L.H. Chang

Study of longitudinal coupled-bunch instabilities in the SRRC storage ring

Longitudinal coupled-bunch instabilities in the SRRC 1.3 GeV storage ring have been studied by analyzing the signals picked up from the stripline electrodes and rf power reflected from the cavities. The current dependence of the instability strengths was measured. As a cross check, the bunch phase jitters as a function of beam current has also been measured by using sampling optical oscilloscope. Threshold currents of a few milliamperes have been determined from both measurements. They agree with each other very well. It is also found that the strength of the instabilities can be reduced significantly by randomly distributing electron bunches around the ring.

Successful Operation of the 500 MHz SRF Module at TLS

A project to replace two existing room-temperature radio frequency (RF) cavities by one CESR-III 500 MHz superconducting radio frequency (SRF) module was initiated for the Taiwan Light Source (TLS) synchrotron ring in 1999. The goals are to double the photon flux of the synchrotron light by doubling the electron beam current and to increase the stability of the electron beam by taking the advantage of the ultra-weak high-order modes (HOM) of the SRF cavity. The SRF module has been routinely operated since February 2005. The NSRRC users have benefited from a very high photon flux stability (Δ I0/I0∼ 0.05%) that had never been achieved previously. Here, we report the initial operational experience of the SRF system.

Stabilization of the spectral intensity fluctuations with the higher order mode frequency tuners

A strong excitation of the longitudinal coupled-bunch instabilities, which is suspected to be driven by the TM/sub 011/-like mode of the DORIS-I cavities, was observed at the higher electron beam current in the storage ring of SRRC. Such instability leads to heavy fluctuations of the photon beam intensity in the horizontal plane and therefore restricted the maximal useful beam current for the user experiments. This restriction has been released by replacing the damping antennae with the additional tuners. Here, we report our experiences after one-year routine operations of the main RF cavities with the second tuners at SRRC.

Performances and characteristics of a prototype symmetric hybrid adjustable phase undulator

Abstract In this work, we construct and measure a prototype symmetric Hybrid Adjustable Phase Undulator (HAPU) with nine poles to compare phase change and gap change in terms of their features and performances. HAPU can also provide the different characteristics between the pure and hybrid adjustable phase undulator. The HAPU has been installed in the Synchrotron Radiation Research Center (SRRC) storage ring to test the performance of the beam dynamics and photon flux behavior. A three-orthogonal Hall probe and a long-loop-coil measurement system are also implemented to take the field measurements. The multipole field and electron trajectory correction are made by shimming, the magic finger, and the active end pole coil corrector. After the field measurements, the vertical integral strength of HAPU is markedly smaller than that of the Hybrid Adjustable Gap Undulator (HAGU) and confirm that no obvious distinction arises between HAPU and HAGU in terms of the spectrum flux. The electron beam dynamic performance in the storage ring indicates that the vertical tune shift of HAPU is three times smaller than that of HAGU. Meanwhile, the energy dynamic tuning with the digital global feedback turned on, the photon position and flux in the bending chamber beam line is invariant. In addition, the injection efficiency remains unaffected at any phase position and the electron beam lifetime does not change when HAPU is in the dynamic tuning.

Elastoplastic Buckling on the Bent Waveguide of CESR-Type SRF Cavity

A superconducting radio-frequency (SRF) cavity module of CESR-type has been adopted for some advanced accelerators. During operation, the pressure in the liquid-helium vessel acts on the cavity wall, but an ultrahigh vacuum must prevail inside the cavity; the cavity structure must be thus pressure-tested at ambient temperature as a standard procedure for safety. During the pressure test elastoplastic buckling might occur on this SRF cavity and its bent waveguide section, being a shell-like structure. A nonlinear finite-element model for computation is established to assess the mechanism of buckling and post-buckling of the bent waveguide. Some test results with copper bent waveguides are presented. The critical pressure and buckling pattern are strongly affected by the material property and the thickness of the structure and the U-channel.

Cryogenic‐related Performance of an SRF Cavity Module in NSRRC

A superconducting 500‐MHz cavity module has been installed into the electron storage ring of NSRRC. This SRF module is tested on both the RF and cryogenic performances, before and after installation into the electron ring. Calibrations and measurements on its cryogenic load at different operating helium bath pressures are described and concluded. The test results of unloaded quality factor are reported. Meanwhile the excellent regulation on helium bath pressure is so advantageous to all these measurements. During normal operation with RF power, fluctuations of the helium bath pressure and liquid helium level are +/− 1.38 mbar (0.02 psi) and +/−0.2%, respectively.

Commissioning and Operations Results of the Industry-Produced CESR-Type SRF Cryomodules

Upon signing a technology transfer agreement with Cornell University, ACCEL began producing turn-key 500 MHz superconducting cavity systems. Five such cryomodules have been delivered and commissioned to date. Four of them are installed in accelerators for operation (two in CESR and one each in Canadian Light Source and Taiwan Light Source) and one serves as an off-line spare at CLS. One more cryomodule is scheduled for testing in early 2005. It will be a spare unit for TLS. Three cryomodules for DIAMOND Light Source are being fabricated at ACCEL. The commissioning results and operational experience with the cryomodules in CESR, CLS and TLS are presented.

Effects of the Passive Harmonic Cavity on the Beam Bunch

In this paper, we present a computer tracking code, which can investigate the bunch length, energy spread and the threshold current of Robinson instability under the influence of the passive harmonic cavity. The effects of the radiation damping, quantum excitation and the beam loading of the harmonic cavity are included in the computation. The calculated result shows that the beam has a constant energy spread and blows up as the beam current increases from below to over the threshold current of the Robinson instability. It also indicates that the shunt impedance of the harmonic cavity is critical for whether the harmonic cavity can reach the designed goal, a stable and lengthening beam at the design beam current.

A coupled-field analysis on RF cavity

A coupled-field analysis process for a RF cavity structure has been established. The commercial code ANSYS is used as the solver which successfully links analyses of different fields, including electromagnetics, heat transfer, and structure mechanics. The computation efficiency is thus dramatically improved. The calculated RF characteristics of a pill-box cavity match the theoretical predictions pretty well. The following thermal and structural computation based on the surface power loss are also performed. This coupled-field analysis process has been applied to a cavity in design. The computed results associated with RF are compared to the ones calculated by other codes.

Effects of magnets with non-unit magnetic permeability on an elliptically polarizing undulator.

This study employs the three-dimensional magnetostatic code TOSCA to assess numerically the effects of NdFeB magnets with non-unit magnetic permeability on an elliptically polarizing undulator. A reduction of a few percent of the on-axis magnetic field strength is predicted. In addition, a deviation of +/-100 G cm uncompensated dipole steering is predicted in a phase shift of 180 degrees for the elliptically polarizing undulator EPU5.6 (having a period length of 56 mm) at the minimum gap of 18 mm, which is related primarily to the configuration of the device end scheme. Results presented herein demonstrate that implementing an active compensation mechanism is a prerequisite for minimizing the orbit distortion during phase-shift adjustment, particularly for operating such a polarizing undulator in a third-generation machine having a median energy similar to that of the 1.5 GeV storage ring at SRRC.