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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

Simulations of multipactor in circular waveguides

Detailed numerical simulations have been done to investigate the properties of multipactor breakdown in circular waveguides operating in the propagating fundamental TE11 mode. Main attention has been given to a comparison between the two fundamental cases corresponding to linear and circular polarizations, respectively, of the propagating electromagnetic wave. It is found that circular polarization is considerably more susceptible to multipactor than linear polarization. The reason for this difference is clarified by a comprehensive study of the electron motion in the waveguide.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Coexistence of mixed mode multipactor

Multipactor is a vacuum discharge based on secondary electron emission, and can manifest in many resonant and non-resonant modes. Where two or more types of multipactor coexist in the same device, it is found analytically that the one with the highest yield or the lowest order dominates.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
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 OSTI2014

Analysis of the multipactor effect in circular waveguides excited by two orthogonal polarization waves

Circular waveguides, either employed as resonant cavities or as irises connecting adjacent guides, are widely present in many passive components used in different applications (i.e., particle accelerators and satellite subsystems). In this paper, we present the study of the multipactor effect in circular waveguides considering the coexistence of the two polarizations of the fundamental TE{sub 11} circular waveguide mode. For a better understanding of the problem, only low multipactor orders have been explored as a function of the polarization ellipse eccentricity. Special attention has been paid to the linear and circular polarizations, but other more general configurations have also been explored.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

The velocity deviations of secondary electrons and the main condition of phase stability in two-sided multipactors

The conditions for secondary emission of electrons and the occurrence of discharge have been analyzed in detail with a plane-parallel model. According to the traditional theory of a resonance discharge (referred to as “simple resonance theory”), the velocity deviation of secondary electrons has no effect on multipactor (MP) zones. In this work, the effect of such deviations from resonance value in two-sided multipactors has been investigated using a different approach. In a previous study [M. Mostajeran and M. Lamehi Rachti, J. Instrum. 5, P08003 (2010)], a stability equation was derived considering only the phase deviation from the resonant phase. Here, the approach used in that paper has been extended to include velocity deviations as well. It is shown that phase and velocity deviations, which may not be considered small, support MP zones. Theoretical calculations are then compared with numerical results.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Machine Learning for Predicting Multipactor Susceptibility in Planar RF Structures

Multipactor discharge is a persistent challenge in high-power microwave (HPM) and accelerator systems, where secondary electron avalanches can cause heating, vacuum degradation, and failure. This work presents the first supervised machine learning (ML) framework for multipactor prediction, trained on high-fidelity 3D Particle-in-Cell (PIC) simulation data in planar geometries. The model maps operational, geometric, and material-dependent secondary electron yield (SEY) parameters to the time-averaged electron growth rate, enabling rapid reconstruction of susceptibility charts. Among the models evaluated, tree-based ensemble methods such as Random Forest and Extra Trees demonstrate superior generalization to unseen materials compared to neural networks such as multilayer perceptron (MLP). Performance metrics, including Intersection over Union (IoU), Structural Similarity Index Measure (SSIM), and Pearson correlation, show close agreement with simulation benchmarks. Principal Component Analysis attributes generalization limits to material feature-space disjointedness.

43 PARTICLE ACCELERATORS↗
DOE OSTI2010

Studies of Multipactor in Dielectric-Loaded Accelerator Structures: Comparison of Simulation Results with Experimental Data

In this paper new results of numerical studies of multipactor in dielectric-loaded accelerator structures are presented. The results are compared with experimental data obtained during recent studies of such structures performed by Argonne National Laboratory, the Naval Research Laboratory, SLAC National Accelerator Laboratory and Euclid TechLabs, LLC. Good agreement between the theory and experiment was observed for the structures with larger inner diameter, however the structures with smaller inner diameter demonstrated a discrepancy between the two. Possible reasons for such discrepancy are discussed.

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

Effects of the energy spread of secondary electrons in a dc-biased single-surface multipactor

The effects of the energy spread of secondary electrons are theoretically investigated for a dc-biased single-surface multipactor. In our previous publication [S. G. Jeon et al., Phys. Plasmas 16, 073101 (2009)], we obtained the conditions for the phase lock of an electron bunch, assuming zero velocity spread of the secondary electrons. In this work, we extended our previous theory to derive a quadratic map, by which the stability and bifurcation of the electron bunch can be systematically investigated. For the study of the energy spread of the secondary electrons, a randomized term was added to this map. The modified map then showed significant smearing-out of the bifurcated branches. The theoretical results were verified by particle-in-cell simulations, which showed good agreement in wide parameter ranges for both cases of monoenergetic and energy-spread secondary electrons.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

How the “main condition” of phase stability can explain the effect of the velocity deviation of secondary electrons in DC-biased single-sided multipactors

In this work, a “main condition” for phase stability has been employed to investigate the effects of the velocity deviation of the electrons in DC-biased single-sided multipactors (MPs). In a previous study [M. Mostajeran, Phys. Plasmas 21, 053108 (2014)], a stability equation was derived, where the secondary electron was assumed to have zero initial velocity and the phase deviation from the resonant phase was considered. In this work, both deviations in phase and velocity from the resonant condition are taken into account, assuming nonzero initial velocity for the secondary electrons. Using the main condition for stability, it is shown that MP discharge can rise in situations, where large velocity deviations from initial velocity and large phase deviations from resonant phase exist. This is contrary to what can be predicted on the basis of the “simple stability condition.” This result is further confirmed by numerical simulations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Higher Harmonics in Multipactor Induced Plasma Ionization Breakdown near a Dielectric Surface

In this Letter, the novel physics of higher harmonic (HH) generation of the normal electric field near a dielectric surface is reported in multipactor induced plasma ionization breakdown, as determined by kinetic particle-in-cell simulations. The observed HH frequency is around ten times the fundamental rf driving frequency, but lower than the electron plasma frequency. A theory is constructed which indicates that stream plasma interaction-induced instability is the mechanism of HH generation in the collisional regime. The HH frequency and its corresponding growth rate of the HH oscillation amplitude from the theory are in good agreement with kinetic particle-in-cell simulations.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2019

Emergence of Novel Multipactor Modes Under Standing Wave Conditions in a Coaxial Line With an RF Window

The multipactor (MP) phenomenon can generate exponentially increasing electron populations in RF vacuum systems, leading to components becoming damaged. This paper uses numerical methods to analyze an RF vacuum window in a coaxial line under various standing wave conditions. Our methods are first benchmarked against the analytic and experimental results for the traveling wave and the numerical results for standing waves that were found in previous work. Second, the effect of standing waves on electron motion in the axial direction is discussed and related to electron trajectories in the presence of an RF window. It is found that standing waves with the magnetic maximum located behind the RF window lead to window collisions that can affect MP depending on the power, frequency, and conductor radii. For a low number of window collisions, MP was highly predictable by earlier work, but as the percentage of collisions occurring on the window surpassed approximately 50%, a novel mode of MP emerged to dominate the phenomenon.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Multipactor in Coaxial Line Induced by DC Bias

High DC voltage applied between outer and inner electrodes of coaxial transmission lines is considered and actually is an effective remedy against multipactor in these lines. However, the high voltage bias being below certain level itself initiates multipacting. In this technical note the experimental manifestation and numerical simulations of the phenomenon are presented.

43 PARTICLE ACCELERATORS↗
DOE OSTI2025

Two-Frequency RF Fields Induced Multipactor in Coaxial Transmission Line

This work investigates multipactor discharge in coaxial geometry with two-frequency rf electric field Vrf [sin(𝜔𝑡+𝜃)+𝛽 sin(n(𝜔𝑡+𝜃)+𝛾)], where Vrf is the peak voltage, 𝛽 is the field strength of the second car-rier mode relative to fundamental mode, n is the ratio of two carrier fre-quencies and 𝛾 is the relative phase of second carrier mode.

43 PARTICLE ACCELERATORS↗