Sunday, December 12, 2010


There are many sources of light in the world.  What do we usually think of when we ask where light comes from?  One obvious answer is the sun!  Another answer is a light bulb!   A third, very common source of light these days is a computer monitor or television screen, often using an LCD screen. A fourth, less common, but beautiful source of light is from some living creatures, in the form of bioluminescence.  Yet another source of light, which is perhaps less well known to the public, but well appreciated by scientists is the so-called synchrotron light sources.  These large scientific complexes produce x-rays that and are used in a variety of methods to help understand the nature and structure of many different things.

Now, physical theory says that light is a travelling electromagnetic field.   In the classical theory, these fields satisfy Maxwell's equations.  A quantum picture shows light as a stream of photons, particles whose energy is related to their wavelength by E=h nu.

What about the processes that create the light to begin with?  In the sun, the matter moves very quickly- at a high temperature- and the light is produced in these collisions, creating blackbody radiation, with the surface temperature being around 5800 K.  A light bulb is essentially a hot filament of metal, radiating as well as a sort of blackbody radiator.  In the case of bioluminescence, the light comes an electron changing energy levels in a molecule in the organism.  Finally, the light from the synchrotron light source comes from high energy electrons bending in a magnetic field.

In each case, the properties of the light depend on the properties of the source.  The light from the sun depends on the temperature of the sun for its spectrum, and on the size and distance of the size for its spatial and angular extent.  For a light bulb, it depends on the power emitted from the bulb, the material of the filament, and the size and shape of the bulb.  For bioluminescence, it depends on the molecules involved which determines the spectrum, and the amount and distribution of that molecule which determines the spatial extent.  Finally, in the case of the synchrotron light source, the properties depend on the energy and trajectory of the electrons for determining the spectrum, and the density  and distribution of the electrons to determine the spatial and angular extent of the resulting light.

The topic of synchrotron radiation from high energy electrons in storage rings is not so well known to the public, nor in fact to most scientists who use the radiation in their experiements.  It developed as an off-shoot to particle physics.  In order to understand the electron beam that is radiating, one needs to know about how the electron beam got there in the first place, and what sets the properties of the electron beam.  The former topic is the topic of accelerator physics.  It typically covers hardware as well as beam dynamics.

This blog will focus on the physics of synchrotron light sources after the acceleration has already occured.  The electrons still need to be maintained- or stored- in the large ring.  And the x-rays produced need to be guided and focussed towards the experiements.  Thus, we will cover some basics of the physics of electron beams in synchrotrons and photon beams following emission. And maybe some generalities on the light emission process that is relevant to many situations where light is produced.  In other words, we situate the physics of synchrotron radiation in storage rings within the broader topic of "light source physics".
Some topics:
Theoretical ways of modeling an electron beam.
Theoretical ways of modeling a light beam.
Software for analyzing electron beam.
Software for analyzing light beam.
Useful properties of photon beams, and their connection to the electron beam.
Ways of measuring electron beams.
Ways of measuring light beams.