Search

Within the 9th European Framework programme, since 2021 EUROfusion is operating five tokamaks under the auspices of a single Task Force called ‘Tokamak Exploitation’. The goal is to benefit from the complementary capabilities of each machine in a coordinated way and help in developing a scientific output scalable to future largre machines. The programme of this Task Force ensures that ASDEX Upgrade, MAST-U, TCV, WEST and JET (since 2022) work together to achieve the objectives of Missions 1 and 2 of the EUROfusion Roadmap: i) demonstrate plasma scenarios that increase the success margin of ITER and satisfy the requirements of DEMO and, ii) demonstrate an integrated approach that can handle the large power leaving ITER and DEMO plasmas. The Tokamak Exploitation task force has therefore organized experiments on these two missions with the goal to strengthen the physics and operational basis for the ITER baseline scenario and for exploiting the recent plasma exhaust enhancements in all four devices (PEX: Plasma EXhaust) for exploring the solution for handling heat and particle exhaust in ITER and develop the conceptual solutions for DEMO. The ITER Baseline scenario has been developed in a similar way in ASDEX Upgrade, TCV and JET. Key risks for ITER such as disruptions and run-aways have been also investigated in TCV, ASDEX Upgrade and JET. Experiments have explored successfully different divertor configurations (standard, super-X, snowflakes) in MAST-U and TCV and studied tungsten melting in WEST and ASDEX Upgrade. The input from the smaller devices to JET has also been proven successful to set-up novel control schemes on disruption avoidance and detachment.

The successful operation of fusion reactors requires plasma scenarios with good core confinement and acceptable first wall heat loads that are stable and robust to external perturbations. This poses both physical and technological challenges. One of the technologies that addresses these challenges is a complex feedback control system that supports advances in physical understanding and helps to ensure stable operating conditions. The operation of marginally stable plasmas often leads to off-normal events (such as disruptions) and feedback control can prevent these to some extent. This contribution gives an overview of the main results of the development and operation of the feedback control algorithms on ASDEX Upgrade (AUG). Fueling actuators, using a combination of gas valves and pellet injection, can simultaneously control neutral density of the divertor and the density of the plasma core above the Greenwald limit. Impurity injection is employed to control the position of the X-point radiator, allowing the creation of an ELM-suppressed H-mode with high radiation fraction. Heating actuators are used to control the plasma energy content, which supports advanced tokamak experiments and enables stable I-mode operation, and the electron temperature control, which supports turbulence studies. In control technology, AUG has pioneered the use of virtual actuators, which allow effective use of the limited number of heating actuators, adaptive control policies, and exception handling. Such technologies will also be used in ITER. Advanced nonlinear state observers (RAPTOR, RAPDENS) and codes to evaluate the power deposition properties (RABBIT, TORBEAM) are available for routine use in the AUG feedback controllers. Extensive use of the AUG discharge control system further enhances the research capabilities of this machine.

In a future fusion reactor, the main fuelling method will likely rely on the injection of solid hydrogen pellets. Current predictions assume that this goal can be achieved, since being based on a technology which is already largely developed. However, this belief is founded on modelling tools that are usually aligned to the observation made in existing devices and then extrapolated to reactor conditions. This approach needs a sound consideration of its intrinsic restrictions and any observed feature not reproduced by the utilised codes should be applied to check their validation and possibly contribute to their refinement. One specific feature still lacking an explanation of a reasonable and self-consistent mechanism in the current models is the appearance of a phenomenon called striations, which are high frequency variations in the radiation emitted during the pellet ablation process. In order to provide a sound and reliable database for further considerations, a dedicated analysis of this effect has been performed on the mid-size tokamak ASDEX Upgrade. Therefore, such cases have been selected with the relevant signal recorded with sufficient temporal resolution during experiments covering a wide variation of plasma and pellet parameters which are regarded to be potentially influential on the striation pattern. In addition, it was ensured that for any specific case the observed behaviour was reproducible for several individual ablation events under identical conditions. In all cases considered, the observed radiation-intensity variations appear with a typical pattern showing a broad peak of frequencies in the range 50 – 150 kHz. This characteristic unveils a notable resilience against any parameter variation. This new collection of data can now act as firm basis to corroborate future modelling code-validation efforts. In addition, the analysis method can provide a relatively simple way of reviewing future modelling predictions.

Experiments were performed that aimed to control the plasma isotopic mixture by delivering supplemental components into the plasma via pellet injection. Pellets, mm-sized solid bodies of frozen fuel, are either formed from a mixture of different hydrogen isotopes or from deuterium (D) with added elements. Mixed HD pellets were produced reliably and reproducibly providing the required 1:1 ratio of the protium (H) and D, mimicking the situation in a future fusion reactor with D and tritium (T). By applying pellet fuelling, control of the plasma isotope composition was established while gaining access to the reactor-relevant high-density regime. However, the approach was mainly to demonstrate the controlling capability. Yet, experiments provided information on the impact of the isotopic composition on the plasma performance. There is a strong indication that adding H to a D reference plasma causes a reduction of both the particle and energy confinement. The deleterious impact of H on the particle confinement is, apparently, even stronger than on the energy confinement. Furthermore, small amounts of H can cause a loss of energy confinement that is stronger than anticipated by the H98(y,2) scaling, which is usually employed in reactor design studies. In the course of this experiment, pellets were demonstrated to be an effective and powerful tool for detailed investigations of the isotope effect. A first test demonstrated that producing and launching pellets that contain a mixture of different elements is a feasible technique. The latter was achieved by doping a D host pellet despite technical reasons limiting the permissible amount of processed N. [ABSTRACT FROM AUTHOR]

This paper describes the extension of the previous experimental database on density control by pellets and ELM control by RMPs in the ASDEX Upgrade tokamak. A stationary low triangularity plasma is sustained by simultaneous pellet fuelling and RMP ELM control at ion pedestal collisionality of ν*i,ped ∼ 0.5. Refuelling of the RMP induced density pump-out requires a pellet rate of Φpel ∼ 0.073 Paux/Tt,ped. The duration of refuelling of density pump-out can be minimised to about 3 energy confinement times. Pellet fuelling increases or preserves ion pedestal pressure with respect to unfuelled ELM suppressed/mitigated phase. Pellets cause transition from ELM suppression to ELMy regime: At low triangularity, the first pellet triggers a transition to an ELMy phase but subsequent ELMs are not modulated by pellets. At elevated triangularity, initial pellets trigger ELM-like events but in-between pellets ELM suppression is preserved. In later phase, as density increases the plasma transitions to an ELMy phase similar to low triangularity case. [ABSTRACT FROM AUTHOR]

One main programme topic at the ASDEX Upgrade all-metal-wall tokamak is development of a high-density regime with central densities at reactor grade level while retaining high-confinement properties. This required development of appropriate control techniques capable of coping with the pellet tool, a powerful means of fuelling but one which presented challenges to the control system for handling of related perturbations. Real-time density profile control was demonstrated, raising the core density well above the Greenwald density while retaining the edge density in order to avoid confinement losses. Recently, a new model-based approach was implemented that allows direct control of the central density. Investigations focussed first on the N-seeding scenario owing to its proven potential to yield confinement enhancements. Combining pellets and N seeding was found to improve the divertor buffering further and enhance the operational range accessible. For core densities up to about the Greenwald density, a clear improvement with respect to the non-seeding reference was achieved; however, at higher densities this benefit is reduced. This behaviour is attributed to recurrence of an outward shift of the edge density profile, resulting in a reduced peeling-ballooning stability. This is similar to the shift seen during strong gas puffing, which is required to prevent impurity influx in ASDEX Upgrade. First tests indicate that highly-shaped plasma configurations like the ITER base-line scenario, respond very well to pellet injection, showing efficient fuelling with no measurable impact on the edge density profile. [ABSTRACT FROM AUTHOR]

The impact of lithium (Li) on plasma performance was investigated at the ASDEX Upgrade tokamak, which features a full tungsten wall. Li pellets containing 1.6  ×  1020 Li atoms were launched with a speed of 600 m s−1 to achieve deep penetration into the plasma and minimize the impact on the first wall. Homogeneous transient Li concentrations in the plasma of up to 15% were established. The Li sustainment time in the plasma decreased with an increasing heating power from 150 to 40 ms. Due to the pellet rate being restricted to 2 Hz, no Li pile-up could take place. No significant positive impact on plasma properties, as reported from other tokamak devices, could be found; the Li pellets rather caused a small reduction in plasma energy, mainly due to enhanced radiation. Due to pellet injection, a short-lived Li layer was formed on the plasma-facing components, which lasted a few discharges and led to moderately beneficial effects during plasma start-up. Most pellets were found to trigger type-I ELMs, either by their direct local perturbation or indirectly by the altered edge conditions; however, reliability was less than 100%. [ABSTRACT FROM AUTHOR]

In ITER, pellets are used for ELM pacing and fueling. More importantly, ELM control and in particular control of the first ELM needs to be demonstrated in the non-nuclear phase of ITER during operation in H or He. Whilst D pellets have been established as an ELM control technique in the stationary phase with D target plasmas in devices with C as plasma-facing component, the behavior of other isotopes in non-stationary phases are not so well known. Here, we report on new pellet triggering experiments in ASDEX Upgrade and JET that mimic specific ITER operating scenarios. Both machines are equipped with an all-metal wall; recent investigations have shown that pellet triggering and pacing become more intricate when an all-metal wall surface is employed. In both machines, ELM triggering has been shown to occur after injection of D pellets into D plasmas during extended ELM-free phases, often following the L → H transition. In both devices the pellets are found to induce ELMs under conditions far from the stability boundary for type-I ELMs. Near the L → H transition, induced ELMs in some cases are more likely to have type-III rather than type-I characteristics. Furthermore, in ASDEX Upgrade this study was conducted during L → H transitions in the current ramp-up phase as envisaged for ITER. In addition, the pellet’s ELM trigger potential has been proven in ASDEX Upgrade with a correct isotopic compilation for the non-nuclear phase in ITER, viz. H pellets into either He or H plasmas.Results from this study are encouraging since they have demonstrated the pellets’ potential to provoke ELMs even under conditions that are quite far from the stability boundaries attributed to the occurrence of spontaneous ELMs. However, with the recent change from carbon to an all-metal plasma-facing component, examples have been found in both machines where pellets failed to establish ELM control under conditions where this would be expected and needed. Consequently, a major task of future investigations in this field will be to shed more light on the underlying physics of the pellet ELM triggering process to allow sound predictions for ITER. [ABSTRACT FROM AUTHOR]

A new high speed gas valve was developed for disruption mitigation studies in the tokamak ASDEX Upgrade. The valve was designed to operate inside the vacuum vessel to reduce the time of flight of the injected gas and to prevent dispersion of the gas cloud before the gas reaches the plasma. A spring-driven mechanism was chosen for the valve as it is robust against the high magnetic fields and electromagnetic disturbances inside the vessel. The internal gas reservoir (128 cm 3 ) of the valve, which holds the mitigation gas, is opened within 1.5 ms, and the maximal stroke between the valve plate and nozzle (diameter 13 mm) is 4.5 mm. This allows a peak flow rate of 72 kPam 3 /s after 1 ms which was determined both analytically and numerically. The highest gas velocity (approximately 560 m/s) is reached 0.6 ms after the valve is opened. The gas cloud expands in a pear shape with an opening angle of 49°.

Experiments have been performed at ASDEX Upgrade, aiming to investigate the impact of lithium in an all-metal-wall tokamak and attempting to enhance the pedestal operational space. For this purpose, a lithium pellet injector has been developed, capable of injecting pellets carrying a particle content ranging from 1.82 × 10(19) atoms (0.21 mg) to 1.64 × 10(20) atoms (1.89 mg). The maximum repetition rate is about 2 Hz. Free flight launch from the torus outboard side without a guiding tube was realized. In such a configuration, angular dispersion and speed scatter are low, and a transfer efficiency exceeding 90% was achieved in the test bed. Pellets are accelerated in a gas gun; hence special care was taken to avoid deleterious effects by the propellant gas pulse. Therefore, the main plasma gas species was applied as propellant gas, leading to speeds ranging from 420 m/s to 700 m/s. In order to minimize the residual amount of gas to be introduced into the plasma vessel, a large expansion volume equipped with a cryopump was added into the flight path. In view of the experiments, an optimal propellant gas pressure of 50 bars was chosen for operation, since at this pressure maximum efficiency and low propellant gas flux coincide. This led to pellet speeds of 585 m/s ± 32 m/s. Lithium injection has been achieved at ASDEX Upgrade, showing deep pellet penetration into the plasma, though pedestal broadening has not been observed yet.