10 research outputs found
Microwave TM010 cavities as versatile 4D electron optical elements
No full textThe realization of high quality ultrashort pulsed beams requires ultrafast time-dependent electron optics. We present derivations of closed expressions both for the longitudinal and transverse focusing powers of resonant microwave TM010 cavities. The derived expressions are validated by particle tracking simulations using realistic cavity fields. For small field amplitudes, in which case the weak lens approximation holds, the focusing powers obtained from simulations are in good agreement with the derived expressions. Furthermore, the required phase and temperature stability for synchronization of electron bunches generated by femtosecond photoemission are discussed
Extreme regimes of femtosecond photoemission from a copper cathode in a dc electron gun
No full textThe femtosecond photoemission yield from a copper cathode and the emittance of the created electron beams has been studied in a 12 MeV/m, 100 keV dc electron gun over a wide range of laser fluence, from the linear photoemission regime until the onset of image charge limitations and cathode damaging. The measured photoemission curves can be described well with available theory which includes the Schottky effect, second-order photoemission, and image charge limitation. The second-order photoemission can be explained by thermally assisted one-photon photoemission (1PPE) and by above-threshold two-photon photoemission (2PPE). Measurements with a fresh cathode suggest that the 2PPE process is dominant. The beam emittance has been measured for the entire range of initial surface charge densities as well. The emittance measurements of space-charge dominated beams can be described well by an envelope equation with generalized perveance. The dc gun produces 0.1 pC bunches with 25 nm rms normalized emittance, corresponding to a normalized brightness usually associated with rf photoguns. In this experimental study the limits of femtosecond photoemission from a copper cathode have been explored and analyzed in great detail, resulting in improved understanding of the underlying mechanisms
Ultrafast/ultracold electron/ion beams
No full textContinuing size reduction in semiconductor manufacturing, and a push toward atomic time and spatial resolution in the field of structural dynamics require, require the development of novel charged-particle sources and instruments. Electron microscopes have recently been combined with pulsed lasers to provide temporal resolution down to sub-ps scale. However, techniques such as radio-frequency cavities provide an alternative. RF fields can also be used to reduce the longitudinal bunch length of pulsed electron beams, such as used for ultrafast electron diffraction (UED). Here the central issue is to gain control over space-charge forces which tend to destroy the quality of the beam. This requires spatial and temporal shaping of the particle bunches from the moment of creation until delivery at the target. I will discuss recent results in which such control allowed single-shot diffraction patterns of crystalline atomic samples to be obtained as well as sub-ps temporal resolution. To extend this technique to biological samples, an electron source that allows for a larger transverse coherence length for a similar target area is being developed, based on trapped atoms. Measurements show that electron temperatures several orders of magnitude lower than for a photo-emission source can be achieved. At the same time, such a source provides unique ion beams with temperatures in the mK range and energy spread well below 1 eV. These may have applications in focused-ion beam instruments, where probesize limitations due to chromatic aberrations can be eliminated
Direct measurement of synchronization between femtosecond laser pulses and a 3 GHz radio frequency electric field inside a resonant cavity
No full text\u3cp\u3eWe demonstrate a method to measure synchronization between femtosecond laser pulses and the electric field inside a resonant 3 GHz radio frequency (RF) cavity. The method utilizes the Pockels effect in a crystal inside the RF cavity by measuring the retardation of the components of polarization as a function of RF phase. Resolution of the setup used is shown to be 29 ± 2 fs (root-mean-square, rms), with timing jitter between the laser pulses and the RF field inside the cavity of 96 ± 7 fs (rms). The method provides a tool to reduce jitter and improve time-resolution in ultrafast electron diffraction experiments.\u3c/p\u3
Compression of sub-relativistic space-charge-dominated electron bunches for singleshot femtosecond electron diffraction
No full textWe demonstrate the compression of 95 keV, space-charge-dominated electron bunches to sub-100 fs durations. These bunches have sufficient charge (200 fC) and are of sufficient quality to capture a diffraction pattern with a single shot, which we demonstrate by a diffraction experiment on a polycrystalline gold foil. Compression is realized by means of velocity bunching by inverting the positive space-charge-induced velocity chirp. This inversion is induced by the oscillatory longitudinal electric field of a 3 GHz radio-frequency cavity. The arrival time jitter is measured to be 80 fs
