PHYSICS PHD DISSERTATION DEFENSE: Siqi Li

Ph.D. Candidate:  Siqi Li
Research Advisor:  Zhirong Huang

Date: Friday, May17, 2019
Time: 9:00am
Location: SLAC B48 Redwood Conference Room C/D.

Title: Laser shaping and streaking for X-ray free-electron lasers

Abstract:
Lasers have been a versatile tool in facilitating x-ray free electron lasers in various components of the machine. At the beginning of photo-cathode injectors, the properties of the drive laser determine the initial properties of the electron bunch. Laser heater has been widely used to modulate electron bunch energy spread in order to suppress collective effects arising from dispersive sections in the accelerator. In the course of undulator radiation, optical lasers provide energy modulations that can be converted into favorable qualities in the x-ray photons. After the x-ray photons have been generated, optical lasers are also utilized to aid diagnosis of the x-ray pulse and various types of user experiments. Laser streaking makes use of the photoelectrons generated by the x-ray pulse to extract valuable information about the x-ray pulse which can be challenging to probe directly.

In this work we examine laser applications specifically in the Linac Coherent Light Source (LCLS). In the first half of the thesis, we focus on laser shaping techniques applied to the injector drive laser and laser heater, in both temporal and spatial dimensions. We further discuss ghost imaging as an application of laser shaping in accelerators. In the second half, we switch gear to laser streaking as a measurement tool for sub-femtosecond x-ray pulses. We present a reconstruction algorithm based on a theoretical understanding of laser streaking and demonstrate experimental results.