Josep Maria Fontdecaba

The low density and high temperature at the pedestal in hybrid scenarios on JET provide a unique test of the neo-classical screening in the periphery of the plasma that has previously been observed on JET in such plasmas. Hybrid scenario plasmas have been further developed to aid in the study of temperature gradient screening of high-Z impurities from the core of the plasma via plasma current, toroidal field, gas fuelling, density and isotope variations. The steps required to adapt the pulses from deuterium to deuterium-tritium are discussed and provide some guidance for future experiments that may operate in deuteriumtritium. The optimisation of the scenario allowed for improved bolometry data compared to the Deuterium Tritium Experiment 2 (DTE2) campaign on JET, hence this work provides vital evidence of impurity screening in a high fusion power, deuterium-tritium plasma.

Screening of high-Z (W) impurities from the confined plasma by the temperature gradient at the plasma periphery of fusion-grade H-mode plasmas was demonstrated for the first time in JET with the Be/W wall [Field A R et al , Nucl. Fusion 63(2023) 016028]. Additional experiments have been performed in JET during 2023, including in DT during the DTE3 campaign, to further optimise the impurity screening in such plasmas, as well as our bolometric measurements of the W impurity fluxes between and during edge-localised modes (ELMs). A decrease in plasma current from 2.3MA to 2.1 MA, reduced the electron density and thereby increased the ion temperature at the H-mode pedestal top, resulting in stronger impurity screening behaviour. The scenario was then successfully transferred to operation in DT by increasing the toroidal field, in order to compensate the lower L/H-threshold power in DT compared to D plasmas. Here, results of detailed analysis and modelling of the neo-classical (NC) W transport infour pulses from these experiments are presented, two in D at 2.3MA and 2.1MA plasma current and a matched pulse pair at 2.1MA in D and DT. Using the FACIT code [D Fajardo et al 2023 Plasma Phys. Control. Fusion 65 035021] to model the NC W transport for these more recent pulses, the outward convection just inside the pedestal top found in our earlier study could not be reproduced. Possible reasons for this discrepancy between experimental observations and our modeling results are discussed, including potential deficiencies in our measurement technique and/or incompleteness of the NC transport modelling.

This work presents a systematic study of the impact of neon seeding on pedestal performance and stability in JET-ITER baseline scenario discharges in deuterium at high input power, extending previous JET impurity seeding experiments. A neon-seeding scan of the JET-ITER baseline at I p = 2.5 MA, B T = 2.7 T, q ₉₅ = 3.2 with high triangularity is studied with a neon pedestal concentration up to 1.7% at the pedestal top. As is typical in these discharges, the global confinement is increased approaching unity and the β N parameter reaches up to 2.3 with the increased radiation fraction. A detailed pedestal profile analysis showed that neon seeding reduces the pedestal density while substantially increasing both the electron and ion pedestal temperatures, resulting in an overall ∼50% increase in pedestal pressure compared to unseeded plasmas. Simultaneously, the total pressure pedestal width broadens, mostly due to the increased T e width ( n e broadening has a negligible impact on the total pressure width). The total pressure gradient develops in a way that stabilizes the pedestal against ideal peeling–ballooning (PB) modes. Ideal magnetohydrodynamics (MHD) stability calculations confirm that unseeded pedestals lie close to the PB boundary, while neon-seeded cases move deep into the PB stable domain, consistent with the observed transition to small or no edge-localized mode (ELM) operation. Comparison with the Europed predictions exhibits good qualitative agreement in pedestal height trends, though systematic offsets and the underestimation of pedestal widths highlight the limitations of the current model.

In the paper we present an overview of interpretive modelling of a database of JET-ILW 2021 D-T discharges using the TRANSP code. The main aim is to assess our capability of computationally reproducing the fusion performance of various D-T plasma scenarios using different external heating and D-T mixtures, and to understand the performance driving mechanisms. We find that interpretive simulations confirm a general power-law relationship between increasing external heating power and fusion output, which is supported by absolutely calibrated neutron yield measurements. A comparison of measured and computed D-T neutron rates shows that the calculations’ discrepancy depends on the absolute neutron yield. The calculations are found to agree well with measurements for higher performing discharges with external heating power above ∼20 M W , while low-neutron shots display an average discrepancy of around +40% compared to measured neutron yields. A similar trend is found for the ratio between thermal and beam-target fusion, where larger discrepancies are seen in shots with dominant beam-driven performance. We compare the observations to studies of JET-ILW D discharges, to find that on average the fusion performance is well modelled over a range of heating power, although an increased unsystematic deviation for lower-performing shots is observed. The ratio between thermal and beam-induced D-T fusion is found to be increasing weakly with growing external heating power, with a maximum value of ≳ 1 achieved in a baseline scenario experiment. An evaluation of the fusion power computational uncertainty shows a strong dependence on the plasma scenario type and fusion drive characteristics, varying between ±25% and 35%. D-T fusion alpha simulations show that the ratio between volume-integrated electron and ion heating from alphas is ≲ 10 for the majority of analysed discharges. Alphas are computed to contribute between ∼15% and 40% to the total electron heating in the core of highest performing D-T discharges. An alternative workflow to TRANSP was employed to model JET D-T plasmas with the highest fusion yield and dominant non-thermal fusion component because of the use of fundamental radio-frequency heating of a large minority in the scenario, which is calculated to have provided ∼10% to the total fusion power.

The JET hybrid scenario has been developed from low plasma current carbon wall discharges to the record-breaking Deuterium-Tritium plasmas obtained in 2021 with the ITER-like Be/W wall. The development started in pure Deuterium with refinement of the plasma current, and toroidal magnetic field choices and succeeded in solving the heat load challenges arising from 37 MW of injected power in the ITER like wall environment, keeping the radiation in the edge and core controlled, avoiding MHD instabilities and reaching high neutron rates. The Deuterium hybrid plasmas have been re-run in Tritium and methods have been found to keep the radiation controlled but not at high fusion performance probably due to time constraints. For the first time this scenario has been run in Deuterium-Tritium (50:50). These plasmas were re-optimised to have a radiation-stable H-mode entry phase, good impurity control through edge T_i gradient screening and optimised performance with fusion power exceeding 10 MW for longer than three alpha particle slow down times, 8.3 MW averaged over 5 s and fusion energy of 45.8 MJ.

As part the DTE2 campaign in the JET tokamak, we conducted a parameter scan in T and D-T complementing existing pulses in H and D. For the different main ion masses, type-I ELMy H-modes at fixed plasma current and magnetic field can have the pedestal pressure varying by a factor of 4 and the total pressure changing from β N = 1.0 to 3.0. We investigated the pedestal and core isotope mass dependencies using this extensive data set. The pedestal shows a strong mass dependence on the density, which influences the core due to the strong coupling between both plasma regions. To better understand the causes for the observed isotope mass dependence in the pedestal, we analysed the interplay between heat and particle transport and the edge localised mode (ELM) stability. For this purpose, we developed a dynamic ELM cycle model with basic transport assumptions and a realistic neutral penetration. The temporal evolution and resulting ELM frequency introduce an additional experimental constraint that conventional quasi-stationary transport analysis cannot provide. Our model shows that a mass dependence in the ELM stability or in the transport alone cannot explain the observations. One requires a mass dependence in the ELM stability as well as one in the particle sources. The core confinement time increases with pedestal pressure for all isotope masses due to profile stiffness and electromagnetic turbulence stabilisation. Interestingly, T and D-T plasmas show an improved core confinement time compared to H and D plasmas even for matched pedestal pressures. For T, this improvement is largely due to the unique pedestal composition of higher densities and lower temperatures than H and D. With a reduced gyroBohm factor at lower temperatures, more turbulent drive in the form of steeper gradients is required to transport the same amount of heat. This picture is supported by quasilinear flux-driven modelling using TGLF-SAT2 within Astra. With the experimental boundary condition TGLF-SAT2 predicts the core profiles well for gyroBohm heat fluxes > 15 , however, overestimates the heat and particle transport closer to the turbulent threshold.

We present an overview of results from a series of L–H transition experiments undertaken at JET since the installation of the ITER-like-wall (JET-ILW), with beryllium wall tiles and a tungsten divertor. Tritium, helium and deuterium plasmas have been investigated. Initial results in tritium show ohmic L–H transitions at low density and the power threshold for the L–H transition (P _LH) is lower in tritium plasmas than in deuterium ones at low densities, while we still lack contrasted data to provide a scaling at high densities. In helium plasmas there is a notable shift of the density at which the power threshold is minimum ( n ¯ e , min ) to higher values relative to deuterium and hydrogen references. Above n ¯ e , min (He) the L–H power threshold at high densities is similar for D and He plasmas. Transport modelling in slab geometry shows that in helium neoclassical transport competes with interchange-driven transport, unlike in hydrogen isotopes. Measurements of the radial electric field in deuterium plasmas show that E _r shear is not a good indicator of proximity to the L–H transition. Transport analysis of ion heat flux in deuterium plasmas show a non-linearity as density is decreased below n ¯ e , min . Lastly, a regression of the JET-ILW deuterium data is compared to the 2008 ITPA scaling law.

Experiments in ASDEX Upgrade (AUG) and JET with the ITER-like wall (JET-ILW) are performed to separate the pedestal and core contributions to confinement in H-modes with different main ion masses. A strong isotope mass dependence in the pedestal is found which is enhanced at high gas puffing. This is because the ELM type changes when going from D to H for matched engineering parameters, which is likely due to differences in the inter ELM transport with isotope mass. The pedestal can be matched in H and D plasmas by varying only the triangularity and keeping the engineering parameters relevant for core transport the same. With matched pedestals Astra/TGLF (Sat1geo) core transport simulations predict the experimental profiles equally well for H and D. These core transport simulations show a negligible mass dependence and no gyro-Bohm scaling is observed. However, to match the experimental observations at medium β it is required to take the fast-ion dilution and rotation into account. This is not enough for high β plasmas where for the first time a profile match between H and D plasmas was achieved experimentally. Under these conditions quasilinear modelling with TGLF over predicts the transport in the core of H and D plasmas alike.