Measurement of electron-beam bunch length and emittance using shot-noise-driven fluctuations in incoherent radiation

It has long been recognized that noise can be used to infer properties of physical systems. Noise by nature carries information without disturbing its system, in contrast to externally applied diagnostics, which often modify the very parameter they attempt to measure. Since the pioneering experiments in the early decades of the century, in which measurements of noise provided the standard for fundamental quantities —electric charge, the Boltzmann constant, and temperature— fluctuation-based diagnostic techniques have been pursued in diverse applications [1]. The range of disciplines, from astrophysics, circuits, acoustics, thermodynamics, material science, industrial engineering, biology, to optics, across which researchers have isolated the signature of essential parameters in noise spectra is reviewed in Ref. [2]. In this Letter, we report first measurements of the full single shot spectra of wiggler spontaneous emission with the resolution required to demonstrate the predicted 100% fluctuation of the spectral intensity and to investigate its detailed properties. The fluctuational characteristics of the wiggler spontaneous emission spectra are studied and utilized to extract electron beam longitudinal and transverse phase space information. The calibrations obtained from the fluctuation measurements rely on a comparison with optical diffraction limited spot size, which provides an absolute calibration standard. It has been recently proposed that properties of a bunch of charged particles can be obtained through measurement of the fluctuations of incoherent emissions from the bunch [3]. Emission can be produced through interaction of the particle with external electromagnetic fields (i.e., wiggler, undulator) or media (i.e., transition radiation, Cerenkov, Smith-Purcell, etc.). The Fourier component, Ev ,o f the electric field of the spontaneous radiation from charged particle beams is given by Ev ev X j e ivt j , (1)