Abstract: A simple Semi‐Analytical method used to fit the experimentally recorded current of the closed switch discharge circuit into the free running under damped LC oscillator model and the arc plasma conductivity, electron density of the plasma and efficiency of the spark gap switch energy transfer have been driven from the model by some simple calculations. The charging voltage, switch pressure and the spark gap has been changed between 6 to 15 kV, 1 to 1.5 bar and 1.4 or 2.8 mm, respectively. The obtained values for plasma conductivity and electron density of plasma are (5‐25)(Ω mm)–1, (0.5–3.5)·1024m–3, respectively. The efficiency of the switch is plotted Vs. Pd and E/P which in both cases the peak value has been about 80 present.

Abstract: A new ignition method of the spark gap based on plasma ejection is proposed in this paper, as the conventional trigatron spark gap performs poorly under the low working coefficient (the ratio of the charging voltage to the self‐breakdown voltage) in air. The plasma is generated by the capillary discharge, which has high pressure, high temperature and high velocity. The capillary discharge device is placed inside the low voltage electrode. As long as the triggering signal is sent to the device, a column of the plasma flow is ejected in axial direction and develops rapidly towards the high voltage electrode. Subsequently, the gap is broken down and a high resistive channel is formed, where the thermal ionization takes place and the arc across the whole gap is generated and develops into a well conductive channel. The process of the thermal ionization of the high resistive channel varies with the change of the spark gap distance. The breakdown delay and the delay jitter of the spark gap increase with the spark gap distance, as both parameters are mainly determined by the developing process of the plasma ejection. The characteristics of the plasma flow determine the possibility of the breakdown of the spark gap under the low working coefficient. The ignition method based on capillary plasma ejection has been proved by the preliminary experiments, which indicate that under the gap length of 8 cm and the working coefficient of less than 3%, the effective ignition is still achievable.

Abstract: Influence of the shear‐layer width on the nonlinear oscillations, spatiotemporal plot of the instantaneous convective flux Qconv with the shear layer‐width dE = 0.02. For details see paper of M. Leconte et al.

Abstract: Implementation of self‐consistent model of plasma recombination into the BIT1 PIC code and the simulation of detached SOL plasma are described. Our simulations indicate that in a strongly recombining plasma edge the sheath properties do not change qualitatively. The most affected parameter is the sheath heat transmission coefficient, which can increase by order of magnitude.

Abstract: Two fluid neutral models are compared with a finite volume solution of the kinetic neutral equation for a simplified 1D detached case. The neutral equations are coupled to the plasma equations. The first fluid neutral model assumes that neutral and ion temperature are equal while the second adds a separate neutral energy equation. It is illustrated that both the AMJUEL‐HYDHEL databases and TRIM code can be incorporated. The first model is already accurate giving a maximum relative error of 10% for the target fluxes. Adding an energy equation further decreases this maximum relative error to 5%.

Abstract: Five ELMy H‐mode Ne seeded JET pulses have been simulated with the self‐consistent core‐SOL model COREDIV. In this five pulse series only the Ne seeding rate was changed shot by shot, allowing a thorough study of the effect of Ne seeding on the total radiated power and of its distribution between core and SOL tobe made. The increase in the simulations of the Ne seeding rate level above that achieved in experiments shows saturation of the total radiated power at a relatively low radiated‐heating power ratio (frad = 0.60) and a further increase of the ratio of SOL to core radiation, in agreement with the reduction of W release at high Ne seeding level. In spite of the uncertainties caused by the simplified SOL model of COREDIV (neutral model, absence of ELMs and slab model for the SOL), the increase of the perpendicular transport in the SOL with increasing Ne seeding rate, which allows to reproduce numerically the experimental distribution core‐SOL of the radiated power, appears to be of general applicability.

Abstract: The 3D ERO code, which simulates plasma–wall interaction and impurity transport in magnetically confined fusion‐relevant devices is described. As application, prompt deposition of eroded tungsten has been simulated at surfaces with shallow magnetic field of 3 T. Dedicated PIC simulations have been performed to calculate the characteristics of the sheath in front of plasma–exposed surfaces to use as input for these ERO simulations. Prompt deposition of tungsten reaches 100% at the highest electron temperature and density. In comparison to more simplified assumptions for the sheath the amount of prompt deposition is in general smaller if the PIC–calculated sheath is used. Due to friction with the background plasma the impact energy of deposited tungsten can be significantly larger than the energy gained in the sheath potential.

Abstract: Kinetic Monte Carlo simulations of coupled atom‐radiation transport in optically thick divertor plasmas can be computationally very demanding, in particular in ITER relevant conditions or even larger devices, e.g. for power plant divertor studies. At high (∼ 1015 cm–3) atomic densities, it can be shown that sufficiently large divertors behave in certain areas like a black body near the first resonance line of hydrogen (Lyman α). This suggests that, at least in part, the use of continuum model (radiation hydrodynamics) can be sufficiently accurate, while being less time consuming. In this work, we report on the development of a hybrid model devoted to switch automatically between a kinetic and a continuum description according to the plasma conditions. Calculations of the photo‐excitation rate in a homogeneous slab are performed as an illustration. The outlined hybrid concept might be also applicable to neutral atom transport, due to mathematical analogy of transport equations for neutrals and radiation.

Abstract: Divertor targets made out of a capillary porous system (CPS) filled with liquid lithium, have been proposed as an alternative to standard, solid state plates. In the current work we simulate the DEMO edge plasma in either a standard single‐null or snowflake divertor configuration. Our tool is the 2D code TECXY.
Lithium ablated from the target plate surface and released into the plasma is shown here to partially screen the incoming heat flux. Lithium's moderate SOL radiation levels suggest additional seeding to be beneficial. Very high heat fluxes to the divertor need to be avoided, as intensive lithium evaporation might unacceptably pollute the plasma.

Abstract: Present computational techniques for coupled finite‐volume/Monte‐Carlo codes for plasma edge modeling under ITER or DEMO conditions face serious challenges with respect to computational time and accuracy. In this paper, scaling laws for different error contributions are assessed and practical procedures for error estimation are proposed. First results on a 1D and a 2D test case are discussed.

Abstract: Power exhaust is one of the major challenges that future devices such as ITER and DEMO will face. Because of the lack of identified scaling parameters, predictions for divertor plasma conditions in these devices have to rely on detailed modelling. Most plasma edge simulations carried out so far rely on transport codes, which most of the times consist of a fluid code for the plasma coupled to a kinetic Monte Carlo (MC) code for neutral particles. One of the main difficulties in interpreting code results is the statistical noise from the MC procedure, which makes it difficult to define a convergence criterion for the simulations. In this work, we elaborate on similarities between noisy transport code simulations and turbulence simulations, and argue that the time averaged solution is a well defined stationary solution for the system. We illustrate these ideas with a simple slab test case with fluid neutrals, to which we add synthetic noise. In this case, the effects of noise are found to be significant only at high noise levels and for large enough correlations times.

Abstract: In this paper numerical simulations with the self‐consistent COREDIV code of the planned JET DT experiments have been performed. First, record shot from the 1997 experiments was simulated and good agreement with experimental data has been found. Direct extrapolation of the carbon wall results to the new ILW configuration (discharge parameters as for the shot #42746) shows very good core plasma performance with even higher fusion power but with too large power to the divertor. However, with the neon seeding the heat load and plate temperatures can be efficiently reduced keeping good the plasma performance. Investigations have been done also for the planned DT operation scenario based on a conventional ELMy H‐mode at high plasma current and magnetic field. Simulations for the reference ELMy H‐mode shot #87412 show good agreement with the experimental data but the direct extrapolation of the DD results to deuterium‐tritium operation shows relatively poor performance in terms of the achieved fusion power. The situation improves, if the highest heating power is assumed (41 MW) and fusion powers in the excess of 12 MW can be achieved. All the high performance shots require the heat load control by neon seeding which shows rather beneficial effect on the plasma performance allowing for relatively wide operational window in terms of the amount of the allowed neon influx.

Abstract: The self‐consistent COREDIV code is used to simulate discharges in a tokamak plasma, especially the influence of impurities during nitrogen and argon seeding on the key plasma parameters. The calculations are performed with and without taking into account the W prompt redeposition in the divertor area and are compared to the experimental results acquired on ASDEX Upgrade tokamak (shots #29254 and #29257).
For both impurities the modeling shows a better agreement with the experiment in the case without prompt redeposition. It is attributed to higher average tungsten concentration, which on the other hand seriously exceeds the experimental value. By turning the prompt redeposition process on, the W concentration is lowered, what, in turn, results in underestimation of the radiative power losses. By analyzing the influence of the transport coefficients on the radiative power loss and average W concentration it is concluded that the way to compromise the opposing tendencies is to include the edge‐localized mode flushing mechanism into the code, which dominates the experimental particle and energy balance. Also performing the calculations with both anomalous and neoclassical diffusion transport mechanisms included is suggested.

Abstract: There is experimental evidence that the pedestal dynamics in type‐I ELMy H‐mode discharges is significantly affected by a change in the recycling conditions at the tungsten plasma‐facing components (W‐PFCs) after an ELM event. The integrated code JINTRAC has been employed to assess the impact of recycling conditions during type‐I ELMs in JET ITER‐like wall H‐mode discharges. By employing a heuristic approach, a model to mimic the physical processes leading to formation and release (i.e. outgassing) of finite near‐surface fuel reservoirs in W‐PFCs has been implemented into the EDGE2D‐EIRENE plasma‐wall interaction code being part of JINTRAC. As main result it is shown, that a delay in the density pedestal build‐up after an ELM event can be provoked by reduced recycling induced by depleted W‐PFC particle near‐surface reservoirs. However the pedestal temperature evolution is barely affected by the change in recycling parameters suggesting that the presented model is incomplete.

Abstract: In this paper we present the comparison of simulations with the numerical codes COREDIV and SOLPS5.0 for JET L‐mode discharges with ITER like wall (ILW). The simulations have been performed for L‐mode shots with and without nitrogen seeding (#82291 ‐ 9) which are characterised by relatively low auxiliary heating power (PNBI = 1.1 MW) and low electron density (ne = 2.35 × 1019 m–3). Comparisons are made to the experimental measurements (e.g. radiation levels, plasma profiles) and the differences between the results from the two codes (e.g. temperature and density profiles at the outer divertor plate) are shown and discussed.