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High-frequency cavity applications and measurements of high-temperature superconductors

Description: A potentially important application of high-temperature superconductors will be high-frequency accelerating cavities. Currently these materials are not competitive with Nb at liquid helium temperature or with Cu at room temperature. However, available data on high-quality single crystals suggest that the relatively poor rf properties (high surface resistance and low surface magnetic field) of bulk and film specimens are due to materials properties that can be eliminated by improved processing techniques. Recent progress in the fabrication of thin films, for example, demonstrates that this is indeed the case. 27 refs., 10 figs., 1 tab.
Date: January 1, 1989
Creator: Cooke, D.W. & Gray, E.R.
Partner: UNT Libraries Government Documents Department

Microwave properties of HTS (high temperature superconductor) films

Description: High-frequency applications of high-temperature superconductors generally fall into two categories: devices that require low values of surface resistance R{sub s} in ambient surface magnetic fields H{sub rf}, and devices that require low R{sub s} in modest fields. Moreover, many applications can be realized with small-surface-area films whereas others require larger areas-radiofrequency (rf) cavities, for example. Regardless of the application, the potential of HTS films is predicated on satisfying one or both of the above-stated requirements. We have measured the surface resistance of small-area (1 cm{sup 2}) and large-area (6.5 cm{sup 2}) YBa{sub 2}Cu{sub 3}O{sub 7} (YBCO) films that have been laser ablated onto LaA{ell}O{sub 3} substrates, large-area (5.1 cm{sup 2}) YBCO films that have been e-beam deposited onto LaA{ell}O{sub 3}, and large-area (11.4 cm{sup 2}) T{ell}-based films that have been magnetron sputtered onto metallic substrates. The best R{sub s} values are obtained from the 1-cm{sup 2} laser-ablated films; they are 40 {mu}{Omega} and 340 {mu}{Omega} at 4 K and 77 K, respectively ({omega}/2{pi} = 10 GHz). Comparable values for Cu are 6 and 13 m{Omega}, respectively. Large-area T{ell}-based films yield typical R{sub s} values of 4 m{Omega} and 14 m{Omega} at 4 K and 77 K, respectively ({omega}/2{pi} = 18 GHz). The dependence of R{sub s} on H{sub rf} for these films indicates that surface fields as large as 55 Oe can be achieved with R{sub s} increasing only by a factor of 10. This field dependence is associated with c-axis texturing.
Date: January 1, 1990
Creator: Cooke, D.W.; Arendt, P.N.; Gray, E.R.; Muenchausen, R.E.; Bennett, B.L.; Foltyn, S.R. et al.
Partner: UNT Libraries Government Documents Department

Microwave characterization of high-temperature superconductors

Description: Thick (10-15 {mu}m) Tl-Ba-Ca-Cu-O films have been deposited onto yttria-stabilized zirconia and Ag substrates by d.c. magnetron sputtering techniques. Direct deposition onto 1'' diameter yttria-stabilized zirconia yields films with typical 22 GHz surface resistance (R{sub s}) values of 5.2 {plus minus} 2 m{Omega} and 52 {plus minus} 2 m{Omega} at 10 K and 77 K, respectively. For comparison, R{sub s} of Cu at this same frequency is 10 m{Omega} at 4 K and 22 m{Omega} at 77 K. Tl-Ba-Ca-Cu-O films have also been deposited onto 1'' diameter Ag substrates using Au/Cu, Cu, and BaF{sub 2} buffer layers. The lowest R{sub s} values were obtained on films with a BaF{sub 2} buffer layer, typical values being 7.8 {plus minus} 2 m{Omega} and 30.6 {plus minus} 2 m{Omega} (measured at 22 GHz) at 10 K and 77 K, respectively. Larger films (1.5'' diameter) with similar R{sub s} values were prepared using this same technique, demonstrating that the fabrication process can be scaled to larger surface areas. These films are promising for radiofrequency cavity applications because they are thick (50-75 times the London penetration depth), have relatively large surface areas, are fabricated on metallic substrates, and have R{sub s} values that are competitive with Cu at 77 K and are lower than Cu at 4 K. Because they are polycrystalline and unoriented, it is anticipated that their R{sub s} values can be lowered by improving the processing technique. High-quality films of YBa{sub 2}Cu{sub 3}O{sub 7} have been electron-beam deposited onto 1'' LaGaO{sub 3} and 1.5'' LaAlO{sub 3} substrates. The 1'' sample is characterized by R{sub s} values of 0.2 {plus minus} 0.1 m{Omega} at 4 K and 18.6 {plus minus} 2 m{Omega} at 77 K. The 4-K value is only 2-4 times higher than Nb. The 1.5'' sample has R{sub s} values (measured ...
Date: January 1, 1989
Creator: Cooke, D.W.; Gray, E.R.; Arendt, P.N.; Beery, J.G.; Bennett, B.L.; Brown, D.R. et al.
Partner: UNT Libraries Government Documents Department

Rf superconductivity research and development at Los Alamos National Laboratory

Description: Los Alamos National Laboratory is engaged in two distinct areas of rf superconductivity research and development. One program is devoted to studying the rf properties of high-temperature superconductors (HTS) and assessing their potential for accelerator-cavity applications. Our best results to date have come from YBa{sub 2}Cu{sub 3}O{sub 7} films (0.8 {mu}m) electron-beam deposited onto LaGaO{sub 3} substrates. A typical surface resistance value is 0.2 {plus minus} 0.1 m{Omega} measured at 22 GHz and 4 K, which is only a factor of 2--4 higher than Nb. A second area of rf superconductivity R D at Los Alamos involves fabricating and testing conventional Nb superconducting rf cavities. Facilities are being constructed for polishing, cleaning,and testing both small (3-GHz) and large (805-MHz) Nb cavities. Much of our effort is being devoted to understanding the proper techniques for welding, cleaning, and handling Nb cavities, to ensure good high-field performance. Nb cavity applications are foreseen in upgrades to LAMPF, compact free-electron lasers, and high-current cw proton linacs. A single 402.5-MHz Nb cavity (in a beam cryostat) is presently being acquired from industry to provide a momentum compactor for the low-energy pion beam at LAMPF. 6 refs., 3 figs.
Date: January 1, 1989
Creator: Cooke, D.W.; Gray, E.R.; Houlton, R.J.; Lawrence, G.P.; McGill, J.A.; Meyer, E. et al.
Partner: UNT Libraries Government Documents Department

High-brightness photoemitter development for electron accelerator injectors

Description: Free-electron-laser (FEL) oscillators require a train of high-brightness bunches. Conventional subharmonic bunchers are currently used with rf linacs to generate pulse trains, but the resulting dilution of the transverse phase space and lower beam brightness are unacceptable for high-performance FELs. Recent developments suggest that photoemitters of high quantum efficiency combined with rapid acceleration can produce pulse trains of higher brightness than has been achieved before.
Date: January 1, 1985
Creator: Fraser, J.S.; Sheffield, R.L. & Gray, E.R.
Partner: UNT Libraries Government Documents Department

New high-brightness electron injector for free-electron lasers driven by rf linacs

Description: A free-electron laser oscillator, driven by an rf linac, requires a train of electron bunches delivered to an undulator. The brightness requirement exceeds that available from a conventional linac with rf bunchers. The demonstrated high brightness of laser-illuminated photoemitters indicates that the conventional buncher system might be eliminated entirely, thereby avoiding the usual loarge loss in brightness that occurs in bunchers. A photoemitter with a current density of about 200 A/cm/sup 2/ is placed on an end wall of an rf cavity to accelerate a 60-ps bunch of electrons to 1 MeV as rapidly as possible. Preliminary experimental work, simulation calculations, and discussions on emittance measurement techniques and positive ion motion in the rf gun are presented.
Date: January 1, 1985
Creator: Fraser, J.S.; Sheffield, R.L. & Gray, E.R.
Partner: UNT Libraries Government Documents Department

Photocathodes in accelerator applications

Description: Some electron accelerator applications require bursts of short pulses at high microscopic repetition rates and high peak brightness. A photocathode, illuminated by a mode-locked laser, is well suited to filling this need. The intrinsic brightness of a photoemitter beam is high; experiments are under way at Los Alamos to study the brightness of short bunches with high space charge after acceleration. A laser-illuminated Cs/sub 3/Sb photoemitter is located in the first rf cavity of an injector linac. Diagnostics include a pepper-pot emittance analyzer, a magnetic spectrometer, and a streak camera.
Date: January 1, 1987
Creator: Fraser, J.S.; Sheffield, R.L.; Gray, E.R.; Giles, P.M.; Springer, R.W. & Loebs, V.A.
Partner: UNT Libraries Government Documents Department

Development of a commissioning plan for the APT linac

Description: The Accelerator Production of Tritium (APT) facility is based on a linac which incorporates both normal-conducting and superconducting RF technology and accelerates a 100-mA cw proton beam to an energy of 1,030 MeV or higher, depending on the desired production rate. Commissioning plans to achieve full power operation with minimum beam-induced activation of components have been evolving. This paper presents the main issues and the basic approaches that are now being discussed.
Date: December 1998
Creator: Funk, L. W.; Crandall, K. R.; Gilpatrick, J. D.; Gray, E. R.; Regan, A. H.; Rohlev, A. et al.
Partner: UNT Libraries Government Documents Department

Simulation studies of the LAMPF proton linac

Description: The LAMPF accelerator consists of two 0.75-MeV injectors, one for H{sup +} and the other for H{sup {minus}}, a separate low-energy beam transport (LEBT) line for each beam species, a 0.75 to 100-MeV drift-tube linac (DTL) operating at 201.25-MHz, a 100-MeV transition region (TR), and a 100 to 800-MeV side-coupled linac (SCL) operating at 805-MHz. Each LEBT line consists of a series of quadrupoles to transport and transversely match the beam. The LEBT also contains a prebuncher, a main buncher, and an electrostatic deflector. The deflector is used to limit the fraction of a macropulse which is seen by the beam diagnostics throughout the linac. The DTL consists of four rf tanks and uses singlet FODO transverse focusing. The focusing period is doubled in the last two tanks by placing a quadrupole only in every other drift-tube. Doublet FDO transverse focusing is used in the SCL. The TR consists of separate transport lines for the H{sup +} and H{sup {minus}} beams. The pathlengths for the two beams differ, by introducing bends, so as to delay arrival of one beam relative to the other and thereby produce the desired macropulse time structure. Peak beam currents typically range from 12 to 18-mA for varying macropulse lengths which give an average beam current of 1-mA. The number of particles per bunch is of the order 10{sup 8}. The work presented here is an extension of previous work. The authors have attempted to do a more complete simulation by including modeling of the LEBT. No measurements of the longitudinal structure of the beam, except phase-scans, are performed at LAMPF. The authors show that, based on simulation results, the primary causes of beam spill are inefficient longitudinal capture and the lack of longitudinal matching. Measurements to support these claims are not presently made at LAMPF. ...
Date: May 1, 1995
Creator: Garnett, R.W.; Gray, E.R.; Rybarcyk, L.J. & Wangler, T.P.
Partner: UNT Libraries Government Documents Department

Design and construction of the photocathode electron gun cavity

Description: A 1300-MHz, two-cell rf accelerator cavity has been constructed for the high-brightness photocathode electron source program. Each cell has an rf drive. Cell one has a replacement photocathode plug on the back wall and has a shape designed for linear radial fields. Cell two has a more standard high-shunt-impedance shape. SUPERFISH values for shunt impedances are, respectively, 29.5 and 45.8 M/Omega//m. Peak surface field maximums are 58.9 and 32.1 MV/m for an electron acceleration of 0.9 and 1.0 MeV. Drive coupling is matched for 55 and 86% beam loading at 1-A average current. The system has vacuum pumping ports, into both cells and is baked at 300/degree/C. Typical operating pressures are in low 10/sup /minus/10//-torr range. Cell frequencies are fine tuned by a combination of operating temperature and cell nose pulling. Cell-to-cell coupling was intended to be low (K = 0.0002); however, because of the high Qs (13,300 and 20,000), substantial coupling effects are seen. Cutting the vacuum-port slots shifted the frequencies by 1.5 MHz and gave an apparent 10% increase in the cavity Qs. Construction of the cavity required a series of four brazes with several annealing cycles. All joints are flat, and the sequence is such that each joint is brazed horizontally; as a result, all joints were successfully brazed on the first attempt. The latest experiment measurements are given in another paper at this conference. 2 refs., 3 figs.
Date: January 1, 1988
Creator: Gray, E.R. & Fraser, J.S.
Partner: UNT Libraries Government Documents Department

Simulated performance of the superconducting section of the APT linac under various fault and error conditions

Description: The current design for the production of tritium uses both normal-conducting (NC) and superconducting (SC) structures. To evaluate the performance of the superconducting part of the linac which constitutes more than 80% of the accelerator, studies have been made to include the effects of various error and fault conditions. Here, the authors present the simulation results of studies such as effects of rf phase and amplitude errors, cavity/klystron failure, quadrupole misalignment errors, quadrupole gradient error, and beam-input mismatches.
Date: August 1, 1997
Creator: Gray, E.R.; Nath, S. & Wangler, T.P.
Partner: UNT Libraries Government Documents Department

Preparation and properties of Y-124 superconductor made by a chemical precipitation method

Description: We have prepared the thermodynamically stable YBCO-124 high temperature superconductor in powder form by a chemical precipitation method and have characterized the material by a number of chemical and physical methods, including carbon content, x-ray diffraction, thermogravimetry, scanning electron microscopy, surface area, thermally stimulated luminescence, and the superconducting transition and magnetization curves. We have also started to consolidate the powders by hot pressing. 21 refs., 6 figs.
Date: January 1, 1990
Creator: Laquer, H.L. (CryoPower Associates, Los Alamos, NM (USA)); Gaines, J.R. Jr.; Brainard, S.; Hutson, S.D.; Pisanelli, J. (Superconductive Components, Inc., Columbus, OH (USA)); Cooke, D.W. et al.
Partner: UNT Libraries Government Documents Department

Beam dynamics aspects for the APT integrated linac

Description: The accelerator-based production of tritium calls for a high-power cw proton linac. The current Los Alamos design uses an integrated approach in terms of accelerating structure. The front part of the accelerator uses normal-conducting (NC) structures while most (>80%) of the linac structure is superconducting (SC). Here, the authors report the beam-dynamics rationale used in the integrated design and present particle simulation results.
Date: August 1, 1997
Creator: Nath, S.; Gray, E.R. & Wangler, T.P.
Partner: UNT Libraries Government Documents Department

RF properties of high temperature superconductors: Cavity methods

Description: A description of cavities used in the study of the microwave properties of the high-temperature superconductors is followed by a lumped-circuit analysis of the coupling of transmission lines and resonators. The frequency dependence of the reflected and transmitted microwave power and the character of transient cavity response are analyzed. Techniques are discussed for the introduction of samples of the high-temperature superconductors into microwave cavities. Following a discussion of sample surface impedance and sample geometry factor, the connection between surface resistance and cavity Q is examined as well as the connection between cavity frequency shift and surface reactance. Measurement techniques that utilize reflected or transmitted power or transient response are described. 35 refs., 1 fig.
Date: January 1, 1990
Creator: Portis, A.M. (California Univ., Berkeley, CA (USA)); Cooke, D.W. & Gray, E.R. (Los Alamos National Lab., NM (USA))
Partner: UNT Libraries Government Documents Department

Overview of the APT high-energy beam transport and beam expanders

Description: The APT high energy beam transport (HEBT) and beam expanders convey the 1700-MeV, 100-mA cw proton beam from the linac to the tritium target/blanket assembly, or a tuning beam stop. The HEBT includes extensive beam diagnostics, collimators, and beam jitter correction, to monitor and control the 170-MW beam prior to expansion. A zero-degree beamline conveys the beam to the beam stop, and an achromatic bend conveys the beam to the tritium production target. Nonlinear beam expanders make use of higher-order multipole magnets and dithering dipoles to expand the beam to a uniform-density, 16-cm wide by 160-cm high rectangular profile on the tritium-production target. The overall optics design will be reviewed, and beam simulations will be presented.
Date: August 1, 1997
Creator: Shafer, R.E.; Blind, B. & Gray, E.R.
Partner: UNT Libraries Government Documents Department

Rf photoelectron gun experimental performance

Description: Free-electron lasers require electrons beams of high peak brightness. In this paper, we describe the performance of a compact high-brightness electron source for driving short-wavelength free-electron lasers. The experiment uses a laser-illuminated photoemitter located in the first rf cavity of a two-cavity linac. The photocathode source and associated hardware are described. The doubled ND:YAG laser (532 nm), which is used to drive the photocathode, produces 75-ps micropulses at a 108-MHz repetition rate and peak powers of approximately 300 kW. Diagnostics include a pepper-pot emittance analyzer, a magnetic spectrometer, and a <4-ps resolution streak camera. Present experiments give the following results: micropulse current amplitudes of 10 mA to 400 A, 75% beam emittances ranging from 10 /center dot/ mm /center dot/ mrad to 40 /center dot/ mm /center dot/ mrad and peak current densities of >600 A/cmS. Results on photocathode lifetime will be presented. Future plans for applications of the photoinjector at Los Alamos National Laboratory are discussed. 7 refs., 4 figs., 2 tabs.
Date: January 1, 1988
Creator: Sheffield, R.L.; Cornelius, W.D.; Nguyen, D.C.; Springer, R.W.; Lamartine, B.C.; Gray, E.R. et al.
Partner: UNT Libraries Government Documents Department

The Los Alamos Photoinjector Program

Description: Free-electron lasers (FELS) require electron beams of high peak brightness. In this presentation, we describe the design of a compact high-brightness electron source for driving short-wavelength FELs. The experiment uses a laser-illuminated Cs/sub 3/Sb photoemitter located in the first rf cavity of an injector linac. The photocathode source and associated hardware are described. The doubled YAG laser (532 nm), which is used to drive the photocathode, produces 75-ps micropulses at 108-MHz repetition rate and peak powers of approximately 300 kW. Diagnostics include a pepper-pot emittance analyzer, a magnetic spectrometer, and a 4-ps resolution streak camera. Present experiments give the following results: micropulse current amplitude of 100 mA to 400 A, beam emittances ranging from 10 n.mm.mrad to 40 n.mm.mrad, an energy spread of +-3%, and peak current densities of 600 A/cm/sup 2/. A brief discussion on the possible applications of this very bright and compact electron source is presented. 16 refs., 5 figs., 1 tab.
Date: September 1, 1987
Creator: Sheffield, R.L.; Gray, E.R. & Fraser, J.S.
Partner: UNT Libraries Government Documents Department

Alignment and steering scenarios for the APT linac

Description: The Accelerator for the Production of Tritium (APT) requires a very high proton beam current (100 mA cw). Requirement for hands-on maintenance limits the beam spill to less than 0.2 nA/m along most of the linac. To achieve this, it is important to understand the effects of fabrication, installation and operational errors, establish realistic tolerances, and develop techniques for mitigating their consequences. A new code, PARTREX, statistically evaluates the effects of alignment, quadrupole field, and rf phase and amplitude errors in the linac. This paper reviews the effects of quadrupole misalignments and present two steering algorithms that minimize the potential for particle loss from the beam halo. These algorithms were tested on the 8-to-20 MeV portion of the APT linac.
Date: September 1, 1996
Creator: Stovall, J.E.; Gray, E.R.; Nath, S.; Takeda, H.; Wood, R.L.; Young, L.M. et al.
Partner: UNT Libraries Government Documents Department

An rf-driven lasertron

Description: An rf-driven lasertron has been built and is being tested. It is designed to produce over 1 MW of power at 1.3 GHz, with over 60% conversion efficiency between the beam's kinetic energy and microwaves. The design and experiment are discussed, and calculations are presented that show lasertrons can operate at frequencies up to 20 GHz. 3 refs., 5 figs.
Date: January 1, 1988
Creator: Tallerico, P.J.; Sheffield, R.L.; Cornelius, W.D.; Gray, E.R.; Wilson, M.T.; Nguyen, D.C. et al.
Partner: UNT Libraries Government Documents Department

Recent developments for high-intensity proton linacs

Description: High-intensity proton linacs are being proposed for new projects around the world, especially for tritium production, and for pulsed spallation neutron sources. Typical requirements for these linacs include peak beam current of about 100 mA, and final energies of 1 GeV and higher, APT, a tritium production linac, requires cw operation to obtain sufficient average tritium production linac, requires cw operation to obtain sufficient average beam power, and H{sup +} ion sources appear capable of providing the required current and emittances. The pulsed spallation neutron source requires a linac as an injector to one or more accumulator rings, and favors the use of an H{sup minus} beam to allow charge-exchange injection into the rings. For both applications high availability is demanded; the fraction of scheduled beam time for actual production must be 75% or more. Such a high availability requires low beam-loss to avoid radioactivation of the accelerator, and to allow hands-on maintenance that will keep the mean repair and maintenance times short. To keep the accelerator activation sufficiently low, the beam loss should not exceed about 0.1 to 1.0 nA/m, except perhaps for a few localized places, where special design adaptations could be made. The requirement of such small beam losses at such a high intensity presents a new beam physics challenge. This challenge will require greater understanding of the beam distribution, including the low- density beam halo, which is believed to be responsible for most of the beam losses. Furthermore, it will be necessary to choose the apertures so the beam losses will be acceptably low, and because large aperture size is generally accompanied by an economic penalty resulting from reduced power efficiency, an optimized choice of the aperture will be desirable.
Date: April 1, 1996
Creator: Wangler, T.P.; Garnett, R.W.; Gray, E.R. & Nath, S.
Partner: UNT Libraries Government Documents Department

Dynamics of beam halo in mismatched beams

Description: High-power proton linacs for nuclear materials transmutation and production, and new accelerator-driven neutron spallation sources must be designed to control beam-halo formation, which leads to beam loss. The study of particle-core models is leading to a better understanding of the causes and characteristics of beam halo produced by space-charge forces in rms mismatched beams. Detailed studies of the models have resulted in predictions of the dependence of the maximum amplitude of halo particles on a mismatch parameter and on the space-charge tune-depression ratio. Scaling formulas have been derived which will provide guidance for choosing the aperture radius to contain the halo without loss.
Date: September 1, 1996
Creator: Wangler, T.P.; Garnett, R.W.; Gray, E.R.; Ryne, R.D. & Wang, T.S.
Partner: UNT Libraries Government Documents Department

Basis for low beam loss in the high-current APT linac

Description: The present evidence that the APT proton linac design will meet its goal of low beam loss operation. The conclusion has three main bases: (1) extrapolation from the understanding of the performance of the 800-MeV LANSCE proton linac at Los Alamos, (2) the theoretical understanding of the dominant halo-forming mechanism in the APT accelerator from physics models and multiparticle simulations, and (3) the conservative approach and key principles underlying the design of the APT linac, which are aimed at minimizing beam halo and providing large apertures to reduce beam loss to a very low value.
Date: December 31, 1998
Creator: Wangler, T.P.; Gray, E.R.; Krawczyk, F.L.; Kurennoy, S.S.; Lawrence, G.P.; Ryne, R.D. et al.
Partner: UNT Libraries Government Documents Department

New high power linacs and beam physics

Description: New high-power proton linacs must be designed to control beam loss, which can lead to radioactivation of the accelerator. The threat of beam loss is increased significantly by the formation of beam halo. Numerical simulation studies have identified the space-charge interactions, especially those that occur in rms mismatched beams, as a major concern for halo growth. The maximum-amplitude predictions of the simulation codes must be subjected to independent tests to confirm the validity of the results. Consequently, the authors compare predictions from the particle-core halo models with computer simulations to test their understanding of the halo mechanisms that are incorporated in the computer codes. They present and discuss scaling laws that provide guidance for high-power linac design.
Date: August 1, 1997
Creator: Wangler, T.P.; Gray, E.R.; Nath, S.; Crandall, K.R. & Hasegawa, K.
Partner: UNT Libraries Government Documents Department