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The performance of heating and cooling systems are affected by their dynamic operating conditions. As a consequence, a sound evaluation of system performance should be done through a dynamic test procedure. However, the complexity, the cost, and the time for such type of experimentation are obstacles to the diffusion of this test method. A new whole system test procedure was developed to reduce its potential failure. The test method considers the emulation of system components that cannot be installed in the laboratory, while the selection of short-yet-significant test sequence is performed classifying the days of the year with clustering analysis. The procedure was applied to a solar-assisted heat pump system in four different European climates. The seasonal performance figures are extrapolated from the test results and compared with the numerical simulations of the entire heating and cooling system. In all test cases the seasonal performance factors are lower than the simulated ones by approximately 10%. The results obtained with dynamic tests confirm the necessity of their implementation, since they provide useful information to improve the system layout and control. In this way, the manufacturers can introduce more efficient products into the market.

This paper discusses the development of a coupled Q-learning/fuzzy control algorithm to be applied to the control of solar domestic hot water systems. The controller brings the benefit of showing performance in line with the best reference controllers without the need for devoting time to modelling and simulations to tune its parameters before deployment. The performance of the proposed control algorithm was analysed in detail concerning the input membership function defining the fuzzy controller. The algorithm was compared to four standard reference control cases using three performance figures: the seasonal performance factor of the solar collectors, the seasonal performance factor of the system and the number of on/off cycles of the primary circulator. The work shows that the reinforced learning controller can find the best performing fuzzy controller within a family of controllers. It also shows how to increase the speed of the learning process by loading the controller with partial pre-existing information. The new controller performed significantly better than the best reference case with regard to the collectors&rsquo, performance factor (between 15% and 115%), and at the same time, to the number of on/off cycles of the primary circulator (1.2 per day down from 30 per day). Regarding the domestic hot water performance factor, the new controller performed about 11% worse than the best reference controller but greatly improved its on/off cycle figure (425 from 11,046). The decrease in performance was due to the choice of reward function, which was not selected for that purpose and it was blind to some of the factors influencing the system performance factor.

A real-time system has been successfully used since 2004 in the RFX-mod nuclear fusion experiment to control the position of the plasma and its Magneto Hydrodynamic (MHD) modes. However, its latency and the limited computation power of the used processors prevented the usage of more aggressive control algorithms. Therefore a new hardware and software architecture have been designed to overcome such limitations and to provide a shorter latency and a much increased computation power. The new system is based on a Linux multi-core server and uses MARTe, a framework for real-time control which is gaining interest in the fusion community.

SPIDER, the ion source test bed in the ITER neutral beam test facility, is under construction and its operation is expected to start in 2014. Control and data acquisition for SPIDER are undergoing final design. SPIDER CODAS, as the control and data acquisition system is referred to, is requested to manage 25 plant units, to acquire 1000 analogue signals with sampling rates ranging from a few S/s to 10 MS/s, to acquire images with up to 100 frames per second, to operate with long pulses lasting up to 1 h, and to sustain 200 MB/s data throughput into the data archive with an annual data storage amount of up to 50 TB. SPIDER CODAS software architecture integrates three open-source software frameworks each addressing specific system requirements. Slow control exploits the synergy among EPICS and Siemens S7 programmable controllers. Data handling is by MDSplus a data-centric framework that is geared towards the collection and organization of scientific data. Diagnostics based on imaging drive the design of data throughput and archive size. Fast control is implemented by using MARTe, a data-driven, object-oriented, real-time environment. The paper will describe in detail the progress of the system hardware and software architecture and will show how the software frameworks interact to provide the functions requested by SPIDER CODAS. The paper will focus on how the performance requirements can be met with the described SPIDER CODAS architecture, describing the progress achieved by carrying out prototyping activities.

The ITER Heating Neutral Beam injectors will be implemented in three steps: development of the ion source prototype, development of the full injector prototype, and, finally, construction of up to three ITER injectors. The first two steps will be carried out in the ITER neutral beam test facility under construction in Italy. The ion source prototype, referred to as SPIDER, which is currently in the development phase, is a complex experiment involving more than 20 plant units and operating with beam-on pulses lasting up to 1 h. As for control and data acquisition it requires fast and slow control (cycle time around 0.1 ms and 10 ms, respectively), synchronization (10 ns resolution), and data acquisition for about 1000 channels (analogue and images) with sampling frequencies up to tens of MS/s, data throughput up to 200 MB/s, and data storage volume of up to tens of TB/year. The paper describes the architecture of the SPIDER control and data acquisition system, discussing the SPIDER requirements and the ITER CODAC interfaces and specifications for plant system instrumentation and control.

The real-time control system of RFX-mod, in operation since 2005, has been successful and has allowed several important achievements in the RFX physics research program. As a consequence of this fact, new control algorithms are under investigation, which are more demanding in terms of both enhanced computing power and reduced system latency, currently around 1.5 ms. For this reason, a major upgrade of the system is being considered, and a new architecture has been proposed, taking advantage of the rapid evolution of computer technology in the last years. The central component of the new architecture is a Linux-based multicore server, where individual cores replace the VME computers. The server is connected to the I/O via PCI-e based bus extenders, and every PCI-e connection is managed by a separate core. The system is supervised by MARTe, a software framework for real-time applications written in C++ and developed at JET and currently used for the JET vertical stabilization and in other fusion devices.

The Ion Source experiment, called SPIDER, is the first experiment to be realized in the ITER Neutral Beam Test Facility under construction at Consorzio RFX, Padova, Italy. Its data acquisition system will provide the collection of around one thousand signals at a sampling rate ranging from a few S/s to 10 MS/s and of diagnostics and inspection images produced at a frame frequency from 10 to 150 frame per second, for beam durations of up to one hour. As the final product of the Test Facility will be the ITER Heating Neutral Beam Injector, we decided to follow the ITER CODAC guidelines. Therefore, the EPICS system provided in the CODAC Core System has been assessed and adopted in plant control. In addition to EPICS, the MDSplus system will be adopted for the overall data management. MDSplus can handle a rich set of data types and offers support for both local and remote data access in a variety of languages, a feature not fully supported by EPICS. All experiment data in SPIDER will be maintained in a single MDSplus database containing both experiment configuration and acquired data. The seamless integration of EPICS and MDSplus in a single system requires the development of some new tools, namely: A Channel Archiver which provides storage into MDSplus pulse files of data produced by EPICS Input Output Controllers (IOC) and exported as Process Variables via Channel Access; A set of IOC records providing direct access from IOC into MDSPlus files; A Channel Access Server to export MDSplus data as EPICS Process Variables, making them directly accessible by IOCs. We expect that the integration of the two frameworks will result in a system that combines the best features from each one, i.e., the management of distributed Process Variables and the IOC concept for EPICS, the sophisticated data management and remote access for MDSplus.

The RFX-mod system for the active control of MHD instabilities has been operating for more than 2 years contributing to improvements of the discharge quality and length. In the past years a dynamic black-box model of the active coils and saddle probes has been developed to analyze and simulate the system electromagnetic behaviour, which is strongly dependent on the frequency due to the presence of passive conductors. Satisfactory results were achieved in reproducing most of experimental responses and the model was used to select and test the gains of operating PID regulators. However, difficulties have emerged in reproducing some flux distributions such as those corresponding to m = 0, low n current patterns. In view of higher current operation a further increase in control system performance and model accuracy is desirable, if not mandatory. To this purpose, new measures have been recently acquired in order to reassess data of critical magnetic couplings and an activity has been started aimed at the verification and integration of data from a white-box model provided by the 3D finite element electromagnetic code Cariddi. The above verification is also a preliminary step for the validation of the CarMa computational tool adaptation to RFX-mod, which has been undertaken to study the plasma resistive wall modes. The paper describes the results of a comparative analysis carried out on experimental and simulation data obtained with the black-box and white-box models.

The recent modifications brought to the RFX experiment, now RFX-mod, pose a strong focus on the active control of the plasma magnetic configuration. The new machine layout features a thin stabilizing shell which makes active control feasible and necessary at the same time. The new set of actuators includes a complex saddle coils system and a redesigned power supply system driving the toroidal field coils. The new toroidal power supply system allows a great flexibility since each sector, composed by four coils, is driven by an independent d.c./d.c. switching converter, in series with the main power supply providing the bias current. The current control on the load, performed by the four quadrant fast switching converter, allows producing the desired waveforms of toroidal field. To fully exploit the capabilities of such a system two different controllers have been designed enabling to track automatically a selected evolution of either the toroidal field at the plasma edge or the reversal F parameter, respectively.The new control systems allow to perform experiments in specified plasma configurations targeting particular physics issues and improved the overall reproducibility of the RFX-mod discharges.

The modified RFX is a very flexible device used for a variety of control schemes for MHD instabilities and for advanced reverse field pinch scenarios. Relative to the previous machine, RFX-mod has a thin Cu shell with vertical field penetration time τS, lowered from 450 to 50 ms and shell/plasma proximity from b/a ≤ 1.24-1.1. Toroidal equilibrium is feedback-controlled and new power supplies provide better Bφ control. Newly designed graphite tiles protect the vessel from localized power deposition. A mesh of 192 external saddle coils, supervised by a digital feedback system, controls radial fields due to field errors and MHD modes. The paper presents an overview of the very encouraging results obtained using both new and 'standard' advanced operational modes in the current range 0.3-1 MA. A dramatic improvement of plasma performance was obtained by using the saddle coils to cancel all the radial field components, an operation mode dubbed virtual shell (VS). The toroidal voltage was lowered by more than 25% and the pulse length was tripled, up to 7 times the τS. Steady-state RFP pulses are now limited only by the applied volt-seconds. The improved magnetic boundary also has an effect on the tearing modes underlying the sustainment of the RFP, whose core amplitude is more than halved. The VS combined with new schemes for the active rotation of the MHD dynamo modes has allowed us to obtain reliable and well-controlled long RFP pulses in the MA current range. This results in a 100% increase in the particle and energy confinement time relative to the previous experiment and opens the possibility of exploring the machine performance in the 2 MA design range. © 2007 IAEA.

A novel representation of the plasma boundary is presented, which employs a B-spline approach to describe the deformation of the plasma surface in an integral way, without resorting to the local properties of the shape as in the gap technique. The evolution of such a curve on the poloidal plane corresponds to that of the plasma shape and a spline controller is designed to counteract disturbances and regulate the plasma shape error during the discharge. The comparison with a gap controller highlights the accuracy in the control capability of the spline controller and its completeness, in the sense that the control of the spline curve implies (but is not implied by) the control of the gaps.

The real-time control system of RFX has been operating since 2004 providing effective control of the plasma position and of the MagnetoHydroDynamic (MHD) modes. The demand for new and more computing-intensive control algorithms and the need for shorter latency pushed the system to its limits and, thus, a complete re-design was carried out in 2012. The new system adopts radically different solutions in hardware, operating system and software management. The VME PowerPC CPUs communicating over Ethernet have been now replaced by a single multicore server. VxWorks, previously used in the VME CPUs has now been replaced by Linux, which can be currently considered a real-time system provided an accurate tuning of the Linux scheduler and interrupt configuration. The previous framework for control and communication has been replaced by MARTe, a modern framework for real-time control gaining interest in the fusion community. The usage of MARTe allowed a rapid development of the control system and, in particular, its intrinsic simulation ability gave us the possibility of carrying out most debugging in simulation, without affecting machine operation. As a result the whole system has been finally commissioned in RFX in only two weeks.

Input–output datasets from two magnetic confinement fusion (MCF) experiments of the reversed-field pinch (RFP) type are examined. The RFP datasets, which are samples of the distributed magnetic field dynamics, are naturally divided into many smaller batches due to the pulsed-plasma operation of the experiments. The two RFP experiments considered are (i) EXTRAP T2R (T2R) with 64 inputs and 64 outputs and (ii) RFX-mod (RFX) with 192 inputs and 192 outputs. Both T2R and RFX are magnetohydrodynamically unstable and operates under magnetic feedback with optional dither injection. Using subspace system identification techniques and randomised cross-validation (CV) methods to minimise the generalisation error, state-space orders of the empirical systems are suggested. These system orders are compared to “stabilisation diagrams” commonly used in experimental modal analysis practice. The relation of the CV system order to the decay of the singular values from the subspace method is observed. Both (i) stable vacuum diffusion and (ii) unstable plasma response datasets are analysed. Apparent simulation and prediction errors are quantified for both cases using a deviation-accounted-for index. These results are purely data-driven. A simple approach towards exploitation of the subspace techniques for finite-element model refinement and data confrontation is presented.

The ITER Neutral Beam Test Facility will be the project's R&D facility for heating neutral beam injectors (HNB) for fusion research operating with H/D negative ions. Its mission is to develop technology to build the HNB prototype injector meeting the stringent HNB requirements (16.5 MW injection power, −1 MVe acceleration energy, 40 A ion current and one hour continuous operation). Two test-beds will be built in sequence in the facility: first SPIDER, the ion source test-bed, to optimize the negative ion source performance, second MITICA, the actual prototype injector, to optimize ion beam acceleration and neutralization. The SPIDER control and data acquisition system is under design. To validate the main architectural choices, a system prototype has been assembled and performance tests have been executed to assess the prototype's capability to meet the control and data acquisition system requirements. The prototype is based on open-source software frameworks running under Linux. EPICS is the slow control engine, MDSplus is the data handler and MARTe is the fast control manager. The prototype addresses low and high-frequency data acquisition, 10 kS/s and 10 MS/s respectively, camera image acquisition, data archiving, data streaming, data retrieval and visualization, real time fast control with 100 µs control cycle and supervisory control. The paper shows how the proposed prototype architecture can address the most critical requirements of SPIDER control.

RFX-mod [Sonato et al., Fusion Eng. Des. 66, 161 (2003)] exploits its 192 active coils in both reversed-field pinch (RFP) and tokamak configurations with varying degrees of 3D shaping, providing also a test bed for validating stellarator codes and 3D nonlinear magnetohydrodynamic codes. This makes RFX-mod a unique and flexible facility for comparative studies on 3D shaping and control. The paper discusses how 3D fields allow access to RFP and tokamak advanced regimes. 3D fields are used to feedback control Single Helicity (SH) RFP equilibria with 1/7 helicity up to similar to 2 MA. They also allow accessing SH regimes with higher density (Greenwald fraction up to 0.5), presently inaccessible in spontaneous SH regimes. Feedback on the 2/1 resistive-wall mode in RFX-mod tokamak plasmas allows for safe operation at q(a) < 2, an almost unexplored promising regime. Forcing the 2/1 mode to saturate at finite but small level, a helical tokamak equilibrium with significant n=1 modulation is produced and a new way to tailor sawteeth is found. The effects of different levels of 3D shaping on momentum transport in both RFP and tokamak helical states are discussed.

A new integrated simulator for experiments of resistive wall mode (RWM) closed-loop control and its benchmark against experimental data are presented. The numerical tool couples in a self-consistent way a full 3D finite element description of the machine boundaries (Cariddi code), a 2D toroidal model of RFP plasma stability (MARS-F code) and a realistic representation of the RFX-mod control system producing an overall dynamic model cast in the state variable space. In this way a full dynamic ‘flight simulator’ of RWM control experiments has been implemented where the interaction of proportional–integral–derivative controller gains and plasma equilibrium parameters can be explored. As an application of the new integrated tool, closed-loop RWM stability analyses have been benchmarked against experimental data. In this way it was possible to experimentally prove that the control simulator correctly reproduces closed-loop RWM growth rates under different control conditions. Time domain simulations were also run to assess the overall accuracy of the model including the presence of non-linear blocks. The achieved results are discussed in connection with the broader international effort towards an effective control of RWMs in both tokamak and RFP configurations.

In this paper, the results of resistive wall mode (RWM) control experiments using several sets of active coils are presented. The control effectiveness as a function of coil number and covered torus surface is studied in the RFX-mod device, thanks to the powerful and flexible MHD active control system. Active control is tried using similar coil number and size both in the reversed field pinch (RFP) and in the tokamak configurations, giving a good experimental set to exploit the differences between the two cases. The experimental results are compared with predictions by a new integrated simulator for closed loop RWM growth. The numerical tool couples, in a self-consistent way, a full 3D finite element description of the machine boundaries, a 2D toroidal model of RFP plasma stability and a model of the RFX-mod control system, producing an overall dynamic model cast in the state variable space. In this way a full dynamic flight simulator of RWM control experiments is implemented, where coil coverage and experimental proportional–integral–derivative gains are explored in simulations. Both in the performed experiments and in simulations wall mode active control is proven to be possible in both the configurations with a very reduced set of active control coils.

Feedback stabilization of the m = 2, n = 1 resistive wall mode in q(a) < 2 plasmas is obtained in the RFX-mod device operated as a circular tokamak. The feedback scheme implements radial sensors within the clean mode control (CMC) algorithm, a technique based on the de-aliasing of the measurements from the high periodicity sidebands produced by the coils. A cylindrical zero-pressure MHD model, which includes a realistic discrete-time description of the feedback, is able to explain the main features of the experimental observation, including the necessity of the removal of sidebands when using radial sensors. The model is also used to show that the radial field CMC presents some advantages in comparison with poloidal field based control schemes, allowing more freedom in the arrangements of the sensors.

SPIDER, the ITER-size ion-source test bed in the ITER Neutral Beam Test Facility, is a fusion device requiring a complex central system to provide control and data acquisition, referred to as CODAS. The CODAS software architecture will rely on EPICS and MDSplus, two open-source, collaborative software frameworks, targeted at control and data acquisition, respectively. EPICS has been selected as ITER CODAC middleware and, as the final deliverable of the Neutral Beam Test Facility is the procurement of the ITER Heating Neutral Beam Injector, we decided to adopt this ITER technology. MDSplus is a software package for data management, supporting advanced concepts, such as platform and underlying hardware independence, self description data, and data driven model. The combined use of EPICS and MDSplus is not new in fusion, but their level of integration will be new in SPIDER, achieved by a more refined data access layer. The paper presents the integration software to use effectively EPICS and MDSplus, including the definition of appropriate EPICS records to interact with MDSplus. The MDSplus and EPICS archive concepts are also compared on the basis of performance tests and data streaming is investigated by ad-hoc measurements.

A closed-loop control system of the reversal parameter has been implemented in the RFX-mod machine on the basis of a model of the toroidal field system including the toroidal power supply circuit, with a description of the coupling with the plasma current and the local control of the power section. In the paper, after a description of the power supply model and the implemented closed-loop control, the outcome of the work will be reported by showing some experimental results on RFX-mod high current dedicated campaigns, with the feedback control system in operation.

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.

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. © 2011 IOP Publishing Ltd.

In this paper insights into the behaviour of resistive wall modes (RWMs) in the RFX-mod reversed field pinch device are given, with a focus on 3D issues in the characterization of the m spectrum of the mode and on the study of multi-harmonic coupling. In the first part of the paper the interaction between multiple unstable RWMs is studied and the presence of a coupling between different poloidal components of the most unstable RWM is demonstrated, taking advantage of the flexibility of the RFX-mod control system. In the second part of the work, the dependence of the growth rates of RWMs on a complete set of plasma parameters is studied in order to create a complete and homogeneous database, which permits a careful validation of stability codes. Finally, the experimental data are compared with the code predictions which take into account the 3D structure of conductors around the plasma. The different effects that modify the simple description, where unstable modes can be identified with single Fourier harmonics, appear to be explained by a mixture of toroidicity-induced and 3D eddy current effects.

This paper summarizes the main achievements of the RFX fusion science program in the period between the 2008 and 2010 IAEA Fusion Energy Conferences. RFX-mod is the largest reversed field pinch in the world, equipped with a system of 192 coils for active control of MHD stability. The discovery and understanding of helical states with electron internal transport barriers and core electron temperature >1.5 keV significantly advances the perspectives of the configuration. Optimized experiments with plasma current up to 1.8 MA have been realized, confirming positive scaling. The first evidence of edge transport barriers is presented. Progress has been made also in the control of first-wall properties and of density profiles, with initial first-wall lithization experiments. Micro-turbulence mechanisms such as ion temperature gradient and micro-tearing are discussed in the framework of understanding gradient-driven transport in low magnetic chaos helical regimes. Both tearing mode and resistive wall mode active control have been optimized and experimental data have been used to benchmark numerical codes. The RFX programme also provides important results for the fusion community and in particular for tokamaks and stellarators on feedback control of MHD stability and on three-dimensional physics. On the latter topic, the result of the application of stellarator codes to describe three-dimensional reversed field pinch physics will be presented. © 2011 IAEA, Vienna.

The paper describes an integrated procedure for the reconstruction of halo currents expected in ITER. The procedure is based on simulation and reconstruction codes, partially adapted from existing codes and partly developed specifically for this purpose. The ultimate goal of this integrated procedure is the assessment of the performance and the possible optimization of the halo current diagnostic system foreseen in ITER. The preliminary results indicate that an alternative distribution of the present layout of halo current sensors could improve the reconstruction capability of this diagnostic system. © 2011 IEEE.

In this work the MHD stability of RFX mode tokamak discharges with q$_{edge}

A spontaneous transition to a helical equilibrium with an electron internal transport barrier is observed in RFX-mod as the plasma current is raised above 1 MA (Lorenzini R et al 2009 Nature Phys. 5 570). The helical magnetic equilibrium can be controlled with external three-dimensional (3D) magnetic fields applied by 192 active coils, providing proper helical boundary conditions either rotating or static. The persistence of the helical equilibrium is strongly increased in this way. A slight reduction in the energy confinement time of about 15% is observed, likely due to the increased plasma-wall interaction associated with the finite radial magnetic field imposed at the edge. A global helical flow develops in these states and is expected to play a role in the helical self-organization. In particular, its shear may contribute to the ITB formation and is observed to increase with the externally applied radial field. The possible origins of this flow, from nonlinear visco-resistive magnetohydrodynamic (MHD) and/or ambipolar electric fields, will be discussed. © 2011 IOP Publishing Ltd.

Plasma current control was experimented in the RFX machine in early nineties with unsatisfactory results due to the frequent onset of unstable regimes leading to increasing voltage demand and saturation of the flat-top power supply. More recently, taking advantage of the new operating conditions disclosed by the active control system of MHD modes and the flexibility of digital systems, the plasma current control has appeared feasible again. A design of the controller according to adaptive control techniques is presented along with the definition of transition rules through the different phases of the pulse. A zero-dimensional model has been used for the design of a regulator for flat-top operation. The digital implementation has allowed a flexible control strategy depending on the actual values of selected plasma parameters. In particular, a real-time estimation of the plasma resistance has been used to drive the control system through different regulators in a stable transfer mode. The paper presents, in addition, the results of experimental campaigns which have been carried out using the new plasma current controller.

The paper addresses two important aspects: event-driven data acquisition and accurate timing reconstruction. Event-driven data acquisition allows reducing the requirements in data storage and transfer bandwidth retaining at the same time all the necessary information for a proper reconstruction of acquired signals. In many cases, in fact, the required bandwidth in data acquisition is not constant, being higher only in correspondence of particular events during the discharge. Event detection is performed in real time and the system has to guarantee a higher sampling speed in a time window centered on the event occurrence. Possible technological solutions are discussed and a new “soft trigger” approach is suggested, which allows using the same hardware for both low and high sampling speed acquisition, as well as using the general network for event transmission in place of dedicated links. Accurate timing reconstruction of acquired signals is necessary, regardless of the duration of the discharge. Care is then required in the implementation of timing systems in order to ensure both time stability and reliability over the whole plasma discharge, and possibly over the whole experiment lifetime. The typical architecture for such systems is based on a GPS clock generator that is connected to all data acquisition devices via a dedicated fiber-optics link carrying the synchronization clock. The paper introduces a different approach in timing reconstruction using IEEE 1588, a recent standard for accurate time synchronization, and discusses its potentialities and limits in data acquisition systems.

The paper discusses the requirements and presents an architectural overview of a new general-purpose framework for real-time control, currently under development at the RFX laboratory. This represents the third generation of control framework developed at RFX. It takes into account the experience gained in the development of the previous frameworks and exploits the evolution of computer technology, in particular, the evolution of Linux towards real-time functionality and the availability of multicore CPUs in most general-purpose computing boards. The framework will supervise data flow among the participating components, which may produce data, consume data, or apply some sort of computation. Some system components have been already developed, while the architecture of others is still being refined. In particular, we are going to integrate the infrastructure layer of the new framework with a real-time framework recently in use at JET. The aim of this collaboration is a general purpose real-time system which can be used on both experiments.

Two issues of the resistive wall mode (RWM) control code maturity are addressed: the inclusion of advanced mode damping physics beyond the ideal MHD description, and the possibility of taking into account the influence of 3D features of the conducting structures on the mode stability and control. Examples of formulations and computational results are given, using the MARS-F/K codes and the CarMa code. The MARS-K calculations for a DIII-D plasma shows that the fast ion contributions, which can give additional drift kinetic stabilization in the perturbative approach, also drive an extra unstable branch of mode in the self-consistent kinetic modelling. The CarMa modelling for the ITER steady state advanced plasmas shows about 20% reduction in the RWM growth rate by the volumetric blanket modules. The multi-mode analysis predicts a weak interaction between the n = 0 and the n = 1 RWMs, due to the 3D ITER walls. The CarMa code is also successfully applied to model the realistic feedback experiments in RFX.

Data acquisition and real-time control in nuclear fusion science are usually demanding with respect to the number of involved components, the amount of handled data, and the accuracy required in time reconstruction. The latter aspect becomes critical in new and future experiments that will be running for hours. In these conditions, the mechanisms used to generate and distribute time information, to keep synchronization in devices such as acquisition boards and processing nodes, and to label with a timestamp the acquired samples play an important role. The IEEE1588 Precise Time Protocol (PTP) is a standard for clock synchronization that can be implemented over an Ethernet network. The technology is being considered for the implementation of the timing system of the ITER Neutral Beam Test Facility to be built in Padova, Italy. A PTP network has been set up using commercial off-the-shelf PTP electronic components, PTP and non-PTP switches, with the purpose of assessing the synchronism in conditions similar to those expected in real application. The capabilities of the PTP standard and of PTP hardware have been assessed with different measurement methodologies. Data have been acquired and analyzed using internal capabilities of the PTP hardware, a histogram-enabled digital oscilloscope, and a couple of digital acquisition boards. In particular, the last measurement method is based on data acquisition hardware that is commonly used in fusion experiments. The different measurements of the synchronization performance are in good agreement. These measurements allow assessing the accuracy in time reconstruction and in particular its dependency on the network switches. Based on the results obtained, the tested PTP implementation can be considered a candidate for time reconstruction of signals with analog bandwidth up to 33 kHz.

With the exploration of the MA plasma current regime in up to 0.5 s long discharges, RFX-mod has opened new and very promising perspectives for the reversed field pinch (RFP) magnetic configuration, and has made significant progress in understanding and improving confinement and in controlling plasma stability. A big leap with respect to previous knowledge and expectations on RFP physics and performance has been made by RFX-mod since the last 2006 IAEA Fusion Energy Conference. A new self-organized helical equilibrium has been experimentally achieved (the Single Helical Axis - SHAx - state), which is the preferred state at high current. Strong core electron transport barriers characterize this regime, with electron temperature gradients comparable to those achieved in tokamaks, and by a factor of 4 improvement in confinement time with respect to the standard RFP. RFX-mod is also providing leading edge results on real-time feedback control of MHD instabilities, of general interest for the fusion community. © 2009 IAEA, Vienna.

The paper discusses the requirements and presents an architectural overview of a new general-purpose framework for real-time control, currently under development at the RFX laboratory. This represents the third generation of control framework developed at RFX. It takes into account the experience gained in the development of the previous frameworks and exploits the evolution of computer technology, in particular the evolution of Linux towards real-time functionality and the availability of multi-core CPUs in most general-purpose computing boards. The framework will supervise data flow among the participating components, which may produce data, consume data, or apply some sort of computation. The lessons learnt after five years of operation and maintenance of the real-time control system of RFX are first presented, outlining in particular which design choices proved successful and which would have required a different approach. The general requirements of a real-time system for fusion devices are then discussed in depth in order to justify the architectural choices presented afterwards. Some system components have been already developed, while the architecture of others is still being refined. In particular, we are going to integrate the infrastructure layer of the new framework with a real-time framework recently in use at JET. The aim of this collaboration is a general purpose real-time system which can be used on both experiments.

The reversed field pinch configuration is characterized by the presence of magnetic structures both in the core and at the edge: in the core, at high plasma current the spontaneous development of a helical structure is accompanied by the appearance of internal electron transport barriers; at the edge strong pressure gradients, identifying an edge transport barrier, are observed too, related to the position of the field reversal surface. The aim of this paper is the experimental characterization of both the internal and edge transport barriers in relation to the magnetic topology, discussing possible analogies and differences with other confinement schemes. © 2009 IOP Publishing Ltd.

A new control scheme, named Clean Mode Control (CMC) and based on the real-time correction of the aliasing of the sideband harmonics produced by the saddle coils, brought about a significant improvement of RFX-mod discharges, allowing the routine operation at 1.5 MA plasma current. A refinement of the electromagnetic model of the system is in progress by integrating new experimental measures and “virtual” measures provided by CARIDDI code. A Multi Input-Multi Output (MIMO) pseudo-decoupler control system was designed and the promising results of numerical simulations suggested starting its real-time implementation. A simplified MIMO decoupler in the mode space, based on a model retaining only the largest cross-coupled mode, is also being developed.

Optimization of machine operation, including plasma position control, density control and especially feedback control on multiple magnetohydrodynamic modes, has led RFX-mod to operate reliably at 1.5 MA, the highest current ever achieved on a reversed field pinch (RFP). At high current and low density the magnetic topology spontaneously self-organizes in an Ohmical helical symmetry, with the new magnetic axis helically twisting around the geometrical axis of the torus. The separatrix of the island disappears leaving a wide and symmetric thermal structure with large gradients in the electron temperature profile. The new topology still displays an intermittent nature but its overall presence has reached 85% of the current flat-top period. The large gradients in the electron temperature profile appear to be marginal for the destabilization of ion temperature gradient modes on the assumption that ions and electrons have the same gradients. There are indications that higher currents could provide the conditions under which to prove the existence of a true helical equilibrium as the standard RFP configuration. © 2008 IOP Publishing Ltd.

Real-time control applications will benefit in the near future from the enhanced performance provided by multi-core processor architectures. Nevertheless real-time communication will continue to be critical in distributed plasma control systems where the plant under control typically is distributed over a wide area. At RFX-mod real-time communication is crucial for hard real-time plasma control, due to the distributed architecture of the system, which consists of several VMEbus stations. The system runs under VxWorks and uses Gigabit Ethernet for sub-millisecond real-time communication. To optimize communication in the system, a set of detailed measurements has been carried out on the target platforms (Motorola MVME5100 and MVME5500) using either the VxWorks User Datagram Protocol (UDP) stack or raw communication based on the data link layer. Measurements have been carried out also under Linux, using its UDP stack or, in alternative, RTnet, an open source hard real-time network protocol stack. RTnet runs under Xenomai or RTAI, two popular real-time extensions based on the Linux kernel. The paper reports on the measurements carried out and compares the results, showing that the performance obtained by using open source code is suitable for sub-millisecond real-time communication in plasma control.

We report on a set of performance measurements executed on VMEbus MVME5500 boards equipped with MPC7455 PowerPC processor, running four different operating systems: Wind River VxWorks, Linux, RTAI, and Xenomai. Some components of RTAI and Xenomai have been ported to the target architecture. Interrupt latency, rescheduling and inter-process communication times are compared in the framework of a sample real-time application. Performance measurements on Gigabit Ethernet network communication have also been carried out on the target boards. To this purpose, we have considered the Linux IP stack and RTnet, an open-source hard real-time network protocol stack for Xenomai and RTAI, which was ported to the considered architecture. Performance measurements show that the tested open-source software is suitable for hard real-time applications.

The RFX-mod machine is equipped with a powerful system for the active control of MHD modes. It consists of 192 active saddle coils and corresponding amplifiers, 192 × 3 magnetic field sensors and a number of passive structures. A control system designed following SISO techniques allowed to achieve a good quality of the static spatial spectrum, but its dynamic behavior was not completely satisfactory in meeting the requirements of tearing mode control. Thus, a new MIMO design procedure was started on the basis of an available and already validated MIMO full electromagnetic model, whose inputs and outputs are the coil currents and the radial field fluxes measured by the sensors, respectively. A dynamic compensator derived from the pseudo-inverse of the system transfer function matrix was designed and a decentralized controller was then implemented. Simulations of mode tracking and mode rejection at different frequencies were run to compare the performances of a SISO-based controller and the newly designed one. Satisfactory results were obtained showing the possibility of designing a controller able to equalise the spatial spectral dynamic response. These results are presented and commented in the paper.

Dramatic improvements in RFX-mod plasma discharges have been achieved after starting operation of the active control system of MHD modes. In view of the design of more sophisticated controllers fully exploiting the system capability, an electromagnetic model of the active system was developed, which takes into account toroidal geometry and effects of passive structures. The model parameters were estimated by processing available data collected in dedicated measurement campaigns. State space representations were adopted to describe the dynamics of the active saddle coil currents and the fluxes measured by saddle probes. Open and closed loop responses were calculated and compared with experimental data. The building of the model along with the results of the extensive validation tests are described in the paper.

RFX-mod is a reversed field pinch (RFP) experiment equipped with a system that actively controls the magnetic boundary. In this paper we describe the results of a new control algorithm, the clean mode control (CMC), in which the aliasing of the sideband harmonics generated by the discrete saddle coils is corrected in real time. CMC operation leads to a smoother (i.e. more axisymmetric) boundary. Tearing modes rotate (up to 100 Hz) and partially unlock. Plasma-wall interaction diminishes due to a decrease of the non-axisymmetric shift of the plasma column. With the ameliorated boundary control, plasma current has been successfully increased to 1.5 MA, the highest for an RFP. In such regimes, the magnetic dynamics is dominated by the innermost resonant mode, the internal magnetic field gets close to a pure helix and confinement improves. © 2007 IOP Publishing Ltd.

Active control of resistive wall modes (RWMs) is a key issue for high-beta operation of future tokamaks, including ITER. RFX has been recently modified to specifically address this issue by means of a new saddle coil system, which is the most powerful and flexible among all fusion devices, including 192 coils covering the whole torus. Unstable RWMs have been experimentally observed during the recent machine operation, showing poloidal and toroidal components m = 1, 2 < |n| < 7 and growth times of some tens of ms, consistent with theoretical predictions. RFX-mod addresses the control of RWMs by acting on the magnetic field configuration at the plasma boundary. The system aims not only at RWM stabilization, but also at interaction with tearing modes. By means of the system, successful experimental campaigns have been executed demonstrating the feasibility of RWM active control. The paper discusses on the system engineering and technological features and presents the results achieved.

The RFX-mod layout is equipped with a complex system for the active control of MHD modes. The system has been fully commissioned and a plasmaless experimental campaign was devoted to its characterization. Several tests were performed in presence of plasma in order to commission the implementation of the main algorithms developed to drive the saddle coils. The results of the tests are very encouraging in terms of plasma performances and are presented in the paper along with discussion of the characterization results and a description of the control schemes adopted

In present day devices, the external kink ideal MHD instability establishes hard operational boundaries for both the tokamak and the Reversed Field Pinch (RFP) configurations. An interesting feature of it is that its growth rate critically depends on the device passive boundary characteristics and this can slow it down to time scales accessible to modern real time feedback control systems, normally using external active coils as actuators. 3D passive structures and external fields play a key role in determining physics and control of this instability. This is in particular true for equilibria with multimodal unstable RWM spectra where modes can couple to specific 3D features of passive and active magnetic boundary. In the paper we will present recent data and simulations from RFX-mod, a medium size (R = 2 m, a = 0.459 m) device able to confine RFP and tokamak plasmas with currents up to 2 MA and 120 kA, respectively. Successful quantitative modeling of multimodal RWM control experiments performed using different actuator configurations will be presented and commented.