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Analyzing measurements of nonlinear transfer functions with Tschebyshev polynomials

Description: Recently, due to advances in computers and data aquisition systems, the following type of measurement has become more common: (1) Impress a given modulation on a device to be tested. (2) Acquire a data stream, usually at equally spaced sample intervals, of the response of the system to the modulation. (3) Fit the data thereby acquired to some nonlinear function set that might (or might not!) describe the response of the device. In this paper it is pointed out that by choosing to modulate the test parameter sinusoidally, and by fast-Fourier transforming the acquired data stream, one unambiguously determines the Tschebyshev expansion of the response function around the working point, potentially yielding quantitative information about high nonlinear orders in the system response. The need for data fitting is thereby eliminated. A detailed example, the analysis of the nonlinear phase-phase transfer function in the Jefferson Lab injector, is presented.
Date: June 1, 1997
Creator: Krafft, G. A.
Partner: UNT Libraries Government Documents Department

A multislit transverse-emittance diagnostic for space-charge-dominated electron beams

Description: Jefferson Lab is developing a 10 MeV injector to provide an electron beam for a high-power free-electron laser (FEL). To characterize the transverse phase space of the space-charged-dominated beam produced by this injector, the authors designed an interceptive multislit emittance diagnostic. It incorporates an algorithm for phase-space reconstruction and subsequent calculation of the Twiss parameters and emittance for both transverse directions at an update rate exceeding 1 Hz, a speed that will facilitate the transverse-phase-space matching between the injector and the FEL`s accelerator that is critical for proper operation. This paper describes issues pertaining to the diagnostic`s design. It also discusses the acquisition system, as well as the software algorithm and its implementation in the FEL control system. First results obtained from testing this diagnostic in Jefferson Lab`s Injector Test Stand are also included.
Date: June 1, 1997
Creator: Piot, P.; Song, J. & Li, R.
Partner: UNT Libraries Government Documents Department

Recirculating accelerator driver for a high-power free-electron laser: A design overview

Description: Jefferson Lab is building a free-electron laser (FEL) to produce continuous-wave (cw), kW-level light at 3-6 {mu}m wavelength. A superconducting linac will drive the laser, generating a 5 mA average current, 42 MeV energy electron beam. A transport lattice will recirculate the beam back to the linac for deceleration and conversion of about 75% of its power into rf power. Bunch charge will range up to 135 pC, and bunch lengths will range down to 1 ps in parts of the transport lattice. Accordingly, space charge in the injector and coherent synchrotron radiation in magnetic bends come into play. The machine will thus enable studying these phenomena as a precursor to designing compact accelerators of high-brightness beams. The FEL is scheduled to be installed in its own facility by 1 October 1997. Given the short schedule, the machine design is conservative, based on modifications of the CEBAF cryomodule and MIT-Bates transport lattice. This paper surveys the machine design.
Date: June 1, 1997
Creator: Bohn, C.L.
Partner: UNT Libraries Government Documents Department

Strategies for minimizing emittance growths in high intensity CW FEL injectors

Description: We discuss beam emittance growths in high intensity CW FEL injectors due to linear and non-linear space charge fields, phase space bifurcation, skew-quad effect, longitudinal momentum modification by space charges in bends, and equipartitioning, etc. We generalize Kim`s RF laser gun theory to DC laser guns. We discuss the traditional free energy theory of emittance growth. The best strategies for designing high-intensity CW FEL injectors are derived, and their application to the design analysis of the CEBAF 10 MeV DC laser gun CW FEL injector test stand is discussed.
Date: July 1996
Creator: Liu, Hongxiu
Partner: UNT Libraries Government Documents Department

Measuring longitudinal distribution and bunch length of femtosecond bunches with RF zero-phasing method

Description: Recently there has been increasing interest in applications of very short electron bunches. Accurately measuring bunch length and profiles becomes essential for characterizing, commissioning, and operating such short bunch machines. The RF zero-phasing method is the only technique that is able to measure bunch length and longitudinal density distribution in the femtosecond regime. In this paper, analytical formulas for calculating longitudinal profile and bunch length are given. using a such technique, bunch lengths as short as 84 fs rms have been measured.
Date: June 1, 1997
Creator: Wang, D.X. & Krafft, G.A.
Partner: UNT Libraries Government Documents Department

Shielded transient self-interaction of a bunch entering a circle from a straight path

Description: When a short (mm-length) bunch with high (nC-regime) charge is transported through a magnetic bending system, self-interaction via coherent synchrotron radiation (CSR) and space charge may alter the bunch dynamics significantly. The authors consider a Gaussian rigid-line-charge bunch following a straight-path trajectory into a circle, with the trajectory centered between two infinite, parallel, perfectly conducting plates. Transients associated with CSR and space charge generated from source particles both on the straight path and the circle are calculated, and their net effect on the radiated power is contrasted with that of shielded steady-state CSR.
Date: June 1, 1997
Creator: Li, R.; Bohn, C.L. & Bisognano, J.J.
Partner: UNT Libraries Government Documents Department

Lattice design for a high-power infrared FEL

Description: A 1 kW infrared FEL, funded by the U.S. Navy, is being built at Jefferson Lab. It will be driven by a compact energy-recovering CW superconducting radio-frequency (SRF)-based linear accelerator. Stringent phase space requirements at the wiggler, low beam energy, and high beam current subject the design to numerous constraints. This report addresses these issues and presents a design solution for an accelerator transport lattice meeting the requirements imposed by physical phenomena and operational necessities.
Date: June 1, 1997
Creator: Douglas, D.R.
Partner: UNT Libraries Government Documents Department

1 nA beam position monitoring system

Description: A system has been developed at Jefferson Lab for measuring transverse position of very low current beams delivered to the Experimental Hall B of the Continuous Electron Beam Accelerator Facility (CEBAF). At the heart of the system is a position sensitive cavity operating at 1497 MHz. The cavity utilizes a unique design which achieves a high sensitivity to beam position at a relatively low cavity Q. The cavity output RF signal is processed using a down-converter and a commercial lock-in amplifier operating at 100 kHz. The system interfaces with a VME based EPICS control system using the IEEE, 488 bus. The main features of the system are simple and robust design, and wide dynamic range capable of handling beam currents from 1 nA to 1000 nA with an expected resolution better than 100 {mu}m. This paper outlines the design of the system.
Date: June 1, 1997
Creator: Ursic, R.; Flood, R. & Piller, C.
Partner: UNT Libraries Government Documents Department

Electron beam instrumentation techniques using coherent radiation

Description: Much progress has been made on coherent radiation research since coherent synchrotron radiation was first observed in 1989. The use of coherent radiation as a bunch length diagnostic tool has been studied by several groups. In this paper, brief introductions to coherent radiation and far-infrared measurement are given, the progress and status of their beam diagnostic application are reviewed, different techniques are described, and their advantages and limitations are discussed.
Date: June 1, 1997
Creator: Wang, D.X.
Partner: UNT Libraries Government Documents Department

Jefferson Lab IR FEL cryomodule modifications and test results

Description: The Infrared Free Electron Laser being constructed at the Thomas Jefferson National Accelerator Facility will require a 42 MeV, 5 mA electron accelerator. The accelerator design requires a 10 MeV injector and a two pass 32 MeV linac, one pass for acceleration and one pass for energy recovery. In order to minimize the cost of the linac, standard CEBAF 1497 MHZ Superconducting Radio Frequency cavities and cryomodules are being used with minimal changes. Two SRF cavities, housed in a quarter cryomodule, operate at a nominal 10 MV/m to provide the injector energy. The linac is composed of one cryomodule, housing eight SRF cavities operating at an average gradient of 8 MV/m. The modifications to the cryomodule are being made to handle the higher beam current, to improve RF control, and to increase machine reliability. The modifications to the higher order mode (HOM) loads, cavity tuners, cavity beam line, warm and cold RF windows, and cryogenic shield are described. Test results from the injector quarter cryomodule are also presented.
Date: June 1, 1997
Creator: Wiseman, M.; Benesch, J.; Drury, M. & Fisher, J.
Partner: UNT Libraries Government Documents Department

The calculation, simulation, and measurement of longitudinal beam dynamics in electron injectors

Description: Polarized electrons are a valuable commodity for nuclear physics research and every effort must be made to preserve them during transport Measurements of the beam emitted from the polarized source at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) have shown a considerable bunch lengthening with increasing beam current. This lengthening leads to unacceptable loss as the beam passes through the injector chopping system. We present an application of the longitudinal envelope equation to describe the bunch lengthening and compare the results to measurements and simulations using PARMELA. In addition, a possible solution to the problem by adding a low power buncher to the beamline is described and initial results are shown.
Date: June 1, 1997
Creator: Dunham, B.; Liu, H. & Kazimi, R.
Partner: UNT Libraries Government Documents Department

Automatic pumpdown of the 2K cold compressors for the CEBAF central helium liquefier

Description: The 2K system at CEBAF includes four stages of centrifugal cold compressors that operate between a suction pressure of 0.031 atm and a discharge of {approximately}1 atm. Starting the cold compressors and pumping the system down to operating pressure is a complex and time consuming operation. Many months of rigorous testing were required to develop a reliable startup procedure. This procedure has been automated through the development of an Autopumpdown program that intelligently coordinates the necessary steps across the boundaries of the existing cryogenic and accelerator control systems. this program has greatly reduced both the time and effort required to recover from failures and restart the system. The paper describes the requirements addressed by the program, the steps it takes while operating, the historical development of the program, the spin-off benefits, and the course of anticipated future developments.
Date: August 1, 1996
Creator: Bevins, B.S.; Chronis, W.C. & Keesee, M.S.
Partner: UNT Libraries Government Documents Department

Conceptual foundation of the Fokker-Planck approach to space-charge effects

Description: An rms-mismatched beam can evolve rapidly to a configuration of quasiequilibrium under the influence of space-charge forces. As sit evolves, its emittance grows and a diffuse halo forms. The beam's distribution function accounts for all the complicated dynamics. Unfortunately, the distribution function is difficult to calculate in as much as the physics lies at the interface between classical mechanics and thermodynamics. This paper presents the foundation for a statistical theory of the dynamics of nonequilibrium space-charge-dominated beams. Within certain approximations, the theory takes on a Fokker-Planck form. Key questions arise concerning the nature of the dynamical friction and diffusion in the beam's phase space and of the quasiequilibrium configuration that ensues.
Date: July 1, 1996
Creator: Bohn, C.L.
Partner: UNT Libraries Government Documents Department

High power electron beam dumps at CEBAF

Description: The CEBAF accelerator produces a very small emittance CW electron beam of up to 200 {mu}A average current. The resulting beam power, up to 1 MW at 5 GeV, and the very high beam power density, pose challenging problems for beam dump design. Two styles of high power dumps have been developed. The first, rated for 100+ kW, is used for beam tune-up and accelerator commissioning. The beam power is entirely contained in metal in this dump, minimizing the problems associated with radioactive water handling. Full power 1 MW dumps are used with the experimental halls. In these dumps, one-third of the beam power is directly absorbed in water. Both dump designs require the beam to be rastered when the smallest beam sizes are used. Design details for each of these dumps will be presented.
Date: June 1, 1997
Creator: Wiseman, M.; Sinclair, C.K.; Whitney, R. & Zarecky, M.
Partner: UNT Libraries Government Documents Department

A portable accelerator control toolkit

Description: In recent years, the expense of creating good control software has led to a number of collaborative efforts among laboratories to share this cost. The EPICS collaboration is a particularly successful example of this trend. More recently another collaborative effort has addressed the need for sophisticated high level software, including model driven accelerator controls. This work builds upon the CDEV (Common DEVice) software framework, which provides a generic abstraction of a control system, and maps that abstraction onto a number of site-specific control systems including EPICS, the SLAC control system, CERN/PS and others. In principle, it is now possible to create portable accelerator control applications which have no knowledge of the underlying and site-specific control system. Applications based on CDEV now provide a growing suite of tools for accelerator operations, including general purpose displays, an on-line accelerator model, beamline steering, machine status displays incorporating both hardware and model information (such as beam positions overlaid with beta functions) and more. A survey of CDEV compatible portable applications will be presented, as well as plans for future development.
Date: June 1, 1997
Creator: Watson, W.A. III
Partner: UNT Libraries Government Documents Department

Improvement of the operational performance of SRF cavities via in situ helium processing and waveguide vacuum processing

Description: The useful performance range of the superconducting rf (SRF) cavities in the CEBAF accelerator at Jefferson Lab is frequently limited by electron field emission and derived phenomena. Improvements are required to support future operation of the accelerator at higher than 5 GeV. Twelve operational cryomodules have been successfully processed to higher useful operating gradients via rf-helium processing. Progress against field emission was evidenced by improved high-field Q, reduced x-ray production and greatly reduced incidence of arcing at the cold ceramic window. There was no difficulty reestablishing beamline vacuum following the processing. Cavities previously limited to 4-6 MV/m are now operating stably at 6-9 MV/m. By applying a pulsed-rf processing technique, we have also improved the pressure stability of the thermal transition region of the input waveguide for several cavities.
Date: June 1, 1997
Creator: Reece, C.E.; Drury, M.; Rao, M.G. & Nguyen-Tuong, V.
Partner: UNT Libraries Government Documents Department

A high-DC-voltage GaAs photoemission gun: Transverse emittance and momentum spread measurements

Description: We have built a high-DC-voltage photoemission gun and a diagnostic beamline permitting us to measure rms transverse emittance ({epsilon}{sub x}) and rms momentum spread ({delta}) of short-duration electron pulses produced by illuminating the cathode with light from a mode-locked, frequency-doubled Nd:YLF laser. The electron gun is a GaAs photocathode source designed to operate at 500kV. We have measured {epsilon}{sub x} and {delta} for conditions ranging from emittance-dominated to space-charge-dominated. We report these measurements as functions of microbunch charge for different beam radii, pulse lengths, and voltages/field gradients at the cathode, and compare them with PARMELA calculations.
Date: June 1, 1997
Creator: Engwall, D.; Bohn, C. & Cardman, L.
Partner: UNT Libraries Government Documents Department

Operating experience and reliability improvements on the 5 kW CW klystron at Jefferson Lab

Description: With substantial operating hours on the RF system, considerable information on reliability of the 5 kW CW klystrons has been obtained. High early failure rates led to examination of the operating conditions and failure modes. Internal ceramic contamination caused premature failure of gun potting material and ultimate tube demise through arcing or ceramic fracture. A planned course of reporting and reconditioning of approximately 300 klystrons, plus careful attention to operating conditions and periodic analysis of operational data, has substantially reduced the failure rate. It is anticipated that implementation of planned supplemental monitoring systems for the klystrons will allow most catastrophic failures to be avoided. By predicting end of life, tubes can be changed out before they fail, thus minimizing unplanned downtime. Initial tests have also been conducted on this same klystron operated at higher voltages with resultant higher output power. The outcome of these tests will provide information to be considered for future upgrades to the accelerator.
Date: June 1, 1997
Creator: Nelson, R. & Holben, S.
Partner: UNT Libraries Government Documents Department

Magnetic measurements of the prototype dipole for the IR-FEL at the Thomas Jefferson National Accelerator Facility

Description: Magnetic measurements have been performed on the prototype dipole for the high power IR-FEL presently under construction at the Thomas Jefferson National Accelerator Facility. The optics-driven requirements for these magnets include low fields, large horizontal apertures, tight field homogeneity, absolute setability of core field & integrated field, and control of the horizontal & vertical focusing terms designed into the magnets. A prototype dipole was fabricated and underwent several iterations of mechanical adjustment and magnetic measurement. Measurements were made to quantify the effects of field clamps on vertical focusing terms and effective length. Additional tests were made using various applications of Purcell gaps and high permeability materials in order to achieve the required homogeneity. Results from the prototype have been integrated into the design of the seven families of dipoles needed for the FEL.
Date: June 1, 1997
Creator: Karn, J.; Biallas, G.; Guerra, A. & Harwood, L.
Partner: UNT Libraries Government Documents Department

Development of digital feedback systems for beam position and energy at the Thomas Jefferson National Accelerator Facility

Description: The development of beam-based digital feedback systems for the CEBAF accelerator has gone through several stages. As the accelerator moved from commissioning to operation for the nuclear physics program, the top priority was to stabilize the beam against slow energy and position drifts (<1 Hz). These slow drifts were corrected using the existing accelerator monitors and actuators driven by software running on top of the EPICS control system. With slow drifts corrected, attention turned to quantifying the higher frequency disturbances on the beam and to designing the required feedback systems needed to achieve the CEBAF design stability requirements. Results from measurements showed the major components in position and energy to be at harmonics of the power line frequencies of 60, 120, and 180 Hz. Hardware and software was installed in two locations of the accelerator as prototypes for the faster feedback systems needed. This paper gives an overview of the measured beam disturbances and the feedback systems developed.
Date: June 1, 1997
Creator: Karn, J.; Chowdhary, M. & Hutton, A.
Partner: UNT Libraries Government Documents Department

Status report on Jefferson Lab`s high-power infrared free-electron laser

Description: Jefferson Lab is building a free-electron laser to produce tunable, continuous-wave (cw), kW-level light at 3-6 {mu}m wavelength. A superconducting accelerator will drive the laser, and a transport lattice will recirculate the beam back through the accelerator for energy recovery. Space charge in the injector and coherent synchrotron radiation in magnetic bends will be present, and the machine is instrumented to study these phenomena during commissioning. The wiggler and optical cavity are conventional; however, significant analysis and testing was needed to ensure mirror heating at 1 kW of outcoupled power would not impede performance. The FEL is being installed in its own facility, and installation will be finished in Fall 1997. This paper surveys the machine, the status of its construction, and plans for its commissioning.
Date: October 1, 1997
Creator: Bohn, C. L.
Partner: UNT Libraries Government Documents Department

Design and implementation of a slow orbit control package at Thomas Jefferson National Accelerator Facility

Description: The authors describe the design and implementation of a C++ client/server based slow orbit and energy control package based on the CDEV software control bus. Several client applications are described and operational experience is given.
Date: June 1, 1997
Creator: Zeijts, J. van; Witherspoon, S. & Watson, W.A.
Partner: UNT Libraries Government Documents Department

Beam breakup in a microtron

Description: In a microtron, the path length change from pass to pass is a fixed multiple of the RF wavelength, and the accelerating system can be reasonably well approximated as a single cavity. Under such circumstances it is possible to derive an analytical formula for the multipass beam breakup threshold current. The threshold current determined by numerical simulations agrees very well with the formula for a machine with a small number of passes. The analytic formula can serve as a useful guide in examining optics designs to improve the BBU threshold.
Date: June 1, 1997
Creator: Yung, B.C. & Merminga, L.
Partner: UNT Libraries Government Documents Department

Linear optics correction in the CEBAF accelerator

Description: During commissioning of the CEBAF accelerator, correcting dispersion, momentum compaction and betatron beam envelopes was essential for robust operation. To speed the diagnostic process we developed a method which allows one to track and correct the machine optics on-line. The method is based on measuring the propagation of 30 Hz modulated betatron oscillations. The beam optics of the accelerator was altered to decrease lattice sensitivity at critical points and to simplify control of the betatron function match. The calculation of the Courant-Snyder invariant from signals of each pair of beam position monitors was used for a correction of the betatron functions. The experience of optics correction and the study of long and short term machine reproducibility obtained during 1996 and early 1997 are also discussed. With minor modifications this method can also be used for on-line optics measurement and correction in circular accelerators.
Date: June 1, 1997
Creator: Lebedev, V.A.; Bickley, M. & Bisognano, J.
Partner: UNT Libraries Government Documents Department