Observation of Fine Structures in Laser-Driven Electron Beams Using Coherent Transition Radiation

We have measured the coherent optical transition radiation emitted by an electron beam from laser-plasma interaction. The measurement of the spectrum of the radiation reveals fine structures of the electron beam in the range 400–1000 nm. These structures are reproduced using an electron distribution from a 3D particle-in-cell simulation and are attributed to microbunching of the electron bunch due to its interaction with the laser field. When the radiator is placed closer to the interaction point, spectral oscillations have also been recorded, signature of the interference of the radiation produced by two electron bunches delayed by 74 fs. The second electron bunch duration is shown to be ultrashort to match the intensity level of the radiation. Whereas transition radiation was used at longer wavelengths in order to estimate the electron bunch length, this study focuses on the ultrashort structures of the electron beam.

Figure 1

OTR energy in the range $0.4–1.0\mu \mathrm{m}$ from the imaging diagnostic as function of the position of the radiator. After correction for the background, the signal has been integrated over the CCD chip and converted into energy using the absolute calibration of the detection system (spectral response from the CCD and the filters). The gray area represents the energy level for an incoherent emission estimated using the electron spectra measured independently.

Figure 2

Examples of OTR spectra obtained experimentally for a radiator placed at 30 mm from the interaction point. These spectra are deconvolved from the instrumental response. An iris limits the collection angle to half angles of (a) 3 mrad and (b) 8 mrad.

Figure 3

Electron distribution from 3D PIC simulation, containing electrons above 100 MeV: (a) perpendicular to the plane of polarization of the laser, (b) in the plane of polarization of the laser. The electrons move from left to right. A structure in the electron distribution can be seen as the electrons overlap with the transverse laser field. This structure is reproduced with a solid line shifted to the right.

Figure 4

OTR spectrum generated at a metal-vacuum boundary placed at $100\mu \mathrm{m}$ from the electron distribution in Fig. 3, using the angular distribution (solid line) and the spatial distribution (dashed line). The ratio between the coherent peak and the incoherent level scales linearly with the number of particles (150 000 electrons are used in the simulations).

Figure 5

(a) OTR spectrum with high frequency modulations in the range 500–600 nm. The very high frequency signal below 450 nm contains only noise from x rays directly detected by the CCD camera. The half angle of collection is 70 mrad. (b) Example of OTR spectrum computed using two electron bunches, as shown in the inset.