The target system of the MAGIX experiment is based on a windowless design: A cryogenic supersonic gas jet target provides a relatively dense gas flow perpendicular to the beam direction, which allows luminosities on the order of 1035cm-2s-1 to be achieved while maintaining beam quality good enough to allow the electron beam to be recaptured by MESA after the interaction - this is necessary for operation in the ERL mode. The target can be operated with most gases, which allows the study of electron scattering processes on various target nuclei.
The target system and its interface to the detector are carefully designed to minimize unwanted interactions, which is necessary to achieve high detector resolution due to the limited energy of the particles involved: One has to prevent energy loss and multiple scattering effects on incoming and scattered particles. Therefore, the material budget is kept at a minimum.
The flowing gas is first pre-cooled by passing it through a vessel filled with liquid nitrogen which cools the gas down to roughly 200 K. The gas is then fed into the main cooling unit, which consists of a series of two cold heads, where it is adiabatically cooled to very low temperatures (the final temperature varies for different gases, e.g. 40 K for hydrogen).
After cooling, the gas is injected into a de Laval nozzle where it is accelerated to supersonic speed. Then, the directed jet is ejected to the scattering chamber. At special conditions, one can even achieve the formation of gas clusters which results in even more stability.
Right underneath the nozzle, there is the compact interaction zone between the electron beam and the gas jet. This is 'where the physics happens'.
After the scattering process the gas streams into a funnel shaped catcher, which is mounted about 10 mm underneath the nozzle. This catcher is connected to a cascade of powerful pumps which can pump away a gas flux of up to 2400 ln/h and thereby prevent that the gas streams into the chamber.
Currently MAGIX investigates in optimizing the nozzle/catcher geometry. Furthermore a gas cleaning and recirculation system will be implemented, which enables reduction of costs and the future usage of rare gases.
