Pohang Light Source Storage Ring Research Articles

Measurement and Analysis of kHz Orbit Fluctuations in the Pohang Light Source

In this article, we report on the orbit stability in the kHz frequency range of the Pohang Light Source (PLS). Fluctuations of the stored beam positions in the PLS storage ring were measured using a digital beam position monitor (BPM) at a fixed location in the ring. Fourier analysis of the measured beam position revealed fluctuation frequencies in the kHz range. The current ripples of the corrector magnet power supplies (MPSs) are suspected to be the sources of these position fluctuations. We surveyed the current ripples of 140 corrector MPSs in the ring to obtain their frequency distribution. The distribution, weighted by the response-matrix elements for each correctors and the BPM, was found to have a strong correlation with the spectrum of the beamposition fluctuations, which substantiates the suspicion.

Quasi-Isochronous Optics for Sub-Picosecond Bunch Length at Pohang Light Source

A lattice design was developed for a quasi-isochronous storage ring, which may produce a sub-picosecond bunch length at the 2.5 GeV Pohang Light Source (PLS) and the analysis of transverse and longitudinal particle dynamics in the designed ring was performed. Small momentum compaction factors in the PLS storage ring were obtained by varying the dispersion function to be negative in bending magnets. The shortest bunch length in the designed ring was estimated to be less than 1 ps, which is three-tenths that of a normal operation. The effects of the second-order momentum compaction factor on the transverse and longitudinal particle dynamics in the quasi-isochronous ring were investigated. The relationship between the momentum compaction factor and the bunch length, which can be obtained in the quasi-isochronous PLS storage ring was also investigated. Multiparticle tracking simulations were performed to investigate the longitudinal beam instability in the quasi-isochronous ring using a broadband impedance model.

Dependence of Dynamic Aperture on Betatron Tune at 2.5 GeV Pohang Light Source Storage Ring

The Pohang Light Source (PLS) storage ring is a synchrotron light source that has been operated using a 2.5 GeV full energy injection from a linac since October 2002. It is necessary to search for betatron tunes that can realize large dynamic apertures in a 2.5 GeV full energy injection. Accordingly, we investigated dependence of dynamic aperture on the betatron tune to obtain large dynamic apertures in the storage ring by a simulation method. We examined dynamic apertures by a tune survey within a specified tune space with and without machine errors. It is also shown how large are the dynamic apertures in the storage ring recovered are after the correction of a closed orbit distortion. The operating tune is examined from the viewpoint of dynamic apertures obtained by a tune survey.

Impedance Estimations from Bunch Lengthening in the Pohang Light Source

Bunch lengthening in the Pohang Light Source storage ring has been investigated. With the three kinds of impedance models, the impedances of the storage ring are estimated by fitting the solutions of the Haïssinski equation and the measured bunch lengthening. The ring impedances estimated by a series R+L impedance model indicate an inductance of L=13.7±3.5 nH and a resistance of R=743±84 Ω. The behaviors of the measured bunch lengthening are compared with those obtained from the estimated impedances. These results show a good agreement. The threshold beam current is estimated to be 7.4 mA. The scaling law for the bunch lengthening above microwave instability threshold is obtained as σx=0.437I1/3.35. Using the Keil–Schnell criterion, longitudinal broadband impedance of the ring is estimated to be 0.24 Ω.

New Energy Ramping Control System in the Pohang Light Source Storage Ring

A new energy ramping control system is developed to increase electron beam energy from 2.0 GeV to 2.5 GeV in the Pohang Light Source storage ring. The control system shows faster energy ramping and more stable beam characteristics than the previous energy ramping control system that consists of an operator interface workstation, subsystem control computer and machine interface units. The previous system controls current settings of the magnet power supplies in multiple steps with different energy increment rate during the energy ramping. In order to improve synchronization of the magnet power supply systems and beam stabilities during the energy ramping, we developed the energy ramping controller hardware that is controlled using a network of optical fibers. Thus, the new ramping system is able to control more synchronously the magnet power supplies with a constant energy increment rate during the energy ramping. With this reliable synchronization, it is also possible to perform the energy de-ramping step from 2.5 GeV to 2.0 GeV on operation. We present the new energy ramping control system and its improved performance on operation.

Suppression of longitudinal coupled bunch instabilities by LFS in PLS storage ring

Abstract The Pohang Light Source (PLS) storage ring uses a longitudinal feedback system (LFS) that is a bunch-by-bunch feedback system employing the digital electronics and the DSP filter algorithm to cure the longitudinal coupled bunch instabilities due to higher order modes of RF cavities. The LFS is able to suppress all longitudinal instabilities in stored beam below 320 mA at 2 GeV. Above 320 mA, however, transverse instabilities appeared and the LFS ceased to control longitudinal instabilities. The performance of the LFS is very reliable and repeatable. This paper describes on the PLS LFS test results of suppressing the longitudinal instabilities.

On Landau Damping by Bunch Filling Pattern in the Pohang Light Source Storage Ring

The stable beam current of the Pohang Light Source (PLS) is normally limited to about 150 mA despite the design value of 400 mA at 2.0 GeV due to the coupled bunch mode instabilities (CBMIs) which are generated by the interaction between higher order modes of RF cavities and circulating beams. In order to improve this situation, a longitudinal feedback system (LFS) using parallel digital signal processors was installed during the summer maintenance period in 1999. Besides the cure of the CBMIs, this programmable LFS is useful for various beam diagnostics such as the synchrotron oscillation phases and their spreads, and the beam pseudo-spectra that are the beam spectra without beam revolution harmonics. With this bunch-to-bunch diagnostic information, it is found that properly arranged bunch filling patterns can increase the stored beam current significantly without an active longitudinal feedback due to Landau damping by bunch-to-bunch synchrotron frequency spreads.

Longitudinal feedback system kicker for the PLS storage ring

The higher order modes (HOMs) of RF cavities at the Pohang Light Source (PLS) storage ring cause longitudinal coupled bunch mode instabilities. A longitudinal feedback system (LFS) is introduced to cure these instabilities. As a key component of the LFS, there is a single-ridged waveguide-overloaded cavity for a longitudinal bunch-by-bunch LFS kicker. Since the RF frequency is 500 MHz, the bandwidth of this kicker should be wider than 250 MHz to damp any coupled bunch mode. Also, a higher shunt impedance of the kicker is selected to use a lower power amplifier. One aluminum kicker is fabricated, and the bandwidth, HOMs, and the shunt impedance of the kicker are measured with a network analyzer. This kicker has several different features from the LFS kicker for DA/spl Phi/NE. First of all, there are four input/output ports to obtain a wider bandwidth. Second, there is a nose cone to obtain a higher shunt impedance. Third, the symmetric frequency response of the shunt impedance around the central frequency is provided. Finally, low beam coupling power (4 W per port at 400 mA) is obtained. According to the simulation result by the high frequency structure simulator (HFSS) code, we obtained the high shunt impedance of 621.3 /spl Omega/ (transit time factor considered value) and the wide bandwidth of 255 MHz. These are compared with the measured result.

Impedance Measurement for Various Vacuum Components in Pohang Light Source Storage Ring

The Pohang Light Source (PLS) is a 2 GeV, third-generation synchrotron light source which was recently commissioned. Loss factor measurements were carried out for various storage ring vacuum components using the synthetic pulse technique. The measurement was done for bunch lengths (σl) ranging from 7.1 to 30 mm. The measured devices were elliptical and circular photon masks, bellows with elliptical and circular cross sections, ellipse-to-diamond transition, ellipse-to-ellipse transition, beam position monitor chamber, and circular T-piece. Broad-band impedances were obtained by fitting the measured loss factors to broad-band resonator model. The total broad-band impedance of 250 devices was 0.17 \\ohm, which is a small fraction of the total machine impedance budget as expected in the calculation.

Effects of Vibration in Storage Ring of Pohang Light Source*

Effects of vibrations in a storage ring are described. For convenience, vibrational sources are categorized as internal and external sources depending on their locations. Internal sources are likely to cause a random vibration of magnets, and their effects on the motion of a circulating beam are described well by the conventional closed orbit amplification factor. External sources cause all or some magnets to vibrate in a coherent manner, and their effect on the beam motion requires separate treatment. In this paper, we discuss the effects of both internal and external vibrational sources on the motion of a stored beam in a storage ring. The results are applied to the Pohang Light Source (PLS) 2 GeV storage ring, currently under construction in Pohang, Korea. Some recent measurement results of the PLS storage ring floor vibration are also presented.