Simulations were performed to estimate the main chamber wall heating in ITER resulting from rapid discharge shutdown by neon massive gas injection (MGI). The TokSys current diffusion model coupled with a simplified impurity transport model was used. Impurity parallel flow was treated with a single-fluid pressure-driven flow model. Impurity cross-field diffusion was treated with an empirical diffusion coefficient estimated from present experiments, while impurity poloidal rotation was included empirically by extrapolation in minor radius from present experiments to ITER. For single-valve neon MGI, maximum wall temperatures of order 1100 K are predicted, somewhat below the melting temperature of beryllium (1560 K). Lower temperature excursions were obtained by increasing the number of gas valves, while higher wall temperatures could be obtained by turning up initial plasma thermal energy or cross-field transport coefficients. Highest wall temperatures tended to occur on the centre ...

Radial profiles of density and temperature of plasmas in tokamak scrape-off layer (SOL) widen along the minor radius at plasma current disruptions and edge-localized modes. It can be a result of polarization (inertial) ion drift, induced by an increase in plasma temperature with time. This paper considers the effect of polarization currents on heat and particle transfer in SOL plasmas, in particular in inertial unipolar arcs observed at transient modes of operation.

A hybrid simulation code for nonlinear magnetohydrodynamics (MHD) and energetic-particle dynamics has been extended to simulate recurrent bursts of Alfvén eigenmodes by implementing the energetic-particle source, collisions and losses. The Alfvén eigenmode bursts with synchronization of multiple modes and beam ion losses at each burst are successfully simulated with nonlinear MHD effects for the physics condition similar to a reduced simulation for a TFTR experiment (Wong et al 1991 Phys. Rev. Lett. 66 1874, Todo et al 2003 Phys. Plasmas 10 2888). It is demonstrated with a comparison between nonlinear MHD and linear MHD simulation results that the nonlinear MHD effects significantly reduce both the saturation amplitude of the Alfvén eigenmodes and the beam ion losses. Two types of time evolution are found depending on the MHD dissipation coefficients, namely viscosity, resistivity and diffusivity. The Alfvén eigenmode bursts take place fo...

We show that collective Thomson scattering (CTS) holds the potential to become a new diagnostic principle for measurements of the fuel ion ratio, n _T/ n _D, in ITER. Fuel ion ratio measurements will be important for plasma control and machine protection in ITER. Measurements of ion cyclotron structures in CTS spectra have been suggested as the basis for a new fuel ion ratio diagnostic which would be well suited for reactor environments and capable of providing spatially resolved measurements in the plasma core. Such measurements were demonstrated in recent experiments in the TEXTOR tokamak. Here we conduct a sensitivity study to investigate the potential measurement accuracy of a CTS fuel ion ratio diagnostic on ITER. The study identifies regions of parameter space in which CTS can be expected to provide useful information on plasma composition, and we find that a CTS fuel ion ratio diagnostic could meet the ITER measurement requirements for a stan...

Plasma–wall interaction is one of the most important issues that present magnetic confinement devices have to face. In the RFX-mod reversed field pinch experiment plasma–wall interaction has become a hard point increasing plasma current up to the RFX-mod maximum design value of 2 MA, since in this case local power deposition can be as high as 10 MW m ⁻². Since the first wall of RFX-mod is entirely covered by graphite tiles different techniques have been tested to control hydrogen wall influx: He glow discharges cleaning, He discharges at high plasma currents, wall boronization and baking. With the best results obtained by boronization, at high plasma currents all such techniques improve the situation but do not allow a complete and stationary hydrogen influx reduction. Furthermore, in the presence of localized high power load the wall still responds providing very high influxes. In order to improve this situation wall conditioning by lithium has been tested. As a first ...

The linear stability of Alfvén eigenmodes (AEs) is studied experimentally in the JET tokamak using its active MHD spectroscopy system, the so-called Alfvén Eigenmode Active Diagnostic (AEAD). Following the optimization of the AEAD system, AEs with toroidal mode numbers ( n) in the low- n and medium- n range were excited systematically. A database was created from the damping rate measurements of toroidal AEs (TAEs), obtained in ohmically heated plasmas with monotonic q-profile. The TAE damping rate measurements were studied as a function of n, focusing on the effects of the edge plasma shape and the q profile and investigating their implications for the importance of the continuum and radiative damping mechanisms. In this paper, the first part of the analysis is presented: a statistical treatment of the damping rate database is performed and the correlations with various plasma parameters and relevant quantities are explored. The analysis sh...

Complementing the database approach that has been presented in part I, this paper probes into individual plasma discharges and investigates the damping rate variations that are driven by the evolving plasma configurations. In addition to confirming the stabilizing effect of elongated plasma shapings on toroidal Alfvén eigenmodes, the significance of the span of the q profile is shown experimentally for the first time. It is found that, under conditions of transient current density profiles, the damping rate of medium- n modes has a dependence on the background plasma parameters that is distinctively different from that of low- n modes. The analysis of the correlation of the damping rate variations with the changes of the q profile shape and the alignment of the gaps of the shear Alfvén continuum leads to an assessment of the effect of the interaction of the modes with the Alfvén continuum as a function of n.

The dispersion relation for high- n ballooning modes is obtained from the customary drift gyro-kinetic equation employing the energy-dependent Lorenz collision operator. There are significant differences between our results and those with the Krook collision operator. The present results show that collision seems to have much weaker stabilizing effects, if any, on the high- n ballooning modes than those with the Krook collision operator when the collisionality, υ _*e, is smaller than a critical value. However, contrary to the phenomenon, the collision shows strong stabilizing effects on the ballooning mode once the collisionality exceeds a critical value. These differences are supposed to arise from boundary layer behaviour which refers to the electron perturbation at the boundary between trapped and passing electrons in velocity space, but the physical process cannot be described by non-conserving Krook collision operators. This destabilizing mechanism of bo...

New measurements of ion energies and currents in type I and mitigated ELMs have been carried out in the ASDEX Upgrade far scrape-off layer using a retarding field analyser (RFA). The ion temperature averaged over an ELM, T _{i ELM} measured 35–60 mm outside the separatrix (i.e. 15–25 mm in front of the outboard limiter) is in the range 20–200 eV, which is 5–50% of the ion temperature at the pedestal top. T _{i ELM} decreases with the separatrix distance with the e-folding length of ~10 mm measured in the far SOL for a particular set of conditions, and increases with the ELM energy W _ELM. Lowest T _{i ELM} is measured during mitigated type I ELMs. Likewise, the ELM-averaged ion current e-folding length increases with W _ELM, similar to the e-folding le...

Recent experiments at ASDEX Upgrade demonstrate the compatibility of ELM mitigation by magnetic perturbations with efficient particle fuelling by inboard pellet injection. ELM mitigation persists in a high-density, high-collisionality regime even with the strongest applied pellet perturbations. Pellets injected into mitigation phases trigger no type-I ELM-like events unlike when launched into unmitigated type-I ELMy plasmas. Furthermore, the absence of ELMs results in improved fuelling efficiency and persistent density build-up. Pellet injection is helpful to access the ELM-mitigation regime by raising the edge density beyond the required threshold level, mostly eliminating the need for strong gas puff. Finally, strong pellet fuelling can be applied to access high densities beyond the density limit encountered with pure gas puffing. Core densities of up to 1.6 times the Greenwald density have been reached while maintaining ELM mitigation. No upper density limit for the ELM-mitiga...