NOvA will use the existing "Neutrinos at the Main Injector" (NuMI) beam at Fermilab that is currently producing neutrinos for the MINOS experiment. Unlike MINOS, which is located on the centerline of the neutrino beam, NOvA will locate its detector slightly off the centerline. This off-axis location produces a large neutrino flux that peaks at 2 GeV, the energy where oscillations to electron neutrinos is expected to be a maximum. The relative narrowness of the off-axis beam aids in rejecting backgrounds to the electron neutrino appearance search. Currently NuMI is running in a configuration optimized for low energy neutrino production which is optimal for the MINOS experiment. The highest neutrino rates for NOvA are achieved with the distance between the two horns is increased to what is called the medium energy configuration. On-axis, this beam produces a neutrino flux peaked at 7 GeV. Off-axis, the peak location is 2 GeV.
Neutrino event rates as a function of energy and off-axis angle. The NOvA detector will be located at 14 mrad which is shown in red.
Accelerator and NuMI Upgrades
As part of the NOvA project, the accelerator infrastructure and the NuMI beam line will be upgraded to provide higher neutrino intensities than are currently possible in the NuMI beam. With the conclusion of the collider run at Fermilab, the Recycler ring, which is currently used to store anti-protons, can be converted to pre-injector to the Main Injector. This allows the cycle time to be reduced from 2 seconds down to 1.33 seconds, yielding an 80% increase in beam power while only increasing the number of protons per bunch by 10%. This upgrade requires the construction of new transfer lines and additional RF stations in the Recycler and Main Injector. In addition, the NuMI line must be upgraded to handle the higher proton beam powers. This involves construction of a new target, upgrades to the cooling systems and improvements to the primary proton line.
These upgrades will occur during two shutdowns. During the first shutdown in summer/fall of 2010 the work to convert the Recycler ring to a proton storage ring will be completed and work will begin on the NuMI modifications. During a second shutdown in summer/fall of 2011 the NuMI horns will be reconfigured to achieve the optimal beam setting in preparation for the start of the NOvA physics run.
Future upgrades to the NuMI proton intensity are possible. One possibility, "SNuMI" is to use the Accumulator ring to momentum stack additional proton batches from the Booster ring. This would achieve a beam power of 1.2 MW. Another possibility is to replace the Booster ring with a new proton linac. This "Project X" would increase the beam intensity to 2.3 MW.