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Investigations of runaway electrons (REs) during induced disruptions are performed at the TEXTOR tokamak. The synchrotron radiation generated by REs in the plasma core is detected using an infrared camera. The measurements enable the observation of the structure and dynamics of the runaway beam. In particular, the runaway beam is investigated as a function of the vertical and horizontal control fields. From the plasma current a runaway number of 4.33 × 1016 was calculated. The number of REs with energies exceeding 25 MeV is 6.4 × 1015 according to the synchrotron measurement at the end of the current quench phase. The mean pitch angle of the >25 MeV runaways is found to be 52 mrad which is smaller than in the low density discharge scenario. In addition, the synchrotron measurements show for the first time that a significant number of the high energy REs can survive after the end of the current plateau phase. In order to understand this, runaway orbit calculations have been performed which are in good agreement with the measurements. [ABSTRACT FROM AUTHOR]

The paper presents results of a first analysis of the divertor performance during the L–H transition in ITER. The integrated model consists of the SOLPS4.3 code suite for the SOL and divertor, and the ASTRA code for the core and pedestal regions. The results of SOLPS4.3 are parametrized and used as the boundary conditions for ASTRA, ensuring a consistent description of the plasma core and the edge. Boundary conditions switch from those for wide (L-mode) to narrow (H-mode) SOL once the transition criterion is met. The results show that, for conditions for which a full-power operational space with acceptable power loading of the targets exists, a transition from the initial L-mode operation to H-mode can be found for the same assumptions, i.e. the full-power H-mode regime is accessible. [ABSTRACT FROM AUTHOR]

In the Large Helical Device (LHD) operated with a metallic (stainless steel) first wall, it is found that the iron density, nFe, at the plasma core is fairly low (nFe ≤ 108 cm-3) in general neutral beam (NB)-heated discharges, while the iron quickly increases with the appearance of impurity accumulation when a multi-hydrogen ice pellet is injected or the NB input power is largely reduced. Although the highest iron density (nFe ≤ 1010 cm-3) at the plasma centre in the LHD is observed from such discharges, it suggests a still low iron concentration (nFe/ne < 10-3). Therefore, the edge iron transport in the ergodic layer, which determines the iron influx to the core plasma, is studied to clarify why the iron density in the core plasma is low. A line ratio of Fe XV located in the vicinity of the last closed flux surface to Fe VIII (or Fe IX) located in the ergodic layer decreases with density. The two-dimensional (2D) edge iron emission of Fe XVI and Fe IX is enhanced in the vicinity of the X-point with a larger number of magnetic field lines directly connected to divertor plates, which suggests that iron ions from the first wall move downstream. The density of edge Fe15+ ions giving the iron influx to the core plasma is analysed with the 2D distribution. The analysis also shows that the iron influx to the core plasma decreases with density. These results clearly indicate that the screening effect developed in the ergodic layer works well for iron ions coming from the first wall. A three-dimensional edge transport simulation with EMC3-EIRENE can also predict an effective impurity screening for heavy impurities compared to light impurities. [ABSTRACT FROM AUTHOR]

Measuring the value of the safety factor (q) in the core during sawtooth cycles is still an open issue. A new method to measure q in Tore Supra plasma core is presented here. It relies on the analysis of the time evolution of a set of MHD modes detected after the sawtooth crashes. These modes are in the frequency range of previously observed Beta-induced Alfvén Eigenmodes, but with a frequency declining in time. The mode frequency analysis shows that the q profile is reversed when we have ICRH, after the sawtooth crash. In high current discharges ( MA), the q-profile remains reversed for a longer time compared with lower plasma current discharges. Non-linear 3D MHD simulations of sawteeth performed with the XTOR-2F code (Lütjens and Luciani 2010 J. Comput. Phys. 229 8130–43) exhibit features that are similar to these observations. [ABSTRACT FROM AUTHOR]

Integrating the plasma core performance with an edge and scrape-off layer (SOL) that leads to tolerable heat and particle loads on the wall is a major challenge. The new European medium size tokamak task force (EU-MST) coordinates research on ASDEX Upgrade (AUG), MAST and TCV. This multi-machine approach within EU-MST, covering a wide parameter range, is instrumental to progress in the field, as ITER and DEMO core/pedestal and SOL parameters are not achievable simultaneously in present day devices. A two prong approach is adopted. On the one hand, scenarios with tolerable transient heat and particle loads, including active edge localised mode (ELM) control are developed. On the other hand, divertor solutions including advanced magnetic configurations are studied. Considerable progress has been made on both approaches, in particular in the fields of: ELM control with resonant magnetic perturbations (RMP), small ELM regimes, detachment onset and control, as well as filamentary scrape-off-layer transport. For example full ELM suppression has now been achieved on AUG at low collisionality with n  =  2 RMP maintaining good confinement . Advances have been made with respect to detachment onset and control. Studies in advanced divertor configurations (Snowflake, Super-X and X-point target divertor) shed new light on SOL physics. Cross field filamentary transport has been characterised in a wide parameter regime on AUG, MAST and TCV progressing the theoretical and experimental understanding crucial for predicting first wall loads in ITER and DEMO. Conditions in the SOL also play a crucial role for ELM stability and access to small ELM regimes. [ABSTRACT FROM AUTHOR]

Beam-driven Alfvén eigenmodes (AEs) have been studied in the TJ-II low-magnetic-shear flexible heliac (B0  =  0.95 T, 〈R〉  =  1.5 m, 〈a〉  =  0.22 m), in L-mode hydrogen plasmas with hydrogen NBI and ECR heating (PNBI  ⩽  1.0 MW, ENBI  =  32 keV, PECRH  ⩽  0.6 MW). In low-density plasmas in the range   =  (0.3–1.5)  ×  1019 m−3, a large variety of AEs have been observed in the plasma core with the heavy ion beam probe diagnostic and Mirnov coils, in the frequency range 50 kHz  <  fAE  <  380 kHz. In experiments in which the vacuum rotational transform is varied during the shot (dynamic configuration scan), some AEs exhibit changes in their nonlinear evolution from bursting-amplitude AEs with chirping frequency to steady-frequency AEs, and back. The range of intervals within which the AEs studied are chirping or steady-state is determined. [ABSTRACT FROM AUTHOR]

Periods of edge localized mode (ELM)-free H-mode with increased pedestal pressure and width were observed in the DIII-D tokamak when density fluctuations localized to the region near the separatrix were present. Injection of a powder of 45 µm diameter lithium particles increased the duration of the enhanced pedestal phases to up to 350 ms, and also increased the likelihood of a transition to the enhanced phase. Lithium injection at a level sufficient for triggering the extended enhanced phases resulted in significant lithium in the plasma core, but carbon and other higher Z impurities as well as radiated power levels were reduced. Recycling of the working deuterium gas appeared unaffected by this level of lithium injection. The ion scale, kθρs ∼ 0.1–0.2, density fluctuations propagated in the electron drift direction with f ∼ 80 kHz and occurred in bursts every ∼1 ms. The fluctuation bursts correlated with plasma loss resulting in a flattening of the pressure profile in a region near the separatrix. This localized flattening allowed higher overall pedestal pressure at the peeling–ballooning stability limit and higher pressure than expected under the EPED model due to reduction of the pressure gradient below the ‘ballooning critical profile’. Reduction of the ion pressure by lithium dilution may contribute to the long ELM-free periods. [ABSTRACT FROM AUTHOR]

The electron component is the main channel for anomalous power loss and the main indicator of transient processes in the tokamak plasma. The electron temperature and density profiles mainly determine the operational mode of the machine. This imposes demanding requirements on the precision and on the spatial and temporal resolution of the Thomson scattering (TS) measurements. Measurements of such high electron temperature with good accuracy in a large fusion device such as ITER using TS encounter a number of physical problems. The 40 keV TS spectrum has a significant blue shift. Due to the transmission functions of the fibres and to their darkening that can occur under a strong neutron irradiation, the operational wavelength range is bounded on the blue side. For example, high temperature measurements become impossible with the 1064 nm probing wavelength since the TS signal within the boundaries of the operational window weakly depends on Te. The second problem is connected with the TS calibration. The TS system for a large fusion machine like ITER will have a set of optical components inaccessible for maintenance, and their spectral characteristics may change with time. Since the present concept of the TS system for ITER relies on the classical approach to measuring the shape of the scattered spectra using wide spectral channels, the diagnostic will be very sensitive to the changes in the optical transmission. The third complication is connected with the deviation of the electron velocity distribution function from a Maxwellian that can happen under a strong ECRH/ECCD, and it may additionally hamper the measurements. This paper analyses the advantages of a ‘multi-laser approach’ implementation for the current design of the core TS system. Such an approach assumes simultaneous plasma probing with different wavelengths that allows the measurement accuracy to be improved significantly and to perform the spectral calibration of the TS system. Comparative analysis of the conservative and advanced approaches is given. [ABSTRACT FROM AUTHOR]

The compatibility of two operational constraints—operation above the L–H power threshold and at low power to divertor—is examined for ITER long pulse H-mode and hybrid scenarios in integrated core–scrape off layer (SOL)–divertor modelling including impurities (intrinsic Be, He, W and seeded Ne). The core thermal, particle and momentum transport is simulated with the GLF23 transport model tested in the self-consistent simulations of temperatures, density and toroidal rotation velocity in JET hybrid discharges and extrapolated to ITER. The beneficial effect of toroidal rotation velocity on fusion gain is shown. The sensitivity studies with respect to operational (separatrix and pedestal density, Ne gas puff) and unknown physics (W convective velocity and perpendicular diffusion in SOL as well as W prompt re-deposition) parameters are performed to determine their influence on the operational window and fusion gain. [ABSTRACT FROM AUTHOR]

In the perspective of operating tungsten monoblocks in WEST, the ongoing major upgrade of the Tore Supra tokamak, a dedicated modelling effort has been carried out to simulate the interaction between the edge plasma and the tungsten wall. A new transport code, SolEdge2D–EIRENE, has been developed with the ability to simulate the plasma up to the first wall. This is especially important for steady state operation, where thermal loads on all the plasma facing components, even remote from the plasma, are of interest. Moreover, main chamber tungsten sources are thought to dominate the contamination of the plasma core. We present here in particular new developments aimed at improving the description of the interface between the plasma and the wall, namely a way to treat sheath physics in a more faithful way using the output of 1D particle in cell simulations. Moreover, different models for prompt redeposition have been implemented and are compared. The latter is shown to play an important role in the balance between divertor and main chamber sources. [ABSTRACT FROM AUTHOR]

Extensive linear and non-linear gyrokinetic simulations and linear magnetohydrodynamic (MHD) analyses performed for JET hybrid discharges with improved confinement have shown that the large population of fast ions found in the plasma core under particular heating conditions has a strong impact on core microturbulence and edge MHD by reducing core ion heat fluxes and increasing pedestal pressure in a feedback mechanism. In the case of the ITER like wall, it is shown how this mechanism plays a decisive role for the transition to plasma regimes with improved confinement and it can explain the weak power degradation obtained in dedicated power scans. The mechanism is found to be highly dependent on plasma triangularity as it changes the balance between the improvement in the plasma core and the edge. The feedback mechanism can play a similar role in the ITER hybrid scenario as in the JET discharges analysed due to its high triangularity plasmas and the large amount of fast ions generated in the core by the heating systems and the alpha power. [ABSTRACT FROM AUTHOR]

A Korean fusion energy development promotion law (FEDPL) was enacted in 2007. As a following step, a conceptual design study for a steady-state Korean fusion demonstration reactor (K-DEMO) was initiated in 2012. After the thorough 0D system analysis, the parameters of the main machine characterized by the major and minor radii of 6.8 and 2.1 m, respectively, were chosen for further study. The analyses of heating and current drives were performed for the development of the plasma operation scenarios. Preliminary results on lower hybrid and neutral beam current drive are included herein. A high performance Nb3Sn-based superconducting conductor is adopted, providing a peak magnetic field approaching 16 T with the magnetic field at the plasma centre above 7 T. Pressurized water is the prominent choice for the main coolant of K-DEMO when the balance of plant development details is considered. The blanket system adopts a ceramic pebble type breeder. Considering plasma performance, a double-null divertor is the reference configuration choice of K-DEMO. For a high availability operation, K-DEMO incorporates a design with vertical maintenance. A design concept for K-DEMO is presented together with the preliminary design parameters. [ABSTRACT FROM AUTHOR]

The effect of resonant magnetic perturbations (RMPs) on particle transport is studied in the J-TEXT tokamak. It is found that for the discharges with an existing saturated 2/1 resistive tearing mode (TM), applied RMPs of moderate amplitude lead to a decrease in electron density with a relative amplitude ranging from −3% to −10% in the plasma core, and the mode stabilization and electron temperature increase are observed simultaneously in this case. Sufficiently large amplitude of RMPs, however, leads to locked modes and much larger decrease in the electron density as well as in the electron temperature, with . For the discharges without 2/1 TMs, applied RMPs cause a relative density decrease (−30%) before (after) field penetration. Using the two-fluid equations and experimental parameters as input, the nonlinear numerical results approximately agree with experimental observations. [ABSTRACT FROM AUTHOR]