Takaaki Fujita

Off-axis current drive and real-time control of current profile in JT-60U

Aiming at optimization of current profile in high-β plasmas for higher confinement and stability, a real-time control system of the minimum of the safety factor (qmin) using the off-axis current drive has been developed. The off-axis current drive can raise the safety factor in the centre and help to avoid instability that limits the performance of the plasma. The system controls the injection power of lower-hybrid waves, and hence its off-axis driven current in order to control qmin. The real-time control of qmin is demonstrated in a high-β plasma, where qmin follows the temporally changing reference qmin,ref from 1.3 to 1.7. Applying the control to another high-β discharge (βN = 1.7, βp = 1.5) with m/n = 2/1 neo-classical tearing mode (NTM), qmin was raised above 2 and the NTM was suppressed. The stored energy increased by 16% with the NTM suppressed, since the resonant rational surface was eliminated. For the future use for current profile control, current density profile for off-axis neutral beam current drive (NBCD) is for the first time measured, using the motional Stark effect diagnostic. Spatially localized NBCD profile was clearly observed at the normalized minor radius ρ of about 0.6–0.8. The location was also confirmed by multi-chordal neutron emission profile measurement. The total amount of the measured beam driven current was consistent with the theoretical calculation using the ACCOME code. The CD location in the calculation was inward shifted than the measurement.

Development of a high-brightness and low-divergence lithium neutral beam for a Zeeman polarimetry on JT-60U

A high-brightness and low-divergence neutral beam is obtained for a Zeeman polarimetry of edge plasmas on JT-60U. The electron density and the pitch angle of the magnetic field line, thus the plasma current density distribution, can be measured by the Zeeman polarimetry using the lithium beam. A thermionic ion source heated by an electron beam is developed in order to obtain the ion beam current extraction over 10 mA. The beam optics is designed after detailed numerical simulation taking the space charge effects into account because a low-divergence angle of the neutral lithium beam leads to a narrow spectrum of the beam emission. It is also necessary to keep the beam radius small for good spatial resolution due to a long beam line of 6.5 m. The newly developed ion gun is operated on a test stand which simulates the diagnostic arrangement on JT-60U. The ion beam current of 10 mA at a beam energy of 10 keV is successfully extracted from the ion source operated at the temperature over 1300 degrees C and focused by Einzel lens. The full width at half maximum radius of the ion beam at the neutralizer is about 9 mm. A sodium vapor neutralizer neutralizes the collimated ion beam fully at the temperature of 300 degrees C. The neutral beam profiles are measured at two locations of the beam line at Z=2.3 m (beam monitor position) and Z=6.5 m (plasma region). The half-width at half maximum radius of the neutral beam of 26 mm and the equivalent beam current of 3 mA with the beam divergence angle of 0.2 deg which is the half-angle divergence are obtained. Those parameters satisfy the requirements of the Zeeman polarimetry. Furthermore, a long pulse extraction with a current of 10 mA and duration of 50 s is attained.

Steady state operation research in JT-60U with extended pulse length

Recent experimental results for steady state operation research in JT-60U are presented with emphasis on extension of sustained duration of high performance. The duration of heating has been extended from 10 to 30 s, and plasma properties and dynamics have been investigated in a long time scale exceeding the current diffusion time and close to the wall saturation time on ELMy H-mode, high βp H-mode and reversed shear H-mode regimes. The duration of sustainment of high beta and/or a large fraction of bootstrap current has been extended. The particle control with the saturated wall has been studied. Development of real-time control of q profile and effects of toroidal rotation on ELMs and the QH-mode are also discussed.

Density fluctuation measurement using motional Stark effect optics in JT-60U

The multichannel motional Stark effect (MSE) diagnostic system in JT-60U has been upgraded to measure density fluctuation profile. A 16-channel fast-sampling digitizer has been added in order to measure photomultiplier-tube signals at measurement frequency of 0.5–1MHz. The new system works as a MSE and beam emission spectroscopy diagnostic. Spatially resolved electron density fluctuation profile measurement in various operation regimes is presented. In the core plasma, density fluctuation induced by rotation of tearing mode islands was observed. Temporal evolution of the fluctuation frequency agrees with that measured by Mirnov coils (poloidal and toroidal mode numbers: 2 and 1, respectively). The phases of the fluctuations on either side of the q=2 surface are inverted, which is consistent with electron cyclotron emission. These measurements show that the density fluctuation is caused by a rotating magnetic island structure induced by the tearing mode. In the scrape-off layer of a H-mode plasma with edge...

Fast dynamics of type I and grassy ELMs in JT-60U

In order to understand the physics of the ELM trigger and determine the ELM size, the fast ELM dynamics of type I and grassy ELMs have been studied in JT-60U, using new fast diagnostics with high spatial and temporal resolutions such as a lithium beam probe (?t ~ 0.5?ms) and a charge exchange recombination spectroscopy (?t ~ 2.5?ms), which can measure the electron density and the ion temperature, respectively. The evolution of the ion pressure profile in the pedestal region has been evaluated for the first time by detailed edge profile measurements. Then, the dynamics of the density, the ion temperature and the ion pressure in the ELM cycle has been investigated. The co-rotating plasmas are compared with the counter (ctr)-rotating plasmas for the understanding of the toroidal rotation effects. Type I ELMs observed in co-rotating plasmas exhibit a larger and wider ELM affected area (?nped/nped ~ 30%, radial extent >15?cm) than ctr-rotating plasmas (?nped/nped ~ 20%, radial extent ~10?cm). Just before a type I ELM crash, the pedestal ion pressure and its maximum gradient in co-rotating plasmas are 20% and 12% higher than those in ctr-rotating plasmas, respectively. It is found that the radial extent of the ion pressure gradient at the pedestal region in co-rotating plasmas is 14% wider than that in ctr-rotating plasmas. The experimental results suggest that the ELM size is connected with the structure of the plasma pressure in the whole pedestal region. As for the dynamics of grassy ELMs, the collapse of density pedestal is smaller (<20%) and narrower (~5?cm) than those of type I ELMs, as observed in the collapse of the electron temperature pedestal. Thus, it is confirmed that both conductive and convective losses due to grassy ELMs are small.

Hydrogen isotope effects on ITG scale length, pedestal and confinement in JT-60 H-mode plasmas

The dependence of heat transport, edge pedestal and confinement on isotopic composition was investigated in conventional H-mode plasmas. Identical profiles for the electron density, electron temperature and ion temperature were obtained for hydrogen and deuterium plasmas, whereas the required power clearly increased for hydrogen, which resulted in a reduction in heat diffusivity for deuterium. The determination of identical temperature profiles, despite the different heating power, suggested that the characteristics of heat conduction essentially differ for hydrogen and deuterium, even at the same scale length of temperature gradient. The self-regulating physics mechanism determining the overall H-mode confinement was also addressed. The inverse of the ion temperature gradient (ITG) scale length, or ∇Ti/Ti, which is required for a given ion heat diffusivity, increased by a factor of approximately 1.2 for deuterium compared with that for hydrogen. The relationship between edge pedestal pressure and global βp holds true consistently regardless of the difference in the isotopic composition. A higher value of βp was obtained for deuterium because of its smaller ITG scale length and because of the additional stored energy in the thermal and fast ion components, the latter due to an increase in the slowing down time with an increase in isotopic mass.

Development of the transport-code framework for self-consistent predictions of rotation and the radial electric field

A toroidal momentum solver, which calculates the evolution of the toroidal angular momentum density summed over thermal species, is modelled and integrated into the 1.5D integrated transport code TOPICS. A non-iterative scheme to uniquely determine the radial electric field Er is developed. It is also used to compute the parallel and toroidal flows for each species based on the neoclassical transport theory. The combination of TOPICS and the orbit-following Monte Carlo code OFMC enables us to self-consistently predict the evolution of not only the density, temperature and safety factor but also the toroidal momentum and Er. The framework developed has been tested against JT-60U experiments and showed the predictive capability of toroidal rotation. Several time-dependent simulations in which toroidal rotation and Er play an important role are carried out, showing that the H-mode confinement considerably depends on the direction of toroidal rotation via the change in Er, as observed in JT-60U. Rapid toroidal rotation can be hardly expected in ITER if rotation is solely driven by neutral beam (NB) injection, whereas the residual stress may potentially generate the torque comparable to or greater than that by NBs.

Magnetic fluctuation profile measurement using optics of motional Stark effect diagnostics in JT-60U

Motional Stark effect (MSE) diagnostics in JT-60U works as polarimeter to measure the pitch angle of magnetic field as well as beam-emission-spectroscopy (BES) monochromator simultaneously at 30 spatial channels. Fluctuation in the BES signal using MSE optics (MSE/BES) contains fluctuations in not only the density but also the pitch angle (or the magnetic field). Correlation analysis of the magnetic fluctuation between two spatial channels is applied to high-beta plasma with a magnetohydrodynamic activity at frequency of about 0.9 kHz. It has been found that the magnetic fluctuation measured by the MSE/BES is spatially localized near the magnetic flux surface having safety factor and that the phase of the fluctuation is inverted at about the surface, suggesting magnetic island structure by tearing mode. The phase of the magnetic fluctuation measured by the MSE/BES at outside of the q=2 surface is consistent with that by the pickup coil placed outside the plasma.

Roles of the double tearing mode on the formation of a current hole

Tokamak plasma with negative central current density is known to be unstable for m = 1/n = O resistive kink magnetohydrodynamic instability and the explanation for the absence of negative neutral current, the so-called current hole Phenomena, is made by the destabilization of the mode, However, a strong reversed magnetic shear configuration has two resonant surfaces for low mode numbers and should be unstable for the double tearing mode (DTM). Here, we examine the effects of DTM on the formation of current hole and show that the occurrence of DTM does not change the situation completely. Tlie growth of DTM flattens the current profile near the minimum-q region: here q is the safety factor, and the current gradient to drive n = 0 mode usually remains after a reconnection event due to DTM and the stability of m = 1/n = O mode is not affected much by the DTM activity.