It is important to generate a high-intensity and high-quality neutrino beam in order to conduct experiments like T2K. The KEK/J-PARC neutrino group, in collaboration with domestic and international universities and institutions, has constructed and has been operating the neutrino experimental facility at the Japan Proton Accelerator Resarch Complex (J-PARC) in Tokai Village, Ibaraki. This facility produces the world’s highest intensity neutrino beam. Furthermore, enhancing its performance is one of the main research objectives in our group.

Overview of the neutrino beamline

Protons accelerated up to 30 GeV in the Main ring accelerator (MR) at J-PARC are extracted, transported, and injected into the neutrino production target to generate a neutrino beam. The components of the facility will be introduced in the following.

Primary proton beamline

The protons extracted from the MR are transported towards the Kamoika direction, bending the proton beam trajectory approximately 90 degrees using a superconducting magnet system (SC), and impinged onto the neutrino production target. The SC consists of 28 magnet units, which are combined-function magnets with dipole and quadrupole magnetic fields, each with a length of 3.3 m, a maximum dipole magnetic field of 2.6 T, and a maximum quadrupole field gradient of 18.6 T/m. The superconducting magnets are cooled by a helium cryogenic system with a cooling capacity of 2kW and an absolute temperature of 4.5 degrees. The preparation section between the MR extraction point and the SC, and the section before the target (the final focusing section) utilize a normal-conductiong magnet system.

Proton beam monitors

In the primary proton beamline, many beam monitors, including CTs, ESMs, SSEMs, and BLMs, are installed to monitor the intensity, position, profile and loss of the proton beam, respectively. These monitors are essential for transporting the proton beam to the target.

Neutrino production target

The neutrino production target is a cylindrical graphite bar. It is 90 cm in length and 2.6 cm in diameter. Many pions and kaons are generated by striking protons on the target. These pions and kaons decay to neutrinos in flight. The target is housed within a titanium alloy container. Helium gas is circulated between the graphite and the titanium container to cool the heat generated by the proton beam. During operation, the temperature at the center of the target reaches approximately 700 C.

Horn mangets

The electromagnetic horn is a device to focus the pions and kaons towards the forward direction. It consists of a double-walled aluminum pipe. A 2 Tesla magnetic field between the inner and outer pipes is generated by a pulsed current of 320,000 A. Three horns are installed.

Target station building

There is a huge helium container (He vessel) with a volume of 1500 m3 inside the target station building. It is integrated with the decay volume. The He vessel contains the three horns, the target, and a baffle. In order to shield radiation, these components are covered by iron and concrete. The He vessel is filled with helium gas.

Decay volume and beam dump

The pions and kaons forward focused by the horns decay in flight inside a tunnel called the decay volume (DV), located underground, to create the neutrino beam. The DV is a tunnel surrounded by approximately 6 m thick concrete, with water cooling pipes laid inside to dissipate the heat generated by secondary particles. At the downstream end of the DV, a beam dump (BD) utilizing graphite blocks is installed. The BD absorbs protons that did not undergo reactions at the target and other particles.

Muon monitor

The muon monitor is a device to measure muons which are generated along with the neutrinos. The purpose of this monitor is to indirectly monitor the direction and stability of the neutrino beam.

Data acquisition and interlock system

Signals from beam monitors, muon monitors etc. are recorded by a data acquisition system synchronized with the MR extraction. During this process, timing measurements using GPS are also performed to record the timing when the neutrino beam was generated at J-PARC. This information is transmitted in real-time to the far detector, Super-Kamiokande (SK). At SK, neutrino events are recorded based on this timing information.

Furthermore, various interlocks have been implemented in order to ensure the safe operation of the high intensity beam.

Upgrade toward future high power operation

The operation of the neutrino beamline has been stable since 2010. As of 2020, operation with 515kW beam power has been achieved. For future operations, a beam power upgrade to 1300kW is planned. Currently, we are performing work to upgrade various beamline components.