Tokamak experiments are usually made with Deuterium as the single main ion specie. Dedicated isotope effect studies sometimes done with Hydrogen in attempt to understand the effect of isotope mass on confinement. Ultimately, the fusion reactor plasma will be a mixture of two hydrogenic isotopes: Deuterium and Tritium. In view of the forthcoming D/T campaign at JET and future experiments at ITER, series of experiments with H/D mixtures were performed to investigate the effects of operations with multiple isotopes on plasma properties and control.
The initial set of experiments were done in 2016, pre-dominantly at lower values of plasma current and toroidal magnetic field (typically 1.4MA and 1.7T). A combination of H-NBI and D-NBI was used, together with H and D gas dosing to achieve a broad range of effective isotope mass Meff=1.1-1.9. It was observed that the energy confinement is almost independent of the isotope mass at this range, but there are more significant changes close to the transition to pure H or D 1. Isotope composition control was not an issue in these gas fuelled plasmas, with any target isotope concentrations achievable within a single pulse by varying H and D gas dosing rates, also using a real time control. In addition to varying plasma H/D composition by altering the gas injection species, additional experiments were done with pellets injection, to mimic ITER plasma fuelling and isotope ratio control .
Further experiments with isotope mixtures were performed recently, together with more thorough analysis of the initial results. Originally, a combination of H-NBI and D-NBI was envisaged to control the core isotope ratio, but it was found that the core isotope composition closely follows the edge values and is virtually independent of the type of species injected by the neutral beams. That was especially pronounced in the extreme case of pure H gas fuelling and pure D-NBI heating, in which case H/D ratio was still the same in the whole plasma, despite significant spatial separation of the sources of two isotopes . That precludes large particle transport coefficients (D and V, diffusion and convection) and seemed to be inconsistent with the earlier results which showed significance of the particle source for the formation of the peaked plasma density profile at JET . To resolve the issue of the apparently opposite observations, a difference in particle transport between the ion and electron plasma species was suggested , and then confirmed in integrated quasi-linear and non-linear modelling [4-6]. The fast isotope transport effect is ubiquitous in plasmas with dominant ITG turbulence regime and greatly facilitates isotope composition control by quickly harmonizing the isotope ratio everywhere in the plasma and may play an important role in helium ash removal from the plasma core.
Insensitivity of plasma core isotope content to the type of specie injected by NBI has motivated more experiments with H/D mixtures. This time the full capacity of JET D-NBI power was available, which opened the operational space of H/D plasmas previously not explored at JET. Experiment with pure D-NBI and pure H-gas dosing was replicated at much larger plasma current and heating power, demonstrating that the isotope mixing is still strong enough to mitigate the accumulation of the NBI specie in the plasma core even for plasma parameters relevant for high performance DT scenarios (see figure 1).
Strong reduction of the neutron rate in H-rich plasmas heated with D-NBI is caused by the high concentration of hydrogen in the core and therefore a deficit of thermal Deuterium ions for the D-D beam-target fusion reactions (reactions between fast ions injected by D-NBI and thermal ions of the plasma). In otherwise similar T-rich plasma with pure D-NBI heating, i.e. where hydrogen is replaced by tritium, the opposite result is expected – large number of fusion reactions between fast D and thermal T will generate high DT neutron rate and fusion power. 
Isotope control with pellets was once again demonstrated to be efficient for changing the core isotope content, this time at large (3MA) plasma current and much more shallow deposition, results will be discussed at this conference separately 
In addition to the confinement and particle transport studies performed in the H/D mixtures, various methods of measuring plasma isotope composition are being tested and validated, both in the core and in the edge. The edge measurements are done by observing the relative intensity of Balmer lines, either at the plasma periphery or in the glow discharges of penning gauges measuring neutral gas pressure outside plasma. Core composition is derived from either the measured neutron rate or directly from observing the active charge exchange spectra [2, 7].
In summary, JET has had a very successful experience with H/D isotope mixture experiments. Isotope composition control capability was proven sufficient to achieve the desired 50/50 DT ratio in future JET DT campaign. Fast wall content changeover allowed to schedule more of the H/D mixture experiments, interleaved with pure D plasmas. Insensitivity of the core isotope content to the NBI species opened path to the isotope effect studies at plasma parameters which were previously deemed inaccessible at JET. More of these experiments are planned for the near future, for isotope effect studies in H/D mixtures and nearly pure H, as well as for 3-ion ICRH heating schemes studies.
References: 1 D B King et al, to be submitted to Nucl. Fusion ;  M Maslov et al 2018 Nucl. Fusion 58 07602;  T Tala et al, submitted to Nucl Fusion ;  C Bourdelle et al 2018 Nucl. Fusion 58 076028 ;  M Marin et al 2019 Nucl. Fusion manuscript accepted;  M Marin et al, submitted to IAEA FEC-2020;  M Valovic et al 2019 Nucl. Fusion 59 106047;  M Valovic et al, submitted to IAEA FEC-2020;  M Maslov et al, 46th EPS conference on Plasma Physics (8-12 July 2019, Milan, Italy) O5.104 http://ocs.ciemat.es/EPS2019PAP/pdf/O5.104.pdf
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