Storage Ring Vacuum Chamber Research Articles

NSLS-II longitudinal impedance budget

From the beginning of the NSLS-II project, the vacuum chamber of the NSLS-II storage ring has been optimized to reduce its impedance. The optimization, performed in close collaboration with the Diagnostic, Vacuum, RF, and Mechanical Engineering groups, has been achieved via calculating the impedance of the vacuum chamber components with the use of both analytical and numerical tools, with the numerical computations carried out with the GdfidL code. For reliable NSLS-II operation with the total beam current of 500 mA, each component of the vacuum chamber has been individually optimized to minimize the beam-induced heating and to prevent single-bunch and coupled-bunch instabilities. In this article, the impedance calculation and optimization know-how are summarized for all the chamber components, and the total longitudinal impedance budget is analyzed. As a result of this extensive research and engineering effort, the design beam current of 500 mA has been recently accumulated in NSLS-II without any major problem caused by the collective effects.

Shielding calculations for the design of new beamlines at ALBA synchrotron

ALBA is a Spanish synchrotron facility generating intense beams of synchrotron radiation from a 3 GeV electron accelerator. At ALBA Synchrotron, electrons are first accelerated in a 110 MeV linear accelerator and then injected in Booster ring which increases the energy up to 3 GeV. Finally, the electron beam is stored in a synchrotron Storage Ring with a current up to 400 mA emitting intense beam of synchrotron radiation. Both the Booster and the Storage Ring are located inside the same concrete building, the Tunnel, while the LINAC is located in a separated concrete building, the Bunker. Outside the Tunnel, and tangentially to the Storage Ring, are located the experimental research laboratories, the Beamlines, where scientists receive synchrotron light for a wide variety of experiments. At present ALBA has 8 Beamlines installed, 3 Beamline under construction, and 1 Beamlines in the design stage.This paper details the design of the shielding elements of the new Beamlines under construction called LOREA, NOTOS and XAIRA. The first one is a soft X-ray and the other two are hard X-ray Beamlines. Shielding calculations are performed using FLUKA Monte Carlo code and aim at quantifying the total dose rate produced by the different sources of radiation at the Beamlines in operation. In particular, it details the dose generated by the gas bremsstrahlung radiation resulting from the interaction of the 3 GeV electron beam with the residual gas of the storage ring vacuum chamber and compares it to the dose rate generated by the insertion device radiation flux. The objective of the FLUKA simulations performed in this work is to establish the requirements for all the Beamline shielding elements to ensure a public dose (dose rate lower than 0.5 μSv/h) outside the shielding during Beamline operation. Results from the simulations set the general requirements for all the Beamline shielding elements, from a radiation protection point of view.

Measurement of the beam yz crabbing tilt due to wakefields using streak camera at CESR

Transverse vertical wakefields can increase the vertical emittance and distort the phase space of a bunch in a storage ring. Recently, we observed charge-dependent vertical beam size growth with a single scraper inserted through the top of the storage ring vacuum chamber. This apparent growth was due in large part to the yz coupling (vertical crabbing) induced by the wake field from the asymmetric scraper configuration. Here, we report a direct measurement of a small beam yz tilt (crabbing) using a streak camera. The recorded images (projected beam profiles in yz plane) are analyzed with three different methods, which yield consistent beam yz tilts. We found the directly-measured current-dependent beam tilts by the streak camera are consistent with the beam tilts calculated from a wakefield model.

Vertical-longitudinal coupling induced by wakefields at the Cornell Electron-Positron Storage Ring Test Accelerator

Transverse vertical wakefields can increase vertical emittance and distort the phase space of a bunch in a storage ring. Here we report recent measurements and simulations of these effects from wakefields from movable scrapers at the Cornell electron-positron storage ring test accelerator. Charge-dependent vertical beam size growth was observed with a single scraper inserted through the top of the storage ring vacuum chamber. No change in the beam size was observed with top and bottom scrapers inserted symmetrically. The apparent growth in the vertical beam size was due in large part to the $yz$ coupling (vertical crabbing) induced by the transverse monopole wake of the asymmetric scraper configuration. We explored this $yz$ coupling and thus the orientation at the observation point by varying the vertical betatron phase advance between the vertical beam size monitor and the scrapers. In addition, we found that existing residual, current-independent $yz$ coupling, perhaps due to nonzero vertical dispersion in the rf cavities, could be compensated by the scraper wake. Predictions from tracking simulations are in good agreement with the measurements. Moreover, the vertical beam size as a function of vertical displacement at narrow-gap chambers (closed scrapers and undulator chamber) was measured. We found the transverse wakefield produced by the off-axis beam could also introduce a $yz$ tilt as well as dilute the beam emittance.

Transparent lattice characterization with gated turn-by-turn data of diagnostic bunch train

Methods of characterization of a storage ring's lattice have traditionally been intrusive to routine operations. More importantly, the lattice seen by particles can drift with the beam current due to collective effects. To circumvent this, we have developed a novel approach for dynamically characterizing a storage ring's lattice that is transparent to operations. Our approach adopts a dedicated filling pattern which has a short, separate diagnostic bunch train (DBT). Through the use of a bunch-by-bunch feedback system, the DBT can be selectively excited on demand. Gated functionality of a beam position monitor system is capable of collecting turn-by-turn data of the DBT, from which the lattice can then be characterized after excitation. As the DBT comprises only about one percent of the total operational bunches, the effects of its excitation are negligible to users. This approach allows us to localize the distributed quadrupolar wakefields generated in the storage ring vacuum chamber during beam accumulation. While effectively transparent to operations, our approach enables us to dynamically control the beta beat and phase beat, and unobtrusively optimize performance of the National Synchrotron Light Source-II accelerator during routine operations.

National Synchrotron Light Source II storage ring vacuum systems

The National Synchrotron Light Source II, completed in 2014, is a 3-GeV synchrotron radiation (SR) facility at Brookhaven National Laboratory and has been in steady operation since. With a design electron current of 500 mA and subnanometer radians horizontal emittance, this 792-m circumference storage ring is providing the highest flux and brightness x-ray beam for SR users. The majority of the storage ring vacuum chambers are made of extruded aluminium. Chamber sections are interconnected using low-impedance radiofrequency shielded bellows. SR from the bending magnets is intercepted by water-cooled compact photon absorbers resided in the storage ring chambers. This paper presents the design of the storage ring vacuum system, the fabrication of vacuum chambers and other hardware, the installation, the commissioning, and the continuing beam conditioning of the vacuum systems.

The system for delivery of IR laser radiaton into high vacuum

The system for insertion of a laser beam into the vacuum chamber of high-energy storage ring is described. The main part of the system is the high-vacuum viewport for the IR radiation, based on ZnSe or GaAs crystals. The design of the viewports is presented.

Force on a storage ring vacuum chamber after sudden turn-off of a magnet power supply

We are commissioning a 2.5 GeV synchrotron radiation source (SRS) where electrons travel in high vacuum inside the vacuum chambers made of aluminum alloys. These chambers are kept between the pole gaps of magnets and are made to facilitate the radiation coming out of the storage ring to the experimental station. These chambers are connected by metallic bellows. During the commissioning phase of the SRS, the metallic bellows became ruptured due to the frequent tripping of the dipole magnet power supply. The machine was down for quite some time. In the case of a power supply trip, the current in the magnets decays exponentially. It was observed experimentally that the fast $B$ field decay generates a large eddy current in the chambers and consequently the chambers are subjected to a huge Lorentz force. This motivated us to develop a theoretical model to study the force acting on a metallic plate when exposed to an exponentially decaying field and then to extend it for a rectangular vacuum chamber. The problem is formulated using Maxwell's equations and converted to the inhomogeneous Helmholtz equation. After taking the Laplace transform, the equation is solved with appropriate boundary conditions. Final results are obtained after taking the appropriate inverse Laplace transform. The expressions for eddy current contour and magnetic field produced by the eddy current are also derived. Variations of the force on chambers of different wall thickness due to spatially varying and exponentially time decaying field are presented. The result is a general theory which can be applied to different geometries and calculation of power loss as well. Comparisons are made with results obtained by simulation using a finite element based code, for quick verification of the theoretical model.

Design of shunt structure to control transverse electric resonance modes in vacuum chamber of storage ring

Abstract It is a significant issue to control the unwanted transverse electric (TE) modes in the vacuum chamber for preventing the position reading noise from the beam position monitor (BPM) of the storage ring. We introduce shorting bars, called as shunt structure, in the shade region of synchrotron radiation to control the frequency distribution of TE resonance modes excited in the sector vacuum chambers of the Pohang Light Source II (PLS-II) storage ring. The design of shunt structure is carried out using full three dimensional finite-difference time-domain (FDTD) eigenmode simulation, in order that the TE resonance modes appear on the outside of the BPM operation frequency band of PLS-II storage ring without large reduction of vacuum conductance, increasing of beam impedance and major mechanical modification. It is verified in the measurement of the RF reflection scattering matrix ( S 11 ) through the pickup electrode of BPM using a vector network analyzer that the peak frequencies in the S 11 graph are found out of the BPM operation frequency band and well matched with the eigenmode frequencies obtained by the FDTD simulation.

Measurement of resonance modes causative of beam position monitor signal noise in vacuum chamber of storage ring

It is known that the position reading obtained from the beam position monitor (BPM) mounted at the storage ring can be corrupted by the resonance mode. We carried out a three dimensional finite-difference time-domain (FDTD) simulation of vacuum chambers of the storage ring of the Pohang Light Source (PLS) without simplified modeling to measure the frequencies of resonance modes excited in the vacuum chamber. The frequencies of resonance modes obtained by the eigenmode simulation are well matched with the peak frequencies of RF transmission scattering matrix ( S 21) graph of sector vacuum chamber measured using a network analyzer. It is found that a transverse electric (TE) resonance mode exists in the operation frequency band of BPM and the vertically oriented electric field of TE resonance mode is linked to the BPM position reading noise. Based on this study, we can easily design a vacuum chamber free from the BPM position reading noise caused by the TE resonance mode.

X-ray generator based on Compton scattering

Nowadays, the sources of the X-rays based on a storage ring with low beam energy and Compton scattering of intense laser beam are under development in several laboratories. In the paper the state-of-art in development and construction of cooperative project of a Kharkov advanced X-ray source NESTOR based on electron storage ring with beam energy 43–225 MeV and Nd:YAG laser are described. The layout of the facility is presented and latest results are described. The designed lattice includes 4 dipole magnets with combined focusing functions, 20 quadrupole magnets and 19 sextupoles with correcting components of magnetic field. At present, a set of quadrupole magnet is under manufacture and bending magnet reconstruction is going on. The main parameters of developed vacuum system providing residual gas pressure in the storage ring vacuum chamber up to 10 −9 Torr are presented. The basic parameters of the X-ray source laser and injection systems are presented. The facility is going to be in operation from the middle of 2006 and generated X-ray flux is expected to be of about 10 13 phot/s.

Gas bremsstrahlung and induced photoneutrons in the NSRRC's electron storage ring: A comparison of measurements and simulations

This study investigated the characteristics of gas bremsstrahlung and induced photoneutrons from the electron storage ring in National Synchrotron Radiation Research Center (NSRRC). Gas bremsstrahlung is generated in the vacuum chamber of an electron storage ring by interactions of the circulating electrons with the atoms of the residual gas. The emitted photons with sufficiently high energies can further produce neutrons by means of photonuclear interactions. Photoneutrons produced in a lead target struck by gas bremsstrahlung were measured as a function of target thickness by using high-efficiency neutron detectors in a well-defined configuration. A Monte Carlo simulation of the whole process including generation of gas bremsstrahlung, production of photoneutrons and subsequent neutron transport has been performed in an integrated manner and the results were directly compared with the measurements on an absolute basis. The agreement between them is very satisfactory and hence validates the evaluation of the characteristics of gas bremsstrahlung and induced photoneutrons in NSRRC.

Patterned metallic coating of a ceramic chamber in storage ring bump magnets

To permit the pulsed field of a bump magnet to penetrate the vacuum chamber of a particle storage ring, a section of ceramic chamber is used. A thin metal coating on the inner surface is usually employed to prevent the static charge build-up on the surface of the ceramic chamber, as well as to reduce the coupling impedance. The metal coating, however, can lead to heating of the chamber walls due to eddy current. To avoid such an unwanted effect, a patterned coating is employed. In this paper, we discuss the electrical power and the field distortion due to the eddy current and the effect of various patterned metallic coatings on the motion of a circulating beam. The results are applied to the storage ring of the Pohang Light Source.

Elettra vacuum system: status and experience with the new Al vacuum chambers

The status and recent developments of the Elettra vacuum system are reported. Due to the fact that in our facility no in-situ bake out is applied, we focused our work to contribute to the theoretical explanation of the beam cleaning efficiency. Gas injection experiments have been performed introducing various gases into the vacuum chamber of the storage ring. These experiments are very useful and help us to analyse better behaviour of vacuum chambers having a higher pressure profile. New aluminium vacuum chambers have been recently installed into the storage ring. Operational experience is also reported.

Measurement of gas bremsstrahlung at the SPring-8 insertion device beamline using PWO scintillator

Gas bremsstrahlung, generated by the interaction of stored electrons with residual gas molecules in a storage ring vacuum chamber, was measured at the insertion device beamlines of SPring-8 by using a new-type scintillator PWO (PbWO 4). The fine energy resolution for the bremsstrahlung photons was obtained, which is about three times as good as that of lead glass. The measurements were made at the two beamlines, BL11XU (high-betatron function beamline) and BL46XU (low-betatron function beamline), in relation to the characteristics of the stored electron beam. The gas bremsstrahlung was measured as a function of the aperture size of the slits, showing the apparent dependence on the conditions of the stored electron beam, as well as the beam current and the vacuum pressure. The measurements of the aperture size dependency were compared with the calculations by using the Monte Carlo code EGS4, to show a fairly good agreement. The total power of the gas bremsstrahlung was estimated as 25.8±0.8 nW/10 −8 Pa/mA and the corresponding equivalent dose rates for a scoring radius of 0.023 cm were calculated to be 15.8 nSv/s/10 −8 Pa/mA for BL11XU, 9.22 nSv/s/10 −8 Pa/mA for BL46XU. Calculations were also made to obtain 25.9 nSV/s/10 −8 Pa/mA neglecting the stored beam conditions at 40 m from the center of the straight section of SPring-8.

New apparatus for precise synchronous phase shift measurements in storage rings

Measuring a synchronous phase shift as a function of beam current is commonly done in accelerator physics to estimate the longitudinal impedance of a storage ring vacuum chamber. This measurement is normally done with RF techniques that do not have enough accuracy to detect small phase shifts typical to the newer storage rings. In this paper we report results from a new method for precise measurement of a synchronous phase shift. Our method involves downmixing from the RF frequency to a kHz range and then using an audio DSP lock-in amplifier for the actual phase detection. This paper describes the idea and the advantages of a new method as well as its practical implementation in the apparatus we build for precise synchronous phase measurements in the Stanford Linear Collider damping rings. The results of those measurements are also presented. [S1098-4402(98)00020-2]

Signal processing, data acquisition, and initial commissioning results of the APS storage ring vertical beam missteering wire monitor

Two wire monitors have been developed and installed in the Advanced Photon Source (APS) storage ring insertion device (ID) front ends to directly detect synchrotron radiation exiting the storage ring vacuum chamber that is missteered by ±5 mm. Each wire monitor employs four in-vacuum tungsten wires positioned above and below the normal x-ray orbit. The temperature change of the wire under x-ray irradiation causes a resistance change that is detected by a local high-sensitivity low-noise signal processing electronics unit with four independent channels. The four outputs of the local unit are linked to a central VME wire resistance measurement and interlock module via 4–20 mA current loops. The central VME module can accept inputs from up to eight wire monitors and has a heartbeat that feeds a MPS summation module. In this paper, an overview of the wire monitor signal processing and data acquisition system design is presented together with recent commissioning results.

Photon stimulated desorption measurement of an extruded aluminum beam chamber for the Advanced Photon Source

The Advanced Photon Source (APS), presently being commissioned, will produce x rays of unprecedented brightness. The high energy ring of the APS is a 7 GeV positron storage ring, 1104 m in circumference designed to operate at less than 10−9 Torr with 300 mA of beam and a greater than 10 h lifetime. The storage ring vacuum chamber is constructed from an extruded 6063 aluminum alloy. During the construction phase, a 2.34-m-long section of the APS extruded aluminum chamber was set up on National Synchrotron Light Source (NSLS) X-ray Beamline X28A and photon stimulated desorption (PSD) was measured. Cleaning and preparation of the chamber was identical to that of the APS construction. In addition to the chamber, small samples of Al, Be, and Cu were also exposed to white light having a critical energy of 5 keV. In addition to PSD, measurements were made of the specular and diffuse scattering of photons. The chamber and samples were each exposed to a dose greater than 1023 photons per m. Desorption yields for H2, CO, CO2, CH4, and H2O are reported as a function of accumulated flux, critical energy, incidence angle, and preparation. These results are compared with previous results for aluminum on NSLS Beamline U10B and PSD results of other laboratories published for aluminum.

Gas bremsstrahlung considerations in the shielding design of the Advanced Photon Source synchrotron radiation beam lines

The Advanced Photon Source (APS) currently under construction at Argonne National Laboratory will be one of the world's brightest synchrotron radiation (SR) facilities. The storage ring, capable of storing currents up to 300 mA at 7.0 GeV, or 200 mA at 7.5 GeV, will produce very intense and energetic synchrotron radiation (Ec = 24 keV for bending magnets, and Ec = 37.4 keV for wigglers, where Ec is the critical energy). The synchrotron radiation beam lines, consisting of experimental enclosures and transport lines, will have to be shielded against synchrotron radiation and gas bremsstrahlung scattered from beam line components. For insertion devices placed in the straight sections (length = 15 m), the gas bremsstrahlung produced by the interaction of the primary stored beam with residual gas molecules or ions in the storage ring vacuum chamber dominates the SR beam line shielding. Gas bremsstrahlung in the forward direction will be stopped by a tungsten beam stop, 18cm thick, located at the back of the experimental enclosure and placed in the median plane of the storage ring. The forward-directed gas bremsstrahlung is characterized, and the effectiveness of the tungsten beam stop is assessed. The Monte Carlo code FLUKA was used to determine the dose equivalent rates from gas bremsstrahlung in a cylindrical tissue phantom with and without the tungsten beam stop. Simulations were performed using an air target of length 15 m at a pressure of 1 atm and 110atm (1 atm = 101.325 kPa = 760 torr) both with and without suppressing positron multiple scattering (M.S.) and Bhabha and Möller scattering in the air target. At a given pressure the dose equivalent rates are higher without positron multiple scattering in the air target than with multiple scattering. For simulations at Ps = 1 atm, the minimization of Bhabha/Möller scattering is important for areas with scoring radii less than 0.4 cm. The maximum dose equivalent rate (for E0 = 7 Gev, I = 300 mA, and P = 0.133 μPa or 10−9 torr) in the phantom (at a depth of 29.5 cm) for a scoring area of 0.0013 cm2 (radius = 0.02 cm) is 2.1 Sv/h, whereas for a scoring area of 0.5 cm2 (radius = 0.4 cm) it is 0.57 Sv/h. Scoring photon fluence and converting it to dose equivalent using Rogers' fluence-to-dose equivalent conversion factors results in an overestimation of dose equivalent. The maximum dose equivalent rate behind the tungsten beam stop, at a depth of 2.5 cm in the tissue, is 0.1 + 0.04 μSv/h for a scoring radius of 1 cm. The corresponding dose equivalent rates without the stop are 32 mSv/h (depth = 2.5 cm) and 0.17 Sv/h (depth = 29.5 cm). A simple expression has been derived for the upper limit on the dose equivalent that an individual can receive due to loss of vacuum in the straight section.

Oxygen discharge cleaning method for aluminum storage ring vacuum chambers

A discharge cleaning method using a dc plasma gun was developed to reduce photon-induced gas desorption by synchrotron radiation in electron storage rings. Hydrocarbon contaminations adsorbed on the inside surface of a long aluminum vacuum chamber (2.5 m) were removed by an oxygen plasma produced by an arc discharge. The plasma density and electron temperature were about 109 cm−3 and 1 eV, respectively, near the inside wall of the vacuum chamber at a discharge current of 5 A. The discharge cleaning was applied for 1 h to an aluminum vacuum chamber manufactured by the EX-extrusion process. The results were as follows: (1) measurements with a quadrupole mass spectrometer determined that 6×1019 CO molecules coming from the hydrocarbon contaminations were removed by the discharge cleaning, (2) in situ Auger electron spectroscopy showed that carbon on the surface of an aluminum sample piece was removed within 1 h of discharge cleaning and (3) the photodesorption yield of the chamber with the discharge cleaning was one order of magnitude smaller than that without cleaning.