RFX-mod Reversed Field Pinch Research Articles

Edge turbulence approaching the density limit in RFX-mod experiment

The boundary region of the RFX-mod reversed field pinch experiment is strongly influenced by the value of the electron density. With increasing density the magnetic topology in the plasma core and in the plasma edge changes, together with the statistical properties of the edge turbulent fluctuations. Using the density normalized to the Greenwald density (n/nG), two density regimes are identified: the low density regime at n/nG < 0.3–0.4, and the high density regime for n/nG > 0.4–0.5, without a clear threshold between them. At low density the plasma is dominated by the (m, n) = (1, −7) magnetic mode, which gives the same shape to the magnetic boundary; at higher density a strong (0, 1) mode rises. When increasing n/nG, edge fluctuations develop strong non-Gaussian tails, turbulent structures become larger and their radial velocity become measurable with the gas puff imaging diagnostic when n/nG > 0.3–0.4. This phenomenology is described in detail for the two density regimes of RFX-mod.

Upgrades of the RFX-mod reversed field pinch and expected scenario improvements

RFX-mod is a Reversed Field Pinch device that allowed performing experiments in regimes with a plasma current up to 2 MA, thanks to its MHD active control system. Experiments have shown that improved plasma performances are obtained when in the resonant part of the m = 1 spectrum one dominant tearing mode is much higher than the other secondary ones (quasi single helicity states). Tearing modes play a crucial role in determining energy and particle transport. Based on the present understanding of the interplay between passive conductive boundaries and tearing modes in an RFP, an upgrade of RFX-mod machine assembly has been designed, dubbed RFX-mod2, and it is now being implemented. The highly resistive Inconel vessel will be removed, graphite tiles will be attached to the copper stabilizing shell and the stainless steel support structure will be modified in order to be vacuum tight. In RFX-mod2, the shell-plasma proximity decreases from b/a = 1.11 to b/a = 1.04 and copper, instead of Inconel, will be the continuous conducting structure nearest to the plasma. MHD non-linear simulations show that secondary tearing modes amplitude and the edge bulging due to their phase locking will decrease; moreover the plasma current threshold for tearing modes wall locking will also significantly increase.

Electron temperature gradient driven instabilities in helical reversed-field pinch plasmas

We describe the occurrence of electron temperature gradient driven micro-instabilities in the helical states of RFX-mod reversed-field pinch plasmas. These plasmas are usually characterized by the presence of large, radially localized electron temperature gradients. Using realistic geometry and profiles, micro-tearing modes and electron temperature gradient modes turn out to regularly co-exist in the region of the temperature barrier. In the paper, we discuss the main features of such instabilities, especially focusing on the distinctive aspects of the helical geometry with respect to the toroidal symmetry.

A novel approach to studying transport in plasmas with magnetic islands

In magnetically confined experiments the presence of rational surfaces modifies the magnetic configuration with one or more islands embedded in the main plasma. The islands strongly modify particle and heat fluxes, hence their role must be included in the study of the transport properties. Two examples of this phenomenology are here presented: a cold pulse propagation in LHD with an RMP (1,1) island highlights a diffusivity reduction inside the island. A second study has been carried out on the RFX-mod reversed field pinch device where the transport on the ITB associated to a spontaneous m = 1 magnetic structure is discussed.To perform these studies a new approach in modeling transport in the presence of islands is presented: this new method, dubbed the multiple domains scheme (MDS), with some simplifying assumptions, is directly applicable to 1.5D transport codes. In the MDS different regions are identified inside the plasma all interfaced by the separatrix that acts as common boundary. In each domain a monotonic radial coordinate can be chosen and the metric elements can be computed accordingly. This scheme is independent of the equilibrium magnetic configuration hence it can be applied to either tokamaks, stellarators or reversed field pinches. The MDS has been implemented in a new numerical tool, dubbed the multiple axis solver.

Control System Optimization Techniques for Real-Time Applications in Fusion Plasmas: The RFX-mod Experience

A series of techniques is presented that has 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 magnetohydrodynamic plasma instabilities. The implementation of simple, linear algebra based, real-time optimization methods will be described along with proof of the beneficial effects sought. The focus of this paper 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.

Light impurity transport studies with solid pellet injections in the RFX-mod reversed-field pinch

The transport of injected impurities is investigated in the multiple helicity (MH) and improved confinement quasi single helicity (QSH) magnetic regimes of RFX-mod reversed-field pinch. Impurities are introduced through injections of room temperature pellets of carbon (in MH and QSH) and lithium (in MH). Simulation of experimental spectroscopic measurements, including line and soft x-rays emission, electron density and radiated power is carried out with a 1D collisional-radiative impurity transport code, allowing to determine the transport parameters: diffusion coefficient and convective velocity. The transport coefficients were found close to those deduced in previous experiments through the introduction of Ni and Ne, confirming the existence of an external velocity barrier also for light impurities.

Lithium wall conditioning by high frequency pellet injection in RFX-mod

In the RFX-mod reversed field pinch experiment, lithium wall conditioning has been tested with multiple scopes: to improve density control, to reduce impurities and to increase energy and particle confinement time. Large single lithium pellet injection, lithium capillary-pore system and lithium evaporation has been used for lithiumization. The last two methods, which presently provide the best results in tokamak devices, have limited applicability in the RFX-mod device due to the magnetic field characteristics and geometrical constraints. On the other side, the first mentioned technique did not allow injecting large amount of lithium. To improve the deposition, recently in RFX-mod small lithium multi-pellets injection has been tested. In this paper we compare lithium multi-pellets injection to the other techniques. Multi-pellets gave more uniform Li deposition than evaporator, but provided similar effects on plasma parameters, showing that further optimizations are required.

Parallel and perpendicular structure of the edge turbulence in a three-dimensional magnetic field

Edge turbulence and blobs are studied in the three-dimensional magnetic topology of the RFX-mod reversed field pinch. The edge of the RFX-mod shows a three-dimensional structure dominated by a helical equilibrium with (1, −7) symmetry, which gives the same space-time modulation to all of the kinetic properties. The interaction between the edge turbulence and this magnetic topology is studied. It is shown that the edge blobs are current-carrying filaments aligned with the magnetic field, and in the perpendicular plane each blob is a positive peak of electron density and a valley of temperature. The inner nature of these blobs is not affected by the presence of the O and X points of the (1, −7) island; however, the statistical properties are sensitive to them, pointing to the influence of the magnetic topology on the edge fluctuations.

Characterization of temperature profiles in the outer region of RFX-mod

The onset of helical states has been shown to strongly affect the transport properties and the plasma performance of both the core region (|R − R0|/a ≲ 0.5) and the plasma edge (|R − R0|/a ≳ 0.95) in the RFX-mod reversed field pinch experiment. Both regions have been widely investigated and a strong effort is still dedicated to deepening the comprehension of the phenomena affecting them. Conversely, the region bounded between the plasma core and the plasma edge (0.6 ≲ |R − R0|/a ≲ 0.95) has been poorly analyzed, despite the large plasma volume enclosed in this domain. Improvements in RFX-mod global plasma performance are thus closely linked to the identification of the physical mechanisms occurring in this region, referred to as the outer region. A specifically conceived method of analysis, which gathers together data from a Thomson scattering system and two edge temperature diagnostics, allows the study of the entire temperature profile of RFX-mod. A statistical analysis performed on a large discharge database shows that the temperature profile in the central region of the plasma column is mainly established by the plasma current and the degree of stochasticity of the magnetic field. On the other hand, the temperature profile in the outer plasma region is found to vary with electron density and plasma equilibrium.

Helical equilibrium reconstruction with V3FIT in the RFX-mod Reversed Field Pinch

Helical states are routinely found in all reversed field pinch experiments. The inherently three-dimensional feature of these states requires an adequate equilibrium reconstruction. In this paper we present the use of the V3FIT code for the RFX-mod experiment. V3FIT uses VMEC as an equilibrium solver and varies reconstruction parameters to minimize the difference between experimental and modelled signals. Both magnetic (local field, saddle coils and flux loops) and kinetic diagnostics (Thomson scattering, interferometric and soft x-ray emissivity data) are used in order to properly deal with the problem of equilibrium degeneracy. From this point of view a sensitivity study of external magnetic measurements on the internal topological structure is presented.As the work deals with fixed-boundary equilibria, a solution to the issue of vacuum fields computation is also presented.A comparison with previous procedures for obtaining both axisymmetric and helical equilibria is presented, with particular attention to the role of pressure and thermal measurements—information which is missing in previous analyses.The results provide a good match with experimental data showing that indeed a consistent inclusion of pressure with temperature and density measurements is necessary. The equilibria obtained are suitable for both transport and stability analysis.

Electrostatic properties and active magnetic topology modification in the RFX-mod edge plasma

A better understanding of the edge phenomena regulating plasma transport and turbulence is of primary importance for the whole fusion community. In particular, in reversed field pinches (RFPs) the edge properties are found to have a strong relation with the magnetic topology. The flexibility of the RFX-mod RFP experiment (Sonato et al 2003 Fusion Eng. Des. 66–68 161–8) enables exploring different topological regimes, and the insertion of a probe equipped with a two-dimensional array of electrostatic sensors allows the characterization of the outermost plasma regions (up to 10% of the minor radius). The electric field radial profiles and its dynamics are found to be highly influenced by the presence of edge magnetic islands modelled with a Field Line Tracing code (FLiT). When a helical perturbation is applied through the saddle coil feedback system a strong modification of the edge electrostatic properties is observed. The E × B velocity field results in a convective cell-like structure according to the externally imposed toroidal periodicity. Finally, an analysis of the long-range correlations in the floating potential fluctuations (measured by two toroidally separated probes) is presented and discussed along with the electric field measurements and the edge topology.

Non-axisymmetric equilibrium reconstruction for stellarators, reversed field pinches and tokamaks

Axisymmetric equilibrium reconstruction using magnetohydrodynamic equilibrium solutions to the Grad–Shafranov equation has long been an important tool for interpreting tokamak experiments. This paper describes recent results in non-axisymmetric (three-dimensional) equilibrium reconstruction of nominally axisymmetric plasmas (tokamaks and reversed field pinches (RFPs)), and fully non-axisymmetric plasmas (stellarators). Results from applying the V3FIT code to CTH and HSX stellarator plasmas, RFX-mod RFP plasmas and the DIII-D tokamak are presented.

3D effects on the RFX-mod boundary

In present fusion research a strong effort is devoted to the studies of non-axisymmetric magnetic perturbations and consequent field ergodization on the external region of the plasma. On this topic interesting results can be drawn from the helical configuration observed in high-current regimes in reversed field pinches (RFPs) where the small edge helical ripple is sufficient to modulate the plasma–wall interaction and the plasma kinetic properties. This paper presents the most recent experimental results and physical interpretation of the phenomena observed in the edge region of the RFX-mod RFP device. Experimental observations indicate that plasma pressure and floating potential are spatially modulated according to the helical deformation. Helical flow is observed at the edge as a consequence of an ambipolar electric field. Emphasis will be devoted to the determination of the actual phase relation between magnetic perturbation and velocity perturbation. Evidence of the influence of the helical ripple on turbulence properties at the edge is also reported.

Experimental investigation of electron temperature dynamics of helical states in the RFX-Mod reversed field pinch

The study of electron temperature profiles of quasi-single helical states (QSH) in RFX-Mod (and, in general, in other RFP machines) was carried out in the past mainly using Thomson scattering, thus being limited to a time resolution of the order of some milliseconds and to the collection of single, isolated cases, which displayed clear transport barriers. A new multichord double filter SXR spectrometer was developed and installed. Temperature profiles are now measured up to a frequency of some kHz, allowing one to determine the entire time evolution of electron temperature during a QSH cycle. A mapping technique, originally developed for Thomson temperature profiles, is adapted to deal with the line-integrated nature of the SXR diagnostic. In particular, temperature gradients related to the presence of transport barriers are reconstructed and analysed in a helical reference system, coherently with the underlying plasma equilibrium. The method is discussed in its capabilities and limits and then applied to a wide QSH database. As a result, a clear difference in temperature gradient behaviour between the rising phase of QSH and the saturated (or flattop) phase is observed. In the rising phase the expected correlation of temperature gradient with magnetic spectrum is confirmed, showing a positive trend between dominant mode amplitude and thermal structure dimension, as well as a negative correlation of secondary mode amplitudes and temperature gradient. On the other hand, in the flattop phase the presence of a thermal structure is intermittent, and much less influenced by the magnetic dynamics. This suggests the simultaneous presence of different mechanisms that enhance energy transport.

The Monte Carlo Simulation of a 1-MW Neutral Beam Injector on RFX-Mod

The megawatt-level TPE-RX neutral beam injector is planned to be installed on the RFX-mod reversed field pinch (RFP) experiment. To properly integrate the TPE-RX injector on RFX-mod, a 3-D Monte Carlo simulation of the injector has been carried out. The code simulates the charged and neutral particle trajectories from the acceleration grids of the beam source to the RFP plasma. The simulation includes the neutralization stage, the interaction between the accelerated particles and the magnetic field produced by RFX and the residual ion dump (RID), and the interactions between the accelerated beam and the background gas. The simulation has allowed to estimate the neutralizer and RID efficiency and specifications, the geometry of the beam duct between the injector and RFX-mod, and the necessary degree of magnetic screening.

RFX-mod wall conditioning by lithium pellet injection

Plasma–wall interaction is one of the most important issues that present magnetic confinement devices have to face. In the RFX-mod reversed field pinch experiment plasma–wall interaction has become a hard point increasing plasma current up to the RFX-mod maximum design value of 2 MA, since in this case local power deposition can be as high as 10 MW m−2. Since the first wall of RFX-mod is entirely covered by graphite tiles different techniques have been tested to control hydrogen wall influx: He glow discharges cleaning, He discharges at high plasma currents, wall boronization and baking. With the best results obtained by boronization, at high plasma currents all such techniques improve the situation but do not allow a complete and stationary hydrogen influx reduction. Furthermore, in the presence of localized high power load the wall still responds providing very high influxes. In order to improve this situation wall conditioning by lithium has been tested. As a first lithization method to deposit a controllable amount of lithium on the wall, a room temperature pellet injector has been used (maximum pellet diameter of 1.8 mm and maximum length of 5 mm). Lithium coatings with a theoretical thickness of about 10 nm have been applied both to clean graphite tiles and over boronized ones. Lithization demonstrated to be effective in lowering hydrogen wall recycling to a value smaller than that of boronized graphite, with the effect lasting 20–30% more than in the boronized case. Compared with boronization, lithization slightly improves (by about 30%) particle confinement time and also clearly affects edge particle transport providing a lower edge density and more peaked density profiles. Lithization also reduces carbon content by about 10% over boronization but still no clear improvement has been observed in terms of energy confinement. Similar results have been obtained performing lithization over boronized graphite.

Alfvén eigenmodes in the RFX-mod reversed-field pinch plasma

High-frequency magnetic activity has been detected at the edge region of the RFX-mod reversed-field pinch (RFP) device. The analysis, performed by means of an insertable probe measuring the magnetic field fluctuations, shows coherent modes clearly depending on the Alfvén velocity, at a frequency in the range 0.1–1.5 MHz. Two main distinct modes are observed, characterized by low (|n| ⩽ 1) toroidal periodicity, with the features of discrete Alfvén eigenmodes. A splitting of the mode frequency is detected, a sign of the relevance of non-linearity in the physical process under study. The experimental observations could suggest an interpretation of the modes in terms of global Alfvén eigenmodes. Some hints about the possible drive mechanisms are also given.

Internal and external electron transport barriers in the RFX-mod reversed field pinch

An interesting result of magnetic chaos reduction in RFX-mod high current discharges is the development of strong electron transport barriers. An internal heat and particle transport barrier is formed when a bifurcation process changes the magnetic configuration into a helical equilibrium and chaos reduction follows, together with the formation of a null in the q shear. Strong temperature gradients develop, corresponding to a decreased thermal and particle transport. Turbulence analysis shows that the large electron temperature gradients are limited by the onset of micro-tearing modes, in addition to residual magnetic chaos. A new type of electron transport barrier with strong temperature gradients develops more externally (r/a = 0.8) accompanied by a 30% improvement of the global confinement time. The mechanism responsible for the formation of such a barrier is still unknown but it is likely associated with a local reduction of magnetic chaos. These external barriers develop primarily in situations of well-conditioned walls so that they might be regarded as attempts towards an L–H transition. Both types of barriers occur in high-current low-collisionality regimes. Analogies with tokamak and stellarators are discussed.

Improved dynamic response of magnetic feedback in RFX-mod and DIII-D

The wall of any magnetic fusion device is characterized by the presence of several 3D structures, such as portholes for diagnostics and for heating and current drive systems, coil feeds and other features. Time-varying magnetic fields induce eddy currents in the wall, whose pattern is modified by these structures, giving rise to magnetic field errors that can be amplified or shielded by the plasma. Two examples will be given on how the dynamic response of a 3D wall to external magnetic fields can be identified and used to optimize magnetic feedback. In the RFX-mod reversed-field pinch, a dynamic decoupler algorithm has been developed, which allows for the production of pure radial magnetic field harmonics inside the wall, reducing the harmonic distortion due to the 3D wall structures. This is applied here to the problem of producing helical boundary conditions to control helical RFP equilibria. In the DIII-D tokamak, a frequency-dependent scheme for the compensation of the magnetic sensors from spurious n = 1 fields due to the coupling with the feedback and axisymmetric coils has been recently implemented in real time and tested with plasma. The possible relevance of these 3D effects for high performance scenarios is discussed.

Magnetohydrodynamic properties of nominally axisymmetric systems with 3D helical core

Magnetohydrodynamic equilibrium states with a three-dimensional helical core are computed to model the MAST spherical tokamak and the RFX-mod reversed field pinch. The boundary is fixed as axisymmetric. The MAST equilibrium state has the appearance of an internal kink mode and is obtained under conditions of weak reversed central shear. The RFX-mod equilibrium state has seven-fold periodicity. An ideal magnetohydrodynamic stability analysis reveals that the reversal of the core magnetic shear can stabilize a periodicity-breaking mode that is dominantly m/n = 1/8 strongly coupled to a m/n = 2/15 component, as long as the central rotational transform does not exceed the value of 8.