Extreme regimes of femtosecond photoemission from a copper cathode in a dc electron gun

Abstract

The femtosecond photoemission yield from a copper cathode and the emittance of the created electron beams has been studied in a 12 MeV=m, 100 keV dc electron gun over a wide range of laser fluence, from the linear photoemission regime until the onset of image charge limitations and cathode damaging. The measured photoemission curves can be described well with available theory which includes the Schottky effect, second-order photoemission, and image charge limitation. The second-order photoemission can be explained by thermally assisted one-photon photoemission (1PPE) and by above-threshold two-photon photoemission (2PPE). Measurements with a fresh cathode suggest that the 2PPE process is dominant. The beam emittance has been measured for the entire range of initial surface charge densities as well. The emittance measurements of space-charge dominated beams can be described well by an envelope equation with generalized perveance. The dc gun produces 0.1 pC bunches with 25 nm rms normalized emittance, corresponding to a normalized brightness usually associated with rf photoguns. In this experimental study the limits of femtosecond photoemission from a copper cathode have been explored and analyzed in great detail, resulting in improved understanding of the underlying mechanisms.

Transient lensing from a photoemitted electron gas imaged by ultrafast electron microscopy

“…The laser pump and electron probe pulses are precisely synchronized in time at a repetition rate of 490 kHz and their relative delay is adjusted by means of an optical delay stage. In this way, we record realspace movies of the charge density dynamics after laser excitation with an integration time of 1 regime 44,51 . The laser fluence is low enough that ablation and plasma formation can be discarded 52 .…”

“…In contrast to frequency-or time-domain THz/microwave spectroscopic investigations, performing such experiments inside an ultrafast electron microscope enables spatially resolving photoexcited electron density variations, similar to previous scanning probe microscopy experiments 31,[34][35][36] but with fs-picosecond (ps) temporal resolution. Furthermore, the capability to image and temporally resolve photoemitted carriers is highly relevant in the plasma physics community [37][38][39][40] , and for the development and characterization of high-brightness electron sources for fourth-generation X-ray facilities or ultrafast electron diffraction and microscopy setups [41][42][43][44][45][46][47][48][49] . The analytic model we develop here allows rough approximation of the number of electrons in the photoemitted gas, which is directly correlated with the electron lens magnification, as well as their velocity spread.…”