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Performance experience with the CEBAF SRF cavities

Description: The full complement of 169 pairs of niobium superconducting cavities has been installed in the CEBAF accelerator. This paper surveys the performance characteristics of these cavities in vertical tests, commissioning in the tunnel, and operational experience to date. Although installed performance exceeds specifications, and 3.2 GeV beam has been delivered on target, present systems do not consistently preserve the high performance obtained in vertical dewar tests as operational capability. Principal sources of these limitations are discussed.
Date: December 31, 1995
Creator: Reece, C.; Benesch, J.; Drury, M.; Hovater, C.; Mammosser, J. & Preble, J.
Partner: UNT Libraries Government Documents Department

Energy stability in recirculating, energy-recovering linacs

Description: Recirculating, energy-recovering linacs can be used as driver accelerators for high power FELs. Instabilities which arise from fluctuations of the cavity fields are investigated. Energy changes can cause beam loss on apertures, or, when coupled to M{sub 56}, phase oscillations. Both effects change the beam induced voltage in the cavities and can lead to unstable variations of the accelerating field. Stability analysis for small perturbations from equilibrium is performed and threshold currents are determined. Furthermore, the analytical model is extended to include amplitude and phase feedback, with the transfer function in the feedback path presently modeled as a low-pass filter. The feedback gain and bandwidth required for stability are calculated for the high power UV FEL proposed for construction at CEBAF. 4 refs.
Date: July 1, 1996
Creator: Merminga, L.; Bisognano, J.J. & Delayen, J.R.
Partner: UNT Libraries Government Documents Department

CEBAF beam loss accounting

Description: This paper describes the design and implementation of a beam loss accounting system for the CEBAF electron accelerator. This system samples the beam curent throughout the beam path and measures the beam current accurately. Personnel Safety and Machine Protection systems use this system to turn off the beam when hazardous beam losses occur.
Date: December 31, 1995
Creator: Ursic, R.; Mahoney, K.; Hovater, C.; Hutton, A. & Sinclair, C.
Partner: UNT Libraries Government Documents Department

Emittance measurements for the Illinois/CEBAF polarized electron source

Description: The transverse thermal properties of the electrons photoemitted from GaAs determine the intrinsic beam emittance, an important quantity in applications such as polarized electron sources and high-brightness sources. In this paper, emittance measurements using the Illinois/CEBAF polarized electron source are described. The emittance was measured as a function of both the laser beam spot size and laser wavelength at low currents. The data was used to infer the transverse thermal energy of the electrons photoemitted from GaAs for wavelengths between 514 and 840 nm. Near the bandgap the transverse energy is {approximately}34 meV, a factor of 3 lower than that of the beam from a typical thermionic electron gun. 8 refs., 2 figs.
Date: January 1, 1995
Creator: Dunham, B.M.; Cardman, L.S. & Sinclair, C.K.
Partner: UNT Libraries Government Documents Department

The CEBAF control system for the CHL

Description: The CEBAF Central Helium Liquefier (CHL) control system consists of independent safety controls located at each subsystem, CAMAC computer interface hardware, and a CEBAF-designed control software called Thaumaturgic Automated Control Logic (TACL). The paper describes how control software was interfaced with the subsystems of the CHL. Topics of configuration, editing, operator interface, datalogging, and internal logic functions are presented as they relate to the operational needs of the helium plant. The paper also describes the effort underway to convert from TACL to the Experimental Physics and Industrial Control System (EPICS), the new control system for the CEBAF accelerator. This software change will require customizing EPICS software to cryogenic process control.
Date: August 1, 1996
Creator: Keesee, M.S. & Bevins, B.S.
Partner: UNT Libraries Government Documents Department

Arcing phenomena on CEBAF RF-windows at cryogenic temperatures

Description: During the CEBAF commissioning tests some of the superconducting cavities had light emitting discharges (arcing) which were observed in the guard vacuum space between a warm polymeric rf window and the cold ceramic rf window. A dedicated off-line test system was implemented to investigate the conditions under which arcing may occur and to gain some understanding of the mechanisms leading to this phenomenon through optical spectral analysis. This paper reports on the photoemission spectra observed during the dedicated tests on a single cell 1500 MHz niobium cavity with a ceramic window operated at 10 MV/m and 2 K. The light emission was detected using a spectrometer with an intensified photodiode array. The effect of moving the window away from the beam line using a waveguide elbow is reported. 12 refs.
Date: December 31, 1995
Creator: Powers, T.; Kneisel, P. & Allen, R.
Partner: UNT Libraries Government Documents Department

Rapid application development using the Tcl/Tk language

Description: During the last year, high level applications at CEBAF were written using the Tcl/Tk scripting language. This language is rapidly gaining in popularity, in part due to ease of constructing programs with X11 graphical user interfaces, and in part to ease of adding compiled user code for specialized purposes. Extensions to the language provide object oriented programming, which was used to develop a hierarchy of classes relevant for high level accelerator control. We describe basic language features, some 3rd party add-on packages, and local additions to the toolbox. Next we describe features of the accelerator object hierarchy, and finally describe applications written using this toolbox such as the ModelServer prototype, Slow Orbit and Energy Lock, the Linac Energy Management System, and other applications.
Date: December 31, 1995
Creator: van Zeijts, J.
Partner: UNT Libraries Government Documents Department

The CEBAF fiber optic phase reference system

Description: The specified phase stability of the CEBAF RF distribution system is 2.9{degree} rms per linac. Stability is achieved through the use of a temperature and pressure regulated coaxial drive line. Purpose of the fiber optic phase reference system is to monitor the relative phase at the beginning and ending of this drive line, between linacs, injector and separator to determine drift due to ambient temperature fluctuations. The system utilizes an Ortel 1310 nm single mode laser driving Sumitumo optical fiber to distribute a reference signal at 1497 MHz. Phase of this reference signal is compared to the 1427 MHz (LO) and the 70 MHz (IF) via a 360{degree} phase detector. The detected information is then routed to the CEBAF control system for display with a specified resolution of {+-}0.2{degree} over a 20{degree} phase delta.
Date: December 31, 1995
Creator: Crawford, K.; Simrock, S.; Hovater, C. & Krycuk, A.
Partner: UNT Libraries Government Documents Department

Accelerator design for the high-power industrial FEL

Description: We have developed a conceptual design for an industrial-use kilowatt UV and IR FEL driven by a recirculating, energy-recovering 200 MeV, 1- 5 mA superconducting rf (SRF) electron accelerator. In this paper we describe the accelerator design of this FEL. The accelerator consists of a 10 MeV injector, a 96 MeV SRF linac with a two-pass transport which accelerates the beam to 200 MeV, followed by energy-recovery deceleration through two passes to the dump. Technical challenges include high-intensity injector development, multi-pass energy- recovery operation, SRF modifications and control for FEL operation, development of tuneable, nearly-isochronous, large-acceptance transports, and matching of the beam to the FEL wiggler. An overview of the accelerator design is presented. 9 refs., 1 fig., 1 tab.
Date: December 31, 1995
Creator: Neuffer, D.V.; Benson, S. & Bisognano, J.
Partner: UNT Libraries Government Documents Department

Advanced surface cleaning methods: three years of experience with high pressure ultrapure water rinsing of superconducting cavaties

Description: In the last three years we have carried out a large number of tests ofn single cell and multi-cell niobium and Nb{sub 3}Sn cavities at L- band frequencies, which as a final surface cleaning step had been rinsed with high pressure jets of ultrapure water. This treatment resulted in an unprecedented quality and reproducibility of cavity performance. Field emission free surfaces up to peak surface electric fields of E{sup peak} {ge} 45 MV/m were achieved nearly routinely after buffered chemical polishing of niobium surfaces. In addition, residual surface resistances below R{sub res} {le} 10 n{Omega} and as low as R{sub res} = 2 n{Omega} were not uncommon. In 5-cell production cavities of the Cornell/CEBAF shape gradients as high as E{sub acc} =21.5 MV/m corresponding to peak surface fields of E{sub peak} {approx} 55 MV/m have been measured after post purification with Ti without the need for rf-processing. Several Nb{sub 3}Sn - cavities exhibited no field emission loading after high pressure ultrapure water rinsing up to the maximum achievable surface fields of E{sup peak} {approx} 33 MV/m; the field limits were given by the available rf-power. The unprecedented reproducibility of the cavities permitted serial testing of various parameters affecting cavity performance such as the influence of residual gas inside the cavities prior to cooldown, the removal of the surface damage layer or the impact of peripheral parts such as rf-windows. The major portion of this paper summarizes several of the results obtained from investigations carried out during the last three years. The second part discusses possibilities for further improvements in cavity cleaning.
Date: January 1, 1995
Creator: Kneisel, P. & Lewis, B.
Partner: UNT Libraries Government Documents Department

Coherent transition radiation produced by a 1.2 MeV electron beam

Description: We describe a method of generating very high-frequency coherent radiation using an electron beam source with a maximum beam energy of 1.2 MeV. We show that, though the high frequency cutoff for the radiation generated when the beam impacts a target at normal incidence is reduced by transverse beam size effects, it is nevertheless possible to generate much higher frequencies by a judicious choice of the angles of incidence and observation. 5 refs., 4 figs., 1 tab.
Date: December 31, 1995
Creator: Benson, S.V.; Dutta, J.M.; Jones, C.R.; Kosai, H. & Swartz, J.
Partner: UNT Libraries Government Documents Department

Measuring and adjusting the path length at CEBAF

Description: Accurately setting the path length around the machine is central to the proper operation of the CEBAF accelerator. The CEBAF main accelerator consists of two recirculating superconducting linacs operating at 1497 MHz fundamental frequency. The electron beam can recirculate up to five times through the two linacs before it is extracted to the experimental halls. In order to obtain maximum energy gain and minimum energy spread through the linacs, all passes should arrive at the beginning of the linacs in phase at the crest of the RF cycle. In this paper we explain how the arrival times of higher pass beams are measured with respect to the first pass to less than one degree of RF phase and how the path length around the machine is adjusted. Following a brief introduction to the CEBAF design and some local nomenclature, these topics will be discussed: differential RF phase measurements of time delay, the energy method of cresting the higher pass beams, results obtained with the measurement techniques, future plans and improvements to the devices, and finally, a set of conclusions. 3 refs., 5 figs., 1 tab.
Date: December 31, 1995
Creator: Krafft, G.A.; Crofford, M. & Douglas, D.R.
Partner: UNT Libraries Government Documents Department

Energy stability in a high average power FEL

Description: Recirculating, energy-recovering linacs can be used as driver accelerators for high power FELs. Instabilities which arise from fluctuations of the cavity fields are investigated. Energy changes can cause beam loss on apertures, or, when coupled to M{sub 56}, phase oscillations. Both effects change the beam induced voltage in the cavities and can lead to unstable variations of the accelerating field. Stability analysis for small perturbations from equilibrium is performed and threshold currents determined. Design strategies to increase the instability threshold are discussed and the high average power FEL proposed for construction at CEBAF is used as an example.
Date: December 31, 1995
Creator: Merminga, L. & Bisognano, J.J.
Partner: UNT Libraries Government Documents Department

Rf system modeling for the high average power FEL at CEBAF

Description: High beam loading and energy recovery compounded by use of superconducting cavities, which requires tight control of microphonic noise, place stringent constraints on the linac rf system design of the proposed high average power FEL at CEBAF. Longitudinal dynamics imposes off-crest operation, which in turn implies a large tuning angle to minimize power requirements. Amplitude and phase stability requirements are consistent with demonstrated performance at CEBAF. A numerical model of the CEBAF rf control system is presented and the response of the system is examined under large parameter variations, microphonic noise, and beam current fluctuations. Studies of the transient behavior lead to a plausible startup and recovery scenario.
Date: December 31, 1995
Creator: Merminga, L.; Fugitt, J.; Neil, G. & Simrock, S.
Partner: UNT Libraries Government Documents Department

A high-average-power FEL for industrial applications

Description: CEBAF has developed a comprehensive conceptual design of an industrial user facility based on a kilowatt UV (150-1000 nm) and IR (2-25 micron) FEL driven by a recirculating, energy-recovering 200 MeV superconducting radio-frequency (SRF) accelerator. FEL users{endash}CEBAF`s partners in the Laser Processing Consortium, including AT&T, DuPont, IBM, Northrop-Grumman, 3M, and Xerox{endash}plan to develop applications such as polymer surface processing, metals and ceramics micromachining, and metal surface processing, with the overall effort leading to later scale-up to industrial systems at 50-100 kW. Representative applications are described. The proposed high-average-power FEL overcomes limitations of conventional laser sources in available power, cost-effectiveness, tunability and pulse structure. 4 refs., 3 figs., 2 tabs.
Date: December 31, 1995
Creator: Dylla, H.F.; Benson, S. & Bisognano, J.
Partner: UNT Libraries Government Documents Department

Precision intercomparison of beam current monitors at CEBAF

Description: The CEBAF accelerator delivers a CW electron beam at fundamental 1497 MHz, with average beam current up to 200 {mu}A. Accurate, stable nonintercepting beam current monitors are required for: setup/control, monitoring of beam current and beam losses for machine protection and personnel safety, and providing beam current information to experimental users. Fundamental frequency stainless steel RF cavities have been chosen for these beam current monitors. This paper reports on precision intercomparison between two such RF cavities, an Unser monitor, and two Faraday cups, all located in the injector area. At the low beam energy in the injector, it is straightforward to verify the high efficiency of the Faraday cups, and the Unser monitor included a wire through it to permit an absolute calibration. The cavity intensity monitors have proven capable of stable, high precision monitoring of the beam current.
Date: December 31, 1995
Creator: Kazimi, R.; Dunham, B.; Krafft, G.A.; Legg, r.; Liang, C.; Sinclair, C. et al.
Partner: UNT Libraries Government Documents Department

Survey and analysis of line-frequency interference in the CEBAF accelerator

Description: Feedthrough of interference from the AC power line into accelerator components is a problem which in pulsed accelerators can be reduced by operation synchronous with the AC line. This means of avoiding line-frequency effects is ineffective for continuous wave machines such as the CEBAF accelerator. We have measured line-frequency perturbations at CEBAF both in beam position and energy by using the beam position monitor system as a multiple-channel sampling oscilloscope. Comparing these data against the measured static optics (taken synchronously with the AC line) we have been able to identify point sources of interference, and resolve line-synchronous variations in the beam energy at a level near 0.001%. 3 refs., 2 figs., 1 tab.
Date: December 31, 1995
Creator: Tiefenback, M.G. & Li, Rui
Partner: UNT Libraries Government Documents Department

Arbitrary order transfer maps for RF cavities

Description: Current modeling of transfer maps for superconducting RF cavities at CEBAF includes only linear effects. Here we extend the transfer mapping modeling capability to include arbitrary order field information generated from the MAFIA field data. We include coupler kicks, normal and skew quadrupole focussing and higher order effects.
Date: December 31, 1995
Creator: van Zeijts, J.
Partner: UNT Libraries Government Documents Department

Design of a High Charge CW Photocathode Injector Test Stand at CEBAF

Description: A 10 MeV high-charge CW electron injector test stand has been designed for the CEBAF UV FEL driver accelerator. It consists of a 500 kV DC photocathode gun, a 1500 MHz room-temperature buncher, a modified CEBAF cryounit (quarter cryomodule) with an SRF accelerating gradient of {approximately}10 MV/m, two solenoids in the 500 kV region and an achromatic, non-isochronous injection transport line delivering 10 MeV beam to the driver accelerator. Experimental work is in progress toward establishing design system performance. 21 refs. , 2 figs., 3 tabs.
Date: August 1, 1996
Creator: Lliu, H.; Kehne, D. & Benson, S.
Partner: UNT Libraries Government Documents Department

Device control at CEBAF

Description: CEBAF has undergone a major conversion of its accelerator control system from TACL to EPICS, affecting device control for the RF system, magnets, the machine protection system, the vacuum and valves, and the diagnostic systems including beam position monitors, harps, and the camera and solenoid devices (beam viewers, faraday cups, optical transition radiation viewers, synchrotron radiation monitor, etc.). Altogether these devices require approximately 125,000 EPICS database records. The majority of these devices are controlled through CAMAC; some use embedded microprocessors (RF and magnets), and newer interfaces are in VME. The standard EPICS toolkit was extended to include a driver for CAMAC which supports dual processors on one serial highway, custom database records for magnets and BPMs, and custom data acquisition tasks for the BPMs. 2 refs., 1 tab.
Date: August 1, 1996
Creator: Schaffner, S.; Barker, D. & Bookwalter, V.
Partner: UNT Libraries Government Documents Department

Integrated on-line accelerator modeling at CEBAF

Description: An on-line accelerator modeling facility is currently under development at CEBAF. The model server, which is integrated with the EPICS control system, provides coupled and 2nd-order matrices for the entire accelerator, and forms the foundation for automated model- based control and diagnostic applications. Four types of machine models are provided, including design, golden or certified, live, and scratch or simulated model. Provisions are also made for the use of multiple lattice modeling programs such as DIMAD, PARMELA, and TLIE. Design and implementation details are discussed. 2 refs., 4 figs.
Date: December 31, 1995
Creator: Bowling, B.A.; Shoaee, H.; Van Zeijts, J.; Witherspoon, S. & Watson, W.
Partner: UNT Libraries Government Documents Department

Operational monitoring of the CEBAF RF system

Description: An EPICS-based control interface has been developed which enables effective monitoring of the 338 independent RF systems that drive CEBAF`s superconducting cavities. Visual screens allow the operator to quickly identify cavities that have faulted, or which are operating out of specification. These screens allow the operator easy access to automated routines for clearing faults, routines for documenting and tracking hardware problems, and expert screens for immediate correction of RF problems. The combination of visual screens and automated scripts has greatly decreased the time required to identify and recover from RF problems, significantly increasing operational uptime. 2 refs., 1 fig., 1 tab.
Date: December 31, 1995
Creator: Karn, J.; Dunham, B. & Tiefenback, M.
Partner: UNT Libraries Government Documents Department

Location and correction of 60 hz in the CEBAF injector

Description: CEBAF produces a continuous electron beam with an emittance of 2-3 nm-rad. Transverse low frequency magnetic oscillations act to dilute this emittance. These fields are typically associated with AC line conductors. The CEBAF injector is approximately 40 m long. To locate the source(s) of the beam motion, measured offsets were back propagated along the beamline using the DIMAD model. Field measurements were then made at the calculated field source positions and correlated with the measured effects. Corrections and final beam measurements were made to verify the corrections. 2 refs., 4 tabs.
Date: August 1, 1996
Creator: Legg, R.; Douglas, D.; Krafft, G.A. & Saulter, Q.
Partner: UNT Libraries Government Documents Department