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Garching bei München, Germany

Li E.,CAS Hefei Institutes of Physical Science | Igochine V.,MPI fur plasma physik | Dumbrajs O.,Institute of Solid State Physics | Xu L.,CAS Hefei Institutes of Physical Science | And 3 more authors.
Plasma Physics and Controlled Fusion | Year: 2014

Evolution of the safety factor (q) profile during L-H transitions in the Experimental Advanced Superconducting Tokamak (EAST) was accompanied by strong core crashes prior to regular sawtooth behavior. These crashes appeared in the absence of q = 1 (q is the safety factor) rational surface inside the plasma. Analysis indicates that the m/n = 2/1 tearing mode is destabilized and phase-locked with the m/n = 1/1 non-resonant kink mode (the q = 1 rational surface is absent) due to the self-consistent evolution of plasma profiles as the L-H transition occurs (m and n are the poloidal and toroidal mode numbers, respectively). The growing m/n = 1/1 mode destabilizes the m/n = 2/2 kink mode which eventually triggers the strong crash due to an anomalous heat conductivity, as predicted by the transport model of stochastic magnetic fields using experimental parameters. It is also shown that the magnetic topology changes with the amplitude of m/n = 2/2 mode and the value of center safety factor in a reasonable range. © 2014 IOP Publishing Ltd. Source

Geraud A.,French Atomic Energy Commission | Lennholm M.,Culham Center for Fusion Energy | Alarcon T.,French Atomic Energy Commission | Bennett P.,Culham Center for Fusion Energy | And 6 more authors.
Fusion Engineering and Design | Year: 2013

A new high frequency pellet injector, part of the JET programme in support of ITER, has been installed on JET at the end of 2007. Its main objective is the mitigation of the Edge Localized Modes (ELMs), responsible for unacceptable thermal loads on the wall when their amplitude is too high. The injector was also required to have the capability to inject pellets for plasma fuelling. To reach this double goal, the injector has to be capable to produce and accelerate either small pellets to trigger ELMs (pace making), allowing to control their frequency and thus their amplitude, or large pellets to fuel the plasma. Operational since the beginning of the 2009 JET experimental campaign, the injector, based on the screw extruder technology, suffered from a general degradation of its performance linked to extrusion instability. After modifications of the nozzle assembly, re-commissioning on plasma has been undertaken during the first half of 2012 and successful pellet ELM pacing was achieved, rising the intrinsic ELM frequency up to 4.5 times. © 2013 EURATOM. Source

Lang P.T.,MPI fur plasma physik | Meyer H.,Culham Center for Fusion Energy | Birkenmeier G.,MPI fur plasma physik | Burckhart A.,MPI fur plasma physik | And 14 more authors.
Plasma Physics and Controlled Fusion | Year: 2015

In ITER, pellets are used for ELM pacing and fueling. More importantly, ELM control and in particular control of the first ELM needs to be demonstrated in the non-nuclear phase of ITER during operation in H or He. Whilst D pellets have been established as an ELM control technique in the stationary phase with D target plasmas in devices with C as plasma-facing component, the behavior of other isotopes in non-stationary phases are not so well known. Here, we report on new pellet triggering experiments in ASDEX Upgrade and JET that mimic specific ITER operating scenarios. Both machines are equipped with an all-metal wall; recent investigations have shown that pellet triggering and pacing become more intricate when an all-metal wall surface is employed. In both machines, ELM triggering has been shown to occur after injection of D pellets into D plasmas during extended ELM-free phases, often following the L → H transition. In both devices the pellets are found to induce ELMs under conditions far from the stability boundary for type-I ELMs. Near the L → H transition, induced ELMs in some cases are more likely to have type-III rather than type-I characteristics. Furthermore, in ASDEX Upgrade this study was conducted during L → H transitions in the current ramp-up phase as envisaged for ITER. In addition, the pellet's ELM trigger potential has been proven in ASDEX Upgrade with a correct isotopic compilation for the non-nuclear phase in ITER, viz. H pellets into either He or H plasmas. Results from this study are encouraging since they have demonstrated the pellets' potential to provoke ELMs even under conditions that are quite far from the stability boundaries attributed to the occurrence of spontaneous ELMs. However, with the recent change from carbon to an all-metal plasma-facing component, examples have been found in both machines where pellets failed to establish ELM control under conditions where this would be expected and needed. Consequently, a major task of future investigations in this field will be to shed more light on the underlying physics of the pellet ELM triggering process to allow sound predictions for ITER. © 2015 IOP Publishing Ltd. Source

Urso L.,MPI fur plasma physik | Fischer R.,MPI fur plasma physik | Isayama A.,Japan Atomic Energy Agency
Plasma Physics and Controlled Fusion | Year: 2010

The determination of the free parameters present in the modified Rutherford equation (MRE), which is routinely used for studying the physics of neoclassical tearing mode (NTM) stabilization, is addressed by making use of the Bayesian probability theory. The evaluation of the free coefficients is particularly sensitive to the assumptions used in the modelled equation, to the correlation of various physical parameters and to the uncertainties of the experimental measurements. A probabilistic method was applied for the consistent evaluation of the coefficients and their uncertainties using a large database of discharges and by considering the correlations and the uncertainties of the multiple physical quantities present in the MRE. The estimated values and uncertainties of the coefficients are related to the precise determination of the minimum amount of electron cyclotron current drive power necessary to stabilize NTMs in the International Thermonuclear Experimental Reactor. © 2010 IOP Publishing Ltd. Source

Li E.,CAS Hefei Institutes of Physical Science | Igochine V.,MPI fur plasma physik | Xu L.,CAS Hefei Institutes of Physical Science | Shi T.,CAS Hefei Institutes of Physical Science | And 12 more authors.
Plasma Physics and Controlled Fusion | Year: 2016

In the 2014 year's campaign of experimental advanced superconducting tokamak (EAST), a series of Magnetohydrodynamics (MHD) instabilities were observed as the launching of Neutral Beam Injection (NBI), the most interesting one of which is the neoclassical tearing mode (NTM). Evidence clearly shows that a kink mode present after a strong sawtooth-like (ST-like) crash leaves a perturbation near the location of the magnetic island, providing the initial seed. The interaction of energetic ions makes the magnetic island oscillate both in island width and in rotation frequency. Analysis indicates that the bulk plasma still dominates the dynamics of NTM, and the orbit excursion of energetic ions induces a polarization current and modifies the width and rotation frequency of the neoclassical magnetic island. © 2016 IOP Publishing Ltd. Source

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