A series of experiments have been executed at JET to assess the efficacy of the newly installed Shattered Pellet Injection (SPI) system in mitigating the effects of disruptions. Issues, important for the ITER disruption mitigation system, such as thermal load mitigation, avoidance of runaway electron formation, radiation asymmetries during thermal quench mitigation, electromagnetic load control and runaway electron energy dissipation have been addressed over a large parameter range. The efficiency of the mitigation has been examined for the various SPI injection strategies. The paper summarises the results from these JET SPI experiments and discusses their implications for the ITER disruption mitigation scheme.

Access to Super H-mode is demonstrated for moderately shaped plasmas in agreement with EPED [Snyder et al., Phys. Plasmas 16, 056118 (2009)] predictions. In particular, Super H-mode is realized in a DIII-D shape that is accessible to the JET tokamak. The reduced triangularity of the JET-compatible shape compared to previous Super H-mode plasma shapes does not prevent deep ascension into the so-called Super H-mode “channel.” Operationally, access is enabled and optimized by delaying the neutral beam power injection and, thus, protracting the L–H transition. In highly shaped DIII-D plasmas, the injection of nitrogen sufficient for the establishment of a radiative divertor is shown to be possible during Super H-mode without pedestal degradation. Due to its increased stored energy and radiative divertor integration capabilities, Super H-mode is a promising candidate as operating regime for JET, ITER, and future fusion reactors.

suppression in ITER high Q DT scenarios since they provide optimum integration features regarding energy and particle confinement, pellet fuelling, radiative divertor operation while eliminating ELM transient power loads and being compatible with low torque input.

Parameters of the post-disruption runaway electron (RE) beam in the low density background plasma achieved after deuterium injection are investigated in DIII-D. The spatially resolved RE energy distribution function is measured for the first time during the RE plateau stage by inverting hard X-ray bremsstrahlung spectra. It has maximum energy up to 20 MeV and a non-monotonic feature at 5-6 MeV observed only in the core of the beam supporting the possibility of kinetic instabilities. Results of Fokker-Plank modelling qualitatively support the formation of the non-monotonic distribution function. The RE current profile is reconstructed for the first time using the spatially resolved RE energy distribution. It is found to be more peaked than the pre-disruption plasma current, with higher internal inductance, suggesting preferential formation of REs in the core plasma or potentially a radially inward motion of REs. The accessed relatively low current (180 kA) RE beam is found to be MHD stable, likely due to its elevated safety factor profile. From this base stable equilibrium, an internal MHD instability is accessed by ramping up the current. The instability leads to a sawtoothlike relaxation of the RE current profile, but drives no RE loss. An internal kink mode proposed as a candidate instability is supported by results of MARS-F modelling. Electron cyclotron emission (ECE) spectrum measured during the low density RE plateau is found to be bifurcated, with a break point at ≈ 100 GHz, suggesting resonant absorption of the ECE at low frequencies.

Effective helium confinement time, τ p * ,He , and its ratio with energy confinement time, τ p * ,He /τ E , are key metrics quantifying the suitability of fusion plasmas for a continuous burn. Comparisons in the DIII-D tokamak of discharges with suppression of edge localized modes (ELMs) by resonant magnetic field perturbations (RMPs) to the corresponding unperturbed ELMy discharges found that both of these metrics were strongly reduced, by a factor of approximately 2, after application of the RMPs. This reduction in τ p * ,He during RMP ELM suppression was observed in the plasma core, edge, and pumping plenum, where higher neutral helium concentration during RMPs was also measured. These findings provide evidence that future devices employing RMP ELM suppression may meet or even exceed the helium exhaust provided by the ELMs themselves, reducing helium ash, and thus maintaining high fusion gain.