Kinetic modeling of ELM-induced tungsten transport in a tokamak plasma

\u3cp\u3eImpurity accumulation in the core plasma leads to fuel dilution and higher radiative losses that can lead to loss of the H-mode, to thermal collapse of the plasma, and eventually even to a disruption in tokamaks. In present experiments, it has been shown that Edge Localized Modes (ELMs) at sufficiently high frequency are required to prevent W accumulation in the core, by expelling impurities from the edge plasma region, thus preventing their penetration into the plasma core. We present a full-orbit particle extension of the MHD code JOREK suitable for simulating impurity transport during ELMs. This model has been applied to the simulation of an ELM crash in ASDEX Upgrade, where we have quantified the displacement of W particles across flux surfaces. The transport mechanism is shown to be the particle E × B-drifts due to the electric field created by the MHD instability underlying the ELM. In- and outward transport is observed as a series of interchange motions, leading to a superdiffusive behavior. This causes not only the particles near the plasma pedestal to move outwards but also the particles outside of the pedestal to move inwards. This has important consequences for operation with W in ITER, where it is expected to be screened in the pedestal, and ELMs are shown here to increase the core W density. A comparison with existing diffusive modeling is made, showing a qualitative agreement and the limitations of this simplified modeling approach.\u3c/p\u3

3D non-linear MHD simulation of the MHD response and density increase as a result of shattered pellet injection

\u3cp\u3eThe MHD response and the penetration of a deuterium shattered pellet into a JET plasma is investigated via the non-linear reduced MHD code JOREK with the neutral gas shielding (NGS) ablation model. The dominant MHD destabilizing mechanism by the injection is identified as the local helical cooling at each rational surface, as opposed to the global current profile contraction. Thus the injected fragments destabilize each rational surface as they pass through them. The injection penetration is found to be much better compared to MGI, with the convective transport caused by core MHD instabilities (e.g. 1/1 kink) contributing significantly to the core penetration. Moreover, the injection with realistic JET SPI system configurations is simulated in order to provide some insights into future operations, and the impact on the total assimilation and penetration depth of varying injection parameters such as the injection velocity or fineness of shattering is assessed. Further, the effect of changing the target equilibrium temperature or q profile on the assimilation and penetration is also investigated. Such analysis will form the basis of further investigation into a desirable configuration for the future SPI system in ITER.\u3c/p\u3

Investigation of MHD activity caused by shattered pellet injection via JOREK 3D non-linear MHD simulations

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Extreme regimes of femtosecond photoemission from a copper cathode in a dc electron gun

\u3cp\u3eThe femtosecond photoemission yield from a copper cathode and the emittance of the created electron beams has been studied in a 12 MeV/m, 100 keV dc electron gun over a wide range of laser fluence, from the linear photoemission regime until the onset of image charge limitations and cathode damaging. The measured photoemission curves can be described well with available theory which includes the Schottky effect, second-order photoemission, and image charge limitation. The second-order photoemission can be explained by thermally assisted one-photon photoemission (1PPE) and by above-threshold two-photon photoemission (2PPE). Measurements with a fresh cathode suggest that the 2PPE process is dominant. The beam emittance has been measured for the entire range of initial surface charge densities as well. The emittance measurements of space-charge dominated beams can be described well by an envelope equation with generalized perveance. The dc gun produces 0.1 pC bunches with 25 nm rms normalized emittance, corresponding to a normalized brightness usually associated with rf photoguns. In this experimental study the limits of femtosecond photoemission from a copper cathode have been explored and analyzed in great detail, resulting in improved understanding of the underlying mechanisms.\u3c/p\u3

Test particles dynamics in the JOREK 3D non-linear MHD code and application to electron transport in a disruption simulation

\u3cp\u3eIn order to contribute to the understanding of runaway electron generation mechanisms during tokamak disruptions, a test particle tracker is introduced in the JOREK 3D non-linear MHD code, able to compute both full and guiding center relativistic orbits. Tests of the module show good conservation of the invariants of motion and consistency between full orbit and guiding center solutions. A first application is presented where test electron confinement properties are investigated in a massive gas injection-triggered disruption simulation in JET-like geometry. It is found that electron populations initialised before the thermal quench (TQ) are typically not fully deconfined in spite of the global stochasticity of the magnetic field during the TQ. The fraction of 'survivors' decreases from a few tens down to a few tenths of percent as the electron energy varies from 1 keV to 10 MeV. The underlying mechanism for electron 'survival' is the prompt reformation of closed magnetic surfaces at the plasma core and, to a smaller extent, the subsequent reappearance of a magnetic surface at the edge. It is also found that electrons are less deconfined at 10 MeV than at 1 MeV, which appears consistent with a phase averaging effect due to orbit shifts at high energy.\u3c/p\u3

Coupled nonlinear MHD-particle simulations for ITER with the JOREK+particle-tracking code

\u3cp\u3eThe JOREK [1] MHD code is coupled to a newly developed particle tracer to simulate heavy impurity radiation and other effects. Particle-fluid sputtering, needed as boundary condition, as well as neoclassical transport is implemented and verified. We show that W sputtering, transport and radiation need to be coupled with MHD for accurate simulation.\u3c/p\u3