Ultracold Electron Source for Ultrafast Electron Diffraction

Single-shot ultrafast electron diffraction (UED) was introduced as a viable way to measure ultrafast processes, such as structural transitions in materials. For UED, the dynamics in a sample are induced by a femtosecond laser pulse and ultrashort electron bunches are used to probe the ultrafast processes in the sample. The ultracold charged particle cloud (UCPC) source researched in this work is an extended electron source with the potential to produce electron bunches with the extreme quality required for single shot UED. The UCPC source uses rubidium atoms that are trapped and cooled in a magneto-optical trap. The rubidium atoms are near-threshold photoionized in a two step ionization process to form the ultracold charged particle cloud. The electrons in the ionization volume are accelerated by a DC electric field. In previous work, nanosecond long, narrow bandwidth ionization pulses were used to create ultracold electron bunches. In this work, measurements of the transverse source temperature are presented for bunches that are ionized with femtosecond long, broad bandwidth ionization pulses. For near-threshold ionization laser wavelengths from 489 nm to 477 nm, temperatures were measured from 25 to 240 K. Very low bunch temperatures were measured despite the broad bandwidth of the ionization laser. Transverse coherence lengths up to 20 nm are reported, which is sufficiently long to perform diffraction experiments on typical protein crystal samples. Individual electron bunches contain typically 10 electrons, less than what is ideally required for single shot UED. The temperature measurements are compared to simulations performed with a semi-analytical model based on electron trajectories in the combined potential energy function of the rubidium atoms and the electric field of the accelerator. The overall shape of the simulated temperature as a function of the ionization laser wavelength mirrors the measured data. From additional measurements, we saw that the transverse source temperature varies sinusoidally as a function of the polarization angle of the ionization laser. The mean value of the oscillations of the transverse source temperature increases monotonously for decreasing ionization laser wavelengths, as does the amplitude of the oscillations. This behavior is also confirmed by the model. The first diffraction patterns that have been measured with the UCPC source setup are also reported in this thesis, measured with a graphene sample. With crystal domains that are several times smaller than the electron beam at the sample, the diffraction pattern consists of rings. The diffraction angles of the first two diffraction rings were measured as a function of the beam energy and match with values calculated using the grating equation. The angular spread determined from the measured diffraction patterns is however many times larger than what is expected based on the transverse source temperature. To improve experiments, a quadrupole lens is added to the setup to counter astigmatism and a second solenoid lens is added. It is left up to future researchers to use the improved setup.