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The objective of this JRA is to advance the science and technology of cryogenically cooled beam sources with applications in various research fields. It is focused on the development of droplet beams of the low-Z elements (H₂, D₂, He) into unique boundary-free targets for hadron physics at storage rings with extensions to heavy noble gases.

Outstanding examples for the many scientific endeavors at FAIR (GSI) and COSY (FZJ) are the highly complex internal-target experiments. The  facility at FAIR represents a state-of-the-art universal detector for antiproton-proton reaction measurements at the high-energy storage ring (HESR) addressing fundamental issues of the strong interaction. Both at HESR and COSY must the internal targets be optimized in terms of maximum luminosity, i.e. the target density times the ion-beam current, and small spatial target extensions. Thus, it is clear that the design and development of dense cryogenic internal target that give precise interaction regions is a scientifically important and technically challenging task that will constitute a major improvement at the research infrastructures. Three types of targets will be pursued in this JRA accompanied with theoretical simulations.

Due to the comparably small cluster sizes of 1-100 nm, cluster beams are ideally suited for internal experiments since they provide a target stream homogeneous in time and space. Large H₂ cluster-jet beam densities have been achieved using Laval-nozzles with a diameter significantly below 30 mm, operated at conditions where the gas is liquid before the nozzle, in the FP6/JRA7 project. However, the results indicate that even higher target densities are possible. The proposed activity is expected to lead to an innovative jet source that provides particles streams with sizes in the nanometer to the micrometer scale. Systematic investigations on the cluster velocities and masses will be performed to gain a deeper insight into the cluster formation process. The challenging production of Laval-nozzles with diameters below 30 mm will allow for investigations of the poorly understood influence of nozzle geometries on the cluster yield.

Larger droplets of He with a size of the order of 1 mm may be produced from a laminar flow that propagating in vacuum as a continuous cylindrical filament. This filament will spontaneously break up, due to Rayleigh instabilities, into a linear stream of nearly mono-disperse spherical droplets. He-droplet beams from sub-10 mm-diameter have been extensively investigated within FP6 and the production of a microscopic liquid He jet in vacuum was demonstrated for the first time. The production of micrometer-size H₂ droplets, proposed here, represents a much more challenging task. This is mainly related to the freezing of the liquid in vacuum as a result of evaporative cooling. Since the nucleation rate of para-hydrogen (p-H₂)was experimentally found to be a factor ≈10³ lower than for normal-hydrogen, freezing of the liquid might be avoided by employing nearly pure liquid p-H₂. This will be investigated.

The use of a micro jet source as internal target would constitute a major improvement for future storage-ring experiments. However, effects as ion-beam heating and losses resulting from ion beam-internal target interaction must be investigated. In particular, the target-induced ion-beam heating must not exceed the beam cooling capacity and this question will be experimentally addressed.

In pellet targets a regular and mono-disperse stream of drops with a size of a few 10 mm is produced in a triple-point chamber. When the drops pass into vacuum, they freeze to pellets due to evaporation, and are accelerated by the gas flux into the vacuum. The time structure of the pellet flux and the flight directions are distorted during this process. Within FP6, optimization of the drop-production processes and minimizing turbulences in the transition unit into the vacuum, pellet-beam diameters of less than a few 100 µm have been achieved at a distance of ~1 m from the nozzle. Further progress on the pellet target systems are necessary to fully exploit them for hadron physics, including the use of heavy gases. A combination of pellet beam performance and pellet tracking developments are therefore foreseen. Within the FP6 it has been demonstrated that it is possible to detect individual pellets using a single line-scan camera. Based on this, it is now proposed to develop a prototype pellet tracking system using several synchronized line-scan cameras. Such a system could to improve the position information of the interaction vertex by one order of magnitude for experiments at HESR (PANDA) and COSY (WASA). The tracking system has also the potential to be useful for other cryogenic targets. Additionally, p-H₂ will be used for pellet production to study its effects on pellet beam performance. Due to its slower freezing it might lead to a new working point where the disturbance on the pellet beam during vacuum injection can be reduced.

Cluster beam parameters are determined by properties of a gas flow inside the nozzle and gas-jet expanding into vacuum. Various factors (e.g., nozzle geometry, gas pressure and temperature) determine the gas flow structure. However, it is not possible to measure the parameters of the jet inside narrow nozzles and close to the nozzle exit. Therefore, detailed calculations for various nozzle and gas parameters will be an important input for the basic design features of the cluster-jet source in this JRA. An attempt will be made to microscopically understand the clusterization process as well as the droplet formation.

In order to better understand the production of a mono-disperse pellet flux, a thermodynamic model for pellet targets has been developed, taking into account capillary decay of cryogenic jets, convective heat exchange, drop acceleration in the gas flow to the vacuum, radiation heat transfer, as well as drop cooling and freezing. This allows one to calculate H₂ drop and pellet temperatures, velocities and sizes, and the deviation from the nominal axis at each point of the trajectory. New experimental results and the inclusion of additional cryogenic materials (N₂, Ar, Kr, Xe) into the model will help to better understand drop formation and pellet propagation and to develop a new generation of pellet targets.

Task 1. Cluster-jet beam source

• Detailed studies on the production of intense cluster-jet beams will be performed at the cluster-jet target installations at the University of Münster (WWU) and GSI Darmstadt. The aim is to provide as high as possible cluster-beam densities and to investigate the properties of the produced cluster-jet beams. Members from INFN-GE and OeAW will participate and contribute to these planned activities.
• Velocity measurements and mass spectroscopic investigations will be made to gain an understanding of the production of cluster-jet beams on a microscopic scale. Mass and velocity distributions will be studied as function of the relevant operation parameters, i.e. the temperature and pressure of the gas before entering the nozzle as well as the nozzle geometry itself. These studies are of high importance for the understanding of the clusterisation process and, therefore, for the optimisation of cluster-jet sources. The measurements will be performed at the University of Münster (WWU) with contributions from OeAW.
• The property of the Laval-type nozzle in a cluster-jet target is of utmost importance for the quality of the cluster-jet beam. Nozzles with the requested shape and minimum diameter are not commercially available and a production of these special nozzles will be started at INFN-GE, GSI and OeAW. Systematic test measurements with these nozzles will be performed at GSI, WWU and OeAW to identify the optimum nozzle type.
• Detailed calculations for various nozzle and gas parameters will be performed at VGKRI to determine basic construction features of the cluster-jet source for the experiments. An attempt will be made to microscopically understand the clusterization process as well as the droplet formation.

• Studies on the liquid helium droplet beam stability will be performed at the cryogenic micro jet beam installation the University Frankfurt (GUF) using laser imaging methods.
• The possibility to improve the properties of liquid hydrogen micro jet beams by using para-hydrogen as input material before entering the micrometer nozzle will be investigated. A catalytic converter will be set up for this purpose to transform ultra-pure normal-hydrogen into better than 99% p-H₂. Detailed measurements on liquid para-hydrogen micro jet beams will then be performed and compared to measurements using normal hydrogen. The investigations will be performed at the University of Frankfurt (GUF).
• Detailed studies of the interaction of ion beams with micro jet beam targets will be made to guide the design studies for a liquid injection system. After setup and the performance tests, the micro jet target device will be installed and operated at the ERS storage ring at GSI for measurements. The studies will be made by members from University Frankfurt (GUF) and GSI.

Task 3 Pellet beam sources

• Detailed investigations on the hydrogen and nitrogen drop formation and optimisation studies on the sluice parameters will be performed at the pellet target prototypes at the Forschungszentrum Jülich (FZJ) and the institutes in Moscow (ITEP and MPEI). Special emphasis is put on the production of pellet beams with minimal transverse width. The production of pellets from heavy gases will also be studied. The optimisation studies will be supported by reference measurements using water droplets.
• Detailed numerical calculations on the optimum sluice geometry and the jet break-up in the triple point chamber will be performed at MPEI to support the optimisation studies on the pellet beam generator. Additionally, pellet beam parameters will be calculated and compared to the measurements to support the optimisation studies.
• Detailed measurements on pellet beam parameters relevant for experiments in hadron physics will be performed at the pellet test station (PTS) in Uppsala (UU). A para-hydrogen converter will be developed for the PTS to study the effects from para-hydrogen on the pellet beam performance.
• A prototype pellet tracking system using line-scan cameras will be build up at UU. The aim is to determine trajectories of each individual pellet and by this greatly improve the definition of the interaction vertex for storage ring experiments.

Advanced internal target technology is a necessity to reach the required high luminosity for precision experiments at storage rings experiments. Hence, it is mandatory to continue the target development initiative on the European level, already started in FP6, to make optimum use of these facilities. The experience gained and the new knowledge that will be accumulated will enhance the output of the infrastructures and set the terrain with innovative ideas.

Antiproton physics will be one of the pillars of the research at FAIR, the Facility for Antiproton and Ion Research. With the new detectors and beams, together with the development of internal target technology outlined in this JRA, a quantum jump in progress of hadron physics can be made. Cluster, droplet and pellet beams as internal targets have their individual advantages with respect to target homogeneity, vertex reconstruction, target localization and luminosity. A prominent example for the great interest of the community on such developments is given by the PANDA collaboration, which considers all of these three target concepts for their planned experimental facility at the future HESR.

The development of the internal target beams goes beyond the capacity of an individual laboratory whereas an integrated initiative on the European level makes such developments possible. The technologies delivered as a result of this JRA will have significant impact on European research infrastructures. Examples are the COSY storage ring at FZJ where the ANKE experimental facility would profit greatly from the expected cluster-jet development. The WASA experiment at COSY the planned PANDA-FAIR experiment would both drastically improve their physics capacity with a pellet tracking system. This would come about from a greatly improved beam-pellet interaction region resolution that would lead to a strong increase in the experimental resolution and thereby also provide very efficient mean to reject background events. In addition, a precise knowledge of the interaction vertex is a pre-requisite for to be able to reconstruct the decay vertex from short-lived particles. The whole program on open charm physics at PANDA is dependent on this capacity.

Both micro jet and pellet beams have also the potential to find applications in laser-plasma physics. For example, the droplet rate is in a micro jet is orders of magnitude higher than the repetition rate of state-of-the-art femto-second lasers and could provide a novel route toward a compact laser-driven ion source.