SLAC, Menlo Park, California
Stanford University, Stanford, Califormia
The LCLS accelerator produces a 14GeV beam with a normalized emittance on the order of one micron RMS, and peak current exceeding 1000 Amps. The design of the beam measurement system relied heavily on optical transition radiation profile monitors, in conjunction with transverse cavities, and conventional energy spectrometers. It has been found that the high peak currents, and small longitudinal phase space of the beam generate strong coherent optical emission that prevents the use of OTR or other prompt optical diagnostics, requiring the use of wire scanners or fluorescent screen based measurements. We present the results of the beam measurements, measurements of the coherent optical effects, and future plans for diagnostics.
NewPath Research L.L.C., Salt Lake City
NewPath, Salt Lake City, Utah
We have shown that a group of sinusoidally-wound non-ferrous coaxial toroids can be used to determine the transverse distribution of a time-dependent current that passes through their common aperture. A single current filament requires one uniformly-wound toroid, and two others having turn densities proportional to the sine and cosine of the azimuthal coordinate. Three simple algebraic equations give the magnitude and phase of the current and its position in terms of the voltages induced on the three toroids, and there is no ill-conditioning. Two current filaments require two additional toroids with turn densities proportional to the sine and cosine of two times the azimuthal coordinate, and the solution may be obtained by using steepest descent to minimize the residuals. Ill-conditioning makes it impractical to use more than two currents. We have tested our algorithms numerically by specifying the magnitudes and phases of the currents and their locations, calculating the voltages that would be induced on the toroids, adding Gaussian noise to these voltages, and then using the algorithms to calculate the currents and their locations from the simulated voltage measurements.
JLAB, Newport News, Virginia
CNU, Newport News, Virginia
Beamline components used for diagnostic elements often rely on thermal stabilization, continual physical maintenance (ie. tuning), and frequent beam-based calibrations to maintain specified performance. Direct-sequence spread spectrum (DSSS) pilot tones injected into a particular element and combined with the beam-derived signal can subsequently be separated and used to assess performance degradation. In addition, the DSSS tone can be used as a real-time calibration signal, without interference to the intended diagnostic signal. This paper demonstrates such a technique in the design of a Beam Current Monitor downconverter system, as an intended upgrade to the CEBAF Beam Loss Accounting system. A brief spread-spectrum primer is included, as well as a description of appropriate spreading codes and their generation.
SLAC, Menlo Park, California
The growth rates of the longitudinal higher-order impedance-driven modes beam have greatly increased since the initial PEP-II design and commissioning. This increase is attributed to the addition of 8 1.2 MW RF klystrons and 12 accelerating cavities, as well as to operations at twice the design current and almost four times the luminosity. As a result, the requirement for the longitudinal feedback has greatly increased since the design, and the feedback filters and control schemes have evolved during PEP-II operations. In this paper growth and damping rate data for the high-order cavity driven beam modes are presented and compared with historical estimates and measurements. The available margins for the 4 A LER and 2.2 HER ring case at PEP-II are estimated, and suggestions for reliable operation are also presented. The effect of noise in the feedback processing channel is also studied. Data for different gain configurations of the filter from PEP and other accelerators are presented. These data show the tradeoff between instability control and higher noise levels.
KEK, Ibaraki
SLAC, Menlo Park, California
Dimtel, San Jose
A bunch-by-bunch feedback system has been developed to suppress longitudinal coupled-bunch instabilities at the KEK-PF. A generalpurpose signal processor, called iGp, has been developed by a collaboration among KEK, SLAC and INFN-LNF. A longitudinal kicker based on the DAΦNE-type over-damped cavity was designed and installed in the ring. The whole feedback loop was closed at the end of June 2007. The longitudinal dipole-mode instabilities are successfully suppressed up to 430 mA. The performance and the details of the system will be presented in this paper.
Northern Illinois University, DeKalb, Illinois
Electrons crossing the boundary between different media generate bursts of transition radiation. In the case of bunches of N electrons, the radiation is coherent and has an N-squared enhancement at wavelengths related to the longitudinal bunch distribution. This coherent transition radiation has therefore attracted attention as an interceptive charged particle beam diagnostic technique. Many analytical descriptions have been devised describing the spectral distribution generated by electron bunches colliding with thin metallic foils making different simplifying assumptions. For typical bunches having lengths in the sub-millimeter range, measurable spectra are generated up into the millimeter range. Analysis of this THz radiation is performed using optical equipment tens of millimeters in size. This gives rise to concern that optical diffraction effects may spread the wavefront of interest into regions larger than the optical elements and partially escape detection, generating a wavelength-dependent instrument response. In this paper we present a model implementing vector diffraction theory to analyze these effects in bunch length diagnostics based on coherent transition radiation.
KEK, Ibaraki
Recent high-intensity machines with a multi-bunched beam demand bunch-by-bunch beam diagnostics. Especially in a colliding machine such as KEKB, the beam-beam effect is one of the main issues from the viewpoint of beam dynamics. Recently, KEKB achieved an effective head-on collision by using of crab cavities and gained a higher specific luminosity. A gated beam-position monitor, being capable of measuring the beam phase as well as the transverse position of a specific bunch in a bunch train, has been developed and is used to measure a beam-beam kick at KEKB. The monitor detects the beam position with a resolution of a few micrometers. The monitor demonstrated the effect of the crab cavities and estimated the effective horizontal beam size at the interaction point from a linear part of a beam-beam kick. Moreover, the monitor detected a displacement of the horizontal beam position along a bunch train under the crabbing collision. We estimate that the horizontal displacement is caused by an asymmetric kick of the crab cavities, which is based on a bunch-by-bunch phase modulation due to transient beam loading.
LANL, Los Alamos, New Mexico
Beam position monitors planned for the LANSCE diagnostics upgrade will be required to measure beam position and phase of the 201.25-MHz bunched beam in the proton linac. One method to do this is direct down conversion to in-phase and quadrature-phase data of the BPM signals using either commercial digitizers or custom designed hardware. We are evaluating selected hardware for systems with emphasis on COTS hardware to the extent practical. Basic system requirements include a beam current range of 26 db, position resolution of 0.05% of beam aperture and relative phase measurement with 0.25 degree resolution. We present our results to date on three approaches including ZTEC Instruments ZT-410 digitizers, a custom four-channel ADC analog front end board combined with National instruments, Inc. digital I/O and some limited data taken with the Instrumentation Technologies Libera system. The Libera system is a stand-alone BPM system. The other two systems use PCI cards in a standard PC running Windows XP. Our primary points of comparison include measured position resolution, phase resolution, phase linearity, minimum cycle rate and approximate cost for these portions of a BPM system.
JLAB, Newport News, Virginia
In this contribution we present results of the timing jitter and energy modulation of the electron beam in the driver energy recovery linac (ERL) for the JLab FEL. Measurements techniques are described. The effects of the timing jitter and the energy modulation on the performance of the FEL are discussed.
Fermilab, Batavia
The Fermilab A0 photoinjector facility consists of an L-band photocathode (PC) gun and a 9-cell SC rf accelerating structure which combine to generate up to 16-MeV electron beams. The drive laser operates at 81.25 MHz, although the micropulse structure is usually counted down to 9 MHz. Bunch length measurements of the laser micropulse and the e-beam micropulse have been done in the past with a single-sweep module of the Hamamatsu C5680 streak camera system with an intrinsic shot-to-shot trigger jitter of 10 to 20 ps. We have upgraded the camera system with the synchroscan module tuned to 81.25 MHz and a phase-locked delay box to provide synchronous summing capability with less than 1.5 ps FWHM trigger jitter. This allows us to measure both the UV laser pulse train at 244 nm and the e-beam via optical transition radiation (OTR). Due to the low OTR signals, we typically summed over 50 micropulses with 1 nC per micropulse. We also identified a significant e-beam micropulse elongation effect from 10 to 30 ps (FWHM) as the charge was varied from 1 to 5 nC. This is attributed to space-charge effects in the PC gun as reproduced by ASTRA calculations.
CERN, Geneva
Wire scanners are instruments to measure the transverse beam profile. The measurement is performed by moving a thin wire across the path of the particle beam while monitoring the secondary particles. One of the limiting factor in application of wire scanners for high-intensity beams is the wire resistance to high temperature. In this work a heat flow equation for a carbon wire passing through a particle beam is solved. The equation contains modeling of wire heating induced by electromagnetic field of the beam and by electronic energy loss of the protons passing through the wire. The cooling processes considered are conduction, radiation, thermionic emission and sublimation enthalpy. Due to the equation nonlinearity a numerical approach based on discretization of the wire movement is used. An estimation of the wire sublimation rate is made. The model is tested on SPS and LEP data. An other limitation of a wire scanner application is a superconducting environment. The energy deposition in the magnet coils of downstream superconducting LHC magnets is estimated using Geant4 simulation package. In conclusions the limits of Wire Scanner operation on LHC beams are drawn.