On the behavior of micro-spheres in a hydrogen pellet target

A pellet target produces micro-spheres of different materials, which are used as an internal target for nuclear and particle physics studies. We will describe the pellet hydrogen behavior by means of fluid dynamics and thermodynamics. In particular one aim is to theoretically understand the cooling effect in order to find an effective method to optimize the working conditions of a pellet target. During the droplet formation the evaporative cooling is best described by a multi-droplet diffusion-controlled model, while in vacuum, the evaporation follows the (revised) Hertz-Knudsen formula. Experimental observations compared with calculations clearly indicated the presence of supercooling, the effect of which is discussed as well.Comment: 22 pages, 8 figures (of which two are significantly compressed for easier download

Insulator charging by contact with metals

In spite of the fact that charge transfer is one of the oldest physical phenomena known, the basic mechanisms involved are not yet quite understood. For experimental investigations of the charge transfer to insulators it is still informative to scrutinize electrification systems consisting of as simple components as possible. The choice of one metallic component is justified by the well-known electronic properties of metals.The charge transferred on (100) oriented surfaces of NaCl crystals from a sliding gold electrode has been measured as a function of sliding velocity and normal force between the electrode and the insulator surface. Besides (100) surfaces other crystallographic orientations of NaCl have been investigated with electrodes of different metals. A series of experiments comparing the sliding and the rolling modes of separation has also been carried out on (100) surfaces and it is shown that in broad outline the different modes of separation are consistent. The results from the investigations on metal-NaCl systems suggest that electrons are transferred into trapping levels in the forbidden energy range of the NaCl crystal.Electrification of insulators by metals is currently explained by uniform carrier injection to a constant depth for each particular material or by the formation of a strictly two-dimensional surface charge. Calculations concerning a more general model are presented, which includes both alternatives as special cases. An expression for the effective surface charge is obtained for the case where an electric field is applied across the insulator. Electrification experiments with polyethylene and various metals have been carried out, and the results are compared with the theoretical predictions. It is found that the assumption of a two-dimensional charge distribution leads to contradiction, and must be rejected. It is shown that the depth of injection in the bulk charge model ma^y be obtained from the experimental data and the formalism developed. The possibility of a non-uniform space charge density is also discussed.By means of special equipment which permits measurements of charging of capacitors due to a step voltage after the capacitors are classically fully charged, the electron injection has been studied as a function of time.This has been done on commercial polymeric capacitors as well as on gold evaporated foils of Teflon (PTFE) or polyethylene. The findings in the case3of polyethylene are correlated with the results on the field dependence of electrostatic charging and support the model of electron injection to a finite depth. An appendix will give some brief considerations concerning the release of electrification charge by exposing the insulator surface to light in the UV-range.digitalisering@um

On a Hydrogen Pellet Target for Antiproton Physics with PANDA

The PANDA experiment is a part of the future FAIR accelerator facility and will study the strong interaction by detecting the reaction products from antiproton-proton annihilations in a near full solid-angle configuration. One option for the internal proton target in PANDA is frozen micro-spheres of hydrogen, so-called pellets. Such a pellet target is interesting because of the unique characteristics it offers; the high target thickness, the small interaction volume, the minimal gas load on the vacuum system, and the possibility of tracking individual pellets. Nevertheless, it is possible to allocate the bulky equipment needed to produce the pellets at a few meters away from the beam. This way particle detectors can be located close and almost fully around the interaction point. This thesis is devoted to the optimization of a pellet target. To perform measurements, a Pellet-Test Station was built at The Svedberg Laboratory, Uppsala. For the first time, experimental results show the pellet distribution in space and time, and in addition, the vacuum along the pellet pipes. Furthermore, dedicated measurements carried out at CELSIUS/WASA demonstrate the existence of pellet heating as a result of beam-target interactions. In performing calculations, the potential problems with pellet heating at PANDA are outlined. Moreover, to look at the consequences for the desired physics, a reaction involving short-lived D-mesons has been used to show the advantages of pellets compared to a more spacious target. In conclusion, these studies lead to a deeper understanding of the pellet properties, which makes it possible to suggest future improvements, such as cooling with no vibrations

High voltage vacuum discharge characteristics

Investigation of the high voltage vacuum discharge characteristics − current derivative (di/dt), current, voltage, and soft as well as hard X-rays − versus time has been carried out. Recorded traces indicate pinching of the interelectrode discharge column two or three times during which the voltage increases to values as high as the initial voltage or even higher. X-ray radiation of high penetration power is emitted during these events. In addition, sausage instabilities have been observed during the pinch giving rise to extreme penetrating X-rays caused partly by the pinch voltage and partly by the sausage voltage. Visible electrode studies show the fact that the discharge is restricted to a narrow column. Finally, a theoretical treatment of the discharge mechanism that seems to be in good agreement with the experimental result is also presented