Photo-triggered pulsed cavity compressor for bright electron bunches in ultrafast electron diffraction

Abstract

Temporally resolved observation of microscopic structural dynamics of solids with ultrafast electron diffraction (UED) requires extremely short pulsed, highly charged, monoenergetic electron beams with sufficient transverse coherence length of several unit cells of the investigated samples. However, Coulomb repulsion defeats these parameters in free propagation of an electron pulse initially bright on the photo cathode. We demonstrate a new electron pulse compressor design based on a simple and compact RF structure incorporating a pair of gallium arsenide photoconductive semiconductor switches that are triggered by femtosecond laser pulses, thereby providing a longitudinal voltage gradient of up to 20 V/ps. Our proof of principle experiment achieved compression of bunches containing 26,000 electrons to a duration of below 750 fs and a beam diameter of 300 μm in the temporal and spatial focus of the device while maintaining the good beam collimation required for time resolved electron diffraction experiments. The simplicity of the compressor provides a strong incentive for its further development toward practical implementation in sub-relativistic UED experiments requiring the highest possible source brightness.

Appendix: A formula for pulsed compressor performance

The compressor described in this paper can be approximated by a parallel plate capacitor with a harmonically ramped axial electric field between the plates separated by distance d, complete with apertures for electron pulses to pass through in the axial direction (see figure and Fig. 6.7 in Ref. 23). Subject to the assumption that the electron pulse transit time through the compressor is less than T/3, where T is the oscillation period of the harmonic electric field dependence, the latter can be considered approximately linear. Furthermore, if the electron pulse duration σ_t is much smaller than T/3, it can be shown that the rms energy spread ΔE _k that is imparted on an electron pulse of rms pulse duration σ_t, is given by [23]

$$ \Updelta E_{\text{k}} = \frac{\gamma }{{\beta^{2} \gamma^{2} + 1}}v_{z} \Updelta E\sigma_{\text{t}} $$

(1)

where γ is the Lorentz factor, β = v _z/c with v _z the electron velocity, and ΔE the electric field difference between the time at which the bunch enters and exits the compressor field respectively. Now, assuming a linear electric field ramp dE/dt such that ΔE = dE/dt × d/v _z, using the fact that the voltage gradient dV/dt = dE/dt × d, and re-writing in terms of the relative rms energy spread ΔE _k,r = ΔE _k/E _k, Eq. 1 yields

$$ \Updelta E_{\text{k,r}} = \frac{\gamma }{{\beta^{2} \gamma^{2} + 1}}\frac{{\sigma_{\text{t}} }}{{E_{\text{k}} }}\frac{dv}{dt} $$

(2)

In the subrelativistic regime, γ/(β²γ² + 1) varies slowly with energy E _k, and consequently the required compressor voltage to achieve a desired ΔE _k,r at fixed compressor frequency and injected pulse duration scales roughly linearly with E _k.