Results are presented from a series of dedicated experiments carried out on JET in tritium, DT, deuterium and hydrogen plasmas to determine the dependence of the H mode power threshold on the plasma isotopic mass. The Pthr ∝ Aeff-1 scaling is established over the whole isotopic range. This result makes it possible for a fusion reactor with a 50:50 DT mixture to access the H mode regime with about 20% less power than that needed in a DD mixture. Results on the first systematic measurements of the power necessary for the transition of the plasma to the type I ELM regime, which occurs after the transition to H mode, are also in agreement with the Aeff-1 scaling. For a subset of discharges, measurements of Te and Ti at the top of the profile pedestal have been obtained, indicating a weak influence of the isotopic mass on the critical edge temperature thought to be necessary for the H mode transition.

The penetration dynamics of the resonant magnetic perturbation (RMP) field is simulated in the full toroidal geometry, under realistic plasma conditions in MAST experiments. The physics associated with several aspects of the RMP penetration-the plasma response and rotational screening, the resonant and non-resonant torques and the toroidal momentum balance-are highlighted. In particular, the plasma response is found to significantly amplify the non-resonant component of the RMP field for some of the MAST plasmas. A fast rotating plasma, in response to static external magnetic fields, experiences a more distributed electromagnetic torque due to the resonance with continuum waves in the plasma. At fast plasma flow (such as for the MAST plasma), the electromagnetic torque is normally dominant over the neoclassical toroidal viscous (NTV) torque. However, at sufficiently slow plasma flow, the NTV torque can play a significant role in the toroidal momentum balance, thanks to the precession drift resonance enhanced, so called superbanana plateau regime.

Sustained ELM mitigation has been achieved using RMPs with a toroidal mode number of n=4 and n=6 in lower single null and with n=3 in connected double null plasmas on MAST. The ELM frequency increases by up to a factor of eight with a similar reduction in ELM energy loss. A threshold current for ELM mitigation is observed above which the ELM frequency increases approximately linearly with current in the coils. A comparison of the filament structures observed during the ELMs in the natural and mitigated stages shows that the mitigated ELMs have the characteristics of type I ELMs even though their frequency is higher, their energy loss is reduced and the pedestal pressure gradient is decreased. During the ELM mitigated stage clear lobe structures are observed in visiblelight imaging of the X-point region. The size of these lobes is correlated with the increase in ELM frequency observed. The RMPs produce a clear 3D distortion to the plasma and it is likely that these distortions explain why ELMs are destabilised and hence why ELM mitigation occurs.

A non-rotating helical magnetic field perturbation grows in the JET plasma when the electron density falls below a minimum value. The growth of this perturbation usually leads to a disruption. The helical phase of the perturbation is reproducible for a given configuration of the applied poloidal and toroidal magnetic fields. Small asymmetries in the applied magnetic fields are shown to cause the growth of the perturbation. Various sources of such field errors in JET have been identified. The existence of a minimum density for a stable plasma can place a severe constraint on the operation of JET. The magnitude of the minimum density depends upon various plasma parameters. Neutral beam injection can be used to reduce the minimum density.

The scaling of the energy confinement in H-mode plasmas with different hydrogenic isotopes (H, D, D-T and T) is investigated in JET. For ELM-free H-modes the thermal energy confinement time τ th is found to decrease weakly with the isotope mass (τ th ~ M-0.25 ± 0.22) whilst in ELMy H-modes the energy confinement time shows practically no mass dependence (τ th ~ M 0.03 ± 0.1). Detailed local transport analysis of the ELMy H-mode plasmas reveals that the confinement in the edge region increases strongly with the isotope mass whereas the confinement in the core region decreases with mass (τ thcore ∝ M-0.16) in approximate agreement with theoretical models of the gyro-Bohm type (τ gB ~ M-0.2).

SOLPS simulations of MAST-U have been carried out to identify in more detail the physics and operational properties of novel divertor configurations such as Super-x divertor (SXD), in particular the physics of detachment. A well diagnosed L-mode discharge from MAST has been utilised to determine L-mode transport coefficients representative for MAST-U L-mode plasmas. Simulations show that under the same core plasma conditions, the MAST-U SXD is strongly detached whilst the conventional divertor (CD) is not (1 eV versus 20 eV at the divertor plate). The detachment and higher power losses (1.6×) in the SXD versus the attached CD lead to a factor of 25 reduction in the target power load and are attributed to changes in radial location of the target. An attached regime can be established for the SXD in L-mode for higher pumping speed and/or heating power. In contrast, the simulation predicts that the MAST-U CD requires 3× higher density or 4× reduced power than the SXD to detach. Comparing two versions of the SXD, each with a different amount of poloidal expansion in the region near the divertor plate, we find that the effect of additional poloidal flux expansion of the SXD on an already detached plasma is small for a change in flux expansion in volume by a factor of 2-3 (target temperature 0.7 eV versus 1.1 eV). The poloidal flux expansion re-arranges the radiation pattern with only a small increase in divertor power losses (1.06×) compared to changing from the CD to SXD topology. By artifically increasing the leakage from the divertor chamber, we confirmed that the tight closure of the divertor region leads to strong increases in neutral density with concomitant power losses.

The JET 2019-2020 scientific and technological programme exploited the results of years of concerted scientific and engineering work, including the ITER-like wall (ILW: Be wall and W divertor) installed in 2010, improved diagnostic capabilities now fully available, a major Neutral Beam Injection (NBI) upgrade providing record power in 2019-2020, and tested the technical & procedural preparation for safe operation with tritium. Research along three complementary axes yielded a wealth of new results. Firstly, the JET plasma programme delivered scenarios suitable for high fusion power and alpha particle physics in the coming D-T campaign (DTE2), with record sustained neutron rates, as well as plasmas for clarifying the impact of isotope mass on plasma core, edge and plasma-wall interactions, and for ITER pre-fusion power operation. The efficacy of the newly installed Shattered Pellet Injector for mitigating disruption forces and runaway electrons was demonstrated. Secondly, research on the consequences of long-term exposure to JET-ILW plasma was completed, with emphasis on wall damage and fuel retention, and with analyses of wall materials and dust particles that will help validate assumptions and codes for design & operation of ITER and DEMO. Thirdly, the nuclear technology programme aiming to deliver maximum technological return from operations in D, T and D-T benefited from the highest D-D neutron yield in years, securing results for validating radiation transport and activation codes, and nuclear data for ITER.

Knowing the ion temperature in the scrape-off layer (SOL) of tokamaks is of great importance for understanding the heat flux to plasma facing components. Few measurements are available for SOL ion temperatures compared with electron temperatures due to the relative complexity of the measurement. Two retarding field energy analysers (RFEAs) have been used in the mega amp spherical tokamak (MAST) to measure the ion temperature at both the midplane and the divertor in ohmic L-mode plasmas with a range of densities. Midplane SOL T i was found to be higher than T e by a factor of about 2 or greater. Divertor T i measurements at the target showed T i ≈ T e having considered the effect of plasma flows on the divertor RFEA measurements.