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Results for “dielectric-loaded accelerating structures”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Complete multipactor suppression in an X-band dielectric-loaded accelerating structure

Multipactor is a major issue limiting the gradient of rf-driven Dielectric-Loaded Accelerating (DLA) structures. Theoretical models have predicted that an axial magnetic field applied to DLA structures may completely block the multipactor discharge. However, previous attempts to demonstrate this magnetic field effect in an X-band traveling-wave DLA structure were inconclusive, due to the axial variation of the applied magnetic field, and showed only partial suppression of the multipactor loading. Here, the present experiment has been performed under improved conditions with a uniform axial magnetic field extending along the length of an X-band standing-wave DLA structure. Multipactor loading began to be continuously reduced starting from 3.5 kG applied magnetic field and was completely suppressed at ~8 kG. Dependence of multipactor suppression on the rf gradient inside the DLA structure was also measured.

43 PARTICLE ACCELERATORS↗
DOE OSTI2013

Observation of multipactor suppression in a dielectric-loaded accelerating structure using an applied axial magnetic field

Efforts by a number of institutions to develop a Dielectric-Loaded Accelerating (DLA) structure capable of supporting high gradient acceleration when driven by an external radio frequency source have been ongoing over the past decade. Single surface resonant multipactor has been previously identified as one of the major limitations on the practical application of DLA structures in electron accelerators. In this paper, we report the results of an experiment that demonstrated suppression of multipactor growth in an X-band DLA structure through the use of an applied axial magnetic field. This represents an advance toward the practical use of DLA structures in many accelerator applications.

43 PARTICLE ACCELERATORS↗
DOE OSTI2016

Multipactor Physics, Acceleration, and Breakdown in Dielectric-Loaded Accelerating Structures

The objective of this 3-year program is to study the physics issues associated with rf acceleration in dielectric-loaded accelerating (DLA) structures, with a focus on the key issue of multipactor loading, which has been found to cause very significant rf power loss in DLA structures whenever the rf pulsewidth exceeds the multipactor risetime (~10 ns). The experiments are carried out in the X-band magnicon laboratory at the Naval Research Laboratory (NRL) in collaboration with Argonne National Laboratory (ANL) and Euclid Techlabs LLC, who develop the test structures with support from the DoE SBIR program. There are two main elements in the research program: (1) high-power tests of DLA structures using the magnicon output (20 MW @11.4 GHz), and (2) tests of electron acceleration in DLA structures using relativistic electrons from a compact X-band accelerator. The work during this period has focused on a study of the use of an axial magnetic field to suppress multipactor in DLA structures, with several new high power tests carried out at NRL, and on preparation of the accelerator for the electron acceleration experiments.

11.4 GHz↗
DOE OSTI2013

Development and Testing of Advanced Accelerator Structures and Technologies at 11.424 GHz (Final Report)

The areas of activity are grouped by Task. Task 1 involved a study of the key physics issues associated with the use of high gradient dielectric-loaded accelerating (DLA) structures in rf linear accelerators, and was carried out in collaboration with Argonne National Laboratory (ANL) and Euclid Techlabs LLC; Task 2 involved a study of high-gain active microwave pulse compressors, and was carried out in collaboration with Omega-P, Inc. and the Institute of Applied Physics of the Russian Academy of Sciences. The experiments were carried out in the X-band Magnicon Laboratory at the Naval Research Laboratory (NRL). The studies under Task 1 were focused on multipactor and breakdown in externally driven DLA structures at the 200-ns timescale. Suppression of multipactor and breakdown are essential to the practical application of dielectric structures in rf linear accelerators. During the first year of this program, we carried out a study of a doubly tapered clamped quartz traveling-wave (TW) structure that operated at up to 15 MV/m without breakdown by eliminating dielectric joints, thus avoiding the joint breakdown that had characterized many earlier experiments. However, strong multipactor loading was observed in the structure. In the second year, we carried out a successful test of a TW DLA structure with a sputtered copper exterior coating that eliminated the need for a clamped metal enclosure to form the outer metallic boundary, thus simplifying the fabrication of the structure. In the second year, we also carried out the first test of a dielectric-loaded standing-wave (SW) accelerating structure. Multipactor loading limited the achievable accelerating gradient in the structure to 9 MV/m, but the SW structure proved to be a useful configuration for studying multipactor physics. To continue this study, a new standing-wave structure is in preparation that will permit direct measurements of the energy stored in the structure for new studies of multipactor physics. Finally, in the third year of the program, we focused on the suppression of multipactor loading using an applied axial magnetic field, as proposed by Chang et al. [C. Chang et al., J. Appl. Phys. 110, 063304 (2011).]. Chang used an analytic model and PIC simulations to show that an axial magnetic field can reduce the period of the secondary electron trajectories on the dielectric surface, and thus spoil the multipactor resonance condition. The optimum effect occurs when the ratio of the gyrofrequency to the rf frequency is in the range of ~ 0.7 to 1. For an X-band structure, this is equivalent to a solenoidal field of 2.8–4 kG. This approach to multipactor suppression is attractive because it maintains the advantages of a DLA structure and is independent of other accelerator parameters.

43 PARTICLE ACCELERATORS↗
DOE OSTI2014

Theory of factors limiting high gradient operation of warm accelerating structures

This final report summarizes the research performed during the time period from 8/1/2010 to 7/31/2013. It consists of two parts describing our studies in two directions: (a) analysis of factors limiting operation of dielectric-loaded accelerating (DLA) structures where the main problem is the occurrence of multipactor on dielectric surfaces, and (b) studies of effects associated with either RF magnetic or RF electric fields which may cause the RF breakdown in high-gradient metallic accelerating structures. In the studies of DLA structures, at least, two accomplishments should be mentioned: the development of a 3D non-stationary, self-consistent code describing the multipactor phenomena and yielding very good agreement with some experimental data obtained in joint ANL/NRL experiments. In the metallic structures, such phenomena as the heating and melting of micro-particles (metallic dust) by RF electric and magnetic fields in single-shot and rep-rate regimes is analyzed. Also, such processes in micro-protrusions on the structure surfaces as heating and melting due to the field emitted current and the Nottingham effect are thoroughly investigated with the account for space charge of emitted current on the field emission from the tip.

43 PARTICLE ACCELERATORS↗
DOE OSTI2012

Dielectric-Lined High-Gradient Accelerator Structure (Final Report)

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 (~2 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 μs 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 10 5 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 MΩ/m for DLA, as compared to 99 MΩ/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.

36 MATERIALS SCIENCE↗