Energy recovery from the spent beam in gyrotrons can enhance their overall efficiency. A technique for converting the rotational motion in the spent beam of a large-orbit gyrotron into axial motion, before leading it into a retarding electrostatic field generated by depressed collectors is proposed. This involves passing the spent beam through a magnetic cusp. A proof-of-principle experiment has been performed which verifies the prediction regarding unwinding of the beam, and its collection at a depressed potential which is higher than would otherwise be possible. The experimental set-up and results are presented. A multi-stage depressed collector for a large-orbit gyrotron has also been designed. Primary electrons representing beamlets of a range of energy values are collected at different depressed potentials after energy sorting. Any turning around of primaries before collection has been eliminated and almost all the back-scattered electrons are re-absorbed either on the same collector or on an adjacent one. A collector efficiency of 70 to 80% has been estimated.
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Singh et al. (1992) studied this question.