UCLA

Ultrafast science is one of the frontiers of modern physics. It allows us to explore intense, out-of-equilibrium process by manipulating laser pulses shorter than the thermal time-scale of materials. Here we extend that capability to MeV electron beams by using the high extraction field of a radio-frequency photoinjector to generate dense bunches of electrons. This gives us a unique source which we take advantage of to explore two different systems: the first is a photonic structure called a dielectric laser accelerator (DLA), and the second is an accelerator based version of an ultrafast electron microscope (UEM).

DLA is an advanced accelerator technology which takes advantage of the high damage threshold of dielectrics (as compared to metals) to sustain GV/m accelerating gradients. It works on the same principle as a conventional RF linac, but it is scaled down 100,000 times from microwave frequencies to optical frequencies. This scaling has important consequences for the beam dynamics of the accelerator, and it leads us to consider a richly nonlinear system in which the stable accelerating region is surrounded by areas of chaos. In order to interrogate the dynamics of this system, we have adapted the beam from an RF photoinjector to fit inside the sub-micron aperture of a DLA. We then perform time-resolved spectroscopic measurements to determine the interaction strength of the accelerator. We record accelerating fields of up to 1.8 GV/m and energy gains as high as 315 keV, but we also find that self-phase modulation can cause dephasing if left uncompensated. Our analysis of the dephasing mechanism, and its compensation, leads us to the design of a DLA with all-optical control of the beam dynamics.

MeV UEM is a branch of ultrafast microscopy which is designed to achieve high spatio-temporal resolution in a single-shot. It requires placing billions of electrons inside a small volume of phase space in order to detect contrast from weakly scattering objects. We show that this is possible using relativistic electrons from an RF photoinjector, but because these electrons are stiff we have to replace the conventional microscope optics with strong permanent magnet quadrupole (PMQ) lenses. We have tested these lenses in a two-stage objective-projector setup and measured a total magnification of 900x. When operating this microscope near the design current of 200 mA we observe a strong distortion of the image. Our analysis suggests that the distortion is caused by nonlinear phase-space correlations from the space-charge kick of a Gaussian bunch. We discuss ways to improve the design and obtain 10 nm-10 ps resolution.