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# CRD - Elektronenstrahldiagnose mit kohärenter Strahlung

**CRD** - **C**oherent **R**adiation **D**iagnostics

**What is CRD?**

Coherent Radiation Diagnostics (CRD) is concerned with accessing characteristics of the longitudinal charge distribution of electron bunches by measuring the spectral distribution of radiation, that the electrons radiate under certain circumstances: synchrotron radiation from dipole magnets, transition radiation from thin screens or foils traversed by the electrons, diffraction radiation from screens with holes or slits, Smith-Purcell radiation from gratings, or undulator radiation. At long wavelengths, comparable to the bunch length or to the length scale of structural features of the bunch, the shape of the emission spectrum is modified by coherent effects. Whereas at short wavelengths the power emitted by a bunch of N electrons is just N times that of a single electron, coherence increases this, eventually to another factor of N for full coherent emission.

Measurements of the spectrum are done with various instruments, for example Martin-Puplett interferometers or grating spectrometers. Access to the bunch charge distribution then requires relatively elaborate mathematical techniques, as the imprint in the spectrum is in fact through the square of the Fourier transform of the charge distribution.

Qualitative information of the longitudinal charge distribution can also be attained with relatively simple setups just monitoring the total, frequency-integrated radiation intensity (so-called bunch compression monitors).

Details on the principles of coherent radiation diagnostics can be found in the report TESLA FEL 2006-03.