Stephen R. Smith

Beam position monitor engineering

The design of beam position monitors often involves challenging system design choices. Position transducers must be robust, accurate, and generate adequate position signal without unduly disturbing the beam. Electronics must be reliable and affordable, usually while meeting tough requirements on precision, accuracy, and dynamic range. These requirements may be difficult to achieve simultaneously, leading the designer into interesting opportunities for optimization or compromise. Some useful techniques and tools are shown. Both finite element analysis and analytic techniques will be used to investigate quasi-static aspects of electromagnetic fields such as the impedance of and the coupling of beam to striplines or buttons. Finite-element tools will be used to understand dynamic aspects of the electromagnetic fields of beams, such as wake fields and transmission-line and cavity effects in vacuum-to-air feedthroughs. Mathematical modeling of electrical signals through a processing chain will be demonstrated,...

Beam position monitor system for PEP-II

The beam position monitor (BPM) system for PEP-II, the B-Factory under construction at SLAC, is described in this paper. The system must measure closed orbit for a 3-A multibunch beam and turn-by-turn position for a low-current single bunch injected in a 200-ns gap in the multibunch beam. A system that combines broadband and narrowband capabilities and provides data at high bandwidth was designed. It includes a filter-isolator box (FIB) that selects a harmonic of the bunch spacing (952 MHz) and absorbs the other frequency components; a CAMAC-based wideband IQ and a calibrator that must work even in presence of beam, correcting for electronic measurement errors. This paper describes the system requirements, the electronics design, and the laboratory tests.

Calibration of the beam-position monitor system for the SLAC PEP-II B Factory

The beam-position monitors (BPM) for the PEP-II B Factory consist of four 1.5-cm diameter button style pickups mounted on the diagonals of the quadrupole vacuum chambers. Before installation of the vacuum chambers in the quadrupole assemblies, the electrical center of the BPMs is measured with respect to the mechanical center in a calibration test stand. In this paper the calibrations test stand is described and the precision and accuracy of the calibrations are presented. After installation of the quadrupole assemblies in the PEP-II tunnel, the passive attenuation for each channel of the system is measured to preserve the accuracy of the calibration. Finally, the active electronics includes an on-board calibrator. Results for these portions of the calibration are presented.

Performance of the beam position monitor system for the SLAC PEP-II B factory

The beam position monitor (BPM) system for the SLAC PEP-II B Factory was designed to measure the positions of single-bunch single-turn to multibunch multi-turn beams in both rings of the facility. Each BPM is based on four button-style pickups. At most locations the buttons are connected to provide single-axis information (x only or y only). Operating at a harmonic (952 MHz) of the bunch spacing, the BPM system combines broadband and narrowband capabilities and provides data at a high rate. The active electronics system is multiplexed for signals from the high-energy ring (HER) and low-energy ring (LER). The system will be briefly described; however, the main purpose of the present paper is to present operational results. The BPM system operated successfully during commissioning of the HER (primarily) and the LER over the past year. Results to be presented include on-line calibration, single-bunch single-turn resolution (<100 μm), and multibunch multi-turn resolution (<3 μm), multiplexing, and absolute ca...

AN X-BAND CAVITY FOR A HIGH PRECISION BEAM POSITION MONITOR

The next generation of accelerators will require increasingly precise control of beam position. For example designs for the next linear collider require beamposition monitors (BPMs) with 300 nm resolution. The accelerator designs also place difficult requirements on accuracy and stability. To meet these requirements a cavity BPM operating at 11.424 GHz was designed. The BPM consists of two cavities: an xy-cavity tuned to the dipole mode and a phase cavity tuned to the monopole mode. The xy-cavity uses a novel-coupling scheme that (in principal) has zero coupling to the monopole mode. This report will present the mechanical design, simulations, and test results of a prototype BPM. In addition BPM designs with even higher precision will be discussed

Design of the button beam position monitor for PEP-II

We use MAFIA to analyze the PEP-II button-type beam position monitor (BPM). Employing proper termination of the BPM into a coaxial cable, the output signal at the BPM is determined. Thus the issues of signal sensitivity and power output can be addressed quantitatively, including all transient e ects and wake elds. Besides this rst quantitative analysis of a true BPM 3D structure, we nd that internal resonant modes are a major source of high value narrow-band impedances. The e ects of these resonances on coupled-bunch instabilities are discussed. An estimate of the power dissipation in the ceramic vacuum seal under high current operation is given. Presented at the Workshop on Collective E ects and Impedance for B-Factories, Tsukuba, Japan, June 12-17, 1995 Work supported by Department of Energy, contract DE-AC03-76SF00515.The beam position monitor (BPM) was designed to provide a robust UHV feedthru and a reliable electromagnetic sensor. Stringent resolution requirements at low beam currents, bunch parameters, along with mechanical and chamber requirements produced challenges in the electrical, thermal, and structural design of the BPMs. Numerical modeling and experimental analyses were used to optimize the design. The higher order modes (HOMs) and beam impedance were modeled using MAFIA. Measurements agreed with the calculated 1 /spl Omega/ transfer impedance at the 952 MHz signal processing frequency, and the first two HOMs found in MAFIA. Tests and analysis both showed the button signal power approaching 40 W. Temperature and stress distributions were analyzed using this power loading with ANSYS. An electronic grade CuNi was selected for the BPM to reliably weld into the copper chambers. Pin seal and compressive joints were considered for the insulator vacuum seals. Both glassy ceramic-to-metal and ceramic-to-metal seals were evaluated.

Design of a Multi‐Bunch BPM for the Next Linear Collider

The Next Linear Collider (NLC) will collide 180‐bunch trains of electrons and positrons with bunch spacing of 1.4 ns. The small spot size (σy < 3 nm) at the interaction point requires precise control of emittance, which in turn requires the alignment of individual bunches in the train to within a fraction of a micron. Multi‐bunch beam position monitors (BPMs) are to determine the bunch‐to‐bunch misalignment on each machine pulse. High bandwidth kickers will then be programmed to bring the train into better alignment on the next machine cycle. A prototype multi‐bunch BPM system with bandwidth (350 MHz) sufficient to distinguish adjacent bunches has been built at SLAC. It is based on 5 G sample/s digitization of analog sum and difference channels. Calibration tone injection and logging of the single bunch impulse response provide the kernel for deconvolution of bunch‐by‐bunch position from the sum and difference waveforms. These multi‐bunch BPMs have been tested in the Accelerator Test Facility at KEK and i...

Beam position mis-measurement due to propagating modes

In many modern storage rings, position monitors process beam signals at frequencies above cutoff for propagating modes in the beam duct. This is common in beam ducts with antechambers. Propagating modes introduce fields at the position monitor pickups unrelated to beam position at the monitor, and therefore can cause errors in indicated position. We discuss issues of generation and propagation of these fields, how they couple to position monitor pickups, and steps that can be taken to minimize their effects. We report some experiences with propagating modes affecting position measurements and our experience with lowering the processing frequency of several BPMs, which must operate in wide beam chamber near the interaction point of the PEP-II ring.

Readout demonstration of 512 TES bolometers using a single microwave SQUID multiplexer (Conference Presentation)

To enable the next-generation of bolometric cameras, we are developing the microwave SQUID multiplexer (μMUX). Upcoming receivers such as Simons Observatory, CCAT-prime, BICEP array, Ali-CPT, and CMB-S4 plan to instrument focal planes with 50,000-500,000 sensors. Sensor count is achieved by tiling many 150 mm-diameter densely packed detector arrays into these focal planes. The fabrication and quality of large-format bolometer arrays has been demonstrated and is now mature. In contrast, the readout technology required for next-generation receivers needs development. The sensitivity, low cross-talk, high multiplexing density, and small component size make the μMUX well-suited for this goal. In this approach, the TES signal modulates the inductance of an rf-SQUID that loads a high-Q microwave resonator. The coupled signal therefore modulates the microwave resonance frequency, which may be read out using homodyne techniques. By coupling each resonator to the same microwave feedline, many detectors can be read out on a single coaxial cable pair. The multiplexing density is in practice limited by signal bandwidth, allowable cross-talk, and the digitization bandwidth of room-temperature readout electronics. We present the design and performance of a scalable 64-channel multiplexer chip optimized for bolometric applications. We utilize a new quarter wave resonator design that increases the physical linear density by a factor of two, therefore achieving a smaller footprint for simplified detector packaging. Measurements of this design show 100 kHz resonator bandwidth, uniform 1.8 MHz frequency spacing, and an input referred current noise of 35 pA/√Hz that is well below the level of an optimized, background-limited TES bolometer. Using 8 daisy-chained and frequency scaled chips, we create a 512-channel multiplexer and use it to readout a 512 TES-bolometer array. We present the results of this large-scale μMUX demonstration including system yield, signal cross-talk, and an analysis of noise in various TES bias configurations. The result demonstrates the multiplexing density required to read out 2,000 sensors between 4-8 GHz.

Some solved problems with the SLAC PEP-II B-Factory beam-position monitor system

The Beam-Position Monitor (BPM) system for the SLAC PEP-II B-Factory has been in operation for over two years. Although the BPM system has met all of its specifications, several problems with the system have been identified and solved. The problems include errors and limitations in both the hardware and software. Solutions of such problems have led to improved performance and reliability. In this paper we will report on this experience. The process of identifying problems is not at an end and we expect continued improvement of the BPM system.