DESY

Resonant Diffraction Radiation from Inclined Targets as a Tool for Bunch Lengths Diagnostics 

ER: Leonid Sukhikh (leonid.sukhikh@desy.de)
Supervisor: Kay Wittenburg  (kay.wittenburg@desy.de)

There exists considerable interest in studying the properties of coherent Smith-Purcell radiation (CSPR) because of the potential possibility to use such a type of radiation from relativistic electrons for “non-invasive” bunch length diagnostics. Radiation with wavelengths λ > σ (σ = bunch length) is emitted coherently, resulting in an emission intensity which scales proportional to N2 (N = number of particles per bunch), and for wavelengths λ < σ the intensity scales only proportional to N.
Due to the grating dispersion relation the wavelength of Smith-Purcell (SP) radiation is defined by the grating period and the photon emission angle. Therefore, the investigation of the SP radiation yield dependence on the polar angle enables to observe the intermediate zone where the transition from incoherent to coherent SP radiation takes place, and as a result to extract a bunch length.

In frame of this project an inclined grating with one/two detectors placed at fixed positions (observation angles) is used to measure the radiation yield as a function of the grating inclination angle. In contrast to CSPR which is emitted from a grating whose surface is oriented parallel to the beam axis the term coherent resonant diffraction radiation (CRDR) seems more appropriate when the grating surface is tilted.

In this diagnostics scheme the information of both individual detectors is available with only one scan of the grating inclination angle. Furthermore, under abandonment of the need for bunch reconstruction and restricting to the case of bunch length measurement the information of both detectors can be combined by taking the intensity ratio R of the signals from detectors D1 and D2. The advantage of such a diagnostics scheme is that one has not to rely on absolute values of the radiation yield, avoiding the need to know the individual sensitivity of each detector to a high level of accuracy.