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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

RF conditioning and beam experiments on 400 keV RFQ accelerator at BARC

A 400 keV Radio-frequency quadrupole accelerator (RFQ) has been designed, developed and tested at BARC. This will be used as a neutron generator (via D-T reaction). The RFQ operates at a resonant frequency of 350 MHz and needs an RF power of ∼ 60 kW to accelerate the deuteron beam to 400 keV within a length of 1.03 m. Though the RFQ is designed for deuteron beam, it was tested by accelerating both the proton and deuteron beams to their designed values of 200 and 400 keV respectively. The proton and deuteron beam experiments required peak RF power of approx. 15 kW and 60 kW respectively at 350 MHz. The RF power from the tetrode amplifier and coaxial transmission lines is coupled to the cavity by a coaxial loop coupler. As the coupler and cavity operated at vacuum of better than 2e-6 torr, extensive RF conditioning of the cavity and coupler was performed to reach at the desired power levels. (author)

43 PARTICLE ACCELERATORS↗
DOE OSTI2014

High power RF conditioning of 2856 MHz, 40 keV prototype buncher cavity system

An on axis biperiodic buncher cavity system at 2856 MHz with three bunching cells plus one power feed cell has been designed and fabricated for 40 keV pulsed electron beam injection from a thermionic gun. The vacuum and high power RF conditioning prior to the beam injection is presented in this paper. Initially the cavity system along with the in-flange fast current transformer and the beam diagnostics chamber has been baked at 60-65 °C for 22 hours under a vacuum ≤ 6 x 10{sup -6} mbar. The final ultimate vacuum was 2.3 x 10{sup -6} mbar and it took 195 seconds to reach to 1 x 10{sup -5} mbar with the gate valve closed. After this the high power RF conditioning was done from 0.3 MW to 1.2 MW input RF power with 4μsec. pulse at 1 Hz PRF. At this point the ultimate vacuum reached to 2.3 x 10{sup -6} mbar and reached to 1 x 10{sup -5} mbar in 195 seconds after closing the gate valve. Finally the system was conditioned with 1MW, 4μsec. RF pulses at 20 Hz PRF. (author)

43 PARTICLE ACCELERATORS↗
DOE OSTI2017

High Average Current Bunched Electron Source for RF Accelerators

High-average current high-quality electron beams are imperative for high-power RF accelerators used in various energy and environmental applications in industry, medicine, and national security as well as for advanced existing and future accelerator systems such as PIP II, particle colliders (eRHIC BNL) for cooling high-energy proton, ion, and hadron beams, storage rings, wakefield accelerators (AWA), and energy recovery linacs. The most common current approach to generate high-peak current high-brightness short electron bunches uses photoemission excited by intense laser radiation. However, to avoid overheating photocathodes are operated at a subharmonic of the linac frequency that in turn restricts the average beam current. There are also limitations in the electron-beam average current due to the average power available from drive lasers. On the other hand, the state of the art thermionic sources that produce high average current electron beams do not have yet good emission gating methods to realize sufficiently short and good quality electron bunches. We develop a robust thermionic electron source (e-source) of >1 A average current with advanced gating of the emission at the linac-frequency and its 3rd harmonic. Central to the approach is a novel scheme for energy efficient feeding of the 3rd-harmonic power in the RF gap. The cathode is attached to the gun cavity through this small emission-gating gap in which the RF modes are excited. The use of thermionic emission allows for the desired high-average current CW electron beams. Adding the 3rd emission gating harmonic shortens the electron bunches and by this means lessens impact of the RF curvature on the beam quality resulting in the smaller energy spread and bunch emittances. Such high current electron beam is suitable for injection into high-power normal- or superconducting linacs in CW mode at a high repetition rate of the linac frequency. In Phase I, we performed modeling and beam dynamics, parametric, and optimization studies. A >1A average current e-source with 200,000hrs lifetime expectancy has been demonstrated in the simulations. We have developed baseline design of the e-source and its components. It is anticipated that the e-source will be in demand for normal- and superconducting particle accelerators and will benefit many commercial and energy and environmental applications such as treating waste and potable water, sludge, stack gases, and medical waste as well as X-ray sources for medicine, sterilization, materials processing, geology, and non-intrusive imaging.

43 PARTICLE ACCELERATORS↗
NASA NTRS2024

Investigation into the Origin of the RF Phase Noise for the LISA Laser Transmitter

The Laser Interferometer Space Antenna (LISA) mission is a collaborative consortium led by the European Space Agency (ESA)in cooperation with the National Aeronautics and Space Administration (NASA). LISA’s goal is to capture gravitational wave events, such as spacetime distortions caused by massive orbiting bodies. Multiple low noise, single frequency master oscillator power amplifier (MOPA) lasers with a polarization-maintaining Yb-fiber amplifier will serve as the light sources for the LISA observatory. The NASA Goddard Space Flight Center (GSFC)is currently developing the laser transmitters for this project. The LISA lasers will carry radio frequency (RF)sidebands for exchanging reference RF clock information between spacecrafts. Excess phase noise, especially carrier-sideband differential phase noise added to the modulation side band by the phase modulator and the fiber amplifier, becomes a concern. In the gravitational wave frequency ranges, leading sources of this RF “differential” phase noise can be both intrinsic and extrinsic. Examples of intrinsic sources include thermal load on the active fiber, thermo-mechanical induced birefringence, and nonlinearities in the various passive and active fibers. Extrinsic sources can include external temperature fluctuations and infrasonic mechanical strain or vibrations. These noise spectra usually include both white and pink (1/f) thermal noise contributions. This work investigates the physical origins and impact of these sources of RF phase noise, and their characteristic spectra and management will be discussed.

Laser Interferometer Space Antenna↗
DOE OSTI2025

Process intensification approach to enhancing heat and mass transfer: Radio frequency (RF) assisted drying of paper and board

Conventional multi-cylinder drying of paper and board typically relies on both conductive drying from steam-heated cylinders and convective drying, where heated air flows over the paper web surface. Conduction primarily contributes to heat transfer, while convection is the main driver of mass transfer. However, conventional drying systems are heavily dependent on steam, usually powered by fossil fuels, and are often energy-inefficient with high levels of waste. Additionally, these surface-driven processes result in a lower percentage of energy absorption compared to the energy supplied, leading to significant energy losses. To improve this long-standing process, an experimental system was developed to investigate a process intensification approach involving the integration of Radio Frequency (RF) heating, a volumetric electromagnetic technology, alongside traditional conduction and convection drying methods. This study also emphasizes the use of sensors to continuously monitor key parameters such as moisture content, supply system temperatures, sample temperatures, air flows, and drying rates in real-time. The effect of RF as an auxiliary energy source in localized environments at varying moisture levels was explored to optimize industrial drying systems, quantify potential improvements, and provide insights for future studies. Experimental results from trials combining RF with convection and with the base case alternating conduction-convection drying processes are presented. It is shown that RF is a viable process intensification approach for paper drying improving the drying rate and energy intensity at higher moisture contents. These findings offer valuable insights for process intensification and contribute to the process development, modeling, and simulation of advanced paper drying techniques.

42 ENGINEERING↗
DOE OSTI2018

Enhancement of effective linear RF surface resistance of superconducting surfaces by microscopic topography

Minimization of Radio-Frequency (RF) dissipation on superconducting surfaces is of interest for many applications. A prominent one is the use of Superconducting RF (SRF) cavities for charged particle acceleration. In addition to our previous investigation which characterized the occurrence of high-field non-linear losses by microscopic surface topography, the topic of increased linear losses as a function of surface topographic character merits consideration. Surfaces with isotropic homogeneous surface topography may be well characterized by power spectral density (PSD) derived from systematic height measurements. PSD characterizations of representative niobium cavity surface treatments have been developed: Electro-Polishing (EP), Nano-Mechanical Polishing (NMP) and Centrifugal Barrel Polishing (CBP) A perturbation model based on PSD statistical analysis is used to calculate additional RF loss on these surfaces when superconducting. The model assesses the penetration depth effects for a superconductor. We thus estimate the RF power dissipation ratio between these rough surfaces and an ideal smooth surface.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

Improved RF measurements of SRF cavity quality factors

SRF cavity quality factors can be accurately measured using RF-power based techniques only when the cavity is very close to critically coupled. This limitation is from systematic errors driven by non-ideal RF components. When the cavity is not close to critically coupled, these systematic effects limit the accuracy of the measurements. The combination of the complex base-band envelopes of the cavity RF signals in combination with a trombone in the circuit allow the relative calibration of the RF signals to be extracted from the data and systematic effects to be characterized and suppressed. The improved calibration allows accurate measurements to be made over a much wider range of couplings. Demonstration of these techniques during testing of a single-spoke resonator with a coupling factor of near 7 will be presented, along with recommendations for application of these techniques.

43 PARTICLE ACCELERATORS↗
DOE OSTI2019

RF sheath modeling of experimentally observed plasma surface interactions with the JET ITER-Like Antenna

Waves in the Ion Cyclotron Range of Frequencies (ICRF) enhance local Plasma-Surface Interactions (PSI) near the wave launchers and magnetically-connected objects via Radio-Frequency (RF) sheath rectification. ITER will use 20MW of ICRF power over long pulses, questioning the long-term impact of RF-enhanced localized erosion on the lifetime of its Beryllium (Be) wall. Recent dedicated ICRF-heated L-mode discharges documented this process on JET for different types of ICRF antennas. Using visible spectroscopy in JET ICRF-heated L-mode discharges, poloidally-localized regions of enhanced (by ~2–4x) Be I and Be II light emission were observed on two outboard limiters magnetically connected to the bottom of the active ITER-Like Antenna (ILA). The observed RF-PSI induced by the ILA was qualitatively comparable to that induced by the JET standard, type-A2 antennas, for similar strap toroidal phasing and connection geometries. The Be II line emission was found more intense when powering the bottom half of the ILA rather than its top half. Conversely, more pronounced SOL density modifications were observed with only top array operation, on field lines connected to the top half of the ILA. So far the near-field modeling of the ILA with antenna code TOPICA (Torino Polytechnic Ion Cyclotron Antenna), using curved antenna model, was partially able to reproduce qualitatively the observed phenomena. A quantitative discrepancy persisted between the observed Be source amplification and the calculated, corresponding increases in E // field at the magnetically connected locations to the ILA when changing from only top to only bottom half antenna operation. This paper revisits these current drive phased and half-ILA powered cases using for the new simulations flat model of the ILA and more realistic antenna feeding to calculate the E// field maps with TOPICA code. Further, the Self-consistent Sheaths and Waves for Ion Cyclotron Heating Slow Wave (SSWICH-SW) code, which couples slow wave evanescence with DC Scrape-Off Layer (SOL) biasing, is used to estimate the poloidal distribution of rectified RF-sheath Direct Current (DC) potential V DC in the private SOL between the ILA poloidal limiters. The approach so far was limited to correlating the observed, enhanced emission regions at the remote limiters to the antenna near-electric fields, as calculated by TOPICA. The present approach includes also a model for the rectification of these near-fields in the private SOL of the ILA. With the improved approach, when comparing only top and only bottom half antenna feeding, we obtained good qualitative correlation between all experimental measurements and the calculated local variations in the E // field and V DC potential.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

RF wave simulation for cold edge plasmas using the MFEM library

A newly developed generic electro-magnetic (EM) simulation tool for modeling RF wave propagation in SOL plasmas is presented. The primary motivation of this development is to extend the domain partitioning approach for incorporating arbitrarily shaped SOL plasmas and antenna to the TORIC core ICRF solver, which was previously demonstrated in the 2D geometry [S. Shiraiwa, et. al., “HISTORIC: extending core ICRF wave simulation to include realistic SOL plasmas”, Nucl. Fusion in press], to larger and more complicated simulations by including a 3D realistic antenna and integrating RF rectified sheath potential model. Such an extension requires a scalable high fidelity 3D edge plasma wave simulation. We used the MFEM [http://mfem.org], open source scalable C++ finite element method library, and developed a Python wrapper for MFEM (PyMFEM), and then a radio frequency (RF) wave physics module in Python. This approach allows for building a physics layer rapidly, while separating the physics implementation being apart from the numerical FEM implementation. An interactive modeling interface was built on pScope [S Shiraiwa, et. al. Fusion Eng. Des. 112, 835] to work with an RF simulation model in a complicated geometry.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2026

Modeling rf sheath formation in turbulent tokamak boundary plasma

During ICRF antenna operation, complex interactions between turbulent density profiles, nonlinear RF sheaths, and RF-induced convective transport are observed to alter plasma density in the tokamak edge [D’Ippolito et al., Nucl. Fusion 38, 1543 (1998)]. In this work, we explore the physics of such interactions via numerical modeling, using a nonlinear EM/plasma/sheath code (VSim) and profiles obtained from a fluid plasma turbulence code (Hermes) in a 3D slab domain containing biased side-wall limiters. RF-rectified sheath formation on antenna and limiter surfaces is observed as electromagnetic waves launched by the antenna are refracted through the turbulent density profile. On transport timescales, such sheath potentials have been shown to influence both the mean species density and its RMS fluctuation spectrum [Smithe et al., these proceedings]. On the faster RF timescales, we demonstrate that the converse is also true – regions of high plasma density near material surfaces give rise to the highest sheath potential amplitudes. When density is turbulent and spatially nonuniform, localized regions of high sheath potential (hotspots) may develop where high-density filaments intersect material surfaces. Such hotspots are of particular concern as sources of impurity sputtering, and we explore their behavior in response to changes both to the local plasma density and to antenna operating parameters and structure. Related results exploring the role of Faraday shields and/or enclosing structures in suppressing high sheath potentials for other devices (e.g. SPARC) will also be shown.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Influence of RF power on magnetron sputtered AZO films

Al-doped Zinc Oxide (AZO) transparent conducting films are prepared on glass substrate by RF magnetron sputtering under different RF power with a 3 inch diameter target of 2 wt%Al{sub 2}O{sub 3} in zinc oxide. The effect of RF power on the structural, optical and electrical properties of AZO films was investigated by X-ray Diffraction (XRD), Hall measurement and UV-Visible spectrophotometry. The XRD data indicates a preferential c-axis orientation for all the films. All films exhibit high transmittance (<90%) in visible region. Films deposited at 60 W power exhibit lowest resistivity of 5.7 Multiplication-Sign 10{sup -4}{omega}cm. Such low-resistivity and high-transmittance AZO films when prepared using low RF power at room temperature could find important applications in flexible electronics.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Dependence of the microwave surface resistance of superconducting niobium on the magnitude of the rf field

Utilizing difference in temperature dependencies we decoupled Bardeen-Cooper-Schrieffer (BCS) and residual components of the microwave surface resistance of superconducting niobium at all rf fields up to B{sub rf}{approx}115 mT. We reveal that the residual resistance decreases with field at B{sub rf} Less-Than-Or-Equivalent-To 40 mT and strongly increases in chemically treated niobium at B{sub rf}>80 mT. We find that BCS surface resistance is weakly dependent on field in the clean limit, whereas a strong and peculiar field dependence emerges after 120 Degree-Sign C vacuum baking.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Superfluid helium cryogenic systems for superconducting RF cavities at KEK

Recent accelerator projects at KEK, such as the Superconducting RF Test Facility (STF) for R and D of the International Linear Collider (ILC) project and the compact Energy Recovery Linac (cERL), employ superconducting RF cavities made of pure niobium, which can generate high gradient acceleration field. Since the operation temperature of these cavities is selected to be 2 K, we have developed two 2 K superfluid helium cryogenic systems for stable operation of superconducting RF cavities for each of STF and cERL. These two 2 K superfluid helium cryogenic systems are identical in principle. Since the operation mode of the cavities is different for STF and cERL, i.e. the pulse mode for STF and the continuous wave mode for cERL, the heat loads from the cavities are quite different. The 2 K superfluid helium cryogenic systems mainly consists of ordinary helium liquefiers/refrigerators, 2 K refrigerator cold boxes, helium gas pumping systems and high-performance transfer lines. The 2 K refrigerators and the high-performance transfer lines are designed by KEK. Some superconducting RF cavity cryomodules have been already connected to the 2 K superfluid helium cryogenic systems for STF and cERL respectively, and cooled down to 2 K successfully.

43 PARTICLE ACCELERATORS↗
DOE OSTI2016

Alternative modeling methods for plasma-based Rf ion sources

Rf-driven ion sources for accelerators and many industrial applications benefit from detailed numerical modeling and simulation of plasma characteristics. For instance, modeling of the Spallation Neutron Source (SNS) internal antenna H{sup −} source has indicated that a large plasma velocity is induced near bends in the antenna where structural failures are often observed. This could lead to improved designs and ion source performance based on simulation and modeling. However, there are significant separations of time and spatial scales inherent to Rf-driven plasma ion sources, which makes it difficult to model ion sources with explicit, kinetic Particle-In-Cell (PIC) simulation codes. In particular, if both electron and ion motions are to be explicitly modeled, then the simulation time step must be very small, and total simulation times must be large enough to capture the evolution of the plasma ions, as well as extending over many Rf periods. Additional physics processes such as plasma chemistry and surface effects such as secondary electron emission increase the computational requirements in such a way that even fully parallel explicit PIC models cannot be used. One alternative method is to develop fluid-based codes coupled with electromagnetics in order to model ion sources. Time-domain fluid models can simulate plasma evolution, plasma chemistry, and surface physics models with reasonable computational resources by not explicitly resolving electron motions, which thereby leads to an increase in the time step. This is achieved by solving fluid motions coupled with electromagnetics using reduced-physics models, such as single-temperature magnetohydrodynamics (MHD), extended, gas dynamic, and Hall MHD, and two-fluid MHD models. We show recent results on modeling the internal antenna H{sup −} ion source for the SNS at Oak Ridge National Laboratory using the fluid plasma modeling code USim. We compare demonstrate plasma temperature equilibration in two-temperature MHD models for the SNS source and present simulation results demonstrating plasma evolution over many Rf periods for different plasma temperatures. We perform the calculations in parallel, on unstructured meshes, using finite-volume solvers in order to obtain results in reasonable time.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Wave induced density modification in RF sheaths and close to wave launchers

With the return to full metal walls - a necessary step towards viable fusion machines - and due to the high power densities of current-day ICRH (Ion Cyclotron Resonance Heating) or RF (radio frequency) antennas, there is ample renewed interest in exploring the reasons for wave-induced sputtering and formation of hot spots. Moreover, there is experimental evidence on various machines that RF waves influence the density profile close to the wave launchers so that waves indirectly influence their own coupling efficiency. The present study presents a return to first principles and describes the wave-particle interaction using a 2-time scale model involving the equation of motion, the continuity equation and the wave equation on each of the time scales. Through the changing density pattern, the fast time scale dynamics is affected by the slow time scale events. In turn, the slow time scale density and flows are modified by the presence of the RF waves through quasilinear terms. Although finite zero order flows are identified, the usual cold plasma dielectric tensor - ignoring such flows - is adopted as a first approximation to describe the wave response to the RF driver. The resulting set of equations is composed of linear and nonlinear equations and is tackled in 1D in the present paper. Whereas the former can be solved using standard numerical techniques, the latter require special handling. At the price of multiple iterations, a simple ’derivative switch-on’ procedure allows to reformulate the nonlinear problem as a sequence of linear problems. Analytical expressions allow a first crude assessment - revealing that the ponderomotive potential plays a role similar to that of the electrostatic potential arising from charge separation - but numerical implementation is required to get a feeling of the full dynamics. A few tentative examples are provided to illustrate the phenomena involved.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Effect of RF power density on micro- and macro-structural properties of PECVD grown hydrogenated nanocrystalline silicon thin films

This contribution provides the comparison between micro- and macro-structure of hydrogenated nanocrystalline silicon (nc-Si:H) thin films grown by plasma enhanced chemical vapor deposition (PECVD) technique under different RF power densities (P{sub RF}: 100−444 mW/cm{sup 2}). Micro-structure is assessed through grazing angle X-ray diffraction (GAXRD), while macro-structure is followed by surface and cross-sectional morphology via field emission scanning electron microscopy (FE-SEM). The nanocrystallite size (∼5 nm) and FE-SEM surface conglomerate size (∼40 nm) decreases with increasing P{sub RF}, crystalline volume fraction reaches maximum at 162 mW/cm{sup 2}, FE-SEM cross-sectional structure is columnar except for the film grown at 162 mW/cm{sup 2}. The dependence of previously determined ‘oxygen content–refractive index’ correlation on obtained macro-structure is investigated. Also, the effect of P{sub RF} is discussed in the light of plasma parameters during film deposition process and nc-Si:H film growth models.

36 MATERIALS SCIENCE↗
DOE OSTI2017

A compact linear accelerator based on a scalable microelectromechanical-system RF-structure

Here, a new approach for a compact radio-frequency (RF) accelerator structure is presented. The new accelerator architecture is based on the Multiple Electrostatic Quadrupole Array Linear Accelerator (MEQALAC) structure that was first developed in the 1980s. The MEQALAC utilized RF resonators producing the accelerating fields and providing for higher beam currents through parallel beamlets focused using arrays of electrostatic quadrupoles (ESQs). While the early work obtained ESQs with lateral dimensions on the order of a few centimeters, using a printed circuit board (PCB), we reduce the characteristic dimension to the millimeter regime, while massively scaling up the potential number of parallel beamlets. Using Microelectromechanical systems scalable fabrication approaches, we are working on further red ucing the characteristic dimension to the sub-millimeter regime. The technology is based on RF-acceleration components and ESQs implemented in the PCB or silicon wafers where each beamlet passes through beam apertures in the wafer. The complete accelerator is then assembled by stacking these wafers. This approach has the potential for fast and inexpensive batch fabrication of the components and flexibility in system design for application specific beam energies and currents. For prototyping the accelerator architecture, the components have been fabricated using the PCB. In this paper, we present proof of concept results of the principal components using the PCB: RF acceleration and ESQ focusing. Finally, ongoing developments on implementing components in silicon and scaling of the accelerator technology to high currents and beam energies are discussed.

43 PARTICLE ACCELERATORS↗
DOE OSTI2020

Recent progress in microscale modeling of RF sheaths

The microscale properties of RF sheaths in the ion cyclotron range of frequencies (ICRF) are investigated by means of analytical theory, nonlinear fluid and particle-in-cell (PIC) code modeling. Previous work that parametrized RF sheath properties, specifically the RF sheath impedance and the rectified (DC) sheath potential, is generalized to include the effect of net DC current flow through the sheath. Analytical results are presented in the low frequency limit where the displacement current is negligible, and tested against results from a fluid numerical model. Here, it is shown that when the sheath draws DC electron current, the voltage rectification is reduced from the zero current case, and the electron admittance is increased. In separate but related work on the microscale model, selected cases have been simulated with PIC codes to validate, further illuminate and extend fluid model results and their parametrizations. Quantitative agreement in trends for voltage rectification and sheath admittance vs. RF driving voltage is found.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗