Yoshiteru Sakamoto

Neoclassical tearing mode control using electron cyclotron current drive and magnetic island evolution in JT-60U

The results of stabilizing neoclassical tearing modes (NTMs) with electron cyclotron current drive (ECCD) in JT-60U are described with emphasis on the effectiveness of the stabilization. The range of the minimum EC wave power needed for complete stabilization of an m/n = 2/1 NTM was experimentally identified for two regimes using unmodulated ECCD to clarify the NTM behaviours with different plasma parameters: 0.2 < jEC/jBS < 0.4 for Wsat/dEC ~ 3 and Wsat/Wmarg ~ 2, and 0.35 < jEC/jBS < 0.46 for Wsat/dEC ~ 1.5 and Wsat/Wmarg ~ 2. Here, m and n are the poloidal and toroidal mode numbers; jEC and jBS the EC-driven current density and bootstrap current density at the mode rational surface; Wsat, Wmarg and dEC the full island width at saturation, marginal island width and full-width at half maximum of the ECCD deposition profile, respectively. Stabilization of a 2/1 NTM using modulated ECCD synchronized with a mode rotation of about 5 kHz was performed, in which it was found that the stabilization effect degrades when the phase of the modulation deviates from that of the ECCD at the island O-point. The decay time of the magnetic perturbation amplitude due to the ECCD increases by 50% with a phase shift of ±50° from the O-point ECCD, thus revealing the importance of the phasing of modulated ECCD. For near X-point ECCD, the NTM amplitude increases, revealing a destabilization effect. It was also found that modulated ECCD at the island O-point has a stronger stabilization effect than unmodulated ECCD by a factor of more than 2.

Momentum transport and plasma rotation profile in toroidal direction in JT-60U L-mode plasmas

The characteristics of momentum transport and plasma rotation in the toroidal direction are studied, using near-perpendicular neutral beam injection (PERP-NBI), co tangential and counter (CTR) tangential NBI in JT-60U. Diffusive and non-diffusive terms of momentum transport are evaluated from the transient analysis by using the momentum source modulation. Fast ion losses due to the toroidal field ripple, which locally induces the edge CTR rotation, are used as a novel momentum source. Parameter dependence of these transport coefficients i.e. the toroidal momentum diffusivity (?) and the convection velocity (Vconv), and the relation between momentum and heat diffusivities (?i) are investigated in L-mode plasmas systematically. The toroidal momentum diffusivity increases with increasing heating power and decreases with increasing plasma current. The relation of ? and ?i to some non-dimensional parameters is investigated. A clear dependence of ?/?i on normalized plasma pressure (?N) is observed. It is also found that toroidal rotation velocity profiles in the case with and without external torque input can be almost reproduced by ? and Vconv estimated from the transient momentum transport analysis at low ? (?N < 0.4).

Plasma models for real-time control of advanced tokamak scenarios

Anintegratedplasmaprofilecontrolstrategy,ARTAEMIS,isbeingdevelopedforextrapolatingpresent-dayadvanced tokamak (AT) scenarios to steady-state operation. The approach is based on semi-empirical modelling and was initiallyexploredonJET(Moreauetal2008Nucl.Fusion48106001). Thispaperdealswiththegeneralapplicability of this strategy for simultaneous magnetic and kinetic control on various tokamaks. The determination of thedevicespecific, control-oriented models that are needed to compute optimal controller matrices for a given operation scenario is discussed. The methodology is generic and can be applied to any device, with different sets of heating and current drive actuators, controlled variables and profiles. The system identification algorithms take advantage of the large ratio between the magnetic and thermal diffusion time scales and have been recently applied to both JT-60U and DIII-D data. On JT-60U, an existing series of high bootstrap current (∼70%), 0.9MA non-inductive AT discharges was used. The actuators consisted of four groups of neutral beam injectors aimed at perpendicular injection (on-axis and off-axis), and co-current tangential injection (also on-axis and off-axis). On DIII-D, dedicated system identification experiments were carried out in the loop voltage (Vext) control mode (as opposed to current control) to avoid feedback in the response data from the primary circuit. The reference plasma state was that of a 0.9MA AT scenario which had been optimized to combine non-inductive current fractions near unity with 3.5 <β N < 3.9, bootstrap current fractions larger than 65% and H98(y,2) = 1.5. Actuators other than Vext were co-current,counter-currentandbalancedneutralbeaminjection,andelectroncyclotroncurrentdrive. Powerandloop voltage modulations resulted in dynamic variations of the plasma current between 0.7 and 1.2MA. It is concluded that the response of essential plasma parameter profiles to specific actuators of a given device can be satisfactorily identified from a small set of experiments. This provides, for control purposes, a readily available alternative to first-principles plasma modelling. (Some figures in this article are in colour only in the electronic version)

Towards an emerging understanding of non-locality phenomena and non-local transport

In this paper, recent progress on experimental analysis and theoretical models for non-local transport (non-Fickian fluxes in real space) is reviewed. The non-locality in the heat and momentum transport observed in the plasma, the departures from linear flux-gradient proportionality, and externally triggered non-local transport phenomena are described in both L-mode and improved-mode plasmas. Ongoing evaluation of ‘fast front’ and ‘intrinsically non-local’ models, and their success in comparisons with experimental data, are discussed

Effects of ripple loss of fast ions on toroidal rotation in JT-60U

The driving mechanism of toroidal rotation and the momentum transport are studied on JT-60U in relation to the toroidal rotation in the direction antiparallel to the plasma current, i.e. counter (CTR) direction, with near-perpendicular neutral beam (PERP-NB) injection. Fast ion losses due to the toroidal field ripple induce CTR rotation in the peripheral region, and the magnitude of CTR rotation with PERP-NBs reduces by installing the ferritic steel tiles as a consequence of the reduction in the ripple losses. It is also found that toroidal rotation velocity profiles in the core region can be explained by momentum transport considering diffusivity and convective velocity estimated from transient momentum transport analysis in L-mode plasmas.

The 2008 public release of the international multi-tokamak confinement profile database

This paper documents the public release PR08 of the International Tokamak Physics Activity (ITPA) profile database, which should be of particular interest to the magnetic confinement fusion community. Data from a wide variety of interesting discharges from many of the worlds leading tokamak experiments are now made available in PR08, which also includes predictive simulations of an initial set of operating scenarios for ITER. In this paper we describe the discharges that have been included and the tools that are available to the reader who is interested in accessing and working with the data. Most discharge descriptions refer to more detailed previous publications. In addition, we review physics analyses that have already made use of the profile database discharges. Public access to PR08 data is unconditional, but this paper should be cited by any publication that makes use of PR08 data.

Dynamics of ion internal transport barrier in LHD heliotron and JT-60U tokamak plasmas

Dynamics of ion internal transport barrier (ITB) formation and impurity transport both in the Large Helical Device (LHD) heliotron and in the JT-60U tokamak are described. Significant differences between heliotron and tokamak plasmas are observed. The location of the ITB moves outwards during the ITB formation regardless of the sign of magnetic shear in JT-60U, and the ITB becomes more localized in plasmas with negative magnetic shear. In LHD, a low Te/Ti ratio (<1) of the target plasma with high power heating is found to be necessary to achieve the ITB plasma and the ITB location tends to expand outwards or inwards depending on the condition of the target plasmas. Associated with the formation of the ITB, the carbon density tends to be peaked due to inward convection in JT-60U while the carbon density becomes hollow due to outward convection in LHD. The outward convection observed in LHD contradicts the prediction by neoclassical theory.

Characteristics and control of the type I edge localized mode in JT-60U

The detailed characteristics of the precursor of the type I edge localized mode (ELM) have been studied in JT-60U using diagnostics with high temporal and spatial resolution such as a microwave reflectometer, electron cyclotron emission (ECE) heterodyne radiometer and grating polychromator. Coherent density and temperature precursors have been observed before the collapse phase of type I ELM. The growth rate of the precursor is evaluated to be γ/ωA ~ 10−3 for several edge pedestal conditions. From the phase delay between ECE signals measured at two toroidal locations and the frequency of the precursor, the toroidal mode number is experimentally evaluated as n = 8–10 or 14–16 assuming that the precursor rotates toroidally with the same toroidal rotation speed of carbon impurity. It is found that the dominant n varies with each ELM under the same plasma condition. The ratio of the pressure gradient inside the pedestal (∇pin) to the pressure gradient within the pedestal (∇pped) has been confirmed as an important parameter in determining the ELM energy loss (ΔWELM) normalized to the pedestal stored energy (Wped), ΔWELM/Wped. From the comparison of the reduction rate in the ion temperature profile due to ELMs, a larger reduction rate within the pedestal and a wider ELM affected area are observed in the plasma with larger ∇pin/∇pped. When the plasma near the top of the pedestal on the high-field side is heated by an electron cyclotron wave (ECW) power of 1.57 MW, the ΔWELM/Wped is reduced by ~35%, together with an increase in the ELM frequency. The increasing rate of the ELM frequency with the heating power is about four times larger in the ECW injection case than the natural power dependence observed in the neutral beam injection case.

Momentum transport studies from multi-machine comparisons

A database of toroidal momentum transport on five tokamaks, Alcator C-Mod, DIII-D, JET, NSTX and JT-60U, has been constructed under a wide range of conditions in order to understand the characteristics of toroidal momentum transport coefficients, namely the toroidal momentum diffusivity (??) and the pinch velocity (Vpinch). Through an inter-machine comparison, the similarities and differences in the properties of ?? and Vpinch among the machines have been clarified. Parametric dependences of these momentum transport coefficients have been investigated over a wide range of plasma parameters taking advantage of the different operation regimes in machines. The approach offers insights into the parametric dependences as follows. The toroidal momentum diffusivity (??) generally increases with increasing heat diffusivity (?i). The correlation is observed over a wide range of ??, covering roughly two orders of magnitude, and within each of the machines over the whole radius. Through the inter-machine comparison, it is found that ?? becomes larger in the outer region of the plasma. Also observed is a general trend for Vpinch in tokamaks; the inward pinch velocity (?Vpinch) increases with increasing ??. The results that are commonly observed in machines will support a toroidal rotation prediction in future devices. On the other hand, differences among machines have been observed. The toroidal momentum diffusivity, ??, is larger than or equal to ?i in JET and JT-60U; on the other hand, ?? is smaller than or equal to ?i in NSTX, DIII-D and Alcator C-Mod. In DIII-D, the ratio ?RVpinch/?? at r/a?=?0.5?0.6 is about 2, which is small compared with that in other tokamaks (?RVpinch/?????5). Based on these different observations, parametric dependences of ??/?i, RVpinch/?? and ?? have been investigated in H-mode plasmas. Across the dataset from all machines, the ratio ??/?i tends to be larger in low at fixed Te/Ti and . An increase in ?? is observed with decreasing ne and/or increasing Te. The pinch number (?RVpinch/??) is observed to increase with increasing at both q95?=?5.5?7.2 and q95?=?3.7?4.5. Here , , Te, Ti, and q95 are, respectively, the normalized effective electron collision frequency, the normalized ion poloidal Larmor radius, the electron and ion temperatures, the inverse ratio of density scale length, , to the major radius, R, and the safety factor at the 95% flux surface.

Effects of edge collisionality on ELM characteristics in the grassy ELM regime

Effects of the edge collisionality on ELM characteristics have been investigated in the grassy ELM regime on JT-60U. Both in the high and low q regions (q95 > 6 and q95 ~ 4.3), ELM amplitude becomes higher at high edge collisionality (density). This collisionality dependence is opposite to the dependence observed in the type I ELM regime, where the ELM amplitude decreases with increasing edge collisionality. The stability analysis suggests that the grassy ELM at q95 ~ 4.3 can be triggered by the infinite-n ballooning mode localized inside the pedestal, when peeling–ballooning modes are stable. In the case of higher collisionality, peeling–ballooning modes can also be unstable, and as a result a larger ELM can be triggered. Since both stability boundaries for infinite-n ballooning mode and peeling–ballooning modes are located near the operational point, a small change in the pedestal condition such as a local pressure gradient and/or a bootstrap current can determine the most unstable mode. This mechanism is considered as a possible candidate for the appearance of a mixture ELM of grassy ELMs and large ELMs.