WorldCat Identities

Nantista, C.

Overview
Works: 8 works in 8 publications in 1 language and 48 library holdings
Publication Timeline
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Most widely held works by C Nantista
A Coherent Compton Backscattering High Gain FEL using an X-Band Microwave Undulator( )

1 edition published in 2005 in English and held by 6 WorldCat member libraries worldwide

High power microwave sources at X-Band, delivering 400 to 500 of megawatts for about 400 ns, have been recently developed. These sources can power a microwave undulator with short period and large gap, and can be used in short wavelength FELs reaching the nm region at a beam energy of about 1 GeV. We present here an experiment designed to demonstrate that microwave undulators have the field quality needed for high gain FELs
Novel Accelerator Structure Couplers( )

1 edition published in 2003 in English and held by 6 WorldCat member libraries worldwide

Recent experience with X-band accelerator structure development has shown the rf input coupler to be the region most prone to rf breakdown and degradation, effectively limiting the operating gradient. A major factor in this appears to be high magnetic fields at the sharp edges of the coupling irises. As a first response to this problem, couplers with rounded and thickened iris horns have been employed, with improved performance. In addition, conceptually new coupler designs have been developed, in which power is coupled through the broadwall of the feed waveguide. A prototype ''mode converter'' coupler, which launches the TM01 mode in circular waveguide before coupling through a matching cell into the main structure, has been tested with great success. With peak surface fields below those in the body of the structure, this coupler represents a break-through in the NLC structure program. The design of this coupler and of variations which use beamline space more efficiently are described here. The latter include a coupler in which power passes directly through an iris in the broad wall of the rectangular waveguide into a matching cell and one which makes the waveguide itself an accelerating cell. We also discuss techniques for matching such couplers
Overmoded Waveguide Components for High-Power RF( )

1 edition published in 2003 in English and held by 6 WorldCat member libraries worldwide

High-power applications of rf often require the use of overmoded waveguide to reduce the probability of rf breakdown by lowering surface fields, as well as to reduce the attenuation due to ohmic losses in transporting the power from the point of generation to the point of use. This is particularly true in the development of warm linear collider designs, such as NLC, JLC, and CLIC, especially the former two which involve extensive rf pulse compression / power distribution systems. Transitioning to and from overmoded waveguide and manipulating rf in such systems requires specialized components in whose design care must be taken to avoid parasitic mode loss and excessive field enhancement. Although fixed frequency operation means modest bandwidth requirements, power levels up to several hundred megawatts must be accommodated, and efficiency is important. Consequently, physicists working at laboratories engaged in the above efforts have produced a number of novel waveguide components in the past several years. These include tapers, mode converters, bends, directional couplers, power splitter/combiners, switches, phase shifters, etc. Often the circular TE01 mode is used for its lack of surface electric field and low attenuation. A class of components using planar geometries for over-height rectangular waveguide has been developed at the Stanford Linear Accelerator Center. The current paper describes a few additional passive high-power components that have not been presented elsewhere a height - taper, a compact mode converter, a four-way power splitter, fractional directional couplers and tap-offs. These were designed at X-band as part of the NLC R & D, but may find wider application
Optimization of the X-Band Structure for the JLC/NLC( )

1 edition published in 2006 in English and held by 6 WorldCat member libraries worldwide

In this paper, we present design considerations to address the high gradient, wakefields, and RF efficiency issues for the JLC/NLC structures. We will present the rationale for the choices of phase advance, structure length, structure aperture, and other design aspects in the optimization of the structure. We will study the impact of parameter choices for the JLC/NLC beam environment and discuss approaches being taken for an optimal JLC/NLC structure design
High Gradient Performance of NLC/GLC X-band Accelerating Structures( )

1 edition published in 2005 in English and held by 6 WorldCat member libraries worldwide

During the past five years, there has been a concerted program at SLAC and KEK to develop accelerator structures that meet the high gradient (65 MV/m) performance requirements for the Next Linear Collider (NLC) and Global Linear Collider (GLC) initiatives. The design that resulted is a 60-cm-long, traveling-wave structure with low group velocity and 150 degree per cell phase advance. It has an average iris size that produces an acceptable short-range wakefield, and dipole mode damping and detuning that adequately suppresses the long-range wakefield. More than eight such structures have operated at a 60 Hz repetition rate over 1000 hours at 65 MV/m with 400 ns long pulses, and have reached breakdown rate levels below the limit for the linear collider. Moreover, the structures are robust in that the rates continue to decrease over time, and if the structures are briefly exposed to air, the rates recover to their low levels within a few days. This paper presents a summary of the results from this program, which effectively ended last August with the selection of ''cold'' technology for an International Linear Collider (ILC)
Design of a Standing-Wave Multi-Cavity Beam-Monitor for Simultaneous Beam Position and Emittance Measurements( )

1 edition published in 2005 in English and held by 6 WorldCat member libraries worldwide

A high precision emittance measurement requires precise beam position at the measurement location. At present there is no existing technique, commercial or otherwise, for non-destructive pulse-to-pulse simultaneous beam position and emittance measurement. FARTECH, Inc. is currently developing a high precision cavity-based beam monitor for simultaneous beam position and emittance measurements pulse-to-pulse, without beam interception and without moving parts. The design and analysis of a multi-cavity standing wave structure for a pulse-to-pulse emittance measurement system in which the quadrupole and the dipole standing wave modes resonate at harmonics of the beam operating frequency is presented. Considering the Next Linear Collider beams, an optimized 9-cavity standing wave system is designed for simultaneous high precision beam position and emittance measurements. It operates with the {pi}-quadrupole mode resonating at 16th harmonic of the NLC bunch frequency, and the 3 {pi}/4 dipole mode at 12th harmonic (8.568 GHz). The 9-cavity system design indicates that the two dipoles resonate almost at the same frequency 8.583 GHz and the quadrupole at 11.427 GHz according to the scattering parameter calculations. The design can be trivially scaled so that the dipole frequency is at 8.568 GHz, and the quadrupole frequency can then be tuned during fabrication to achieve the desired 11.424 GHz. The output powers from these modes are estimated for the NLC beams. An estimated rms-beam size resolution is sub micro-meters and beam positions in sub nano-meters
Tests on MgB2 for Application to SRF Cavities( )

1 edition published in 2006 in English and held by 6 WorldCat member libraries worldwide

Magnesium diboride (MgB{sub 2}) has a transition temperature (T{sub c}) of {approx} 40 K, i.e., about 4 times higher than niobium (Nb). Studies in the last 3 years have shown that it could have about one order of magnitude less RF surface resistance (R{sub s}) than Nb at 4 K and seems to have much less power dependence than high-T{sub c} materials such as YBCO. However, it was also found that it will depend on the way you deposit the film. The result from on-axis pulsed laser deposition (PLD) showed rapid increase in R{sub s} with higher surface magnetic fields compared to the film deposited with reactive evaporation method
Test Bed for Superconducting Materials( )

1 edition published in 2005 in English and held by 6 WorldCat member libraries worldwide

Superconducting rf cavities are increasingly used in accelerators. Gradient is a parameter of particular importance for the ILC. Much progress in gradient has been made over the past decade, overcoming problems of multipacting, field emission, and breakdown triggered by surface impurities. However, the quenching limit of the surface magnetic field for niobium remains a hard limitation on cavity fields sustainable with this technology. Further exploration of materials and preparation may offer a path to surpassing the current limit. For this purpose, we have designed a resonant test cavity. One wall of the cavity is formed by a flat sample of superconducting material; the rest of the cavity is copper or niobium. The H field on the sample wall is 75% higher than on any other surface. Multipacting is avoided by use of a mode with no surface electric field. The cavity will be resonated through a coupling iris with high-power rf at superconducting temperature until the sample wall quenches, as detected by a change in the quality factor. This experiment will allow us to measure critical magnetic fields up to well above that of niobium with minimal cost and effort
 
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