Table of contents

The application of fast pulse, high intensity lasers to drive low cost DT point neutron sources for fusion materials testing at high flux/fluence is investigated. At present, high power bench-top lasers with intensities of 10¹⁸W/cm² are routinely employed and systems capable of ⩾ 10²¹ W/cm² are becoming available. These potentially offer sufficient energy density for efficient neutron production in DT targets with dimensions of around 100 μm. Two different target concepts are analysed - a hot ion, beam-target system and an exploding pusher target system - and neutron emission rates are evaluated as a function of laser and target conditions. Compared with conventional beam-target neutron sources with steady state liquid cooling, the driver energy here is removed by sacrificial vaporization of a small target spot. The resulting small source volumes offer the potential for a low cost, high flux source of 14 MeV neutrons at close coupled, micro (⩽ 1 mm) test specimens. In particular, it is shown that a laser driven target with ∼100 J/pulse at 100 Hz (i.e. ∼10 kW average power) and laser irradiances in the range Iλ²∼10¹⁷-10¹⁹ W μm²/cm² could produce primary, uncollided neutron fluxes at the test specimen in the 10¹⁴-10¹⁵ n cm^-2 s^-2 range. While focusing on the laser-plasma interaction physics and resulting neutron production, the materials science required to validate computational damage models utilizing ⩾ 100 dpa irradiation of such specimens is also examined; this may provide a multiscale predictive capability for the behaviour of engineering scale components in fusion reactor applications.

Measurement and analysis of the energy distribution of the neutron emission from the nuclear burnup of tritons produced at 1 MeV in d + d → t + p reactions are reported. The results refer to deuterium plasmas with a strongly pulsed neutron production attained with neutral beam heating in the JET tokamak representing both quasi-steady and transient plasma conditions. The measured triton produced neutron spectrum is described with reference to the triton slowing down behaviour in the plasma and pertinent parameter dependences as predicted with a time dependent model. The first study of the triton burnup neutron spectrum for both transient and quasi-steady-state plasma conditions is presented and a description based on classical confinement and slowing down of fast tritons is largely supported.

The merits of a large ion heating fraction for the performance of a tokamak reactor are investigated. The abilities of different ICRF heating scenarios in providing significant ion heating are discussed. The ³He minority scenario is found to be the most suitable scenario for a reactor. Simulations of ITER relevant scenarios suggest that the bulk ion heating fraction is in the range of 70% for most of the heating phase. The benefits of a large ion heating fraction are then discussed in the framework of transport simulations. In particular, a large ion heating fraction is shown to lead to an improved control over the path to ignition. Finally, the implications of a reduced size ITER are discussed. The steady state performance with ³He minority scenario is improved by 30% as compared with pure electron heating schemes.

Resistive neoclassical tearing modes (NTMs) can cause the stable beta value in long pulse, high confinement plasmas to fall significantly below that predicted by ideal theory. The NTM islands which degrade confinement and limit beta are induced and sustained by helically perturbed bootstrap currents. A combination of shaping and q-profile modification is used in the DIII-D tokamak to increase this beta limit.

Ignition conditions in axially magnetized cylindrical targets are investigated by examining the thermal balance of assembled DT fuel configurations at stagnation. Special care is taken to adequately evaluate the energy fraction of 3.5 MeV alpha particles deposited in magnetized DT cylinders. A detailed analysis of the ignition boundaries in the ρR,T parametric plane is presented. It is shown that the fuel magnetization allows a significant reduction of the ρR ignition threshold only when the condition BR ≳ 6 × 10⁵G cm is fulfilled (B is the magnetic field strength and R is the fuel radius).

The effects of changing beam and plasma species on the edge transport barrier are investigated for ELM-free hot ion H mode discharges from the recent DT experiments on JET. The measured pressure at the top of the pedestal is higher for mixed deuterium and tritium and pure tritium plasmas over and above the level measured in pure deuterium plasmas at the same heating power. The pedestal pressure increases with beam tritium concentration for mixed deuterium-tritium beam injection into deuterium plasmas where the measured edge tritium concentration remains low. Alpha heating plays a significant role in the core of such plasmas, and the possible impact on the edge is discussed together with possible direct isotopic effects. Heuristic models for the transport barrier width are proposed, and used to explore a wider range of edge measurements including full power DD and DT pulses. This analysis supports the plasma current and mass dependence for a barrier width set by the orbit loss of either thermal or fast ions, though it does not unambiguously distinguish between them. The fast ion hypothesis could well account for some of the JET observations, though more theoretical work and direct experimental measurement would be required to confirm this. An ad hoc model for the power loss through the separatrix, P_loss ∝ n_edge² Z_eff,edgeI_p^-1, is proposed based on neoclassical theory, a ballooning limit to the edge gradient and a barrier width set by the poloidal ion gyroradius. Such a model is compared with experimental data from JET. In particular, the model ascribes the systematic difference in loss power between the Mark I and Mark II divertors to the change in the measured Z_eff. This change in Z_eff is consistent with the observed change in impurity production, which is described in some detail, together with a possible explanation provided by the temperature dependence of chemical sputtering.

Alpha particle physics experiments were done on TFTR during its DT run from 1993 to 1997. These experiments utilized several new alpha particle diagnostics and hundreds of DT discharges to characterize the alpha particle confinement and wave-particle interactions. In general, the results from the alpha particle diagnostics agreed with the classical single particle confinement model in MHD quiescent discharges. The alpha loss due to toroidal field ripple was identified in some cases, and the low radial diffusivity inferred for high energy alphas was consistent with orbit averaging over small scale turbulence. Finally, the observed alpha particle interactions with sawteeth, toroidal Alfvén eigenmodes and ICRF waves were approximately consistent with theoretical modelling. What was learned is reviewed and what remains to be understood is identified.