(Presented XXXXX; received XXXXX; accepted XXXXX; published online XXXXX) (Dates appearing here are provided by the Editorial Office) The B-dot probe diagnostic suite on the ASDEX Upgrade tokamak has recently been upgraded with a new 125 MHz, 14 Bit resolution digitizer to study ion cyclotron emission (ICE). While classic edge emission from the low field side plasma is often observed, we also measure waves originating from the core with fast fusion protons or beam injected deuterons being possible emission driver. Comparing the measured frequency values with ion cyclotron harmonics present in the plasma places the origin of this emission on the magnetic axis, with the fundamental hydrogen/second deuterium cyclotron harmonic matching the observed values. The actual values range from ~27 MHz at on-axis toroidal field B T = -1.79 T to ~40 MHz at B T = -2.62 T. When the magnetic axis position evolves during this emission, the measured frequency values track the changes in the estimated on-axis cyclotron frequency values. Core ICE is usually a transient event lasting ~100 ms during the neutral beam startup phase. However, in some cases core emission occurs in steady-state plasmas and lasts for longer than 1 s. These observations suggest an attractive possibility of using a non-perturbing ICE-based diagnostic to passively monitor fusion alpha particles at the location of their birth in the plasma core, in deuterium-tritium burning devices such as ITER and DEMO.

A new 1D divertor plasma code, SD1D, has been used to examine the role of recombination, radiation, and momentum exchange in detachment. Neither momentum or power losses by themselves are found to be sufficient to produce a reduction in target ion flux in detachment (flux rollover); radiative power losses are required to a) limit and reduce the ionization source and b) access low-target temperature, T target , conditions for volumetric momentum losses. Recombination is found to play little role at flux rollover, but as T target drops to temperatures around 1eV, it becomes a strong ion sink. In the case where radiative losses are dominated by hydrogen, the detachment threshold is identified as a minimum gradient of the energy cost per ionisation with respect to T target . This is also linked to thresholds in T target and in the ratio of upstream pressure to power flux.A system of determining the detached condition is developed such that the divertor solution at a given T target (or lack of one) is determined by the simultaneous solution of two equations for target ion current one dependent on power losses and the other on momentum. Depending on the detailed momentum and power loss dependence on temperature there are regions of T target where there is no solution and the plasma jumps from high to low T target states. The novel analysis methods developed here provide an intuitive way to understand complex detachment phenomena, and can potentially be used to predict how changes in the seeding impurity used or recycling aspects of the divertor can be utilised to modify the development of detachment.

A reduction of the pedestal pressure with increasing separatrix density over pedestal density (n e sep/n e ped) has been observed in JET. The physics behind this correlation is investigated. The correlation is due to two distinct mechanisms. The increase of n e sep/n e ped till ≈0.4 shifts the pedestal pressure radially outwards, decreasing the peeling-balloning stability and reducing the pressure height. The effect of the position saturates above n e sep/n e ped ≈ 0.4. For higher values, the reduction of the pedestal pressure is ascribed to increased turbulent transport and, likely, to resistive MHD effects. The increase of n e sep/n e ped above ≈0.4 reduces ∇n e /n e, increasing η e and the pedestal turbulent transport. This reduces the pressure gradient and the pedestal temperature, producing an increase in the pedestal resistivity. The work suggests that the increase in resistivity might destabilize resistive balloning modes, further reducing the pedestal stability.

We present the results of GENE gyrokinetic calculations based on a series of JET-ILW type I ELMy H-mode discharges operating with similar experimental inputs but at different levels of power and gas fuelling. We show that turbulence due to electron-temperature-gradient modes (ETGs) produces a significant amount of heat flux in four JET-ILW discharges, and, when combined with Neoclassical simulations, is able to reproduce the experimental heat flux for the two low gas pulses. The simulations plausibly reproduce the high-gas heat fluxes as well, although power balance analysis is complicated by short ELM cycles. By independently varying the normalised temperature gradients ωTe and normalised density gradients ωne around their experimental values, we demonstrate that it is the ratio of these two quantities ηe = ωTe / ωne that determines the location of the peak in the ETG growth rate and heat flux spectra. The heat flux increases rapidly as ηe increases above the experimental point, suggesting that ETGs limit the temperature gradient in these pulses. When quantities are normalised using the minor radius, only increases in ωTe produce appreciable increases in the ETG growth rates, as well as the largest increases in turbulent heat flux which follow scalings similar to that of critical balance theory. However, when the heat flux is normalised to the electron Gyro-Bohm heat flux using the temperature gradient scale length LTe , it follows a linear trend in correspondence with previous work by different authors.