Tau+: an upgrade to the UCNτ neutron lifetime experiment*

Tau+ is a planned upgrade to the existing UCNτ experiment that measures the free neutron lifetime.  Tau+ will share many of the features of UCNτ, including storing ultracold neutrons (UCN) produced by the LANSCE UCN source in an asymmetric trap closed by a Halbach array of permanent magnets on the bottom and sides and by gravity on the top; using an in situ scintillating detector to count the surviving neutrons after storage for different periods of time; and using large area moveable plastic sheets to remove any loaded UCNs above the gravitationally trappable energy range.  Tau+ will differ by improving the method used to load UCNs into the trap: instead of loading them through a moveable trapdoor in the bottom of the Halbach array, it will load them by lowering them into the trap from above in a moving “elevator” volume.  Simulations show that for a given production rate of UCNs from the LANSCE source, a factor of 9 greater number of UCNs will be loaded into the trap using this technique versus the trap door of UCNτ.  Since the UCNtau results published thus far have been statistically limited in overall precision, we expect an improvement in sensitivity in Tau+ of a factor of 3.  The status of the simulations and engineering design will be presented.