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Compact, low power radio frequency cavity for femtosecond electron microscopy
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Image of FIG. 1.
FIG. 1.

A general schematic of how an RF cavity and slit are used in tandem with an electron microscope to create a regular train of electron bunches.

Image of FIG. 2.
FIG. 2.

An ideal dielectric filled pillbox cavity.

Image of FIG. 3.
FIG. 3.

A cross-sectional view demonstrating the electric and magnetic fields of the TM110 mode in a pillbox cavity.

Image of FIG. 4.
FIG. 4.

A contour plot of P d /P vac as a function of tan δ and ɛ r . The lines represent values of constant P d /P vac . The symbols represent various common dielectric materials and their typical values measured at 3 GHz. The blue square represents the material chosen for filling the pillbox, while the red star represents our measured value.

Image of FIG. 5.
FIG. 5.

Realistic dielectric filled cavity. The expanded radius in one direction further reduces frequency to the intended operating frequency.

Image of FIG. 6.
FIG. 6.

Two TM110 modal frequencies after alterations to cavity.

Image of FIG. 7.
FIG. 7.

The magnetic field profile along the longitudinal axis of the cavity at r = 0 for the realistic cavity geometry (red solid line) and the ideal pillbox cavity (black dotted line).

Image of FIG. 8.
FIG. 8.

The absorption spectrum of the cavity as a function of frequency with a Lorentzian fit.

Image of FIG. 9.
FIG. 9.

Field profile measurement (black circles) and calculated field profile (red solid curve) of the TM110 mode.

Image of FIG. 10.
FIG. 10.

Frequency shift versus the depth of the dielectric tuning stub, measurement (black dots) plotted against CST Microwave Studio simulation (red solid curve).

Image of FIG. 11.
FIG. 11.

General schematics demonstrating the beamline that the cavity was designed for and tested in.

Image of FIG. 12.
FIG. 12.

(a) Electron spot on detector with no power fed to the cavity. (b) Streak electron beam on detector with Prf = 3.2 W power fed to the cavity.

Image of FIG. 13.
FIG. 13.

Theoretical (red solid line) and measured (black dots) streak length of the electron beam at the detector as a function of RF input power.

Image of FIG. 14.
FIG. 14.

Waist scan measurements of (a) the dc beam and (b) the chopped beam. The red curves are fits using Eq. (15). The fits yield normalized transverse emittances of 0.9 and 9 nm rad for the dc and the chopped beams, respectively.


Generic image for table
Table I.

Comparison of estimated characteristics between vacuum and dielectric filled 3 GHz pillbox cavity with a magnetic field amplitude B 0 = 3 mT, relative permittivity ɛ r = 36.5–38, and loss tangent tan δ = 2 × 10−4.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Compact, low power radio frequency cavity for femtosecond electron microscopy