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Future fusion reactors require a safe, steady-state divertor operation. With deep divertor detachment, which is typically induced by impurity seeding, the radiation concentrates in a small region at the X-point or on closed flux surfaces above the X-point. This so-called X-point radiator (XPR) moves further inside the confined region with increasing seeding and the location can be actively controlled. At AUG, the parameter space for operation with an XPR was significantly extended, using active feedback on the XPR location. The XPR is observed in nearly the whole operational space of AUG in the high-densities or high collisionality regime. ELM suppression is consistently observed in all cases where the XPR was moved to a significant height above the X-point. Direct measurements of density and temperature from the region around the XPR using the new divertor Thomson scattering system at AUG indicate that the temperature at the location of the XPR remains high (>30eV) and only the region towards the X-point cools down further. In this cold XPR core, the temperature reduces to about 1eV. An XPR is also observed in TCV by the injection of nitrogen as extrinsic impurity. This highlights that the wall material (W for AUG, C for TCV) or machine size does not play a significant role for the existence of the regime. However, the scenario appears to be less stable in TCV. First experiments show the necessity of an active control for the XPR: Depending on the wall conditions and the nitrogen wall storage, the required nitrogen seeding level to achieve an XPR changes. Both, the low temperatures measured radially outside of the radiation zone at AUG, and the lower stability of the XPR regime at TCV with the presence of carbon are consistent with the predictions of a one-dimensional model of the XPR. However, the model would predict the development of the cold XPR core, and significant radiation at the X-point might already exist before reaching this cold temperature solution.

Evidence that density shoulder broadening is dependent on high main-chamber neutral density is presented. Shoulder broadening does notoccur when the sources for main-chamber neutrals are minimized using divertor baffles and wide gaps to the first wall (similar to 3x the densitydecay length). Removing the baffles or reducing the gap to the inner wall both act to increase the density shoulder amplitude in otherwiseidentical TCV discharges. Radial turbulent transport is correlated with shoulder amplitude. (C) Published under an exclusive license by AIP Publishing

High resolution spectroscopy on the Tokamak à Configuration Variable (TCV) divertor plasma provided Doppler broadening measurements to infer the ion and neutral temperature of injected helium gas. This paper presents the Divertor Spectroscopy System’s (DSS) access to He II ion temperature measurements over a broad range, ≈0.5–15 eV, with an uncertainty of i(He II) ≃ T e, making this diagnostic a reliable thermometer along the divertor leg plasma over the wide range of magnetic equilibria and divertor configurations achievable in TCV. A detailed description of the diagnostic hardware, data analysis, and sources of uncertainty is presented.

Nitrogen seeded detachment has been achieved in the tokamak a configuration variable (TCV) in alternative divertor configurations (ADCs), namely X-divertor and X-point target, with and without baffles in H-mode plasmas with high core confinement. Both ADCs show a remarkable reduction in the inter-ELM particle and heat fluxes to the target compared to the standard divertor configuration. 95%–98% of the inter-ELM peak heat flux to the target is mitigated as a synergetic effect of ADCs, baffling, and nitrogen seeded detachment. The effect of divertor geometry and baffles on core-divertor compatibility is investigated in detail. The power balance in these experiments is also investigated to explore the physics behind the observed reduction in heat fluxes in the ADCs.

Multi-spectral imaging of helium atomic emission (HeMSI) has been used to create 2D poloidal maps of T e and n e in TCV’s divertor. To achieve these measurements, TCV’s MANTIS multispectral cameras (Perek et al 2019 Rev. Sci. Instrum. 90 123514) simultaneously imaged four He I lines (two singlet and two triplet) and a He II line (468 nm) from passively present He and He+. The images, which were absolutely calibrated and covered the whole divertor region, were inverted through the assumption of toroidal symmetry to create emissivity profiles and, consequently, line-ratio profiles. A collisional-radiative model (CRM) was applied to the line-ratio profiles to produce 2D poloidal maps of T e and n e . The collisional-radiative modeling was accomplished with the Goto helium CRM code (Zholobenko et al 2018 Nucl. Fusion 58 126006, Zholobenko et al 2018 Technical Report, Goto 2003 J. Quant. Spectrosc. Radiat. Transfer 76 331–44) which accounts for electron-impact excitation (EIE) and deexcitation, and electron–ion recombination (EIR) with He + . The HeMSI T e and n e measurements were compared with co-local Thomson scattering measurements. The two sets of measurements exhibited good agreement for ionizing plasmas: ( 5 eV ⩽ T e ⩽ 60 eV , and 2 × 10 18 m − 3 ⩽ n e ⩽ 3 × 10 19 m − 3 ) in the case of majority helium plasmas, and ( 10 eV ⩽ T e ⩽ 40 eV , 2 × 10 18 m − 3 ⩽ n e ⩽ 3 × 10 19 m − 3 ) in the case of majority deuterium plasmas. However, there were instances where HeMSI measurements diverged from Thomson scattering. When T e ⩽ 10 eV in majority deuterium plasmas, HeMSI deduced inaccurately high values of T e . This disagreement cannot be rectified within the CRM’s EIE and EIR framework. Second, on sporadic occasions within the private flux region, HeMSI produced erroneously high measurements of n e . Multi-spectral imaging of Helium emission has been demonstrated to produce accurate 2D poloidal maps of T e and n e within the divertor of a tokamak for plasma conditions relevant to contemporary divertor studies.

This paper presents the first result using nitrogen-seeded exhaust feedback control of the NII impurity emission front in TCV. The NII emission front position is consistently located below its commonly used CIII counterpart, indicating the NII emission front is representative of a colder plasma region. We demonstrate control of the NII impurity emission front position for two cases: (a) using nitrogen seeding as the sole actuator, and (b) using deuterium fueling as an actuator while injecting a small amount of nitrogen that remains a trace impurity. For sole nitrogen actuation, peak target current density is significantly reduced when the NII emission front approaches the x-point (≈50% for the NII front at the halfway point). When actuating with deuterium, peak target current density is less affected, which is explained by changes in fueling engendering a different scrape-off-layer plasma density. Perturbative (system identification) experiments show that nitrogen actuation induces a stronger, but slower, response of the NII emission front than deuterium actuation. Moving the NII emission front back to the target after pushing it towards the x-point has proven difficult, where both the NII front position and total radiated power do not reach pre-seeding conditions within the discharge time following termination of nitrogen injection. This result highlights the need to account for impurity retention for such seeded discharges in exhaust control strategies.

The Tokamak a Configuration Variable (TCV) has recently been equipped with gas baffles to increase its divertor closure for a broad range of divertor magnetic geometries. First experimental results reported in Reimerdes et al. (2021) demonstrated compatibility with a broad range of divertor magnetic geometries and confirmed the main design constraints of the baffles, in particular an increased divertor neutral pressure. The present article presents a more in-depths analysis and extended experiments of this first baffle assessment on the TCV boundary plasma. It is shown that the divertor neutral pressure increased following the installation of the baffles, as predicted by SOLPS-ITER and SolEdge2D-EIRENE simulations. Varying the divertor closure by changing the position of the plasma showed that the plasma equilibrium designed to assess the baffle effect was not far from the optimal trade-off between plasma plugging and recycling on the baffles. The baffles facilitate access to a partially detached regime in both L- and H-modes. In L-Mode, with the ion ∇ B -drift directed from the X-Point to the plasma core, a reduction of the line-averaged density detachment threshold by approximately 20% is observed at the outer target, while inner strike point detachment is only achieved in the presence of baffles. Multispectral imaging shows that the CIII front moves from the outer target towards the X-Point at a lower line-averaged plasma density, indicating a colder outer leg. In H-mode, the CIII front is generally located near the X-Point between the ELMs, while without baffles, N 2 -seeding is required to move the front up to that location.

Plasma emission of hydrogen and impurities is measured with use of filtered camera imaging in TCV divertor. The measurement technique is discussed in detail including: selection choice of filters, the absolute calibration procedure and the tomographic inversion. The process is applied to an unseeded L-mode density ramp discharge in an unfavorable for H-mode access magnetic field direction with divertor baffles. Obtained poloidal emission profiles are used to split Balmer emission into excitation and recombination parts revealing a recombination emission maximum on the high field side. The optical emission is further studied by selecting radial profiles in cross sections of interest finding an opposite radial shift direction for high and low n Balmer line emission maxima in detached conditions.

Detachment, an important mechanism for reducing target heat deposition, is achieved through reductions in power, particle and momentum; which are induced through plasma–atom and plasma–molecule interactions. Experimental research in how those reactions precisely contribute to detachment is limited. Both plasma–atom as well as plasma–molecule interactions can result in excited hydrogen atoms which emit atomic line emission. In this work, we investigate a new Balmer Spectroscopy technique for Plasma–Molecule Interaction—BaSPMI. This first disentangles the Balmer line emission from the various plasma–atom and plasma–molecule interactions and secondly quantifies their contributions to particle (ionisation and recombination) and power balance (radiative power losses). Its performance is verified using synthetic diagnostic techniques of both attached and detached TCV and MAST-U SOLPS-ITER simulations. We find that H 2 plasma chemistry involving H 2 + and/or H − can substantially elevate the Hα emission during detachment, which we show is an important precursor for Molecular Activated Recombination. An example illustration analysis of the full BaSPMI technique shows that the hydrogenic line series, even Lyα as well as the medium-n Balmer lines, can be significantly influenced by plasma–molecule interactions by tens ofpercent. That has important implications for using atomic hydrogen spectroscopy for diagnosing divertor plasmas.

Triggerless 2/1 neoclassical tearing modes (NTMs), i.e. 2/1 NTMs that originate from unstable safety factor profiles (with positive classical stability index at zero island width, i.e.Delta(t)(0) > 0) and saturate neoclassically under the effect of perturbed bootstrap current, have been observed reproducibly in TCV discharges with a strong near-axis electron cyclotron current drive (ECCD). An unexpected density dependence of the onset of these NTMs is newly observed based on the statistics of many TCV discharges, suggesting a certain range of density within which the NTMs can occur. The range is found to broaden with increasing near-axis ECCD power. Based on a different set of experiments and corresponding interpretative simulations with the modified Rutherford equation (MRE), a simple analytical model for Delta(t)(0) has been developed. This model proves to explain well the observed density dependence of mode onset, resulting from the density dependence of the stability of ohmic plasmas and of the ECCD efficiency. The simulations have also quantified various effects and reproduced self-consistently the entire island width evolution of the triggerless NTM, from the onset as a tearing mode at zero island width to the neoclassical saturation as an NTM. The standard terms of the MRE model used in the paper are relevant for NTMs that are seeded by other mechanisms.

A detailed cross-device investigation on the role of filamentary dynamics in high-density regimes has been performed within the EUROfusion framework, comparing the ASDEX Upgrade (AUG) and TCV tokamaks. Both devices run density ramp experiments at different levels of plasma current, keeping the toroidal field or q 95 constant in order to disentangle the role of the parallel connection length and the current. During the scan at a constant toroidal field, in both devices the scrape-off layer (SOL) profiles tend to develop a clear SOL density shoulder at a lower edge density whenever the current is reduced. Different current behaviour is substantially reconciled in terms of the edge density normalized to the Greenwald fraction. During the scan at constant q 95 AUG exhibits similar behaviour, whereas in TCV no upstream profile modification signature has been observed at lower current levels. The latter behaviour has been ascribed to the lack of target density rollover. The relation between the upstream density profile modification and detachment condition has been investigated. For both devices the relation between blob size and the SOL density e-folding length is found independent of the plasma current, with the observation of a clear increase in blob size and the edge density normalized to a Greenwald fraction. ASDEX Upgrade has also explored filamentary behaviour in the H-mode. The experiments in AUG have focused on the role of neutrals, performing discharges with and without cryogenic pumps, highlighting how high neutral pressure, not only in the divertor but also at the midplane, is needed in order to develop an H-mode SOL profile shoulder in AUG.

Synchrotron radiation observed in a quiescent TCV runaway discharge is studied using filtered camera images targeting three distinct wavelength intervals. Through the tomographic SART procedure the high momentum, high pitch angle part of the spatial and momentum distribution of these relativistic particles is reconstructed. Experimental estimates of the distribution are important for verification and refinement of formation-, decay- and transport-models underlying runaway avoidance and mitigation strategy design. Using a test distribution it is demonstrated that the inversion procedure provides estimates accurate to within a few tens of percent in the region of phase-space contributing most to the synchrotron image. We find that combining images filtered around different parts of the emission spectrum widens the probed part of momentum-space and reduces reconstruction errors. Next, the SART algorithm is used to obtain information on the spatiotemporal runaway momentum distribution in a selected TCV discharge. The momentum distribution is found to relax towards an avalanche-like exponentially decaying profile. Anomalously high pitch angles and a radial profile increasing towards the edge are found for the most strongly emitting particles in the distribution. Pitch angle scattering by toroidal magnetic field ripple is consistent with this picture. An alternative explanation is the presence of high frequency instabilities in combination with the formation of a runaway shell at the edge of the plasma., 18 pages, 13 figures, submitted to Nuclear Fusion

This paper introduces a recent innovation in dealing with non-periodic behavior often referred to as transients in perturbative experiments. These transients can be the result from the unforced response due to the initial condition and other slow trends in the measurement data and are a source of error when performing and interpreting Fourier spectra. Fourier analysis is particularly relevant in system identification used to build feedback controllers and the analysis of various pulsed experiments such as heat pulse propagation studies. The basic idea behind the methodology is that transients are continuous complex-valued smooth functions in the Fourier domain which can be estimated from the Fourier data. Then, these smooth functions can be subtracted from the data such that only periodic components are retained. The merit of the approach is shown in two experimental examples, i.e. heat pulse propagation (core transport analysis) and radiation front movement due to gas puffing in the divertor. The examples show that the quality of the data is significantly improved such that it allows for new interpretation of the results even for non-ideal measurements.

The effect of the upcoming TCV divertor upgrade on the distribution of neutrals and the onset of detachment is studied using 2D transport code simulations. The divertor upgrade is centered around the installation of a gas baffle to form a divertor chamber of variable closure. SOLPS-ITER simulations predict that the baffle geometry selected to be installed in TCV in 2019 increases the divertor neutral density by a factor ∼5 and the neutral compression by one order of magnitude in typical TCV single null, Ohmic heated scenarios (330 kW). The compression increases further with the addition of auxiliary heating systems (1.2 MW). Simulations show that volumetric power losses in the divertor increase giving access to deeper detachment for given upstream densities and heating power. Predictions for observations by various TCV diagnostics, including baratrons, divertor spectrometer and visible camera systems, are presented to guide the experimental verification of the efficiency of the divertor baffles.

In ITER and DEMO, various control objectives related to plasma control must be simultaneously achieved by the plasma control system (PCS), in both normal operation as well as off-normal conditions. The PCS must act on off-normal events and deviations from the target scenario, since certain sequences (chains) of events can precede disruptions. It is important that these decisions are made while maintaining a coherent prioritization between the real-time control tasks to ensure high-performance operation. In this paper, a generic architecture for task-based integrated plasma control is proposed. The architecture is characterized by the separation of state estimation, event detection, decisions and task execution among different algorithms, with standardized signal interfaces. Central to the architecture are a plasma state monitor and supervisory controller. In the plasma state monitor, discrete events in the continuous-valued plasma state are modeled using finite state machines. This provides a high-level representation of the plasma state. The supervisory controller coordinates the execution of multiple plasma control tasks by assigning task priorities, based on the finite states of the plasma and the pulse schedule. These algorithms were implemented on the TCV digital control system and integrated with actuator resource management and existing state estimation algorithms and controllers. The plasma state monitor on TCV can track a multitude of plasma events, related to plasma current, rotating and locked neoclassical tearing modes, and position displacements. In TCV experiments on simultaneous control of plasma pressure, safety factor profile and NTMs using electron cyclotron heating (ECH) and current drive (ECCD), the supervisory controller assigns priorities to the relevant control tasks. The tasks are then executed by feedback controllers and actuator allocation management. This work forms a significant step forward in the ongoing integration of control capabilities in experiments on TCV, in support of tokamak reactor operation. EURATOM 633053 Netherlands Organization for Scientific Research 680.47.436

Recent experiments on Tokamak à Configuration Variable have made significant progress toward partial detachment of the outer divertor in neutral beam heated H-mode plasmas in conventional and alternative divertor configurations. The heating power required to enter H-mode was measured in a range of divertor configurations. It is found that at the core densities most favourable for H-mode access, the L–H threshold power is largely independent of the poloidal flux expansion and major radius of the outer divertor, and in the snowflake minus configuration. A factor 2 reduction in the outer divertor power load was achieved in ELM-free (using a fuelling and nitrogen seeding) and ELMy (using nitrogen seeding) H-mode plasmas. No significant reduction in the outer divertor particle flux was achieved in the ELM-free scenarios, compared with ~30% reduction in the most strongly detached ELMy cases. The poloidal flux expansion at the outer divertor was not found to significantly alter the cooling of the divertor in the ELM-free scenarios.

The research program of the TCV tokamak ranges from conventional to advanced-tokamak scenarios and alternative divertor configurations, to exploratory plasmas driven by theoretical insight, exploiting the device’s unique shaping capabilities. Disruption avoidance by real-time locked mode prevention or unlocking with ECRH was thoroughly documented, using magnetic and radiation triggers. Runaway generation with high-Z noble-gas injection and runaway dissipation by subsequent Ne or Ar injection were studied for model validation. The new 1-MW NBI has expanded the parameter range, now encompassing ELMy H-modes in an ITER-like shape and stationary non-inductive discharges sustained by ECCD and NBCD. In H-mode, the pedestal pressure and plasma stored energy are insensitive to fueling, whereas nitrogen seeding moves the pedestal outwards and increases the stored energy. High fueling at high triangularity is key to accessing the attractive smallELM (type-II) regime. Turbulence is reduced in the core at negative triangularity, consistent with increased confinement and in accord with global gyrokinetic simulations. The GAM, possibly coupled with avalanche events, has been linked with particle flow to the wall in diverted plasmas. Detachment, SOL transport, and turbulence were studied in L- and H-mode in both standard and alternative configurations (snowflake, super-X, and beyond). The detachment process is caused by power “starvation” reducing the ionization source, with volume recombination playing only a minor role. Partial detachment in Hmode is obtained with impurity seeding and is insensitive to divertor geometry. In the attached L-mode phase, increasing the outer connection length reduces the in-out heat-flow asymmetry. A doublet plasma, featuring an internal X-point, was achieved successfully, and a transport barrier was observed in the mantle just outside the internal separatrix. In the near future variable configuration baffles and cryo-pumping will be introduced to investigate the effect of divertor closure on exhaust and performance, and 3.5-MW ECRH and 1-MW NBI heating will be added., The 27th IAEA Fusion Energy Conference

This paper investigates the power loads at the inner and outer divertor targets of attached, Ohmic L-mode, deuterium plasmas in the TCV tokamak, in various experimental situations using an Infrared thermography system. The study comprises variations of the outer divertor leg length and target flux expansion, the plasma current and a reversal of the magnetic field direction. The direct impact of the divertor magnetic geometry on scrape-off layer (SOL) transport -parameterised by the SOL power fall-off length lambda_{q, u}, the divertor spreading factor S u and the in-out power asymmetry- is reported for constant core properties. The in-out power asymmetry increases, either with the divertor leg length, or the target flux expansion. The SOL width lambda_{q, u} scales positively with divertor leg length, with a strength that depends on the field direction and differs between the inner and outer divertor. This implies a parametric dependence of lambda_{q, u} that is not explicitly included in current multi-machine scaling laws. The divertor spreading factor at the target S = S_u f_x , where f_x is the target flux expansion, appears unaffected by changes in the divertor geometry and in the plasma current, is independent of the magnetic field direction and is similar between inner and outer divertor. Possible interpretations of these observations using an ad-hoc analytical purely conductive model for the SOL, by ion drifts or by asymmetric turbulent cross-field transport in the divertor are presented. The observed values of lambda_{q, u} are related to existing L-mode and H-mode scaling laws and to similar studies in other tokamaks. Finally, potential implications of these findings for future larger fusion machines are discussed.

International audience; Edge intrinsic rotation was investigated in Ohmic L-mode discharges on ă the Tokamak a Configuration Variable, scanning the major radial position ă of the X point, R-X. Edge rotation decreased linearly with increasing ă RX, vanishing or becoming countercurrent for an outboard X point, in ă agreement with theoretical expectations. The core rotation profile ă shifted fairly rigidly with the edge rotation, changing the central ă rotation speed by more than a factor of two. Core rotation reversals had ă little effect on the edge rotation velocity. Edge rotation was modestly ă more countercurrent in unfavorable than favorable del B shots.

56th Annual Meeting of the APS Division of Plasma Physics, New Orleans, ă LA, OCT 27-31, 2014; International audience; Recent theoretical work predicts intrinsic toroidal rotation in the ă tokamak edge to depend strongly on the normalized major radial position ă of the X-point. With this motivation, we conducted a series of Ohmic ă L-mode shots on the Tokamak a Configuration Variable, moving the X-point ă from the inboard to the outboard edge of the last closed flux surface in ă both lower and upper single null configurations. The edge toroidal ă rotation evolved from strongly co-current for an inboard X-point to ă either vanishing or counter-current for an outboard X-point, in ă agreement with the theoretical expectations. The whole rotation profile ă shifted roughly rigidly with the edge rotation, resulting in variation ă of the peak core rotation by more than a factor of two. Core rotation ă reversals had little effect on the edge rotation. Edge rotation was ă slightly more counter-current for unfavorable than favorable del B drift ă discharges. (c) 2015 AIP Publishing LLC.

JT-60SA envisions electron cyclotron wall conditioning (ECWC), as wall conditioning method in the presence of the toroidal field to control fuel and impurity recycling and to improve plasma performance and reproducibility. This paper reports on Helium ECWC experiments on TCV in support of JT-60SA operation. Nearly sixty Helium conditioning discharges have been successfully produced in TCV, at a toroidal field BT = 1.3 or 1.54 T, with gyrotrons at 82.7 GHz in X2 mode, mimicking ECWC operation in JT-60SA at the second harmonic of the EC wave. Discharge parameters were tuned in order to (i) minimize the time for the onset of ECWC plasmas, thus minimizing absorption of stray radiation by in-vessel components, (ii) improve discharge homogeneity by extending the discharge vertically and radially, and wall coverage, in particular of inboard surfaces where JT-60SA plasmas will be initiated, (iii) assess the efficiency of He-ECWC to deplete carbon walls from fuel. An optimized combination of vertical and radial magnetic fields, with amplitudes typically 0.1 to 0.6% of that of BT, has been determined, which resulted in lowest breakdown time, improved wall coverage and enhanced fuel removal. A standard ohmic D2-plasma could be then sustained, whereas it would not have been possible without He-ECWC.

Measurements of various plasma parameters at the divertor targets of snowflake (SF) and conventional single-null configurations indicate an enhanced effect of the -->E*-->B drift in the scrape-off layer of plasmas in the SF configuration. Plasma boundary transport simulations using the EMC3-Eirene code show that the poloidal gradients of the kinetic profiles in the vicinity of the null-point of a SF divertor are substantially larger than those of a conventional single-null configuration. These gradients are expected to drive larger -->E*-->B flows in the SF divertor and are thought to be responsible for the formation of the double-peaked particle and heat flux target profiles observed experimentally. Experiments in forward and reversed toroidal magnetic field directions further support this conclusion. The formation of such a double-peaked profiles is enhanced at higher plasma densities and may have beneficial effects on the divertor heat loads since they lead to broader target profiles and lower peak heat fluxes.

One of the key features of the new digital plasma control system installed on the TCV tokamak is the possibility to rapidly design, test and deploy real-time algorithms. With this flexibility the new control system has been used for a large number of new experiments which exploit TCV's powerful actuators consisting of 16 individually controllable poloidal field coils and 7 real-time steerable electron cyclotron (EC) launchers. The system has been used for various applications, ranging from event-based real-time MHD control to real-time current diffusion simulations. These advances have propelled real-time control to one of the cornerstones of the TCV experimental program. Use of the Simulink graphical programming language to directly program the control system has greatly facilitated algorithm development and allowed a multitude of different algorithms to be deployed in a short time. This paper will give an overview of the developed algorithms and their application in physics experiments.

TCV experiments demonstrate the basic power exhaust properties of the snowflake (SF) plus and SF minus divertor configurations by measuring the heat fluxes at each of their four divertor legs. The measurements indicate an enhanced transport into the private flux region and a reduction of peak heat fluxes compared to a similar single null configuration. There are indications that this enhanced transport cannot be explained by the modified field line geometry alone and likely requires an additional or enhanced cross-field transport channel. The measurements, however, do not show a broadening of the scrape-off layer (SOL) and, hence, no increased cross-field transport in the common flux region. The observations are consistent with the spatial limitation of several characteristic SF properties, such as a low poloidal magnetic field in the divertor region and a long connection length to the inner part of the SOL closest to the separatrix. Although this limitation is typical in a medium sized tokamak like TCV, it does not apply to significantly larger devices where the SF properties are enhanced across the entire expected extent of the SOL.

International audience

International audience; The geodesic acoustic mode is a coherently oscillating mode, related to the zonal flows that can regulate turbulence in magnetized toroidal plasmas. Modes possessing geodesic acoustic character have been widely observed in the TCV tokamak. A transition has been observed in the course of a single discharge from a continuum regime to a radially extended single-frequency regime. The mode has been also observed and characterized for the first time in the scrape-off layer, primarily by Langmuir probes, suggesting a particle flow to the edge modulated at the mode frequency. These experimental observations are consistent with nonlinear global gyrokinetic simulations, reported in a companion paper (Merlo et al 2017 Plasma Phys. Control. Fusion). These also suggest a possible coupling with radial avalanche phenomena.

A new modular, digital, distributed feedback control system has been developed and installed to control the TCV plasma. With many more inputs and outputs, it provides the possibility to build control algorithms using far more information on the plasma state than previously possible as well as the ability to control many more actuators, including the multi-megawatt, multi-launcher electron cyclotron heating and current drive system. This paper provides an overview of the new control system, its integration into the TCV systems and its successful application to control the TCV plasma discharge