Disruptions represent the highest concern for next-step fusion devices based on the tokamak principle. Active disruption avoidance and off-normal event handling need to be developed in Plasma Control Systems (PCS) to predict and react when the plasma approaches dangerous operational boundaries. EUROfusion programme has put strong emphasis on disruption research, focusing on their mitigation and prevention, as well as on the study of relevant disruption paths, such as H-mode density limits. Future fusion power plants are foreseen to operate at high densities in the high confinement mode (H-mode). At densities close to the Greenwald limit, the plasma can exhibit a back transition from the H-mode to the low confinement mode (L-mode). In addition to confinement degradation, a radiation instability, the MARFE, can develop, showing a poloidally localized volume of "cold" and dense plasma. The onset of this edge radiative instability is a non-negligible issue during plasma current ramp down on devices like ITER and DEMO. After the installation of the baffles in the divertor, the development of the H-mode density limit on TCV has been found to be consistent with the one observed on ASDEX Upgrade (AUG). Similarly, to the framework implemented in this latter, a disruption avoidance tool to handle H-mode density limits has been ported to TCV and integrated in the real-time control system. Such an approach relies on a proximity measure with respect to the operational boundary defined in the H98y,2 vs edge normalized density state space, and allows identifying the energy confinement degradation with increasing density that is associated with approaching the density limit, MARFE formation and disruption. Main concepts and schemes for active avoidance of the density limits will be described with reference to the generic framework implemented in the two machines to handle off-normal events potentially leading to disruption.

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Abstract--In the frame of the TCV Tokamak upgrade, two 84/126 GHz/2 s dual frequency Gyrotrons designed by SPC and KIT and manufactured by THALES MSI will be added to the existing EC-System. The first unit has been delivered to EPFL-SPC and tested. In the commissionning configuration, a matching optics unit (MOU) is connected to the gyrotron window. The RF is then coupled to the HE11 mode of a 63.5mm corrugated waveguide and dissipated in a load procured by CNR after 4m of waveguides and 2 miter bends. Owing to the flexible triode gun design giving the possibility to adjust the pitch angle parameter, the specifications were met at both frequencies. At 84 GHz (TE17,5 mode), a power of 0.930MW was measured in the calorimeter, with a pulse duration of 1.1s. At the high frequency (126 GHz, TE26,7 mode), a power of 1.04 MW was reached for a pulse length of 0.5 s. Accounting for the load reflection and the ohmic losses in the various subcomponents of the transmission line and the tube, it is estimated that the output power at the gyrotron window is in excess of 1MW at both frequencies, with an electronic efficiency of 32% and 34% at 84 GHz and 126 GHz respectively. The gyrotron behaviour is remarkably robust and reproducible, suggesting that the ongoing implementation in the gyrotron control and protection system of an already tested novel collector sweeping modulation scheme will permit the straightforward extension of the pulse length to 2s.

In inertial confinement fusion (ICF) experiments the reaction rate is one of the key parameters to assess the plasma performance and is usually determined by measurements of the 14 MeV neutron emission. As the plasma approaches ignition conditions, however, the 14 MeV neutrons experience significant energy degradation by scattering in the plasma, which makes the number of those unscattered no longer proportional to the fusion rate. Under these conditions, measurements of 17 MeV gamma-rays born from the 105 less likely t(d,g)5He reactions could be used instead. In magnetic confinement fusion (MCF), on the other hand, the plasma is always transparent to neutron propagation, but there is here need to have at least two independent methods to determine the fusion power for nuclear licensing of the power plant. This, similarly to ICF, makes t(d,g)5He gamma-ray measurements a potential choice. In this work, we describe the challenges that must be faced to determine the fusion power from t(d,g)5He gamma-ray measurements in ICF and MCF. These range from the development of absolutely calibrated detectors to aspects of the reaction cross sections and are both tackled by a recently approved project in the context of MCF. Proof of principle experiments in forthcoming deuterium-tritium MCF campaigns will be also discussed.

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The EU 1 MW gyrotron for the ITER Electron Cyclotron Heating and Current Drive (EC H&CD) system has been developed in coordinated efforts from several EU institutions (EGYC) and with support of F4E. In a first experimental campaign the tube has been optimized in short pulse operation and has been conditioned for long pulse operation at the KIT teststand up to 180 s. In a second experimental campaign the tube has been transferred to SPC, EPFL, for operation in a dedicated super-conducting magnet at increased pulse length. This paper presents the main experimental results of the prototype tube achieved at KIT.

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The ASDEX Upgrade (AUG) programme, jointly run with the EUROfusion MST1 task force, continues to enhance significantly the physics base of ITER and DEMO. Here, the full tungsten wall is a key asset for extrapolating to future devices. The high overall heating power, flexible heating mix and comprehensive diagnostic set allows studies ranging from mimicking the scrape-off-layer (SOL) and divertor conditions of ITER and DEMO to investigations low at collisionality testing ITER/DEMO pedestal and core physics. This paper summarises the research in key areas of divertor and edge physics, operation without edge localised modes (ELM), fast ion physics, disruption handling and core transport over the last two years.

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 small-ELM (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 H-mode 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.

Core density profile peaking and electron particle transport have been extensively studied by performing several dimensionally matched collisionality (?*) scans in various plasma operation scenarios on JET and on DIII-D and a 2-point ?*scan in I-mode on C-Mod. The experimental results from the various JET H-mode as well as the DIII-D H-mode scans show that density peaking increases with decreasing ?*. In JET, the NBI particle source is contributing 50-60% to the peaking in plasmas where Te/Ti~1 and at ?*=0.1-0.5 (averaged between r/a=0.3-0.8) independent of ?*. On DIII-D, the density peaking is solely a transport effect at low ?* while the NBI contributes to density peaking around 30-40% at higher ?*. The reason for JET and DIII-D being different with respect to NBI fueling versus inward pinch is under investigation. These dimensionally matched ?* scans give the best data for model validation. TGLF simulations are in excellent agreement with the experimental results with respect to the role of NBI versus inward pinch in JET and higher ?* discharges on DIII-D. GENE predicts larger role for the NBI fueling in JET than observed in experiment but is in good agreement within the DIII-D scan, justifying the use of these models/codes in predicting density peaking. In L-mode plasma conditions, the role of the NBI source is small, typically 10-20% and the electron particle transport coefficients are large. On C-Mod, the I-mode density peaking database indicated that in the I-mode plasmas, there is no ?* dependence in density peaking. This result indicates that particle transport characteristics are more analogous to those of L-mode than H-mode and similar to L-mode ones observed in JET and DIII-D.

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