Operation of Multiple Accelerating Structures in an X-band High-gradient Test Stand PDF Download
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Author: Lee Millar
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Languages : en
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Author: Lee Millar
Publisher:
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Languages : en
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Author: Wei Gai
Publisher: World Scientific
ISBN: 9814602116
Category : Science
Languages : en
Pages : 176
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This proceedings volume, for the symposium in honor of Junwen Wang's 70th anniversary, is dedicated to his many important achievements in the field of accelerator physics.It includes the discussions of recent advances and challenging problems in the field of high gradient accelerating structure development.
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Languages : en
Pages : 10
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In the second phase of a program to develop a compact accelerator based on a dielectric-loaded accelerating structure, we have conducted high power tests on a traveling-wave and a standing-wave prototype. Indications are that the traveling-wave structure achieved an accelerating gradient of 3-5 MV/m before the input coupling window failed, while the standing wave structure was poorly matched at high power due to contamination of copper residue on its coupling window. To solve both of these problems, a new method for coupling RF into the structures has been developed. The new couplers and the rest of the modular structure are currently under construction and will be tested at the Naval Research Laboratory shortly.
Author: J Rossbach
Publisher: World Scientific
ISBN: 9814554073
Category :
Languages : en
Pages : 1288
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Book Description
The High Energy Accelerator Conference has always been the monitor of the state of the art and the new trends in planning, construction and operation of large particle accelerators. It is held every three years. The 1992 conference is devoted to High Energy Hadron Accelerators and Colliders, Linear Colliders, e⁺e⁻ Storage Rings and related Technologies for these machines. In addition to status reports and contributed papers, the program features twelve survey talks which include summaries of individual poster papers.
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Category :
Languages : en
Pages : 8
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Accelerating gradient is a key parameter of the accelerating structure in large linac facilities, like future Linear Collider. In room temperature accelerating structures the gradient is limited mostly by breakdown phenomena, caused by high surface electric fields or pulse surface heating. High power processing is a necessary procedure to clean surface and improve the gradient. In the best tested X-band structures the achieved gradient is exceed 100 MV/m in of (almost equal to)200 ns pulses for breakdown rate of (almost equal to) 10−7. Gradient limit depends on number of factors and no one theory which can explain all sets of experimental results and predict gradient in new accelerating structure. In paper we briefly overview the recent experimental results of breakdown studies, progress in understanding of gradient limitations and scaling laws. Although superconducting rf technology has been adopted throughout the world for ILC, it has frequently been difficult to reach the predicted performance in these structures due to a number of factors: multipactoring, field emission, Q-slope, thermal breakdown. In paper we are discussing all these phenomena and the ways to increase accelerating gradient in SC cavity, which are a part of worldwide R & D program.
Author: S. Doebert
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Languages : en
Pages :
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Inspired by the very high gradients (150-195 MV/m) achieved at CERN in 30 GHz accelerator structures made with tungsten and molybdenum irises and operated with short (16 ns) rf pulses [1], an X-band (11.4 GHz) version of this structure design was built at CERN and tested at SLAC. The goals of this experiment were to provide frequency scaling data on high gradient phenomena at similar pulse lengths, and to measure the structure performance at the longer pulse lengths available at SLAC (the CLIC test facility, CTF II, could provide only 16 ns pulses for high power operation and 32 ns pulses for medium power operation). Earlier high gradient tests of 21 GHz to 39 GHz standing-wave, single cells, indicated no significant frequency dependence of the maximum obtainable surface field [2]. The X-band scaling test would check if this was true for travelling-wave, multi-cell structures as well. For the experiment, the CLIC group at CERN built a 30 cell accelerating structure that consisted of copper cells and molybdenum irises that were clamped together. The structure was mounted in a vacuum tank and installed in the Next Linear Collider Test Accelerator (NLCTA) beam line at SLAC where it was operated at high power for more than 700 hours.
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Languages : en
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The design, construction, and bench testing of an X-band travelling-wave accelerating structure loaded with a permittivity=20 dielectric has been published recently by the Argonne Advanced Accelerator Group[1]. Here we describe a new program to build a test accelerator using this structure. The accelerator will be powered using high-power 11.424-GHz radiation available at the Magnicon Facility at the Naval Research Lab[2]. The magnicon is expected to provide up to 30 MW from each of two WR-90 output waveguide arms in pulses of up to 1-[micro]s duration, permitting tests of the dielectric-loaded X-band device at gradients of[approximately]40 MV/m. The use of higher power pulses (100-500 MW) eventually available at the output of an active pulse compressor[3] driven by the magnicon will permit gradients in excess of 100 MV/m to be achieved.
Author:
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Category :
Languages : en
Pages : 26
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Rectangular particle accelerator structures with internal planar dielectric elements have been studied, with a view towards devising structures with lower surface fields for a given accelerating field, as compared with structures without dielectrics. Success with this concept is expected to allow operation at higher accelerating gradients than otherwise on account of reduced breakdown probabilities. The project involves studies of RF breakdown on amorphous dielectrics in test cavities that could enable high-gradient structures to be built for a future multi-TeV collider. The aim is to determine what the limits are for RF fields at the surfaces of selected dielectrics, and the resulting acceleration gradient that could be achieved in a working structure. The dielectric of principal interest in this study is artificial CVD diamond, on account of its advertised high breakdown field (H" GV/m for dc), low loss tangent, and high thermal conductivity. Experimental studies at mm-wavelengths on materials and structures for achieving high acceleration gradient were based on the availability of the 34.3 GHz third-harmonic magnicon amplifier developed by Omega-P, and installed at the Yale University Beam Physics Laboratory. Peak power from the magnicon was measured to be about 20 MW in 0.5?s pulses, with a gain of 54 dB. Experiments for studying RF high-field effects on CVD diamond samples failed to show any evidence after more than 105 RF pulses of RF breakdown up to a tangential surface field strength of 153 MV/m; studies at higher fields were not possible due to a degradation in magnicon performance. A rebuild of the tube is underway at this writing. Computed performance for a dielectric-loaded rectangular accelerator structure (DLA) shows highly competitive properties, as compared with an existing all-metal structure. For example, comparisons were made of a DLA structure having two planar CVD diamond elements with a all-metal CERN structure HDS operating at 30 GHz. It was shown that the ratio of maximum surface electric field to accelerating field at the metal wall is only 0.35-0.4 for DLA, much smaller than the value 2.2 for HDS; and the ratio of surface magnetic field to accelerating field is 3.0 mA/V for DLA, compared with 3.45 mA/V for HDS. These values bode well for DLA in helping to avoid breakdown and to reducing pulsed surface heating and fatigue. The shunt impedance is found to be 160-175 M?/m for DLA, as compared to 99 M?/m for HDS. Conclusions are reached from this project that CVD diamond appears promising as a dielectric with a high threshold for RF breakdown, and that rectangular accelerator structures can be devised using planar CVD diamond elements that could be operated at higher acceleration gradients with low probability of RF breakdown, as compared with corresponding all-metallic structures.
Author: Sergey A. Arsenyev
Publisher:
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Category :
Languages : en
Pages : 182
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This thesis presents the design and testing of the first multi-cell superconducting accelerating cavity with a photonic band gap (PBG) coupler cell. The structure serves as a building block for superconducting radio-frequency (SRF) electron accelerators. It has five accelerating cells: four cells of elliptical shape, commonly used for SRF cavities, and one PBG cell in the middle. The purpose of the PBG cell is to damp unwanted Higher-Order electromagnetic Modes (HOMs) in the structure. Strong HOM damping is highly desirable for SRF cavities because it increases maximum achievable beam current by reducing the negative effect that HOMs have on the propagating electron beam. In the presented structure, effective HOM damping is achieved because of the inherent frequency selective properties of the PBG cell. The HOM spectrum in the five-cell cavity was carefully analyzed using eigenmode and wakefield simulations with good agreement between the two methods. The simulations showed that most of the dangerous HOMs were damped to fairly low external quality factors on the order of 102-104. This in principle implies that the new multicell cavity will support much higher beam currents than achievable in conventional SRF cavities that are not optimized for high-current operation. The improved HOM damping does not significantly compromise the accelerating properties of the cavity which are comparable to those of the cavities that only use the elliptical cells. Additionally, the PBG cavity does not need HOM couplers on the beam-pipe sections of the structure, and hence for the same amount of acceleration has a shorter length in the direction of the propagating beam. The five-cell cavity was fabricated of high purity niobium. Fabrication and tuning mechanisms were successfully tested on a copper prototype before being implemented for the niobium cavity. The accelerating gradient profile in the tuned niobium cavity matched the desired profile within a 5% accuracy. Two cryogenic tests were conducted with the five-cell cavity. The first test did not succeed due to a problem with the low quality factor of the cavity's accelerating mode. The problem was identified as a poor waveguide joint in the fundamental power coupler. Modifications were made to the waveguide joint and a second cryogenic test was conducted. In the second test, the high cavity quality factor was demonstrated at the temperature of 4.2 K for accelerating gradients up to 3 MV/m. The measured value of the cavity's quality factor with all ports closed was 1.55 x 108, in agreement with the prediction. This agreement indicated that the implemented surface treatment was effective in the cavity, including the complex PBG cell. No cavity leaks were observed during the tests in superfluid helium, proving the reliability of the fabrication process which included difficult electron-beam welds. No hard barriers in the accelerating gradient were observed during the test, indicating the absence of fundamental limits to cavity's operation for the gradient of at least several MV/m. A series of room-temperature experiments were conducted to measure external quality factors of six dangerous HOMs in the fabricated five-cell cavity. The measurements agreed with the simulations, showing all of the measured Q-factors below 3 x 103. Effective HOM damping, together with the ability to support accelerating gradients of multiple MV/m at cryogenic temperatures, makes the cavity an attractive candidate for future high-current accelerators.
Author:
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Languages : en
Pages : 5
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Accelerating gradient is one of the crucial parameters affecting design, construction and cost of next-generation linear accelerators. For a specified final energy, the gradient sets the accelerator length, and for a given accelerating structure and pulse repetition rate it determines power consumption. Accelerating gradients on the order of 100 MV/m have been reached in short ((almost equal to) 20cm) standing wave and traveling wave X-band accelerating structures [1, 2, 3]. But recent experiments have shown damage to traveling wave accelerating structures at gradients as low as 50 MV/m after 1000 hours of operation [4]. RF breakdown is a probable cause of this damage. An extensive experimental and theoretical program to determine a safe operating gradient for the Next Linear Collider (NLC) is under way in SLAC. The present work is a part of that program.
