Theory and particle tracking simulations of a resonant radiofrequency deflection cavity in TM 110 mode for ultrafast electron microscopy

Abstract

We present a theoretical description of resonant radiofrequency (RF) deflecting cavities in TM 110 mode as dynamic optical elements for ultrafast electron microscopy. We first derive the optical transfer matrix of an ideal pillbox cavity and use a Courant-Snyder formalism to calculate the 6D phase space propagation of a Gaussian electron distribution through the cavity. We derive closed, analytic expressions for the increase in transverse emittance and energy spread of the electron distribution. We demonstrate that for the special case of a beam focused in the center of the cavity, the low emittance and low energy spread of a high quality beam can be maintained, which allows high-repetition rate, ultrafast electron microscopy with 100 fs temporal resolution combined with the atomic resolution of a high-end TEM. This is confirmed by charged particle tracking simulations using a realistic cavity geometry, including fringe fields at the cavity entrance and exit apertures.

Electron-beam spectroscopy for nanophotonics

“…A new design shows that 200-fs electron pulses may be created with an energy width below 500 meV. 153 Using a sequence of GHz RF cavities even promises energy resolutions down to 20 meV, far better than the energy spread in the source itself. These designs come within a factor 10 of the Heisenberg limit of the variance in beam energy and position.…”

Compact ultracold electron source based on a grating magneto-optical trap

“…Higher beam quality is generally provided by sharp-tipped sources, developed for electron microscopy. By sideways femtosecond photoemission from a nanometer-sized field emission tip 14 (or by RF chopping [15][16][17] ) the same beam quality can be achieved as in conventional electron microscopy, enabling imaging with atomic spatial and temporal resolution. However, this results on average in less than one electron per pulse.…”

Time-of-flight electron energy loss spectroscopy by longitudinal phase space manipulation with microwave cavities

“…By combining this with a TM 110 streak cavity to create ultrashort pulses with a high beam quality [19,20] and a second streak cavity to detect the flight times [21,22], pulse creation, modulation, and detection can easily be synchronized. A temporal resolution of a few fs has already been demonstrated experimentally with microwave cavities [22], which would translate into a few tens of meV energy resolution for a 30 keV beam and a drift space of 1 m. Figure 1 tungsten filament gun is chopped into pulses using a dielectrically loaded TM 110 cavity [23].…”

“…By focusing the continuous beam at the center of the first cavity the emittance of the beam is conserved [19,20]. In the simulations, this is done with a half-angle of 50 µrad, after which the beam is deflected at a magnetic field amplitude of 3 mT over a 10 µm slit, placed at a distance of 10 cm.…”