Beam Duct Research Articles

Vacuum design for the 3 GeV TPS synchrotron light source

The 3 GeV Taiwan Photon Source (TPS) is a synchrotron light accelerator which is designed for a beam emittance of < 2 nmrad at a beam current of 400 mA. The vacuum system for the electron storage ring provides a pressure of < 1 nTorr for the electron beam with long beam life time and small disturbance from trapped ions. Due to the limited space for the pumps and the poor conductance of the beam ducts, the vacuum chambers should be manufactured with a clean surface of very low outgassing rate. Aluminum alloys vacuum chambers and the structures of confined-pumping for the localized photo-desorption are selected to meet the design requirement. The CNC-machining for the bending chambers in pure alcohol spraying, followed by ozonated water cleaning and TIG welding in clean room, provides a clean surface and simple structure which lowers the outgassing rate and the chamber impedance. Combinations of the lumped non-evaporable getters (NEG) and the sputtering ion pumps are located near the absorbers for higher efficiency of localized pumping capability. Either the NEG-strips or interior NEG-coating will be used for the Undulator chambers for providing a good linear pumping feature. The design concept will be described in this paper. Besides, the simulation of the pressure distribution and some test results of outgassing measurement for the aluminum chambers will be discussed.

Soft-lithography-based Inter-chip Optical Interconnects

The increasing performance of microprocessors leads to higher bandwidth require- ments for the data ∞ow to and from the processor. Today, all signaling on a PCB is performed electrically, using copper lines that are integrated in the board. However, issues such as prop- agation loss and inter-channel crosstalk limit the scalability of electrical interconnects to ever higher bandwidth densities. Optical interconnects feature a higher bandwidth £ length prod- uct, are more power-e-cient and enable a higher bandwidth density than electrical interconnects do. This paper describes a kind of two-dimensional monolayer optical interconnects providing interconnections between chips on conventional PCB. We have designed a soft-lithography-based, versatile coupling structure with a 45 - total internal re∞ector (TIR), a beam duct, and a polymer waveguide in order to vertically couple light beams between transmitter (or receiver) and the waveguide layer. This proposed integrated architecture of a polymeric optical interconnection has been demonstrated to be advantageous in the aspects of misalignment tolerance, ease and low cost of fabrication, as well as relative simplicity in deployment. We also investigated the characteristics of in-plane connections including cross-over and branching nodes in the optical interconnects with experimental and theoretical analysis. The theoretical crosstalk, as calculated by a function of crossing angle, was determined for a set of interconnect pairs with varying cross- sections, and was compared with experimental measurements. Furthermore, a suitable branching angle was found for branching node and the efiects of short-distance mode scrambling in highly multimode polymer waveguides were studied in detail in this paper too.

Preliminary Experiments on Accelerator-Driven Subcritical Reactor with Pulsed Neutron Generator in Kyoto University Critical Assembly

A series of preliminary experiments on an accelerator-driven subcritical reactor (ADSR) with 14 MeV neutrons were conducted at Kyoto University Critical Assembly (KUCA) with the prospect of establishing a new neutron source for research. A critical assembly of a solid-moderated and -reflected core was combined with a Cockcroft-Walton-type accelerator. A neutron shield and a beam duct were installed in the reflector region for directing as large a number as possible of the high-energy 14MeV neutrons generated by deuteron-tritium (D-T) reactions to the fuel region, since the tritium target is located outside the core. And then, neutrons (14MeV) were injected into a subcritical system through a polyethylene reflector. The objectives of this paper are to investigate the neutron design accuracy of the ADSR with 14MeV neutrons and to examine experimentally the neutronic properties of the ADSR with 14MeV neutrons at KUCA. The reaction rate distribution and the neutron spectrum were measured by the foil activation method for investigating the neutronic properties of the ADSR with 14 MeV neutrons. The eigenvalue and fixed-source calculations were executed using a continuous-energy Monte Carlo calculation code MCNP-4C3 with ENDF/B-VI.2 for the subcriticality and the reaction rate distribution, respectively; the unfolding calculation was done using the SAND-II code coupled with JENDL Activation Cross Section File 96 for the neutron spectrum. The values of the calculated subcriticality and the reaction rate distribution were in good agreement with those of the experiments. The results of the experiments and the calculations demonstrated that the installation of the neutron shield and the beam duct was experimentally valid and that the MCNP-4C3 calculations were accurately carried out for analyzing the neutronic properties of the ADSR with 14MeV neutrons at KUCA.

Nuclear Assessment of Final Optics of a KrF Laser Driven Fusion Power Plant

In the HAPL program, power plant designs are assessed with targets driven by 40 KrF laser beams. The final optics system that focuses the laser onto the target may include a grazing incidence metallic mirror (GIMM) located at 24 m from the target with 85° angle of incidence. The GIMM is in direct line of sight of the target and has a 50 micron thick aluminum coating. Two options were considered for the substrate material; SiC and AlBeMet. The impact of the GIMM design options on the nuclear environment at the dielectric focusing and turning mirrors was assessed. Using AlBeMet results in about a factor of two higher neutron flux. We considered beam duct configuration modifications such as utilizing neutron traps to reduce radiation streaming. In addition, we investigated the impact of lining the beam ducts and neutron traps with different materials that help slowing down and absorbing neutrons.

Influence of electron irradiation and heating on secondary electron yields from non-evaporable getter films observed within situx-ray photoelectron spectroscopy

Nonevaporable getter (NEG) film has been used for the beam ducts of particle accelerators as a pump having a large area. NEG film has been considered to have a low outgas rate induced by energetic particle irradiation and a low secondary electron yield (SEY). In this article, we focused on SEY measurements and in situ surface characterization of four NEG film samples using x-ray photoelectron spectroscopy (XPS). The NEG samples were TiZrV thin films deposited by magnetron sputtering at 100 or 300°C on stainless steel. In addition, NEG samples saturated by CO gas exposure were prepared. SEY and XPS measurements of the surfaces of NEG samples were carried out under the conditions of as received, after electron beam irradiation, and after heating at 200°C for 24h. The maximum SEY values of the primary electron energy dependence, δmax, of all NEG samples decreased to around 1 by electron beam irradiation owing to a change in the carbon impurities, such as carbon oxide, carbon hydroxide, and hydrocarbon, to graphite state (graphitization) during the irradiation. After heating, δmax values of the NEG samples without CO gas exposure were also around 1 owing to the carbonization of Ti, Zr, and V. The δmax≈1 was remarkably lower than that of copper baked under the same conditions. However, in saturated NEG samples, metal carbides were not produced to a significant extent by heating, and the δmax values did not decrease, showing values of 1.5–1.7.

Recent studies on photoelectron and secondary electron yields of TiN and NEG coatings using the KEKB positron ring

In order to obtain a method to suppress electron-cloud instability (ECI), the photoelectron and the secondary electron yields (PEY and SEY) of a TiN coating and an NEG (Ti–Zr–V) coating on copper have been studied so far by using the KEK B-factory (KEKB) positron ring. Recently, test chambers with these coatings were installed at a straight section of the ring where the irradiated photon density was considerably smaller than that at the arc section of a previous experiment. The number of electrons around beams was measured by an electron current monitor; this measurement was performed up to a stored beam current of approximately 1700 mA (1389 bunches). For the entire range of the beam current, the electron currents of the NEG-coated and the TiN-coated chambers were clearly smaller as compared to those of the uncoated copper chamber by the factors of 2–3 and 3–4, respectively. The small photon density, that is, the weak effect of photoelectrons, elucidated the differences in the SEYs of these coatings when compared to the measurements at the arc section. By assuming almost the same PEY ( η e) values obtained in the previous study, the maximum SEY ( δ max) for the TiN and NEG coatings and the copper chamber was again estimated based on a previously developed simulation. The evaluated δ max values for these three surfaces were in the ranges of 0.8–1.0, 1.0–1.15, and 1.1–1.25, respectively. These values were consistent with the values obtained so far. As an application of the simulation, the effective η e, η e−eff (which included the geometrical effect of the antechamber) and δ max values were also estimated for copper chambers with one or two antechambers. These chambers were installed in an arc section and a wiggler section, respectively. The evaluated η e−eff and δ max values were approximately 0.008 and 1.2, and 0.04 and 1.2, respectively, where η e=0.28 was assumed on the side wall. As expected, the η e−eff values were considerably smaller than those obtained in the case of a simple circular chamber ( η e=0.28–0.3). Further, the δ max values were consistent with those obtained so far. With regard to the uncertainty in the simulation, the effect of the SEY spectrum on the estimation of δ max values is briefly discussed. As the next step in our study, we plan to combine beam ducts with antechambers and TiN coatings; this combination is the most promising solution to ECI at present.

Design of Cryogen-Free Mini-Pole Superconducting Undulator

A cryogen-free cryostat for a superconducting undulator with a period of 15 mm (SU15) was developed at the National Synchrotron Radiation Research Center (NSRRC). A 0.44 mm-thin superconducting NbTi wire was wound on iron pole with a racetrack profile. The design field strength of SU15 was 1.4 T and the coil had 148 turns/pole at 230 A; the magnet gap was 5.6 mm. The liquid helium (LHe) recondensation cryostat was designed for SU15. The cryostat provides the advantage of uniform cooling and reduces the amount of LHe consumed. A0.3mm-thick stainless steel (SS) beam duct was formed to separate the vacuum in the storage ring from the magnet. Therefore, all of dirty material is produced out of storage ring vacuum. Most of the heat leaked from the LHe vessel leaks from the LHe-fill port heat leak and the 300 K-4.2 K beam duct or was associated with image current power dissipation (ICPD) and bending magnet power dispersion (BMPD). The leaking of heat from the main coil was prevented by the high-temperature superconducting current leads (HTSC). The beam duct was coated with a copper layer with a high-RRR ratio to reduce image current heating. The heat loads on the magnet were cooled using two 1.5 W Gifford-MacMahon (GM) type cryocoolers via LHe medium.

Design, Construction, and Performance Testing of a 6.5 T Superconducting Wavelength Shifter

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A compact three-pole superconducting magnet with an aluminum warm beam duct was designed and fabricated as an X-ray source in a 1.5 GeV Taiwan Light Source (TLS) or the 3 GeV Taiwan Photon Source (TPS). Three pairs of racetrack NbTi superconducting coils were connected to one main power supply to create a central field of over 6.5 T. Two low current trim power supplies were connected in parallel to the two side pairs of the coil to eliminate the first and second field integrals. The wavelength shifter magnet was cooled in a pool boiling helium bath. Helium boils off at 1.3 L/hr. The vapor-cooled current lead is used to pass the 350 A excitation current. The magnetic field strength was measured at room temperature using a Hall probe and a stretched wire system. The magnet was tested successfully to 308 A, at which the central field exceeded 6.5 T, and the peak field on coil was 8.2 T. The design and construction of the magnet and the cryostat, the quenching protection, and field measurement results will be presented and discussed. </para>

Analysis of Heat Load in a Superconducting Wiggler With a Semi-Cold UHV Beam Duct

A superconducting wiggler with a magnetic period of 6.0 cm (SW6) and a peak field of 3.2 T has been designed and fabricated in the National Synchrotron Radiation Research Center (NSRRC). The beam duct separates the electron beam from the cryogenic system of the magnet. The heat load on the beam duct should be low to stabilize the operation of the superconducting magnets. However, outgassing caused by synchrotron radiation at an electron energy of 1.5 GeV and a current of 200 mA must be reduced. Accordingly, operating the system at a higher temperature can minimize the adsorption of molecules on the beam duct. Therefore, the beam duct system and its connection by finite element analysis are designed to optimize the operating temperature of the beam duct at between 100 and 120 K. Performance of the beam duct is established to comply with specifications during the operation of magnet

Vacuum System Developments at the National Synchrotron Radiation Research Center -from the 1.5 GeV TLS to the 3.3 GeV TPS

This article describes the design and operation of the vacuum system of the 1.5 GeV Taiwan Light Source (TLS) at the National Synchrotron Radiation Research Center (NSRRC). The TLS vacuum system has aluminum beam ducts, an oil-less fabrication process, a low impedance structure, an oil-less pumping system, little dust and an effective vacuum safety interlock system. Operating already for 12 years, TLS vacuum system has been highly reliable. A low-emittance 3.3 GeV synchrotron light source, the Taiwan Photon Source (TPS, with a circumference of 518.4 m), at the NSRRC is proposed to meet the requirements of advanced scientific study. Given the strictness of these requirements, the TPS vacuum system must be constructed to perform well in many areas, with a low vacuum pressure, little dust, a stable mechanical structure and a low risk of material failure caused by a high heat load or radiation damage. The vacuum system of the TPS has been preliminarily designed based on experience gained at the TLS. This work presents the technical design of the TPS vacuum chambers, the pumping configuration and some special components, emphasizing the considerations in obtaining an ultrahigh vacuum, the factors that influence beam quality and the reliability of the vacuum system.

ＭＯ型フランジの加速器ビームダクトへの応用 その２―実機へ適用―

ＭＯ型フランジの加速器ビームダクトへの応用 その２―実機へ適用―

Present Status of The KEK B-Factory Vacuum System

The KEK B-Factory (KEKB) is a two-ring electron-positron collider with asymmetric energies dedicated to proving the CP violation. The vacuum system has been working satisfactorily form the beginning of commissioning in December, 1998. The maximum stored beam currents are 1.3 A for the 8 GeV electron ring (High Energy Ring, HER), and 1.8 A for the 3.5 GeV positron ring (Low Energy Ring, LER). Most of the beam ducts are made of oxygen-free copper, owing to its ability to withstand an intense heat load and to effectively shield the radiation from high beam currents. The pumping scheme is a combination of Non-Evaporable Getter (NEG) pumps and auxiliary sputter ion pumps. An average pressure of about 1×10-7 Pa has been achieved for both rings at the maximum beam currents for both rings. The coefficient of photon-stimulated gas desorption has been decreasing steadily to less than 5×10-7 molecules photon-1 at an integrated linear photon density of about 1×1026 photons m-1. Although special care was taken to make the inside of the beam ducts as smooth as possible in order to avoid the excitation of higher order modes (HOM), the presence of high-current bunched beams has created several problems so far. The intense HOM has sometimes caused the heating of components near to some big HOM sources. Electron cloud around positron bunches led to an instability, which resulted in a vertical beam size blow-up in LER. A discussion about upgrading KEKB has just started aiming at a higher luminosity. The R & D of the vacuum components has also begun based on various experiences so far.

Shielding analysis at the upper section of the accelerator-driven system

The proton beam duct of the accelerator-driven system (ADS) acts as a streaming path for spallation neutrons and photons and causes the activation of the magnets and other devices above the subcritical core. We have performed a streaming analysis at the upper section of the lead-bismuth target/cooled ADS (800 MWth). MCNPX was used to calculate the radiation dose from streamed neutrons and photons through the beam duct. For the secondary photon production calculation, cross sections for several actinides were substituted with plutonium because of the lack of gamma production cross section. From the results of this analysis, the neutron dose from the beam duct is seen to be about 20 orders higher than that of the bulk shield. The magnets and shield plug are heavily irradiated by streaming neutrons according to the DCHAIN-SP analysis.

Superconducting wiggler with semi-cold beam duct at Taiwan light source

A 3.2 T superconducting wiggler with a periodic length of 6 cm and 32 poles was designed and fabricated as an X-ray source. The beam duct of this magnet is a semi-cold, ultra-high vacuum chamber that consists of an aluminum and stainless steel taper. The number of poles in this magnet design is even, to minimize the integral strengths of the multipole components. Two measurement systems—involving room-temperature and cryogenic Hall probes—were set up to measure the field of the superconducting wiggler. A cryogenic plant that supplied liquid helium and nitrogen to the superconducting wiggler has already been established. The performance of magnet construction is good and the commissioning of the superconducting wiggler in the storage ring has been successful. No trim coil compensation on the magnet is required to adjust the electron beam orbit. Furthermore, the electron beams exhibit no loss and remain highly stable after the superconducting wiggler has been quenched.

Continuing study on the photoelectron and secondary electron yield of TiN coating and NEG (Ti–Zr–V) coating under intense photon irradiation at the KEKB positron ring

In order to investigate a way to suppress the electron-cloud instability (ECI), the secondary electron and photoelectron yields (SEY and PEY) of a TiN coating were studied with an intense positron beam at the KEK B-Factory (KEKB), following up our previous study on a NEG (Ti–Zr–V) coating. A TiN-coated chamber was installed at an arc section of the KEKB positron ring, where photons with a line density of 6.5×10 14 photons m −1 s −1 mA −1 were directly irradiated. The number of electrons around the positron bunches was measured by a special electron-current monitor, up to a stored beam current of about 1700 mA (1284 bunches). The electron current of the TiN-coated chamber was clearly smaller than those of the NEG-coated and the non-coated copper chambers by a factor of 2 for all beam currents. Using the previous results of the NEG-coated and the non-coated copper chambers as well as the TiN-coated one here, the maximum SEY ( δ max) and the PEY ( η e) of the TiN coating, the NEG coating and the copper were again estimated based on a simulation. The evaluated δ max values for these three surfaces were 0.8–1.0, 0.9–1.1 and 1.1–1.3, and the η e values were 0.13–0.15, 0.22–0.27 and 0.28–0.31, respectively. It was found that the TiN coating had an SEY ( δ max∼0.9) as low as the NEG coating ( δ max∼1.0), but the electron current was clearly smaller than that of the NEG coating, due to its lower photoelectron yield ( η e∼0.14). This study again indicated that the suppression of photoelectrons is required to make effective use of a surface with a low SEY, such as a TiN or a NEG coating. As an application of the simulation code, the electron current of a beam duct with an antechamber was calculated for the case of a NEG coating or a TiN coating ( δ max = 0.9 – 1.0 ). The calculated electron current for a copper duct with an antechamber was about 1/4 of that of a simple circular copper duct ( δ max = 1.2 for both cases) at a high current (∼1700 mA, 1284 bunches), which was in good agreement with the measurement. By combining with a surface with a low SEY ( δ max = 1.0 – 0.9 ), that is, a TiN coating or a NEG coating, a further reduction of the electron current by a factor of about 2 was obtained.

Application of a Matsumoto-Ohtsuka-type vacuum flange to beam ducts for future accelerators

The Matsumoto-Ohtsuka (MO)-type vacuum flange, which can provide a gapless connection and a highly reliable electric contact between flanges, was studied experimentally for a possible application to beam ducts for high-current accelerators, where the apertures have a complicated structure, such as the combination of a beam channel and one or two flat rectangular antechambers. In spite of the complex aperture, test flanges showed a good vacuum-sealing property. Vacuum sealing was successfully achieved with a reasonable fastening torque (15–20Nm), and remained sealed after baking at 250°C for 24h. A twist between flanges of up to about 5mrad was also manageable. A pseudoelastic structural analysis well reproduced the observed deformation of the flange. This analysis should be available for optimizing the flange structure. The MO-type vacuum flange was found to be promising for connecting flanges of the beam duct with a complicated aperture in future accelerators. The flange will also be available as a compact vacuum flange in cases where a complex aperture is required.

First experimental and simulation study on the secondary electron and photoelectron yield of NEG materials (Ti–Zr–V) coating under intense photon irradiation

A beam duct coated with NEG materials (Ti, Zr, V), which had been known to have a low secondary electron yield (SEY), was studied for the first time under intense photon irradiation using a positron beam at the KEK B-Factory (KEKB) to investigate a way to suppress the electron cloud instability (ECI). A 2.56 m test copper chamber was coated with the NEG materials (we call it NEG coating here) by magnetron sputtering. It was installed at an arc section of the KEKB positron ring, where the chamber was irradiated by direct photons with a line density of 6.5×10 14 photons m −1 s −1 mA −1. The vacuum pressure around the test chamber during a usual beam operation was lower than the case of non-coated copper chambers by a factor of 4–5. The number of electrons around positron bunches was measured by a special electron monitor up to a stored beam current of 1600 mA. The measured electron current, however, was almost the same as a non-coated copper chamber, especially at low-beam currents, and the effect of the NEG coating was smaller than expected. A simulation explained the result that abundant photoelectrons in the positron ring reduce the effect of the low SEY. The maximum SEYs of the NEG coating and non-coated copper were evaluated using a simulation as about 0.9–1.0 and 1.1–1.3, respectively, which were consistent with the values after a sufficient electron bombardment. Their photoelectron yields were also estimated as 0.22–0.28 and 0.26–0.34, respectively, and were in good agreement with the previous experimental results. The study indicates that the suppression of photoelectrons, by a beam duct with an antechamber, for example, is indispensable to make effective use of a surface with a low SEY, such as the NEG coating.

ＭＯ型フランジの加速器ビームダクトへの応用

The MO-type vacuum flange, which can provide a gapless connection between flanges, was studied experimentally aiming to apply it to the beam duct of high-current accelerators. A test flange showed a good vacuum sealing property although the flange had a complex aperture, that is, a combination of a circle and a rectangle. The vacuum sealing was successfully achieved with a reasonable fastening force, and was kept even after a baking at 250°C for 24 hours. The structural analysis well reproduced the observed deformation of flange, and utilized to optimize the flange structure. The MO-type vacuum flange was found to be a promising one for the connection flanges of the beam duct with a complicated aperture in the future accelerators.

Soft-lithography-based optical interconnection with high misalignment tolerance

A low-cost, versatile optical coupling structure featuring a monolithic integration of a polymeric waveguide, beam ducts, and end-reflectors has been designed, prototyped, and demonstrated to be capable of sustaining high misalignment errors whilst maintaining a reasonable coupling efficiency. The soft lithography fabrication process of this interconnection design allows for significant advantages over traditional designs in terms of misalignment tolerance, manufacture cost and speed, as well as 3-D integration capability.

R&D of copper beam duct with antechamber scheme for high-current accelerators

A beam duct with an antechamber scheme for high-current accelerators was designed and the test chambers were studied experimentally. The duct consists of two channels, i.e., a beam channel where a beam circulates and a Synchrotron Radiation (SR) channel (antechamber) aside where the SR passes through. By using the antechamber scheme, the maximum power density of SR can be diluted at the side wall. The impedance is small owing to the pumping ports not being at the beam channel, but at the SR channel. Photoelectrons inside the beam channel are also expected to be reduced, which would be a big merit for a positron ring to suppress the electron cloud effect since the photoelectron is a major source of electrons composing the cloud. Two copper test chambers were manufactured with different methods, by pressing and by drawing. These chambers showed a good static vacuum property, i.e., gas desorption rates with less than 3.5 × 10 - 9 Pa m 3 s - 1 m - 2 after baking. After the installation to the positron ring of the KEK B-factory (KEKB), electron numbers in the beam channel, temperatures and pressures were measured during beam operation. The electrons in the beam channel were found to be reduced by a factor of 4 at 1.5 A compared to the case of the usual circular chamber. The reduction, however, was much larger, about 1/300, at a beam current of about 20 mA where the photoelectrons were dominant and the multiplication of electrons by the multipactoring was small. The temperatures were almost in agreement with the expectation. Vacuum scrubbing by photons proceeded almost smoothly, although pressure bursts were sometimes observed, especially for one test chamber, which was possibly due to discharges at the transverse joints in the beam chamber. Various instructive information had been obtained for future practical beam ducts for high-intensity accelerators.