arXiv subjects

J. F. M. van Rens

Publications and source records attributed to J. F. M. van Rens.

6 recordsLinked to original sources

arXiv2019

Dual mode microwave deflection cavities for ultrafast electron microscopy

This paper presents the experimental realization of an ultrafast electron microscope operating at a repetition rate of 75 MHz based on a single compact resonant microwave cavity operating in dual mode. This elliptical cavity supports two orthogonal TM$_{110}$ modes with different resonance frequencies that are driven independently. The microwave signals used to drive the two cavity modes are generated from higher harmonics of the same Ti:Sapphire laser oscillator. Therefore the modes are accurately phase-locked, resulting in periodic transverse deflection of electrons described by a Lissajous pattern. By sending the periodically deflected beam through an aperture, ultrashort electron pulses are created at a repetition rate of 75 MHz. Electron pulses with $τ=(750\pm10)$ fs pulse duration are created with only $(2.4\pm0.1)$ W of microwave input power; with normalized rms emittances of $ε_{n,x}=(2.1\pm0.2)$ pm rad and $ε_{n,y}=(1.3\pm0.2)$ pm rad for a peak current of $I_p=(0.4\pm0.1)$ nA. This corresponds to an rms normalized peak brightness of $B_{np,\textrm{rms}}=(7\pm1)\times10^6$ A/m$^2$ sr V, equal to previous measurements for the continuous beam. In addition, the FWHM energy spread of $ΔU = (0.90\pm0.05)$ eV is also unaffected by the dual mode cavity. This allows for ultrafast pump-probe experiments at the same spatial resolution of the original TEM in which a 75 MHz Ti:Sapphire oscillator can be used for exciting the sample. Moreover, the dual mode cavity can be used as a streak camera or time-of-flight EELS detector with a dynamic range $>10^4$.

physics.app-ph↗

arXiv2018

Design and characterization of dielectric filled TM$_{110}$ microwave cavities for ultrafast electron microscopy

Microwave cavities oscillating in the TM$_{110}$ mode can be used as dynamic electron-optical elements inside an electron microscope. By filling the cavity with a dielectric material it becomes more compact and power efficient, facilitating the implementation in an electron microscope. However, the incorporation of the dielectric material makes the manufacturing process more difficult. Presented here are the steps taken to characterize the dielectric material, and to reproducibly fabricate dielectric filled cavities. Also presented are two versions with improved capabilities. The first, called a dual-mode cavity, is designed to support two modes simultaneously. The second has been optimized for low power consumption. With this optimized cavity a magnetic field strength of 2.84 $\pm$ 0.07 mT was generated at an input power of 14.2 $\pm$ 0.2 W. Due to the low input powers and small dimensions, these dielectric cavities are ideal as electron-optical elements for electron microscopy setups.

physics.ins-det↗

arXiv2018

Time-of-Flight Electron Energy Loss Spectroscopy by Longitudinal Phase Space Manipulation with Microwave Cavities

The possibility to perform high-resolution time-resolved electron energy loss spectroscopy has the potential to impact a broad range of research fields. Resolving small energy losses with ultrashort electron pulses, however, is an enormous challenge due to the low average brightness of a pulsed beam. In this letter, we propose to use time-of-flight measurements combined with longitudinal phase space manipulation using resonant microwave cavities. This allows for both an accurate detection of energy losses with a high current throughput, and efficient monochromation. First, a proof-of-principle experiment is presented, showing that with the incorporation of a compression cavity the flight time resolution can be improved significantly. Then, it is shown through simulations that by adding a cavity-based monochromation technique, a full-width-at-half-maximum energy resolution of 22 meV can be achieved with 3.1 ps pulses at a beam energy of 30 keV with currently available technology. By combining state-of-the-art energy resolutions with a pulsed electron beam, the technique proposed here opens up the way to detecting short-lived excitations within the regime of highly collective physics.

physics.acc-ph↗

arXiv2018

High quality ultrafast transmission electron microscopy using resonant microwave cavities

Ultrashort, low-emittance electron pulses can be created at a high repetition rate by using a TM$_{110}$ deflection cavity to sweep a continuous beam across an aperture. These pulses can be used for time-resolved electron microscopy with atomic spatial and temporal resolution at relatively large average currents. In order to demonstrate this, a cavity has been inserted in a transmission electron microscope, and picosecond pulses have been created. No significant increase of either emittance or energy spread has been measured for these pulses. At a peak current of $814\pm2$ pA, the root-mean-square transverse normalized emittance of the electron pulses is $\varepsilon_{n,x}=(2.7\pm0.1)\cdot 10^{-12}$ m rad in the direction parallel to the streak of the cavity, and $\varepsilon_{n,y}=(2.5\pm0.1)\cdot 10^{-12}$ m rad in the perpendicular direction for pulses with a pulse length of 1.1-1.3 ps. Under the same conditions, the emittance of the continuous beam is $\varepsilon_{n,x}=\varepsilon_{n,y}=(2.5\pm0.1)\cdot 10^{-12}$ m rad. Furthermore, for both the pulsed and the continuous beam a full width at half maximum energy spread of $0.95\pm0.05$ eV has been measured.

physics.app-ph↗

arXiv2017

Theory and particle tracking simulations of a resonant radiofrequency deflection cavity in TM$_{110}$ mode for ultrafast electron microscopy

We present a theoretical description of resonant radiofrequency (RF) deflecting cavities in TM$_{110}$ mode as dynamic optical elements for ultrafast electron microscopy. We first derive the optical transfer matrix of an ideal pillbox cavity and use a Courant-Snyder formalism to calculate the 6D phase space propagation of a Gaussian electron distribution through the cavity. We derive closed, analytic expressions for the increase in transverse emittance and energy spread of the electron distribution. We demonstrate that for the special case of a beam focused in the center of the cavity, the low emittance and low energy spread of a high quality beam can be maintained, which allows high-repetition rate, ultrafast electron microscopy with 100 fs temporal resolution combined with the atomic resolution of a high-end TEM. This is confirmed by charged particle tracking simulations using a realistic cavity geometry, including fringe fields at the cavity entrance and exit apertures.

physics.acc-ph↗

arXiv2017

Direct magneto-optical compression of an effusive atomic beam for high-resolution focused ion beam application

An atomic rubidium beam formed in a 70 mm long two-dimensional magneto-optical trap (2D MOT), directly loaded from a collimated Knudsen source, is analyzed using laser-induced fluorescence. The longitudinal velocity distribution, the transverse temperature and the flux of the atomic beam are reported. The equivalent transverse reduced brightness of an ion beam with similar properties as the atomic beam is calculated because the beam is developed to be photoionized and applied in a focused ion beam. In a single two-dimensional magneto-optical trapping step an equivalent transverse reduced brightness of $(1.0\substack{+0.8-0.4})$ $\times 10^6$ A/(m$^2$ sr eV) was achieved with a beam flux equivalent to $(0.6\substack{+0.3-0.2})$ nA. The temperature of the beam is further reduced with an optical molasses after the 2D MOT. This increased the equivalent brightness to $(6\substack{+5-2})$$\times 10^6$ A/(m$^2$ sr eV). For currents below 10 pA, for which disorder-induced heating can be suppressed, this number is also a good estimate of the ion beam brightness that can be expected. Such an ion beam brightness would be a six times improvement over the liquid metal ion source and could improve the resolution in focused ion beam nanofabrication.

physics.atom-ph↗