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International audience; The Borexino detector was built starting from 1996 in the underground hall C of Gran Sasso National Laboratory (LNGS) in Italy under about 1400 m of rock (3800 m.w.e) and it is mostly aimed to the study in real-time of the low-energy solar neutrinos.Since the beginning of data taking, in May 2007, the unprecedented detector radio-purity made the performances of the detector unique: a milestone has been very recently achieved with the measurement of solar pp neutrino flux, providing the first direct observation in real time of the key fusion reaction powering the Sun.In this contribution the most important Borexino achievements to the fields of solar, geo-neutrino and particle physics are reviewed and the future perspectives discussed, emphasizing in particular the unique possibility of Borexino to cover at the end of its program the entire solar neutrino spectrum and to exploit the possible existence of a fourth sterile neutrino (SOX project).

The GERmanium Detector Array (GERDA) is an experiment designed to investigate the neutrinoless double beta decay ( 0 ν β β ) in 76 Ge. An array of high purity germanium detectors isotopically enriched to 87% of 76 Ge is operated within 64 m 3 of liquid argon (LAr) at the Laboratori Nazionali del Gran Sasso (LNGS). The experiment aims to explore the 0 ν β β half-life up to 1.4×10 26 yr with a collected exposure of 100 kg yr separated into two physics phases. The data taking of Phase I started in November 2011 and finished in May 2013 with 21.6 kg yr of exposure and a background index (BI) of 2×10 −2 cts/(kg yr keV) around the Q -value of 2039 keV before pulse shape cuts. Phase II of the experiment is being prepared with additional 30 Broad Energy Germanium (BEGe) detectors and an instrumentation of the LAr, aiming at a BI reduction by a factor of 10 w.r. to Phase I. This paper will present the GERDA setup and the latest results of the experiment including a new measurement of the 2 ν β β spectrum of 76 Ge and the decomposition of the background spectrum. The 0 ν β β analysis, finished in the meanwhile, will be briefly mentioned. Furthermore, the major improvements planned for Phase II will be discussed.

The plasma-neutral gas balance is considered in the partially ionized transition region situated between a fully ionized magnetically confined plasma and an adjacent neutral gas cloud. A well-defined boundary layer can develop between the plasma and the neutral gas, but only within certain ranges of the relevant parameters such as the plasma density and the magnetic field strength. Consequently, such layers are expected to appear in the atmospheres of normal and magnetic variable stars and in other more dense cosmic clouds of matter, as well as in impermeable laboratory plasmas. On the other hand, sharply defined boundary layers are unlikely to appear in the low-density regions of interstellar and interplanetary space where the plasma becomes permeable to neutral gas and the partially ionized transition region has a minimum depth of the order of 5×1018/nb m, where nb is the corresponding characteristic plasma density.

It may become possible to create powerful neutron sources from a number of magnetic plasma confinement schemes being investigated in fusion research. Among the one- and two- energy-component plasmas discussed so far, special attention is drawn in this paper to quasi-static systems such as the Spherator with magnetically shielded supports, as well as to rotating plasma systems such as the closed Tornado bottle and open bottles. Provided that sufficiently high ion and electron temperatures can be reached, these systems may be operated at higher plasma densities and smaller dimensions than those of mirror and Tokamak neutron sources.

The “Extrap” (External Ring Trap) scheme consists of a toroidal Z-pinch immersed in a transverse (poloidal) magnetic field B v which is produced by currents in a set of external ring-shaped conductors1–4. This scheme has two characteristic features. First, a purely transverse confinement field.B = B p + B v is obtained by generating a plasma current j = curlB p/µo along the zero-field region of the externally imposed transverse “vacuum” field B v. Second, the field B v is strongly inhomogeneous which leads to a non-circular plasma cross section and to a stabilizing effect on a large class of free-boundary and internal modes.

Rotating plasma operation in spiral coil “Tornado” traps provides a unique combination of confinement and heating properties. Such a system consists of a closed and compact magnetic bottle to which the crossed-field technique can be applied, in absence of end insulators and their critical velocity limitation effect. This is expected to lead to the generation and heating of fully ionized plasmas within a large range of ion densities, temperatures, and rotational velocities. Provided that stable operation becomes possible at high temperatures and the effects due to the asymmetries of the spiral coil structure can be neglected, it is likely that Tornado traps can be used as strong neutron sources of moderately large dimensions and technically realizable parameter values. Some detailed data and operation ranges are outlined for the “Tornado 6 650” device in Leningrad, and for a somewhat larger device which may be operated as a neutron source.

The heat balance is considered of a rotating plasma which is permeable to neutral gas. The neutral particles which enter the plasma and fill its confinement volume are assumed to have small velocities before interacting with ions and electrons. Plasma-neutral gas interaction then gives rise to a type of frictional heating of the plasma. The conditions are analysed under which this effect becomes the dominant plasma heating mechanism. It is found that thermonuclear ion temperatures become available at rotational velocities of the order of 106m/s. The result rests on two crucial questions. First, it requires the densities of the neutral gas and the plasma as well as the geometry of the magnetic configuration and its boundary conditions to be chosen such as to avoid critical velocity limitation by plasma-neutral gas interaction. Such a possibility seems to be supported by recent experiments, but requires further and more detailed investigations. Second, an excessively large power input is required to reach these temperatures, unless the plasma and neutral gas densities are chosen to be small. Thus, the necessary power input by far exceeds the thermonuclear energy production, but the present system could still become useful as a neutron source.

A review is given on the features of magnetic traps with a spherical separatrix, with special emphasis on Tornado spiral coil configurations. The confinement and heating of static plasmas in Tornado traps is treated, including the topology of the magnetic field structure, the magneto-mechanical properties of the magnetic coil system, as well as the particle orbits and plasma behaviour in these traps. In addition, the mode of rotating plasma operation by crossed electric and magnetic fields is described. The results of experiments on static and rotating plasmas are summarized, and conclusions are drawn about future possibilities of Tornado traps in the creation and containment of hot plasmas.

In order to study the instubilities of a pinched gas discharge, a liquid scale model of the discharge column is constructed. The model consists of a freely fulling vertical, cylindrical stream of mercury. The stream is carrying a direct current, and its stability to electromagnetic disturbances is studied. Different types of instability are found in cases with and without a longitudinal magnetic field. A 3000 frames per second high speed camera is used, and the films taken show the growth of sausage'' and lateral instabilities. A possible analogy with gas discharge pinch intstabilities is discussed. (auth)

The motion of a charged particle in a magnetic field may be described as the sum of a gyration around the field lines, a drift motion across the same lines and a motion parallel to the field. External force fields, magnetic field inhomogeneities and inertia forces are responsible for the drift motion in this single-particle picture. In an ionized gas the picture has to be modified by particle interactions, such as collisions and phenomena caused by space charges. Only encounters between nonidentical particles produce a diffusion in first order across the magnetic field.» The macroscopic correspondence to a situation where the transverse drift due to particle interactions and inertia forces can be neglected and where the motions are relatively slow is an ionized gas nearly "frozen to" the magnetic field lines. This strong coupling between ionized matter and magnetic fields is of great importance, not only in astrophysics but also for the problem of producing controlled thermonuclear energy in a plasma confined by a magnetic field. However, it has been emphasized by Bohm, Burhop, Massey and Williams that random fluctuations of charge density and plasma oscillations may produce electric fields which in their turn give rise to drift motions across the magnetic field lines. This ' "drain" diffusion provides an additional mechanism for ionized matter to "slip" across a magnetic field and may, when it dominates over collision diffusion, introduce considerable difficulties into the physics of a plasma in a magnetic field. Bohm and collaborators performed experiments with an arc plasma in a magnetic field and came to the conclusion that the diffusion was not consistent with collision phenomena but could be explained by "drain". This interpretation was criticized by Simon, who pointed out that the transverse diffusion of the plasma is not necessarily ambipolar. Thus, in the arc experiment, space-charge neutralization can be maintained by the conducting end walls which produce an electron short-circuit across the magnetic field. With this new interpretation Simon was able to show that the arc experiments did not conflict with the binary collision theory. The purpose of the present investigation is to study diffusion across a magnetic field in a configuration which is free from short-circuiting effects such as those described by Simon. It provides the possibility of deciding whether collision or "drain" diffusion is operative. For the purpose a long cylindrical plasma column with a homogeneous magnetic field along the axis has been chosen as described in Section 5. The theoretical treatment is given in Sections 2 to 4. On the basis of the collision diffusion theory Tonks, Rokhlin, Cummings and Tonks and Fataliev have pointed out that a longitudinal magnetic field will reduce the losses of particles to the walls. Consequently, when the magnetic field is present, a lower electron temperature and a smaller potential drop along the plasma column should be required to sustain a certain ion density. The same conclusions have recently been drawn by Bickertonf and by Bickerton and von Engel in theoretical and experimental investigations which also include probe measurements in a magnetic field. In the range of the apparatus used by these authors the positive column was found to behave in good agreement with the collision theory. In the absence of a magnetic field the behaviour of the column was consistent with the theory of Tonks and Langmuir, and when the field was present a modified Schottky theory was applicable (next section). The present experiment forms an extension of that of Bickerton and von Engel into a range where the Schottky theory is applicable in the absence of a magnetic field and where the applied magnetic field is still made strong enough to influence the electron temperature.