Applications of THz detection: Single shot electron bunch length measurements

Applications of THz detection techniques in accelerator and free electron laser science

NEW: We demonstrate the spectral upconversion of a unipolar subpicosecond terahertz (THz) pulse, where the THz pulse is the Coulomb field of a single relativistic electron bunch. The upconversion to the optical allows remotely located detection of long wavelength and nonpropagating components of the THz spectrum, as required for ultrafast electron bunch diagnostics [Appl. Phys. Lett. 2010]

HIGHLIGHT: benchmarking electro-optic detection for ultrashort THz pulses. An ultra-short relativistic electron bunch is precisely characterized by a state-of-the-art transverse deflecting cavity. The transient electric field of the electron bunch is used as a known THz pulse for calibration and validation of electro-optic detection using GaP as crystal and a 30 fs Ti:S laser pulse as probe. [Phys. Rev. Lett. 2007]

The output from a far-infrared (wavelengths longer than 50 micrometers) free electron laser (FEL) can be characterized with electro-optic detection techniques that are commonly used in THz science. At the FELIX free electron laser we use single-shot detection techniques since the time jitter between the FEL far-infrared pulse and the optical probe pulse is on the order of 400 fs.

Figure right: Single shot electro-optic measurement of a quasi-monochromatic FEL pulse. The wavelength is 130 micrometer, and one cycle therefore corresponds to 430 fs. The image has been obtained by subtracting an image without the presence of an FEL pulse (background) from an image where an FEL pulse was present. The graph has been obtained by vertically binning the image. More information can be found in our paper presented at the FEL2004 conference.

Electro-optic detection of the electric field of electron bunches, which can be regarded as a unipolar THz pulse, is a promising technique for the measurement of the bunch length and shape in the sub-picosecond time domain. The electro-optic detection method makes use of the fact that the local electric field of a highly relativistic electron bunch moving in a straight line is almost entirely concentrated perpendicular to its direction of motion. This electric field induces birefringence in an electro-optic crystal placed in the vicinity of the beam. The amount of birefringence depends on the electric field and is probed by monitoring the change of polarization of a short optical laser pulse.

At FELIX we develop schemes for real-time nondestructive, single-shot measurements of the Coulomb field of sub-picosecond electron bunches. More information can be found on the electron bunch webpage.

In addition, recent FELIX experiments have used single-shot electro-optic detection to measure the temporal profile of the far-infrared electric field pulse of coherent synchrotron radiation (CSR), initial results of which are reported here. Such time-resolved CSR measurements have the potential for a completely non-invasive bunch longitudinal profile determination, without the ambiguity in profile that is present in CSR spectral measurements. [Berden et al. DIPAC2005, Jamison et al. NIMA 2006]

Figure: The electro-optic signal of the CSR emitted from the entrance to the dipole magnet. Two separate measurements are shown to demonstrate the level of reproducibility.