Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session BP11: Poster Session I:
On Demand
Poster
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Hide Abstracts |
Room: Hall A |
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BP11.00001: : MFE: LOW ASPECT RATIO TOKAMAKS
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BP11.00002: Status and Plans for the New Solenoid-Free PEGASUS-III Spherical Tokamak Joshua A Reusch, Michael W Bongard, Steffi J Diem, Raymond J Fonck, John A Goetz, Armand K Keyhani, Mark D Nornberg, Jilliann K Peery, Christopher Pierren, Alexander T Rhodes, Nathan J Richner, Cuauhtemoc Rodriguez Sanchez, Rachel K Sassella, Carolyn E Schaefer, Aaron C Sontag, Timothy N Tierney, Justin D Weberski Identifying attractive means of initiating current without using induction from a central solenoid remains a critical challenge facing the spherical tokamak (ST) concept and is desirable for tokamaks in general. The PEGASUS-III facility is under construction to provide a dedicated US platform for non-solenoidal tokamak startup and sustainment studies. This new solenoid-free, extremely low aspect ratio ST (A ≥ 1.22, Ip ≤ 0.3 MA, BT ≤ 0.6 T, pulse length ~ 100 ms) is focused on comparing, contrasting, and combining several of the leading non-solenoidal startup techniques, including: local helicity injection (LHI); coaxial helicity injection (CHI); RF-aided and RF-only startup; and PF induction. It will feature: new LHI systems capable of Ip ~ 0.3 MA and a dedicated port for testing the efficacy of non-circular injectors; a novel dual floating electrode CHI system capable of both transient and sustained CHI operation; a 28 GHz RF system for synergy and, eventually, RF only startup experiments; and an expanded PF system with improved control. The long-term goal of these studies is to provide an optimized scheme for achieving MA-class plasma startup that is compatible with non-inductive sustainment in NSTX-U and beyond. |
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BP11.00003: Local Helicity Injection Studies on the PEGASUS-III Experiment Michael W Bongard, Steffi J Diem, Raymond J Fonck, John A Goetz, Armand K Keyhani, Mark D Nornberg, Jilliann K Peery, Christopher Pierren, Joshua A Reusch, Alexander T Rhodes, Nathan J Richner, Cuauhtemoc Rodriguez Sanchez, Carolyn E Schaefer, Aaron C Sontag, Justin D Weberski Initiating high-Ip plasmas without a central solenoid benefits both the ST and AT concepts. The PEGASUS program improved the physics basis and predictive models for non-solenoidal startup using local helicity injection (LHI), demonstrating plasmas with Ip > 0.2 MA. Major upgrades have converted the PEGASUS facility into the solenoid-free PEGASUS-III ST. They include: increased BT = 0.6 T; improved shape control; and retaining ultralow A ~ 1.2. This BT supports a new mission to expand the breadth and range of solenoid-free startup research on the facility with multiple techniques. Initial LHI experiments will extend Ip and Te scaling studies to BT > 0.15 T and test projections of access to Ip = 0.3 MA using two pairs of compact injectors (Iinj = 4 kA, Vinj ~ 1 kV, Ainj = 4 cm2 each) on the low-field-side of the tokamak and new voltage-controlled power supplies. A port-mounted, non-circular injector with equivalent helicity drive to the LFS arrays will test the feasibility of a compact, active current source geometry technically attractive to NSTX-U and beyond. Other planned studies include: beam-driven instabilities and dynamo current drive; impurity assessment; fueling; and scenario development for RF heating and CD. |
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BP11.00004: A Radiofrequency System for the PEGASUS-III Experiment Jilliann K Peery, Stephanie J Diem, Tim S Bigelow, Michael W Bongard, Raymond J Fonck, John A Goetz, Robert W Harvey, Cornwall H Lau, Mark D Nornberg, Yuri V Petrov, Joshua A Reusch, Aaron C Sontag Development of radiofrequency (RF) injection and other non-solenoidal heating and current drive (CD) techniques is critical to the design of compact, electricity-producing fusion power plants–especially for spherical tokamak (ST) based designs. The PEGASUS-III Experiment is a new, solenoid-free, low A ST focused on non-solenoidal plasma startup and sustainment techniques with local helicity injection (LHI), coaxial helicity injection (CHI) and RF. PEGASUS-III will use a 28 GHz gyrotron based system for EC heating and EBW heating and CD. Modeling of EC & EBW injection in PEGASUS-III has been performed on projected LHI- and CHI-produced target plasmas to study propagation, absorption, heating, and CD. GENRAY calculations were used to obtain the optimal parameters for 2nd harmonic X-mode ECH for LHI plasmas with Te = 15–300 eV resulting in 15–70% first-pass absorption as Te is increased. EBW simulations in overdense plasmas show core heating near the 2nd harmonic EC resonance. RF on PEGASUS-III will provide key enabling reactor relevant technologies to directly test proposed startup and ramp-up scenarios for larger scale STs and investigate synergistic improvements to startup plasmas for bootstrap and NBI sustainment. |
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BP11.00005: Transient CHI Research on PEGASUS-III Roger Raman, Kyle D Morgan, Michael W Bongard, Stephanie J Diem, Raymond J Fonck, Alan C Palmer, Joshua A Reusch, Justin D Weberski, Gregory R Winz, Fatima Ebrahimi The spherical tokamak (ST) has the potential for high bootstrap current driven operation, which is necessary to reduce the reactor recirculating power, if the aspect ratio could be sufficiently reduced. This requires the capability for a method, other than the solenoid, to generate a substantial portion of the initial startup current. PEGASUS-III is a ST non-solenoidal startup development station dedicated to solving this problem. One method being explored is transient co-axial helicity injection (T-CHI). T-CHI has shown promising capability on the HIT-II and NSTX STs. However, in both these machines the vacuum vessel was electrically cut. For reactor applications a simpler biased electrode configuration is required in which the insulator is not part of the external vacuum vessel. To develop this capability PEGASUS-III will use a double (floating) biased electrode configuration, which will be a first of its kind for the reactor-relevant development of the CHI concept. The system is projected to generate plasma start-up currents at the levels that can be supported by the external poloidal field coils, ~0.3 MA, with an initial 0.15 MA capability using a 30 kA, 2 kV electrolytic capacitor bank power supply. |
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BP11.00006: Active Feedback and Control of High Power Ćuk Converter for Fusion Applications Timothy Ziemba, James R Prager, Kevin Muggli, Huatsern Yeager, Steven Wilson, Zach Mulalley Eagle Harbor Technologies, Inc. (EHT) has developed a control module for active feedback and control a high power Ćuk converter. This control module will be used at Pegasus Toroidal Experiment to maintain a precision current flat top on the helicity injectors. The EHT controller allows for high-power arbitrary waveform on microsecond-timescales. EHT modeled the control algorithm and explored edge case scenarios for the Pegasus application. A hardware solution was designed around the STM32 microcontroller, and software was developed, including a graphical interface. EHT demonstrated precision waveform control with the controller on a small-scale Ćuk converter. We will present the modeling work, system capabilities, and initial data. This controller will allow Pegasus to extract the maximum energy from their capacitor bank while maintaining a flat-top current profile. |
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BP11.00007: Extending the low-recycling regime to higher performance discharges and liquid lithium walls in the Lithium Tokamak Experiment-β Dennis P Boyle, Joveria Baig, Santanu Banerjee, Grant Bodner, Ronald E Bell, Stepan N Gorelenkov, Paul E Hughes, Robert Kaita, Benoit P LeBlanc, Anurag Maan, Dick Majeski, George J Wilkie, Theodore M Biewer, Drew B Elliott, Filippo Scotti, Vlad Soukhanovskii, Shigeyuki Kubota, Terry L Rhodes, Jay K Anderson, William J Capecchi, Christopher J Hansen, Shota Abe, Bruce E Koel, Evan Ostrowski, David C Donovan, Leonid Zakharov In order to extend the low-recycling regime first observed in the Lithium Tokamak Experiment to higher performance, steadier discharges with solid and liquid lithium wall coatings, and to study its unique physics in more detail, LTX was upgraded to the Lithium Tokamak Experiment-β. The upgrade improves the lithium evaporation system, roughly doubles the toroidal field and ohmic heating power supplies, adds neutral beam injection for heating and core fueling, and adds or improves numerous diagnostics. Recent experiments have extended the duration and performance of the low-recycling regime in ohmic discharges with solid and liquid Li walls, with record values of current, temperature, pressure, and confinement. Upcoming experiments will also investigate beam heating at high current. |
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BP11.00008: Diagnostic upgrades for the Lithium Tokamak eXperiment-β enable recycling analysis and scrape-off layer characterization Anurag Maan, George J Wilkie, Dennis P Boyle, Robert Kaita, Dick Majeski, Santanu Banerjee, William J Capecchi, Vlad Soukhanovskii, Christopher J Hansen, Evan Ostrowski, Bruce E Koel, Paul E Hughes, Shigeyuki Kubota The Lithium Tokamak eXperiment (LTX) and its upgrade LTX-?? is the only tokamak in the world capable of operating with near complete coverage of lithium on its PFCs. LTX and LTX-?? have demonstrated plasma performance improvement due to flat electron temperature profiles, lowered recycling and reduced radiative loss from impurities. Experiments on LTX demonstrated that high edge temperatures and low edge density, likely due to a low recycling boundary, lead to a low collisionality scrape-off layer (SOL). However, the connection between flattening electron temperature profiles and recycling, particularly as it relates to the SOL characteristics, remains to be made. To study this unique SOL and to quantify fuel recycling in LTX-??, a new, movable, low field side, off-midplane, swept single Langmuir probe was installed to measure SOL edge density and temperature. Additionally, the neutral particle influx from the high field side limiter was measured using a hydrogen Lyman-?? array. Core electron temperature and density were measured using the LTX-?? Thomson scattering system. The data from these diagnostics indicate a reduction in line integrated Lyman-?? emission intensity and an increase in edge temperature and energy confinement times after fueling termination at higher plasma currents compared to LTX. The DEGAS2 code is being used to assess how sources of neutrals can affect the interpretation of the data. These results will be presented, along with a preliminary characterization of the SOL as it relates to recycling analysis. |
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BP11.00009: Interferometry and Far-Forward Scattering Analysis of Electron Density Fluctuations in LTX-β Shigeyuki Kubota, Dick Majeski, Dennis P Boyle, Paul E Hughes, Anurag Maan, Terry L Rhodes, Christopher J Hansen Investigating the mechanisms for fluctuation suppression due to low-recycling Li walls and their impact on the confinement properties are key research topics for exploring the potential of Li as a surface coating for plasma facing components. With increased BT and Ip, and the addition of NBI, LTX-β extends the investigation of low-recycling regimes to high performance plasmas. Target discharges with longer duration and higher electron densities (with correspondingly larger fluctuation amplitudes) create both opportunities and difficulties for the f=288 GHz radial-chord interferometer. Increased fluctuation levels enhance the far-forward scattering (FFS) component of the probe beam, which can severely degrade the interferometry signal. Interferometry is sensitive to k⊥<0.67 cm-1 while the FFS response has a range of 0.4<k⊥<1.7 cm-1. Fluctuations are characterized by using a physical optics model to synthesize the FFS contribution to the interferometer signal. A correction is then generated to recover the electron line density. Fluctuation analysis from recent discharges using this method will be presented. Additional measurements from the profile and fluctuation reflectometers (13.5-33 GHz) are expected to provide better localization for further characterization of internal electron density fluctuations. |
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BP11.00010: Validation of diamagnetic flux loop measurement with kinetic equilibrium reconstruction on LTX-β Santanu Banerjee, Christopher J Hansen, Ronald E Bell, Dennis P Boyle, Anurag Maan, Dick Majeski, Paul E Hughes, William J Capecchi, Drew B Elliott During discharges in the Lithium Tokamak eXperiment -- β (LTX-β) significant eddy currents are driven in the first wall or shell, which is constructed of 1.6 mm stainless steel backed with a 10 mm copper layer, as well as the stainless steel vacuum vessel. These eddy currents, whose distributions are strongly 3D, complicate the use and interpretation of magnetic diagnostics in LTX-β. In particular, the diamagnetic flux loop, which is a valuable diagnostic of the stored energy and its evolution, exhibits pickup due to axisymmetric and 3D eddy currents that limit its accuracy. Experiments on LTX-β are intended to demonstrate improved confinement with lithium coatings on the plasma-facing shell surfaces, and experimental estimation of the stored energy and its evolution is crucial for that. Work to predict and compensate for eddy current pickup on these diagnostics will be presented. Eddy currents will be modeled using reconstructions, PSI-Tri, and time-dependent simulations, PSI-Tet. Studies of MHD activity and its effect on high performance discharges will also be presented. |
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BP11.00011: Neutral beam prompt loss in LTX-β William J Capecchi, Jay K Anderson, Dennis P Boyle, Paul E Hughes, Anurag Maan, Dick Majeski, Drew B Elliott, Christopher J Hansen Prompt loss of beam injected fast ions approaches 100% in LTX-β discharges, though significantly improved confinement is expected for the higher current plasmas made available by a recent upgrade to the Ohmic Heating Power Supply. Modeling of fast ions using TRANSP/NUBEAM finds a maximum coupled beam fraction of 76% at the near-term limits of the LTX-β operating space. The full ion orbit code POET is employed to validate NUBEAM results against possible non-adiabatic effects on fast ion orbits, but corrections to the prompt loss fraction due to collisionless transport are found to be small. The graphical method code CONBEAM is used to investigate the topology of fast ion phase space as it relates to neutral beam deposition, and counter-injected NBI is considered as a way to access a region of high field side beam deposition. A metric is developed within the CONBEAM using a beam filament model to estimate the prompt loss fraction and shown to agree well with both POET and NUBEAM, enabling near real-time analysis and potential feedback to operators between plasma discharges. |
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BP11.00012: Negative triangularity and diverted operation of LTX-β Dick Majeski, A. Brooks, W. Capecchi, Christopher J Hansen, L. Zakharov LTX-β, the upgrade to the Lithium Tokamak Experiment, is a high-field side limited, low aspect ratio tokamak (with R/a nominally 1.6). We have recently been exploring the range of equilibria accessible in LTX-β. These modifications include the use of a pair of high field side coils, which were installed to compensate for vacuum vessel eddy currents during breakdown, as divertor coils (with positive triangularity). We have also explored the use of a pair of low field side poloidal field coils, which are internal to the vacuum vessel, but external to the lithium-coated liner system, to generate tokamak discharges with negative triangularity. Diverted, positive triangularity equilibria offer a closer comparison to NSTX-U diverted discharges. Negative triangularity discharges offer the possibility of reduced drive for trapped electron modes, which in combination with flat temperature profiles, may further reduce transport in a tokamak. While the collisionless scrape-off layer (SOL) in conventional, positive triangularity discharges (with lithium walls) is expected to have a large, mirror trapped population, the SOL in negative triangularity discharges should have no trapped population. Here we discuss both positive and negative triangularity equilibria which can be achieved with the LTX-β coilset. We will also briefly discuss the possible reactor implications of low recycling, negative triangularity tokamaks, with a hot, collisionless SOL. |
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BP11.00013: Initial Transport Studies in the Spherical Tokamak Pi3 at General Fusion Inc Celso Ribeiro, Rouslan Ivanov, Carl Dunela, Filiberto Braglia, Ivan Khalzov, William Young, Patrick Carle, Akbar Rohollahi, Kelly Epp, Adrian Wong, Aaron Froese, Ryan Zindler, Daymon Krotez, Alex Mossman, Michel Laberge, General Fusion Team The Spherical Tokamak Pi3 is a relatively new device in operation at General Fusion Inc. aiming to guide the Fusion Demonstration Plant (FDP) to be constructed at Culham Laboratory campus, UK[1]. |
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BP11.00014: Flat Thomson Temperature Profiles in SPECTOR with Li-coated walls Rouslan Ivanov, Patrick Carle, Aaron Froese, Alex Mossman, Michel Laberge, Akbar Rohollahi, Adrian Wong, William Young SPECTOR is built as a small spherical tokamak device, (R = 12 cm, a = 8.5 cm, B0 = 0.3 T, Ipl = 220 kA) supplied with Li-coated walls. It can generate the relatively dense (ne ~ 1x1020 m-3) and hot (Te = 300 – 400 eV) plasma. The plasma current is induced by injecting helicity, using a magnetized coaxial Marshall gun. The machine is fueled by a brief initial gas pulse into the gun prior to the plasma breakdown. The radial profiles of electron temperature and the density have been measured by the Thomson laser scattering at 6 radially resolved points. Flat or nearly flat radial profiles with a hot edge (up to Te = 300 eV) have been observed when no gas was injected after the plasma formation, and the plasma was continuously losing particles. The low recycling mode was established in the deuterium plasma. The experiments with helium plasma did not show flat Te profiles, since helium ions were poorly retained by the lithium-coated wall, and the low recycling mode likely did not establish. |
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BP11.00015: Tokamak Energy Inc lands in America with 5 INFUSE projects underway at U.S. national labs Mark E Koepke, D. Kingham, D. Wilson, Larry R Baylor, Steven McNamara, Walter Guttenfelder, V. Shevchenko, Timothy S Bigelow, M. Romanelli, Choongseok Chang, T. Davis, C. Taylor Tokamak Energy Inc, launched a year ago as a subsidiary of Tokamak Energy Ltd, participates in DoE's INFUSE program. These projects and our plans will be described. TE designs and builds high-performance prototypes and rapidly innovates while building a team to design and develop fusion pilot plants and power modules (500 MWh, 150 MWe), with the aim of 100 MKelvin in 2021). Our technology, destined to show that the energy confinement time in ST is sufficient to enable a compact device to produce continuous power output at high gain, is based on compact spherical tokamaks (ST) with high-temperature superconducting (HTS) magnets. ST40 has attracted U.S. and Japanese collaborators and is aiming for 100 million K plasma (ion) temperature in 2021. TE is developing HTS magnets via prototypes operating at increasingly higher B fields. Our record field is over 24 T on the conductor in a test magnet operating at 21K. This field strength is already sufficient for a fusion power plant, and these magnets will be scaled up further over the next few years. TE has built up a patent portfolio of greater than 50 patent application families, many related to HTS magnets. TE welcomes technical competition from other private fusion ventures, while appreciating the collective and cooperative push toward a commercial fusion culture, and values the opportunity to collaborate with national laboratories through programs such as INFUSE. |
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BP11.00016: Equilibrium Reconstructions, Stability Calculations, and Disruption Event Characterization of Plasmas in the MAST and MAST Upgrade Spherical Tokamaks John W Berkery, Steven A Sabbagh, Yanzheng Jiang, Veronika Klevarova, Lucy Kogan, David Ryan, Guoliang Xia, Christopher Ham The MAST-U spherical tokamak experiment, an upgrade of the MAST device, is in its first physics campaign. Disruptions in MAST-U, as well as in the database of MAST discharges, are studied to characterize the events that cause them. Loss of vertical stability control was not found to be common in MAST. MAST discharges were able to somewhat exceed the Greenwald density limit, with many density limit disruptions occurring in the current rampdown. Reconstructions of plasma equilibria using kinetic profiles are necessary for stability and disruption analysis. The VALEN code was used to determine effective resistances of the MAST and MAST-U conducting structures to provide estimated currents in those structures for EFIT reconstruction and to separate the plasma current measurement from Rogowski coil sets that enclose those currents. Ideal stability calculations of MAST plasmas, assisted also with machine learning techniques, indicate that the no-wall beta limit can be reliably determined. Projections of stability for MAST-U show that a larger stability gap between the no-wall and with-wall limits should be expected in MAST-U than in MAST, due to passive stabilization plates. Experiments using resonant field amplification to characterize MAST-U stability are planned. |
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BP11.00017: Proton Detection in MAST-U using Silicon and Diamond Surface Barrier Detectors Ashgan Aboutaleb, Werner U Boeglin, Leonel Martinez A silicon based surface barrier detector (SSB) array has previously been used at MAST (Perez et al.) to detect unconfined energetic 3MeV fusion protons. In a tokamak environment detector heating can strongly affect the energy resolution and detection efficiency. To study these we have used an Am-241 α particle source to evaluate the behavior of a Si-based SSB detector with an active area of 50 mm2 and a depletion depth of 300 μm and a diamond detector with an active area of 3 mm2 and 100 μm depletion depth over a temperature range of up to 50oC. Both types of detectors will be installed in MAST-U and first results will be shown if available. The diamond detectors are planned to be used in |
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BP11.00018: Linear analysis of outboard scrape-off layer instability in MAST-U Derek A Baver, James R Myra, Fulvio Militello, David Moulton The ArbiTER1 linear eigenvalue code is a useful tool for analyzing not only the sources of turbulence and filamentary structures in the scrape-off layer of tokamaks, but also their dynamics and spatial structure. Past examples include penetration of filamentary structures into the divertor leg in single-null configuration, and analysis of instability in the private scrape-off layer in double-null configuration. |
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BP11.00019: First measurements of the radiated power in MAST Upgrade using bolometry Jack Lovell, Fabio Federici, Matthew L Reinke, Anthony R Field MAST Upgrade features a suite of diagnostics to measure the power radiated by the plasma. There are arrays of resistive bolometers which provide both poloidal and toroidal coverage of the main chamber. There are also arrays of resistive bolometers in the lower Super-X divertor chamber which measure the radiated power from the outer leg of the divertor in both conventional and Super-X configurations. The bolometer sensors are connected to an FPGA-based electronics system which manages the excitation and calibration of the sensors as well as data acquisition. |
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BP11.00020: A new combined Doppler backscattering and cross-polarization scattering system for measurement of density and magnetic turbulence in the MAST-U spherical tokamak Stephen Storment, Rory Scannell, Peng Shi, Clive A Michael, Quinn Pratt, Troy A Carter, Terry L Rhodes We report on the design and laboratory testing of a new combined Doppler backscattering (DBS) and cross-polarization scattering (CPS) diagnostic that is currently being installed on the MAST-U spherical tokamak at the Culham Centre for Fusion Energy in Oxfordshire, UK. This system will allow for simultaneous measurement of local density turbulence, flows, and magnetic turbulence. The systems are based on an eight channel millimeter wave source, where DBS and CPS share a probing beam. A DBS analysis module in OMFIT will also be presented, which will be used to analyze MAST DBS data from 2013 as well as to be the primary tool with which to analyze new MAST-U DBS data as it becomes available. Initial scoping and design of experiments to investigate the relationship between collisionality, confinement, and turbulent transport will be reported. |
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BP11.00021: Analysis and sensitivity study of an HTS spherical tokamak pilot plant design using the PROCESS 0-D systems code Charles Swanson, Walter Guttenfelder PPPL and CCFE are collaborating on the use of the PROCESS systems code to explore innovative pilot plant and Fusion Nuclear Science Facility (FNSF) designs. We have used PROCESS to benchmark a spherical tokamak pilot plant design published in Menard et. al. (2016). Previously, the copper-centerstack FNSF design in that paper was benchmarked by Muldrew et. al. (2020). |
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BP11.00022: The NSTX-U Research Program Stanley M Kaye NSTX-U is a high-powered Spherical Tokamak (ST) whose mission is to establish the physics basis for next-step ST facilities and broaden the scientific understanding of plasma confinement for ITER. In particular, the research in NSTX-U will be critical for informing the design of a Compact Fusion Pilot Plant. NSTX-U will have a mission-oriented research program that addresses key gaps over the next five years. 1) Extend confinement and stability studies to low collisionality and high-β. NSTX-U will operate at collisionalities 5x lower than NSTX, allowing validation of the strong improvement of core and pedestal confinement with decreasing collisionality. 2) Develop operation at large bootstrap fraction and advance the physics basis required for non-inductive, high-performance and low-disruptivity operation. NSTX-U will develop operational scenarios with fBS=60–90% and βN=4-6 for multiple current redistribution times, and it will operate fully non-inductively for plasma currents up to 1 MA. 3) Develop and evaluate conventional and innovative power and particle handling techniques to optimize plasma exhaust in high performance scenarios. Research in this area will evolve towards implementation of flowing liquid lithium components. |
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BP11.00023: Low aspect ratio next-step tokamak design studies at PPPL Jonathan E Menard, Brian A Grierson, Tom Brown, Rajesh Maingi, Jacob Schwartz, Chirag Rana, Yuhu Zhai Recent U.S. magnetic fusion strategic planning reports all recommend that the U.S. should pursue innovative science and technology to enable construction of a Fusion Pilot Plant (FPP) that produces net electricity from fusion at reduced capital cost. Compact tokamaks have been proposed as a means of potentially reducing the capital cost of an FPP. However, compact steady-state tokamak FPPs face the challenge of integrating a high fraction of self-driven current with high core confinement, plasma pressure, and high divertor parallel heat flux. Such integration has not been previously accessed nor is it presently planned to be accessed. This integration is sufficiently challenging that construction and operation of a dedicated sustained-high-power-density (SHPD) tokamak facility has been proposed by the U.S. community to close this integration gap. In this presentation recent systems studies for superconducting low aspect ratio FPPs will be summarized, a preliminary SHPD configuration with substantial flexibility in aspect ratio, shape, and divertor will be discussed, and possible future FPP R&D in the U.S. program and at PPPL will be considered. |
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BP11.00024: Radiation transport effects in lithium divertors. Vlad Soukhanovskii, Aleksandr I Khrabryi, Howard A Scott, Thomas D Rognlien Lithium vapor properties are key in lithium-based tokamak divertor concepts, such as the vapor box divertor and the vapor-shielding open divertor. Some of these concepts are to be tested in NSTX-U with divertor power densities up to tens of MW/m$^2$. Effects of deuterium and lithium line radiation trapping in divertor plasmas with high lithium density are studied with the radiation transport code CRETIN and the multi-fluid transport code UEDGE. Lithium atomic data from the FAC code is used. Parametric plasma temperature and density scans in deuterium and lithium plasma slabs and boxes with lithium densities $10^{18}-10^{22}$ m^${-3}$ are used to study the Li I, Li II, and Li III emission line opacities, broadband spectra, and total radiation fluxes. A UEDGE based divertor plasma model with an NSTX lower single null divertor configuration is used with CRETIN calculations to study lithium opacity effects: modified radiated power and radiation flux asymmetries, shifted lithium ionization balance and redistribution of lithium in the divertor. |
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BP11.00025: COTSIM-based Optimization of Transport Diffusivity Models for Enhanced Predictive COTSIM Simulations of NSTX-U Discharges Hassan R Al Khawaldeh, Andres Pajares, Tariq Rafiq, Eugenio Schuster An optimizer has been developed to adjust the set of coefficients associated with parameterized analytical transport models such as Bohm/Gyro-Bohm and Coppi-Tang to better represent specific NSTX-U scenarios. The parameterized transport models in the Control Oriented Transport SIMulator (COTSIM) are tuned by the optimizer in order to minimize a user-defined cost function measuring the mismatch between experimental and COTSIM-predicted plasma states based on the associated experimental inputs (total plasma current, line-average density, heating and current-drive powers, etc.). For instance, the cost function could be the integral over space of the squared difference between experimental and predicted representative plasma profiles such as the safety factor, the electron temperature, and the angular momentum. This integral could be evaluated at a specific time, at a set of predefined times, or over a time interval. Constraints on the possible range of the transport coefficients are imposed during the optimization. The resulting Nonlinear Programming problem is solved by using Sequential Quadratic Programming (SQP), which is predicated on determining a local minimizer of the original nonlinear program by iteratively solving a sequence of approximated Quadratic Programing problems. |
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BP11.00026: Transport Modeling of NSTX/NSTX-U discharges Galina Avdeeva, Kathreen E Thome, Sterling P Smith, Orso-Maria O Meneghini, Joseph Mcclenaghan, Devon J Battaglia, Walter Guttenfelder, Stanley M Kaye The spherical tokamak (ST) provides critical information required for the optimization of the |
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BP11.00027: Development of Electron Temperature Gradient Transport Model for Tokamak Plasmas Tariq Rafiq, Jan Weiland, Eugenio Schuster, Alexei Pankin Progress in the development of a new model for the electron-temperature-gradient (ETG) modes in tokamaks is presented for a general geometry. The ETG modes can be unstable in conventional and low-aspect ratio tokamaks, resulting in turbulence-driven electron thermal transport. Burning plasma discharges, where collisions with fast alpha particles primarily heat electrons, would also benefit from a better understanding of ETG turbulence and transport. This new model will replace an old model in the Multi-Mode Module that employs empirical coefficients for ETG-driven transport. The new model includes new significant plasma-parameter dependences but does not contain empirical coefficients. Although the ETG modes at the electron gyroradius scale are essentially electrostatic, electromagnetic effects in high beta tokamak plasmas must be taken into account. A system of equations is presented governing the dynamics of low-frequency short-wavelength electromagnetic ETG-driven drift modes in the presence of density/temperature/magnetic field gradients, curvature, finite beta, collisionality, and reverse/low magnetic shear. Non-adiabatic ion effects, including E×B and polarization drifts, are taken into account without including ion-temperature-gradient mode fluctuations. |
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BP11.00028: Linear stability analysis of ion-scale microturbulence in low and high collisionality NSTX discharges Cesar F Clauser, Walter Guttenfelder, Tariq Rafiq, Eugenio Schuster Future NSTX-U discharges will focus on low collisionality regimes where turbulence-driven thermal transport could play a critical role. The gyrokinetic code CGYRO is used to analyze a high-beta and high-collisionality NSTX discharge in both electrostatic and electromagnetic limits to better understand the transition from high to low collisionality regimes and the influence of anomalous thermal transport on the energy confinement time. Microinstabilities such as ion temperature gradient and trapped electron modes are shown to be stable in the electrostatic limit due to the effects of flow shear. In the electromagnetic limit, on the other hand, kinetic ballooning modes and microtearing modes are shown to be unstable, with the latter being the dominant instability when experimental profile values are used. Scans of a variety of parameters, including ion temperature gradient, pressure gradient, electron beta, and wavenumber, are performed to examine these instabilities and their stability thresholds. The results of a low collisionality NSTX discharge are also compared to those of a high collisionality discharge. Finally, the NSTX-U low collisionality expecting conditions are investigated. The results are contrasted to gain a better understanding of the impact of collisionality on these regimes. |
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BP11.00029: Comparison of KBM stabilization by shaping effects at NSTX and conventional aspect ratios Amil Sharma, Michael Cole, Yang Chen, David R Hatch, Benjamin J Sturdevant, Robert Hager, Seung Hoe Ku, Choongseok Chang, Walter Guttenfelder Linear electromagnetic simulations of KBM stabilization by shaping effects at NSTX aspect ratio and high plasma beta are performed using the global gyrokinetic PIC code XGC [1,2]. Linear KBM stability at NSTX aspect ratio is found to be highly sensitive to shaping effects. Magnetic geometries with high elongation and triangularity are included. The effects of compressional magnetic perturbations are approximated via a modification to the particle drifts, and this approximation is used in a benchmark between the global gyrokinetic codes XGC, GEM [3], and GENE [4]. Profile-consistent magnetic equilibria are generated for each beta value and geometry, and single toroidal mode numbers are considered. KBM stabilization by shaping effects is compared at NSTX and conventional aspect ratios. |
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BP11.00030: Impact of edge harmonic oscillations on the divertor heat flux in NSTX Kaifu Gan, Travis K Gray, Stewart J Zweben, Eric D Fredrickson, Rajesh Maingi, Adam G McLean, Brian D Wirth Previously, ELM-free and inter-ELM divertor peak heat flux reduction induced by an edge harmonic oscillation (EHO) was observed in NSTX (K.F. Gan et al., 2017 Nucl. Fusion 57 126053). This paper introduces new analysis of the EHO impact on the divertor heat flux. It was found that enhanced edge turbulence significantly increases the divertor hear flux footprint width in NSTX, relative to ELM-free discharges. The EHO at low edge fluctuation level will significantly increase the heat flux width and decrease the divertor peak heat flux, while the EHO at high edge fluctuation level will increase the divertor peak heat flux. It was also found that the heat flux width increased with the frequency of the EHO. |
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BP11.00031: Extended MHD studies of reconnection physics in spherical tokamaks Fatima Ebrahimi, Roger Raman Fast axisymmetric reconnection has been shown to be crucial for helicity injection current-drive techniques in spherical tokamaks. In recent years, it has been demonstrated that the transient coaxial helicity injection could take advantage of fast plasmoid reconnection to produce startup current in NSTX/NSTX-U (Ebrahimi&Raman PRL, NF 2015,2016, Ebrahimi PoP 2019). Although in more conventional helicity injection techniques, generated current carrying filaments require relaxation due the dynamo terms in 3-D, here merging of axisymmetric plasmoids would lead to a final stable equilibrium startup current formation. By incorporating the final vessel configuration of PEGASUS III, as well as the coil currents in the NIMROD code, we have performed new sets of extended MHD simulations for this configuration. Preliminary simulations show large-volume flux closure of about 20mWb, We present simulations with further optimizing coil currents for Pegasus III, as well as the reconnecting edge stability analysis for this configuration. This work was done in collaboration with the Pegasus III team. Work supported by DOE grants DE-AC02-09CHI1466, and DE-SC0010565. |
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BP11.00032: Low frequency MHD activity in NSTX and NSTX-U discharges Stefano Munaretto, Devon J Battaglia, Nathaniel M Ferraro, Stefan P Gerhardt, Walter Guttenfelder, Jong-Kyu Park, Zhirui Wang
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BP11.00033: Chirping Ion Cyclotron Emission (ICE) on NSTX-U Eric D Fredrickson We report here on the discovery on NSTX-U of a qualitatively new type of non-thermal emission from plasmas in the ion-cyclotron frequency range (ICE). This new type of ICE is weakly damped, É¡damp/??ICE ≈ 0.14%, low order eigenmodes, excited by a fast ion population radially located near an internal transport barrier. The ICE appears as repetitive bursts with durations of ≈ 1 ms, and the bursts exhibit frequency chirps upwards and/or downwards with maximum δf/f ≈ 1%. We refer to this chirping ICE as ch-ICE. The bursts are longer than the typical ICE bursts of ≈ 100 µs reported previously for NSTX and NSTX-U, for simplicity, NSTX(-U). The chirping resembles that seen in modeling of weakly unstable fast particle driven instabilities [cf., Berk, et al., Phys. Lett. A 234, (1997) 213]. These data also provide some preliminary indications of non-linear coupling between the ch-ICE harmonics. Despite extensive experimental and theoretical studies of ICE, theoretical understanding of ICE remains qualitative at best. The detailed experimental observations of ICE from NSTX(-U) find a wide variation in the ICE characteristics. The new observations of ch-ICE presented here, combined with the characteristics of ICE on NSTX(-U) previously reported [Fredrickson, et al., Phys. Plasmas 26 (2019), 032111], could guide the development of more complete theoretical models of ICE. |
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BP11.00034: Nonlinear simulations of GAEs for NSTX and NSTX-U Elena Belova, Eric D Fredrickson, Neal A Crocker Nonlinear simulations of subcyclotron-frequency global Alfven eigenmodes (GAEs) has been performed for National Spherical Torus Experiment (NSTX/NSTX-U) conditions to study dependence of saturation amplitude on beam parameters and compare with experimental scaling. GAEs are frequently excited during neutral beam injection (NBI) in the NSTX(-U), as well as other beam-heated devices such as MAST and DIII-D. These modes are driven unstable through the Doppler shifted cyclotron resonance with the NBI ions and can be excited in ITER due to super-Alfvenic velocities and strong anisotropy of the beam ions. Numerical simulations using the HYM code have been performed to study the excitation and the saturation of GAEs for a single toroidal mode number, and also investigating the nonlinear evolution in the presence of multiple unstable modes. The scaling of the saturation amplitude with the linear growth rate and the enhancement of the saturation level due to resonance overlapping in the presence of multiple unstable modes are shown. The nonlinear simulations show the evolution of the beam ion distribution function towards reduction of anisotropy; the effect is stronger for larger normalized NBI injection velocity. |
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BP11.00035: HHFW coupling resistance assessment for different NSTX-U plasma scenarios using Petra-M FEM framework Nicola Bertelli, Syun'ichi Shiraiwa NSTX-U will be equipped with the high harmonic fast wave (HHFW) antenna system with an injected power up to 6 MW. HHFW could play a major role in NSTX-U plasma particularly for driving current in the plasma ramping phase and providing a significant core electron heating. An evaluation of the antenna coupling resistance for different NSTX-U plasma scenarios, such as different scrape-off layer (SOL) density profiles and magnetic field values, is very desirable before the upcoming NSTX-U experimental campaign in order to assess the potential HHFW antenna performance. Here we employ the Petra-M FEM framework, which has been used to simulate the full NSTX-U torus including the SOL and the realistic antenna geometry. Full field plasma scenarios, which correspond to a toroidal magnetic field of BT =1 T, will be examined in detail and compared with lower BT field cases. A scan of SOL density values and antenna phasing will be also performed. Finally, an attempt will be made to relate the current numerical results shown in this work with previous HHFW coupling resistance measurements and predictions. |
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BP11.00036: Progress in the development of RF diagnostics for NSTX-U and WEST Raymond Diab, Seung Gyou Baek, Nicola Bertelli, Paul T Bonoli, William Burke, James P Gunn, Julien Hillairet, Earl S Marmar, Masayuki Ono, Syun'ichi Shiraiwa, Gregory M Wallace, John C Wright Recent advances in computational tools have made it possible to locally compare the RF wave-field in scrape-off-layer (SOL) plasmas with simulations, which motivated the need for new RF diagnostics for wave characterization in the SOL of NSTX-U and WEST. To characterize and mitigate the high harmonic fast wave (HHFW) power flow in the SOL of NSTX-U, we have been developing an 8-channel homodyne RF detection system that will be connected to the RF Langmuir probes in the antenna and divertor regions of NSTX-U. This diagnostic will allow us to locally measure the RF voltage in the SOL to study RF rectification effects (R. J. Perkins et al., PoP 22, 042506 (2015)) and will support spectral analysis to study parametric decay instabilities (J. R. Wilson et al., AIP Conf. Proc. 787, 66 (2005)). In parallel, we have been assessing the possibility of installing an RF B-dot probe to a reciprocating probe system on WEST. COMSOL modeling of the probe's response at the WEST ICRF frequency (50 MHz) allowed us to optimize the probe's dimensions, and a prototype probe has been built and calibrated in the lab. This diagnostic will yield crucial data to validate recent full-wave simulations on the WEST tokamak and may support a study of parametric decay instabilities in the SOL of WEST. |
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BP11.00037: Faraday-effect polarimetry diagnostic for NSTX-U spherical tokamak Jie Chen, David L Brower, Weixing Ding, Brentley C Stratton, William Harris A Faraday-effect polarimetry-interferometry diagnostic is under design for the NSTX-U spherical tokamak. Aiming to address issues such as model validation and plasma control, the diagnostic will provide internal magnetic field and magnetic fluctuation measurements associated with MHD instabilities, energetic particle driven modes, broadband magnetic turbulence, and 3D non-axisymmetric effects. The diagnostic will utilize 3-wave technique to accomplish simultaneous Faraday-effect and line-integrated density measurements with fast time response (0.1 µs) and low phase noise (0.01 degree). The probe beams will access the plasma along a horizontal chord via a mid-plane port, to directly detect internal radial magnetic fluctuations via the Faraday effect. Solid state sources and planar diode mixers at 930 GHz will be used to minimize contamination from Cotton-Mouton effect. Probe beam diameter of ~22 mm infers a corresponding upper bound wavenumber of measurable fluctuations at kρs~1, for NSTX-U plasmas with BT=1 T and Te=2 keV. Various optical designs are under consideration, including a dual-chord design with toroidal offset to measure toroidal mode number up to n=60 and a single-pass design to reduce feedback. |
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BP11.00038: Poloidal High-k Scattering on NSTX-U Yilun Zhu, Calvin W Domier, Jon Dannenberg, Guanying Yu, Xianzi Liu, Yang Ren, Brentley C Stratton, N C Luhmann A poloidal high-k scattering system is under development by UC Davis and PPPL to study high-k electron density fluctuations on NSTX-U. The 8-channel diagnostic system, which operates at 693 GHz, replaces a 280 GHz, 5-channel toroidal high-k scattering system on NSTX. The probe beam is launched from Bay G towards Bay L where large aperture optics collect radiation at 8 simultaneous scattering angles ranging from 2 to 15°, corresponding to poloidal wavenumbers from 7 cm-1 to >40 cm-1. The receiver optics and subharmonic mixer array are mounted on a 5-axis receiver carriage which is translated and rotated as a whole to position the scattering region as desired between r/a= 0.1 and the plasma edge (r/a = 0.99). Upwards and downwards scattering options are available, and can be easily switched from one mode to another between discharges. Details of the diagnostic, including installation and commissioning schedules, will be presented. |
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BP11.00039: Towards In-Between-Discharges Model-Based Scenario Planning in NSTX-U Via Fast Nonlinear Optimization Brian R Leard, Sai Tej Paruchuri, Tariq Rafiq, Eugenio Schuster The realization of advanced scenarios in tokamaks is achieved by carefully selecting the actuator trajectory waveforms, which defines a feedforward control problem. As an alternative to the usual “trial-and-error” approach, a more systematic approach to scenario planning via model-based optimization has been proposed [1]. By parameterizing the actuator trajectories, the feedforward control inputs are determined by minimizing a cost function measuring the distance between actual and desired plasma state. This arbitrary cost function, which can weigh different properties of the desired plasma state, is minimized subject to plasma-dynamics, actuator, and state constraints by using Sequential Quadratic Programming. To avoid spending time in numerically computing the gradients of the cost function with respect to the to-be-optimized parameters, analytical expressions of these gradients are pre-calculated in this work. These expressions require the integration of a plasma transport model for NSTX-U, which is provided in this case by the Control Oriented Transport SIMulator (COTSIM). This fast feedforward-control optimizer has the potential of being used routinely for in-between-discharges scenario planning at NSTX-U. |
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BP11.00040: Feedforward shape control and neural net equilibrium modeling on NSTX-U Josiah T Wai, Mark D Boyer, Egemen Kolemen The NSTX-U shape control algorithm relies entirely on feedback control to track target shapes. Control performance can be improved by instead using feedback to adjust coil currents around a reference trajectory, i.e. feedback plus feedforward control. To this end, a design tool that translates target shape evolutions into coil current trajectories has been developed and validated with previous campaign data. Additionally, we report on development of several neural networks related to equilibrium modeling in NSTX-U. These networks can be used as standalone tools, or in conjunction with the feedforward coil currents planner to perform fast simulations. The networks include: (1) a neural network free-boundary solver that identifies equilibrium flux surfaces given the coil currents and internal profiles, (2) a convolutional neural network that performs the inverse problem, identifying coil currents from the flux surfaces, and (3) a neural net estimator of the nonrigid plasma response. The ground truth data for the nonrigid plasma response is obtained using the gspert code. This quantity is an estimate of how the plasma redistributes in response to external coil current perturbations or changes in Ip, Betap, and Li, and is important for shape control algorithm design. |
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BP11.00041: : COHERENT RADIATION AND INTENSE LASER DRIVEN X-RAY SOURCES
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BP11.00042: Self-consistent modeling of coherent synchrotron radiation from electron beams in bunch compressors Chengkun Huang, Feiyu Li, Hoby N Rakotoarivelo, Bruce E Carlsten, Thomas J Kwan, Rao Garimella, Gary A Dilts, Robert Robey The self-consistent nonlinear dynamics of a relativistic particle beam interacting with its complete self-fields is a fundamental problem underpinning many of the accelerator design issues in high brightness beam applications, as well as the development of advanced accelerators. Particularly, synchrotron radiation induced effects can lead to collective beam instabilities and emittance growth. We are developing a novel Lagrangian method for the calculation of the particles’ radiation near-fields on adaptive meshes, which are then interpolated onto a global mesh. This method allows simulation of radiation co-propagation and interaction with the beam at greatly reduced errors. Multiple levels of parallelisms inherent in this method are implemented in our code CoSyR [1] to enable at-scale simulations of the nonlinear beam dynamics on modern computing platforms using MPI, multi-threading, and GPUs. CoSyR has been benchmarked with other coherent synchrotron radiation models and used to evaluate the transverse and longitudinal effects on the beam. Beam optics designs proposed for the mitigation of beam brightness degradation in a magnetic compressor are also investigated and discussed. |
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BP11.00043: High intensity, monochromatic THz emission from obliquely colliding laser pulses in a plasma Min Sup Hur, Hyung Seon Song, Teyoun Kang, Manoj Kumar, Jaeho Lee, Dohyun Park Terahertz emission from plasmas driven by short-duration laser pulses appears mostly as a wideband, burst of a few cycles, as the emission mechanism is determined by the generation of transient current of comparable duration with the driving pulses. In that case, control of the emission spectra is relatively difficult compared to other mechanisms that use oscillating currents. Previously we proposed a novel mechanism of generating terahertz emission using a plasma dipole oscillation (PDO) [1-3], that is generated by collision of two detuned, short laser pulses. As the emission utilizes the plasma oscillation, the spectrum is narrow and frequency is readily controllable. Here, we discuss several new aspects of PDO-based emission: increased efficiency by obliquely colliding, long laser pulses, control of high harmonics generation by driving pulse shape, and mixture of dipole and quadrupole emission with different polarizations in a magnetized condition. We discuss how those features of PDO can be relevant to astrophysical radio bursts and plasma diagnostics as well as strong light sources in the terahertz band. |
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BP11.00044: THz photonics of stratified plasmas: Transient Cherenkov radiation from laser wakes Serge Y Kalmykov, Jennifer A Elle, Andreas Schmitt-Sody A terawatt laser pulse, propagating in a stratified gas, instantly ionizes it, forming a nonhomogeneous plasma column, concurrently driving a partly electromagnetic wake wave. Ramping up the plasma density along the drive pulse path increases the wake phase velocity beyond the vacuum speed of light, making the wake emit a broadband, positively chirped THz Cherenkov signal. At any locality within the ramp, the wake phase velocity increases continuously, becoming singular within a finite interval of time, changing sign afterwards, eventually becoming sub-luminal by absolute value. In the course of this wake `reversal,' the Cherenkov wave vector rotates by 180○. The signal in the wave zone is thus an expanding spherical shell, the signal length increasing with the observation angle from almost zero (forward emission) to a few tens of picoseconds (backward emission.) The signal frequency increases in time from the Langmuir frequency at the foot of the column to the one at the top. The THZ energy flow at 90○, emanating from the infinitely superluminal wake, is the highest, a few kW. Experimentally capturing the details of this transient THz emission must shed light onto the plasma wake dynamics. Approved for public release; distribution is unlimited. PA release approval AFRL-2021-1933 |
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BP11.00045: Relativistic Harmonic Generation with Flat Spectrum: Boosted Coherent Synchrotron Emission from Continuously Accelerated Electron Nanobunches Xiaofei Shen, Alexander Pukhov, Bin Qiao We demonstrate analytically and numerically that relativistic harmonics with a flat spectrum as In/I0=n-2/3 can be achieved when an electron nanobunch obtains considerable acceleration during the photon emission process. Here In is the intensity of the nth harmonic. Different from the γ-spike distribution in relativistically oscillating mirror and coherent synchrotron emission (CSE), in this mechanism, the electron relativistic factor γ is rising significantly even during the stationary phase point, leading to boosted radiation bursts. Using two-dimensional particle-in-cell simulations, we find an approach to produce harmonics with such a flat spectrum, where an intense femtosecond laser pulse is incident on the front edge of a thick target. The excited strong surface plasma wave field supports the continuous longitudinal acceleration for electrons. The obtained spectrum is much flatter than the -4/3-power law of the conventional CSE mechanism. This paves the way to unprecedentedly large energy attosecond pulses. |
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BP11.00046: Development of High Fluence X-Ray Sources on the NIF Using Laser Heated Novel Nano-Wire Metal Foams Mark J May, Gregory E Kemp, Russ Benjamin, Patrick Poole, Klaus Widmann, Jeff D Colvin, Tyler Fears, Fang Qian, Brent E Blue High fluence K-shell and L-shell x-ray sources are desired for various high energy density physics experiments. One efficient method for creating such a source is the laser heating of materials that are underdense to laser light. Nanowire foams are an ideal choice for an underdense material and have average densities of 6-15 mg/cc. The manufacture of robust Cu, Ag and Au nano-wire foams into millimeter scale targets is possible through a technique of freeze casting an aqueous suspension of nano-wires. Cylindrical targets with sizes between 2 to 4 mm have been shot on both the NIF and the Omega laser facilities. For example, x-ray conversion efficiencies (XRCE) from silver nano-wire foams have been measured to be ~1.0 when heated with ~400 TW of 3w laser light in a 2.5 ns square pulse from the NIF laser system. The XRCE from foam targets have been found to be ~2 times that observed in metal lined cylindrical cavity targets and ~5 times that observed in prepulsed metal foils. Experimental results and comparisons with simulations will be presented. |
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BP11.00047: High repetition-rate Kα x-ray source from a low-density gas target Jon Murphy, Milos Burger, John Nees, Nicholas J Peskosky, Karl M Krushelnick Potential applications for laser-driven x-ray sources benefit from operation at high repetition-rate. Here, 15 mJ CPA pulses are generated at 480 Hz repetition-rate and tightly focused onto a gas target for the generation of Kα x-rays from a number of noble gases. The continuously-flowing nature of the gas jet meant that the target density was below the threshold for clustering and ensured an easier-to-implement target design. A robust experimental analysis of this debris-free x-ray source is presented including measurements of its performance while varying a number of parameters and how the source’s output could scale under different experimental conditions. Investigation of potential applications of the source and future improvements are discussed as well. |
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BP11.00048: Generation of Collimated MeV γ-rays from Intense Poincaré Beam-Plasma Interaction Daniel Younis, Asher Davidson, Bahman Hafizi, Daniel F Gordon The production of a petawatt γ-ray beam is demonstrated using a novel configuration based on fully-structured light irradiating a dense plasma. We study how the relativistic pulse efficiently confines and accelerates plasma electrons, driving a quasi-static field that stimulates magneto-bremsstrahlung radiation. The emitted γ-rays are highly collimated and account for upwards of 20% of the incident field energy. The essential characteristics of the interaction are validated using three-dimensional particle-in-cell simulations. |
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BP11.00049: Laser-generated γ-ray flash interactions with high-Z target study employing Monte Carlo simulations David Kolenaty, Prokopis Hadjisolomou, Roberto Versaci, Tae Moon Jeong, Petr Valenta, Veronika Olšovcová, Sergei V Bulanov It is known that high-power laser pulse interaction with solid targets can result in high-power γ-ray flash emission. We used the γ-photons obtained from PIC (EPOCH) simulation [1] of single-cycle tightly focused laser pulse, reproducing the λ3 regime, irradiating the solid target as a source particles for MC (FLUKA) simulations of their interactions with solid matter. MC simulations can be used to estimate the energy and angular spectra of the post-PIC interacted and generated particles and the radioactive nuclides produced. These simulations can also provide feedback regarding the optimal γ-photons spectrum required for a particular purpose. The lead was chosen as the target material owing to the high cross section of giant dipole resonance and pair production. Potential applications of generated high-energy high-intensity neutrons and positrons are nuclear waste management and applications in medicine, neutron diffraction, positron annihilation lifetime spectroscopy, positron source for further acceleration, electron-positron plasma studies, etc. Activation of residual nuclides can be used for nuclear physics study, astrophysics study, and for direct applications as nuclear medicine. |
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BP11.00050: Extreme Gamma-Ray Flash in the λ3 Regime Prokopis Hadjisolomou, Tae Moon Jeong, Petr Valenta, David Kolenaty, Roberto Versaci, Veronika Olsovcova, Christopher P Ridgers, Sergei V Bulanov During the next decades lasers with power approximately 100 petawatt are expected. These |
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BP11.00051: Development of a High Energy Gamma-Ray Spectrometer for Laser-Plasma Experiments Rebecca J Fitzgarrald, Elias Gerstmayr, Eva E Los, Alexander G Thomas, Stuart Mangles There is an abundance of existing gamma-ray detectors, but for laser-plasma experiments where ultrashort high-energy gamma rays are produced, existing detectors lack the response time to measure the energy spectrum through single hit detection. An initial scintillator-based design for a new spectrometer was created to infer the spectrum from the shape of the electromagnetic shower in the detector as gamma rays pass through an array of CsI scintillating rods. However, noise in the system and poor distinction between the responses to high-energy photons in the 200-500 MeV range have limited the spectral retrieval of this design to forward fitting methods that require prior knowledge of the spectral shape. The goal of this project is to develop a spectrometer that is less sensitive to noise and can retrieve the full spectrum without requiring existing knowledge. To this end, we used FLUKA simulations and codes in MATLAB to investigate how different materials and geometries impact the susceptibility of the spectral reconstruction to noise. We have explored new scintillator geometries to improve the conditioning of the matrix response. The spectrometer design will be built and used for ZEUS experiments to provide a full picture of the energy distribution of gamma rays. |
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BP11.00052: LASER PLASMA ION ACCELERATORS
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BP11.00053: Design Optimization of Permanent-Magnet Based Compact Transport Systems for Laser-Driven Proton Beams Jared T De Chant, Kei Nakamura, Qing Ji, Lieselotte Obst-Huebl, Samuel Barber, Antoine M Snijders, Thomas Schenkel, Jeroen van Tilborg, Cameron R Geddes, Carl B Schroeder, Eric H Esarey Laser-driven (LD) ion acceleration will be explored in a newly constructed short-focal length beamline at the BELLA petawatt facility (iP2). For applications utilizing such LD ion beams, a beam transport system is required, which for reasons of compactness be ideally contained within 3 m. The large divergence and energy spread of LD ion beams present a unique challenge to transporting them compared to beams from conventional accelerators. This presentation gives an overview of various compact transport designs for the iP2 proton beamline that respond to various needs from applications and have different advantages and disadvantages. Here we considered the use of permanent magnet elements, which can provide high magnetic field gradients on a small footprint. The performance of each design was evaluated based on high order particle tracking simulations of typical LD proton beams. |
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BP11.00054: IP2: High Intensity Experiment Platform at the BELLA Petawatt Laser Kei Nakamura, Lieselotte Obst-Huebl, Sahel Hakimi, Jared De Chant, Zachary Kober, Tobias M Ostermayr, Stepan S Bulanov, Axel Huebl, Anthony J Gonsalves, Csaba Toth, Thomas Schenkel, Jeroen van Tilborg, Carl B Schroeder, Eric Esarey, Cameron R Geddes In this presentation, we will report on the status of a newly constructed high intensity laser beamline at the BELLA petawatt facility with a short-focal length OAP (f\2.5), where expected laser focus intensity is >1021 W/cm2 with a repetition rate up to 1 Hz. The first commissioning experiment is scheduled in fall 2021, and the results will be discussed in the presentation. |
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BP11.00055: 3D Particle-In-Cell Simulations of Double Pulse Enhanced Target Normal Sheath Acceleration Ricky Oropeza, Joseph R Smith, Nashad Rahman, Chris Orban An interesting approach to enhance proton acceleration with intense laser experiments involves irradiating a target simultaneously with two overlapping laser pulses. This phenomenon has been studied extensively with 2D PIC simulations but at present only a few 3D PIC simulations of this phenomenon have been published. We present a suite of 3D PIC simulations made with EPOCH in an effort to better understand the basic physics involved. |
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BP11.00056: Jitter characterization and flux scaling of a novel mass-limited liquid target for ultrashort laser-driven fast neutrons Nicholas J Peskosky, John Nees, Karl M Krushelnick Recent advances in high-power laser engineering and non-linear ultrafast pulse compression will soon yield functional relativistic laser drivers with terawatt (TW) peak powers and kilowatt average powers. Logarithmic increases to driver repetition rates demand a paradigm shift in the way experimenters must approach shot-to-shot targetry at kHz/MHz repetition rates and have set the stage for the preeminence of tabletop laser-driven fusion neutron sources with high-brightness. We report on initial mass-limited target spatial-temporal jitter for micron-scale deuterated liquid targets electrohydrodynamically dispensed at multi-kHz repetition rates. Spatial expansion dynamics of critical-density fusion plasma plumes resulting from irradiation of these targets by sub-relativistic and relativistic Ti:Sapphire laser pulses is time-resolved via an off-harmonic back-lighter generated via optical parametric amplification. Isotropic fast neutron yield scaling trends for this mass-limited target are presented for various pre-pulse temporal delay parameters (ps-to-ns) and driver pulse durations spanning from 38-350 femtoseconds. Data from long-pulse (150-350 fs) interactions is extrapolated in terms of expected neutron flux performance realized from TW-class ultrashort Yb-doped fiber laser systems leveraging coherent spatial and spectral beam combining techniques. |
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BP11.00057: Fusion Driven Transmutation of Transuranics in a Molten Salt with Multiple Sources Joshua Tanner, Ales Necas, Sydney Gales, Gerard Mourou, Toshiki Tajima A first set of computational studies of transmutation of spent nuclear fuel using compact tunable 14 MeV D-T fusion driven neutron sources is presented. Where we study the controllability, time evolution, as well as effects of spatial distribution of the neutronics in the transmutation in the subcritical operations regime of a transmutator, in which our neutron sources are small, distributed, and can be monitored. Source neutrons are generated via beam-target fusion whereas a deuteron beam is created by laser irradiation of nanometric foils, through the Coherent Acceleration of Ions by Laser (CAIL) process, onto a tritium soaked target. This can be accomplished using relatively cheap fiber lasers terminating onto small scale targets which makes possible the use of multiple tunable and distributable neutron sources. This source is then combined with a molten salt core whose liquid state allows: homogeneity by mixing, safety, in-situ processing, and monitoring. Such a source and molten salt combination allows for the introduction of rapid feedback or feedforward control of the system's operation that have not previously been considered. This encourages an investigation with the aid of AI into new spatial and operation control strategies as done here. |
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BP11.00058: Divergence reduction and enhanced energies of protons from double-layer target irradiated by intense shaped laser Camilla Willim, Jorge Vieira, Victor Malka, Luis O Silva Multi-MeV proton beams find various applications such as "fast ignition" of inertial confinement fusion targets or modification of material parameters. Target normal sheath acceleration (TNSA) driven by short intense laser pulses is, in this context, a well-established proton acceleration model. The generation of multi-MeV proton beams with ultrashort duration (ps) and a high number of protons in a bunch (1011 – 1013) has been successfully demonstrated in experiments, but beam properties still need improvement for applications. Improving the divergence of TNSA generated proton bunches, for example, is an important open question for future progress. Here, we demonstrate that the divergence of protons accelerated from double-layer targets irradiated by intense lasers with orbital angular momentum (OAM) can be significantly smaller (up to factor 7) than when considering pure Gaussian modes. We find that such reduction is closely connected with the laser self-focusing and absorption in the near-critical plasma of the double-layer target, which impacts on hot electron generation and subsequent proton acceleration at the rear side. The self-consistent laser—plasma dynamics is investigated analytically and by relying on three-dimensional particle-in-cell simulations in OSIRIS. |
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BP11.00059: RELATIVISTIC HIGH-ENGERY DENSITY PHYSICS AND HIGH FIELD PHYSICS
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BP11.00060: Exploring the interaction between ultra-relativistic fireball beams and magnetized plasmas Cinzia Chiappetta, James Gold, Mariaelena Innocenti, Nitin Shukla, Kevin M. Schoeffler, Elisabetta Boella Ultra-relativistic particle jets composed of electrons and positrons are common in astrophysical environments and are associated with some of the most luminous astrophysical objects, such as gamma-ray bursts, supernova explosions and active galactic nuclei [1]. The interaction of these jets with the surrounding media drives a variety of plasma phenomena including the generation of intense magnetic fields and particle acceleration. |
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BP11.00061: Magnetic field amplification in relativistic-laser-driven implosion of a preformed cylindrical shock wave structure in a gas Jesse Griff-McMahon, Julia M Mikhailova Laser-driven generation and amplification of quasistatic magnetic fields have seen an increased interest in the last few years. Using three-dimensional particle-in-cell simulations, we demonstrate the amplification of a seed magnetic field in a converging cylindrical shock-wave structure at gas densities. The shock wave is formed by ionizing a gas jet with a structured laser beam of sub-relativistic intensity (high-order Bessel beam at 1015 W/cm2) and subsequently heated by a relativistic-intensity laser pulse (>1018 W/cm2 at 800 nm wavelength). Our simulation results suggest that considerable enhancement of magnetic fields may be possible with this renewable, debris-free approach. |
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BP11.00062: Investigating Self-Induced Relativistic Transparency in Plasmas with Ultrafast High Intensity Laser Pulses Brendan Stassel, Brandon K Russell, Paul T Campbell, Hongmei Tang, Louise Willingale We model high intensity laser plasma interactions on thin film and solid targets in the self-induced relativistic transparency (RT) regime using 2D OSIRIS 4.0 particle-in-cell simulations. Intense laser fields produce relativistic electrons such that the critical density is increased and laser fields can penetrate and interact with the dense plasma regions, potentially affecting the electron heating mechanisms and efficiency. In preparation for our upcoming LaserNetUS experiment at the Scarlet Laser Facility, we model a 30 fs, 800 nm wavelength pulse with the normalized field strength a0 varied between 1 and 50. The solid target was modeled after 8CB liquid crystal (4-N-octyl-cyanobiphenyl, C21H26N). Different initial target density profiles were considered to model a pre-expanded target, while conserving the areal density for 8CB and varying the target thicknesses from 30 to 200 nm. An analysis of the data will study the transmitted and reflected laser characteristics, and electron heating. |
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BP11.00063: Structured Laser Pulses to Accentuate Signatures of Collective QED-Plasma Effects Alec Griffith, Kenan Qu, Nathaniel J Fisch An intense laser experiences a change in frequency when it passes through plasma with a time-varying plasma frequency, such as through plasma undergoing ionization. Alternatively, passing through a plasma composed of relativistic electrons, that are slowing down or accelerating , will similarly cause a change in frequency. These effects might be exploited by counterpropagating a relativistic electron beam with an intense laser in order both to produce (via a QED cascade) and to observe (via laser frequency shifts) collective behavior in a pair plasma [1]. We explore here whether introducing temporal or spatial structure to the counterpropagating laser might make these collective pair plasma effects more easily detectable. [1] Qu, K., Meuren, S. and Fisch, N.J., 2020. Signature of Collective Plasma Effects in Beam-Driven QED Cascades. arXiv preprint arXiv:2001.02590. |
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BP11.00064: On the generation of superluminal or subluminal orbital angular momentum femtosecond laser pulse Tae Moon Jeong, Sergei V Bulanov, Prokopis Hadjisolomou, Timur Esirkepov The superluminal electromagnetic radiation is one of intriguing concepts after Einstein’s relativity theory [1-2]. Recently, the superluminal behavior of a high-power ultrashort laser pulse is of great interest [3-5]. In this presentation, analytic expression for an electric field describing a superluminal/subluminal femtosecond(fs) laser focus has been presented for a fs laser pulse with an orbital angular momentum (OAM). The analytic expression for a spatio-temporal field distribution is obtained through a diffraction integral plugged by a time-dependent longitudinal chromatic aberration (LCA) under the paraxial approximation and its Fourier transformation. It shows that the speed and the pulse duration of a laser focus depends on the group delay dispersion and a chromaticity parameter determined by the LCA of a focusing optic. The expression is applied to show the generation of a several fs superluminal OAM laser focus. |
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BP11.00065: Status report on the construction of Zettawatt-Equivalent Ultrashort pulse laser System (ZEUS) at the University of Michigan Anatoly M Maksimchuk, John Nees, Galina Kalinchenko, Bixue Hou, Yong Ma, Andrew McKelvey, Tan Shi, Paul T Campbell, Andre F Antoine, Mario Balcazar, Jason A Cardarelli, Nicholas Ernst, Rebecca Fitzgarrald, Colton Graham, Qian Qian, Igor Jovanovic, Carolyn C Kuranz, Alexander G Thomas, Louise Willingale, Karl M Krushelnick The Zettawatt-Equivalent Ultrashort pulse laser System (ZEUS) is being built at the University of Michigan as the NSF mid-scale user facility. The ZEUS facility will include a repetitive dual-beamline 3 PW laser system, a 100 J programmable shape multi-ns pulse driver, three radiation shielded experimental areas and will provide unique new capabilities to explore nonlinear quantum electrodynamics, relativistic plasmas, particles acceleration, extreme laboratory astrophysics, basic plasma physics and nuclear photonics. The construction status of the ZEUS facility including building renovation, laser, beam lines, experimental chambers, radiation shielding and the future experimental plans will be reported. |
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BP11.00066: Micron-scale ambient-temperature liquid jets for high repetition rate laser-matter interactions Griffin Glenn, Christopher Crissman, Chandra Breanne Curry, Daniel Deponte, Jake Koralek, Mianzhen Mo, Franziska Treffert, Siegfried Glenzer, Maxence Gauthier To take full advantage of the growing set of PW-class lasers capable of operating at 1 Hz or greater, high repetition rate-compatible target technologies are necessary. We have developed a target for high-intensity laser interactions based on micron-scale sheet jets of ambient-temperature, low-surface-tension liquid developed at SLAC National Accelerator Laboratory.1,2 These sheets are created at the convergence of two liquid flows and can be tuned in situ to have thicknesses ranging from less than one micron to several tens of microns. Here we describe an experimental platform using these jets3 and show that they withstand 0.5 Hz operation with laser intensities exceeding 1021 W/cm2. We also illustrate the application of this platform to several topics in high-intensity laser-matter interactions, including laser-driven neutron production, streaming instabilities in laser-driven ion beams, and data-driven real-time optimization of laser-matter interactions. |
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