Abstract The work describes the pedestal structure, transport and stability in an effective mass ( A eff ) scan from pure deuterium to pure tritium plasmas using a type I ELMy H-mode dataset in which ...key parameters that affect the pedestal behaviour (normalized pressure, ratio of the separatrix density to the pedestal density, pedestal ion Larmor radius, pedestal collisionality and rotation) are kept as constant as possible. Experimental results show a significant increase of the density at the pedestal top with increasing A eff , a modest reduction in the temperature and an increase in the pressure. The variations in the pedestal heights are mainly due to a change in the pedestal gradients while only small differences are observed in the pedestal width. A clear increase in the pedestal density and pressure gradients are observed from deuterium to tritium. The experimental results suggest a reduction of the pedestal inter-edge localized mode (inter-ELM) transport from deuterium to tritium. The reduction is likely in the pedestal inter-ELM particle transport, as suggested by the clear increase of the pedestal density gradients. The experimental results suggest also a possible reduction of the pedestal inter-ELM heat transport, however, the large experimental uncertainties do not allow conclusive claims on the heat diffusivity. The clear experimental reduction of η e (the ratio between density and temperature gradient lengths) in the middle/top of the pedestal with increasing A eff suggests that there may be a link between increasing A eff and the reduction of electron scale turbulent transport. From the modelling point of view, an initial characterization of the behaviour of pedestal microinstabilities shows that the tritium plasma is characterized by growth rates lower than the deuterium plasmas. The pedestal stability of peeling-ballooning modes is assessed with both ideal and resistive magnetohydrodynamics (MHD). No significant effect of the isotope mass on the pedestal stability is observed using ideal MHD. Instead, resistive MHD shows a clear increase of the stability with increasing isotope mass. The resistive MHD results are in reasonable agreement with the experimental results of the normalized pedestal pressure gradient. The experimental and modelling results suggest that the main candidates to explain the change in the pedestal are a reduction in the inter-ELM transport and an improvement of the pedestal stability from deuterium to tritium.
ITER needs a Disruption Mitigation System (DMS) for protection from the consequences of plasma disruptions during high-power operation. The current DMS for ITER is based on the Shattered Pellet ...Injection (SPI) technology. This works on the basic principle of a cryogenic pipe gun with around a half-meter-long acceleration barrel. It is important to avoid any contact between the pellet and the internal surfaces of the flight line to minimize the risk of premature pellet breakage. Therefore, the internal dimensions of the flight tube of the SPI are relatively large compared to the pellet hence providing a bypass for propellant gas resulting in a significant quantity arriving in the plasma ahead of the pellet. This will cause an undesirable instability in the plasma and compromise the effectiveness of disruption mitigation. The described events appear on a scale of a few milliseconds. One possible solution for this issue could be using a fast-acting shutter in the flight line, which will close after the pellet has passed through to hold back the propellant gas. Due to the environmental requirements, the shutter needs to close an approximately 40 mm aperture within a few ms, the shutter has limited space and limited access and acts under a magnetic field, therefore it should endure a high number of cycles and should perform well in a very demanding situation. This paper describes the optioneering study of the shutter and possible solutions for the demanding criteria. The laboratory testing of the Electromechanical Test Bench and the Prototype design are also detailed in separate papers.
The ITER Disruption Mitigation System (DMS) Support Laboratory at the Centre for Energy Research, Budapest, Hungary aims at the characterization of the de-sublimation, acceleration and shattering of ...the cryogenic pellets, which will be fired upon request into the ITER plasma to mitigate the detrimental consequences of disruptions. The test bench includes a fast-acting valve for pellet acceleration, which releases high-pressure gas on a millisecond time scale. The valve is a new development since no valve on the market existed which would have met the requirements. The electromechanical modelling of the valve actuator has been validated against a laboratory experiment, the final system was built and thoroughly tested on a test bench to characterize the valve properties, and to demonstrate that the system fulfils the requirements.
Abstract An analytical formula systematically predicts the observed frequency of pressure relaxation limit cycle oscillations in the vicinity of the transition to high confinement in four tokamaks ...(JET, ASDEX Upgrade, COMPASS, Globus-M). The experimental dataset spans the widest available range of frequencies, machine sizes and plasma ion species. The machine size dependence is explained by the connection length scale of plasma flows parallel to the magnetic field. The model also explains the observed up-down poloidal current asymmetry and the impact of the plasma ion species mass and charge.
•Revealing trends in pellet acceleration by developing a zero-dimensional model.•The results are compared to the measurements of two different SPI test laboratory.•Low computational demand, thus fast ...design of injection scenarios.•Adiabatic propellant gas expansion without supersonic flow and wave propagation.
Shattered Pellet Injection (SPI) is considered as a method to effectively mitigate the effect of severe disruptions in tokamaks. For the development of SPI technology in ITER, the Centre for Energy Research, in collaboration with H-ion Kft and VTMT Kft, designed, constructed and operates a Disruption Mitigation System (DMS) Support Laboratory. To simulate pellet acceleration, we developed a zero-dimensional model. The model uses the parameters of pellet material, propellant gas characteristics, dimension of the barrel and fast valve (FV), opening characteristics, temperature, and filling pressure of the FV in order to calculate the time dependent pellet position, pellet velocity, FV pressure and barrel pressure during the acceleration process. The goal of the acceleration model is to forecast velocities for parameters where no data is available as of yet. This paper describes the model and compares its results to two significantly different setups: the SPI measurements at the ASDEX Upgrade test laboratory and the ITER DMS Support Laboratory. We show that our model is capable of predicting tendencies and revealing the sticking stress of pellets to the barrel.
•Formation and launch of H and Ne pellets for the ITER DMS system are demonstrated.•Dark (snow) layer formation is observed on the pellet surface.•All pellets can be launched without a mechanical ...punch mechanism.•Pellets can fly straight through the flight tube in ITER to the shattering head.•First shattering results obtained with a laser curtain diagnostic.
A support laboratory for the ITER Disruption Mitigation System has been set up at the Centre for Energy Research, Budapest, Hungary. It consists of a cryogenic pellet injector capable of producing, launching and shattering 19 × 38 and 28.5 × 55 mm (d x L) cryogenic pellets. Up to now more than 300 pellets made of hydrogen, deuterium, neon and mixtures thereof have been formed and launched . Nearly all pellets flew through the 4 m long, 60 mm diameter flight tube, which represents the ITER flight path between the propellant recovery chamber and the shattering head. Initial analysis of the fragment plumes produced by an open shattering head indicate that they extend 3 times further parallel to the shattering plate than perpendicularly to it. The results show that the large pellets needed for the ITER SPI system can be produced and launched.
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Abstract A baseline scenario of deuterium–tritium (D–T) plasma with peripheral high-field-side fuelling pellets has been produced in JET in order to mimic the situation in ITER. The isotope mix ratio ...is controlled in order to target the value of 50%–50% by a combination of tritium gas puffing and deuterium pellet injection. Multiple factors controlling the fuelling efficiency of individual pellets are analysed, with the following findings: (1) prompt particle losses due to pellet-triggered edge-localised modes (ELMs) are detected, (2) the plasmoid drift velocity might be smaller than that predicted by simulation, (3) post-pellet particle loss is controlled by transient phases with ELMs.The overall pellet particle flux normalised to the heat flux is similar to that in previous pellet fuelling experiments in AUG and JET.
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