11 research outputs found
Magnetic nanoparticles for Magnetic Particle Imaging : from particle characterization to image simulations
No full text
Command and control in stressful conditions
No full textThe author describes his personal experiences during the time he was Commander of Dutchbat 2/UNPROFOR. He describes the general situation in the area of operations and then will go on to discuss the "difficult circumstances" in Srebrenica. His aim is to sketch the circumstances under which the operation had to be carried out and his perceptions of them. He will also deal with a number of operational issues in addition to other aspects of the operation and personal experiences. Finally he discusses the question of how these circumstances were dealt with in relation to command and control
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
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
