Electron transport in stellarator fields

In developing the Spiral Line Induction Accelerator, experiments have begun on the transport of a beam drifting through straight and 180° curved beamline segments in the presence of a guide field, a continuously rotating quadrupole field (for strong focusing), and a bending field. The goal is to accelerate an electron beam through multiple induction cavities several times while controlling the beam offset in the presence of energy spread, to minimize beam breakup interaction with the cavity. In preliminary work, the authors measured the charge and current transport of a 900-keV, 600-A electron beam around a 180° bend as a function of the quadrupole focusing gradient. The position of the beam centroid at the bend exit is in good agreement with the calculated position as the bending field is varied. For a guide field of 1870 G, the 90% beam transmission fraction tolerance to bending field mismatch is ±20% for zero quadrupole gradient, broadening to -50% and more than +130% for a gradient of 500 G/cm