Shaped electron bunches from ultracold plasma
AuthorSheludko, David V.
AffiliationSchool of Physics
Document TypePhD thesis
CitationsSheludko, D. V. (2010). Shaped electron bunches from ultracold plasma. PhD thesis, School of Physics, The University of Melbourne.
Access StatusOpen Access
© 2010 Dr. David V. Sheludko
This thesis presents the development of a new apparatus, and imaging tech- niques, used to produce shaped cold electron bunches from ultracold plasma (UCP). Due to their low temperature, cold electron bunches from UCP have the potential to provide a compact electron source with sufficient brightness and coherence to enable single-shot ultrafast diffractive imaging of nano-scale samples, such as bio-molecules. To create the electron bunch, a cold (70 μK) cloud of 108 rubidium-85 atoms was prepared in a magneto optical trap (MOT). Some of the atoms were excited to an intermediate state using a laser with a spatially varying in- tensity profile, then ionised by a second laser pulse. The shaped excitation laser allowed two-dimensional control of the electron density. The ionisation laser could, in principle, also be spatially varying, to allow three dimensional manipulation of the electron bunch shape. Due to Coulomb repulsion betweeen electrons within a bunch, the brightness of the source is critically dependent on its initial shape, and therefore the shape of the atom cloud. Uniform density bunches are particularly desirable due to their linear space-charge expansion. To optimise the source through production of uniform bunches, knowledge of the spatial density distribution of atoms in the cloud is required. Conventional imaging techniques for cold atoms are either technically demanding or destructive to the atom cloud, and are unsuitable for this application, providing direct motivation for the development of new methods. I have developed two new imaging techniques for use in shaped electron bunch production. The new techniques are particularly suited to our application, but are applicable to the cold atom research community in general, offer- ing several advantages over conventional methods. The first method adapts a phase-contrast imaging technique to measure the spatial distribution of atoms in a specific excited state. The second approach allows single-shot imaging of inhomogeneous atom clouds; that is, where both the density and refractive index may be spatially varying. The method uses a perturbative approach in conjunction with phase retrieval based on the transport of in- tensity equation. This technique is also potentially valuable for studies of atomic coherence effects in cold atoms and was demonstrated using spatially modulated electromagnetically induced transparency. In collaboration with other members of our research group and in parallel with the imaging research, I also designed and constructed a new appara- tus to produce UCP. The experiment first produced electron bunches in late 2009, and using the imaging techniques I developed, the results of the first shaped electron bunches from UCP are presented here. Due to the low tem- perature of the electrons, such shaped bunches can only be produced and observed using a cold electron source. Conventional thermal sources, includ- ing photoemission and field emission sources, produce hot electrons whose high temperature immediately diffuses any initial structure. The effects of increased electron temperature on the quality of the bunch shape are inves- tigated here, resulting in the conclusion that cold electrons are essential for observing, and thus optimising, shaped bunches. Arbitrarily shaped bunches are demonstrated for the first time, using an intensity shaped excitation laser beam followed by uniform ionisation of the excited atom distribution. The laser intensity profile is adjusted using a spatial light modulator (SLM). Variations in atomic density which would degrade the bunch shape are measured using the imaging techniques developed. Adjustment of the excitation laser intensity to compensate for atomic density is shown to produce uniform bunches in two dimensions. Quantitative analysis of the acuity of the bunch edge provides an upper limit to the electron temperature of T = 35 K. Unlike photoemission sources, the electron bunch shaping mechanism demonstrated here can easily be generalised to three dimensions. In addition, field ionisa- tion of Rydberg atoms is observed to play an important role in the ionisation process, and suggested as a future avenue of research.
Keywordslaser cooling; ultracold plasma; electron sources
- Click on "Export Reference in RIS Format" and choose "open with... Endnote".
- Click on "Export Reference in RIS Format". Login to Refworks, go to References => Import References