Just to be different, today we will talk about physics. Mostly because I have done nothing of interest in the past few days, so I have nothing else to talk about. I previously mentioned the PRL cover we just got, so I thought I would explain the results a little better. It is, after all, a cover, so I feel it is well within my rights as the lowly graduate student to brag.
This explanation builds on some of the other previous entries. Specifically, the general introduction to acceleration and the specific introduction to plasma-based acceleration.
Okay, back to the basics. Sorry, I am using a slide from defense's powerpoint presentation. I am lazy. You gotta problem with it? Tough.
The top image is the schematic of what happens when the beam enters the plasma. The bottom image is the charge distribution of the incoming beam in maroon, as well as, the energy distribution of the beam before and after the plasma. The incoming beam (green) already has an energy chirp (i.e. the head particles are more energetic that the tail particles). When the beam goes through the plasma, the bulk of the particles lose energy to drive the plasma wake and then the tail particles gain energy when the plasma electrons land behind the beam – the green shaded region in the picture above.
Our diagnostic looks at the energy axis. Imagine that you take the blue and green lines and look at their energy profiles. In other words, you take a time integrated picture of the energy profiles with and without the plasma. This picture might help with that visualization (the green is profiled on the left and the blue is profiled on the right). It is a crappy picture, but you get the idea (hopefully).
Now to look at the data:
The image on the left is like the green profile, whereas the image on the right is the blue profile. Note that the particles that have been accelerated are all the particles above the top line. This means a peak energy gain of about 2.5 GeV over a 10 cm plasma or 25 GeV/m (presently, SLAC acceleration is about 20 MeV/m). The particles accelerated in the plasma are not helpful in a real particle physics experiment because there is a huge smear in the energy. This smear is because of the way we do our experiment – we should be able to improve this by beam shaping, but that will be in the future. Also, for the skeptics, about 8% of the beam's electrons are accelerated. There could be (really, should be) more particles accelerated, but since we cannot resolve the beam in time we can't actually measure it. In the future we hope to improve that number too.
Okay, I think that's enough physics. This should make up for my month traveling and doing strictly non-physics related things.
Tomorrow is my first day on the job. And I think I can safely say, I am scared s**tless. I feel like a little bit of a poser. Yeah, sure, I have a Ph.D., but do I really know what the hell I'm doing? Does a thesis in an über-specialized field make me more qualified as a physicist? It is not totally obvious to me that it should, hence the semi-permanent feeling of dread. I am hoping the worry will start to dissipate once I start; if not, there will certainly be an ulcer in my future.
