[en] Rotation effects on the linear stability of the resistive wall mode (RWM) are investigated numerically based on the ideal magnetohydrodynamics (MHD) model. One topic is impact of poloidal rotation in a toroidally rotating plasma. Since the difference between mode frequency and rotation frequency is important to change the MHD stability by rotation, modulation of the Doppler-shifted frequency by poloidal rotation can play an important role even when the poloidal rotation velocity is significantly lower than the toroidal one. In addition, when poloidal rotation is fixed, the absolute value of the Doppler-shifted frequency relies on the direction of toroidal rotation, and hence, linear MHD stability can depend on this rotation direction. Such a dependence is sometimes observed experimentally. The other topic is the re-destabilization of the RWM by toroidal rotation in a reversed shear tokamak. Several numerical results imply that a candidate of this re-destabilized mode is a stable MHD eigenmode, which becomes unstable due to coupling with the RWM when rotation frequency is similar to the frequency of this stable eigenmode in a static equilibrium. (paper)

[en] The modelling of tokamak scenarios requires the simultaneous solution of both the time evolution of the plasma kinetic profiles and of the magnetic equilibrium. Their dynamical coupling involves additional complications, which are not present when the two physical problems are solved separately. Difficulties arise in maintaining consistency in the time evolution among quantities which appear in both the transport and the Grad–Shafranov equations, specifically the poloidal and toroidal magnetic fluxes as a function of each other and of the geometry. The required consistency can be obtained by means of iteration cycles, which are performed outside the equilibrium code and which can have different convergence properties depending on the chosen numerical scheme. When these external iterations are performed, the stability of the coupled system becomes a concern. In contrast, if these iterations are not performed, the coupled system is numerically stable, but can become physically inconsistent. By employing a novel scheme (Fable E et al 2012 Nucl. Fusion submitted), which ensures stability and physical consistency among the same quantities that appear in both the transport and magnetic equilibrium equations, a newly developed version of the ASTRA transport code (Pereverzev G V et al 1991 IPP Report 5/42), which is coupled to the SPIDER equilibrium code (Ivanov A A et al 2005 32nd EPS Conf. on Plasma Physics (Tarragona, 27 June–1 July) vol 29C (ECA) P-5.063), in both prescribed- and free-boundary modes is presented here for the first time. The ASTRA–SPIDER coupled system is then applied to the specific study of the modelling of controlled current ramp-up in ASDEX Upgrade discharges. (paper)

[en] Global gyrokinetic simulations of ion temperature gradient (ITG) driven turbulence in an ideal MHD ITER equilibrium plasma are performed with the ORB5 code. The noise control and field-aligned Fourier filtering procedures implemented in ORB5 are essential in obtaining numerically healthy results with a reasonable amount of computational effort: typical simulations require 10⁹ grid points, 10⁹ particles and, despite a particle per cell ratio of unity, achieve a signal to noise ratio larger than 50. As compared with a circular concentric configuration with otherwise similar parameters (same ρ_* = 1/720), the effective heat diffusivity is considerably reduced for the ITER MHD equilibrium. A self-organized radial structure appears, with long-lived zonal flows (ZF), modulating turbulence heat transport and resulting in a corrugated temperature gradient profile. The ratio of long-lived ZF to the fluctuating ZF is markedly higher for the ITER MHD equilibrium as compared with circular configurations, thereby producing a more effective ITG turbulence suppression, in spite of a higher linear growth rate. As a result, the nonlinear critical temperature gradient, R/L_Tcrit,NL, is about twice the linear critical temperature gradient, R/L_Tcrit,lin. Moreover, the heat transport stiffness above the nonlinear threshold is considerably reduced as compared with circular cases. Plasma elongation is probably one of the essential causes of this behaviour: indeed, undamped ZF residual levels and geodesic acoustic mode damping are both increasing with elongation. Other possible causes of the difference, such as magnetic shear profile effects, are also investigated. (paper)

[en] The numerical simulation of the dynamics of fast ions coming from neutral beam injection (NBI) heating is an important task in fusion devices, since these particles are used as sources to heat and fuel the plasma and their uncontrolled losses can damage the walls of the reactor. This paper shows a new application that simulates these dynamics on the grid: FastDEP. FastDEP plugs together two Monte Carlo codes used in fusion science, namely FAFNER2 and ISDEP, and add new functionalities. Physically, FAFNER2 provides the fast ion initial state in the device while ISDEP calculates their evolution in time; as a result, the fast ion distribution function in TJ-II stellerator has been estimated, but the code can be used on any other device. In this paper a comparison between the physics of the two NBI injectors in TJ-II is presented, together with the differences between fast ion confinement and the driven momentum in the two cases. The simulations have been obtained using Montera, a framework developed for achieving grid efficient executions of Monte Carlo applications. (paper)

[en] The shock ignition concept for inertial confinement fusion includes launching a strong shock with a high-intensity laser spike into an imploding shell. The laser intensity in the plasma corona is above the threshold for parametric instabilities, thus providing conditions for strong non-linear effects. Here we present a series of one-dimensional kinetic simulations of laser–plasma interactions in such a regime. After a transient period of strong non-stationary scattering, the laser–plasma interaction enters an asymptotic regime where a significant part of the incident laser flux is absorbed in the plasma and is transformed into hot electrons. The repartition of the absorbed energy and spectral characteristics of the scattered radiation are presented for laser intensities in the range 2.4–24 PW cm⁻². For a laser intensity of 8 PW cm⁻², the total absorption is 69% ; about 50% of absorption takes place at quarter critical density and the remaining 19% at 1/16th of the critical density. 52% of the total laser pulse energy are absorbed due to stimulated Raman scattering, which produces electrons with a temperature of about 30 keV, and 17% is absorbed due to cavitation, which produces a more isotropic distribution of hot electrons with a temperature of about 10 keV. (paper)

[en] An analytic form to describe the boundary of an axisymmetric plasma is proposed. This new form uses a generalization of the family of superellipses. The plasma boundaries of existing tokamaks are well described using the compact notation. The form employs eleven parameters of which five are standard, two are generalizations of a standard parameter and four are introduced here. With these same parameters, a closed-form analytic solution can be used to generate new boundaries without x-points. If the desired boundary has x-points, the analytic form can be extended in a manner for which a closed-form solution has not been found, but does have an exact solution that can be found numerically. This new form should be useful for variety of physics studies that use magnetohydrodynamic equilibria, such as the dependence of plasma stability on shape and design of poloidal field coil sets that can support a defined range of shapes. (paper)

[en] This paper will discuss simulations of the full ionization process (i.e. plasma burn-through), fundamental to creating high temperature plasma. By means of an applied electric field, the gas is partially ionized by the electron avalanche process. In order for the electron temperature to increase, the remaining neutrals need to be fully ionized in the plasma burn-through phase, as radiation is the main contribution to the electron power loss. The radiated power loss can be significantly affected by impurities resulting from interaction with the plasma facing components. The DYON code is a plasma burn-through simulator developed at Joint European Torus (JET) (Kim et al and EFDA-JET Contributors 2012 Nucl. Fusion 52 103016, Kim, Sips and EFDA-JET Contributors 2013 Nucl. Fusion 53 083024). The dynamic evolution of the plasma temperature and plasma densities including the impurity content is calculated in a self-consistent way using plasma wall interaction models. The recent installation of a beryllium wall at JET enabled validation of the plasma burn-through model in the presence of new, metallic plasma facing components. The simulation results of the plasma burn-through phase show a consistent good agreement against experiments at JET, and explain differences observed during plasma initiation with the old carbon plasma facing components. In the International Thermonuclear Experimental Reactor (ITER), the allowable toroidal electric field is restricted to 0.35 (V m⁻¹), which is significantly lower compared to the typical value (∼1 (V m⁻¹)) used in the present devices. The limitation on toroidal electric field also reduces the range of other operation parameters during plasma formation in ITER. Thus, predictive simulations of plasma burn-through in ITER using validated model is of crucial importance. This paper provides an overview of the DYON code and the validation, together with new predictive simulations for ITER using the DYON code. (paper)

[en] The lithium beam emission spectroscopy (Li-BES) is a powerful diagnostic to resolve the plasma edge density with high temporal and spatial resolution. The recent upgrades of the Li-BES at ASDEX Upgrade and the resulting gain in photon flux allow the plasma edge density to be determined with an advanced level of accuracy. Furthermore, electron density fluctuations are measured using Li-BES. The Li-BES capabilities and limitations to measure electron density profiles as well as density fluctuations are presented. It is well suited to characterize electron density turbulence in the scrape off layer (SOL) with decreasing sensitivity towards the plasma core. This is demonstrated by simulations as well as by comparisons with other diagnostics. The Li-BES is an appropriate tool to study transport phenomena in the SOL over a wide range of plasma parameters due to its robustness and routine usage. (paper)

[en] QuaLiKiz, a model based on a local gyrokinetic eigenvalue solver (Bourdelle et al 2002 Nucl. Fusion 42 892–902) is expanded to include momentum flux modeling in addition to heat and particle fluxes (Bourdelle et al 2007 Phys. Plasmas 14 112501, Casati et al 2009 Nucl. Fusion 49 085012). Essential for accurate momentum flux predictions, the parallel asymmetrization of the eigenfunctions is successfully recovered by an analytical fluid model. This is tested against self-consistent gyrokinetic calculations and allows for a correct prediction of the E × B shear impact on the saturated potential amplitude by means of a mixing length rule. Hence, the effect of the E × B shear is recovered on all the transport channels including the induced residual stress. Including these additions, QuaLiKiz remains ∼10 000 faster than non-linear gyrokinetic codes allowing for comparisons with experiments without resorting to high performance computing. The example is given of momentum pinch calculations in NBI modulation experiments (Tala et al 2009 Phys. Rev. Lett. 102 075001) for which the inward convection of the momentum is correctly predicted. (paper)

[en] TCV experiments demonstrate the basic power exhaust properties of the snowflake (SF) plus and SF minus divertor configurations by measuring the heat fluxes at each of their four divertor legs. The measurements indicate an enhanced transport into the private flux region and a reduction of peak heat fluxes compared to a similar single null configuration. There are indications that this enhanced transport cannot be explained by the modified field line geometry alone and likely requires an additional or enhanced cross-field transport channel. The measurements, however, do not show a broadening of the scrape-off layer (SOL) and, hence, no increased cross-field transport in the common flux region. The observations are consistent with the spatial limitation of several characteristic SF properties, such as a low poloidal magnetic field in the divertor region and a long connection length to the inner part of the SOL closest to the separatrix. Although this limitation is typical in a medium sized tokamak like TCV, it does not apply to significantly larger devices where the SF properties are enhanced across the entire expected extent of the SOL. (paper)