Target at HESR

One of the scientific caterpillars at the future FAIR facility at GSI is represented by the highly complex internal-target experiments at the high-energy storage ring (HESR). Here, experiments foreseen within the SPARC and PANDA collaborations aim at addressing fundamental issues of the atomic and strong interactions and novel tests of QED and QCD. In this regard, the use of the internal targets provides the highest efficiency in terms of the luminosity, a particularly crucial aspect for the investigation of processes characterized by tiny cross sections.

In recent years considerable experimental efforts have enabled to improve the internal target features, especially in terms of the highest achievable density for the low-Z targets hydrogen and helium. Owing to their boundary-free and self-replenishing nature, cluster beams produced by expanding a gas initially at stagnation source conditions (pressure and temperature) through a nozzle into vacuum offer significant advantages. Indeed, hydrogen target densities close to 1015 cm-2 have been demonstrated in the group of A. Khoukaz in Münster by employing a cluster target beam in PANDA geometry. Here, Laval-type nozzles as those formerly developed at CERN were found to provide the highest densities for a given orifice diameter. Most recently, target densities exceeding the 1015 cm-2 threshold, yet accompanied by unexplained density fluctuation phenomena, have been observed in the same geometry by cooling the source further down into the two-phase supercritical regime, and eventually well into the liquid phase. These studies further confirm the observation that the vacuum expansion of a supercritical fluid or even a liquid is fundamentally different in nature compared with a gas jet expansion.

Indeed, the fact that the vacuum expansion of a liquid at significantly lower temperatures may indeed provide a valid alternative to cluster jet targets has been shown recently by our group at the experimental storage ring (ESR) at GSI by employing a cryogenically cooled liquid microjet target beam source. In order to understand the underlying physical mechanism, we show in Fig. 1 a typical physical situation occurring when a liquid beam expands in vacuum. The expanding liquid, which initially propagates as a continuous filament, rapidly undergoes a disruptive, cavitation-induced fragmentation, delivering broad distributions of microscopic droplet sizes and directions. Experimentally, one observes a significant narrowing of the beam divergence either by further decreasing the source temperature or by increasing the source pressure. Eventually, the laminar regime is achieved, in which the continuous liquid filament breaks up as a result of Rayleigh induced oscillations into a monodirectional stream of nearly monodisperse droplets.

Effects related to the particular nozzle geometry strongly affect the internal target beam features. For example, it is known, both experimentally and numerically, that the cluster formation process during a supersonic gas jet expansion is favoured by the Laval-type nozzle geometry compared to thin-walled nozzles. More specifically, the Laval exit geometry greatly enhances the collisions between the expanding atoms, leading to the formation of larger clusters along the jet propagation axis, which thus carry a higher mass and consequently provide an increase in the target density further downstream. However, the physical processes occurring during the expansion of a liquid are radically different to those governing the cluster formation during a gas expansion. In the latter case the clusters are formed as a result of collisional processes, whereas a droplet beam is the result of a disruptive fragmentation of the liquid state. Yet, whether the Laval-Type geometry still offers the ideal choice in this latter case is an open question. The nozzle geometry is here certainly still playing a major role, but the precise dependence of fundamental beam features such as the droplet size and droplet size distribution on the nozzle geometry is poorly understood. The situation appears to be even more challenging in the supercritical regime, where unpredictable effects related to the interplay of the gas and liquid phases make possible predictions even more challenging.

Given the broad importance of the above topics, most of our efforts are currently devoted to the planning of experiments that aim at characterizing the droplet beam features under different regimes of interest for internal target development, for instance by means of light scattering techniques, which are very useful for the in situ characterization of the droplet target beam by investigating the light scattered off the target. The suitability of this technique for the detailed characterization of droplet beams has been proven during recent proof-of-principle experiments performed in our laboratory at IKF.