Thesis defense of Arjun Radha Krishnan

The generation and characterization of ultrashort, coherent radiation pulses in the extreme ultraviolet (EUV) and X-ray regimes are critical for investigating ultrafast dynamic processes in matter. This thesis advances these capabilities through two complementary research directions: the development of deep-learning-based methods for advanced beam diagnostics and the first experimental implementation of echo-enabled harmonic generation (EEHG) in an electron storage ring.
To overcome the limitations of conventional diagnostics, deep-learning-based methods were developed for both storage ring and free-electron laser (FEL) applications. At the 1.5-GeV electron storage ring DELTA, operated by the Center for Synchrotron Radiation of TU Dortmund University, coherent harmonic generation (CHG) is employed, a technique where an external laser seed interacts with the electron beam to generate ultrashort radiation pulses at harmonics of the laser frequency. A convolutional neural network (CNN) was trained to extract the spectral phase properties of the seed laser pulses from the CHG radiation spectra. This method successfully identified a large uncompensated third-order dispersion in the seed pulse, explaining previously unresolved spectral asymmetries. Furthermore, two CNN-based methods were implemented to reconstruct photon pulse profiles from longitudinal electron phase space images at FLASH, an FEL in Hamburg. This work involved the application of a standard U-Net model and a specialized dual-input design (Y-Net) developed specifically for this task. The deep learning models demonstrate the capability to perform single-shot reconstruction of the photon pulse profiles without requiring frequent reference measurements and reduce non-physical artifacts.
In the experimental domain, this work presents the commissioning of the SPEED (Short-Pulse Emission via Echo at DELTA) facility at DELTA. This project utilizes the EEHG mechanism that employs a two-stage seeding scheme to manipulate electrons in the longitudinal phase space for harmonic generation. By reconfiguring a 4.75 m electromagnetic undulator, the world’s first proof-of-principle demonstration of EEHG in a storage ring was achieved. Coherent emission was successfully observed and characterized up to the 11th harmonic (73 nm) of an 800 nm seed laser. These results confirm the robustness of the EEHG mechanism in a circular accelerator. Building on this success, a conceptual design for an optimized and compact permanent-magnet-based EEHG setup is presented. Simulation studies indicate that this design, fitting within a typical 5-meter straight section, can extend efficient harmonic generation up to the 29th harmonic (≈ 27 nm), showing a path forward for future short-pulse sources based on electron storage rings.