Paper | Title | Page |
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MOOCA01 | R&D of a Super-compact SLED System at SLAC | 39 |
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Funding: Work supported by Department of Energy contract DE-AC03-76SF00515. We have successfully designed, fabricated, installed and tested a super-compact X-Band SLED system at SLAC. It is composed of an elegant mode converter/polarizer and a single sphere energy store cavity with high Q of 94000 and diameter less than 12 cm. The available RF peak power of 50 MW can be compressed to peak average power of more than 200 MW in order to double the kick for the electron bunches in a RF transverse deflector system and greatly improve the measurement resolution for both the electron bunch and the x-ray FEL pulse. High power operation has demonstrated the excellent performance of this RF compression system without any problems in RF breakdown, pulse heating and radiation. The design physics and fabrication as well as the measurement results will be presented in detail. |
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Slides MOOCA01 [20.278 MB] | |
DOI • | reference for this paper ※ DOI:10.18429/JACoW-IPAC2016-MOOCA01 | |
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MOPMR056 | Single-shot THz Spectrometer for Measurement of RF Breakdown in mm-wave Accelerators | 374 |
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Funding: This work was supported by the U.S. Department of Energy, Office of High Energy Physics, under contract DE-SC0013684 We present a new instrument designed to detect RF pulse shortening caused by vacuum RF breakdown in mm-wave particle accelerators. RF breakdown limits the performance of high gradient RF accelerators. To understand the properties of these breakdowns, it is necessary to have diagnostics that reliably detect RF breakdowns. In X-band or S-band accelerators, RF breakdowns are detected by measuring RF pulse shortening, vacuum burst, or, if current monitors are available, spikes in the field-emitted currents. In mm-wave accelerators, all of these methods are difficult to use. In our experiments, we could not measure RF pulse shortening directly with a crystal detector because the RF pulse is very short'just a few nanoseconds'and changes in the measured signal were masked by RF amplitude jitter. To overcome this limitation, we built a single-shot spectrometer with a frequency range of 117-125 GHz and a resolution of 0.1 GHz. The spectrometer should be able to measure the widening of the spectrum caused by the shortening of nanosecond-long pulses. We present design considerations, first experimental results obtained at FACET, and planned future improvements for the spectrometer. |
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DOI • | reference for this paper ※ DOI:10.18429/JACoW-IPAC2016-MOPMR056 | |
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MOPMW038 | Measurements of Copper RF Surface Resistance at Cryogenic Temperatures for Applications to X-Band and S-Band Accelerators | 487 |
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Funding: Funding from DOE SCGSR and DOE/SU Contract DE-AC02-76-SF00515 Recent SLAC experiments with cryogenically cooled X-Band standing wave copper accelerating cavities have shown that these structures can operate with accelerating gradients of ~250 MV/m and low breakdown rates. These results prompted us to perform systematic studies of copper rf properties at cryogenic temperatures and low rf power. We placed copper cavities into a cryostat cooled by a pulse tube cryocooler, so cavities could be cooled to 4K. We used different shapes of cavities for the X-Band and S-Band measurements. Properties of the cavities were measured using a network analyzer. We calculated rf surface resistance from measured Q0 and Q external of the cavity at temperatures from 4 K to room temperature. The results were then compared to the theory proposed by Reuter and Sondheimer. These measurements are a part of studies with the goal of reaching very high operational accelerating gradients in normal conducting rf structures. |
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DOI • | reference for this paper ※ DOI:10.18429/JACoW-IPAC2016-MOPMW038 | |
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MOPMW039 | TM01 Mode Launcher for Use in High Brightness Photoguns | 491 |
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Funding: DOE SCGSR and DOE/SU Contract DE-AC02-76-SF00515 Photo rf guns are a source of electron beams for X-ray FELs such as LCLS and European XFEL. In existing photoguns power is coupled into the cavity by waveguides through the cell walls, like LCLS, or through coaxial coupling, at the European XFEL. We are considering feeding a gun using a circular waveguide with the TM01 mode. To do that we need a mode launcher, a matched device that couples the rectangular TE01 mode waveguide to a TM01 mode in a circular waveguide. Use of the mode launcher reduces complexity of the gun cavity and increases flexibility of positioning the input waveguide relative to the gun body. Mode launchers have been successfully used at SLAC and elsewhere for X-band high gradient tests. Because the existing mode launchers were not built for high brightness guns, they have a significant quadrupole field component. High brightness rf guns have tight requirements on output beam properties, and this quadrupole component adversely affects the beam. We have designed a mode launcher free of this disadvantage. We present design considerations, methodology, and an example S-band mode launcher. |
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DOI • | reference for this paper ※ DOI:10.18429/JACoW-IPAC2016-MOPMW039 | |
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MOPMW040 | Electron Beam Excitation of a Surface Wave in mm-Wave Open Accelerating Structures | 494 |
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Funding: Work supported by the US DOE under contract DE-AC02-76SF00515. As part of research on the physics of rf breakdowns we performed experiments with high gradient traveling-wave mm-wave accelerating structures. The accelerating structures are open, composed of two identical halves separated by an adjustable gap. The electromagnetic fields are excited by an ultra-relativistic electron beam. We observed that a confined travelling-wave mode exists in half of the accelerating structure. The experiments were conducted at FACET facility at SLAC National Accelerator Laboratory. Depending on the gap width, the accelerating structure had beam-synchronous frequencies that vary from 90 to 140 GHz. When we opened the gap by more than half wavelength the synchronous wave remains trapped. Its behavior is consistent with the so called "surface wave". We characterized this beam-wave interaction by several methods: measurement of the radiated rf energy with the pyro-detector, measurement of the spectrum with an interferometer, measurement of the beam deflection by using the beam position monitors and profile monitor. |
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DOI • | reference for this paper ※ DOI:10.18429/JACoW-IPAC2016-MOPMW040 | |
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MOPMW041 | Measurements of RF Breakdowns in Beam Driven mm-Wave Accelerating Structures | 497 |
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Funding: Work supported by the US DOE under contract DE-AC02-76SF00515 We studied the physics and properties of rf breakdowns in high gradient traveling-wave accelerating structures at 100 GHz. The structures are open, made of two halves with a gap in between. The rf fields were excited in the structure by an ultra-relativistic electron beam generated by the FACET facility at the SLAC National Accelerator Laboratory. We observed rf breakdowns generated in the presence of GV/m scale electric fields. We varied the rf fields excited by the FACET bunch by moving structure relative to the beam and by changing the gap between structure halves. Reliable breakdowns detectors allowed us to measure the rf breakdown rate at these different rf parameters. We measured radiated rf energy with a pyro-detector. When the beam was off-axis, we observed beam deflection in the beam position monitors and on the screen of a magnetic spectrometer. The measurements of the deflection allowed us to verify our calculation of the accelerating gradient. |
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DOI • | reference for this paper ※ DOI:10.18429/JACoW-IPAC2016-MOPMW041 | |
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MOPMY036 | High-harmonic mm-Wave Frequency Multiplication using a Gyrocon-like Device | 579 |
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Funding: This project was funded by U.S. Department of Energy under Contract No. DE-AC02-76SF00515, and the National Science Foundation. Traditional linear interaction RF sources, such as Klystrons and Traveling Wave Tubes, fail to produce significant power levels at millimeter wavelengths. This is because their critical dimensions are small compared to the wavelength, and the output power scales as the square of the wavelength. We present a vacuum tube technology, where the device size is inherently larger than the operating wavelength. We designed a low–voltage mm–wave source, with an output interaction circuit based on a spherical sector cavity. This device was configured as a phased-locked frequency multiplier. We report the design and cold test results of a proof-of-principle fifth harmonic frequency multiplier with an output frequency of 57.12 GHz. |
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DOI • | reference for this paper ※ DOI:10.18429/JACoW-IPAC2016-MOPMY036 | |
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MOPOW046 | RadiaBeam/SLAC Dechirper as a Passive Deflector | 817 |
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Funding: This work was supported by Department of Energy Contract No. DE-AC02-76SF00515. We discuss the possibility of using the RadiBeam/SLAC dechirper recently installed at LCLS for measuring the bunch length of very short bunches, less than 1 fs perhaps as short as 100 atto second. When a bunch travels close to one of the jaws the particles of the bunch get a transverse kick depends upon the position of a particle in a bunch. The tail particles get more kick. The transverse force also gets a nonlinear dependence on the transverse position. The stretched bunch can be measured at the YAG screen that is 100 m downstream the dechirper. The most important aspect of this measurement is that that no synchronization is needed. The Green's function for the transverse kick was evaluated based on the precise wake field calculations of the dechirper corrugated structure*. Using this function we can restore the longitudinal shape of the bunch. This may also help to see if a bunch has any micro-bunch structure. * A. Noovokhatski "Wakefield potentials of corrugated structures",Phys. Rev. ST Accel. Beams 18, 104402 (2015) |
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DOI • | reference for this paper ※ DOI:10.18429/JACoW-IPAC2016-MOPOW046 | |
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WEPOY025 | High Power RF Generation From a W-Band Corrugated Structure Excited by a Train of Electron Bunches | 3040 |
SUPSS062 | use link to see paper's listing under its alternate paper code | |
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We report on the generation of multi-megawatt peak RF power at 91textGHz, using an ultrarelativistic electron bunch train to excite electromagnetic fields in a high-impedance metallic corrugated structure. This device can be used as a power source for high gradient acceleration of electrons. To achieve precise control of the wakefield phase, a long range wakefield interferometry method was developed in which the RF energy due to the interference of the wakefields from two bunches was measured as a function of the bunch separation. | ||
DOI • | reference for this paper ※ DOI:10.18429/JACoW-IPAC2016-WEPOY025 | |
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THPOR042 | New Quantity Describing the Pulse Shape Dependence of the High Gradient Limit in Single Cell Standing-Wave Accelerating Structures | 3878 |
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A new quantity has been developed to study the relationship among the breakdown rate, the pulse width and the gradient. Difference pulse shapes can be treated by introducing a Green's function. This paper describes the quantity and the results while it is applied to the data of many high-power test runs of different single-cell standing wave accelerating structures. A remarkably similar relationship between the new quantity and breakdown rate is observed from all of the test results. | ||
DOI • | reference for this paper ※ DOI:10.18429/JACoW-IPAC2016-THPOR042 | |
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THPOR044 | mm-Wave Standing-Wave Accelerating Structures for High-Gradient Tests | 3884 |
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We present the design and parameters of single-cell accelerating structures for high-gradient testing at 110 GHz. The purpose of this work is to study the basic physics of ultrahigh vacuum RF breakdown in high-gradient RF accelerators. The accelerating structures consist of pi-mode standing-wave cavities fed with TM01 circular waveguide mode. The geometry and field shape of these accelerating structures is as close as practical to single-cell standing-wave X-band accelerating structures, more than 40 of which were tested at SLAC. This wealth of X-band data will serve as a baseline for these 110 GHz tests. The structures will be powered from a pulsed MW gyrotron oscillator. One MW of RF power from the gyrotron may allow us to reach a peak accelerating gradient of 400 MeV/m. | ||
DOI • | reference for this paper ※ DOI:10.18429/JACoW-IPAC2016-THPOR044 | |
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THPOW028 | Automated Design for Standing Wave Electron Photoguns: TOPGUN RF Design | 3999 |
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Funding: DOE SCGSR and DOE/SU Contract DE-AC02-76-SF00515 Systematic design of RF photoguns involves multiple RF simulations in conjunction with beam dynamic simulations. RF simulations include tuning gun frequency, matching the gun to the feeding RF circuit, balancing the on axis electric fields between gun cells, minimizing surface electric and magnetic fields and power consumption, and optimizing separation of resonant mode frequencies. We created a tool that allows this multiple parameter optimization to be done automatically. We used SUPERFISH to accomplish the RF simulations. We present an example of the rf photogun TOPGUN design using these tools. |
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DOI • | reference for this paper ※ DOI:10.18429/JACoW-IPAC2016-THPOW028 | |
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