Event

 

Name: Kazi Sharmeen Rashid

Committee:

Professor Thomas Antonsen, Chair

Professor Phillip Sprangle

Professor Patrick O'Shea

Date/time: Friday, August 21 at 10 AM 

Location: AVW 2460

Title: ADJOINT SENSITIVITY ANALYSIS AND OPTIMIZATION OF 1D FREE-ELECTRON LASER WITH SPACE CHARGE EFFECTS

Abstract:  Free-electron lasers (FELs) are versatile sources of coherent electromagnetic radiation with applications ranging from materials science and chemistry to medicine, and accelerator physics. Their performance depends critically on the interaction between the electron beam, radiation field, and wiggler magnetic field. As increasingly realistic physical effects are incorporated into FEL models, analyzing and optimizing FEL performance becomes significantly more challenging. This research develops and validates a computational framework for the sensitivity analysis and optimization of one-dimensional FEL models with progressively increasing physical complexity.

Three hierarchical FEL models are investigated. The first is a simplified one-dimensional model that captures the fundamental resonant interaction between the electron beam, radiation field, and externally applied wiggler field. The second extends this model by incorporating the exact relativistic longitudinal momentum, the accumulated wiggler phase, and the complete nonlinear interaction between the radiation and wiggler fields through both the field components and their complex conjugates, thereby retaining counter-rotating and higher-order coupling terms neglected in the simplified formulation. The third further incorporates longitudinal space-charge effects through coupled transverse-electric (TE) and transverse-magnetic (TM) waveguide modes, yielding a more complete self-consistent description of FEL dynamics. This hierarchical framework enables a systematic assessment of how increasingly realistic beam and field physics influence FEL gain, radiation intensity, efficiency, particle dynamics, and the optimization of wiggler taper profiles.

Global optimization is first performed using differential evolution, a derivative-free optimization algorithm that explores the design space without requiring gradient information. Differential evolution is used to identify favorable operating points together with optimized wiggler field-strength and wiggler wavenumber profiles for each FEL model. The optimized solutions obtained from this global search provide the starting point for further refinement.

To efficiently improve these solutions, an adjoint sensitivity formulation is developed for each FEL model. Rather than repeatedly perturbing individual design variables, the adjoint approach computes the sensitivity of a chosen objective function with respect to many design variables simultaneously, with a computational cost that is nearly independent of the number of control parameters. This makes it particularly well suited for large optimization problems. The computed sensitivity values are then validated against finite-difference calculations and Taylor remainder tests before being used in gradient-based local optimization to further refine the solutions obtained from differential evolution.

The proposed framework provides an efficient approach for the analysis and optimization of increasingly realistic free-electron laser models. The results give insight into the influence of higher-order physical effects, especially the influence of longitudinal space charge, on FEL performance and parameter sensitivity.