Distinct stages of radio frequency emission at the onset of pedestal collapse in KSTAR H-mode plasmas
Using a high-speed and broadband radio frequency (RF) (0.1–1 GHz) spectrum analyzer developed on the KSTAR tokamak, it is found that several distinct stages of RF emission appear at the pedestal collapse in high confinement discharges. Comparison with 2D electron cyclotron emission (ECE) images has revealed that each stage is related to the instantaneous condition at the outboard mid-plane edge. First, high-harmonic ion cyclotron emissions (ICE) are intensified with the appearance of a non-modal filamentary perturbation in the edge within several tens of microseconds before the collapse. Then, the RF emission becomes broad toward high-frequency range (<500 MHz) at the burst onset of the non-modal filament. During the pedestal collapse initiated by the filament burst, rapid chirping (1–3 μs) appear with additional filament bursts. The strong correlation between the RF spectra and the perturbation structure provides important clues on the stability of edge-localized modes and on the ion dynamics in the plasma boundary.
Plasma wall interactions in ITER
Designing the interface between a thermonuclear plasma and the solid material environment is arguably one of the highest technical challenges of ITER and of the successful development of future fusion power reactors. Carbon-based materials are recognized to have superior thermomechanical properties and do not melt, but they retain high levels of tritium by co-deposition with eroded carbon that could severely constrain plasma operations. A carbon-fibre-composite (CFC) divertor target in ITER would survive long enough only with methods of mitigation/suppression of large edge localised modes (ELMs). Metallic materials, such as tungsten, would avoid the tritium retention issue, but melt layer losses due to large ELMs and disruptions may lead to severe damage and unacceptably short lifetimes. Maintaining plasma purity with high-Z materials remains a concern. A mix of several different plasma-facing materials is proposed in ITER to optimize the requirements of areas with different power and particle flux characteristics (i.e., Be for the first wall, CFC for the divertor strike point tiles and W elsewhere in the divertor). However, this will inevitably lead to cross-material contamination and the formation of material mixtures, whose behaviour remains uncertain and requires further investigation. Some of the most outstanding plasma–wall interaction problems that are still at issue in the design of ITER are briefly discussed in this paper, together with prospects for their resolution, by either R&D or design changes and/or dedicated operation provisions. They include: (i) plasma thermal loads and material damage during type I ELMs and disruptions, and prospects for mitigation, (ii) control of co-deposited tritium inventory in the carbon films, (iii) effects of mixed-materials and (iv) plasma operation with tungsten plasma-facing components. Directions and priorities of future research are proposed to narrow the remaining uncertainties and improve confidence in solutions in these areas.
THE PERFORMANCE OF STEADY DETONATION ENGINES
Propulsion systems driven by steady normal detonations are studied. The practical di‐culties associated with stabilizing a detonation wave are highlighted. The requirement on the freestream total enthalpy is considered in parallel with efiects such as condensation or auto-ignition of the fuel-air mixture and limitations associated with fuel sensitivity to detonation. A criterion for detonation stabilization is formulated and applied to an analytical treatment of a detonation ramjet and a detonation turbojet, which are the respective analogs of the ramjet and the turbojet using detonative combustion. The performance of these engines is evaluated and compared to the ideal ramjet and turbojet models for hydrogen and JP10. A limitation is placed on the maximum total temperature allowed in the combustor, based on material considerations. The results show that steady detonation engines have a small thrust-producing range, due to the requirements for detonation stabilization. Their performance is always lower than that of the conventional ramjet and turbojet because of the total pressure loss across the detonation.