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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

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

Preliminary Statistical Analysis of CEBAF's Cavity Pair Assembly Process Data

Description: CEBAF has been collecting much data during the cavity pair assembly process. Some process data has been entered and analyzed during the last two years as part of our attempt to apply statistical process control methods. Analysis is presented here on mechanical tolerances achieved by the industrial fabricator of the CEBAF superconducting rf cavities (Siemens). Suggestions for tolerances obtainable in future procurements are made. Influence of cooldown conditions during vertical test on field emission onset gradient is discussed. An increase in the mean gradient of 2 MV/m was seen after a simple change in procedure.
Date: December 1, 1993
Creator: Benesch, Jay
Partner: UNT Libraries Government Documents Department

Study of Higher Order Modes in High Current Multicell SRF Cavities.

Description: The electron cooling project for RHIC luminosity upgrade and future projects such as eRHIC (electron-ion collider) demand cavities operating at high average current and high bunch charge in CW energy recovery mode. This paper describes the investigation of Higher Order Modes (HOMs) in such a cavity. This work is part of an ongoing effort to develop a 5-cell superconducting cavity for such high current and high bunch charge energy-recovery superconducting linac. The frequency of the cavity is 703.75 MHz with an iris of 17cm and two ferrite absorbers for HOM damping. The main focus of this paper is to identify and investigate possible trapped HOM modes that might result in multibunch instabilities. Detailed MAFIA calculations were performed using the e-module for different end cell geometries. Results from these calculations will be presented. Beam breakup results using TDBBU due to dipole HOM's will also be presented. A beam breakup threshold of above 1.8 A was calculated.
Date: September 8, 2003
Creator: Calaga, R.; Ben-Zvi, Ilan; Zhao, Y.; Wang, D. & Wu, G.
Partner: UNT Libraries Government Documents Department

Status of the first batch of niobium resonator production for the New Delhi booster linac.

Description: This paper reports the status and details of the costs of construction of niobium superconducting resonant cavities for a linear accelerator, presently being built as a booster for the 15 UD tandem Pelletron accelerator at the Nuclear Science Centre, New Delhi. The linear accelerator will have three cryostat modules, each holding eight quarter-wave resonators. Construction of a batch of ten resonators for the linac started at Argonne National Laboratory in May 1997. For production, all fabrication and all electron beam welding is being done through commercial vendors. Details of construction and present status of the project are presented.
Date: March 16, 1999
Creator: Potukuchi, P. N.
Partner: UNT Libraries Government Documents Department

Recent progress on photonic band gap accelerator cavities

Description: We report on the current status of our program to apply Photonic Band Gap (PBG) concepts to produce novel high-energy, high-intensity accelerator cavities. The PBG design on which we have concentrated our initial efforts consists of a square array of metal cylinders, terminated by conducting or superconducting sheets, and surrounded by microwave absorber on the periphery of the structure. A removed cylinder from the center of the array constitutes a site defect where a localized electromagnetic mode can occur. In previous work, we have proposed that this structure could be utilized as an accelerator cavity, with advantageous properties over conventional cavity designs. In the present work, we present further studies, including MAFIA-based numerical calculations and experimental measurements, demonstrating the feasibility of using the proposed structure in a real accelerator application.
Date: February 1997
Creator: Smith, D. R.; Li, D. & Vier, D. C.
Partner: UNT Libraries Government Documents Department

A brief history of high power RF proton linear accelerators

Description: The first mention of linear acceleration was in a paper by G. Ising in 1924 in which he postulated the acceleration of positive ions induced by spark discharges which produced electric fields in gaps between a series of {open_quotes}drift tubes{close_quotes}. Ising apparently was not able to demonstrate his concept, most likely due to the limited state of electronic devices. Ising`s work was followed by a seminal paper by R. Wideroe in 1928 in which he demonstrated the first linear accelerator. Wideroe was able to accelerate sodium or potassium ions to 50 keV of energy using drift tubes connected alternately to high frequency waves and to ground. Nuclear physics during this period was interested in accelerating protons, deuterons, electrons and alpha particles and not heavy ions like sodium or potassium. To accelerate the light ions required much higher frequencies than available at that time. So linear accelerators were not pursued heavily at that time. Research continued during the 1930s but the development of high frequency RF tubes for radar applications in World War 2 opened the potential for RF linear accelerators after the war. The Berkeley laboratory of E. 0. Lawrence under the leadership of Luis Alvarez developed a new linear proton accelerator concept that utilized drift tubes that required a full RF period to pass through as compared to the earlier concepts. This development resulted in the historic Berkeley 32 MeV proton linear accelerator which incorporated the {open_quotes}Alvarez drift tube{close_quotes} as the basic acceleration scheme using surplus 200 MHz radar components.
Date: December 31, 1996
Creator: Browne, J.C.
Partner: UNT Libraries Government Documents Department

Conceptual design of a superconducting high-intensity proton linac

Description: A SCRF (superconducting RF linac) has been developed for a high-intensity proton linac which will be used as the driver for neutron sources. This design is conservative, using current SCRF technologies. As well as lowering operating cost, the design offers performance advantages in availability, beam loss, and upgradability, which are important for the application as a neutron source.
Date: September 1, 1996
Creator: Dominic Chan, K.C.
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

Superconducting cavities for the APT accelerator

Description: The design of an Accelerator Production of Tritium (APT) facility being investigated at Los Alamos includes a linear accelerator using superconducting rf-cavities for the acceleration of a high-current cw proton beam. For electron accelerators with particles moving at the speed of light ({beta} {approx} 1.0), resonators with a rounded shape, consisting of ellipsoidal and cylindrical sections, are well established. They are referred to as elliptical cavities. For the APT-design, this shape has been adapted for much slower proton beams with {beta} ranging from 0.60 to 0.94. This is a new energy range, in which resonators of an elliptical type have never been used before. Simulations with the well-proven electromagnetic modeling tools MAFIA and SUPERFISH were performed. The structures have been optimized for their rf and mechanical properties as well as for beam dynamics requirements. The TRAK-RF simulation code is used to investigate potential multipacting in these structures. All the simulations will be put to a final test in experiments performed on single cell cavities that have started in the structures laboratory.
Date: October 1, 1997
Creator: Krawczyk, F.L.; Gentzlinger, R.C.; Haynes, B.; Montoya, D.I.; Rusnak, B. & Shapiro, A.H.
Partner: UNT Libraries Government Documents Department

Niobium quarter-wave cavity for the New Delhi booster linac

Description: This paper reports the completion of development of a 97 Mhz niobium coaxial quarter-wave cavity to be used in a booster linac for the New Delhi 16UD pellatron electrostatic accelerator. A prototype cavity, which incorporates a niobium-bellows tuning device, has been completed and operated at 4.2 K at accelerating gradients above 4 MV/m for extended periods of time.
Date: September 1, 1997
Creator: Shepard, K.W.; Roy, A. & Potukuchi, P.N.
Partner: UNT Libraries Government Documents Department

Tests of a prototype magnetostrictive tuner for superconducting cavities

Description: The Continuous Electron Beam Accelerator (CEBA) uses mechanical tuners at 2 K driven by room temperature stepping motors in a feedback loop to maintain cavity frequency at 1497 MHz. Modification of the system was designed, replacing a passive section of the mechanical tuner with a magnetostrictive tuning element consisting of a Ni rod and an industrially supplied 0.25 T superconducting solenoid. This assembly was tested with several magnetic shield configurations designed to keep the stray flux at the Nb cavity below 1 {mu}T when the cavity was normal, to maintain cavity Q. Results of the tests, including change in cavity performance when the cavity was locally quenched near the end of the solenoid, showed that the a multi-layer shield of 6mm steel, with sheets of mu metal, niobium and my metal spaced appropriately outside the thick steel, was effective in containing the flux, both remanent and current-driven, preventing any change in cavity Q upon cooldown or quench with an external heater near the solenoid end. Hysteresis attributed to the Ni magnetostrictive element was observed.
Date: October 1, 1996
Creator: Benesch, J.F. & Wiseman, M.
Partner: UNT Libraries Government Documents Department

Medium-beta superconducting cavity tests at Los Alamos National Laboratory for high-current, proton accelerators

Description: Single-cell superconducting cavities are currently being evaluated for use in high-current proton accelerator applications being developed at Los Alamos National Laboratory (LANL). The designs that have been evaluated so far include 0.48 and 0.64 beta cavities. The parameters that have been checked are: peak surface electric field, magnetic quench field, multipacting levels, cavity Q, and propensity for Q disease. In limited tests to date, peak surface fields of 43 MV/m, and quench fields up to 103 mT have been achieved. Q{sub 0} values have been typically 1 {times} 10{sup 10} at 2 K, with a reduction of about 30% after being held at 150 K for two hours. While some conditioning barriers were eliminated, no obvious multipactor zones were found.
Date: August 1, 1998
Creator: Haynes, W.B.; Rusnak, B.; Chan, K.C.D.; Krawcyzk, F.; Shapiro, A.; Bibeau, R. et al.
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

Operational Optimization of Large-Scale SRF Accelerators

Description: Unlike other types of accelerator subsystems, because of the flexibility in setting the gradient in each cavity, an SRF linac has many operational degrees of freedom. The overall linac has an operational envelope (beam voltage and current) that depends on acceptable reliability, cryogenic capacity, and RF power budget. For economic and end-user physics reasons, one typically wants to run as close to the edge of the operational envelope as possible. With about 160 cavities in each of the CEBAF linacs, we have been forced to treat this problem in a very general way, and satisfy other non-fundamental needs as energy lock and rapid recovery from failures. We present a description of the relevant diverse constraints and the solution developed for CEBAF.
Date: January 1, 1999
Creator: Delayen, J. R.; Doolittle, L. R. & Reece, C. E.
Partner: UNT Libraries Government Documents Department

Analysis of performance limitations for superconducting cavities

Description: The performance of superconducting cavities in accelerators can be limited by several factors, such as: field emission, quenches, arcing, rf power; and the maximum gradient at which a cavity can operate will be determined by the lowest of these limitations for that particular cavity. The CEBAF accelerator operates with over 300 cavities and, for each of them, the authors have determined the maximum operating gradient and its limiting factor. They have developed a model that allows them to determine the distribution of gradients that could be achieved for each of these limitations independently of the others. The result of this analysis can guide an R&D program to achieve the best overall performance improvement. The same model can be used to relate the performance of single-cell and multi-cell cavities.
Date: August 1, 1998
Creator: Delayen, J. R.; Doolittle, L. R. & Reece, C. E.
Partner: UNT Libraries Government Documents Department

Contamination free helium leak detection of sensitive systems

Description: High Technology Systems (HTS) with sensitive surfaces, such as superconducting radio frequency (SRF) accelerating cavities, polarized electron sources (PES) for accelerators and many others, are prone to degradation when subjected to particulate or hydrocarbon contaminants. Particulate contamination control of SRF cavity surfaces and vacuum components have been discussed by several authors at this contamination workshop. Hydrocarbon contamination mainly results from prolonged evacuation with conventional oil lubricated pumping systems and/or prolonged leak detection with conventional leak detectors. The sensitivity of the conventional leak detectors suffers due to the back-streaming of atmospheric helium (5 x 10{sup {minus}1} Pa) through the pumping systems and/or the trapping of helium in the O-rings and oils of the pumping systems. This reduced sensitivity leads to the use of the leak detectors over long periods of time for detecting small (1 x 10{sup {minus}10} atm. cc s{sup {minus}1}) leaks in HTS thereby exposing the sensitive surfaces to contamination. In this paper, a review of the work in progress, at Thomas Jefferson Accelerator Facility (Jefferson Lab), in reducing the contamination of sensitive surfaces is presented.
Date: January 1, 1997
Creator: Myneni, G.
Partner: UNT Libraries Government Documents Department

RF control studies for moderate beamtime coupling between SRF cavities

Description: When an SRF accelerator is designed, there is motivation to move the cavities close together on the beamline. Assuming the beamline apertures are not shrunk as well, this compaction (which will increase the overall accelerating gradient and/or lower the dynamic cryogenic heat load) increases the inter-cavity coupling. Within certain limits, the control system can compensate for this coupling by retuning each of the cavities. This paper describes constraints on the RF system, tuners, couplers, and control systems that are required to provide stable operation of cavities in the presence of inter-cavity coupling that exceeds the loaded bandwidth of an individual cavity.
Date: August 1, 1998
Creator: Doolittle, L. R. & Wang, D. X.
Partner: UNT Libraries Government Documents Department

TESTING PROCEDURES AND RESULTS OF THE PROTOTYPE FUNDAMENTAL POWER COUPLER FOR THE SPALLATION NEUTRON SOURCE*

Description: High-power RF testing with peak power in excess of 500 kW has been performed on prototype Fundamental Power Couplers (FPC) for the Spallation Neutron Source superconducting (SNS) cavities. The testing followed the development of procedures for cleaning, assembling and preparing the FPC for installation in the test stand. The qualification of the couplers has occurred for the time being only in a limited set of conditions (travelling wave, 20 pps) as the available RF system and control instrumentation are under improvement.
Date: June 18, 2001
Creator: Stirbet, M.
Partner: UNT Libraries Government Documents Department

Electronic Damping of Microphonics in Superconducting Cavities

Description: In previous applications of high-velocity superconducting cavities the accelerated beam currents were sufficiently high that the microphonics-induced frequency excursions were significantly less than the loaded bandwidth, and the power absorbed by the beam dominated the total power requirement. In new applications (CEBAF Upgrade, RIA) the beam currents will be sufficiently low that the RF power requirements will be dominated by the control of the cavity fields in the presence of microphonics. Active electronic damping of microphonics by modulation of the cavity field amplitude has been occasionally used in the past in small, low-velocity, low-gradient superconducting structures; its application to much larger, high-velocity, high-gradient structures could result in a substantial reduction of the RF power requirements. This paper presents an analytical study of various schemes for electronic damping and presents formulae that quantify the reduction of microphonics as a function of RF field amplitude modulation.
Date: June 1, 2001
Creator: Delayen, J.R.
Partner: UNT Libraries Government Documents Department

CEBAF Upgrade Cryomodule Component Testing in the Horizontal Test Bed (HTB)

Description: The planned upgrade of the CEBAF electron accelerator includes the development of an improved cryomodule. Several components differ substantially from the original CEBAF cryomodule; these include: the new 7-cell, 1.5 GHz cavities with integral helium vessel, a new, backlash-free cavity tuner, the waveguide coupler with its room-temperature ceramic window, and the HOM damping filters. In order to test the design features and performance of the new components, a horizontal cryostat (Horizontal Test Bed) has been constructed which allows testing with a turn around time of less than three weeks. This cryostat provides the environment for testing one or two cavities, with associated auxiliary components, in a condition similar to that of a real cryomodule. A series of tests has been performed on a prototype 7-cell cavity and the above-mentioned systems. In this paper the results of the tests on the cryostat, on the cavity performance, on its coupler, on the tuner characteristics, and on the microphonics behavior will be reported.
Date: June 1, 2001
Creator: Campisi, I.E.; Carpenter, B.; Davis, G.K.; Delayen, J.; Drury, M.; Feldl, E. et al.
Partner: UNT Libraries Government Documents Department

TESTING PROCEDURES AND RESULTS OF THE PROTOTYPE FUNDAMENTAL POWER COUPLER FOR THE SPALLATION NEUTRON SOURCE

Description: High-power RF testing with peak power in excess of 500 kW has been performed on prototype Fundamental Power Couplers (FPC) for the Spallation Neutron Source superconducting (SNS) cavities. The testing followed the development of procedures for cleaning, assembling and preparing the FPC for installation in the test stand. The qualification of the couplers has occurred for the time being only in a limited set of conditions (travelling wave, 20 pps) as the available RF system and control instrumentation are under improvement.
Date: June 1, 2001
Creator: STIRBET, M.; CAMPISI, I.E. & AL, ET
Partner: UNT Libraries Government Documents Department