The electron accelerator

The Mainz Energy-Recovering Superconducting Accelerator MESA will be the world's first superconducting energy-recovering accelerator dedicated to research. "With the high intensity and quality of its beam, MESA provides a unique platform for a forward-looking experimental program to study and test the limits of currently known phenomena in elementary particle physics. This is one of the core objectives of PRISMA," said Professor Hartmut Wittig, Spokesperson for the PRISMA Cluster of Excellence, from whose resources the development and construction of MESA is funded.

Electrons are provided by a photo-gun and are pre-accelerated to 5 MeV with a beam time structure of 1.3 GHz (quasi continuous wave beam). The heart of the accelerator, however, are the two cryomodules each with two superconducting cavities. With each pass the electrons gain up to 25 MeV more energy. Actually, the accelerator has two different operating modes.

In extracted-beam mode, the beam can be directed to the so-called P2 experiment after up to three circulations. Main objective of this parity-violation experiment is to precisely measure the weak mixing angle. Behind P2, the beam is then stopped in a massive beam dump. One may reinterprete this beamdump also as a massive target for production of dark matter particles, which might then be detected by our dedicated beam dump experiment, DarkMESA.

There is also a little beamdump that can be used for MAGIX in external beam mode (attractive in the early stage of the accelerator, or when thick targets are used), but the real beauty of that accelerator is that it can be operated in energy-recovering mode, where after passing the MAGIX target the beam is guided back to the cavities, now with a phase shift of 180 degrees. As a result, the electrons are not accelerated, but they are decelerated... so they return energy to the accelerating structures! Again, up to 25 MeV for each pass. The beam makes up to two turns until it reaches 5 MeV and is then disposed in a tiny beam dump, which is far away from our experiment (excellent for a minimal background situation at the experimental side). Overall, this energy recovery mode provides a very energy efficient acceleration, and in return enormous beam currents of 1000 uA or more (up to 10000 uA at a later stage...!) can be achieved. For this to work, the beam must not be disturbed too much by the interaction with the target, and therefore, we need a low-density target. Just like the gas jet target we have developed for this very purpose, which provides a very compact interaction zone between beam electrons and our target nuclei.

Detailed information on the accelerator can be found on the website of the ⇒ MESA collaboration.