Variable grid gap for a wider operational space of NBI-heating at ASDEX upgrade

Abstract

A novel extraction system with variable grid gap has been developed for the Neutral Beam Injection (NBI) at ASDEX Upgrade to be able to adjust beam energy and power independently.The ASDEX Upgrade NBI is composed of two beamlines (one operating at 60 kV, one at 93 kV) with four ion sources each, delivering a total heating power up to 20 MW. To achieve low beam divergence which reduces transmission losses in the duct, the system has to be operated, like any NBI, at the perveance optimum Πopt which is proportional to Iext/Uext3/2, (Iext = extracted current, Uext = extraction voltage). The extracted beam power Pext = Iext Uext = Πopt Uext5/2 has therefore a strong dependence on the extraction voltage. If beam energies lower than the nominal values are required, the extracted current has to be reduced accordingly, leading to a drastic reduction of the heating power.The optimum perveance Πopt is specific for each extraction system and defined by its geometry, approximately by the ratio of the grid aperture diameter to the gap between the first and the second grid (extraction gap).We have developed an extraction system with a “variable gap“ between those grids which can be changed in situ and remotely between shots. Varying this gap modifies Πopt which allows to set beam energy and beam current independently in a wide range while maintaining minimum divergence. This extends the operational space of NBI significantly and adds experimental versatility for ASDEX Upgrade.A conventional extraction grid system was modified while maintaining interchangeability with the existing systems and keeping the impact on beam optics minimal. The paper discusses the consequences of such variable gap for beam optics and describes the technical boundary conditions, the final design and prototype tests.