SHAPELY ELECTRONS?

An electron bunch. (<10K)

As per the latest issues of Nature, there’s been some progress in the way of electron diffraction, specifically in ultrafast beam pulses and generation of “arbitrary shaped” electron bunches, and opening doors to new improvements in the study of biological molecules.  Using selective photo-ionization and acceleration of cold (~10K) Rubidium gas “ultra-fast” pulses are able to be generated yielding millions of cold electrons in a beam. These types of pulses may be useful for studying small biological molecules.

Biological molecules are small, obviously; they’re in t he order of a few nanometres sometimes and in order to study these by the method of electron diffraction, pulses, from a point-like source, are required to have intensities allowing for a large number of electrons. This feat has appeared almost impossible as short pulses of a few femtoseconds usually generate only a few electrons at a time.

Couldn’t one just up the intensity?

Apparently not. When generating pulses of a useful intensity so as to deliver more electrons, it is here where Coloumbic effects become significant (ie repulsion).  These effects cause the beam to “blow apart” and degrade the beam’s coherence – “Coloumb expansion”. (NB – coherence of the beam is a measure of how constant the phase remains throughout. Coherence length, Lc, is the distance from source to a point where coherence disintegrates.)

An ellipsoid electron "bunch".

The above was remedied by emitting the electron pulse from a large area rather than a point-like source, and also by tailoring the shape of the pulses to be 3D-ellipsoids – “electron bunches”.  These bunches are uniformly distributed in charge and have internal “self fields” which are linear. These internal fields allow for manipulation of the phase-space distribution of the electrons without compromising coherence of the beam.

Be that as it may, ellipsoid bunches are difficult to achieve, although it is possible to generate quasi-2D-pancake-like and 1-D-cigar-like approximations which are rough projections of the ellipsoid.  

More study in this field is definitely needed. However, it does indicate the possibility of significant improvements in electron diffraction.

Any thoughts on this?

Vredenbregt, E. & Luiten, J. (2011) "Electron Diffraction - Cool Beams in Great Shape". Nature Physics, Vol.7, October 2011, pp 747-748.