The Unbearable Business of Thesis Writing.

I figure if I'm going to bitch about my thesis on this blog, I might as well explain what exactly my thesis is about. So let's discuss.

As a sort of a foundation, I've already talked a bit about the fundamentals of acceleration and a little bit about plasma wake field acceleration. Now on to the wonderful world of field ionization ...

First things first, the electron beam we use is not a continuous beam (like a laser), instead it is composed of little beamlets or bunches of electrons. For our experiment, each bunch is really tiny. It has a radius (actually, it's the r.m.s. value, if that means anything to you, but let's not be picky) of 15 microns (that's 0.000015 meters), a length of about 30 microns (r.m.s. again) and has about 2x10^{10} electrons. That is a REALLY dense amount of electrons. Since the incoming bunch is so dense, it has really strong electric fields associated with it. These fields are sufficiently strong to rip off the outer most electron of the lithium atoms in our oven -- this process is called field ionization. When the beam's electric field ionizes the lithium vapor, the beam not only generates its own plasma, it then generates the plasma wake which does the accelerating.

This is actually really nice. Before, we had this clunker of a laser which we used to photo-ionize the vapor and make the plasma. This was always a pain in the ass because you have to align the laser, time it properly with the incoming bunch and propagate it through the vapor. Now our system is wonderfully contained and elegant.

My thesis focuses on this field ionization process and, in particular, the energy loss associated with the mechanism. Here are a few pictures of the energy of the beam after exiting the plasma. The picture on the left shows the beam without ionizing the vapor and the picture on the right is after the beam fully ionized the vapor. The picture in the middle shows you the transition.

Okay, I think that's enough science for one day, don't you?

Comments

[little miss demosthenes]

just making sure - rms == root mean square?

[Caolionn]

Yes. rms == root mean square. I can't figure out how to do symbols on this blog, otherwise I would have used a sigma. Sorry.

[little miss demosthenes]

http://www.chaos.org.uk/~eddy/bits/chars.html should do the trick

[Caolionn]

Excellent!! Thanks.

[Carlos]

I'm not sure that I'm reading the graphs correctly. Do they show that the energy of the electrons after field ionization has a larger spread (including a lower minimum) but with a higher average?

[Caolionn]

Carlos,

At threshold the plasma isn't ionized until the middle of the bunch (peak of the Gaussian). Once ionized, the beam then expels the plasma electrons to generate the plasma wake. The problem is, at threshold, the wake starts too late so no particle get accelerated since the wake crashes too far in back of the bunch. After we are way past threshold, ionization occurs earlier and earlier, until the wake is recovered and particles accelerated.

So in the picture I have shown, it is just energy loss no energy gain. Although you are correct that when the wake is recovered, there is a large energy spread. Later I will show the energy gain. But excellent point which I should have made clearer.