Author: Gai, W.
Paper Title Page
MOP008 Upgrade of the Argonne Wakefield Accelerator Facility (AWA) and Commissioning of a New RF Gun for Drive Beam Generation 115
 
  • M.E. Conde, D.S. Doran, W. Gai, R. Konecny, W. Liu, J.G. Power, Z.M. Yusof
    ANL, Argonne, USA
  • S.P. Antipov, C.-J. Jing
    Euclid TechLabs, LLC, Solon, Ohio, USA
  • E.E. Wisniewski
    Illinois Institute of Technology, Chicago, Illinois, USA
 
  Funding: Work supported by the U.S. Department of Energy under contract No. DE-AC02-06CH11357.
The AWA Facility is presently undergoing several upgrades that will enable it to further study wakefield acceleration driven by high charge electron beams. The facility employs an L-band photocathode RF gun to generate high charge short electron bunches, which are used to drive wakefields in dielectric loaded structures as well as in metallic structures (iris loaded, photonic band gap, etc). Several facility upgrades are underway: (a) a new RF gun with a higher quantum efficiency photocathode will replace the RF gun that has been used to generate the drive bunches; (b) the existing RF gun will be used to generate a witness beam to probe the wakefields; (c) three new L-band RF power stations, each providing 25 MW, will be added to the facility; (d) five linac structures will be added to the drive beamline, bringing the beam energy up from 15 MeV to 75 MeV. The drive beam will consist of bunch trains of up to 32 bunches spaced by 0.77 ns with up to 100 nC per bunch. The goal of future experiments is to reach accelerating gradients of several hundred MV/m and to extract RF pulses with GW power level.
 
 
MOP046 RF Breakdown Studies Using Pressurized Cavities 184
 
  • R. Sah, A. Dudas, R.P. Johnson, M.L. Neubauer
    Muons, Inc, Batavia, USA
  • M. BastaniNejad, A.A. Elmustafa
    Old Dominion University, Norfolk, Virginia, USA
  • J.M. Byrd, D. Li
    LBNL, Berkeley, California, USA
  • M.E. Conde, W. Gai
    ANL, Argonne, USA
  • A. Moretti, M. Popovic, K. Yonehara
    Fermilab, Batavia, USA
  • D. Rose
    Voss Scientific, Albuquerque, New Mexico, USA
 
  Funding: Supported in part by USDOE STTR Grant DE-FG02-08ER86352 and FRA DOE Contract DE-AC02-07CH11359
Many present and future particle accelerators are limited by the maximum electric gradient and peak surface fields that can be realized in RF cavities. Despite considerable effort, a comprehensive theory of RF breakdown has not been achieved, and mitigation techniques to improve practical maximum accelerating gradients have had only limited success. Recent studies have shown that high gradients can be achieved quickly in 805 MHz RF cavities pressurized with dense hydrogen gas without the need for long conditioning times, because the dense gas can dramatically reduce dark currents and multipacting. In this project we use this high pressure technique to suppress effects of residual gas and geometry found in evacuated cavities to isolate and study the role of the metallic surfaces in RF cavity breakdown as a function of radiofrequency and surface preparation. A 1.3-GHz RF test cell with replaceable electrodes (e.g. Mo, Cu, Be, W, and Nb) has been built, and a series of detailed experiments is planned at the Argonne Wakefield Accelerator. These experiments will be followed by additional experiments using a second test cell operating at 402.5 MHz.
 
 
MOP107 Status of Dielectric-Lined Two-Channel Rectangular High Transformer Ratio Accelerator Structure Experiment 298
 
  • S.V. Shchelkunov, M.A. LaPointe
    Yale University, Beam Physics Laboratory, New Haven, Connecticut, USA
  • M.E. Conde, W. Gai, J.G. Power, Z.M. Yusof
    ANL, Argonne, USA
  • J.L. Hirshfield
    Omega-P, Inc., New Haven, Connecticut, USA
  • T.C. Marshall
    Columbia University, New York, USA
  • D. Mihalcea
    Northern Illinois University, DeKalb, Illinois, USA
  • G.V. Sotnikov
    NSC/KIPT, Kharkov, Ukraine
 
  Funding: This work is supported by DoE, Office of High Energy Physics
Recent tests of a two-channel rectangular dielectric lined accelerator structure are described; comparison with theory and related issues are presented. The structure (with channel width ratio 6:1) is designed to have a maximum transformer ratio of ~12.5:1. It operates mainly in the LSM31 mode (~ 30GHz). The dielectric liner is cordierite (dielectric constant ~4.76). The acceleration gradient is 1.2 MV/m for each 10nC of the drive bunch for the first acceleration peak of the wakefield, and 0.92 MV/m for the second peak. The structure is installed into the AWA beam-line (Argonne National Lab) and is excited by a single 10-50nC, 14MeV drive bunch. Both the drive bunch and a delayed witness bunch are produced at the same photocathode. This is the first experiment to test a two-channel dielectric rectangular wakefield device where the accelerated bunch may be continuously energized by the drive bunch. The immediate experimental objective is to observe the energy gain and spread, and thereby draw conclusions from the experimental results and the theory model predictions. The observed energy change of the test bunch might be well explained*.
* G. V. Sotnikov, et al., Advanced Accelerator Concepts: 13th Workshop, Carl B. Schroeder, Wim Leemans and Eric Esarey, editors, AIP Conf. Proc. 1086), pp. 415–420 (AIP, New York, 2009).
 
 
MOP115 Progress on Multipactor Studies in Dielectric-Loaded Accelerating Structures 310
 
  • S.P. Antipov, C.-J. Jing, A. Kanareykin, P. Schoessow
    Euclid TechLabs, LLC, Solon, Ohio, USA
  • D.S. Doran, W. Gai, J.G. Power
    ANL, Argonne, USA
  • B. Feng
    IIT, Chicago, Illinois, USA
 
  Funding: DOE SBIR
Significant progress has been made in the development of high gradient rf driven dielectric accelerating structures (DLA). One principal effect limiting further advances in this technology is the problem of multipactor. The fraction of the power absorbed at saturation in DLA experiments was found to increase with the incident power, with more than 30% of the incident power per unit length being absorbed. We studied a possibility of multipactor mitigation by introduction of surface grooves (transverse and longitudinal) to interrupt the resonant trajectories of electrons in the multipactor discharge. Four DLA structures based on quartz tubes with transverse and longitudinal grooves of various dimensions were designed. In this paper we report simulation results and plans for high-power tests of these structures.
 
 
MOP116 Development of an X-Band Dielectric-Based Wakefield Power Extractor for Potential CLIC Applications 313
 
  • C.-J. Jing, S.P. Antipov, A. Kanareykin, P. Schoessow
    Euclid TechLabs, LLC, Solon, Ohio, USA
  • M.E. Conde, W. Gai, J.G. Power
    ANL, Argonne, USA
  • I. Syratchev
    CERN, Geneva, Switzerland
 
  Funding: Work is funded by DoE SBIR PhaseI.
In the past decade, tremendous efforts have been put into the development of the CLIC Power Extraction and Transfer Structure (PETS), and significant progress has been made. However, one concern remains the manufacturing cost of the PETS, particularly considering the quantities needed for a TeV machine. A dielectric-based wakefield power extractor in principle is much cheaper to build. A low surface electric field to gradient ratio is another big advantage of the dielectric-loaded accelerating/decelerating structure. We are currently investigating the possibility of using a cost-effective dielectric-based wakefield power extractor as an alternative to the CLIC PETS. We designed a 12 GHz dielectric-based power extractor which has a similar performance to CLIC PETS with parameters 23 mm beam channel, 240 ns pulse duration, 135 MW output per structure using the CLIC drive beam. In order to study potential rf breakdown issues, as a first step we are building a 11.424 GHz dielectric-based power extractor scaled from the 12 GHz version, and plan to perform a high power rf test using the SLAC 11.424 GHz high power rf source.
 
 
MOP117 Beam Test of a Tunable Dielectric Wakefield Accelerator 316
 
  • C.-J. Jing, S.P. Antipov, A. Kanareykin, P. Schoessow
    Euclid TechLabs, LLC, Solon, Ohio, USA
  • M.E. Conde, W. Gai, J.G. Power
    ANL, Argonne, USA
 
  Funding: Work supported by US DoE SBIR Grant under Contract # DE-FG02-07ER84822
We report on a collinear wakefield experiment using the first tunable dielectric loaded accelerating structure. Dielectric-based accelerators are generally lacking in approaches to tune the frequency after fabrication. However, by introducing an extra layer of nonlinear ferroelectric which has a dielectric constant sensitive to temperature and DC voltage, the frequency of a DLA structure can be tuned on the fly by controlling the temperature or DC bias. The experiment demonstrated that by varying the temperature of the structure over a 50°C temperature range, the energy of a witness bunch at a fixed delay with respect to the drive beam could be changed by an amount corresponding to more than half of the nominal structure wavelength.
 
 
MOP119 The Dielectric Wakefield Accelerating Structure 319
 
  • A. Kanareykin, S.P. Antipov, J.B. Butler, C.-J. Jing, P. Schoessow
    Euclid TechLabs, LLC, Solon, Ohio, USA
  • W. Gai
    ANL, Argonne, USA
 
  Funding: US Department of Energy
We report here on the development of THz diamond wakefield structures produced using Chemical Vapor Deposition (CVD) technology*. The diamond structures would be used in a THz generation experiment at the new FACET facility at SLAC. We consider a dielectric based accelerating structure to study of the physical limitations encountered driving >GV/m wakefields in the cylindrical and planar geometries of a dielectric wakefield accelerator (DWA). In a DWA, an ultrashort drive bunch traverses the evacuated central region of the structure, creating Cherenkov wakefields in the dielectric to accelerate a witness bunch. A diamond-based DWA structure will allow a sustained accelerating gradient exceeding breakdown threshold demonstrated with the FFTB experiments**. The electrical and mechanical properties of diamond make it an ideal candidate material for use in dielectric rf structures: high breakdown voltage, extremely low dielectric losses and the highest thermoconductive coefficient available for removing waste heat from the device.
*R. J. Barker et al., Modern Microwave and Millimeter-Wave Power Electronics, IEEE Press/Wiley-Interscience, Piscataway NJ 2005, Chapter 7
**M.C. Thompson et al. Phys. Rev.Lett.100:214801, 2008.
 
 
MOP130 New Studies of X-band Dielectric-loaded Accelerating Structures 337
 
  • S.H. Gold
    NRL, Washington, DC, USA
  • S.P. Antipov, W. Gai, C.-J. Jing, R. Konecny, J.G. Power
    ANL, Argonne, USA
  • A. Kanareykin
    Euclid TechLabs, LLC, Solon, Ohio, USA
  • A.K. Kinkead
    Icarus Research, Inc., Bethesda, Maryland, USA
 
  Funding: Work supported by the DoE Office of High Energy Physics and ONR.
A joint program is under way to study externally driven X-band dielectric-loaded accelerating (DLA) structures and CLIC-type power extraction structures. The structures are designed and fabricated by Argonne National Laboratory and Euclid Techlabs and tested at up to 20 MW drive power using the X-band Magnicon Facility at the Naval Research Laboratory, with additional tests carried out at SLAC. Thus far, tests have been carried out on a large variety of structures fabricated from quartz, alumina, and MCT-20, and the principal problems have been multipactor loading and rf breakdown.* Multipactor loading occurs on the inner surface of the dielectric in a region of strong normal and tangential rf electric fields; rf breakdown occurs principally at discontinuities in the dielectric. Gap-free DLA structures have been tested at 15 MV/m without breakdown. New tests are being prepared to address these two issues. New gap-free structures will make use of a metallic coating on the outer surface of the dielectric in order to permit tapering both the inner and outer diameters for rf matching, while new multipactor studies will examine the use of grooved surfaces to suppress multipactor.
* C. Jing, W. Gai, J.G. Power, R. Konecny, W. Liu, S.H. Gold, A.K. Kinkead, S.G. Tantawi, V. Dolgashev, and A. Kanareykin, IEEE Trans. Plasma Sci., vol. 38, pp. 1354–1360, June 2010.
 
 
WEP100 Energy Spread Compensation for Multi-Bunch Linac Operation Mode 1662
 
  • D. Mihalcea
    Northern Illinois University, DeKalb, Illinois, USA
  • W. Gai, J.G. Power
    ANL, Argonne, USA
 
  Funding: This work was supported under the U.S. Department of Energy contract number: DE-AC02-06CH11357 with Argonne National Laboratory.
Higher wakefield gradients can be achieved by increasing the total beam charge which is passed through a dielectric-loaded structure and by reducing the transverse size of the beam. Currently, the Argonne AWA photoinjector operates with electron bunches of up to 100 nC and the goal is to raise the total beam charge to about 1000 nC and to improve the beam focusing to a few 100's microns transverse spot size. The increase of the beam charge can be done by superimposing electron bunches that fill up several consecutive RF buckets. Although the energy stored in a single 7-cell linac is by design large the multi-bunch operation with short bunch trains (~10 ns) is still plagued by large energy spread due to significant beam loading effects. In this paper we present a technique intended to reduce the energy spread for a high charge bunch train by properly choosing the time delay between consecutive bunches. The simulations show that the energy spread can be lowered to about 2.8% from about 6.0% for a 10-bunch train of total charge 1000 nC and kinetic energy of about 70 MeV.
 
 
WEP111 Beam Breakup in Dielectric Wakefield Accelerating Structures: Modeling and Experiments 1689
 
  • P. Schoessow, C.-J. Jing, A. Kanareykin, A.L. Kustov
    Euclid TechLabs, LLC, Solon, Ohio, USA
  • A. Altmark
    LETI, Saint-Petersburg, Russia
  • W. Gai, J.G. Power
    ANL, Argonne, USA
 
  Funding: Work supported by USDOE SBIR program.
Beam breakup (BBU) effects resulting from parasitic wakefields limit considerably the intensity of the drive beam that can be transported through a dielectric accelerating structure and hence the accelerating field that can be achieved. We have been developing techniques to control BBU effects using a quadrupole channel or solenoid surrounding the wakefield device. We report here on the status of simulations and experiments on BBU and its mitigation, emphasizing an experiment at the Argonne Wakefield Accelerator facility using a 26 GHz dielectric wakefield device fitted with a solenoid to control BBU. We present calculations based on a particle-Green’s function beam dynamics code (BBU-3000) that we are developing. The code allows rapid, efficient simulation of BBU effects in advanced linear accelerators.
 
 
WEP196 Single-Shot Longitudinal Phase Space Measurement Diagnostics Beamline Status at the Argonne Wakefield Accelerator 1858
 
  • M.M. Rihaoui, D. Mihalcea, P. Piot
    Northern Illinois University, DeKalb, Illinois, USA
  • W. Gai, J.G. Power
    ANL, Argonne, USA
 
  A single-shot longitudinal phase space diagnostics experiment is currently being commissioned at Argonne Wakefield Accelerator. The diagnostic beamline consists of two magnetic dipoles that bend the beam horizontally followed by an rf deflecting cavity that streaks the beam vertically. Using this configuration, the incoming longitudinal phase space can be mapped to a final (x,y) plane which can be directly measured, e.g., using a YAG screen. In this paper we discuss the limitations of such longitudinal phase space diagnostics and present some preliminary measurements.  
 
THOBN6 Wakefield Breakdown Test of a Diamond-Loaded Accelerating Structure 2074
 
  • S.P. Antipov, C.-J. Jing, A. Kanareykin, P. Schoessow
    Euclid TechLabs, LLC, Solon, Ohio, USA
  • M.E. Conde, D.S. Doran, W. Gai, J.G. Power, Z.M. Yusof
    ANL, Argonne, USA
 
  Funding: DOE SBIR
Diamond has been proposed as a dielectric material for dielectric loaded accelerating (DLA) structures. It has a very low microwave loss tangent, the highest available thermoconductive coefficient and high RF breakdown field. In this paper we report the results from a wakefield breakdown test of diamond-loaded rectangular accelerating structure and development of a cylindrical diamond DLA. We expect to achieve field levels on the order of 100 MV/m in the structure using the 100nC beam at the Argonne Wakefield Accelerator Facility. Single crystal diamond plates produced by chemical vapor deposition (CVD) are used in the structure. The structure is designed to yield up to 0.5 GV/m fields on the diamond surface to test it for breakdown. A surface analysis of the diamond is performed before and after the beam test.
 
slides icon Slides THOBN6 [1.629 MB]  
 
THP078 Study of a TeV Level Linear Collider Using Short rf Pulse (~20ns) Two Beam Accelerator Concept 2279
 
  • C.-J. Jing, S.P. Antipov, A. Kanareykin, P. Schoessow
    Euclid TechLabs, LLC, Solon, Ohio, USA
  • M.E. Conde, W. Gai, J.G. Power
    ANL, Argonne, USA
 
  Funding: Work is supported by DOE SBIR grant under contract No. DE-SC0004320.
In a general sense, a high gradient is desirable for a TeV level linear collider design because it can reduce the total linac length. More importantly, the efficiency and the cost to sustain such a gradient should be considered as well in the optimization process of an overall design. We propose a high energy linear collider based on a short rf pulse (~22ns flat top), high gradient (~267MV/m loaded gradient), high frequency (26GHz) dielectric two beam accelerator scheme. This scheme is a modular design and its unique locally repetitive drive beam structure allows a flexible configuration to meet different needs. Major parameters of a conceptual 3-TeV linear collider are presented. This preliminary study shows an efficient (~7% overall ) short pulse collider may be achievable. As the first step, a dielectric based broadband accelerating structure is under development.