The Working Group 2 is dedicated to the production of very intense neutrino beams.
Conventional muon neutrino beams are produced through the decay of pions and kaons generated by a high-energy proton beam hitting targets. These targets need to be thin in order for the generated hadrons to escape out of the target.
The positive or negative mesons thus generated are sign-selected and focused, and respectively defocused, by large-acceptance magnetic lenses into a long evacuated decay tunnel where muon neutrinos/antineutrinos are generated.
The challenge here is the focusing of pions and kaons produced with different energies and different angles by the protons striking the target. The first focusing system was the ‘horn’ proposed in 1961 by Simon Van der Meer1.
Fig1. Van der Meer’s sketch of magnetic horn1
Figure 1 shows the drawing of the horn from Van der Meer’s original paper1. Van der Meer’s horn consists of two metallic conductors shaped as truncated cones, disposed around the cylindrical target hit by the proton beam.
Fig 2. Example of magnetic horn
The magnetic field is confined in the volume between the inner and outer conductors and the field lines are circles around the axis. Depending on the current direction, the field would focus pions of one sign or another. The special shape of the conductors allows maximizing the focusing of pions of different momenta, by statistically taking into account the angle-energy correlation of the hadrons produced by the protons hitting the target.
In the case of positive-charge selection, the neutrino beam typically has a contamination of antineutrinos at the few-per-cent level (from the decay of the residual p–, K– and K0) and contains about 1% of electron neutrinos and antineutrinos coming from three-body kaon and muon decays.
Increasing the neutrino beam intensity using existing facilities is not straightforward especially as this kind of upgrade had not been foreseen during the construction of the facility.
All existing and other planned future facilities use circular accelerators, for which space-charge problems severely limit the achievable beam power as compared to what can be achieved with linear accelerators.
Furthermore, serious safety problems can be encountered due to the significantly higher radiation levels, needing substantially more shielding and protection. The already radioactive environment in these facilities could render their power upgrade challenging or in the worst case impossible. Many other issues would have to be taken into account during the redesign of these facilities in order to be able to go to higher intensities, such as cooling, power supplies, reliability, etc.
1Van der Meer S. 1961. A directive device for charged particles and its use in an enhanced neutrino beam CERN Report 61-07
Source: arXiv:1805.01373v1 [physics.acc-ph] May 2018