Abstract:High confinement and stability are obtained simultaneously in stationary conditions in improved H-mode discharges at ASDEX Upgrade. The improved H-mode discharges are typically composed of two different phases: ‘lower heating phase’, where H98(y, 2) is similar to standard H-modes (H98(y, 2) ∼ 1), and ‘fully developed improved H-mode phase’, where H98(y, 2) is higher than standard H-modes (H98(y, 2) up to 1.4). In this paper, the confinement physics is studied by comparing these two different phases using ASTRA… Show more
“…There are several uncertainties in the interpretive analysis such as assumptions of the effective ion charge, , neoclassical transport models, and fitting of kinetic profiles. We scanned the effective charge along with different radial profile shapes 21 , tried other neoclassical models 22 , 23 and modified the kinetic profile fittings in several different ways, but the discrepancy has persisted nevertheless. Fig.…”
Section: Resultsmentioning
confidence: 99%
“…During the main numerical simulations, the effective charge is assumed with the Carbon impurity only. Additional simulations to verify the effect of were also done with different radial profile shapes 21 within .…”
A tokamak, a torus-shaped nuclear fusion device, needs an electric current in the plasma to produce magnetic field in the poloidal direction for confining fusion plasmas. Plasma current is conventionally generated by electromagnetic induction. However, for a steady-state fusion reactor, minimizing the inductive current is essential to extend the tokamak operating duration. Several non-inductive current drive schemes have been developed for steady-state operations such as radio-frequency waves and neutral beams. However, commercial reactors require minimal use of these external sources to maximize the fusion gain, Q, the ratio of the fusion power to the external power. Apart from these external current drives, a self-generated current, so-called bootstrap current, was predicted theoretically and demonstrated experimentally. Here, we reveal another self-generated current that can exist in a tokamak and this has not yet been discussed by present theories. We report conclusive experimental evidence of this self-generated current observed in the KSTAR tokamak.
“…There are several uncertainties in the interpretive analysis such as assumptions of the effective ion charge, , neoclassical transport models, and fitting of kinetic profiles. We scanned the effective charge along with different radial profile shapes 21 , tried other neoclassical models 22 , 23 and modified the kinetic profile fittings in several different ways, but the discrepancy has persisted nevertheless. Fig.…”
Section: Resultsmentioning
confidence: 99%
“…During the main numerical simulations, the effective charge is assumed with the Carbon impurity only. Additional simulations to verify the effect of were also done with different radial profile shapes 21 within .…”
A tokamak, a torus-shaped nuclear fusion device, needs an electric current in the plasma to produce magnetic field in the poloidal direction for confining fusion plasmas. Plasma current is conventionally generated by electromagnetic induction. However, for a steady-state fusion reactor, minimizing the inductive current is essential to extend the tokamak operating duration. Several non-inductive current drive schemes have been developed for steady-state operations such as radio-frequency waves and neutral beams. However, commercial reactors require minimal use of these external sources to maximize the fusion gain, Q, the ratio of the fusion power to the external power. Apart from these external current drives, a self-generated current, so-called bootstrap current, was predicted theoretically and demonstrated experimentally. Here, we reveal another self-generated current that can exist in a tokamak and this has not yet been discussed by present theories. We report conclusive experimental evidence of this self-generated current observed in the KSTAR tokamak.
“…As input for the simulations, profiles of the ion and the electron temperature, the electron density, and the toroidal rotation are taken from experiments. In case of ASDEX Upgrade, the toroidal rotation is estimated from the other pulse 17870 [37] and the poloidal rotation is assumed to be neoclassical. In case of TCV, both the toroidal and the poloidal rotation can be negligible compared with ASDEX Upgrade because they are Ohmic discharges.…”
Section: Benchmark and Applications Of The Ntm Modelingmentioning
An integrated numerical system is established to model time-dependent behavior of the neoclassical tearing mode (NTM) in a tokamak which solves the modified Rutherford equation (MRE) by coupling with plasma transport, equilibrium, heating and current drive self-consistently. The MRE is formulated by considering the electron cyclotron heating (ECH) effect as well as the electron cyclotron current drive (ECCD) effect to stabilize the NTM which is well-suited for time-dependent simulations including a predictive purpose. The integrated numerical system is applied to experiments in which NTMs are stabilized by ECCD and by ECH at ASDEX Upgrade and TCV, respectively. It is found that the simulation results catch the main experimental trend.
“…Thus, the E×B shear (largely determined by toroidal rotation velocity) contributes significantly to the energy confinement improvement in DIII-D hybrid scenarios as has been shown via predictive modelling with the TGLF transport model [11]. However, the modelling of ASDEX Upgrade hybrid [7] and improved H-mode discharges [13] with the GLF23 and Weiland models shows a relatively weak stabilising effect of the E×B shear in these plasmas.…”
The E×B shear stabilisation of anomalous transport in JET hybrid discharges is studied via selfconsistent predictive modelling of electron and ion temperature, ion density and toroidal rotation velocity performed with the GLF23 model. The E×B shear stabilisation factor (parameter a E in the GLF23 model) is adjusted to predict accurately the four simulated quantities under different experimental conditions, and the uncertainty in a E determined by 15% deviation between simulated and measured quantities is estimated. A correlation of a E with toroidal rotation velocity and E×B shearing rate is found in the low density plasmas, suggesting that the turbulence quench rule may be more complicated than assumed in the GLF23 model with constant a E . For the selected discharges the best predictive accuracy is obtained by using weak/no E×B shear stabilisation (i.e. a E ≈ 0) at low toroidal angular frequency (W < 60krad/s), even in the scenarios with the current overshoot, and a E = 0.9 at high frequency (W > 100krad/s). Interestingly, a weak E×B shear stabilisation of anomalous transport is found in the medium density strongly rotating discharge. An importance of linear b e stabilisation in this discharge is estimated and compared to the low density discharge with equally high b e . The toroidal rotation velocity is well predicted here by assuming that the momentum diffusion coefficient is a fraction of thermal ion diffusivity. Taking into account the a E and Prandtl number with their uncertainties determined in the modelling of JET hybrid discharges, the performance of ITER hybrid scenario with optimised heat mix (33MW of NBI and 20MW of ECCD) is estimated showing the importance of toroidal rotation for achieving Q > 5.
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