[en] An overview of the studies by collinear laser spectroscopy in the island of inversion is presented with emphasis on the β-detection method. The spin and magnetic-moment measurements of ³¹Mg and ³³Mg are described in detail. Comparison with the spherical and deformed shell models provides evidence for prolate shapes and indicates the ground-state parities. The results are considered in the context of other experimental studies in the region. The potential of the β-detection technique for studying rms charge radii is highlighted.

[en] Combining measurements of the extracted currents with probe and laser-photodetachment diagnostics, the study is an extension of recent tests of factors and gas-discharge conditions stimulating the extraction of volume produced negative ions. The experiment is in a single element of a rf source with the design of a matrix of small-radius inductively driven discharges. The results are for the electron and negative-ion densities, for the plasma potential and for the electronegativity in the vicinity of the plasma electrode as well as for the currents of the extracted negative ions and electrons. The plasma-electrode bias and the rf power have been varied. Necessity of a high bias to the plasma electrode and stable linear increase of the extracted currents with the rf power are the main conclusions

[en] Shielding the bias applied to the probe by the sheath formed around it and determination of parameters of unperturbed plasmas are in the basis of the probe diagnostics. The results from a two-dimensional model of a discharge with a probe inserted in it show that the probe influences the spatial distribution of the plasma parameters in the entire discharge. The increase (although slight) in the electron temperature, due to the increased losses of charged particles on the additional wall in the discharge (mainly the probe holder), leads to redistribution of the plasma density and plasma potential, as shown by the results obtained at the floating potential of the probe. The deviations due to the bias applied to the probe tip are stronger in the ion saturation region of the probe characteristics. The pattern of the spatial redistribution of the plasma parameters advances together with the movement of the probe deeper in the discharge. Although probe sheaths and probe characteristics resulting from the model are shown, the study does not aim at discussions on the theories for determination of the plasma density from the ion saturation current. Regardless of the modifications in the plasma behavior in the entire discharge, the deviations of the plasma parameters at the position of the probe tip and, respectively, the uncertainty which should be added as an error when the accuracy of the probe diagnostics is estimated do not exceed 10%. Consequently, the electron density and temperature obtained, respectively, at the position of the plasma potential on the probe characteristics and from its transition region are in reasonable agreement with the results from the model of the discharge without a probe. Being in the scope of research on a source of negative hydrogen ions with the design of a matrix of small radius inductive discharges, the model is specified for a low-pressure hydrogen discharge sustained in a small-radius tube.

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[en] Concepts for the extraction of volume-produced negative hydrogen ions from a rf matrix source (a matrix of small-radius discharges with a planar-coil inductive driving) are presented and discussed based on experimental results for the current densities of the extracted ions and the co-extracted electrons. The experiment has been carried out in a single discharge of the source: a rf discharge with a radius of 2.25 cm inductively driven by a 3.5-turn planar coil. The length of the discharge tube, the area of the reference electrode inserted in the discharge volume, the discharge modes, the magnetic filter and its position along the discharge length, the position of the permanent magnets for the separation of the co-extracted electrons from the extracted ions in the extraction device and the bias applied to its first electrode are considered as factors influencing the extracted currents of negative ions

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[en] For the D-T operational phase of the international fusion experiment ITER, starting in 2035, pulses are planned with Q > 10 and a duration of 400 s and pulses with Q = 5 for up to 3600 s. The two neutral beam injection (NBI) systems will deliver 33.3 MW of heating power to the plasma and will also be an important source for non-inductive current drive. An essential part of the NBI systems is a large RF driven source for negative hydrogen or deuterium ions. In order to fulfil the requirements imposed to the NBI system, the ion source has to deliver an intense, stable and homogeneous large negative ion beam over pulse lengths of 400 s or 3600 s. Till now, long pulses in deuterium at the extracted negative ion current density required for ITER could not be demonstrated. The achievable performance is limited by a high current and a pronounced vertical asymmetry of the co-extracted electrons, both strongly increasing with time, whereas the stability of the negative deuterium ions is not an issue. For reduced performance, i.e. reduced negative ion current, lower and more stable co-extracted electron currents are observed, making possible pulses up to 3600 s (using pulsed extraction). One main aim of ongoing investigations at the two test facilities BATMAN Upgrade (Bavarian test machine for negative ions) and ELISE (extraction from a large ion source experiment) is the development of scenarios stabilizing and symmetrizing the co-extracted electron current in deuterium. These investigations are discussed and the current status of hardware upgrades ongoing at BATMAN Upgrade and ELISE towards the ITER scenario of one-hour steady state extraction is presented. (paper)

[en] Neutral Beam Injection (NBI) requires high particle energies if one of its aims is to contribute to current drive in large fusion tokamaks. For example, 1 MeV D is foreseen for the ITER NBI. At such energies, the NBI must be based on a source of negative hydrogen ions (N-NBI) due to their higher neutralization efficiency of up to 60% in a gas neutralizer. Negative hydrogen ions are produced on low-work function surfaces, for which caesium is evaporated continuously into the ion source. The strong technological development resulted in an ion source that operates technically reliably and is in principle capable of running continuously (RF plasma generation, RF coupling, high voltage for extraction and acceleration, cooling etc.), where the only technical limit for the operating time is the vacuum pumping capacity. However, the strong dynamics of the Cs layers caused by the plasma-surface interaction creates a steadily increasing amount of inevitably co-extracted electrons, limiting the pulse duration at present. The vacuum pumping and further aspects regarding the neutralizer, beam duct components, etc. are discussed in [1]. The N-NBI test facilities BATMAN Upgrade and ELISE (1/8 and 1/2 size of the ITER NBI source, respectively) contribute to the development program of the ITER NBI; while full size prototype sources are hosted by Consorzio RFX at the Neutral Beam Test Facility (Padova, Italy). Conditioning recipes and further measures (e.g. biased surfaces close to the extraction system) have been developed to stabilize and/or reduce the current of co-extracted electrons. These optimizations resulted for the first time in almost 90% of the targeted extracted negative ion current (30 A) reproducibly achievable in 600 s hydrogen pulses at ELISE, demonstrating that the requirements for the first operational phase of the ITER NBI are in reach for large scale N-NBI sources. Deuterium operation remains more challenging, since the co-extracted electrons increase more strongly during a pulse. A newly developed measurement of the work function proved that the work function degrades after long pulses. Simulations give hints that the limited flux of Cs onto the surface during the plasma discharge causes the degradation. In order to increase the flux of neutral Cs, an alternative Cs evaporation concept (“Cs shower”) is tested at BATMAN Upgrade. With the Cs shower, a steady state with extremely stable performance has been reached in long deuterium pulses for the first time in a caesiated negative ion source. Further improvements of the concept are required in order to make it applicable to large sources. This contribution reports on the significant progress of N-NBI ion sources towards long pulse operation. Results from plasma and beam diagnostics are presented and possible solutions to bring large ion sources into a steady state are discussed.