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Sunday, December 19, 2010

light source physics

I've been writing about some frustration with the field of accelerator physics and where I fit into it recently. I came up with something of a tentative solution to this long standing problem, which I want to say a little about here. It may look like just words, but it does represent a change of orientation, and perhaps will lead to a better fit between what I am interested in and can do well, and what I am doing and asked to be doing for my job/career.

Outside of the question of how well research is supported, there has been an additional problem.
My thesis work was on electron storage rings and equilibrium electron distributions. But to actually continue on with that sort of topic is typically defined as accelerator physics, or beam physics, or even machine physics. The problem is that none of these really excites me that much. I was basically interested in the classical mechanics, or the non-linear dynamics, or the statistical mechanics. But not in making particles go as fast as possible. Beam physics is more interesting to me, but if it is defined so narrowly and with such little research support, its still not great. Machine physics also seems like a somewhat derogatory way of describing the topic. It doesn't describe what the physics is about, but only where it takes place. It is the stuff back there, beyond where the real science is happening, inside that big machine.

So I decided to define my own field of work/research as light source physics. This is meant to both exclude and include. One can say that light source physics (for synchrotron light sources, anyway) relies on accelerator physics and beam physics. But one could also say there is some overlap with accelerator physics and beam physics. First of all, one needs to get the beam there in the first place. That's the accelerator physics (but of course, its much more. Its engineering, its control systems, its infrastructure...) Then one needs to know about general behavior of relativistic beams of charged particles. This would be beam physics. Light source physics implies that the purpose of this electron beam is the radiation it produces. And furthermore, the dynamics of this beam is only half the story. The other half is the light that is produced. The electrons produce electromagnetic fields, or perhaps photons, or perhaps a distribution of light. I'd say that until this light exits the front end and heads down the beamline, we are in the realm of light source physics. The source of the light.
Thus both accelerator physics and other applications of beams are excluded. Colliding beams are used for particle physics. There are also medical purposes for beams. There is electron microscopy using electron beams.

Finally, whereas beam physics is shared between synchrotron light sources and colliders, on the radiation end, we could say its shared with xray optics particularly, and optics more generally. So flashlights and LED's and the sun, and fluorescent molecules are also light sources. And its within the realm of these topics that the definitions of brightness, brilliance, flux, and all that has been developed. So its not cheating to say that they are part of the field.

As for its need, one can look to a site such as and one finds all about the applications, but not too much about synchrotron radiation and even less about the electron beam. So, though unorthodox, it seems to me a gap that could use development, but with different emphasis and theory than comes to mind with accelerator physics or beam physics or machine physics. In particular, both single pass and multipass is included. FEL's can be included... for now, just my own personal definition. But I think it makes sense. Talman's "Accelerator X-Ray Sources" is I think a good reference to orient some of this.

Just briefly, so, the picture is going from an electron beam to a photon beam. The electron beam may be described with Twiss parameters, and more general coupling formalism. I believe one can describe the photon beam in the same way.

Anyway, this blog has been somewhat a strange mix of personal and professional stuff. Since I hope to see how well I can do as a light source physicist, I decided to create a separate blog called Light Source Physics. The present blog will stay a bit more personal, and amateurish, venturing briefly into topics I know little about, but find interesting. Perhaps the other will develop more substantially. Or perhaps by splitting into two, I'll lose interest in blogging on either one... we shall see...

Another idea is to focus more on the philosophical aspects of things here, and more technical on the other one. Light may be described with a Wigner function for example. I'm curious what it means. Its supposed to be the closest to a representation of the distribution of photons. But it can go negative. Its interpretation is also difficult in quantum mechanics. So I'm curious about it with light. Is it a quantum mechanics issue? Is light transport a good context for thinking about basic non-relativisitic quantum mechanics? Is symplecticity an important concept in light transport?

The other thing to do with this blog is to continue with the mess I've been working on and describing related to accelerator physics, and beam physics. But hopefully continuing in a positive direction. Oriented towards getting a good code, and a reasonable set of references to help understand these things. It is a part of light source physics, after all.

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