During pellet injection in tokamaks, a rapid movement of pellet ablation substance towards the low-field or outward major radius R direction is observed, favoring pellet injection from the high-field-side in order to promote deeper fuel penetration. The motion has been attributed to a vertical curvature and ∇B drift current induced inside the ionized ablated material by the 1/R toroidal field variation. The uncompensated vertical drift current inside the weakly diamagnetic (β < 0.1) ablation cloud will cause charge separation at the boundary. The resulting electrostatic field induces the E × B drift to the largeR side of the torus. The calculated fuel penetration depth is consistent with inside launched pellet experiments on the DIII-D tokamak [J.L. Luxon and L.G. Davis, Fusion Technol. 8, 441 (1985)]. The dependence of the penetration depth with plasma parameters suggests that low-velocity inside launched pellets may provide a unique solution to the refueling problem in larger and hotter machines of the future.

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In this paper, we discuss the use of a 3 rd harmonic component to the drive voltage on the grid of an inductive output tube (IOT). High-efficiency inductive output tubes (IOTs) are typically characterized by efficiencies up to 70 − 75%. However, the achievement of efficiencies greater than 80% would substantially reduce the operating costs of next-generation accelerators. In order to achieve this goal, we consider the addition of a 3 rd harmonic component to the drive signal on the grid of an inductive output tube. The use of a 3 rd harmonic drive component in IOT guns has been considered in order to apply such a gun as the injector [1] of radio frequency linear accelerators (RF linacs). Here we consider that the IOT will be used to provide the rf power to drive RF linacs and apply the 3 rd harmonic with the intention of increasing the efficiency of the RF output of the IOT. We consider a model IOT with a 700 MHz resonant cavity and using an annular beam with a voltage of 30 kV, an average current of 6.67 A yielding a perveance of about 1.3 P. We simulate this IOT using the NEMESIS simulation code [2] which has been successfully validated by comparison with the K5H90W-2 IOT developed by Communications and Power Industries. It is found that the effect of the 3 rd harmonic on the efficiency is greatest when the phase of the 3 rd harmonic is shifted by  radians with respect to the fundamental drive signal and with 3 rd harmonic powers greater than about 50% that of the fundamental drive power. For the present example, we show that efficiencies approaching 86% are possible by this means.

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