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Physical Review Letters

Ultracold Electron Source

Brigitte J. C. Claessens, S.B. van der Geer, G. Taban, Edgar J. D. Vredenbregt, O.J. Luiten

Published Oct 12, 2005• 138 citations
Publisher

Abstract

We propose a technique for producing electron bunches that has the potential for advancing the state-of-the-art in brightness of pulsed electron sources by orders of magnitude. In addition, this method leads to femtosecond bunch lengths without the use of ultrafast lasers or magnetic compression. The electron source we propose is an ultracold plasma with electron temperatures down to 10 K, which can be fashioned from a cloud of laser-cooled atoms by photoionization just above threshold. Here we present results of simulations in a realistic setting, showing that an ultracold plasma has an enormous potential as a bright electron source.

DOI
10.1103/physrevlett.95.164801

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We propose a technique for producing electron bunches that has the potential for advancing the state-of-the-art in brightness of pulsed electron sources by orders of magnitude. In addition, this method leads to femtosecond bunch lengths without the use of ultrafast lasers or magnetic compression. The electron source we propose is an ultracold plasma with electron temperatures down to 10 K, which can be fashioned from a cloud of laser-cooled atoms by photoionization just above threshold. Here we present results of simulations in a realistic setting, showing that an ultracold plasma has an enormous potential as a bright electron source.
Ultra-cold plasmas with electron temperatures of ~10 K can be created by photo-ionization just above threshold of a cloud of laser-cooled atoms. Recently it was shown 7 by GPT particle tracking simulations that an ultra-cold plasma has an enormous potential as a pulsed bright electron source. Here we discuss these results in the framework of normalized 6D brightness, which allows us to make a proper comparison both with the performance of pulsed, radio-frequency photo-emission sources and with the performance of continuous, needle-like field-emission sources. In addition we speculate on the possibility of using ultra-cold plasmas to realize quantum degenerate electron beams, constituting the ultimate limit in electron beam brightness.
We present the first observation of subpicosecond electron bunches from an ultracold electron source. This source is based on near-threshold, two-step, femtosecond photoionization of laser-cooled rubidium gas in a grating magneto-optical trap. Bunch lengths as short as 735±7 fs (rms) have been measured in the self-compression point of the source by means of ponderomotive scattering of the electrons by a 25 fs, 800 nm laser pulse. The observed temporal structure of the electron bunch depends on the central wavelength of the ionization laser pulse, in agreement with detailed simulations of the atomic photoionization process. This shows that the bunch length limit imposed by the atomic photoionization process has been reached.
Electron microscopy and diffraction have long been used to study the structure of materials. A recent development is the achievement of ultrafast time resolution using high-repetition rate single electron sources and single-shot many-electron sources. Achieving large coherence lengths under these conditions is challenging due to intrinsic heating mechanisms including inter-particle Coulombic forces. Near-threshold, pulsed photoionization of atoms trapped and cooled with lasers is being investigated as a means to produce intense electron pulses with large intrinsic coherence length due to a low electron temperature. Photo-ionizaton sources are, of course, not an entirely new idea but recent implementations using trapped atoms offer new possibilities. Several properties of electron beams extracted from such ultracold sources have already been investigated, showing in particular that temperatures of a few Kelvin and emittances in the nanometer-radian range can be achieved with tens of thousands electrons per pulse for nanosecond pulse lengths. This can be combined with delicate shaping of the charge distribution to counteract emittance growth due to non-linear self fields. Using ultrafast lasers for the ionization step, shorter electron pulses can be achieved, but an interesting question is whether the concomitant spread in photon energy still allows for low electron temperature. Recent experiments have shown that few-Kelvin temperatures combined with picosecond pulse lengths are indeed possible. It turns out that this can be understood with a classical model for electron trajectories resulting from photoionization. In addition, experimentally observed polarization effects are in general agreement with model results. --------------------------------------------------------------------------------
Ultracold plasmas (UCPs) represent a marriage between the seemingly distinct fields of cold atom physics and plasma physics. In cold atom physics, dilute clouds of atoms can be trapped with laser radiation and cooled to J.1Ktemperatures. In contrast, in plasma physics the constituents generally have temperatures in the 1,000K-1O,000K range as well as much higher density. In 1999,Rolston and co-workers [1] first converted a cloud of laser-cooled atoms into a plasma by photo-ionizing the atoms just above threshold. This provides very unusual circumstances:the temperature of ions and neutrals is O.OOIK,that of electrons 10K! Such ultra-cold plasmas therefore are interesting subjects for fundamentalplasma physics studies.\nIn a recent paper [2], we argued that ultracold plasmas (UCPs) may in addition provide an entirely new way of generatinghigh-brightnesscharged-particlebeams, such as used in focused-ion beams, laser synchrotrons, and ultra-fast electron microscopy.\nHere, a low temperature T is beneficial for the intrinsic (transverse) brightness B of a\nsource since B = II(A1), where I is the current and A the cross-sectional area. I will discuss an experimental investigation of the properties of ion and electron beams extracted from ultracold plasmas, which shows that such beams indeed have low temperatures, and, as a result, high angular intensity.
We report the creation of an ultracold neutral plasma by photoionization of laser-cooled xenon atoms. The charge carrier density is as high as $2\ifmmode\times\else\texttimes\fi{}{10}^{9}{\mathrm{cm}}^{\ensuremath{-}3}$, and the temperatures of electrons and ions are as low as 100 mK and $10\ensuremath{\mu}\mathrm{K}$, respectively. Plasma behavior is evident in the trapping of electrons by the positive ion cloud when the Debye screening length becomes smaller than the size of the sample. We produce plasmas with parameters such that both electrons and ions are strongly coupled.