Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “rf source”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,279 records · Page 71
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

A Drive Laser for Multi-Bunch Photoinjector Operation

Numerous electron beam applications would benefit from increased average current without sacrificing beam brightness. Work is underway at LLNL to investigate the performance of X-band photoinjectors that would generate electron bunches at a rate matching the RF drive frequency, i.e. one bunch per RF cycle. A critical part of this effort involves development of photo-cathode drive laser technology. Here we present a new laser architecture that can generate pulse trains at repetition rates up to several GHz. This compact, fiber-based system is driven directly by the accelerator RF and so is inherently synchronized with the accelerating fields, and scales readily over a wide range of drive frequencies (L-band through X-band). The system will be required to produce 0.5 {mu}J, {approx}200 fs rise time, spatially and temporally shaped UV pulses designed to optimize the electron beam brightness. Presented is the current status of this system, producing 2 ps pulses from a continuous-wave source.

42 ENGINEERING↗
DOE OSTI2019

Validation Studies using ELK and the Open-Source MOOSE Framework Application Zapdos for Electromagnetic Coupled Plasma Simulation

An open source simulation platform that captures electromagnetics, plasma chemistry, and fluid treatment of the plasma domain would be a powerful tool for studying low temperature plasma (LTP) phenomena. Within the Multiphysics Object-Oriented Simulation Environment (MOOSE) open source framework, ELK, CRANE, and Zapdos are combined to provide open source simulation capability for the LTP community. This talk presents continued validation of the general electromagnetic (EM) simulation tool ELK (Electromagnetic Library for Kinetics & fluids). Previous validation efforts focused on simulating standard waveguide and antenna benchmarks in vacuum and linear dielectrics, but ELK has since been coupled with the MOOSE-based plasma fluid application Zapdos [Lindsay et al 2016 J. Phys. D: Appl. Phys. 49 235204] to perform radio-frequency (rf) capacitively coupled plasma (CCP) system simulation. Plasma validation efforts initially focused on electrostatic benchmarks to demonstrate ELK-Zapdos code coupling and then moved to electromagnetic benchmarks based on interesting geometries and conditions (VHF effects in a CCP reactor, microwave heating, etc.). Results and progress in these efforts will be summarized and discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Collaboration with NSTX in Calculations of Radiofrequency and Neutral Beam Heating and Current Drive

The overall objective of this project was to advance the understanding of heating and current drive necessary for toroidal magnetic confinement fusion energy reactors, and to provide accurate models for these sources to aid in understanding magneto-hydrodynamic, transport and boundary physics issues. Increased understanding is provided by computer simulation using the underlying physics laws, and will lead to greater reliability and increased efficiency of a power producing fusion reactor. The proposed work entailed applications of existing CompX Fokker-Planck, ray tracing, full-wave microwave, and particle diffusion calculator codes to the National Spherical Torus Experiment at Princeton Plasma Physics Laboratory. Special features of NSTX include that large ion orbits are emphasized in its geometry and thus offer a major opportunity within fusion research for focused exploration of their ubiquitous effects, including comparison with simulation codes. The CompX CQL3D Fokker-Planck code was be further adapted to the finite-orbit-width physics. Comparison with experiment ensured that the computer codes have sufficient accuracy to accurately model the experiments. Nonthermal distributions of energetic ions are produced in experiments-- and modeled with the codes-- exhibiting the signatures of auxiliary heating, transport, and radiation processes within the plasma. Using physics-based modeling, in conjunction with experimental data, nonthermal distributions can provide keys to validation and quantification of auxiliary heating physics. Nonthermal distributions also hold particular information on radial transport processes, not obscured by the smoothing Maxwellization effects of Coulomb collisions. Generally, radial transport is not well understood but is a key to efficient fusion energy production. By comparison with Fokker-Planck modeling, the velocity dependence of radial transport coefficients becomes experimentally accessible, particularly in cases of well-modeled sources such as neutral beams, fast wave power, or the toroidal electric field. It is clear from modeling of fast-wave interactions with ions that finite-orbit-width effects play a significant role. For example, a different set of ions is computed to intersect a given cyclotron resonance depending on whether or not radial orbit drifts are taken into account. Also, radial transport, associated bootstrap current, and alpha channeling depend on radial drifts. Ion cyclotron heating above the first harmonic interacts preferentially with higher velocity ions, giving large radial drifts. The CQL3D Fokker-Planck code, maintained by the proposers, is a physics based model used extensively within the US fusion community to compare with and interpret experimental observations. At the beginning of this grant, bounce-averages of the rf quasilinear and other operators have been performed neglecting the radial drifts which give finite orbit widths to the particle orbits. The proposed work included a major upgrade to CQL3D to include finite-orbit-width effects be accounted for by performing the bounce-averages of rf and neutral beam fast ions along guiding center orbits. This CQL3D enhancement provided microwave and neutral beam contributions to neoclassical radial transport coefficients for nonthermal distributions, and substantially increased the comprehensiveness of the ion model. Additional work validated Fokker-Planck and ray tracing modeling of neutral beam and high harmonic fast wave absorption and current drive against the NSTX experiment, as had been very successful for electron cyclotron current drive in the DIII-D tokamak at General Atomics and lower hybrid waves on C-Mod at MIT. The validation work was in conjunction with staff and graduate students at Princeton Plasma Physics Laboratory. This DOE grant supported fusion energy research, a potential long-term solution to the world's energy needs. The research grant under consideration, DE-SC0006614, provided research support at the 0.3 Full-Time-Equivalent level for application of computer models to aid in understanding and projecting efficacy of heating and current drive sources in the National Spherical Torus Experiment, a tokamak variant, at the Princeton Plasma Physics Laboratory. The NSTX experiment explores the physics of very tight aspect ratio, almost spherical tokamaks, aiming at producing steady-state fusion plasmas. The current drive is an integral part of the steady-state concept, maintaining the magnetic geometry in the steady-state tokamak.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

NMIS with Imaging and Gamma Ray Spectrometry for Pu, HEU, HE and Other Materials

The Nuclear Material Identification System (NMIS) has been under development at ORNL and the National Nuclear Security Administration (NNSA) Y-12 National Security Complex since 1984. In the mid-1990s, what is now the US Department of Energy (DOE) Office of Nuclear Verification (ONV) realized that it was a useful technology for future arms control treaty applications and supported further development of the system. In 2004, fast-neutron imaging was incorporated into the system. In 2007, the ONV decided to develop a fieldable version of the system, designated as FNMIS, for potential use in future treaties. The FNMIS is being developed to be compatible with the eventual incorporation of gamma-ray spectrometry and an information barrier. This report addresses how and what attributes could be determined by the FNMIS system with gamma-ray spectrometry. The NMIS is a time-dependent coincidence system that incorporates tomographic imaging (including mapping of the fission sites) and gamma-ray spectrometry. It utilizes a small, lightweight (30 lb), portable deuterium-tritium (DT) neutron (14.1 MeV) generator (4 x 10 7 neutrons/second) for active interrogation and can also perform passive interrogation. A high-purity germanium (HPGe) gamma-ray detector with multichannel analysis can be utilized in conjunction with the source for active interrogation or passively. The system uses proton recoil scintillators: 32 small 2.5 x 2.5 x 10.2-cm-thick plastic scintillators for imaging and at least two 2 x 2 arrays of 27 x 27 x 10-cm-thick plastic scintillators that detect induced fission radiation. The DT generator contains an alpha detector that time and directionally tags a fan beam of some of the neutrons emitted and subdivides it into pixels. A fast (1 GHz) time correlation processor measures the time-dependent coincidence among all detectors in the system. A computer-controlled scanner moves the small detectors and the source appropriately for scanning a target object for imaging. The system is based on detection of transmitted 14.1 MeV neutrons, fission neutrons, and gamma rays from spontaneous, inherent source fission of the target, fission neutrons and gamma rays induced by the external DT source, gamma rays from natural emissions of uranium and plutonium, and induced gamma-ray emission by the interaction of the 14.1 MeV neutrons from the DT source. The NMIS can and has been used with a time-tagged californium spontaneous fission source. It has also been used with pulsed interrogation sources such as LINACs, DT, and deuterium-deuterium (DD) sources. This system is uniquely suited for detection of shielded highly enriched uranium (HEU), plutonium, and other special nuclear materials and detection of high explosives (HE) and chemical agents. The NMIS will be adapted to utilize a trusted processor that incorporates information barrier and authentication techniques using open software and then be useful in some international applications for materials whose characteristics may be classified. The proposed information barrier version of the NMIS system would consist of detectors and cables, the red (classified side) computer system, which processes the data, and the black (unclassified side) computer, which handles the computer interface. The system could use the 'IB wrapper' concept proposed by Los Alamos National Laboratory and the software integrity (digital signatures) system proposed by Sandia. Since it is based entirely on commercially available components, the entire system, including NMIS data acquisition boards, can be built with commercial off-the-shelf components. This system is being developed into a fieldable system (FNMIS) for potential arms control treaties by the ONV. The system will be modularly constructed with the RF shielded modules connected to the processor by appropriate control and signal cable in metal conduit. The FNMIS is presently being designed for eventual incorporation of gamma-ray spectrometry and an information barrier to protect classified information. The system hardware and software can be configured to obtain the following: plutonium presence, plutonium mass, Pu-240/239 ratio, plutonium geometry, plutonium metal vs non-metallic (absence of metal), time (age) since processing for plutonium (or last purification), uranium presence, uranium mass, uranium enrichment, uranium geometry, uranium metal vs non-metallic compound (absence of metal), beryllium presence and mass, tritium and deuterium gas bottle presence, HE, and chemical weapons. A matrix of the quantities determined, the method of determination, whether active (external neutron source) or passive, and the measurement equipment involved is given in the Tables 1-4. Some of these attributes can be obtained by multiple data analysis methods. The gamma-ray spectrometry methods for HEU, plutonium, and HE have been developed by other laboratories, are well known, and will be incorporated.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2024

Observation of tantalum deposition and growth on TiB2 and ZrB2 from PISCES-RF deuterium and helium plasma exposures

Deuterium and helium plasma exposures on bulk TiB2 and ZrB2 samples were performed using the PISCES-RF linear plasma device. 40 and 90 eV deuterium ion plasma exposures were performed at 240 and 800 °C sample temperatures, and 80 eV helium ion plasma exposures were performed at 800 °C sample temperatures. Following plasma exposures, it was discovered that two plasma conditions (90 eV deuterium and 80 eV helium at 800 °C) resulted in thick (>200 nm) tantalum-rich (>10 at%) surface features on the targets, presumably from tantalum sourced from a tantalum adapter mask or cap used as part of the target holder. This work aims to characterize these tantalum-rich features and examine the mechanisms of impurity deposition.Plasma-induced surface morphology of the tantalum-rich surface layers depends on plasma properties and target temperature and chemistry. Greater titanium sputtering compared to zirconium resulted in more distinct surface features in the TiB2 samples compared to the ZrB2 samples via increased, prompt deposition onto tantalum surface impurities. There is still uncertainty as to why thick tantalum deposition only occurred under some plasma exposure conditions but not others; it is likely due to tantalum sputtering by a combination of boron molecules from the targets and carbon-impurities in the tantalum mask or targets. Impurity driven surface features are a well-documented phenomena in samples exposed to plasma from linear plasma device facilities—this work confirms the occurrence of this and emphasizes the need for chemistry characterization of isolated post-mortem surface features in plasma-exposed samples.

Nuckols, Lauren↗
NASA NTRS1999

A Versatile Planetary Radio Science Microreceiver

We have developed a low-power. programmable radio "microreceiver" that combines the functionality of two science instruments: a Relative Ionospheric Opacity Meter (riometer) and a swept-frequency, VTF/HF radio spectrometer. The radio receiver, calibration noise source, data acquisition and processing, and command and control functions are all contained on a single circuit board. This design is suitable for miniaturizing as a complete flight instrument. Several of the subsystems were implemented in a field-programmable gate array (FPGA), including the receiver detector, the control logic, and the data acquisition and processing blocks. Considerable efforts were made to reduce the power consumption of the instrument, and eliminate or minimize RF noise and spurious emissions generated by the receiver's digital circuitry. A prototype instrument was deployed at McMurdo Station, Antarctica, and operated in parallel with a traditional riometer instrument for approximately three weeks. The attached paper (accepted for publication by Radio Science) describes in detail the microreceiver theory of operation, performance specifications and test results.

Fry, Craig D.↗
DOE OSTI2015

Commissioning of NSLS-II

NSLS-II, the new 3rd generation light source at BNL was designed for a brightness of 10 22 photons s -1 mm -2 mrad -2 (0.1%BW) -1 . It was constructed between 2009 and 2014. The storage ring was commissioned in April 2014 which was followed by insertion device and beamline commissioning in the fall of 2014. All ambitious design parameters of the facility have already been achieved except for commissioning the full beam intensity of 500mA which requires more RF installation. This paper reports on the results of commissioning.

43 PARTICLE ACCELERATORS↗
DOE OSTI2017

SCIDAC Center for simulation of wave particle interactions CompX participation

Harnessing the energy that is released in fusion reactions would provide a safe and abundant source of power to meet the growing energy needs of the world population. The next step toward the development of fusion as a practical energy source is the construction of ITER, a device capable of producing and controlling the high performance plasma required for self-sustaining fusion reactions, or “burning” plasma. The input power required to drive the ITER plasma into the burning regime will be supplied primarily with a combination of external power from radio frequency waves in the ion cyclotron range of frequencies and energetic ions from neutral beam injection sources, in addition to internally generated Ohmic heating from the induced plasma current that also serves to create the magnetic equilibrium for the discharge. The ITER project is a large multi-billion dollar international project in which the US participates. The success of the ITER project depends critically on the ability to create and maintain burning plasma conditions, it is absolutely necessary to have physics-based models that can accurately simulate the RF processes that affect the dynamical evolution of the ITER discharge. The Center for Simulation of WavePlasma Interactions (CSWPI), also known as RF-SciDAC, is a multi-institutional collaboration that has conducted ongoing research aimed at developing: (1) Coupled core-to-edge simulations that will lead to an increased understanding of parasitic losses of the applied RF power in the boundary plasma between the RF antenna and the core plasma; (2) Development of models for core interactions of RF waves with energetic electrons and ions (including fusion alpha particles and fast neutral beam ions) that include a more accurate representation of the particle dynamics in the combined equilibrium and wave fields; and (3) Development of improved algorithms that will take advantage of massively parallel computing platforms at the petascale level and beyond to achieve the needed physics, resolution, and/or statistics to address these issues. CompX provides computer codes and analysis for the calculation of the electron and ion distributions in velocity-space and plasma radius which are necessary for reliable calculations of power deposition and toroidal current drive due to combined radiofrequency and neutral beam at high injected powers. It has also contributed to ray tracing modeling of injected radiofrequency powers, and to coupling between full-wave radiofrequency wave models and the distribution function calculations. In the course of this research, the Fokker-Planck distribution function calculation was made substantially more realistic by inclusion of finite-width drift-orbit effects (FOW). FOW effects were also implemented in a calculation of the phase-space diffusion resulting from radiofrequency full-wave models. Average level of funding for CompX was approximately three man-months per year.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS2003

A 94 GHz RF Electronics Subsystem for the CloudSat Cloud Profiling Radar

The CloudSat spacecraft, scheduled for launch in 2004, will carry the 94 GHz Cloud Profiling Radar (CPR) instrument. The design, assembly and test of the flight Radio Frequency Electronics Subsystem (RFES) for this instrument has been completed and is presented here. The RFES consists of an Upconverter (which includes an Exciter and two Drive Amplifiers (DA's)), a Receiver, and a Transmitter Calibrator assembly. Some key performance parameters of the RFES are as follows: dual 100 mW pulse-modulated drive outputs at 94 GHz, overall Receiver noise figure < 5.0 dB, a highly stable W-band noise source to provide knowledge accuracy of Receiver gain of < 0.4 dB over the 2 year mission life, and a W-band peak power detector to monitor the transmitter output power to within 0.5 dB over life. Some recent monolithic microwave integrated circuit (MMIC) designs were utilized which implement the DA's in 0.1 micron GaAs high electron-mobility transistor (HEMT) technology and the Receiver low-noise amplifier (LNA) in 0.1 micron InP HEMT technology.

radio frequency electronics subsystem (RFES)↗
DOE OSTI2023

Basic Research in Edge and Scrape-Off Layer Fusion Plasmas (Final Report)

U.S. Department of Energy (DOE) supported work at Lodestar Research Corporation under grant DE-FG02-97ER54392 is summarized in this final report. The research conducted under this grant addressed key scientific issues associated with boundary plasmas in fusion energy experimental devices. Harnessing the energy that is released in fusion reactions would provide a safe and abundant source of power to meet the growing energy needs of the world population. Lodestar investigated the important cm-scale region near the edge of these devices where hot plasma, close to one million degrees, meets material surfaces in what is known as a scrape-off layer (SOL). This critical region is subject to plasma turbulence and plasma flows which must be understood to optimize fusion performance on future machines. Strong interactions of applied RF waves is also observed to occur in this region, and must be understood to mitigate unwanted effects and enhance overall performance. The work employed theoretical models and theory-based computational models for validation. The models were applied to understand experimental results and to carry out basic theoretical investigations relevant to the production of fusion energy.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1980

SPS solid state antenna power combiner

A concept for a solar power satellite antenna power combiner which utilizes solid state dc-rf converters is described. To avoid the power combining losses associated with circuit hybrids it is proposed that the power from multiple solid state amplifiers be combined by direct coupling of each amplifier's output to the radiating antenna structure. The selected power-combining antenna consists of a printed (metalized) microstrip circuit on a ceramic type dielectric substrate which is backed by a shallow lightweight aluminum cavity which sums the power of four microwave sources. The antenna behaves like two one-half wavelength slot-line antennas coupled together via their common cavity structure. A significant feature of the antenna configuration selected is that the radiated energy is summed to yield a single radiated output phase which represents the average insertion phase of the four power amplifiers. This energy may be sampled and, by comparison with the input signal, one can phase error correct to maintain the insertion phase of all solid state power combining modules at exactly the same value. This insures that the insertion phase of each SPS power combining antenna module is identical. An experiment verification program is described.

Fitzsimmons, G. W.↗
DOE OSTI2021

Preliminary RAM-RODD results for the MUSiC subcritical configurations

The Measurement of Uranium Subcritical and Critical (MUSiC) was performed at the DOE’s National Criticality Experiments Research Center (NCERC) located in the Nevada National Security Site (NNSS). The measurement utilized the Rocky Flats shells to perform benchmark measurements of similar highly enriched uranium (HEU) systems that span a wide range of reactivities. The Rocky Flats (RF) shells are 93.16% U-235 enriched metal hemishells that can be stacked concentrically. Ten configurations were measured with effective multiplication factors spanning between deeply subcritical (~ 0.64) through delayed critical. Details of the measured configurations are listed in Table 1. This unique set of measurements with its large span of reactivies is being used to determine the range over which neutron noise techniques such as Feynman variance-to-mean, Rossi-alpha, and pulsed neutron source techniques can be accurately employed for a bare HEU system. The results of the measurements will be published as a benchmark in The International Criticality Safety Benchmark Evaluation Project (ICSBEP) Handbook. The results will support the growing amount of subcritical benchmark data, such as the SCRaP measurements, that is available to the community. The measurements were performed using several different detector systems for the purposes of cross-validation and obtaining detector independent results. Four detector systems were deployed, three by the NCERC team and one from the University of Michigan. The detector systems included a Neutron Multiplicity Array Detector (NoMAD) system (similar to the MC-15), four small volume 0.635 cm (Ø) × 7.59 cm 3 He detectors (ideal for measuring prompt neutron decay constants due to their fast recovery speed), the Rossi Alpha Measurements – Rapid Organic (n, γ) Discrimination Detector (RAM-RODD), and the Organic Scintillator Array (OSCAR). RAM-RODD is an array of eight 5.08 cm (Ø) × 5.08 cm EJ-309 organic scintillator detectors. OSCAR is a University of Michigan system and is an array of twelve 5.08 cm (Ø) × 5.08 cm stilbene detectors. Details of measurements performed with OSCAR will be discussed in a separate talk. The focus of this work is preliminary results obtained by RAM-RODD for the 8 subcritical configurations. Additional details on the measurements and on the setup and deployment of RAM-RODD will be discussed. Preliminary neutron noise analysis results including Rossi-alpha and Feynman-alpha will also be presented.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2024

Observational benchmarks inform representation of soil organic carbon dynamics in land surface models

Abstract. Representing soil organic carbon (SOC) dynamics in Earth system models (ESMs) is a key source of uncertainty in predicting carbon–climate feedbacks. Machine learning models can help identify dominant environmental controllers and establish their functional relationships with SOC stocks. The resulting knowledge can be integrated into ESMs to reduce uncertainty and improve predictions of SOC dynamics over space and time. In this study, we used a large number of SOC field observations (n=54 000), geospatial datasets of environmental factors (n=46), and two machine learning approaches (namely random forest, RF, and generalized additive modeling, GAM) to (1) identify dominant environmental controllers of global and biome-specific SOC stocks, (2) derive functional relationships between environmental controllers and SOC stocks, and (3) compare the identified environmental controllers and predictive relationships with those in models used in Phase 6 of the Coupled Model Intercomparison Project (CMIP6). Our results showed that the diurnal temperature, drought index, cation exchange capacity, and precipitation were important observed environmental predictors of global SOC stocks. While the RF model identified 14 environmental factors that describe climatic, vegetation, and edaphic conditions as important predictors of global SOC stocks (R2=0.61, RMSE = 0.46 kg m−2), current ESMs oversimplify the relationships between environmental factors and SOC, with precipitation, temperature, and net primary productivity explaining > 96 % of the variability in ESM-modeled SOC stocks. Further, our study revealed notable disparities among the functional relationships between environmental factors and SOC stocks simulated by ESMs compared with observed relationships. To improve SOC representations in ESMs, it is imperative to incorporate additional environmental controls, such as the cation exchange capacity, and refine the functional relationships to align more closely with observations.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2011

Performance Study of K2CsSb Photocathode Inside a DC High Voltage Gun

In the past decade, there has been considerable interest in the generation of tens of mA average current in a photoinjector. Until recently, GaAs:Cs cathodes and K{sub 2}CsSb cathodes have been tested successfully in DC and RF injectors respectively for this application. Our goal is to test the K{sub 2}CsSb photocathode inside a DC gun. Since the multialkali cathode is a compound with constant characteristics over its entire thickness, we anticipate that the lifetime issues seen in GaAs:Cs due to surface damage by ion bombardment would be minimized. Hence successful operation of the K{sub 2}CsSb cathode in a DC gun could lead to a relatively robust electron source capable of delivering ampere level currents. In order to test the performance of a K{sub 2}CsSb cathode in a DC gun, we have designed and built a load lock system that allows the fabrication of the cathode at Brookhaven National Lab (BNL) and its testing at Jefferson Lab (JLab). In this paper, we will present the performance of the K{sub 2}CsSb photocathode in the preparation chamber and in the DC gun.

43 PARTICLE ACCELERATORS↗
DOE OSTI2017

Techniques for Achieving High Reliability Operation of the Spallation Neutron Source High Power Radio-Frequency System

The Spallation Neutron Source (SNS) high power radiofrequency (HPRF) system operates with high reliability to support the goals of the SNS user program. In recent operational periods the availability of the HPRF system has exceeded 97 percent while the neutron source availability overall is typically greater than 90 percent. SNS has a unique set of 92 HPRF stations that operate at either 402.5 MHz or 805 MHz with peak output power ranging from 550 kW to 5 MW and average power ranging from 49.5 kW to 450 kW. The HPRF transmitters consist of chassis-mounted power supplies, solid-state amplifiers and other equipment that support the operation of the klystrons that ultimately provide the RF power to the accelerating structures. Management of the operation and maintenance of the HPRF system has increasingly focused on reliability and sustainability in recent years. Techniques for klystron lifetime preservation and optimization of transmitter reliability have been developed and will be described.

Moss, John↗
DOE OSTI2013

The local structure, magnetic, and transport properties of Cr-doped In{sub 2}O{sub 3} films

Cr-doped In{sub 2}O{sub 3} films were deposited on Si (100) substrates by RF-magnetron sputtering technique. The local structure, magnetic, and transport properties of films are investigated by X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption fine structure, Hall effect, R-T, and magnetic measurements. Structural analysis clearly indicates that Cr ions substitute for In{sup 3+} sites of the In{sub 2}O{sub 3} lattice in the valence of +2 states and Cr-related secondary phases or clusters as the source of ferromagnetism is safely ruled out. The films with low Cr concentration show a crossover from semiconducting to metallic transport behavior, whereas only semiconducting behavior is observed in high Cr concentration films. The transport property of all films is governed by Mott variable range hopping behavior, suggesting that the carriers are strongly localized. Magnetic characterizations show that the saturated magnetization of films increases first, and then decreases with Cr doping, while carrier concentration n{sub c} decreases monotonically, implying that the ferromagnetism is not directly induced by the mediated carriers. It can be concluded the ferromagnetism of films is intrinsic and originates from electrons bound in defect states associated with oxygen vacancies.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Photocathode Research for Electron Accelerators

This dissertation presents studies of two different types of photocathodes used at electron accelerators: GaAs and K2CsSb. The spin polarization of photoelectrons extracted from GaAs photocathodes was evaluated using traditional one-photon absorption and two-photon absorption, to determine the validity of speculation that two-photon absorption would provide significantly higher polarization. To accomplish this study, a novel compact, retarding-field Mott polarimeter was designed, built and commissioned. The second type of photocathode, K2CsSb, was manufactured at Brookhaven National Laboratory and transported to Jefferson Lab in an ultrahigh vacuum "suitcase" and installed within a DC high voltage gun. Charge lifetime measurements were made using a K2CsSb photocathode and compared to those of GaAs under identical operating conditions. The surface morphology of the used K2CsSb was also studied. Many important atomic, nuclear and high energy physics experiments rely on using highly spin-polarized electron beams obtained via photoemission from GaAs photocathodes. Remarkably, even after 30 years of GaAs photogun operation, there are still some lingering questions associated with the beam polarization values being lower than expected. Bulk GaAs can theoretically provide 50% polarization but typically provides ~ 35%. Reduced polarization is attributed to a number of proposed depolarization mechanisms. Recently, it was proposed that two-photon absorption could be exploited to selectively populate the conduction band with electrons of just one spin state providing 100% polarization. These claims were refuted by others on theoretical grounds. In this thesis, for the first time, two-photon photoemission was demonstrated with beam delivered to a novel polarimeter constructed for this experiment. Polarization was found to be comparable to that obtained with conventional one-photon absorption, and to converge to the same value with a reduction in thickness of the active layer, thereby providing additional experimental insight to the polarized photoemission process an depolarization mechanisms in GaAs. Many proposed accelerators (light sources, energy recovery linacs and electron cooling machines) require extremely high average beam current (~ 100mA). The highest average current accelerator today, the FEL at JLab, can operate at up to 10mA and relies on photoemission from GaAs, even though spin polarization is not required. GaAs is an extremely delicate material, prone to failure and requiring ultrahigh vacuum conditions. In the early 1990s, the multi-alkali photocathode K2CsSb was used to demonstrate 32mA average current inside an RF gun with poor vacuum characteristics. This experiment lasted just minutes and was terminated shortly after the high current milestone was met. Based on this early work, many people believe K2CsSb represents a much better photocathode choice for unpolarized accelerator applications, providing high current for long periods of time and under less stringent operating conditions, but a detailed comparative study using the same apparatus has not been done. To address the high current reliability issue, two K2CsSb photocathodes were manufactured at Brookhaven National Lab and transported to Jefferson Lab via an ultrahigh vacuum transport chamber. They were installed within a 200kV DC high voltage photogun. Systematic charge lifetime measurements were performed using both photocathode materials under identical conditions. The performance of K2CsSb was shown to be highly dependent on extracted current. By combining this decay behavior with surface science measurements of the used photocathodes, it was shown that heat quickly disrupts the capability of the K2CsSb photocathode.

McCarter, James↗
DOE OSTI2016

Flat electron beam sources for DLA accelerators

In this study we discuss the application of the flat beam transform to generate beams suitable for injection into slab-symmetric dielectric laser-driven accelerators (DLAs). A study of the focusing requirements to keep the particles within the tight apertures characterizing these accelerators shows the benefits of employing ultralow beam emittances. The slab geometry of the many dielectric accelerating structures strongly favors the use of flat beams with large ratio between vertical and horizontal emittances. We employ particle tracking simulations to study the application of the flat beam transform for two injector designs, a DC non relativistic photogun and a 1.6 cell S-band RF photoinjector, obtaining in both cases emittance ratios between the horizontal and vertical plane in excess of 100 in agreement with simple analytical estimates. The 4 MeV RF photoinjector study-case can be directly applied to the UCLA Pegasus beamline and shows normalized emittances down to < 3 nm in the vertical dimension for beam charges up to 20 fC, enabling a two-stage DLA experiment.

43 PARTICLE ACCELERATORS↗