A nonlinear verification benchmark is reported between the three-dimensional magneto-hydrodynamic (3D MHD) codes SPECYL [Cappello and Biskamp, Nucl. Fusion 36, 571 (1996)] and PIXIE3D [Chacón, Phys. Plasmas, 15, 056103 (2008)]. This work substantially extends a former successful verification study between the same two codes [Bonfiglio et al., Phys. Plasmas, 17, 082501 (2010)] and focuses on the verification of thin-shell resistive-wall boundary conditions, recently implemented in both codes. Such boundary conditions feature a thin resistive shell in contact with the plasma and an ideal wall placed at a finite distance, separated from the resistive shell by a vacuum region, along with a 3D boundary flow consistent with Ohm's law. This setup allows the study of MHD modes that are influenced by the plasma magnetic boundary, such as external kink modes. The linear growth and nonlinear saturation of external kink modes are studied in both the tokamak and reversed-field pinch magnetic configurations, demonstrating excellent agreement between the two codes. For the tokamak, we present a comparison with analytical linear stability results for the external kink mode, demonstrating remarkable agreement between numerical and analytical growth rates.

Modeling of molecular kinetics in plasmas.

Several studies pointed out the joint role of resistivity g and viscosity v in determining the dynamics and the emergence of helical regimes of reversed-field pinch (RFP) plasmas. In this framework, the self-consistent time evolution of the g and v coefficients still lacks of a fully satisfying modeling, being constrained by many approximations. In this work, the hypothesis of a flat viscosity profile is relaxed: A viscosity profile inspired by the Braginskii perpendicular viscosity is implemented in the code. This choice is motivated by the fact that the magnetohydrodynamics field instabilities relevant for the RFP configuration dynamics (resistive-kink/tearing modes) are active in the direction perpendicular to the magnetic field. Such a non-monotonous profile causes a localized damping of plasma flow in the regions, where the viscosity is stronger, close to the plasma edge. This results in the reduction of the flow shear, in turn allowing the enhancement of edge magnetic field modes amplitude. The impact on the magnetic topology and on connection length to the wall is also analyzed.

This paper concerns the kinematic viscosity in reversed-field pinch fusion plasmas, including both the study of numerical magneto-hydrodynamics (MHD) simulations and the analysis of RFX-mod experimental data. In the first part, we study the role of non-uniform time-constant radial viscosity profiles in 3D non-linear visco-resistive MHD simulations. The new profiles induce a moderate damp (for the velocity field) and a correspondent enhancement (for the magnetic field) of the spectral components resonating in the regions where the viscosity is higher. In the second part, we evaluate the kinematic viscosity coefficient on a wide database of RFX-mod shots according to the transport theories of Braginskii (considering parallel, perpendicular and gyro viscosity coefficients), considering the action on viscosity of ITG modes (ion temperature gradient) and according to the transport theory of Finn. We then exploit the comparison with the visco-resistive MHD simulations (where the visco-resistive dissipation rules the MHD activity) to show that the classical Braginskii perpendicular viscosity produces the best agreement between simulations and data, followed by the Braginskii gyro-viscosity.

Present contribution aims at comparing the different kinds of disruptions that occurred in the last JET with ITER-like wall (ILW) campaigns with Tritium and Deuterium-Tritium fuels. Last campaigns performed in JET-ILW with a D fuel showed that the majority (around 80%) of disruptions follow two main paths [1]. The first path (temperature hollowing, TH) is strictly related with the influx of high Z impurities, which can accumulate in the plasma core increasing the radiative losses and deteriorating the electron temperature (Te) profile [2]. The second path (edge cooling, EC) is instead related to the erosion of the edge Te profile; the contraction of Te profiles looks similar to that of a "density limit" disruption [3]. Both paths are found [4] to modify differently the current density profile but always in a way to destabilize a 2/1 mode, which locks before disruption. In T and in DT campaigns, around the 90% of disruptions can be explained by the occurrence of TH or EC. Furthermore, it is found that in DT the two main scenarios developed at JET [5] are characterized by disruptions following mainly one out of the two paths. In the DT experiments performed in baseline scenario (?N~1.8, q95~3), 12 disruptions out of 13 follow an EC; while in the DT hybrid scenario (?N~2-3, q95~4) 13 disruptions out of 15 occur after a TH in the ramp-down phase. The two different kinds of disruptions will be compared for the two high performance scenarios and for the different isotopes contents. The comparison will be performed considering the conditions at onset of the 2/1 modes before the disruption, in terms of the power balance and of the density levels.

Alpha particles with energies on the order of megaelectronvolts will be the main source of plasma heating in future magnetic confinement fusion reactors. Instead of heating fuel ions, most of the energy of alpha particles is transferred to electrons in the plasma. Furthermore, alpha particles can also excite Alfvénic instabilities, which were previously considered to be detrimental to the performance of the fusion device. Here we report improved thermal ion confinement in the presence of megaelectronvolts ions and strong fast ion-driven Alfvénic instabilities in recent experiments on the Joint European Torus. Detailed transport analysis of these experiments reveals turbulence suppression through a complex multi-scale mechanism that generates large-scale zonal flows. This holds promise for more economical operation of fusion reactors with dominant alpha particle heating and ultimately cheaper fusion electricity.

At present, the only method for assessing the fusion power throughput of a reactor relies on the absolute measurement of 14 MeV neutrons produced in the D-T nuclear reaction. [1] For ITER and DEMO, however, at least another independent measurement of the fusion power is required. The 5Henucleus produced in the D-T fusion reaction has two de-excitation channels. The most likely is its disintegration in a particle and a neutron, D+T->5He->?+n, by means of the nuclear force. There is however also an electromagnetic channel, with a branching ratio ~10-5, which leads to the emission of a 17 MeV gamma-ray, i.e. D+T->5He*-> 5He+?. [2] The detection of this gamma-ray emission could serve as an independent method to determine the fusion power. In order to enable 17 MeV gamma-ray measurements, there is need for a detector with some coarse energy discrimination and, most importantly, capable to work in a neutron rich environment. Conventional inorganic scintillators, such as LaBr3(Ce), have comparable efficiencies to neutrons and gamma rays and they cannot be used for 17 MeV gamma-ray measurements without significant neutron shielding. In order to overcome this limitation, we here propose the conceptual design of a gamma ray counter with a variable energy threshold based on the Cherenkov effect and designed to operate in intense neutron fields. The detector geometry has been optimized using Geant4 so to achieve a gamma-ray to neutron efficiency ratio better than 105. The design is based on a gas Cherenkov detector and uses a CsI coated scintillating GEM (Gas Electron Multiplier) as photon pre-amplifier, together with a wavelength shifter to minimize the sensitivity to neutrons. Photons produced in the GEM are collected by an optical window and a bundle of optical fibers, which guides them towards an array of silicon photomultipliers (SiPMs) located further away from the plasma, in a region at low nuclear radiation.

In the RFX-mod2 experiment presently under construction, devoted to the study of magnetic confinement of fusion relevant plasmas, significant electric fields, in the kV/mm range, are expected to form in between in-vessel conductive plasma facing components during transient plasma current phases (start-up and termination). While such electric fields are of no concern for components in vacuum, the presence of a scrape-off plasma at the edge (electron density ne 1016 ÷1018 m-3, electron temperature Te of few eV) can create the conditions for potentially dangerous arc formation. For this reason part of the plasma facing components (in particular the graphite 'first wall' tiles covering the copper 'stabilizing shell' placed within the plasma chamber) require a proper conditioning technique capable of maintaining the insulation between conductive components even in presence of the scrape-off plasma. An experimental apparatus has been developed in order to test the conditions for the arc formation and prevention between two electrodes immersed in a plasma generated by a hot emitting filament. The results of an extensive experimental campaign will be presented, aimed at demonstrating the possibility of gaining a sufficient electrical conditioning by applying the standard conditioning technique usually employed for higher voltage ranges. It consists of a sequence of high voltage pulses applied to the pair of electrodes with current limitation, in the presence of a background cold plasma with low ionization degree. The experimental procedure is such that the voltage of the pulses is slightly increased when arcing ceases, until the final desired voltage level is achieved (2.5 kV). Different electrode materials have been tested in a variety of plasma conditions in terms of electron density and working gas pressure.

A series of techniques are presented that have been developed to optimize the output magnetic field of the feedback control system on the RFX-mod reversed field pinch device. With the aim of minimizing the harmonic distortion and correcting localized error fields, these methods should be lightweight for real-time application and effective in improving the performance of a system that is routinely used for active control of magneto- hydro-dynamic plasma instabilities. The implementation of sim- ple, linear algebra based, real-time optimization methods will be described along with proof of the sought beneficial effects. Focus of the work is set on a spurious harmonics reduction technique based on the decoupling of sensors and actuators, a description of its derivation will be given together with the implementation in the control loop. A similar procedure for the compensation of faulted actuators will also be mentioned.

RFX - mod is a medium size (R=2m, a=0.459m) toroidal device dedicated to studying the magnetic confinement of fusion relevant plasmas. It is equipped with an advanced feedback system for the control of magneto - hydro - dynamic instabilities: 192 active saddle coils and over 600 magnetic sensors. A set of simple optimization techniques will be shown, allowing to assess the system external action on a given plasma and improve its effectiveness. Real time applicability is one of the main requirements.

With this internal note we want to make available to the RFX colleagues the study performed for the invited talk "Effect of helical magnetic perturbations on the 3D MHD self-organization of fusion plasmas" given by D. Bonfiglio at the 7th IAEA Technical Meeting on "Theory of Plasmas Instabilities" held in Frascati, Italy on 4-6 March 2015. The proceeding papers of invited and contributed talks, including a version of this internal note, will be published on the conference website http://www.iaeatm-inst2015.enea.it/

With the increasing impact of scientific discovery via advanced computation, there is presently a strong emphasis on ensuring the mathematical correctness of computational simulation tools. Such endeavor, termed verification, is now at the center of most serious code development efforts. In this study, we address a cross-benchmark nonlinear verification study between two three-dimensional magnetohydrodynamics (3D MHD) codes for fluid modeling of fusion plasmas, SPECYL [S. Cappello and D. Biskamp, Nucl. Fusion 36, 571 (1996)] and PIXIE3D [L. Chacon, Phys. Plasmas 15, 056103 (2008)], in their common limit of application: the simple viscoresistive cylindrical approximation. SPECYL is a serial code in cylindrical geometry that features a spectral formulation in space and a semi-implicit temporal advance, and has been used extensively to date for reversed-field pinch studies. PIXIE3D is a massively parallel code in arbitrary curvilinear geometry that features a conservative, solenoidal finite-volume discretization in space, and a fully implicit temporal advance. The present study is, in our view, a first mandatory step in assessing the potential of any numerical 3D MHD code for fluid modeling of fusion plasmas. Excellent agreement is demonstrated over a wide range of parameters for several fusion-relevant cases in both two-and three-dimensional geometries.