Taina Kurki-Suonio

Formation and detection of internal transport barriers in low-current tokamaks

In low-current tokamaks, in the absence of radial electric fields (Er), the widths of the drift orbits are large and the direct orbit losses can extend deep into the plasma. Furthermore, in such a plasma even a modest Er can produce rotation with a poloidal Mach number (Mp) that exceeds unity. Using the Monte Carlo code ASCOT, which follows charged particle orbits in the five-dimensional phase space, the formation of an internal transport barrier (ITB) in such a tokamak is investigated. Carrying out the simulations for the geometry corresponding to the FT-2 tokamak, it is shown that if, under these conditions, a steep density gradient is created in the plasma, the plasma responds by generating a strong (much stronger than needed to compensate the diamagnetic drift) Er in the region of the strong gradient. The generation appears to be a pure neoclassical effect, but a global solution over the entire plasma cross section is required to fully identify it. As a result, an ITB-like situation with a strongly sheared E×B flow is obtained inside the plasma. In these circumstances Mp>1, and thus the orbits of the majority of ions become strongly squeezed. The neutral fluxes observed by neutral particle analysers are also simulated to find out if the neutral spectra can be utilized to estimate the Er values across the plasma cross section in the FT-2 tokamak.

Comparison of fast ion collective Thomson scattering measurements at ASDEX Upgrade with numerical simulations

Collective Thomson scattering (CTS) experiments were carried out at ASDEX Upgrade to measure the one-dimensional velocity distribution functions of fast ion populations. These measurements are compared with simulations using the codes TRANSP/NUBEAM and ASCOT for two different neutral beam injection (NBI) configurations: two NBI sources and only one NBI source. The measured CTS spectra as well as the inferred one-dimensional fast ion velocity distribution functions are clearly asymmetric as a consequence of the anisotropy of the beam ion populations and the selected geometry of the experiment. As expected, the one-beam configuration can clearly be distinguished from the two-beam configuration. The fast ion population is smaller and the asymmetry is less pronounced for the one-beam configuration. Salient features of the numerical simulation results agree with the CTS measurements while quantitative discrepancies in absolute values and gradients are found.

Comparison of collective Thomson scattering signals due to fast ions in ITER scenarios with fusion and auxiliary heating

Auxiliary heating such as neutral beam injection (NBI) and ion cyclotron resonance heating (ICRH) will accelerate ions in ITER up to energies in the MeV range, i.e. energies which are also typical for alpha particles. Fast ions of any of these populations will elevate the collective Thomson scattering (CTS) signal for the proposed CTS diagnostic in ITER. It is of interest to determine the contributions of these fast ion populations to the CTS signal for large Doppler shifts of the scattered radiation since conclusions can mostly be drawn for the dominant contributor. In this study, distribution functions of fast ions generated by NBI and ICRH are calculated for a steady-state ITER burning plasma equilibrium with the ASCOT and PION codes, respectively. The parameters for the auxiliary heating systems correspond to the design currently foreseen for ITER. The geometry of the CTS system for ITER is chosen such that near perpendicular and near parallel velocity components are resolved. In the investigated ICRH scenario, waves at 50 MHz resonate with tritium at the second harmonic off-axis on the low field side. Effects of a minority heating scheme with 3He are also considered. CTS scattering functions for fast deuterons, fast tritons, fast 3He and the fusion born alphas are presented, revealing that fusion alphas dominate the measurable signal by an order of magnitude or more in the Doppler shift frequency ranges typical for fast ions. Hence the observable CTS signal can mostly be attributed to the alpha population in these frequency ranges. The exceptions are limited regions in space with some non-negligible signal due to beam ions or fast 3He which give rise to about 30% and 10–20% of the CTS signal, respectively. In turn, the dominance of the alpha contribution implies that the effects of other fast ion contributions will be difficult to observe by CTS.

ASCOT Simulations of Fast Ion Power Loads to the Plasma-facing Components in ITER

The wall loads due to fusion alphas as well as neutral beam injection-and ICRF-generated fast ions were simulated for ITER reference scenario-2 and scenario-4 including the effects of ferritic inse ...

Narrow power profiles seen at JET and their relation to ion orbit losses

Abstract Recent JET measurements of heat load profiles during NBI-heated, natural density, ELMy H-modes revealed highly peaked profiles. In this study, ion orbit loss (IOL) from the core periphery was simulated numerically on four selected JET discharges (two high power H-modes, a fuelled H-mode and an L-mode), using realistic magnetic geometry and 2-D plasma/neutral background. The simulated and measured profiles agree quite well in all discharges, suggesting that IOL is indeed the mechanism responsible for the narrow profiles. The interaction between energetic ions and scrape-off layer (SOL) ions/atoms was also investigated, resulting in SOL power stopping factors of 60–80%. The momentum transfer via IOL to background ions was found to be in the same direction as the observed SOL flows (towards the inner target) but too small to be their primary cause. Preliminary simulation of IOL on ITER indicate peak heat loads

Interpretation of recent power width measurements in JET MkIIGB ELMy h-modes

Results of a novel technique of measuring the deposited power profile based on thermocouples embedded in the divertor plates are presented and discussed. Power profiles were obtained for a series of 2.5 MA/2.4 T discharges involving a scan in the NBI heating power (4-18 MW, where PL-H~5 MW) and a scan in D2 fuelling rate ((0-3)×1022 s-1). In H-mode plasmas, a narrow feature was observed in the power profile, which scaled inversely with power entering the scrape-off-layer, λq,nw∝PSOL-0.4±0.1, while the base profile showed virtually no power dependence. At constant input power, the peak heat flux was strongly reduced by D2 puffing (from ~20 to 4 MW m-2), with the narrow feature in the power profile effectively suppressed. Based on numerical analysis using a guiding-centre Monte-Carlo code ASCOT, coupled to a fluid-like plasma-neutral code package OSM2/EIRENE, the above results are interpreted as a consequence of inter-ELM ion orbit loss from the pedestal region.

Guiding-center simulations of nonlocal and negative inertia effects on rotation in a tokamak

The magnitude of the radial electric field (Er), resulting from nonambipolar fluxes of neoclassical origin, is evaluated using Monte Carlo guiding-center simulations for a low-current plasma corresponding to the FT-2 tokamak {Fisichiskii Tokamak-2, Ioffe Institute, St. Petersburg [S. I. Lashkul, V. N. Budnikov, E. O. Vekshina et al., Plasma Phys. Rep. 27, 1001 (2001)]}. The Er-values are found to significantly exceed those given by the standard neoclassical theory, based on thin-orbit assumption, when the plasma current is sufficiently low and the pressure gradient is sufficiently high. Strong Er-structures are found to form in the same low plasma-current range where enhanced confinement is reported in the FT-2 tokamak. In the simulations, the physics behind the strong increase in the field values is intimately related to the poloidal Mach-number, together with the wide ion orbits.

Bifurcation of the radial electric field in the presence of edge polarization in tokamaks

In guiding-center simulations of a tokamak plasma edge, bifurcations of both the radial electric field and the electrode current are observed for a threshold value of the electrode bias voltage, in qualitative agreement with experiments. The simulations are neoclassical and Monte Carlo based, also containing the perpendicular viscosity effect. Above the voltage threshold, an inward moving, solitary radial electric field structure is generated with a concomitant drop in the electrode current. The position, width, and height of the soliton depend on the magnitude and radial variation of the conductivity (and, thus, on local plasma parameters), finite orbit effects, and also on the perpendicular viscosity and neutral density. The obtained Er structures are narrower and higher than the ones measured in the edge polarization experiments in the Julich tokamak TEXTOR-94 [S. Jachmich et al., Plasma Phys. Controlled Fusion 40, 1105 (1998)].

Power loads to ITER first wall structures due to fusion alphas in a non-axisymmetric magnetic field including the presence of MHD modes

We use the orbit-following Monte Carlo code ASCOT to calculate the wall power loads in ITER caused by fusion alphas. The simulations are carried out for a realistic 3D magnetic field that includes the effect of both ferritic inserts and the test blanket modules, both causing aberrations in the magnetic field structure, particularly at the edge. In addition to an magnetohydrodynamic (MHD)-quiescent plasma we now also address the power loads in the presence of relevant MHD events: both neoclassical tearing modes (NTMs) and toroidal Alfven eigenmodes (TAEs) are included in the simulation model. In the case of NTMs, the total power load to the wall is found to depend on the perturbation amplitude. Even with the strongest perturbation, however, the power load density stays within the design limit of the ITER wall materials. In the case of TAEs, while the wall power load density stays at the MHD-quiescent level, significant redistribution of alphas inside the plasma was observed. This was also found to affect the alpha heating profile.

Monte Carlo simulations of the heat load asymmetries on JET divertor plates

JET divertor load measurements using embedded thermocouples have indicated a large inner/outer target load asymmetry and a significant non-uniformity in the load to the outer target. Furthermore, under H-mode conditions, the load appears to be dominated by the ion component. While the inner/outer target load asymmetry can be understood in terms of the conventional fluid picture, the sharp structures in the deposition profile as well as the ion dominance of the load cannot be readily explained by the fluid approach. The guiding-centre orbit-following Monte Carlo code ASCOT is used to simulate the ion contribution to the target deposition profiles, and this is the first time a narrow structure in the divertor loads has been reproduced in simulations. The importance of the plasma edge collisionality, the atomic, molecular and ionic collisions in the scrape-off layer, and the radial electric field both inside and outside the separatrix are all studied individually in detail.