.2018.8659381.