Abstract
We present an RF gun design for single shot ultrafast electron diffraction experiments that can produce sub-100 fs high-charge electron bunches in the 130 keV energy range. Our simulations show that our proposed half-cell RF cavity is capable of producing 137 keV, 27 fs rms (60 fs FWHM), 106 electron bunches with an rms spot size of 276 μm and a transverse coherence length of 2.0 nm. The required operation power is 9.2 kW, significantly lower than conventional rf cavity designs and a key design feature. This electron source further relies on high electric field gradients at the cathode to simultaneously accelerate and compress the electron bunch to open up new space-time resolution domains for atomically resolved dynamics.
Highlights
Observing atomic motions during structural transitions is one of the great challenges in science
We present an RF gun design for single shot ultrafast electron diffraction experiments that can produce sub-100 fs high-charge electron bunches in the 130 keV energy range
Our simulations show that our proposed half-cell RF cavity is capable of producing 137 keV, 27 fs rms (60 fs FWHM), 106 electron bunches with an rms spot size of 276 lm and a transverse coherence length of 2.0 nm
Summary
Observing atomic motions during structural transitions is one of the great challenges in science. The time resolution in these early studies was sufficient for resolving the structure of intermediates but not for observing the specific atomic motion direction of the chemistry and passage through the barrier crossing region These early experiments used beam geometries that required low electron bunch charge and were subject to velocity mismatch time broadening issues.. This work effectively showed that despite coulomb repulsion or space charge issues, it was possible to design electron guns with sufficient spatial coherence, short pulse durations, and brightness (with respect to casting the challenge as an imaging problem) to resolve atomic motions in real time. One technique to counteract space charge effects is to reduce the number of electrons to one electron while increasing the repetition rate to several megahertz This technique, requires the sample to be reproducibly pumped and probed $106 times to obtain diffraction patterns of sufficient quality.
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