Sometimes, though not often, I actually talk about science. I have gotten excellent questions from readers, but I have generally answered them in the comment section. Since I figure most people aren't reading the comments, I thought I would consolidate some of the questions here. I am also preoccupied with editing chapter 4, so forgive a little re-hashing.
For a plasma wakefield refresher see acceleration basics, plasma wakefield basics and my thesis basics.
Q: Energy. Everyone keeps asking about energy. Basically, does this experiment obey the laws of physics and conserve energy? Provided we are not crackpot scientists and, in fact, conserve energy, how do we do that?
A: We are not crackpots. Odd, maybe, but not crackpots. Our experiment fully obeys the laws and regulations set forth in the universe. The system is a transformer. Yes, I say this every time, but, well, it's a damn transformer. The head particles lose energy such that the tail particles can gain energy. The way the energy is transferred is through the plasma wake. Some energy might be dissipated into the plasma, so it won't be a perfect 100% transfer, but you get the idea.
Q: At this point, people ask why the tail particles don't crash into the now less energetic head particles.
A: We are working with ultra-relativistic particles (28.5 GeV - if that means anything to you). Basically, the electrons in the bunch are going the speed of light. So when the head particle lose energy and tail particles gain energy, they are still basically going the speed of light. So there is no concern that the head particles will stop and tail particles crash into them.
Q: What is going on with the plasma ions and electrons?
A: The ions don't move on the time scale that the beam passes through the plasma, so they are not part of the wake. The wake is only composed of electrons from the ionized plasma. Having the ions stationary means they exert a nice focusing force on the electron bunch (also very good). The electrons in the wake all land behind the electron bunch, so there is a huge density spike on axis behind the bunch (sorry, that blue shaded region is supposed to be A LOT of plasma electrons, not ions, which I now realize it looks like). The density spike then administers the "kick in the ass".
Q: What is the maximum energy gain and what is the limit?
A: The maximum wave we can get is called the wave-breaking field. This maximum wave determines the energy *gradient* not gain. In other words, the limit on how large the plasma wake can get. The wave-breaking field is given by the following formula:
E_peak [V/m] = 96*sqrt(n_p[cm^-3])
where n_p is the density of the plasma. For a plasma with density 10^17 cm^-3, the maximum acceleration gradient is 30 GeV/m. The maximum amount of energy gained by the particles is then 30 GeV/m * (length of the plasma).
Q: Are you a proof-of-principle experiment?
A: Yes. We have gotten excellent results (hopefully to be published soon), so the experiment is progressing a little bit into more "are-you-practical?" type questions. This is still unclear, however, optimism remains high.
Q: What's your efficiency?
A: A good question, but one I hate. Mostly because we do not have a good answer. First and foremost, we are a proof-of-principle experiment, so proving good efficiency is a bit beyond our call at the moment. That's the cheap answer; the real answer is we basically don't know. We work with short bunches (~ 25 microns) and there doesn't exist a diagnostic which can time-resolve something that short. We know we lose a lot of energy in many particle and we know we gain a lot of energy in a few particles. We have a lower limit in knowing how many particles have gained energy, but we don't know the true amount. The not-knowing leaves us in a tough position to properly answer the efficiency question.
Q: Is it scalable for use on the International Linear Collider (ILC) if it is built with the cold design (superconductivity)?
A: I have no clue, but this question has been briefly looked at in advanced accelerator workshops. No solid results except that we have potential.
Q: Do any of your colleagues feel that the selection of the German approach (the cold design) was primarily a political decision, especially in light of the continuing quagmire over the ITER site?
A: Disclaimer: I do not work on the ILC, I have absolutely no knowledge about the inner workings on the technology decision for ILC, also I am not a particle physicist and I am in some weird branch of advanced accelerator physics, so I think we can all assume where my interests lie. With that large grain of salt in mind, I have had discussions with people on the subject. The most succinct commentary on the decision I heard: "They chose intensity over energy." As for the political ramifications of such a decision, I do not know. I do think that high energy physics community has realized that in order for the ILC to be built, the community needs to be supportive of any decision made. The moment there are internal fractures, it becomes too easy for the government not to support it financially. Whether they execute the cooperation fully will remain to be seen.
Okay, I think that is way too much science for one day. Any more questions, bring it on. I promise to answer them in time, which is heavily dependent on how much I want to avoid the responsibilities of the thesis writing.