109 research outputs found
Dynamics of resistive double tearing modes with broad linear spectra
The nonlinear evolution of resistive double tearing modes (DTMs) with safety
factor values q=1 and q=3 is studied in a reduced cylindrical model of a
tokamak plasma. We focus on cases where the resonant surfaces are a small
distance apart. Recent numerical studies have shown that in such configurations
high-m modes are strongly unstable. In this paper, it is first demonstrated
that linear DTM theory predicts the dominance of high-m DTMs. A semi-empirical
formula for estimating the poloidal mode number of the fastest growing mode,
m_peak, is obtained from the existing linear theory. Second, using nonlinear
simulations, it is shown that the presence of fast growing high-m modes leads
to a rapid turbulent collapse in an annular region, whereby small magnetic
island structures form. Furthermore, consideration is given to the evolution of
low-m modes, in particular the global m=1 internal kink, which can undergo
nonlinear driving through coupling to fast growing linear high-m DTMs. Factors
influencing the details of the dynamics are discussed. These results may be
relevant for the understanding of the magnetohydrodynamic (MHD) activity near
the minimum of q and may thus be of interest to studies concerned with
stability and confinement in advanced tokamaks.Comment: 11 pages, 10 figure
Comparison between resistive and collisionless double tearing modes for nearby resonant surfaces
Multi-Phase Simulation of Fast Ion Profile Flattening due to Alfven Eigenmodes in a DIII-D Experiment
Comparison between resistive and collisionless double tearing modes for nearby resonant surfaces
The linear instability and nonlinear dynamics of collisional (resistive) and
collisionless (due to electron inertia) double tearing modes (DTMs) are
compared with the use of a reduced cylindrical model of a tokamak plasma. We
focus on cases where two q = 2 resonant surfaces are located a small distance
apart. It is found that regardless of the magnetic reconnection mechanism,
resistivity or electron inertia, the fastest growing linear eigenmodes may have
high poloidal mode numbers m ~ 10. The spectrum of unstable modes tends to be
broader in the collisionless case. In the nonlinear regime, it is shown that in
both cases fast growing high-m DTMs lead to an annular collapse involving small
magnetic island structures. In addition, collisionless DTMs exhibit multiple
reconnection cycles due to reversibility of collisionless reconnection and
strong ExB flows. Collisionless reconnection leads to a saturated stable state,
while in the collisional case resistive decay keeps the system weakly dynamic
by driving it back towards the unstable equilibrium maintained by a source
term.Comment: 15 pages, 9 figure
Plasma physics and control studies planned in JT-60SA for ITER and DEMO operations and risk mitigation
| openaire: EC/H2020/633053/EU//EUROfusionA large superconducting machine, JT-60SA has been constructed to provide major contributions to the ITER program and DEMO design. For the success of the ITER project and fusion reactor, understanding and development of plasma controllability in ITER and DEMO relevant higher beta regimes are essential. JT-60SA has focused the program on the plasma controllability for scenario development and risk mitigation in ITER as well as on investigating DEMO relevant regimes. This paper summarizes the high research priorities and strategy for the JT-60SA project. Recent works on simulation studies to prepare the plasma physics and control experiments are presented, such as plasma breakdown and equilibrium controls, hybrid and steady-state scenario development, and risk mitigation techniques. Contributions of JT-60SA to ITER and DEMO have been clarified through those studies.Peer reviewe
Shattered pellet injection experiments at JET in support of the ITER disruption mitigation system design
A series of experiments have been executed at JET to assess the efficacy of the newly installed shattered pellet injection (SPI) system in mitigating the effects of disruptions. Issues, important for the ITER disruption mitigation system, such as thermal load mitigation, avoidance of runaway electron (RE) formation, radiation asymmetries during thermal quench mitigation, electromagnetic load control and RE energy dissipation have been addressed over a large parameter range. The efficiency of the mitigation has been examined for the various SPI injection strategies. The paper summarises the results from these JET SPI experiments and discusses their implications for the ITER disruption mitigation scheme
Spectroscopic camera analysis of the roles of molecularly assisted reaction chains during detachment in JET L-mode plasmas
The roles of the molecularly assisted ionization (MAI), recombination (MAR) and dissociation (MAD) reaction chains with respect to the purely atomic ionization and recombination processes were studied experimentally during detachment in low-confinement mode (L-mode) plasmas in JET with the help of experimentally inferred divertor plasma and neutral conditions, extracted previously from filtered camera observations of deuterium Balmer emission, and the reaction coefficients provided by the ADAS, AMJUEL and H2VIBR atomic and molecular databases. The direct contribution of MAI and MAR in the outer divertor particle balance was found to be inferior to the electron-atom ionization (EAI) and electron-ion recombination (EIR). Near the outer strike point, a strong atom source due to the D+2-driven MAD was, however, observed to correlate with the onset of detachment at outer strike point temperatures of Te,osp = 0.9-2.0 eV via increased plasma-neutral interactions before the increasing dominance of EIR at Te,osp < 0.9 eV, followed by increasing degree of detachment. The analysis was supported by predictions from EDGE2D-EIRENE simulations which were in qualitative agreement with the experimental observations
New H-mode regimes with small ELMs and high thermal confinement in the Joint European Torus
New H-mode regimes with high confinement, low core impurity accumulation, and small edge-localized mode perturbations have been obtained in magnetically confined plasmas at the Joint European Torus tokamak. Such regimes are achieved by means of optimized particle fueling conditions at high input power, current, and magnetic field, which lead to a self-organized state with a strong increase in rotation and ion temperature and a decrease in the edge density. An interplay between core and edge plasma regions leads to reduced turbulence levels and outward impurity convection. These results pave the way to an attractive alternative to the standard plasmas considered for fusion energy generation in a tokamak with a metallic wall environment such as the ones expected in ITER.& nbsp;Published under an exclusive license by AIP Publishing
The role of ETG modes in JET-ILW pedestals with varying levels of power and fuelling
We present the results of GENE gyrokinetic calculations based on a series of JET-ITER-like-wall (ILW) type I ELMy H-mode discharges operating with similar experimental inputs but at different levels of power and gas fuelling. We show that turbulence due to electron-temperature-gradient (ETGs) modes produces a significant amount of heat flux in four JET-ILW discharges, and, when combined with neoclassical simulations, is able to reproduce the experimental heat flux for the two low gas pulses. The simulations plausibly reproduce the high-gas heat fluxes as well, although power balance analysis is complicated by short ELM cycles. By independently varying the normalised temperature gradients (omega(T)(e)) and normalised density gradients (omega(ne )) around their experimental values, we demonstrate that it is the ratio of these two quantities eta(e) = omega(Te)/omega(ne) that determines the location of the peak in the ETG growth rate and heat flux spectra. The heat flux increases rapidly as eta(e) increases above the experimental point, suggesting that ETGs limit the temperature gradient in these pulses. When quantities are normalised using the minor radius, only increases in omega(Te) produce appreciable increases in the ETG growth rates, as well as the largest increases in turbulent heat flux which follow scalings similar to that of critical balance theory. However, when the heat flux is normalised to the electron gyro-Bohm heat flux using the temperature gradient scale length L-Te, it follows a linear trend in correspondence with previous work by different authors
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