SLAC Research Articles

A practical method of evaluating work functions

In this report we present the experimental data and the step-by-step technique to evaluate the work functions of thermionic cathodes. The thermionic cathodes utilized here were small, almost button size, and were provided by the Spectra-Mat corporation [1]. The experiments were performed by one of us, Erik Jongewaard, at Stanford Linear Accelerator Laboratory (SLAC). We would like to emphasize that among other parameters, the cathode electron current emission work function is of paramount importance for selecting the cathode for many applications and specifically for the injector of linear induction accelerators similar to DARHT II and most recently for the new radiographic accelerators under consideration.

Design Criteria for High-Gradient Radio-Frequency Linacs

This article will review methods used at the SLAC National Accelerator Laboratory and other world accelerator laboratories to design high-gradient normal conducting accelerating structures. A quest for compact radio-frequency linacs fueled decades of studies toward a higher accelerating gradient. A major phenomena limiting the increase of the gradient is vacuum radio-frequency breakdown; therefore, this paper will address the breakdown physics and discuss approaches that reduce the breakdown probability. This discussion will cover both the electrical design and fabrication technology of the accelerating structures to achieve practical operating accelerating gradients in excess of 100 MV/m. Most of the data described here were obtained during the development of 11 GHz linacs for electron–positron linear colliders, so extrapolation of the results to other frequencies should be performed cautiously.

Shielding Analysis of a Preclinical Bremsstrahlung X-ray FLASH Radiotherapy System within a Clinical Radiation Therapy Vault.

A preclinical radiotherapy system producing FLASH dose rates with 12 MV bremsstrahlung x rays is being developed at Stanford University and SLAC National Accelerator Laboratory. Because of the high expected workload of 6,800 Gy w -1 at the isocenter, an efficient shielding methodology is needed to protect operators and the public while the preclinical system is operated in a radiation therapy vault designed for 6 MV x rays. In this study, an analysis is performed to assess the shielding of the local treatment head and radiation vault using the Monte Carlo code FLUKA and the empirical methodology given in the National Council on Radiation Protection and Measurements Report 151. Two different treatment head shielding designs were created to compare single-layer and multilayer shielding methodologies using high-Z and low-Z materials. The multilayered shielding methodology produced designs with a 17% reduction in neutron fluence leaking from the treatment head compared to the single layered design of the same size, resulting in a decreased effective dose to operators and the public. The conservative assumptions used in the empirical methods can lead to over-shielding when treatment heads use polyethylene or multilayered shielding. High-Z/Low-Z multilayered shielding optimized via Monte Carlo is shown to be effective in the case of treatment head shielding and provide more effective shielding design for external beam radiotherapy systems that use 12 MV bremsstrahlung photons. Modifications to empirical methods used in the assessment of MV radiotherapy systems may be warranted to capture the effects of polyethylene in treatment head shielding.

Ben Ofori-Okai

Ben Ofori-Okai, a spectroscopist at the SLAC National Accelerator Laboratory, has some big questions about what’s happening at the center of planets. The secret lives of high-pressure and high-temperature environments hold the key to scientists’ long-standing questions about the evolution of our universe and even the elusive chemistry within nuclear reactors. To explore these exotic conditions, Ofori-Okai is taking spectroscopy to the extreme. Ofori-Okai specializes in a field called terahertz spectroscopy, which measures the vibrational energy of molecules as they interact with and emit terahertz radiation. This region of the electromagnetic spectrum, which lies roughly between infrared and microwave wavelengths, captures distinct vibrational modes of molecules that other spectroscopic measurements miss. For example, chemists can use terahertz spectra to reveal the chemical composition of molecular mixtures that might be indistinguishable by nuclear magnetic resonance or infrared spectroscopy, Ofori-Okai says. Yet chemists have been slow to adopt terahertz spectroscopy because “it’s

Study of the reactions e+e−→K+K−π0π0π0 , e+e−→KS0K±π∓π0π0 , and e+e−→KS0K±π∓π+π− at center-of-mass energies from threshold to 4.5 GeV using initial-state radiation

We study the processes e+e−→K+K−π0π0π0γ, KS0K±π∓π0π0γ, and KS0K±π∓π+π−γ in which an energetic photon is radiated from the initial state. The data were collected with the BABAR detector at the SLAC National Accelerator Laboratory. About 1200, 2600, and 6000 events, respectively, are selected from a data sample corresponding to an integrated luminosity of 469 fb−1. The invariant mass of the hadronic final state defines the effective e+e− center-of-mass energy. The center-of-mass energies range from threshold to 4.5 GeV. From the mass spectra, the first ever measurements of the e+e−→K+K−π0π0π0, e+e−→KS0K±π∓π0π0, and e+e−→KS0K±π∓π+π− cross sections are performed. The contributions from the intermediate states that include η, ϕ, ρ, K*(892), and other resonances are presented. We observe the J/ψ and ψ(2S) in most of these final states and measure the corresponding branching fractions, many of them for the first time.10 MoreReceived 23 July 2022Accepted 10 August 2022DOI:https://doi.org/10.1103/PhysRevD.107.072001Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasForm factorsParticle interactionsParticle productionParticles & Fields

Monte Carlo simulation of shielding designs for a cabinet form factor preclinical MV-energy photon FLASH radiotherapy system.

A preclinical MV-energy photon FLASH radiotherapy system is being designed at Stanford and SLAC National Accelerator Laboratory. Because of the higher energy and dose rate compared to conventional kV-energy photon laboratory-scale irradiators, adequate shielding in a stand-alone cabinet form factor is more challenging to achieve. We present a Monte Carlo simulation of multilayered shielding for a compact self-shielding system without the need for a radiation therapyvault. A multilayered shielding approach using multiple alternating layers of high-Z and low-Z materials is applied to the self-shielded cabinet to effectively mitigate the secondary radiation produced and to allow the device to be housed in a Controlled Radiation Area outside of a radiation vault. The multilayered shielding approach takes advantage of the properties of high-Z and low-Z radiation shielding materials such as density, cross-section, atomic number of the shielding elements, and products of radiation interactions within each layer. The Monte Carlo radiation transport code, FLUKA, is used to simulate the total effective dose produced by theoperation. The multilayered shielding designs proposed and simulated produced effective dose rates significantly lower than monolayer designs with the same total material thickness at the regulatory boundary; this is accomplished through the manipulation of the locations where secondary radiation is produced and reactions due to material properties such as neutron back reflection in hydrogen. Borated polyethylene at five weight percent significantly increased the shielding performance as compared to regular polyethylene, with the magnitude of the reduction depending upon the order of the shieldingmaterial. The multilayered shielding provides a path for shielding preclinical FLASH systems that deliver MV-energy bremsstrahlung photons. This approach promises to be more efficient with respect to the shielding material mass and space claim as compared to shielded vaults typically required for clinical radiation therapy with MVphotons. This article is protected by copyright. All rights reserved.

Pseudoscalar and scalar meson photoproduction interpreted by Regge phenomenology

We have evaluated pseudoscalar and scalar neutral pion photoproduction in $\stackrel{P\vec}{\ensuremath{\gamma}}p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}p$ and ${a}_{0}^{0}p$ above the resonance region and within Regge phenomenology. Our fit, including GlueX $\mathrm{\ensuremath{\Sigma}}$ pseudoscalar photoproduction data, shows that previous Stanford Linear Accelerator Center (SLAC) $\mathrm{\ensuremath{\Sigma}}$ measurements for $\stackrel{P\vec}{\ensuremath{\gamma}}p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}p$ above ${E}_{\ensuremath{\gamma}}=4\phantom{\rule{0.16em}{0ex}}\mathrm{GeV}$ vary substantially from SLAC data with more recent measurements made by GlueX in the vicinity of ${E}_{\ensuremath{\gamma}}=9\phantom{\rule{0.16em}{0ex}}\mathrm{GeV}$. The Regge model predicts that the beam polarization asymmetry $\mathrm{\ensuremath{\Sigma}}$ of the scalar meson is opposite to that of pseudoscalar meson photoproduction; however, the cross sections are similar. While the natural parity vector meson exchange is dominant in both cases, the contribution of the unnatural parity pseudovector meson exchange is very small. Using Regge phenomenology, we predicted high-energy behavior for double polarized observables $\mathbb{E}$, $\mathbb{F}$, $\mathbb{G}$, and $\mathbb{H}$ for the reactions $\ensuremath{\gamma}p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}p$ and ${a}_{0}^{0}p$.

Electromagnetic flavor-changing lepton decays from Lorentz and CPT violation

Lorentz- and $CPT$-violating effects initiating two-body electromagnetic flavor-changing decays of charged leptons are studied in the framework of Lorentz-violating effective field theory. An analysis of data from experiments at the Paul Scherrer Institute and at the Stanford Linear Accelerator measuring the branching ratios of these decays provides 576 constraints on independent flavor-changing effects in the charged-lepton sector, consistent with no Lorentz and $CPT$ violation at the level of parts in ${10}^{\ensuremath{-}13}--{10}^{\ensuremath{-}9}\text{ }\text{ }{\mathrm{GeV}}^{\ensuremath{-}1}$.

Impact of Solvent-Solute Hydrogen Bonding on Ultrafast Electron Transfer in a Trimetallic Iron-Ruthenium Complex

Transition metal complexes (TMCs) are widely used as light absorbers and catalysts for photochemical energy conversion processes such as photocatalytic reactions in liquid environments. Of particular interest are TMCs which incorporate multiple bridged metal centres because they represent a step towards more complex supramolecular assemblies that can ultimately enable control of the charge flow in solution-based photo-redox processes. However, the development of design guidelines for such molecular assemblies is impeded by an incomplete understanding of the coupling between their electronic and nuclear dynamics, including the interplay of TMCs with their liquid environment upon photoexcitation. Probing the involved processes requires high spatial sensitivity towards electronic motion and the ability to capture atomic motion on ultrafast timescales – a combination which is not readily available for optical transient spectroscopic techniques.X-ray free electron lasers (XFELs) have revolutionized the field of ultrafast science in recent years. XFELs generate tuneable and extremely bright X-ray pulses with a typical pulse duration of <30 fs, which are well-suited as an element-specific probe of ultrafast electronic and atomic motion in molecules and materials.1,2 Applied to TMCs, XFEL probes are sensitive to the local electronic structure in such molecular systems as well as to intra- and inter-molecular structural reorganization at an atomic level.3 In this talk, I will present work on the interaction of a linear trimetallic cyanide-bridged iron-ruthenium complex (FeRuFe) with its solvent environment, carried out using the LCLS XFEL facility at SLAC National Accelerator Laboratory. The focus of this study lies on elucidating how photoinduced intramolecular electron transfer processes in FeRuFe are affected by hydrogen bonding interactions between its cyanide ligands and the surrounding solvent molecules. In water, such solvent-solute hydrogen bonding interactions are stronger for Fe(II) than Fe(III), leading to substantial changes in solute-solvent interactions upon photoinduced electron transfer between metal centres.The experiment is performed in a pump-probe geometry on a liquid jet, where a visible/near-infrared pump induces a metal-to-metal charge transfer (MMCT) between Ru and Fe which is followed by an ultrafast back-electron transfer (BET). The excited state dynamics are monitored using hard X-ray pulses as a function of time delay between the laser pump and the X-ray probe pulses. To resolve and correlate electronic and structural dynamics upon MMCT excitation, we simultaneously perform ultrafast Fe Kβ X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS) measurements, where XES is sensitive to the oxidation and spin state of the Fe centres and XSS provides information on atomic motion in solute and solvent, as demonstrated in previous work on a similar system.4 To evaluate the effect of solute-solvent hydrogen bonding interactions on the electron transfer dynamics, we perform measurements in a series of solvents with different hydrogen bonding properties, including water, methanol, and acetonitrile. We find that the BET increases as a function of decreasing hydrogen bonding ability of the solvent, and that distinct signatures of solvation dynamics are present in the scattering data. The resulting molecular level understanding of solvent reorganization coupled to electron transfer demonstrates that the strength and type of solute-solvent interactions are a central factor in determining the outcome of photoinduced charge transfer processes in TMCs.References Bergmann, U. et al. Using X-ray free-electron lasers for spectroscopy of molecular catalysts and metalloenzymes. Nat. Rev. Phys. 3, 264–282 (2021).Schoenlein, R. et al. Recent advances in ultrafast X-ray sources. Philos. Trans. R. Soc. Math. Phys. Eng. Sci. 377, 20180384 (2019).Gaffney, K. J. Capturing photochemical and photophysical transformations in iron complexes with ultrafast X-ray spectroscopy and scattering. Chem. Sci. 12, 8010–8025 (2021).Biasin, E. et al. Direct observation of coherent femtosecond solvent reorganization coupled to intramolecular electron transfer. Nat. Chem. 13, 343–349 (2021).

The time-resolved atomic, molecular and optical science instrument at the Linac Coherent Light Source.

The newly constructed time-resolved atomic, molecular and optical science instrument (TMO) is configured to take full advantage of both linear accelerators at SLAC National Accelerator Laboratory, the copper accelerator operating at a repetition rate of 120 Hz providing high per-pulse energy as well as the superconducting accelerator operating at a repetition rate of about 1 MHz providing high average intensity. Both accelerators power a soft X-ray free-electron laser with the new variable-gap undulator section. With this flexible light source, TMO supports many experimental techniques not previously available at LCLS and will have two X-ray beam focus spots in line. Thereby, TMO supports atomic, molecular and optical, strong-field and nonlinear science and will also host a designated new dynamic reaction microscope with a sub-micrometer X-ray focus spot. The flexible instrument design is optimized for studying ultrafast electronic and molecular phenomena and can take full advantage of the sub-femtosecond soft X-ray pulse generation program.

LCLS-II-HE Cryomodule Production Status at Fermilab

The Linac Coherent Light Source (LCLS) is an X-ray science facility at SLAC National Accelerator Laboratory. The LCLS-II project (an upgrade to LCLS) is in its final stages, and the LCLS-II-HE (High Energy) project is another upgrade to the facility enabling higher energy operation. An electron beam is accelerated using superconducting radio frequency (SRF) cavities built into cryomodules. It is planned to build twenty-five modules total for the LCLS-II-HE project. Fourteen of these modules are planned to be assembled and tested at Fermilab. Procurements for production modules have been awarded, and parts are arriving. The design of the LCLS-II-HE modules is similar to the module design used in LCLS-II. The major cryomodule design change from LCLS-II is the nominal gradient required per cavity, increasing from 16 to 21 MV/m. The additional gradient yields additional heat loads. The first LCLS-II-HE cryomodule, referred to as the verification cryomodule (vCM), was assembled using parts from an LCLS-II module with new SRF cavities. The assembly is complete, and this module is currently being tested at Fermilab. The vCM cavities perform well, exceeding the required gradient and quality factor specifications. The first article module assembly is planned to start later in 2021. The status of the production as well as some of the design and assembly changes will be presented.

Kelly N. Chacón

The first time Kelly N. Chacón went to the synchrotron at the SLAC National Accelerator Laboratory during graduate school, they were hooked. All that time spent in their home laboratory preparing samples, freezing them with liquid nitrogen, and shipping them to the national laboratory was about to pay off. Before starting the experiments, Chacón was “not really sure” what they were going to see, they say. But when the beam was fired up and the data started coming in, they could “literally see the atoms interacting with other atoms.” The minute that happened, Chacón says, “I just knew that this was what I wanted to do.” Chacón describes themself as “very nontraditional—in a lot of senses of the word.” A desire for independence led them to drop out of high school at age 15. They worked in food service for a number of years before—and while—getting their GED. A high

Single particle detection system for strong-field QED experiments

Measuring signatures of strong-field quantum electrodynamics (SF-QED) processes in an intense laser field is an experimental challenge: it requires detectors to be highly sensitive to single electrons and positrons in the presence of the typically very strong x-ray and γ-photon background levels. In this paper, we describe a particle detector capable of diagnosing single leptons from SF-QED interactions and discuss the background level simulations for the upcoming Experiment-320 at FACET-II (SLAC National Accelerator Laboratory). The single particle detection system described here combines pixelated scintillation LYSO screens and a Cherenkov calorimeter. We detail the performance of the system using simulations and a calibration of the Cherenkov detector at the ELBE accelerator. Single 3 GeV leptons are expected to produce approximately 537 detectable photons in a single calorimeter channel. This signal is compared to Monte-Carlo simulations of the experiment. A signal-to-noise ratio of 18 in a single Cherenkov calorimeter detector is expected and a spectral resolution of 2% is achieved using the pixelated LYSO screens.

Study of the reactions e+e−→π+π−π0π0π0π0 and π+π−π0π0π0η at center-of-mass energies from threshold to 4.5 GeV using initial-state radiation

We study the processes e+e−→π+π−π0π0π0π0γ and π+π−π0π0π0ηγ in which an energetic photon is radiated from the initial state. The data were collected with the BABAR detector at the SLAC National Accelerator Laboratory. About 7300 and 870 events, respectively, are selected from a data sample corresponding to an integrated luminosity of 469 fb−1. The invariant mass of the hadronic final state defines the effective e+e− center-of-mass energy. The center-of-mass energies range from threshold to 4.5 GeV. From the mass spectra, the first ever measurements of the e+e−→π+π−π0π0π0π0 and the e+e−→π+π−π0π0π0η cross sections are performed. The contributions from ωπ0π0π0, ηπ+π−π0, ωη, and other intermediate states are presented. We observe the J/ψ and ψ(2S) in most of these final states and measure the corresponding branching fractions, many of them for the first time.20 MoreReceived 4 October 2021Accepted 16 November 2021DOI:https://doi.org/10.1103/PhysRevD.104.112004Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasParticle interactionsTechniquesParticle productionParticles & Fields

Getting beneath the surface of water

It may be everywhere, but there’s plenty that scientists don’t yet understand about water’s properties at the atomic scale. A group of researchers has made the first direct observation of how the atoms within water molecules move when excited—a finding that may lead to a more accurate picture of how energy flows during chemical reactions ( Nature 2021, DOI: 10.1038/s41586-021-03793-9 ). The researchers used the mega-electron-volt ultrafast electron diffraction instrument at SLAC National Accelerator Laboratory to capture the motion of molecular bonds within the water. They started with jets of water 100 nm thick, excited them with infrared laser light, and then fired bursts of high-energy electrons from the instrument into the stream to detect the vibrations. The team found that when a water molecule starts to vibrate, its hydrogen atom initially pulls the oxygen atoms in neighboring molecules toward it. Then, as the system heats up, the atoms move

Redox-Active Polymers Designed for the Circular Economy of Energy Storage Devices

A.G. and A.S. acknowledge funding from the TomKat Center for Sustainable Energy at Stanford University and the StorageX initiative. A.S. and S.T.M.T. gratefully acknowledge support from the National Science Foundation Award CBET #1804915. T.J.Q. and G.L. acknowledge support from the NSF Graduate Research Fellowship Program under grant DGE-1656518. Part of this work was performed at the Stanford Nanofabrication Facilities (SNF) and Stanford Nano Shared Facilities (SNSF), supported by the National Science Foundation as part of the National Nanotechnology Coordinated Infrastructure under award ECCS-1542152. The authors acknowledge financial support from KAUST, including the Office of Sponsored Research (OSR) award nos. OSR-2018-CRG/CCF-3079, OSR-2019-CRG8-4086, and OSR-2018-CRG7-3749. The authors acknowledge funding from an ERC Synergy Grant SC2 (610115). Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-76SF00515.

High-gradient rf tests of welded X -band accelerating cavities

Linacs for high-energy physics, as well as for industry and medicine, require accelerating structures which are compact, robust, and cost-effective. Small foot-print linacs require high-accelerating gradients. Currently, stable-operating gradients, exceeding $100\text{ }\text{ }\mathrm{MV}/\mathrm{m}$, have been demonstrated at SLAC National Accelerator Laboratory, CERN, and KEK at X-band frequencies. Recent experiments show that accelerating cavities made out of hard copper alloys achieve better high-gradient performance as compared with soft copper cavities. In the scope of a decade-long collaboration between SLAC, INFN-Frascati, and KEK on the development of innovative high-gradient structures, this particular study focuses on the technological developments directed to show the viability of novel welding techniques. Two novel X-band accelerating structures, made out of hard copper, were fabricated at INFN-Frascati by means of clamping and welding. One cavity was welded with the electron beam and the other one with the tungsten inert gas welding process. In the technological development of the construction methods of high-gradient accelerating structures, high-power testing is a critical step for the verification of their viability. Here, we present the outcome of this step---the results of the high-power rf tests of these two structures. These tests include the measurements of the breakdown rate probability used to characterize the behavior of vacuum rf breakdowns, one of the major factors limiting the operating accelerating gradients. The electron beam welded structure demonstrated accelerating gradients of $90\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ at a breakdown rate of ${10}^{\ensuremath{-}3}/(\mathrm{pulse}\text{ }\mathrm{meter})$ using a shaped pulse with a 150 ns flat part. Nevertheless, it did not achieve its ultimate performance because of arcing in the mode launcher power coupler. On the other hand, the tungsten inert gas welded structure reached its ultimate performance and operated at about a $150\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ gradient at a breakdown rate of ${10}^{\ensuremath{-}3}/(\mathrm{pulse}\text{ }\mathrm{meter})$ using a shaped pulse with a 150 ns flat part. The results of both experiments show that welding, a robust, and low-cost alternative to brazing or diffusion bonding, is viable for high-gradient operation. This approach enables the construction of multicell standing and traveling-wave accelerating structures.

Cathode Electrolyte Interface Stabilization and Transition Metal Dissolution Mitigation Via Sodium Boron Salt Utilization

Renewable energy storage devices have revolutionized the application of modern electronics and electrical infrastructures. The ever-increasing demand for large-scale energy storage has steered investigative interest to the rising potential of cost-efficient sodium ion batteries. However, the long-term cycling instability of cathode materials impedes wide-scale commercialization of sodium ion batteries. The interfacial reactions between the cathode and electrolyte has been shown to lead to prevalent surface reconstruction, transition metal dissolution, and the formation of intragranular nanocracks in the cathode particle, reducing overall cathode stability and cycling performance. Transitional metal dissolution can play a significant role in in performance degradation and further accelerating cathode particle instability. Investigating and improving the surface chemistry between the cathode and electrolyte is paramount to improving cathode stability and performance. We have designed a series of electrolytes that utilize select sodium boron salts that can passivate the O3-NaNi1/3Fe1/3Mn1/3O2 cathode and help form a robust cathode-electrolyte interphase (CEI). Through the utilization, and combination, of the sodium boron electrolytes, we observe improved cathode stability and long-term performance. We demonstrate enhanced cathode-electrolyte stability and cyclability at various temperatures ranging from 0°C - 45°C. Through improving the cathode surface passivation, we reduce interfacial reactions and observe a mitigation of transitional metal dissolution and migration from the cathode to separator or anode. Through this study, we demonstrate the importance of understanding the interfacial cathode-electrolyte surface chemistries and that there is further room for improving the electrolyte formulation for sodium layered oxide cathode materials. L. Mu, X. Feng, R. Kou, Y. Zhang, H. Guo, C. Tian, C.J. Sun, X.W. Du, D. Nordllund, H.L. Xin, and F. Lin Adv. Energy Mater., 8, 1801975 (2018)R. Jung, F. Linsenmann, R. Thomas, J. Wandt, S. Solchenbach, F. Maglia, C. Stinner, M. Tromp, H.A. Gasteiger, J. Electrochem. Soc., 166, A378–A389 (2019)J. Chen, Z. Huang, C. Wang, S. Porter, B. Wang, W. Lie, H.K. Liu Chem. Commun., 51, 9809–9812 (2015). The work was supported by the Virginia Tech Department of Chemistry Startup Funds, 4-VA Collaborative Research, and National Science Foundation (No. CBET-1912885). Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-76SF00515. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory, under Contract No. DE-AC02- 06CH11357.

Test and operation of high-power RF system for KSTAR helicon current drive system

The helicon current drive system for the efficient off-axis current drive of KSTAR is currently being upgraded with 476 MHz, 1.2 MW klystrons and a new high-voltage power supply (HVPS). Four klystrons, which were used in PEP-II, were transferred to NFRI from the Stanford Linear Accelerator Center. Each klystron can generate 1.2 MW of CW RF power at 476 MHz with an 84 kV 24 A beam. Two klystrons will be powered in parallel using a single pulse step modulator HVPS with a rating of 92 kV and 64 A. Herein, a detailed description of the klystron test setup and results obtained during its test is presented. A high-power test of the klystrons is performed, wherein an RF power of 620 kW is generated by a 70 kV and 18.8 A beam.

Electro-optic sampling beam position monitor for relativistic electron beams

An electro-optic sampling beam position monitor for installation at the SLAC National Accelerator Laboratory FACET-II facility is described. Simulations of the detector signal were performed using realistic electron beam current profiles, demonstrating that this single-shot, non-destructive, compact detector is capable of resolving the relative transverse offset and longitudinal separation of an ultra-relativistic, two-bunch electron beam to order 1 μm and 10 fs, respectively. A design study was performed to optimize the detector’s response to the ultra-relativistic two-bunch electron beam utilized in plasma wakefield accelerator experiments. In addition, the expected sensitivity to beam tilt is studied for typical electron beams used in free electron lasers.