Performance of multi-aperture grid extraction systems for an ITER-relevant RF-driven negative hydrogen ion source

The ITER neutral beam system requires a negative hydrogen ion beam of 48 A with an energy of 0.87 MeV, and a negative deuterium beam of 40 A with an energy of 1 MeV. The beam is extracted from a large ion source of dimension 1.9 × 0.9 m2 by an acceleration system consisting of seven grids with 1280 apertures each. Currently, apertures with a diameter of 14 mm in the first grid are foreseen. In 2007, the IPP RF source was chosen as the ITER reference source due to its reduced maintenance compared with arc-driven sources and the successful development at the BATMAN test facility of being equipped with the small IPP prototype RF source ( of the area of the ITER NBI source). These results, however, were obtained with an extraction system with 8 mm diameter apertures. This paper reports on the comparison of the source performance at BATMAN of an ITER-relevant extraction system equipped with chamfered apertures with a 14 mm diameter and 8 mm diameter aperture extraction system. The most important result is that there is almost no difference in the achieved current density—being consistent with ion trajectory calculations—and the amount of co-extracted electrons. Furthermore, some aspects of the beam optics of both extraction systems are discussed.</jats:p

What limits supercurrents in high temperature superconductors? A microscopic model of cuprate grain boundaries

The interface properties of high-temperature cuprate superconductors have been of interest for many years, and play an essential role in Josephson junctions, superconducting cables, and microwave electronics. In particular, the maximum critical current achievable in high-Tc wires and tapes is well known to be limited by the presence of grain boundaries, regions of mismatch between crystallites with misoriented crystalline axes. In studies of single, artificially fabricated grain boundaries the striking observation has been made that the critical current Jc of a grain boundary junction depends exponentially on the misorientation angle. Until now microscopic understanding of this apparently universal behavior has been lacking. We present here the results of a microscopic evaluation based on a construction of fully 3D YBCO grain boundaries by molecular dynamics. With these structures, we calculate an effective tight-binding Hamiltonian for the d-wave superconductor with a grain boundary. The critical current is then shown to follow an exponential suppression with grain boundary angle. We identify the buildup of charge inhomogeneities as the dominant mechanism for the suppression of the supercurrent.Comment: 28 pages, 12 figure

Evidence for recycling of N from plants to soil during the growing season.

In spite of the known below-ground biomass production of plant roots that concurrently introduce significant amounts of carbon and nitrogen into the soil, the effects of these inputs on N cycling in the soil-plant system are seldom considered. Here, we report on two field experiments carried out between 1995 and 1997 at the FAM Research Station Scheyern: (1) a N-turnover experiment to determine the N fluxes derived from N-15-labeled clover residues incorporated into the plough layer of defined plots, and (2) a root production experiment to assess the above (shoot) and below ground (gross and net root) biomass production of winter wheat in different fields, but nearby the N-15 plots. An initial 50% decrease in soil organic N-15 at 0-20-cm soil depth was recorded between fall, 1996 (incorporation of clover straw) and spring, 1997 (138 days after incorporation), which was then followed by a period of stability in N-15 levels in the soil organic N until the harvest of winter wheat (286 days after incorporation). This stability may be explained in two ways: (a) actual stability of clover-derived N-15 remaining in the second phase, e.g., due to recalcitrant compounds or microbial immobilization; or (b) apparent stability, e.g., because the actual mineralization of clover-derived N-15 in the soil was compensated by secondary inputs of organic 1 5 N (recycling). Further results showed that the first explanation was unlikely, as (1) between 138 and 286 days after clover incorporation, the mean N-15 signature in soil mineral N was 2.1 at.%, indicating a persistent mineralization of clover residues; and (2) a decrease in soil microbial biomass N-15 occurred in the second phase, indicating a continued N turnover in the soil, The amount of clover-derived N-15 accumulated below the plough layer at 20-110-cm soil depth (11.5%) between early spring and the harvest of wheat also corroborated the return of mineralized N-15 into the soil being due to the root N inputs by winter wheat. Based on the depth distribution of winter wheat net root biomass (root production experiment) and on soil organic N-15 depth distribution (1 5 N-turnover experiment), the root N input into soil was estimated to be 282 kg ha(-1), equivalent to 54% of total net N assimilation of winter wheat. Thus, the results of this study give substantial evidence for a N loop between soil and growing plants, whereby a part of the net mineralized N taken up by plants is continuously returned into the soil by their roots. The implications of this N loop for the interpretation of N-15 experiments and for plant nutrition are discussed