JRA11-CRYOJET: Cryogenically cooled particle streams from nano- to micrometer size for internal targets at accelerators

The objective of this JRA is to significantly develop the science and technology of cryogenically cooled target beam sources for applications in present and planned complex internal-target experiments, which require target beams of highest quality, such as PANDA@FAIR. In addition to hadron physics experiments, cryogenically cooled cluster/pellet/ microjet beams have recently been discovered to be perfectly suited as targets for laser-particle interaction. New and innovative techniques will be addressed and developed within this project to allow for enhanced performance of present and future infrastructures.

The comparably small cluster sizes of, e.g., 1-100 nm, of cluster beams make them ideally suited for internal storage ring experiments, targets at electron accelerators, or laser-driven hadron accelerators. Nozzle production techniques will be improved and extended with the aim to achieve higher target beam thicknesses and higher nozzle production yield. Furthermore, measurements and developments will be initiated get cryogenic targets with highest performance, longterm stability, within known and suitable vacuum conditions. While hydrogen as target material is of primary interest, also clusters from heavier gases will be investigated. This will be from highest interest to adapt the target optimally to the requirements of the different experiments.

A key issue for laser-induced particle acceleration to multi-MeV kinetic energies is to build targets that can make use of the high laser repetition rate (up to kHz), targets that contain only those elements to be accelerated (e.g. hydrogen), and that have a limited density (to assure a high acceleration efficiency). A cheap and portable alternative to generate MeV hadrons is the interaction of high-power laser pulses with a suitable cluster target. This represents one further objective of this activity.

An additional goal of this JRA is to develop cryogenic droplet beam sources, both for hadron physics experiments at storage rings, electron accelerators, and the exciting novel possibility to use them for intense laser-driven proton acceleration. One focus will be the challenging generation of stable periodic droplet beams of cryogenic elements hydrogen, for which a novel concept for a droplet injector is developed. Special issues are the avoidance of nozzle clogging and the development of a nozzle alignment system to be used during cryogenic target operation. Within this JRA it is planned to prepare a prototype for a real-time pellet tracking system which can predict the time dependent position of an individual pellet in the target region. Such a device could be used for the accelerator beam monitoring in a hadron physics accelerator or ion acceleration in laser generated relativistic plasmas. The production of curtain shaped targets will be studied as well as cryogenic hydrogen fibers, which could be investigated with the help of the pellet tracking equipment.

In pellet targets, a regular and mono-disperse stream of droplets is produced which freeze in vacuum to pellets. The main focus is the production of small pellets with the diameters 10-15 μm. This requires the development of the techniques for the nozzle fabrication and development of the new diagnostics system for measurement of the parameters of the extra small pellets. New regimes for the production of target streams will be developed.