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In the upcoming PANDA experiment at FAIR in Darmstadt (Germany) the annihilation of antiprotons with protons will be investigated in order to study the QCD, i.e., the theory explaining the strong interaction [1]. The antiprotons will be produced with a series of linear and ring accelerators and reach a momentum of up to 15 GeV/c in the storage ring HESR. The protons for the annihilation experiment will be provided by a cluster-jet target operated with hydrogen. Around the interaction point a 4π detector will be mounted; hence the cluster source and the beam dump will be located in more than 2 m away from the interaction point (Fig. 1). Narrow pipes will connect the interaction point with the source and the beam dump. To achieve the desired luminosities high target thicknesses are required which have never been realized before in such large distances to the cluster source. Thus, a new state-of-the-art cluster-jet target is under development at the University of Münster.
The core piece of the cluster-jet target is a convergent-divergent Laval nozzle with a narrowest cross-section of 30 μm that produces clusters, i.e., hydrogen spheres with diameters of <20 μm [2]. At temperatures between 20 to 50 K and pressures of up to 20 bar hydrogen is pressed through the Laval nozzle. Via different cluster building processes that depend on the operating parameters and the phase of the hydrogen upstream of the nozzle, clusters are produced. The resulting cluster jet is separated from the residual gas and tailored by two orifices, the skimmer and the collimator. By using movable orifices and a nozzle tilting system, the cluster jet can be aligned to pass through narrow pipes to the IP and the beam dump.
At the Institute of Nuclear Physics at the University of Münster, a new nozzle design is developed, that allows precise control of the beam shape (Fig. 2). It also allows for target densities that exceed the needs of the PANDA experi-ment. Nozzles with even smaller narrowest cross-sections of 10 μm and 20 μm are currently in production, and the latest results will be presented.
As the accelerator facilities at FAIR are still under construction, the PANDA cluster-jet target was operated at the accelerator facility COSY in Jülich (Germany) until the last beam time in 2023 [3]. The provided proton beam with a beam momentum of up to 3.7 GeV/c was utilized to perform studies on the ion beam induced evaporation of the clusters during their interaction with the accelerator beam by measuring the increase of the residual gas background [4]. Combined with calculations on flash evaporation, vacuum simulations are conducted on cluster-jet targets to predict the vacuum conditions in the PANDA experiment or in any other cluster-jet setup.
[1] G. Barucca et al., “PANDA Phase One”, European Physics Journal A57, 44, arXiv: 2101.11877 (2021).
[2] H. Eick et al., “Determination of hydrogen cluster size distributions of a cluster-jet target using shadowgraphy”, Nuclear Instruments and Methods in Physics Research Section A, DOI: 10.1016/j.nima.2025.171245 (2026).
[3] P. Brand et al., “Operation of the PANDA cluster-jet target with the HESR stochastic cooling at COSY”, Phys. Rev. Accel. Beams, DOI: 10.1103/3g6s-qm12 (2026).
[4] U. Bechstedt et al., “Progress and developments at the cooler Synchrotron Cosy”, Proceedings of EPAC (2002).