Wednesday, May 22, 2019
Ultrashort Hard X-Ray Pulses from Laser Plasma
Vojtěch Horný, Institute of Plasma Physics, Czech Academy of SciencesThe pulses of hard X-ray light are demanded by various fields such as fundamental research, medicine, industry or public security. The major advances in the field of ultraintense laser pulse interaction with matter achieved within last decades brought conceptually new sources of X-ray generation. The laser wakefield acceleration of electrons in the gaseous target provides a convenient source of so called betatron radiation, which is emitted due to the transverse oscillations of accelerated electrons. This theoretical talk will introduce the method how to construct the spectrogram which comprises both information about the frequency and temporal profile of generated X-ray signal. Using this method, it was shown that the duration of such pulses can be few femtoseconds or even less, which is beneficial for sampling the fundamental physical processes such as lattice vibrations, chemical reaction or phase transition, which occur on the time scale of about tens of femtoseconds.
Return to the 2018-2019 AMOP Seminar ScheduleWednesday, Sept. 19, 2018
Imaging Coherent Nuclear Motion in Photoexcited Molecules with Ultrafast Electron Diffraction
Kyle Wilkin, Department of Physics and Astronomy, University of Nebraska-LincolnWe have used ultrafast electron diffraction (UED) to capture the structural dynamics during the UV photodissociation reaction of 1,2-Diiodotetrafluoroethane (C2F4I2). The experiment was performed at the SLAC UED facility with a 3.7 MeV electron beam where previous experiments have shown femtosecond resolution [1][2][3]. Previously, the Zewail group used UED with 5 ps resolution to study the evolution of photo-dissociated C2F4I2 and found the structure of the transient state C2F4I to be the classical non-bridged structure [4]. No evidence of a bridged structure was found; however, a question remained on whether a bridged structure with a femtosecond lifetime also existed. We have observed that the non-bridged structure forms within 200 fs of the laser excitation, one vibrational period of the C-I bond. After dissociation an oscillation with a 250 fs period is observed in the inter-atomic distances of the transient. Through comparison to numerical simulations, this oscillation has been determined to be a combination of coherent vibrations and rotations in the isolated CF2 group.
Return to the 2018-2019 AMOP Seminar ScheduleWednesday, Nov. 14, 2018
Electron Trapping from Interactions between Laser-Driven Relativistic Plasma Waves
Grigory Golovin, Senior Research Associate, Extreme Light LaboratoryControllable injection of electrons into an accelerating plasma wake remains one of the biggest issues of laser and beam-driven wakefield acceleration. Final properties of the accelerated electron beam, such as charge, energy spread, pulse duration, emittance, and stability, are often limited by the injection process. To control and optimize these parameters, electrons have to be placed at a correct phase of the wake with high precision. At Extreme Light Laboratory we have recently experimentally demonstrated two novel injection mechanisms capable of such precise control: injection via ponderomotive drift and wake-wake interference. In our experiments, two laser pulses (drive and injector) were propagated through a plasma in crossing directions. The injector pulse of a very high intensity (up to 1.7x10^20 W/cm^2) pushed electrons via its ponderomotive force into the drive-pulse wake, causing their trapping (injection via ponderomotive drift). In addition, it created its own wake, which interfered with the drive-pulse wake, also resulting in electron trapping (injection via wake-wake interference). The demonstrated injection mechanisms have important features which can lead to novel ways to engineer wakefield-accelerated electron beams and enhance their quality. The injection via ponderomotive drift can occur at any point where the injector laser beam intersects the drive wake. One can, therefore, inject electrons at the optimal location and phase with respect to the wake for maximal accelerator performance and beam quality. Since injection via wake-wake interference occurs periodically, multiple electron bunches, separated by a plasma period, can be injected, forming a bunch train, which is of high interest for numerous applications, including time-resolved pump-probe studies of ultrafast phenomena. In addition, both mechanisms can be used not only in laser-wakefield accelerators, but in beam-driven ones also.
Return to the 2018-2019 AMOP Seminar Schedule