Accelerating through accelerator physics

When I applied to the International Summer Student Programme at the Helmholtz-Zentrum Berlin, I never thought that I would be diving into a whole new, and to me previously unknown, area of physics. Though I have had courses on related topics like mechanics and electromagnetism, accelerator physics also includes special relativity, optics and a whole lot of maths. All these areas of physics are combined in order to calculate, as precisely as possible, the movements of the particles inside the accelerator.

My internship started with a whole lot of learning. With books and research papers as my tools, I entered into the world of particle accelerators. One of the first and most exciting thing I learned was that the only force used in the accelerators is the Lorentz force, given by:

This was fascinating to me. A whole area of physics for which humans have built particle accelerators as large as the LHC at CERN relies on one single force! I want to explain quickly what the Lorentz force (F_L) allows us to do.
The Lorenz force lets us change the trajectory of a particle with the charge q and the velocity v. Using an electric field E, we can accelerate the particle, and by using a magnetic field B, we can bend the path of the charged particle.

With these two fields alone, and the resulting Lorenz force, we can accelerate charged particles AND we can steer them! This is (almost) all we need for an accelerator. 

BESSY II, found at the HZB Adlershof campus, is a source for radiation. It consists of a linear accelerator, or linac, for initial acceleration, a booster ring, which accelerates the particles up to a specific velocity, and a storage ring, where the particles continue to circulate at near constant velocity. All of this takes quite a lot of space and so physicists went on the hunt for new acceleration methods. What they found was Laser Plasma Acceleration, or LPA. The basic principle is this: when a laser pulse is shot at a plasma, this creates an electric field that is strong enough to accelerate electrons, in so-called bunches, to very high velocities over very short distances. The problem with this method is that the bunch lengths are too short and the energy spread is too big for the particles to be injected directly into a storage ring. What this means is that the energies of all the particles are not the same, they are “spread out” over too large of an interval and this is problematic when we want to inject the bunches into a storage ring. 

Luckily, there is a solution to the problem; it’s decompression! This can be done by using something called a chicane, and this is also the focus of my internship project. We can steer the bunches with a specific set of magnets in order to create a correlation between the bunch length and the energy spread. Then we can reduce the energy spread by lengthening the bunch.

What you see in figure 1 is a bunch compressor. It allows for the particles at the end of a very long bunch to catch up with the particles at the very front, and as a result shortening the bunch length. A chicane, or decompressor, works the same way, but in reverse. My project is to model such a chicane to meet the criteria that would allow for the bunch to be injected into a storage ring. This can then hopefully one day be used in combination with a LPA. 

For the time being, the work that I do consists of immersing myself in the physics of accelerators and also playing with the modeling software to learn how to use it. The software used is a Python package called pyAT that is the interface to Accelerator Toolbox. It lets the user model configurations of different magnets and then track how charged particles would move through the configuration.

So far it has been very fun to learn the use of the package that I hope to be able to use well throughout my internship. I’m looking forward to creating many models and to improve upon them as I improve upon my skills alongside. I’m excited to continue on this fast-path through accelerator physics!