Abstract
Compact DC photoelectron guns of high-voltage are highly desired to output ultrabright and ultrashort electron pulses for accessing irreversible processes by using the ultrafast electron diffraction (UED) technique. The high-voltage breakdown, however, is a major technical barrier to providing an intense electric field strength in a condensed space between the photocathode and the anode when the voltage is over 120 kV. In this work, by adopting the concept of voltage division, we propose a novel design of ultrabright near-relativistic DC photoelectron guns for UED. The electric field breakdown mainly caused by micro-particle collision could be avoided such that an optimized three-level acceleration DC gun can work with an electron energy of up to 270 keV and an electric field strength of up to 15 MV/m. N-particle simulations of the electron pulse propagation show that, with such a DC electron gun, it is possible to have ultrabright and ultrashort electron probe pulses with no jitter issue.
Highlights
The temporal resolution of Ultrafast electron diffraction (UED) depends on the optical pump and electron probe pulse durations, as well as the time jitter and the velocity mismatch between them
The brightness of the electron pulse gives another limit for applications in probing irreversible processes
We proposed a novel scheme of high-voltage DC electron guns as an alternative option for table-top single-shot femtosecond electron diffraction instruments
Summary
Ultrafast electron diffraction (UED), as a table-top in situ probe tool, has been widely used to study on lattice dynamics and chemical reaction dynamics with an atomic resolution due to the high scattering cross-section of the electron–atom interaction. Roughly speaking, the temporal resolution of UED depends on the optical pump and electron probe pulse durations, as well as the time jitter and the velocity mismatch between them. Another option is to use an RF cavity to compress the nonrelativistic electron pulse generated by DC photoelectron guns, based on its chirping property. The curiosity arises if it is possible to obtain brighter and shorter electron pulses by further increasing the electron energy of DC guns up to the near relativistic region In this scheme, the synchronization issue of RF photoelectron guns mentioned above could be inherently avoided; an alternative ultrabright, ultrashort, and ultrastable electron source could be developed for single-shot UED instruments. An optimized multilevel acceleration photoelectron gun is designed and simulated to successively accelerate electrons up to 270 keV and generate 160 fs (or 251 fs) electron pulses of 105 (or 106) electrons per pulse Such an ultrabright DC photoelectron gun, could be employed in UED for probing irreversible processes with reliable long-term stability. It demonstrates clearly that the field enhancement at both the boundaries and the center of electrodes is acceptable, much smaller than 20 MV/m
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