Micha Dehler

Full scale simulation of a field-emitter arrays based electron source for free-electron lasers

We describe the computer modeling of relativistic electron guns (0.5MeV) using pulsed field emitter arrays. The special challenge lies in the fact, that current distributions vary at a submicron scale, whereas structural dimensions are in the millimeter range. The general approach uses two steps. The first one is the computation of individual field-emitter tips including gate and focusing layers. Real world influences as, e.g., the effect of adsorbates on the emitted currents are taken into account by parameterizing the phase space of the tips. Together with a stochastic distribution of emitter properties, this leads to an equivalent current distribution on the cathode itself, which is used in the second step for the calculation of the electron dynamics in the gun itself. We present results for a source using a field-emitter array of 17 700 tips. For the current field-emitter geometry, we see a large high base line effect resulting from nonlinear focusing forces inside the emitter itself. Of special inter...

Commissioning of the Swiss Light Source

The Swiss Light Source (SLS) at the Paul Scherrer Institute (PSI) consists of a turn key 100 MeV linac, a novel type of booster synchrotron and a 12-TBA storage ring providing 5 nm-rad natural emittance at 2.4 GeV, The SLS project was approved by Swiss Government in Sept. 1997. By June 1999 the building was finished. Linac and booster commissioning concluded by April, resp. Sept. 2000. First beam in the ring was stored Dec. 15, 2000. By June 2001 storage ring commissioning entered the final phase: The design current of 400 mA was reached, an excellent agreement of lattice functions with design calculations was achieved and first undulator spectra were measured. Commissioning of booster and storage ring included commissioning of the innovative subsystems like the digital BPM system, the digital power supplies, the high stability injection system and the CORBA based beam dynamics software.

Static and optical field enhancement in metallic nanotips studied by two-photon photoemission microscopy and spectroscopy excited by picosecond laser pulses

The authors studied the photoemission from molybdenum nanotip arrays with controlled apex curvatures by photoelectron microscopy and spectroscopy excited by 10 ps visible laser pulses under dc electric field. While two-photon photoemission microscopy demonstrates the enhancement of photoabsorption at the sharp tip apex, spatially resolved analysis of the photoelectron energy revealed anomalous energy distribution that is ascribed to the dc field enhancement and resultant barrier reduction at the tip apex. The results show the applicability of photoelectron microscopy and spectroscopy to study the distribution of dc and optical electric field enhancement in field-emitter arrays.

Nanoseconds field emitted current pulses from ZrC needles and field emitter arrays

The properties of the electron source define the ultimate limit of the beam quality in linear accelerators such as free electron lasers (FELs). The goal is to develop an electron gun delivering beam emittance lower than the current state of the art. Such a gun should reduce the cost and size of an x-ray FEL (XFEL). In this article we present two concepts of field emitter cathodes which could potentially produce low emittance beam. The first challenging parameter for such cathode is to emit peak current as high as 5 A. This is the minimum current requirement for the XFEL concept from Paul Scherrer Institut (http://leg.web.psi.ch). Maximum currents of 0.12 and 0.58 A have been reached, respectively, with field emitter arrays and single needle cathodes. Laser assisted field emission gave encouraging results to reach even higher peak current and to prebunch the beam.

Pulsed field emitted current from different commercial samples in the purpose of a free electron laser application

The development of an electron gun with the lowest possible emittance would help reduce the total length and cost of a free electron laser. Recent progress in vacuum microelectronics makes field emitter tips an attractive technology to explore for high brightness electron sources. In order to be a good candidate for a low emittance gun, field emission cathodes must provide at least the peak current, stability, and homogeneity of current state of the art electron sources. In this article, we report on current voltage measurements of commercially available field emitter samples in both continuous and pulsed mode. Pulsed electron emission is of particular interest for a free electron laser application. As mentioned by other authors [F. Charbonnier, Appl. Surf. Sci. 94/95, 26 (1996); P. R. Schwoebol et al., J. Vac. Sci. Technol B 19, 980 (2001)], higher peak current and more stable emission can be achieved when using short square voltage pulses at low frequency. We present maximum peak currents achieved with ...

Field emitter arrays for a free electron laser application

In this paper, the results on current performance of some field emitter materials showed high and stable current pulses at low frequency can be emitted with less contamination problems. For a free electron laser application, peak current values are still too small but by using shorter pulses and small internal field emitter array resistance, a required current was expected to be reached.

Characterization of metallic field emitter array devices fabricated by molding for x-ray free electron laser applications

A low-emittance and high-brightness electron source is a prerequisite for the successful development of sub-nm wavelength x-ray free electron lasers (XFEL). For that application, a field emitter array (FEA) device equipped with a focusing gate is potentially advantageous over the state-of-the-art photocathode. In the low-emittance gun design of the PSI-XFEL project at the Paul Scherrer Institut, the cathode is assumed to emit above 0.2 nC within 10-40 ps, or -10 A, from an array of total area below 1 mm in diameter. So far, the current of the commercially available FEA device is limited to -0.1 A/mm2. To reach higher currents, we explore the field-emission properties of pyramidal-shaped molybdenum FEAs based on the molding-technique. For high-current applications, the pyramidal tip with low-aspect ratio is advantageous over a conical/cylindrical one because of the higher thermal conductance and thermal spread. Here we present fabrication and characterization of pyramidal-shaped molybdenum FEAs with relatively small numbers of tips (1) to measure the maximum current Imax per tip for single-gate devices, and (2) to explore the spatially- and energy-resolved photoemission in FEAs without gate electrodes.

Real-time control of beam parameters

This article gives an overview of the theory and application of real-time control of accelerator beams. The design and structure of orbit feedbacks are described, going from basic local feedbacks to modern state-of-the art global systems. The time domain behaviour is analysed for the building blocks of the systems as well as from the spectrum of random sources driving the orbit perturbations. The use of predictive filtering is shown for the design of the control algorithm. A second important class is the control of tunes and chromaticities. Advanced tune measurements are performed using a digital phase-locked loop. The feedback systems are typically hybrid, simultaneously working on tune and coupling and chromaticity. Adaptive feed-forward algorithms are shown to be a suitable approach for use in energy ramping. For application in a high-speed bunch-by-bunch feedback system, efficient low-noise data processing is presented for a digital filter. Also here, predictive filtering is shown to give well-adapted high-order filters.

Development of a low emittance electron gun based on field emission cathodes

Field emitters are investigated for use as a low emittance electron gun. Two available field emitter technologies are currently explored; the field emitter arrays (FEAs) with individual focusing and single tip cathode with robust and fairly blunt apex. The challenge is to achieve several amperes of peak current without tip destructions. Very good cathode and environmental conditioning procedures with extremely short emission duration (ns) at low repetition rate (10 Hz) gave encouraging results to reach high peak current emission. For a free electron laser application, very short emission durations are preferred. Such operation regime should prevent the tip from overheating so that higher current densities could be reached. Another possible low emittance electron sources are single needle tips made from etched wires and which can be coated and formed in order to carry high current emission. One way to achieve short emission duration is to use pulsed laser light illuminating the tip while high electric field is applied.

Energy efficiency studies for dual-grating dielectric laser-driven accelerators

Abstract Dielectric laser-driven accelerators (DLAs) can provide high accelerating gradients in the GV/m range due to their having higher breakdown thresholds than metals, which opens the way for the miniaturization of the next generation of particle accelerator facilities. Two kinds of scheme, the addition of a Bragg reflector and the use of pulse-front-tilted (PFT) laser illumination, have been studied separately to improve the energy efficiency for dual-grating DLAs. The Bragg reflector enhances the accelerating gradient of the structure, while the PFT increases the effective interaction length. In this paper, we investigate numerically the advantages of using the two schemes in conjunction. Our calculations show that, for a 100-period structure with a period of 2 μ m, such a design effectively increases the energy gain by more than 100 % when compared to employing the Bragg reflector with a normal laser, and by about 50 % when using standard structures with a PFT laser. A total energy gain of as much as 2.6 MeV can be obtained for a PFT laser beam when illuminating a 2000-period dual-grating structure with a Bragg reflector.