Demonstration of Three-dimensional Spiral Injection for the J-PARC Muon g – 2/EDM Experiment

In the J-PARC Muon g-2/EDM experiment, to measure muon anomalous magnetic moment (g-2) and electrical dipole moment (EDM), it is necessary to accumulate 300MeV/c muon beams with a 66 cm diameter region with a 3 T solenoid-type magnetic field. A new three-dimensional spiral injection scheme has been invented to achieve this target. Since this is the first instance to employ this injection scheme, a scale-down experiment with an electron beam of 297 keV/c and storage beam diameter of 24 cm is established at KEK. A simplified storage beam monitor using scintillating fiber has been designed and fabricated to measure the stored beam. The 100 ns width pulsed beam is injected and the signal maintains a few microseconds by the stored beam observed. According to this result, the beam storage is confirmed. The recent result implies that the stored beam deviated from the design orbit and caused betatron oscillations. To measure the beam deviation quantitatively and tune the beam, the storage monitor has been updated. The data from this stored beam monitor are the primary data for considering the conceptual design of the beam monitor for the muon g-2/EDM experiment. This poster will discuss the measurement of beam storage by three-dimensional spiral injection and beam tuning using a scintillating fiber monitor.

1. INTRODUCTION 1.1.J-PARC muon g-2/EDM experiment The muon anomalous magnetic moment (g-2) is one of the physical quantities for which large discrepancies between predictions by the Standard Model of particle physics and experimental measurements have been reported [1].In addition, the electric dipole moment (EDM) of elementary particles is greatly suppressed in the Standard Model, with values O 10 −42 e • cm.If EDM with larger values is observed, it will be evidence of CP -violation in the lepton sector.The J-PARC muon g-2/EDM experiment (E34) [2] aims to verify the muon g-2 anomaly and to search for muon EDM, and preparations are underway to start physics data-taking in JFY2028 to perform simultaneous measurements with a precision of 450 ppb for g-2 and 1.5 × 10 −21 e • cm (90%C.L.) for EDM, respectively.

Demonstration experiment of three-dimensional spiral injection
In the E34 experiment, a muon beam of 300 MeV/c must be injected and stored in a solenoid magnetic field of 3 T.The beam storage region, which is designed to have a magnetic field uniformity of less than 0.1 ppm to surpress g-2 systematic uncertainty has a diameter of 66 cm, and a brand new method, three-dimensional spiral injection [3], will be employed to achieve beam storage in such a compact region.In this technique, the beam is injected in a solenoidal magnetic field and applying vertical kick by the pulsed magnetic field of the kicker system, able to accumulate the beam in a weak focusing magnetic field.
Since this method is the first attempt in the world, a demonstration beamline has been constructed at KEK [4,5].Figure 1 shows an overview of the experimental beamline.This demonstration experiment uses electron beams.Both the beam momentum and the storage solenoid field strength are scaled down from the E34: 297 keV/c and 82.5 × 10 −4 T, respectively.Under these conditions, the cyclotron radius is 12 cm and cyclotron period is 5 ns.The beam storage region has a weak focusing magnetic field as in E34, and it n-value is n = 1.8 × 10 −2 , two orders of magnitude stronger than E34 for easier beam accumulation.The DC beam generated by the electron gun can be switched by the chopper system to a pulsed beam with a pulse width of 100 ns and a repetition rate < 50 Hz (variable).The beam phase space is adjusted by three rotatable quadrupole magnets (Q1, Q2, Q3) to make an XY-coupled beam suitable for injection [6].The beam is deflected by the bending magnets (B1, B2) and injected into the storage magnet from bottom of the storage magnet.The kicker system is installed in the storage magnet.In the beam diagnostics area, beam diagnostics can be performed by measuring the DC beam cross-section.

CONFIRMING BEAM ACCUMULATION
A single scintillation fiber (Sci-Fi) shown in Fig. 2 is installed inside the storage magnet to confirm the beam accumulation by measuring the signal from stores electrons.
Here, a cylindrical coordinate system (R, θ, z) is used to explain the geometrical relationship inside the storage chamber, where R is the distance from the center of the solenoid magnetic field, θ is the azimuthal angle, and z is the position along the solenoid axis.z = 0 cm is the storage plane.
To allow measurement by changing the Sci-Fi position in the R-direction, a rotary inductor was installed at the center of the port, and the Sci-Fi was attached to the tip of it with an offset of 3 mm from the rotation axis.The z-direction length of Sci-Fi (−8.5 cm ≤ z ≤ +8.5 cm) is sufficiently longer than the range of the beam storage region (−6 cm ≤ z ≤ +6 cm) determined  by the weak focusing magnetic field.So, the betatron oscillation in the z-direction does not effect the measured signal.

Inside of storage chamber
The signal shown on the left of Fig. 3 is one of the measurement results.The stored beamderived signal fluctuates over time.This time fluctuation can be derived from the R-directional oscillation of the stored beam.If a beam is accumulated with an R-directional deviation from the designed orbit, it causes eccentric motion due to the distribution of the magnetic field in the storage region, and it generates oscillation in the R-direction.

STORAGE BEAM MONITOR
Measurement results indicate that the center of the stored beam is estimated to be in eccentric motion.This eccentric motion broadens the R-directional distribution of the stored beam and also causes time fluctuation.The stored beam shifts from the designed orbit causing this motion.A storage beam monitor to measure the R-directional distribution of the stored beam and to adjust the beam based on the measurement results is discussed.

Conceptual design
The conceptual design of the storage beam monitor is shown in Fig. 4.
The storage beam monitor detects the beam using six Sci-Fi arrays in the R-direction.To reduce the effect on the stored beam, Sci-Fi with a diameter of 200 µm is used and sparsely aligned in the R-direction.In order to perform the measurement while a kicker device (current peak: 45 A, duration: 140 ns) is operating nearby, the light signal from each Sci-Fi is transmitted by optical fiber to a location far enough outside the vacuum chamber and converted into an electrical signal using a photodetector.Since what we want to measure in this beam monitor is the signal derived from the radial oscillation of the stored beam, the z-directional length of each Sci-Fi is set to −7.5 cm ≤ z ≤ +7.5 cm, which sufficiently covers the z-directional range of the beam storage area determined from the weak focusing magnetic field.The beam monitor is mounted on a copper support rod that is connected to the rotary induction motor at the top

Figure 1 .
Figure1.An overview of the demonstration beamline.The DC beam generated by the electron gun can be switched by the chopper system to a pulsed beam with a pulse width of 100 ns and a repetition rate < 50 Hz (variable).The beam phase space is adjusted by three rotatable quadrupole magnets (Q1, Q2, Q3) to make an XY-coupled beam suitable for injection[6].The beam is deflected by the bending magnets (B1, B2) and injected into the storage magnet from bottom of the storage magnet.The kicker system is installed in the storage magnet.In the beam diagnostics area, beam diagnostics can be performed by measuring the DC beam cross-section.

Figure 2 .
Figure 2. Left: The chamber port at the top of the storage chamber and single Sci-Fi using measurement.Right: Measurement setup.Sci-Fi was inserted into the beam storage region through the port at the top of the vacuum chamber, R = 12 cm axis.