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Register a new userNew method for a continuous determination of the spin tune in storage rings and implications for precision experiments
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A new method to determine the spin tune is described and tested. In an ideal planar magnetic ring, the spin tune - defined as the number of spin precessions per turn - is given by $ u_s = gamma G$ (gamma is the Lorentz factor, $G$ the magnetic anomaly). For 970 MeV/c deuterons coherently precessing with a frequency of ~120 kHz in the Cooler Synchrotron COSY, the spin tune is deduced from the up-down asymmetry of deuteron carbon scattering. In a time interval of 2.6 s, the spin tune was determined with a precision of the order $10^{-8}$, and to $1 cdot 10^{-10}$ for a continuous 100 s accelerator cycle. This renders the presented method a new precision tool for accelerator physics: controlling the spin motion of particles to high precision is mandatory, in particular, for the measurement of electric dipole moments of charged particles in a storage ring.
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Precision experiments, such as the search for electric dipole moments of charged particles using storage rings, demand for an understanding of the spin dynamics with unprecedented accuracy. The ultimate aim is to measure the electric dipole moments with a sensitivity up to 15 orders in magnitude better than the magnetic dipole moment of the stored particles. This formidable task requires an understanding of the background to the signal of the electric dipole from rotations of the spins in the spurious magnetic fields of a storage ring. One of the observables, especially sensitive to the imperfection magnetic fields in the ring is the angular orientation of stable spin axis. Up to now, the stable spin axis has never been determined experimentally, and in addition, the JEDI collaboration for the first time succeeded to quantify the background signals that stem from false rotations of the magnetic dipole moments in the horizontal and longitudinal imperfection magnetic fields of the storage ring. To this end, we developed a new method based on the spin tune response of a machine to artificially applied longitudinal magnetic fields. This novel technique, called textit{spin tune mapping}, emerges as a very powerful tool to probe the spin dynamics in storage rings. The technique was experimentally tested in 2014 at the cooler synchrotron COSY, and for the first time, the angular orientation of the stable spin axis at two different locations in the ring has been determined to an unprecedented accuracy of better than $2.8mu$rad.
The Continuous Electron Beam Accelerator Facility (CEBAF) was built with a thermionic electron source and the three original experimental hall lines reflected this. A few years after beam delivery began a parity violation experiment was approved and two polarimeters were installed in the Hall A beam line without consultation with the accelerator physics group. The beam raster system was placed after the new Compton polarimeter, before one accelerator quadrupole and four quadrupoles in the new Moller polarimeter. It was very difficult to meet experimental requirements on envelope functions and raster shape with this arrangement so a member of the accelerator physics group had a sixth quadrupole installed downstream of the Moller polarimeter. All of the parity experiments in Hall A have been run with this still-unsatisfactory configuration. The MOLLER experiment is predicated on achieving a 2% error on a 32 ppb asymmetry. Beam line changes are required to meet the systematic error budget. This paper documents the existing beam line, an interim change which can be accomplished during a annual maintenance down, and the final configuration for MOLLER and subsequent experiments.
This paper reports the first spin tune measurement at high energies (24 GeV and 255 GeV) with a driven coherent spin motion. To maintain polarization in a polarized proton collider, it is important to know the spin tune of the polarized proton beam, which is defined as the number of full spin precessions per revolution. A nine-magnet spin flipper has demonstrated high spin-flip efficiency in the presence of two Siberian snakes [1]. The spin flipper drives a spin resonance with a given frequency (or tune) and strength. When the drive tune is close to the spin tune, the proton spin direction is not vertical anymore, but precesses around the vertical direction. By measuring the precession frequency of the horizontal component the spin tune can be precisely measured. A driven coherent spin motion and fast turn-by-turn polarization measurement are keys to the measurement. The vertical spin direction is restored after turning the spin flipper off and the polarization value is not affected by the measurement. The fact that this manipulation preserves the polarization makes it possible to measure the spin tune during operation of a high energy accelerator.
The paper describes the commissioning of the experimental equipment and the machine studies required for the first spin-filtering experiment with protons at a beam kinetic energy of $49.3,$MeV in COSY. The implementation of a low-$beta$ insertion made it possible to achieve beam lifetimes of $tau_{rm{b}}=8000,$s in the presence of a dense polarized hydrogen storage-cell target of areal density $d_{rm t}=(5.5pm 0.2)times 10^{13},mathrm{atoms/cm^{2}}$. The developed techniques can be directly applied to antiproton machines and allow for the determination of the spin-dependent $bar{p}p$ cross sections via spin filtering.
The unique global feature of COSY is its ability to accelerate, store and manipulate polarized proton and deuteron beams. In the recent past, these beams have been used primarily for precision measurements, in particular in connection with the study of charged particle EDMs (Electric Dipole Moment) in storage rings. The role of COSY as a R&D facility and for initial (static and oscillating) EDM measurements can hardly be overestimated. Unfortunately, as a consequence of the strategic decisions of Forschungszentrum Julich and the subsequent TransFAIR agreement between FZJ and GSI Darmstadt, it is currently planned to stop the operation of COSY by the end of 2024. The various groups working with polarized beams at COSY felt it important to collect information on essential measurements to be performed until the termination of machine operation. These experiments, briefly described in this document along with an estimate of the beam time required, serve as pathfinder investigations toward an EDM storage ring and Spin for FAIR.
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