Electron pulse broadening due to space charge effects in a photoelectron gun for electron diffraction and streak camera systems

Abstract

The electron pulse broadening and energy spread, caused by space charge effects, in a photoelectron gun are studied analytically using a fluid model. The model is applicable in both the photocathode-to-mesh region and the postanode electron drift region. It is found that space charge effects in the photocathode-to-mesh region are generally unimportant even for subpicosecond pulses. However, because of the long drift distance, electron pulse broadening due to space charge effects in the drift region is usually significant and could be much larger than the initial electron pulse duration for a subpicosecond electron pulse. Space charge effects can also lead to a considerable electron energy spread in the drift region. Temporal broadening is calculated for an initial electron pulse as short as 50 fs with different electron densities, final electron energies, and drift distances. The results can be used to design electron guns producing subpicosecond pulses for streak cameras as well as for time resolved electron diffraction.

Double-mode electrostatic dispersing prism for electron pulse time-domain compression

“…It is worth mentioning that one must account for this initial distribution when calculating the time of flight for electron pulse duration at the prism exit. An important point to realize here is that space charge effect induced broadening in the prism can be safely ignored since the incident electron pulse has a much reduced space charge effect after a certain distance of propagation outside [4][5][6].…”

“…The space charge effect refers to the self-broadening due to the Coulomb repulsion between electrons in an electron pulse, which is strengthened when the electron density is getting higher. Due to that, time duration for a pulse of 50 fs containing only 10,000 electrons after 10 cm of travel at 30 keV is limited to several picoseconds [5,6]. So exploring techniques of electron pulse modulation is always in the forefront of UED since its advent.…”

Double-mode electrostatic dispersive prism for compressing electron pulse

“…Until now there still remains a big technological challenge in improving time-resolved performance of UED since temporal broadening of electron pulse induced by both space charge effect and its initial energy spread prevents obtaining such electron pulses with sub-100 fs durations, limiting the range of phenomena that can be studied by this technique. Due to that, time duration for a pulse of 50 fs containing only 10000 electrons after 10 cm of travel at 30 keV is limited to several picoseconds 4,5 . So exploring techniques of electron pulse modulation is always in the forefront of UED since its advent.…”

Laser-excited acoustical phonons probed by ultrashort pulses from a laser-driven x-ray diode

“…This kind of emission takes place at a laser fluence exceeding 0.7-1.1 mJ/ cm 2 . 21 We have measured an upper limit of the pulse duration of 26 ps ͑full width at half maximum͒ with an x-ray streak camera, which is the resolution limit of our camera. In spite of the good longtime stability, fluctuations of almost 50% have been observed, due to the strong nonlinear process.…”