Table of contents

Erosion of hydrocarbon films is investigated in a particle-beam experiment employing sources for argon ions and hydrogen atoms. Sputtering by argon ions sets in above a threshold of 58 eV and reaches a yield of 0.5 at an ion energy of 800 eV. Sputtering by argon ions with an additional flux of thermal hydrogen atoms towards the surface occurs above a threshold of 1.3 eV and reaches a yield of about three at an ion energy of 800 eV and a hydrogen atom to argon ion flux ratio of 400. A pronounced dependence of the yield on this flux density ratio is observed. It is proposed that the shift of the threshold energy as well as the change in the absolute yields is due to the process of chemical sputtering: within a collision cascade of the incident ions, broken bonds are instantaneously passivated by the abundant flux of atomic hydrogen. This leads to the formation of hydrocarbon molecules within the ion penetration range, which diffuse to the surface and desorb. This has important implications for the lifetime of plasma wall components in the divertor region of next step nuclear fusion devices.

In inertial fusion energy (IFE) power plant designs, the fuel is a spherical layer of frozen DT contained in a target that is injected at high velocity into the reaction chamber. For direct drive, typically laser beams converge at the centre of the chamber (CC) to compress and heat the target to fusion conditions. To obtain the maximum energy yield from the fusion reaction, the frozen DT layer must be at about 18.5 K and the target must maintain a high degree of spherical symmetry and surface smoothness when it reaches the CC. During its transit in the chamber the cryogenic target is heated by radiation from the hot chamber wall. The target is also heated by convection as it passes through the rarefied fill-gas used to control chamber wall damage by x-rays and debris from the target explosion. This article addresses the temperature limits at the target surface beyond which target uniformity may be damaged. It concentrates on direct drive targets because fuel warm up during injection is not currently thought to be an issue for present indirect drive designs and chamber concepts. Detailed results of parametric radiative and convective heating calculations are presented for direct-drive targets during injection into a dry-wall reaction chamber. The baseline approach to target survival utilizes highly reflective targets along with a substantially lower chamber wall temperature and fill-gas pressure than previously assumed. Recently developed high-Z material coatings with high heat reflectivity are discussed and characterized. The article also presents alternate target protection methods that could be developed if targets with inherent survival features cannot be obtained within a reasonable time span.

The use of mixed(monopole–dipole)-phasing of a set of ion cyclotron range of frequency antennas is potentially useful to optimize tokamak performance. However, recent mixed-phasing experiments on JET, described here, showed undesirable antenna–plasma interactions under certain circumstances. A possible physical mechanism to explain the experimental results is discussed, namely, rf-driven dc parallel currents flowing between adjacent antennas with different phasings can lead to arcing on the antenna with the largest sheath voltage. Means of controlling the interaction are discussed.

Electron cyclotron current drive (ECCD) experiments on the DIII-D tokamak are solidifying the physics basis for localized, off-axis current drive, the goal being to validate a predictive model for ECCD. The ECCD profiles are determined from the magnetic field pitch angles measured by motional Stark effect polarimetry. The measured ECCD switches from the co- to the counter-direction as the toroidal injection angle is varied with a profile width that is in accordance with ray tracing calculations. Tests of electron trapping in low beta plasmas show that the ECCD efficiency decreases rapidly as the deposition is moved off-axis and towards the outboard side of the plasma, but the detrimental effects of electron trapping on the current drive are greatly reduced in high beta plasmas. Overall, the measured ECCD is in good agreement with theoretical calculations using a quasilinear Fokker–Planck code over a wide range of injection angles and plasma parameters.

The confinement physics of the improved H-mode is analysed in detail. It is shown that this scenario can be largely unified with that of the standard H-mode. Transport barriers exist in the early phase of the discharge just after the switch on of the heating. In steady state, however, no clear internal transport barriers can be indentified in the ion temperature profiles. The profiles are then found to be stiff and the improved H-mode follows the same ion temperature scaling as the standard H-mode. The improvement of the confinement against the H-mode scaling is partly due to the dependence of this scaling on the line averaged density, and is partly obtained through density peaking appearing at low densities. It is shown that pre-heating is not essential in reaching the good confinement.

Fusion alpha parameters are calculated for tokamaks with high DT fusion rates using the TRANSP plasma analysis code. Parameters include the fast alpha density n_α, fast alpha pressure normalized to magnetic field energy β_α, and its normalized gradient −R×∇(β_α). The plasma conditions are taken from the plasmas in TFTR and JET with the highest DT fusion rates, and from plasmas in the proposed IGNITOR, FIRE, and ITER-FEAT tokamaks.

List of articles ordered by subject.

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