We report a combined experimental and theoretical investigation of the magnetic structure of the honeycomb-lattice magnet Na${}_{2}$IrO${}_{3}$, a candidate for a realization of a gapless spin liquid. Using resonant x-ray magnetic scattering at the Ir ${\mathrm{L}}_{3}$ edge, we find three-dimensional long-range antiferromagnetic order below ${T}_{N}=13.3$ K. From the azimuthal dependence of the magnetic Bragg peak, the ordered moment is determined to be predominantly along the $a$ axis. Combining the experimental data with first-principles calculations, we propose that the most likely spin structure is a zig-zag structure.

Long-range magnetic ordering in Na 2 IrO 3

Structural Organic Chemistry

The ion storage ring, S-LSR has been constructed to study the beam cooling, especially for the physics of the extreme cold ion beam. The magnetic rigidity is 1 Tm and the circumference is 22.557 m. The lattice of the ring was designed to suppress the beam heating as small as possible. The construction was completed in the summer of 2005 and the beam commissioning using 7 MeV protons was started in October, 2005. The electron cooler at S-LSR has an effective cooling length of 0.44 m (2 % of the circumference). The 7 MeV proton beam was successfully cooled in November, 2005. After the improvements and the optimization of the electron cooler, the maximum cooling force of 0.12 eV/m is achieved with the electron energy of 3.8 keV and the electron current of 56 mA. The estimated electron temperature is 25 μeV. Since February, 2006, the one-dimensional ordering experiment of protons has been performed using the electron cooling. The first proton ordering was successfully confirmed in July, 2006. Abrupt drop in the momentum spread and the Schottky noise power have been observed at a proton number of ∼2000 with electron currents of 25 mA, 50 mA and 100 mA. The transition temperatures of the proton ordering are 0.17 meV and 1 meV in the longitudinal and transverse direction, respectively. The transverse proton temperature is much below the transverse electron temperature, which is the result of the magnetized electron cooling. Below the transition, the longitudinal proton temperature is cooled down to 26 μeV, which is close to the longitudinal electron temperature. The transition temperature of the proton ordering is much lower than those of the highly charged heavy ions at ESR because of the weak Coulomb interaction. Nevertheless, the particle reflection model agrees well with the experimental result of the proton ordering. The reflection probability of the proton at the transition is close to those of the heavy ions. It shows that the one-dimensional ordering of the proton occurs by the same mechanism as the highly charged heavy ions. The beam simulations using the molecular dynamics method also agree well with the experimental results.

/pdf/one-dimensional-beam-ordering-of-protons-at-ion-storage-ring-1ftmxrtfnn.pdf

One-dimensional Beam Ordering of Protons at Ion Storage Ring, S-LSR

The efficiency of the multiturn ion injection with electron cooling depends on two parameters, namely, cooling efficiency and ion lifetime. The lifetime of freshly injected ions is usually shorter than the lifetime of strongly cooled stacked ions. Freshly injected ions are lost in the vertical direction because the vertical acceptance of the synchrotron is usually a few times smaller than the horizontal acceptance. Incoherent vertical losses of freshly injected ions arise from their multiple scattering by residual gas atoms and transverse diffusion caused by stack noise. Reduced ion lifetime limits the multiturn injection efficiency. Analytical estimations and BETACOOL-based numerical evaluations of the vertical ion losses during multiturn injection are presented in comparison with the experimental data obtained at the HIMAC synchrotron and the S-LSR storage ring.

Incoherent vertical ion losses during multiturn stacking cooling beam injection

The motivation for formation of bunched electron beam at the electron cooler of CSRm is based on the three requirements. Firstly, the high energy electron cooling, especially, the ion beam with TeV energy, the bunched electron beam for cooling would be easier than the DC operating mode. Secondly, the electric field induced by the intensity modulated electron beam will be used for the suppression of instability developed in the high intensity ion beam after accumulation with the help of electron cooling, Thirdly, the electron beam was required to turn on and off in the different period of the atomic physics experiments. Some initial design and consideration were presented in this paper. And also the current situation and condition of CSRm electron cooler were described here. An off-line testbench will be established in the laboratory, and the test and the optimization will be explored in this experimentation. The validity of this system will be verified in the near future. The procedure of the modulation on the voltage of control electrode in the electron gun of the CSRm cooler was discussed. The scheme of off-line measurement was devised according to the progress.

FORMATION OF BUNCHED ELECTRON BEAM AT THE ELECTRON COOLER OF CSRm

The electron cooler at S-LSR was designed to maximize the effective cooling length in the limited space. The magnetic field of the cooler was calculated precisely by the 3-D program code and the effective cooling length became 0.44 m while the total length of the cooler is 1.8 m. The electrostatic deflectors are used in the troid to decrease the electron beam loss rate, which is 2x10 -4 now. The commissioning of the electron cooling was started from November 2005. In the first day, the cooling of 7MeV proton beam was observed. The maximum cooling force is 0.12 eV/m with the electron current of 56 mA. The attainable lowest momentum spread of protons is 5x10 -6 at 10 4 protons. Two kinds of the ion beam instabilities were observed. The vertical coherent instability was succeeded to be damped by the feedback and the stored current of 1.2mA was achieved. The maximum current is limited by the initial loss now.

Commissioning of electron beam cooling at s-lsr*

Experimental plan at S-LSR to realize ordered state with ultra-low temperature by application of laser cooling for 24 Mg + ions with kinetic energy of 35 keV has been surveyed. By normal mode with finite dispersion, 1 dimensional ordering and 2 dimensional zigzag structure of crystalline beam are expected if strong enough laser cooling force is applied with resonant coupling method. Three dimensional crystal is expected to be attained by the dispersion free mode with coupling cavity although limited at 1 layer level. With “tapered cooling” utilizing Wien Filter, multi-layer 3 dimensional crystal state is expected to be realized.

Experimental strategy for realization of 3-d beam ordering with use of tapered cooling at s-lsr *

Closed orbit correction will be applied to S-LSR for dispersion-free mode. Gradual transfer from normal mode to dispersion free one is needed and addition of electric field and correction of closed orbit distortion(COD) is shifted step by step to guide the ions through 30 26mm aperture in bending sections. Maximum COD is reduced from 2.5mm to 0.8mm using a proton beam. Based on this result, possible process to realize the dispersion-free mode is simulated. During the process, COD will be kept below 9mm, smaller than vertical aperture of 13mm.

https://epaper.kek.jp/e06/papers/wepch032.pdf

Orbit correction system for s-lsr dispersion-free mode

A compact electron cooler was designed and constructed at Kyoto university. The commissioning of the compact electron cooler of the S-LSR in Kyoto university was carried out with successful observation of both longitudinal and horizontal cooling of a 7 Me V proton beam. By varying the electric potential on the Pierce electrode in the gun, we have investigated the possibility of generating a hollow shaped electron beam, and studied its effect on the electron cooling process. The design of the electron cooler and the results of the first electron cooling experiments will be presented.

/pdf/design-and-commissioning-of-a-compact-electron-cooler-for-593k4if7mh.pdf

Design and Commissioning of a Compact Electron Cooler for the S-LSR

Brief review of experimental studies of the electron cooling of a proton beam at COSY (Juelich, Germany) and S-LSR (Kyoto) are presented. Intensity of the proton beam is limited by two general effects: particle loss directly after the injection and development of instability in a deep cooled ion beam. Results of the instability investigations and the methods of the instability suppression, which allow increasing the cooled beam intensity, are described. Recent attempts of proton beam ordering in both rings are presented and discussed as well. This work is supported by RFBR grant # 05-02-16320 and INTAS grant #03-54-5584.

Electron cooling of proton beam in cosy and s-lsr

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