COMPRESSION CAVITY

The compression cavity is a compact, single-cell, power-efficient resonant microwave cavity, supporting a TM010 mode at a resonance frequency of 2.9985 GHz with an unloaded quality factor Q≈6000. The special ‘Omega’ shape of the cylindrical cavity enables the concentration of the oscillating electric field on the symmetry axis, which coincides with the path of the electron beam passing through. In this way, electric field strengths up to 10 MV/m can be achieved on-axis over a length of 6 mm with 1 kW of RF power.

The RF power is fed into the cavity through N-type coaxial connectors. The compression cavity was originally developed for compressing 100 keV electron bunches from several 10-ps to sub-100-fs pulse lengths. 100 W of RF power is sufficient to compress bunches to 100 fs at a distance of 20-40 cm downstream from the cavity.

To enable 100 fs temporal resolution, the RF phase has to be synchronized to within 100 fs with the femtosecond laser, which can be done with our Synchronizer .  In addition temperature stabilization of the cavity is required to within 10 mK , which can be done with our Temperature Controller . The resonant frequency is fixed, nominally at 2.9985 GHz at a temperature of 40° C, but other resonant frequencies are possible on demand. The compression cavity is part of our Crystallography system , in which electron bunches generated by femtosecond photoemission are accelerated to 100 keV in a 100 kV DC electron gun and subsequently simultaneously focused and compressed onto a sample.

The compression cavity can also be used to accelerate or decelerate electron bunches. For example, with 1 kW of RF power a 100 keV electron bunch can be accelerated to 125 keV.

Finally, the compression cavity can be used at the opposite RF phase to stretch the electron pulses instead of compressing them. In this way the uncorrelated energy spread can be decreased substantially, which is used in our ToF EELS spectrometer.

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