Tune drifts on the Tevatron front porch

It has long been known that the tunes of the Tevatron drift on the 150 GeV front porch. The drift is observed to have the same time dependence as the drift in the chromaticity. The variation in the chromaticity is due to the change in the b ₂ of the superconducting dipoles, which represents the integrated sextupole moment of the magnet. It is reasonable to assume that the tune drift is due to the feed down from the changing b ₂- Calculations based on this assumption, both here and in earlier attempts to explain the tune drift, show, absent unreasonable assumptions about the closed orbit, that the simple models of the variation of the sextupole moment will not explain the tune drift. An explanation, for both the tune drift, and the tune split observed when the Tevatron was first operated, is proposed. The suggestion is based on the longitudinal variation of the time varying sextupole component of the dipoles and the fact that the dipoles are not perfect sector magnets.