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ITER is of key importance in the European fusion roadmap as it aims to prove the scientific and technological feasibility of fusion as a future energy source. The EUROfusion consortium of labs within Europe is contributing to the preparation of ITER scientific exploitation and operation and aspires to exploit ITER outcomes in view of DEMO. The paper provides an overview of the major progress obtained recently, carried out in the frame of the new (initiated in 2021) EUROfusion work-package called ' Pr eparation of I TER O peration' (PrIO). The overview paper is directly supported by the eleven EUROfusion PrIO contributions given at the 29th Fusion Energy Conference (16–21 October 2023) London, UK [www.iaea.org/events/fec2023]. The paper covers the following topics: (i) development and validation of tools in support to ITER operation (plasma breakdown/burn-through with evolving plasma volume, new infra-red synthetic diagnostic for off-line analysis and wall monitoring using Artificial Intelligence techniques, synthetic diagnostics development, development and exploitation of multi-machine databases); (ii) R&D for the radio-frequency ITER neutral beam sources leading to long duration of negative deuterium/hydrogen ions current extraction at ELISE and participation in the neutral beam test facility with progress on the ITER source SPIDER, and, the commissioning of the 1 MV high voltage accelerator (MITICA) with lessons learned for ITER; (iii) validation of neutronic tools for ITER nuclear operation following the second JET deuterium–tritium experimental campaigns carried out in 2021 and in 2023 (neutron streaming and shutdown dose rate calculation, water activation and activated corrosion products with advanced fluid dynamic simulation; irradiation of several materials under 14.1 MeV neutron flux etc). [ABSTRACT FROM AUTHOR]

An improvement for the vacuum system of the multidriver radio frequency (RF) prototype negative ion source SPIDER has been developed, to allow operating at high RF power, while minimizing the breakdown probability on the outside of the plasma source. A first-of-its-kind large nonevaporable getter (NEG) pump, based on a modular design of 384 cartridges totaling about 350 kg of ZAO® alloy (composed by Zr-Ti-V-Al) with an installed pumping speed at a room temperature of 330 m3/s for hydrogen, will complement the existing pumping system, based on eight cryogenic pumps and four turbomolecular pumps totaling about 90 m3/s in hydrogen. The vessel pressure during absorption is expected to be between 20 and 40 mPa, while during the getter regeneration, the peak operating pressure will be around 2 Pa. The NEG pump will use an additional vessel module, with integrated thermal shields to protect the in-vessel components during activation and regeneration of the pump, to be carried out at temperatures in the range of 550–600 °C. Integrated thermal analyses were carried out to verify the low heat load on pre-existing in-vessel components with a low limit of acceptable temperature, and to provide boundary conditions for the mechanical verifications of the pump structure. The scenario of cyclic hydrogen load was calculated considering the SPIDER operation modes, the expected gas throughput, and the cumulated load to the pump, to determine the regeneration temperature and auxiliary pumping necessary to make the regeneration duration compatible with the high availability of the system. The upgrade of the auxiliary pumping system is also described, as well as the mitigation of air or water exposure risk during regeneration of the NEG pump. [ABSTRACT FROM AUTHOR]

The requirements of ITER neutral beam injectors (1 MeV, 40 A negative deuterium ion current for 1 h) have never been simultaneously attained; therefore, a dedicated Neutral Beam Test Facility (NBTF) was set up at Consorzio RFX (Padova, Italy). The NBTF includes two experiments: SPIDER (Source for the Production of Ions of Deuterium Extracted from Rf plasma), the full-scale prototype of the source of ITER injectors, with a 100 keV accelerator, to investigate and optimize the properties of the ion source; and MITICA, the full-scale prototype of the entire injector, devoted to the issues related to the accelerator, including voltage holding at low gas pressure. The present paper gives an account of the status of the procurements, of the timeline, and of the voltage holding tests and experiments for MITICA. As for SPIDER, the first year of operation is described, regarding the solution of some issues connected with the radiofrequency power, the source operation, and the characterization of the first negative ion beam. [ABSTRACT FROM AUTHOR]

High current (Ip = 1-1.7 MA) operations in the ohmic reversed field pinch RFX-mod spontaneously access the quasi-single helicity (QSH) regime, in which the magnetic dynamics is dominated by the innermost resonant mode (m = 1, n = -7), the magnetic chaos level is reduced and the internal magnetic field configuration is close to a pure helix. During the QSH state strong electron temperature (Te) gradients can show up, identifying an electron transport barrier (Puiatti 2011 Nucl. Fusion 51 073038). Results and advancements obtained in recent RFX-mod high current campaigns are reported in this paper. The best plasma performances at high Ip (i.e. high power) in QSH regimes have been reached with marginally reversed values of the safety factor q at the edge (-0.01 < qa < 0) when the lowest amplitudes of secondary m = 0, 1 modes are obtained. The QSH magnetic states are not stationary and show back transitions to the multiple helicity regime; however, the total persistency of the QSH is found to increase with the plasma current and at Ip > 1.5MA it exceeds 90% of the plasma current flat-top. The high electron temperature gradients (>2 keVm-1) do not persist throughout the magnetic QSH cycle, as documented by a new high time resolution SXR double-filter multichord system, however, they last 5 to 10 ms, more than the energy confinement time. In the barrier region the profile of the electron heat diffusivity Χe falls around 2-20m² s-1, well below ~40-100m-2 s-1 typical of the outer regions. The experimental evaluation of Χe across the thermal barrier is consistent with a microtearing mode driven heat transport. [ABSTRACT FROM AUTHOR]

In the reversed field pinch RFX-mod at the highest plasma current of 2 MA, when error fields are not effectively feedback controlled, localized thermal loads up to tens of MWm-2 can be produced. The graphite tiles withstand such high power loads, but the high hydrogen retention makes density control extremely difficult. Several wall conditioning techniques have been optimized in the last campaigns, including helium glow discharge cleaning and wall boronization by diborane glow discharges. More recently, lithium conditioning has been applied for the first time in a reversed field pinch by the evaporation technique. The main results are discussed in this paper. Lithization leads to important operational advantages: a significant improvement of the density control is obtained. Densities up to n/nG ≈ 0.5 can be produced in a controlled way. At the same value of input power, plasmas at higher densities can be sustained. However, due to the short particle confinement time, such densities are reached with high rates of gas puffing and the resulting profiles at high density are edge peaked. A lithium multipellet injector, to be applied in order to obtain a more uniform deposition, has been tested. [ABSTRACT FROM AUTHOR]

The reversed field pinch (RFP) is a magnetic configuration alternative to the tokamak that can be considered for a second generation of reactors. In this paper new remarkable results obtained in the RFP experiment RFX-mod are presented, showing that an internal transport barrier delimitates a large fraction of the plasma volume in a RFP when the current is raised to ∼1.5 MA. The formation of this transport barrier is related to a profound, spontaneous modification of the magnetic topology. Due to the occurrence of a saddle node bifurcation the plasma enters in the single helical axis state, which is theoretically known to be more resilient to chaos. This bifurcation is driven by the amplitude of the helical perturbation which dominates the mode spectrum. [ABSTRACT FROM AUTHOR]

The RFX reversed field pinch (RFP) is undergoing major modifications of the load assembly and will be equipped with a large number of external coils for active magnetohydrodynamics mode control. The investigation of plasma parameters is then of paramount importance to study the effects on the plasma of both the new passive structure and the action of the external active coils. The study of turbulence is also very important because of its influence on plasma transport. A system of electrostatic probes is needed, fulfilling very challenging requirements in terms of spatial and temporal resolution, which are characteristic features of RFP plasmas. This system should also be integrated with other in-vessel sensors (magnetic and calorimetrical probes) and with other diagnostics placed outside the vessel. A description of the diagnostic system is presented. The global layout of the probes along the first wall is shown: The system allows a two-dimensional investigation of the plasma; both single and triple electrostatic probes have been constructed. Each sensor is embedded in the graphite tiles which completely cover the vessel. The design of the probes is displayed, describing the original solutions adopted to cope with several technological issues. In particular, due to the high number of signals and to limited space available within the vessel, the probe dimensions were minimized. Since RFX possesses no iron core, care has been devoted to avoid pick-up of stray time-varying magnetic fields. Under particularly high thermal loads onto the wall, due to the absence of limiters and to phase-locked magnetic modes, thermal requirements are provided by a suitable choice of materials, at the same time meeting the necessary standards of electrical insulation. Mechanical tests on the carbon tiles modified for inserting the probes were also needed; thermal and electrical tests on prototypes were performed. [ABSTRACT FROM AUTHOR]

In negative ion electrostatic accelerators a considerable fraction of extracted ions is lost by collision processes causing efficiency loss and heat deposition over the components. Stripping is proportional to the local density of gas, which is steadily injected in the plasma source; its pumping from the extraction and acceleration stages is a key functionality for the prototype of the ITER Neutral Beam Injector, and it can be simulated with the 3D code AVOCADO. Different geometric solutions were tested aiming at the reduction of the gas density. The parameter space considered is limited by constraints given by optics, aiming, voltage holding, beam uniformity, and mechanical feasibility. The guidelines of the optimization process are presented together with the proposed solutions and the results of numerical simulations. [ABSTRACT FROM AUTHOR]