Engineering topics

Fox, J D

Publications and source records attributed to Fox, J D.

10 records

DOE OSTI2012

Radio Frequency Noise Effects on the CERN Large Hadron Collider Beam Diffusion

Radio frequency (rf) accelerating system noise can have a detrimental impact on the Large Hadron Collider (LHC) performance through longitudinal motion and longitudinal emittance growth. A theoretical formalism has been developed to relate the beam and rf station dynamics with the bunch length growth. Measurements were conducted at LHC to determine the performance limiting rf components and validate the formalism through studies of the beam diffusion dependence on rf noise. As a result, a noise threshold was established for acceptable performance which provides the foundation for beam diffusion estimates for higher energies and intensities. Measurements were also conducted to determine the low level rf noise spectrum and its major contributions, as well as to validate models and simulations of this system.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2012

Suppression of Longitudinal Coupled-Bunch Instabilities at the KEK-PF

A bunch-by-bunch feedback system has been developed to suppress longitudinal coupled-bunch instabilities at the KEK-PF. A longitudinal kicker based on a DAFNE-type overdamped cavity has been designed and installed in the ring, and a general purpose signal processor, called iGp, has been developed by the collaboration of the KEK, SLAC, and INFN-LNF. The entire feedback loop has been closed by the end of June 2007, and the feedback system has successfully suppressed the longitudinal dipole-mode instabilities up to 430 mA.

43 PARTICLE ACCELERATORS↗

DOE OSTI2011

Commissioning of the IGp Feedback System at DAFNE

The iGp (Integrated Gigasample Processor) is an innovative digital bunch-by-bunch feedback system developed by a KEK / SLAC / INFN-LNF joint collaboration. The processing unit can sample at 500 MHz and compute the bunch-by-bunch output signal for up to {approx}5000 bunches. The feedback gateware code is implemented inside just one FPGA (Field Programmable Gate Array) chip, a Xilinx Virtex-II. The FPGA implements two banks of 16-tap FIR (Finite Impulse Response) filters. Each filter is realtime programmable through the operator interface. At DA{Phi}NE, the Frascati {Phi}-Factory, two iGp units have been commissioned in the April 2007. The iGp systems have substituted the previous betatron feedback systems. This insertion has been very fast and has shown no problems involving just a substitution of the old, less flexible, digital systems, letting unchanged the baseband analog frontend and backend. The commissioning has been very simple, due to the complete and powerful EPICS operator interface, working well in local and remote operations. The software includes also tools for analyzing post processor data. A description of the commissioning with the operations done is reported.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗

DOE OSTI2011

Studies of RF Noise Induced Bunch Lengthening at the LHC

Radio Frequency (RF) noise induced bunch lengthening can strongly affect the Large Hadron Collider (LHC) performance through luminosity reduction, particle loss, and other effects. This work presents measurements from the LHC that better quantify the relationship between the RF noise and longitudinal emittance blowup and identify the performance limiting RF components. The experiments presented in this paper confirmed the predicted effects on the LHC bunch length growth. Dedicated measurements were conducted in the LHC to gain insight in the effect of RF noise to the longitudinal beam diffusion. It was evident that the growth rate of the bunch length is strongly related to the accelerating voltage phase noise power spectral density around f{sub s} + kf{sub rev}, as predicted in [4]. The noise threshold for 2.5 ps/hr growth was estimated to -101 dBc/Hz (SSB flat noise spectral density from f{sub s} to the edge of the closed loop bandwidth). A 9 dB margin is achieved with the current RF configuration and the BPL on. With this formalism it is now possible to estimate the effect of different operational and technical RF configurations on the LHC beam diffusion. This formalism could also be useful for the design of future RF systems and the budgeting of the allowed noise.

43 PARTICLE ACCELERATORS↗

DOE OSTI2011

RF system models for the CERN Large Hadron Collider with application to longitudinal dynamics

The LHC RF station-beam interaction strongly influences the longitudinal beam dynamics, both single bunch and collective effects. Non-linearities and noise generated within the Radio Frequency (RF) accelerating system interact with the beam and contribute to beam motion and longitudinal emittance blowup. Thus, the noise power spectrum of the RF accelerating voltage strongly affects the longitudinal beam distribution. Furthermore, the coupled-bunch instabilities are also directly affected by the RF components and the configuration of the Low Level RF (LLRF) feedback loops. In this work we present a formalism relating the longitudinal beam dynamics with the RF system configurations, an estimation of collective effects stability margins, and an evaluation of longitudinal sensitivity to various LLRF parameters and configurations.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2010

Self-Consistant Numerical Modeling of E-Cloud Driven Instability of a Bunch Train in the CERN SPS

The simulation package WARP-POSINST was recently upgraded for handling multiple bunches and modeling concurrently the electron cloud buildup and its effect on the beam, allowing for direct self-consistent simulation of bunch trains generating, and interacting with, electron clouds. We have used the WARP-POSINST package on massively parallel supercomputers to study the growth rate and frequency patterns in space-time of the electron cloud driven transverse instability for a proton bunch train in the CERN SPS accelerator. Results suggest that a positive feedback mechanism exists between the electron buildup and the e-cloud driven transverse instability, leading to a net increase in predicted electron density. Comparisons to selected experimental data are also given. Electron clouds have been shown to trigger fast growing instabilities on proton beams circulating in the SPS and other accelerators. So far, simulations of electron cloud buildup and their effects on beam dynamics have been performed separately. This is a consequence of the large computational cost of the combined calculation due to large space and time scale disparities between the two processes. We have presented the latest improvements of the simulation package WARP-POSINST for the simulation of self-consistent ecloud effects, including mesh refinement, and generation of electrons from gas ionization and impact at the pipe walls. We also presented simulations of two consecutive bunches interacting with electrons clouds in the SPS, which included generation of secondary electrons. The distribution of electrons in front of the first beam was initialized from a dump taken from a preceding buildup calculation using the POSINST code. In this paper, we present an extension of this work where one full batch of 72 bunches is simulated in the SPS, including the entire buildup calculation and the self-consistent interaction between the bunches and the electrons. Comparisons to experimental data are also given.

43 PARTICLE ACCELERATORS↗

DOE OSTI2010

11.424 Ghz Stripline Transversal Filter for Sub-Picosecond Bunch Timing Measurements

Measurement of time-of-arrival or instantaneous longitudinal position is a fundamental beam diagnostic. We present results from a stripline transversal periodic coupler structure which forms the heart of a sub-ps beam timing detector. This filter structure approximates a sinx/x response in the frequency domain which corresponds to a limited pulse length response in the time domain. These techniques have been used extensively in beam feedback systems at 3 GHz center frequencies with operational single-shot resolutions of 200 fs [1]. We present a new design, based on a 11.424 GHz center frequency, which is intended to offer a factor of four improvement in time resolution. Two-dimensional electromagnetic simulation results are shown, and the design optimization approach leading to the final circuit implementation is illustrated. The prototype circuit has been fabricated on 64mil Rogers 4003 and lab frequency domain and time domain data are compared to the 2-D simulation results. Performance of the prototype circuit is shown with applicability to sub-ps beam measurements in LINAC and FEL applications.

43 PARTICLE ACCELERATORS↗

DOE OSTI2010

Lessons Learned from PEP-II LLRF and Longitudinal Feedback

The PEP-II B-Factory collider ended the final phase of operation at nearly twice the design current and 4X the design luminosity. To highlight the evolution from the original conceptual design through to the 1.2E34 final machine we choose one example each from the broadband feedback and from the LLRF system. They illustrate the original design estimation missed some very significant details, and how in the course of PEP-II operation unexpected difficulties led to significant insights and new approaches which allowed higher machine performance. We present valuable 'lessons learned' which are of interest to designers of next generation feedback and impedance controlled LLRF systems.

43 PARTICLE ACCELERATORS↗

DOE OSTI2010

SIMULATION OF E-CLOUD DRIVEN INSTABILITY AND ITS ATTENUATION USING A FEEDBACK SYSTEM IN THE CERN SPS

Electron clouds have been shown to trigger fast growing instabilities on proton beams circulating in the SPS [1], and a feedback system to control the instabilities is under active development [2]. We present the latest improvements to the Warp-Posinst simulation framework and feedback model, and its application to the self-consistent simulations of two consecutive bunches interacting with an electron cloud in the SPS.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2010

SPS ECLOUD INSTABILITIES - ANALYSIS OF MACHINE STUDIES AND IMPLICATIONS FOR ECLOUD FEEDBACK

The SPS at high intensities exhibits transverse single-bunch instabilities with signatures consistent with an Ecloud driven instability. They present recent MD data from the SPS, details of the instrument technique and spectral analysis methods which help reveal complex vertical motion that develops within a subset of the injected bunch trains. The beam motion is detected via wideband exponential taper striplines and delta-sigma hybrids. The raw sum and difference data is sampled at 50 GHz with 1.8 GHz bandwidth. sliding window FFT techniques and RMS motion techniques show the development of large vertical tune shifts on portions of the bunch of nearly 0.025 from the base tune of 0.185. Results are presented via spectrograms and bunch slice trajectories to illustrate development of the unstable beam and time scale of development along the injected bunch train. The study shows that the growing unstable motion occupies a very broad frequency band of 1.2 GHz. These measurements are compared to numerical simulation results, and the system parameter implications for an Ecloud feedback system are outlined.

60 APPLIED LIFE SCIENCES↗