Ilmoon Hwang

Top-up operation at Pohang Light Source-II

After three years of upgrading work, PLS-II (S. Shin, Commissioning of the PLS-II, JINST, January 2013) is now successfully operating. The top-up operation of the 3 GeV linear accelerator had to be delayed because of some challenges encountered, and PLS-II was run in decay mode at the beginning in March 2012. The main difficulties encountered in the top-up operation of PLS-II are different levels between the linear accelerator and the storage ring, the 14 narrow gap in-vacuum undulators in operation, and the full energy injection by 3 GeV linear accelerator. Large vertical emittance and energy jitter of the linac were the major obstacles that called for careful control of injected beam to reduce beam loss in the storage ring during injection. The following measures were taken to resolve these problems: (1) The high resolution Libera BPM (see http://www.i-tech.si) was implemented to measure the beam trajectory and energy. (2) Three slit systems were installed to filter the beam edge. (3) De-Qing circuit was applied to the modulator system to improve the energy stability of injected beam. As a result, the radiation by beam loss during injection is reduced drastically, and the top-up mode has been successfully operating since 19th March 2013. In this paper, we describe the experimental results of the PLS-II top-up operation and the improvement plan.

Effects of the frequency detuning in Raman backscattering of infinitely long laser pulses in plasmas

Raman backscattering (RBS) in an infinite homogeneous laser-plasma system was investigated with the three-wave fluid model and averaged particle-in-cell (aPIC) simulations in the nonrelativistic and low temperature regime. It was found that the periodic boundary condition for the electrostatic potential, which is commonly used in an infinite homogeneous plasma, induces a numerical frequency shift of the plasma wave. The initial frequency detuning between the three waves is modified by the frequency shift, leading to a significantly wrong result in the RBS system. An alternative boundary condition based on the Maxwell equation is presented. The aPIC simulations with the modified boundary condition show that the pump depletion level depends sensitively on the frequency mismatch between the three waves. This sensitivity is closely related with the erroneous RBS: the numerical frequency shift is very minor (a few percent of the plasma frequency or less than that) but RBS can be greatly affected even by such a...

Studies and optimization of Pohang Light Source-II superconducting radio frequency system at stable top-up operation with beam current of 400 mA

After three years of upgrading work, the Pohang Light Source-II (PLS-II) is now successfully operating. The final quantitative goal of PLS-II is a top-up user-service operation with beam current of 400 mA to be completed by the end of 2014. During the beam store test up to 400 mA in the storage ring (SR), it was observed that the vacuum pressure around the radio frequency (RF) window of the superconducting cavity rapidly increases over the interlock level limiting the availability of the maximum beam current storing. Although available beam current is enhanced by setting a higher RF accelerating voltage, it is better to keep the RF accelerating voltage as low as possible in the long time top-up operation. We investigated the cause of the window vacuum pressure increment by studying the changes in the electric field distribution at the superconducting cavity and waveguide according to the beam current. In our simulation, an equivalent physical modeling was developed using a finite-difference time-domain code. The simulation revealed that the electric field amplitude at the RF window is exponentially increased as the beam current increases, thus this high electric field amplitude causes a RF breakdown at the RF window, which comes with the rapid increase of window vacuum pressure. The RF accelerating voltage of PLS-II RF system was set to 4.95 MV, which was estimated using the maximum available beam current that works as a function of RF voltage, and the top-up operation test with the beam current of 400 mA was successfully carried out.

Beam emittance measurement for the PLS-II linac

The second-generation Pohang Light Source (PLS-II) has a 100-MeV pre-injector for the 3-GeV linac. A thermionic gun produces an electron charge of 200 pC with a bunch duration of 500 ps by using a 250-ps triggering pulser. At the pre-injector, one of the most important beam parameters for identifying the beam quality is the transverse emittance of electron bunches. Therefore, we measure the beam emittance and the Twiss functions at 100 MeV in order to match the beam optics to the beam transport line and go through it to the storage ring. To measure the transverse emittance, we used a well-known technique, the quadrupole scan, at the pre-injector. The emittances were 0.591 mm-mrad in the horizontal direction and 0.774 mm-mrad in the vertical direction.

Study on Vertical Position Reading Noise from Beam Position Monitor in Pohang Light Source Storage Ring Vacuum Chamber

A sudden step change is observed in the vertical position readings from beam position monitors (BPMs) mounted at the several sector vacuum chambers of Pohang Light Source. To study the source of this sudden step change, we measured the RF transmission scattering matrix (S₂₁) through the pickup electrodes of BPMs mounted at the both ends of the sector vacuum chamber. The measured S₂₁ graph of the sector vacuum chambers suffering sudden step change has a peak in the BPM operation frequency bandwidth. Otherwise that of the other sector vacuum chambers doesn’t have a peak. It is shown by the numerical simulation that the peak found in the BPM operation frequency bandwidth corresponds to the longitudinal harmonic of transverse electric resonance mode.

Enhancement of Electron Beam Generation by Using a Steep Downward Density Gradient

By using a preplasma which has a steep downward density gradient, the electron beam generation by the self-injection scheme was performed. Tens of pC electrons with 69 MeV central peak energy and 20 MeV FWHM energy spread were generated. Also we could observe the enhancement of its reproducibility by using it.

Impedance Analysis of Longitudinal Bunch Shape Measurements at PLS

We measured the longitudinal bunch shape by a streak camera at 2.5 GeV Pohang Light Source (PLS). The impedances estimated by a series R+L model indicate a resistance R=0.96 kΩ, an inductance L=80 nH and a longitudinal impedance | Z/n|eff= 0.53 Ω . The scaling law for the bunch lengthening is expressed as I0.22. The effects of insertion device in the ring on the impedance, particularly the effect of the vertical height of the in-vacuum undulator currently placed in the PLS ring is also presented.

Suppression of the Longitudinal Coupled-Bunch Instabilities by the RF Phase Modulation in the Pohang Light Source

In the 2.5 GeV Pohang Light Source, we have investigated the suppression of the longitudinal coupled instabilities (CBI) caused by higher order modes (HOMs) of RF cavities. At higher beam current than 170 mA the 758 MHz or 1300 MHz HOMs occurred and the beam could be unstable. The longitudinal CBI could be suppressed by modulating the phase of an RF accelerating voltage at a frequency of 2 times the synchrotron oscillation frequency and by adjusting the water temperatures of the RF cavities. The longitudinal beam oscillations measured by streak camera in synchro-scan mode were shown. The experiment results were compared with the macro particle tracking simulation.

Strong-strong beam-beam simulation for a muon-collider ring

We have performed a strong-strong simulation to investigate the effect of a beam-beam interaction on the motion of circulating beams in a 50 GeV×50 GeV muon-collider ring. The phase-slip factor η1 in the muon-collider ring is approximately -1×10-6 so that the synchrotron motion is nearly frozen during the muon-storage time. Thus, the energy variation of a particle in the beam due to the beam-beam interaction accumulates from turn to turn, resulting in an increase in the energy spread of the beam. Through the strong-strong simulation, we have investigated the parametric dependencies of the energy spread and the beam size induced by the beam-beam interaction on various parameters. It is shown that the induced energy spread due to the beam-beam interaction affects the growth of the beam size. It is also shown that compensation of the energy spread by introducing one additional rf cavity in the ring can reduce the growth of the beam size.

Design of a Transverse Ionization-Cooling Channel in Solenoidal Focusing Field

In this study, ionization cooling in solenoidal channels is investigated for a neutrino factory. Newly designed Super-FOFO cooling channels are considered and it is shown that the transverse beam emittance in the cooling channels is reduced by a factor of six. Design and simulation of the cooling channel are performed using the ICOOL program. We describe design concepts and engineering constraints for the cooling channels. The particle losses, cooling performance, bunch lengthening and growth of momentum spread in the cooling channels are examined. It is shown that the cooling channels provide cooling performance satisfying the so-called Palmer, Johnson and Keil (PJK) design parameters for a neutrino factory. Engineering feasibility for superconducting solenoids shows that the super-FOFO cooling channel has a lower value of BJR (magnetic field B at coils × current density J × stress R on the conductor) than several cooling channels that have been designed for a neutrino factory. The simulation result for transverse cooling is also compared with that obtained using an analytical formula. It is shown that the numerical result agrees well with that of the analytical formula.