Relativity and Accelerator Engineering

TL;DR

This paper explores how the theory of relativity, especially four-dimensional geometry and relativistic kinematics, is crucial for understanding and improving the design and operation of X-ray Free Electron Lasers (XFELs).

Contribution

It introduces a four-dimensional geometrical approach to analyze relativistic effects in XFELs, emphasizing the importance of relativistic kinematics and Wigner rotation for accurate modeling.

Findings

Relativistic kinematics significantly affect XFEL radiation emission.

Wigner rotation of the modulation wave vector is essential for correct predictions.

Ignoring relativistic effects leads to discrepancies between theory and experiments.

Abstract

From a geometrical viewpoint, according to the theory of relativity, space and time constitute a four-dimensional continuum with pseudo-Euclidean structure. This has recently begun to be a practically important statement in accelerator physics. An X-ray Free Electron Laser (XFEL) is in fact the best, exciting example of an engineering system where improvements in accelerator technology makes it possible to develop ultrarelativistic macroscopic objects with an internal fine structure, and the theory of relativity plays an essential role in their description. An ultrarelativistic electron bunch modulated at nanometer-scale in XFELs has indeed a macroscopic finite-size of order of 10 $\mu$m. Its internal, collective structure is characterized in terms of a wave number vector. Here we will show that a four-dimensional geometrical approach, unusual in accelerator physics, is needed to solve problems involving the emission of radiation from an ultrarelativistic modulated electron beam accelerating along a curved trajectory. We will see that relativistic kinematics enters XFEL physics in a most fundamental way through the so-called Wigner rotation of the modulation wave number vector, which is closely associated to the relativity of simultaneity. If not taken into account, relativistic kinematics effects would lead to a strong qualitative disagreement between theory and experiments. In this paper, several examples of relativistic kinematics effects, which are important for current and future XFEL operation, are studied. The theory of relativity is applied by providing details of the clock synchronization procedure within the laboratory frame. This approach, exploited here but unusual in literature, is rather "practical", and should be acceptable to accelerator physicists.