Search

The ITER tokamak requires two heating Neutral Beam Injectors (NBIs), based on the neutralisation of 40A of negative hydrogen/deuterium ions accelerated to 1MeV with a pulse length of one hour. As these simultaneous parameters were never experimentally achieved, in order to optimise source operation and to assess beam properties, a specific test facility ("PRIMA") was established in Padova, comprising two experiments: SPIDER (full-size negative ion source with 40A beam, 100keV particle energy) and MITICA, the prototype of ITER NBIs. A key NBI component is the particle accelerator, where electrostatic and magnetic fields produce the beam optics. The design of SPIDER and MITICA accelerators was based on the most advanced numerical codes available for the investigation of the expected beam properties and on the most up-to-date experience of various research groups. Several diagnostic systems will characterise source and beam during the experiments. In view of the operation, effort is presently devoted (within bilateral collaboration frameworks between Italian and Japanese agencies) to the preparation for the synergistic employment of simulations and experiments. Specifically, a characterisation of the beam properties is carried out in the NIFS-R&D multi-cusp negative ion source (NIFS-RNIS) by a sophisticated set of source and beam diagnostic systems. Simulations are mainly performed at Consorzio RFX, by a suite of numerical tools to compute or investigate various aspects of beam physics. Preliminary comparisons showed very good agreement between simulations and measured data regarding the basic features of the accelerator. The present contribution describes the synergy between numerical codes and diagnostic techniques. Both are applied to real-time characterisation of the beam, with the main scopes of providing interpretation of the measured results and of supplying data for successive choices during the operation. These activities allow also extensive validation of numerical models and the choice of the most useful set of codes to be used in future experiments like DEMO.

Development of the high performance negative hydrogen ion source is a fundamental demand in realizing fusion reactor. In order to clarify the extraction mechanism of H´, temporal and spatial variations of the negative ions and electrons in the extraction region are intensively surveyed at NIFS. In addition, the beam acceleration experiments have been performed by changing the accelerator configuration in order to improve the voltage holding capability and to study the negative ion beam optics. In a caesiated hydrogen plasma, it was observed that the negative hydrogen ion density (nH´) becomes one order higher in magnitude than the electron density (n e) in the vicinity of the plasma grid (PG). The response of negative-ion rich plasmas to the extraction field was investigated by measuring the plasma potential (V p) profiles in the axis perpendicular to the PG before and during beam extraction. The V p increases with applying the extraction field, and the influence of the extraction field on the V p was observed at 30 mm from the PG. As for the nH´, it was also observed that the extraction field affects on the nH´ at 30 mm from the PG, where the nH´ decreases simultaneously with the beam extraction. These observations indicate that the extraction field affects the particle dynamics in the wide region extending over 30 mm from the PG. This feature is completely different from that of electron-ion plasmas. We also found that the negative ion production efficiency becomes twice higher by changing the shape of grounded grid holes without any modification on the plasma chamber. We assumed that the behaviour of the back-streaming ion was affected by changing the GG. The back-streaming ion trajectory was analyzed with the beam trajectory simulation, and it was found that the back-streaming ion distributes in the larger area on the back plate with the slot GG. This implies that some part of condensed Cs on the back plate was more effectively evaporated by the back-streaming ion with the slot GG and that Cs flux onto the PG surface increased, which resulted in the enhancement of the negative ion production. This result suggests that the accelerator configuration is one of the key factors to determine the negative ion production efficiency and that the Cs consumption can be reduced by the Cs recycling from the wall of the ion source chamber.

The driving and damping mechanism of plasma flow is an important issue because flow shear has a significant impact on turbulence in a plasma, which determines the transport in the magnetized plasma. Here we report clear evidence of the flow damping due to stochastization of the magnetic field. Abrupt damping of the toroidal flow associated with a transition from a nested magnetic flux surface to a stochastic magnetic field is observed when the magnetic shear at the rational surface decreases to 0.5 in the large helical device. This flow damping and resulting profile flattening are much stronger than expected from the Rechester-Rosenbluth model. The toroidal flow shear shows a linear decay, while the ion temperature gradient shows an exponential decay. This observation suggests that the flow damping is due to the change in the non-diffusive term of momentum transport.

Neutral beam injector (NBI) is an indispensable device to produce high-density and hot plasmas in fusion development machine. We have developed negative-ion-based NBI (N- NBI) in LHD, that have been operated continuously since 1998. Three N-NBIs are arranged tangentially, and was equipped with a negative hydrogen ion (H-) source which enabled high energy and high power beam injection. The nominal specication of beam energy and beam power for each N-NBI was 180 keV and 5 MW, respectively. We had achieved 16 MW beam injection by three N-NBIs in LHD experiment. Moreover, we installed two positive-ion-based neutral beam injector (P-NBI) arranged perpendicularly to the LHD. High-power (12 MW) with lower energy (40~60 keV) beam injection allowed effective ion heating and profile measurement of ion temperature by charge exchange recombination spectroscopy. A hydrogen negative ion source is a key component in N-NBI system, that will also use future NBI in ITER. Understanding of H- behavior from cesiated metal production surface to extraction aperture is an important issue to maintain high power negative ion beam. We have researched distribution of H- ions and its extraction behavior by cavity ring-down spectroscopy and a spectrally-selective imaging system, respectively, in the 1/3 scaled ion source (NIFS-RNIS). We found that H- ions widely distributed in the extraction region, and these ions constituted H- beam. Recently, we have a lot of domestic and international collaboration program for H- source. We applied a spectrally-selective imaging system in RF hydrogen negative on source in ELISE test facility in Max-Planck-Institut fur Plasmaphysik (IPP) Garching (Germany). We found clear signal response on H? emission during beam extraction. This observation will contribute to obtain behavior of H- ions in RF source which is variable output for production high power beam and optimization of beam shape in ITER source. We also observed an extracted H- beam shape in NIFS-RNIS used the mini-STRIKE system which consists of a carbon- fibre-carbon-composite (CFC) tiles and an infrared camera collaborated with Consorzio RFX (Italy). We found the clear dependence between the beam shape and operating parameters. These findings have contributed to the high performance and safety NBI operation in the LHD.

The experimentally optimized magnetic field configuration of the Large Helical Device [A. Iiyoshi et al., Nucl. Fusion 39, 1245 (1999)], where the magnetic axis is shifted inward by 15 cm from the early theoretical prediction, reveals 50% better global energy confinement than the prediction of the scaling law. This configuration has been investigated further from the viewpoints of high energy particle confinement and magnetohydrodynamic (MHD) stability. The confinement of high energy ions is improved as expected. The minority heating of ion cyclotron range of frequency was successful and the heating efficiency was improved by the inward shift. The confinement of passing particles by neutral beam injection was also improved under low magnetic field strength, and there could be obtained an almost steady high beta discharge up to 3% in volume average. This was a surprising result because the observed pressure gradient exceeded the Mercier unstable limit. The observed MHD activities became as high as beta but they did not grow enough to deteriorate the confinement of high energy ions or the performance of the bulk plasma, which was still 50% better than the scaling. According to these favorable results, better performance would be expected by increasing the heating power because the neoclassical transport can also be improved there.

A group of fusion safety professionals contribute to a Joint Working Group (JWG) that performs occupational safety walkthroughs of US and Japanese fusion experiments on a routine basis to enhance the safety of visiting researchers. The most recent walkthrough was completed in Japan in March 2008 by the US Safety Monitor team. This paper gives the general conclusions on fusion facility personnel safety that can be drawn from the series of walkthroughs.

The progress in the understanding of the physics and the concurrent parameter extension in the large helical device since the last IAEA-FEC, in 2012 (Kaneko O et al 2013 Nucl. Fusion 53 095024), is reviewed. Plasma with high ion and electron temperatures (Ti(0) ~ Te(0) ~ 6 keV) with simultaneous ion and electron internal transport barriers is obtained by controlling recycling and heating deposition. A sign flip of the nondiffusive term of impurity/momentum transport (residual stress and convection flow) is observed, which is associated with the formation of a transport barrier. The impact of the topology of three-dimensional magnetic fields (stochastic magnetic fields and magnetic islands) on heat momentum, particle/impurity transport and magnetohydrodynamic stability is also discussed. In the steady state operation, a 48 min discharge with a line-averaged electron density of 1 × 1019 m−3 and with high electron and ion temperatures (Ti(0) ~ Te(0) ~ 2 keV), resulting in 3.36 GJ of input energy, is achieved.