Point mode

[Bill Tivol]

On Jun 10, 2004, at 3:43 AM, Mikhail.Eltsov at lau.unil.ch wrote:

> The reference data for CM120 mentioned below were obtained with so 
> called "point mode" of LaB6 suggested in Ruiz et al. (2003). Even 
> after reading of this paper it is not very much clear for me what is a 
> principle of this mode and how it improves coherence. Can anyone 
> explain?
>
Dear Mikhail,
	The tip of a LaB6 filament is a truncated pyramid, and the small, flat 
area--the point--is in a particular crystallographic orientation.  The 
work function for the point is lower than that for the rest of the 
filament--the sides of the pyramid and the edges.  As the voltage 
heating the filament is increased, there is a value for which a 
significant number of electrons have energies larger than the work 
function of the point but not larger than the other work functions; 
this is the condition for point mode.  Since the temperature of the 
filament is lower for point mode than for the usual mode of operation, 
where electrons are emitted from the sides and edges of the pyramid in 
addition to the point, the energy spread of the emitted electrons is 
less, so the beam has better temporal coherence, and, since the 
emitting area is smaller, the beam has better spatial coherence.  You 
can see the areas from which the electrons are emitted by observing the 
beam at crossover as the excitation of the filament is raised and 
lowered.  For a properly aligned filament you should see the emission 
from the point at a low voltage, then four spots should appear around 
the point emission as the voltage is raised.  The four spots increase 
in brightness and size until they merge with the point emission and 
each other, until at saturation the emission spot appears uniform.
Yours,
Bill Tivol, PhD
EM Scientist and Manager
Cryo-Electron Microscopy Facility
Broad Center, Mail Code 114-96
California Institute of Technology
Pasadena CA 91125
(626) 395-8833
tivol at caltech.edu