Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session PP8: Poster Session VI: NSTX, Field- Reversed Configurations, Other Magnetic Confinement; Intense LPI and FI; Beams and Coherent Radiation |
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Room: Hall BC |
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PP8.00001: NSTX, FIELD-REVERSED CONFIGURATIONS, OTHER MAGNETIC CONFINEMENT |
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PP8.00002: New plasma techniques for mass separation Renaud Gueroult, Nathaniel J. Fisch Plasma filters offer theoretically an advantage over other types of mass separation in that velocities can be large, elements are dissociated and the costs of ionization are small compared to the cost of chemical separation. Although most of the past studies have been devoted to isotope separation, rotating plasma configurations might also be promising techniques to discriminate elements with large mass difference too. About a decade ago, plasma mass filtering techniques were introduced in order to process nuclear waste. In the Ohkawa filter, as well as in other separation devices based on centrifugal forces, the unconfined heavy stream is collected at the radial outer surfaces, which makes more difficult the collection of the heavy elements. The new mass filter proposed here utilizes centrifugal and magnetic confinement of ions in a way similar to the asymmetric centrifugal trap. This magnetic centrifugal mass filter is shown to be more proliferation resistant than other separation technology and the separation is largely axial, rather than radial, which makes easier the collection of separated elements and reduces the overall contamination of the device. [Preview Abstract] |
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PP8.00003: Toroidal current induced by particle trapping inside a new toroidal laboratory magnetized plasma device with poloidal magnetic field induced by a central conductor Th. Pierre, X. Caron, E. Gravier, G. Antar In the laboratory toroidal device MISTOR, a poloidal field is created by a current (1200 A) flowing along a central toroidal conductor. A security factor q=1 is obtained at radial position r = 5 cm. Helium plasma is produced by electric discharge using a tungsten filament. When Bpol= 0, the whole plasma is turbulent that is a standard in a Simple Magnetized Torus. As Bpol is increased, the turbulence level decreases and a stable plasma is obtained. This is correlated with the decrease of the radial electric field. The confinement time is estimated in the afterglow decaying plasma. It increases from 50 microsec. without poloidal field to 0.5 millisec. when q=1 at mid-radius. The trapping of the particles inside the mirror-traps inherent in this topology (banana orbits) is studied. The precession of the banana orbits seems to be the dominant mechanism. The measurement of the toroidal current using a probe indicates that the electrons experience an oriented toroidal drift along the field lines. Detailed measurements of the EEDF are compared with theory. The influence of detrapping is investigated in order to estimate the bootstrap current induced in this device. [Preview Abstract] |
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PP8.00004: Fuelling and plasma flow change by compact torus injection into the STOR-M Tokamak Takumi Onchi, Yelu Liu, Mykola Dreval, David McColl, Chijin Xiao, Akira Hirose, Tomohiko Asai, Sean Wolfe The Saskatchewan TORus Modified (STOR-M) tokamak is equipped with a Compact Torus (CT) injector for tangential (toroidal) injection of a high density plasmoid at a velocity of 150 km/s. The objectives of CT injection (CTI) are to fuel the core region of tokamak and optimize the bootstrap current in future reactors by control of the plasma pressure gradient. After CTI, the line averaged density along central chord increases from $n_e \sim \times$ 10$^{12}$ to 1.5 $\times$ 10$^{13}$ [cm$^{-3}$]. Measurement of soft X-ray bremsstrahlung emission profile indicates a steeper density gradient is generated after the asymmetric density profile is formed and the profile become symmetry again in STOR-M. Intrinsic impurity ion flows have been measured with ion Doppler spectroscopy. Significant radial velocity shear from center to edge region is observed even in Ohmic discharges. The toroidal flow direction is found to depend on the plasma current direction. CTI also modifies toroidal plasma flow. The edge plasma flow increases by 5 km/s 1millisecond after CTI. During these milliseconds of time, toroidal flow shear is also increased from 214.3 to 285.7 [10$^{3} \times 1$/s]. A few milliseconds later than that time, plasma flow slows down, but plasma confinement is improved. H$\alpha $ emission decreases by 50{\%}. [Preview Abstract] |
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PP8.00005: Active feedback stabilization of the flute instability in a mirror machine using field-aligned coils Assaf Lifshitz, Ilan Be'ery, Amnon Fruchtman, Amnon Fisher, Amiram Ron Plasma confined in mirror machine is unstable even at low $\beta $, mainly because of the flute instability. One possible way to stabilize the plasma is to use active feedback to correct the plasma shape in real-time. The investigated apparatus consists of feedback coils aligned with the magnetic field, immersed in cold plasma around the hot core. When the current through the feedback coils changes, the plasma moves to conserve magnetic flux via compressional Alfven waves. An analytical model is used to find a robust feedback algorithm with zero residual currents. It is shown that due to the plasma's rotation, maximal stability is obtained with large phase angle between the perturbations' modes and the feedback integral-like term. Lastly, a 2-dimentional MHD simulation implementing the above algorithm indeed shows stabilization of the plasma with zero residual currents. [Preview Abstract] |
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PP8.00006: Overview of NSTX Facility Upgrades Status and Research Plans M. Ono The National Spherical Torus eXperiment (NSTX) is undergoing a major facility upgrade. The major mission of NSTX-U is to develop physics basis for an ST-based Fusion Nuclear Science Facility (FNSF). The ST-based FNSF has a promise of achieving high neutron fluence needed for reactor component testing with a relatively modest tritium consumption. At the same time, the unique operating regimes of NSTX-U provide high leverage to address several important issues in the physics of burning plasmas to optimize the performance of ITER. The NSTX-U program further aims to determine the attractiveness of the compact ST for addressing key research needs on the path toward a fusion demonstration power plant (Demo). The upgrade project will double the toroidal field, plasma current, and NBI heating power and increase the pulse length from 1-1.5s to 5-8s. More tangential NBI system is designed to attain full non-inductive operation. Innovative plasma start-up and ramp-up techniques without the central solenoid operation which is needed for a compact FNSF design will be explored. With higher fields and heating power, the NSTX-U plasma collisionality will be reduced by a factor of 3-6 to help explore the transport trend toward the low collisionality regimes expected in FNSF, ITER, and Demo. [Preview Abstract] |
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PP8.00007: The effect of progressively increasing lithium coatings on plasma performance, and the underlying role of collisionality, in the NSTX R. Maingi, S.M. Kaye, D.P. Boyle, J.M. Canik, T.H. Osborne, P.B. Snyder Lithium wall coatings have been shown to both improve energy confinement and eliminate ELMs in NSTX. Here, we present analysis of variable pre-discharge lithium evaporation from multiple experiments. First, a nearly continuous improvement of several discharge characteristics, e.g. reduced recycling, ELM frequency, and edge electron transport, with increasing pre-discharge lithium evaporation has been identified [Maingi, \textit{NF} \textbf{52} (2012) 083001, and refs. therein]. Profile and stability analysis has clarified the mechanism responsible for ELM avoidance and the role of lithium: lithium coatings reduce recycling, core fueling, and thus the density and its gradient near the separatrix. The temperature gradient near the separatrix is unaffected; hence the pressure gradient and bootstrap current near the separatrix are reduced, leading to stabilization of kink/peeling modes thought to be responsible for the NSTX ELMs; the ELM-free pedestal is seen to expand by 100{\%}. Furthermore, the normalized core confinement scalings are consistent with boronized wall results when viewed as a function of collisionality. *Research sponsored in part by U.S. Dept. of Energy under contracts DE-AC05-00OR22725, DE-AC02-09CH11466, DE-FC02-04ER54698, DE-AC52-07NA27344, DE-FG03-99ER54527 and DE-FG02-99ER54524. [Preview Abstract] |
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PP8.00008: Response of Electron-scale Turbulence and Thermal Transport to Continuous ExB Shear Ramping-up in NSTX Y. Ren, W. Guttenfelder, S.M. Kaye, E. Mazzucato, K.C. Lee, C.W. Domier Here we present the first observation of the change in electron-scale turbulence wavenumber spectrum (measured by a high-k scattering system) and thermal transport responding to continuous ExB shear ramping-up at the edge of a set of NSTX NBI-heated L-mode plasmas (r/a $\sim$ 0.66-0.78). We observed that as the ExB shearing rate is continuously ramped up, the ratio between the ExB shearing rate and the maximum ITG mode growth rate continuously increases and the maximum power of the measured electron-scale turbulence wavenumber spectra decreases. Meanwhile, both the electron and ion thermal transports are also reduced as long as MHD activities are not important. These observations are consistent with that some of the observed electron-scale turbulence is nonlinearly driven by ITG turbulence and its power decreases as ITG turbulence is progressively suppressed by ExB shear. Heat fluxes predicted by local nonlinear ITG simulations at different radial locations can be larger or significantly smaller than the corresponding local experimental heat fluxes depending on the local ExB shearing rate, which indicates that global effects may have to be included in future simulations. Comparison with gyrokinetic simulations of L-mode plasmas of conventional tokamaks will be also presented. [Preview Abstract] |
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PP8.00009: Survey of microinstability and simulated turbulent transport in NSTX Walter Guttenfelder, Jeff Candy, Stanley Kaye, Ronald Bell, Benoit LeBlanc, Yang Ren, Howard Yuh Linear gyrokinetic simulations demonstrate a large variety of microinstabilities are possible in NSTX. Microtearing modes are often unstable in the core region (r/a=0.5-0.8) of NBI heated H-modes. In cases without Lithium wall conditioning, the local E$\times $B shearing rates are larger than linear growth rates (r/a=0.5-0.6). Instead, the ETG instability (at electron scales) is unstable; nonlinear simulations in this region will be presented. Farther out (r/a=0.7-0.8), and in plasmas with Lithium wall conditioning, other ion scale instabilities can co-exist with, or dominate, microtearing modes. The nature of these ballooning modes is complicated and can exhibit ITG/TEM or KBM behavior depending on the MHD alpha parameter ($\alpha _{MHD} =-q^2R\nabla \beta )$. In limited cases tearing-parity ITG modes have also been identified. While non-linear simulations of these ``mixed-mode'' conditions are challenging, first attempts are underway. This work is supported by US DOE contract DE-AC02-09CH11466. [Preview Abstract] |
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PP8.00010: Thermal Transport and Characterization of Fluctuations at the Pedestal Top using Gyrokinetic Simulations in ELMy H-mode on NSTX Ahmed Diallo, S-H. Ku, W. Guttenfelder, B. LeBlanc, F. Scotti, R. Maingi, C.-S. Chang Early experimental work on NSTX has reported measurements of the spatial structure of turbulence fluctuations during an ELM cycle in the pedestal region. These measurements showed spatial structures with scales $k_{\theta}\rho_i^{ped}$ ranging from 0.1 to 0.2 propagating in the ion diamagnetic drift direction. These propagating spatial scales structures are found to have a large poloidal extent ($\sim$ 18$\rho_i^{ped}$) and are found to be consistent with ion-scale microturbulence of the type ion temperature gradient (ITG), hybrid ITG and trapped electron mode, and/or kinetic ballooning modes (KBM). Motivated by these experimental observations, we seek to identify the role of pedestal transport between type I ELMs and compare it with microturbulence-induced transport in the pedestal region. Using TRANSP, we show that both the ion and electrons heat diffusivities at the pedestal top remains unchanged between ELMs. Preliminary simulations during the last part of the ELM cycle, using XGC1 code in a delta-f mode shows localized fluctuations consistent with experimental level radial and poloidal correlation lengths. Extension of these simulations to full-f mode will be presented. In addition, other gyrokinetics simulations (e.g., GS2, GYRO) will be performed to identify the unstable modes in the pedestal top and associated heat fluxes The turbulence and neoclassical contributions to these fluxes will also be discussed. Work supported by US DOE contracts DE-AC02-09CH11466. [Preview Abstract] |
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PP8.00011: Comparison of Pedestal Stability on the Power Scrape-Off Layer Width in NSTX Travis Gray, Rajesh Maingi, Joon-Wook Ahn, Thomas Osborne, Adam McLean Recent analysis shows that the pedestal ballooning parameter, $\alpha_{MHD}$ [1] can be used as a dimensionless parameter to describe NSTX H-mode pedestals across a wide range of plasma current and power. Similar analysis has been performed on Alcator C-MOD [2] and DIII-D [3]. $\alpha_{MHD}$ can also be used to describe the pedestal changes that occur when varying amounts of pre-discharge lithium evaporation are used [4]. Varying amounts of lithium deposition has also been shown to directly effect the power scrape-off layer width, $\lambda_q$ on NSTX [5]. Preliminary results indicate a corresponding correlation between $\alpha_{MHD}$ and $\lambda_q$. This suggests that the H-mode pedestal plays a critical role in setting $\lambda_q$. [Preview Abstract] |
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PP8.00012: Impact of rotation and kinetic damping on the NSTX ideal-wall limit Jonathan Menard, Yueqiang Liu Kinetic resonances have previously been identified to play an important role in the stability of the resistive wall mode (RWM) in high-beta NSTX plasmas operating above the ideal no-wall stability limit. Under such circumstances, the ``plasma mode'' which determines the with-wall limit is typically assumed to be stable. To assess this assumption, systematic kinetic stability analysis of the with-wall limit has been carried out for the first time using the MARS-K code for NSTX. The stability of the plasma mode is found to be a sensitive function of rotation and dissipation. Specifically, calculations indicate that as the rotation is increased to experimental values in the absence of dissipation, the n=1 mode becomes unstable at the experimental nominal wall position of the wall which is inconsistent with experiment. Inclusion of strong parallel sound-wave damping does not substantially change the ideal plasma mode stability. Including perpendicular damping through precession resonances can recover plasma mode stability at high rotation, but only for small plasma-wall gap. Importantly, the inclusion of full kinetic damping model (precession + bounce + transit resonances) can significantly increase plasma mode stability consistent with the experiment. [Preview Abstract] |
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PP8.00013: Kinetic RWM Stabilization Physics and RWM State-Space Control in NSTX High Beta Plasmas J. Bialek, J. Berkery, S. Sabbagh, O. Katsuro-Hopkins, R. Betti, R. Bell, S. Gerhardt, B. LeBlanc, Y. Liu Steady-state operation of spherical torus fusion devices can be disrupted by resistive wall modes (RWMs). Present research on NSTX aims for a greater understanding of passive kinetic stabilization physics and improved active control techniques to reduce disruptions. Calculations with MISK indicate that resonance between the mode and precession of thermal ions can explain experimental marginal stability. The stabilizing effect from energetic particles depends on their anisotropic distribution. MISK has been benchmarked with other codes, including MARS-K, and the physics is shown to be equivalent through comparison of results from Solov'ev and ITER equilibria. An RWM state-space controller has been used in long-pulse discharges that have exceeded \textit{$\beta $}$_{N}$ = 6.4, and \textit{$\beta $}$_{N}$/$l_{i}$ = 13. It includes a 3D model of the unstable RWM eigenfunction and currents induced in nearby conducting structures. This model is reduced using optimal control techniques to less than 20 states for use in real-time. Effects of varying the gain matrix and feedback phase are experimentally examined. Comparisons between sensor measurements and the model show agreement with a sufficient number of states and details of the 3D wall. The system can allow for $n >$ 1 plasma control through inclusion of $n >$ 1 eigenfunctions. [Preview Abstract] |
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PP8.00014: Multiple harmonic oscillations and magnetic islands in NSTX King-Lap Wong, Ronald E. Bell, Benoit P. LeBlanc, Joel P. Hosea Multiple harmonic oscillations (MHOs) appear in the 10 - 100 kHz range in NSTX plasmas. They are observed by Mirnov coils located near the plasma edge. Their frequency spectrum has a distinct feature of multiple peaks with equal spacing $\Delta $f between neighboring peaks, i.e., the n-th peak has frequency fn = n$\Delta $f = nf1. They may last from a few to tens of milliseconds in NSTX. Since Mirnov coils have no spatial resolution, these frequently observed oscillations remain unexplained for many years. In this paper, we report the first successful explanation based on the good correlation between the measured magnetic island rotation frequency and the MHO frequencies. The island location can vary from the plasma core to the plasma edge. Enhanced electron transport appears in the vicinity of the island causing redistribution of electron thermal energy. Deterioration of global plasma confinement happens when multiple islands overlap. When the island appears at the edge of a H-mode plasma, it can suppress the edge-localized modes (ELMs) while the global energy confinement continues to improve. This is the first observation of such quiescent (no ELMs) H-mode plasma in spherical tokamaks. The unambiguous presence of magnetic islands at the plasma edge clearly reveals the trigger process for the first time. It provides a natural explanation for ELM suppression based on the known mechanism of stochastic magnetic fields. [Preview Abstract] |
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PP8.00015: Error Field in Ideal Magnetically Symmetric Tokamaks Generated through Asymmetric Shadows Cast by Neutral Beams on Divertor Floor Hironori Takahashi, Eric Fredrickson, Stefan Gerhardt The neutral beam intersects open field lines as it traverses the Scrape-Off-Layer (SOL), and casts its ``shadows'' on the divertor floor, where beam particles and heat lost in transit are deposited. These shadows are toroidally asymmetric in shape, reflecting the localized nature of the beam geometry and, unlike in the main plasma, a lack of symmetrizing field-line property (irrational surfaces) in the SOL. Thermoelectrically driven Scrape-Off-Layer Current (SOLC) due to a Te difference between these shadows is also toroidally asymmetric, and, when considered on a single flux-surface basis, generates an error field in an otherwise ideal magnetically symmetric tokamak. Spreading of the SOLC over flux surfaces has a symmetrizing effect on magnetic field produced due to field-line shear, except around a ``sweet spot'' midway between primary and secondary separatrices, necessitating calculations along the entire SOL beam path for a reliable field estimate. This study explores the possibility that error field due to a SOLC in the beam shadows may contribute to strong plasma rotation braking often observed when the SOL magnetic structure rapidly evolves in an early discharge phase. Similar considerations may apply to pellet paths, gas puff clouds, and other operational asymmetries. [Preview Abstract] |
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PP8.00016: Characterization of Disruption Halo Currents in NSTX S.P. Gerhardt, J. Breslau, E. Fredrickson, S. Jardin, R. Kaita, J. Manickam, J. Menard, S. Sabbagh, F. Scotti, H. Takahashi, A.H. Boozer Since 2007, an ever-expanding set of disruption halo current diagnostics has been installed in NSTX. These sensors include partial rogowski coils wrapped around the center column, discrete toroidal field detectors, tiles instrumented with resistive shunts, and small rogowski sensors on the PFC supports. Halo current fractions greater than 30{\%} have been measured, with the largest currents often coming from Ohmic and L-mode vertically unstable disruptions. The largest halo currents are often associated with large plasma current quench rates. Halo currents are sometimes observed to flow when q{\_}edge$>$3, but a rapid increase in the amplitude occurs when q{\_}edge drops below 2. These halo currents can have a significant toroidal asymmetry, and this asymmetry is observed to rotate toroidally. Up to 7 toroidal transits have been observed in rare cases, with 2-3 revolutions fairly common. The toroidal rotation frequencies are typically 0.5-2 kHz, though the rotation tends to be non-steady, often with a low rotation phase followed by a period of rapid rotation. The rotation frequency tends to be anti-correlated with the halo current fraction. The non-axisymmetric part of the halo current typically decays before the n=0 part, and filament modeling of the plasma indicates that the loss of the non-axiymmetric current corresponds to the time when the last closed magnetic surface vanishes. [Preview Abstract] |
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PP8.00017: MHD Calculation of halo currents and vessel forces in NSTX VDEs J.A. Breslau, H.R. Strauss, R. Paccagnella Research tokamaks such as ITER must be designed to tolerate a limited number of disruptions without sustaining significant damage. It is therefore vital to have numerical tools that can accurately predict the effects of these events. The 3D nonlinear extended MHD code M3D [1] can be used to simulate disruptions and calculate the associated wall currents and forces. It has now been validated against halo current data from NSTX experiments in which vertical displacement events (VDEs) were deliberately induced by turning off vertical feedback control. The results of high-resolution numerical simulations at realistic Lundquist numbers show reasonable agreement with the data, supporting a model in which the most dangerously asymmetric currents and heat loads, and the largest horizontal forces, arise in situations where a fast-growing ideal 2,1 external kink mode is destabilized by the scraping-off of flux surfaces with safety factor $q>2$ during the course of the VDE. \\[4pt] [1] W. Park, et al., Phys. Plasmas {\bf 6} (1999) 1796. [Preview Abstract] |
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PP8.00018: Predator-prey modeling of the coupling of co-propagating CAE to kink modes Eric Fredrickson Co-propagating Compressional Alfven eigenmodes (CAE) with shorter wavelength and higher frequency than the counter-propagating CAE and Global Alfven eigenmodes (GAE) often accompany a low frequency n=1 kink. The lower frequency CAE and GAE are excited through a Doppler-shifted cyclotron resonance; the high frequency CAE (hfCAE) through a simple parallel resonance. We present measurements of the mode structure and spectrum of the hfCAE, and compare those measurements to predictions of a simple model for CAE. The modes are bursting with a typical burst frequency on the order of a few kHz. The n=1 kink frequency is usually higher than this, but when the kink frequency does drop towards the hfCAE burst frequency, the hfCAE burst frequency can become locked with the kink frequency. A simple predator-prey model to simulate the hfCAE bursting demonstrates that a modulation of the growth or damping rate by a few percent, at a frequency near the natural burst frequency, can lock the burst frequency to the modulation frequency. The modulation of the damping rate is postulated to be through a coupling of the kink with a symmetry-breaking error field. The deeper question is how the kink interaction with a locked mode can affect the damping/growth rates of the CAE. [Preview Abstract] |
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PP8.00019: M3D-K simulations of wave-particle interaction with realistic fast ion distribution D. Liu, W.W. Heidbrink, G.Y. Fu, J.A. Breslau, M. Podesta, F. Wang, N.A. Crocker, S. Kubota The stability properties of MHD modes in the presence of fast ions are often studied with kinetic/MHD hybrid codes such as M3D-K or NOVA-K, which employs the ideal MHD description for the background plasmas and kinetic model for the fast ion kinetic response. In these codes, a simple analytic energetic particle distribution is typically used to specify the initial fast ion distribution. But this could limit the simulation accuracy especially in the cases when the analytic distribution is a poor fit to the actual fast ion distribution. In this work, an interface between the output of the tokamak fast ion Monte Carlo modeling code NUBEAM and M3D-K is developed. The realistic fast ion distribution in (R, Z, $\lambda $, E) from NUBEAM output is converted to a set of continuously and differentiable 2D cubic B-splines in the (P$_{\phi }$, E) directions with a set of discrete bins in $\mu $ direction, and then used as an input for the unperturbed fast ion distribution in M3D-K code. Initial M3D-K simulations of non-resonant kink mode in NSTX with NUBEAM fast ion distribution shows that fast ions from NBI have a weakly stabilizing effect on the mode. M3D-K simulations of TAE on NSTX with NUBEAM fast ion distribution will also be performed to study the effects of fast ion distribution on TAE growth rate, frequency and mode structure and compare with experimental data. Examples will illustrate the quality of the spline fit to the NUBEAM fast ion distribution and benefits of using the more accurate fast ion distribution. *Work supported by US DOE [Preview Abstract] |
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PP8.00020: Temporal Dynamics of NSTX NBI+HHFW Discharges using CQL3D-Hybrid-FOW R.W. (Bob) Harvey, Yu. V. Petrov, D. Liu, W.W. Heidbrink, G. Taylor, P.T. Bonoli The Fast Ion Diagnostic FIDA [1] signal resulting from neutral beam injection (NBI) and high harmonic fast wave (HHFW) RF power injected into the NSTX spherical tokamak has been simulated with the CQL3D Fokker-Planck code [2,3,4]. Motivated by a radial inward shift of the FIDA signal simulated with zero-orbit-width (ZOW) CQL3D compared to the large finite-orbit-width (FOW) NSTX experimental results [5], a 1st order correction was added which gave too large an outward shift [3]. More recent CQL3D FOW simulations based on guiding center orbits (plus gyro-width for losses) [4] produces quite accurate comparison with experiment. The modulated NBI and time-dependent background plasma variations are accounted for, also giving temporal neutron variation in rough agreement with NSTX observations.\\[4pt] [1] W. W. Heidbrink, et al. Plasma Phys. Controlled Fusion 46, 1855 (2004).\\[0pt] [2] R.W. Harvey and M. McCoy, ``The CQL3D Fokker Planck Code,'' http://www.compxco.com/cql3d.html.\\[0pt] [3] R.W. Harvey, et al., EPS, Strasbourg, Fr. (2011).\\[0pt] [4] Yu. Petrov and R.W. Harvey, this APS-DPP mtg (2012).\\[0pt] [5] M. Choi, et al., Phys. of Plasmas 17, 056102 (2010). [Preview Abstract] |
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PP8.00021: Effects of TAE avalanches and energetic particle mode bursts in NSTX on neutral beam ion confinement, loss, and current drive Douglass Darrow, Alessandro Bortolon, Neal Crocker, Eric Fredrickson, Nikolai Gorelenkov, Marina Gorelenkova, Shigeyuki Kubota, Deyong Liu, Mario Podest\`a, Roscoe White Brief ``avalanches'' of toroidal Alfv\'{e}n eigenmodes (TAEs) are observed in NSTX plasmas having several different n numbers simultaneously present. Modeling has shown that these cause stochastic loss of neutral beam ions along with rapid reduction in the energy of some beam ions. Here we make initial comparisons of the modeled distributions with confined beam ion measurements, including FIDA and ssNPA. In addition, initial results of modeled changes to the beam driven current profile will be shown. Finally, we describe efforts to apply the measurement and modeling methods to energetic particle mode bursts that are ubiquitous in the plasma current ramp up phase of NSTX discharges. [Preview Abstract] |
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PP8.00022: Numerical Simulations of NBI-driven CAE modes in H-mode Discharges in NSTX E.V. Belova, N.N. Gorelenkov, N.A. Crocker, E.D. Fredrickson Excitation of co- and counter-propagating compressional Alfven modes (CAEs) have been observed for H-mode NSTX discharges. Hybrid 3D code HYM has been used to investigate properties of beam ion driven CAE modes in NSTX. The HYM code is a nonlinear, global stability code in toroidal geometry, which includes fully kinetic ion description. Numerical simulations have been performed for the NSTX shots with strong CAE activity. It is shown that co-propagating CAE mode can be excited for large toroidal mode numbers n$\ge $8 and frequency range f$>$0.4f$_{ci}$, consistent with observations. In contrast to GAE modes, large compressional magnetic perturbation is seen both in the core and at the plasma edge for CAE modes. Conditions which are favorable for CAE instability are studied, including the effects of the plasma density profiles, beam ion parameters, and magnetic profiles. It is shown that lower energy beam ions satisfy regular resonance conditions, whereas high energy beam ions satisfy Doppler-shifted cyclotron resonance condition. Relative contributions of different groups of resonant particles, their location in the phase-space and corresponding types of particle orbits are investigated. [Preview Abstract] |
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PP8.00023: Initial predictions of linear TAE stability in NSTX-U M. Podesta, N.N. Gorelenkov, E.D. Fredrickson, S.P. Gerhardt, G.J. Kramer A second Neutral Beam (NB) injection line is being installed on the NSTX Upgrade device, resulting in six NB sources with different tangency radii available for current profile control. Optimization of NSTX-U discharges toward high performance requires an accurate knowledge of the NB driven current, especially when the behavior of the injected NB fast ions deviates from classical predictions. In particular, Toroidal Alfven Eigenmodes (TAE) destabilized by fast ions are known to affect NB driven current by inducing fast ion redistribution and loss. This work explores the stability of TAE modes for NSTX-U scenarios with various NB injection geometries, from more perpendicular to more tangential. Initial predictions, based on linear stability analysis through the ideal MHD code NOVA-K, are presented. For the scenarios considered in this work, TAE are marginally stable, with continuum damping and ion/electron Landau damping representing the dominant damping mechanisms. Because of the higher magnetic field in NSTX-U (up to 1T) with respect to NSTX, the spectrum of less stable modes shifts to higher toroidal mode numbers, peaking at n=5-8. The sensitivity of these results to parameters such as toroidal field, plasma temperature and density, and safety factor profile is also discussed. [Preview Abstract] |
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PP8.00024: Feasibility Study for Local Helicity Injection Startup in the NSTX Upgrade Device A.J. Redd, M.W. Bongard, R.J. Fonck, S.C. Jardin Helicity injection using localized current sources in the tokamak edge region can initiate and sustain plasmas without using a central solenoid. Using this technique, Pegasus Toroidal Experiment plasmas with I$_{p}\le $ 0.17 MA have been created using $\sim $4 kA of injected current, and the method appears scalable to larger devices. We explore the feasibility of using a local helicity injection system for MA-class startup on the NSTX Upgrade (NSTX-U) device. Two key aspects of this effort are: development of accurate computational models to predictively simulate NSTX-U local helicity injection discharges, through the plasma startup, growth and sustainment phases; and, development of a compact injector assembly and corresponding power supplies appropriate for deployment at NSTX-U. The predictive modeling effort uses the Tokamak Simulation Code (TSC) to test theory-based models of the detailed physics underlying helicity injection discharges against experimental Pegasus results. Effects studied include the effective toroidal current drive, the confinement/dissipation in these plasmas, and neoclassical effects in ultralow aspect ratio Pegasus plasmas. The conceptual NSTX-U injector structure includes a plasma injector with active gas control and a shaped electrode to optimize current drive with respect to the observed helicity balance and magnetic relaxation limits on plasma performance. The existing Pegasus injector is a test of both of these features. [Preview Abstract] |
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PP8.00025: Flux-surface closure during resistive-MHD simulations of Coaxial Helicity Injection (CHI) in NSTX E.B. Hooper, C.R. Sovinec, R. Raman, Fatima Ebrahimi, J.E. Menard CHI in STs offers considerable promise for generating startup plasmas, with NSTX experiments demonstrating coupling to Ohmic drive with magnetic flux savings.\footnote{R. Raman et al., Phys. Rev. Letters 104, 095003 (2010).} Success in these experiments depends in part on the achievement of flux closure following CHI voltage crowbarring. Flux closure is demonstrated here in whole-device, resistive MHD simulations using the NIMROD code. In axisymmetric plasmas significant closure due to resistive effects requires the injection slot to be narrow (e.g. 4 cm vs. 11 cm) in agreement with experiment. In simulations reduction of the applied injector flux following the crowbar forms an X-point close to the bottom of NSTX that significantly enlarges the closed volume; closure is not seen if the flux is held constant. The physics of closure will be discussed and applied to maximizing the volume. Effects of a background plasma in simulations of flux formation and closure will also be described. [Preview Abstract] |
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PP8.00026: SOL Properties of HHFW Electron Heating Generated H-modes in NSTX Joel Hosea, R.E. Bell, A. Diallo, S. Gerhardt, M.A. Jaworski, G.J. Kramer, B.P. LeBlanc, R.J. Perkins, C.K. Phillips, L. Roquemore, G. Taylor, J.R. Wilson, J-W. Ahn, T.K. Gray, R. Maingi, A. McLean, P.M. Ryan, S. Sabbagh In neutral beam generated H-modes, it has been shown that high harmonic fast wave power lost to the divertor regions flows along the magnetic field lines passing in front of the antenna [1]. Here we extend this power flow study to the case of HHFW generated H-modes [2]. Using the field strike point spiral from the Spiral code as a guide (Langmuir probe characteristics near the outer vessel strike radius are used to specify the best equilibrium for the code), it is found that for comparable launched RF powers the power loss in the outer scrape off layer (SOL) is generally much less for the HHFW generated H-mode case. Also, much of the heating in the lower divertor region is at/near the outer vessel strike radius as expected for low RF power loss in the SOL. The dependence of the loss at the outer vessel strike radius on the possible presence of ETG turbulence will be discussed.\\[4pt] [1] R. Perkins et al., to be published in Phys Rev Letters.\\[0pt] [2] J. Hosea et al, EPS Conf. Proc. (Strasbourg 2011) paper P2-098. [Preview Abstract] |
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PP8.00027: Divertor scenario development for NSTX Upgrade V.A. Soukhanovskii, A.G. McLean, E.T. Meier, T.D. Rognlien, D.D. Ryutov, R.E. Bell, A. Diallo, S.P. Gerhardt, R. Kaita, E. Kolemen, B.P. Leblanc, J.E. Menard, M. Podesta, F. Scotti In the NSTX-U tokamak, initial plans for divertor plasma-facing components (PFCs) include lithium and boron coated graphite, with a staged transition to molybdenum. Steady-state peak divertor heat fluxes are projected to reach 20-30 MW/m$^2$ in 2~MA, 12~MW NBI-heated discharges of up to 5 s duration, thus challenging PFC thermal limits. Based on the recent NSTX divertor experiments and modeling with edge transport code UEDGE, a favorable basis for divertor power handling in NSTX-U is developed. The snowflake divertor geometry and feedback-controlled divertor impurity seeding applied to the lower and upper divertors are presently envisioned. In the NSTX snowflake experiments with lithium-coated graphite PFCs, the peak divertor heat fluxes from Type I ELMs and between ELMs were significantly reduced due to geometry effects, increased volumetric losses and null-point convective redistribution between strike points. H-mode core confinement was maintained at H98(y,2)$\leq1$ albeit the radiative detachment. Additional CD$_4$ seeding demonstrated potential for a further increase of divertor radiation. [Preview Abstract] |
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PP8.00028: UEDGE modeling of NSTX and NSTX-U snowflake divertor configurations E.T. Meier, V.A. Soukhanovskii, A.G. McLean, T.D. Rognlien, D.D. Ryutov, R.E. Bell, A. Diallo, S. Gerhardt, R. Kaita, B.P. LeBlanc, J.E. Menard, M. Podesta, F. Scotti The 2D multi-fluid edge transport code UEDGE is applied to interpret NSTX snowflake divertor (SFD) experiments and to project SFD behavior in upcoming NSTX-U operation. The SFD reduces divertor target heat flux mainly by direct flux-expansion broadening of the heat flux profile and by volumetric losses due to increased connection length and larger divertor volume. Experimental SFD discharges in NSTX showed significant peak heat flux reduction as compared to standard divertor (STD) discharges ($q_{pk,SFD}\approx 1$MW/m$^2$ vs. $q_{pk,STD}\approx 7$MW/m$^2$). Core carbon impurity reduction of 30-70\% was observed in the SFD experiments. UEDGE modeling gives insight into the physics that underlie these results. For heat flux reduction,the relative contributions of direct profile broadening and volumetric effects are examined. In connection with carbon impurity reduction, the roles of sputtering reduction and carbon transport modification are studied. Also presented are results from UEDGE modeling of high-power NSTX-U scenarios in which the SFD is expected to play a key role in keeping heat fluxes below material limits. [Preview Abstract] |
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PP8.00029: Local effects of biased electrodes in the divertor of NSTX Lane Roquemore, Stewart Zweben, Michael Campanell, Brendan Lyons, Ricardo Maqueda, Yevgeny Raitses, Filippo Scotti, Hiro Takahashi One proposed solution to the problem of high scrape-off layer heat flux in tokamaks is to generate non-axisymmetric convective cells near the divertor plate to modify the local heating pattern [1]. To test this theory, four small rectangular electrodes were installed into the outer divertor plates of NSTX. When the electrodes were located near the outer strike point and biased positively, there was an prompt increase in the nearby probe currents and probe potentials and an increase in the LiI light emission at the large major radius end of these electrodes. When an electrode located farther outward from the outer strike point was biased positively, there was sometimes a significant decrease in the LiI light emission at the small major radius end of this electrode, but there were no clear effects on the nearby probes. These changes are qualitatively consistent with the expected vertical motion due to a convective cell generated by the electrodes. Possible applications of this technique to future tokamaks will be discussed. This work is supported by DOE Contracts DE-AC02-09CH11466.\\[4pt] [1] R.H. Cohen and D.D. Ryutov 1997\textit{ Nucl. Fusion }\textbf{37}, 621\\[0pt] [2] S.J. Zweben et al, Plasma Phys. Cont. Fusion \textbf{51} 105012 (2009) [Preview Abstract] |
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PP8.00030: Physics design of a cryo-pumping system for NSTX-U J.M. Canik, R. Maingi, S.P. Gerhardt, M.A. Jaworski, J.E. Menard, D.P. Stotler, E. Meier, V.A. Soukhanovskii The NSTX Upgrade is anticipated to require active particle pumping to achieve stationary conditions over increased pulse durations (up to $\sim $5 seconds), and to control the plasma density to access reduced collisionality regimes. While pumping by lithium wall coatings is also being explored, here we present an analysis of a complementary cryo-pumping system to provide particle control. A semi-analytic pumping model [Maingi, \textit{NF} \textbf{39} (1999) 1187] has been extended, and used with NSTX heat flux and divertor temperature measurements to project the particle removal rates of candidate systems. The geometry and position of the pumping volume entrance have been optimized based on these projections to provide sufficient pumping over a wide range of plasma configurations. Assuming a balance between neutral beam fueling and cryo-pumping, results indicate that low plasma densities (down to $\sim $50{\%} of the Greenwald density) can be obtained. The optimized pump is compatible with high flux expansion Snowflake divertors, which show stronger pumping than conventional geometries. Analysis of the optimized pump geometry using SOLPS [Schneider, \textit{CPP} \textbf{46} (2006) 3], which uses a rigorous neutral transport model, will be presented. [Preview Abstract] |
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PP8.00031: A mechanism for large divertor plasma energy loss via lithium radiation in tokamaks T.D. Rognlien, E.T. Meier, V.A. Soukhanovskii Lithium has been used as a wall-conditioning element in a number of tokamaks over the years, including TFTR, FTU, and NSTX, where core plasma energy confinement and particle control are often found to improve following such conditioning. Here the possible role of Li in providing substantial energy loss for divertor plasmas via line radiation is reported. A multi-charge-state 2D UEDGE fluid model is used where the hydrogenic and Li ions and neutrals are each evolved as separate species and separate equations are solved for the electron and ion temperatures. It is shown that a sufficient level of Li neutrals evolving from the divertor surface via sputtering or evaporation can induce energy detachment of the divertor plasma, yielding a strongly radiating zone near the divertor where ionization and recombination from/to neutral Li can radiate most of the power flowing into the scrape-off layer while maintaining low core contamination. A local peaking of Li emissivity for electron temperatures near 1 eV appears to play an important role in the detachment of the mixed deuterium/Li plasma. Evidence of such behavior from NSTX discharges will be discussed. [Preview Abstract] |
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PP8.00032: Collaborative Research and Development on Liquid Metal Plasma Facing Components M.A. Jaworski, T. Abrams, R. Ellis, A. Khodak, B. LeBlanc, J. Menard, M. Ono, C.H. Skinner, D.P. Stotler, G. DeTemmerman, M.A. Gleeson, A.R. Lof, J. Scholten, M.A. van den Berg, H.J. van den Meiden, T.K. Gray, S.A. Sabbagh, V.A. Soukhanovskii, J. Hu, L. Wang, G. Zuo Liquid metal plasma facing components (PFCs) provide the potential to avoid component replacement by continually replenishing the plasma-facing surface. Data during the NSTX liquid lithium divertor (LLD) campaign indicate that impurity accumulation on the static lithium resulted in a mixed-material surface. However, no lithium ejection nor substrate influx was observed during normal operation. This motivates research on flowing systems for near-term machines. Experiments on the Magnum-PSI linear test-stand and EAST tokamak have begun to explore issues related to near-surface lithium transport, surface evolution and coating lifetime for exposures of 5-10s. Technology development for a fully-flowing liquid lithium PFC is being conducted including construction of a liquid lithium flow loop and thermal-hydraulic studies of novel, capillary-restrained lithium PFCs for possible use on EAST and NSTX-U. [Preview Abstract] |
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PP8.00033: Compositional changes of lithium coatings on TZM molybdenum during plasma bombardment T. Abrams, M.A. Jaworski, R. Kaita, G. De Temmerman, M.A. Gleeson, A.R. Lof, J. Scholten, M.A. van den Berg, H.J. van der Meiden, P. Raman, D.N. Ruzic The Titanium-Zirconium-Molybdenum alloy TZM has previously been used as a metallic plasma-facing component in Alcator C-Mod is being considered for use in NSTX-Upgrade. The time evolution of lithium (Li) coatings on TZM are studied in Magnum-PSI, a linear plasma device capable of ion fluxes up to 10$^{25}$ m$^{-2}$s$^{-1}$ at electron temperatures $<$ 5 eV. A series of 5 s exposures were run on a bare TZM sample then repeated after an evaporation of 100 nm of Li. The temporal and spatial variation of neutral Li and oxygen (O) radiation were monitored using optical emission spectroscopy (OES) and a fast camera with a Li-I (671 nm) filter. The O-I (777 nm) line intensity decreased during discharges while the Li-I line intensity increased. The ionization mean free path (MFP) of Li was calculated and validations against the ADAS collisional-radiative model (CRM) will be reported. Separate measurements with a 100-1000 eV D$^{+}$ ion beam incident on Li-coated TZM were also obtained and compared with theoretical predictions of physical sputtering rates. [Preview Abstract] |
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PP8.00034: Deuterium retention via highly oxygenenated lithium coatings C.N. Taylor, K.E. Luitjohan, J.P. Allain, P.S. Krstic Lithium wall conditioning is applied in numerous fusion devices as a means of improving plasma performance. Previously, researchers suspected that these improvements have come from a direct lithium-deuterium interaction, however recent results have shown that lithium catalyzed oxygen plays a more significant role in retaining deuterium. The oxygen concentration increases from 5 at. {\%} in virgin graphite to as much as 45 at. {\%} throughout the process of retaining deuterium in lithiated graphite. This large increase of oxygen is stimulated by and during ion bombardment. While oxygen from within the bulk sample and the ambient vacuum contribute slightly to the high oxygen concentration, pre-gettered oxygen from the lithium deposit contributes more significantly as the source of oxygen. Details of lithium catalyzed oxygen-deuterium retention are presented. [Preview Abstract] |
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PP8.00035: Algorithm Development for Multi-Energy SXR based Electron Temperature Profile Reconstruction D.J. Clayton, K. Tritz, M. Finkenthal, D. Kumar, D. Stutman New techniques utilizing computational tools such as neural networks and genetic algorithms are being developed to infer plasma electron temperature profiles on fast time scales ($>$ 10 kHz) from multi-energy soft-x-ray (ME-SXR) diagnostics. Traditionally, a two-foil SXR technique, using the ratio of filtered continuum emission measured by two SXR detectors, has been employed on fusion devices as an indirect method of measuring electron temperature. However, these measurements can be susceptible to large errors due to uncertainties in time-evolving impurity density profiles, leading to unreliable temperature measurements. To correct this problem, measurements using ME-SXR diagnostics, which use three or more filtered SXR arrays to distinguish line and continuum emission from various impurities, in conjunction with constraints from spectroscopic diagnostics, can be used to account for unknown or time evolving impurity profiles [K. Tritz et al, Bull. Am. Phys. Soc. Vol. 56, No. 12 (2011), PP9.00067]. On NSTX, ME-SXR diagnostics can be used for fast (10-100 kHz) temperature profile measurements, using a Thomson scattering diagnostic (60 Hz) for periodic normalization. The use of more advanced algorithms, such as neural network processing, can decouple the reconstruction of the temperature profile from spectral modeling. [Preview Abstract] |
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PP8.00036: Utilization of passive emission contributing to charge exchange spectra in NSTX R.E. Bell, A. Diallo, B.P. Leblanc, M. Podesta, F. Scotti The passive emission of C$^{5+}$ ions from independent views is routinely measured on NSTX to subtract the passive contribution from active views across the neutrals beams used for charge exchange recombination spectroscopy. The passive emission can provide further useful information in the region of the C$^{5+}$ emitting shell near the plasma edge. Inversion of the fitted spectrum of the line-integrated C$^{5+}$ brightness from the passive views yields profiles of local ion temperature, velocity, and emissivity. Using rate coefficients for electron excitation, recombination, and thermal charge exchange, the relative contributions to the C$^{5+}$ emission and the C$^{5+}$ density profile could be uniquely determined if the local neutral deuterium density were known. Using the radial force balance equation and both active and passive measurements, the profile shape of the C$^{5+}$ density can be determined. In turn, the amplitude of the C$^{5+}$ density can be related to the neutral deuterium density profile, thereby establishing a range for the neutral deuterium density near the plasma edge. Independent information on the ratio of the C$^{5+}$/C$^{6+}$ densities, from an impurity transport code or possibly from the C$^{5+ }$emissivity profile shape, can improve the accuracy of the inferred neutral deuterium profile. [Preview Abstract] |
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PP8.00037: The NSTX-U Thomson scattering diagnostic system B.P. LeBlanc, A. Diallo, H. Feder, G. Labik, D.R. Stevens, R. Upcavage The NSTX-U upgrade consists primarily of two elements: (1) a new Center Stack (CS) of larger diameter, doubling of the toroidal field ($\le $ 1T) and quintupling the flat-top duration; (2) a second Neutral Beam Injector (NBI) doubling the NBI heating and current drive power. These two hardware modifications necessitate rerouting the laser beam path of the Multi-Point Thomson Scattering (MPTS) diagnostic: (1) In order to avoid ablating the CS tile material, the laser beam path has been re-aimed to a larger tangency radius; (2) Since a straight unobstructed laser-beam escape route is precluded by the presence of the new NBI box, a multi-mirror arrangement is being designed. Since it is unpractical to relocate the mirror collection optics to a different machine port, the new laser beam path has been kept as close as possible to the original route. Ray-tracing calculations complemented with in-situ test of principle verification have shown that the original collection optics can reconfigured to image the new laser beam path with acceptable resolution. The NSTX-U MPTS system will benefit from a recent upgrade to 42 radial channels and will continue using two 30-Hz Nd:YAG lasers, although the laser input and exit flight tubes will be given a larger bore in order to permit a straightforward addition of a third laser beam. Progress report will be given of this continuing work. [Preview Abstract] |
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PP8.00038: High-k poloidal scattering and FIR tangential interferometer/polarimeter diagnostics on NSTX-U R. Barchfeld, C.W. Domier, C.M. Muscatello, C. Robertson, N.C. Luhmann, R. Kaita, Y. Ren, B.C. Stratton The addition of a 2nd neutral beam injector on NSTX during the current shut-down period necessitates reconfiguration of the toroidal scattering and far-infrared tangential interferometer/polarimeter (FIReTIP) systems. During this shut-down period, the laser systems are undergoing laboratory characterization, maintenance, and upgrade at UC Davis. The 280GHz tangential scattering diagnostic will be reconfigured into a steerable 600GHz poloidal high-k scattering system, providing enhanced fluctuation measurements of radial and poloidal k spectra. The workhorse of the scattering diagnostic consists of a CO2-pumped FIR laser, whose output provides the probing beam. This laser system is extensively characterized, and a concept for a scattering test target is demonstrated. FIReTIP will be reconfigured as a 3-channel system, employed for core density monitoring/feedback control as well as core/edge fluctuation measurements. Besides the spatial rearrangement of the laser system, optics, and electronics, a significant upgrade to FIReTIP involves modification of its constituent Stark laser, in particular, the shape of its waveguide and electrodes. With the new design, simulations indicate improved mode quality and power output which directly translates to improved FIReTIP signal quality. [Preview Abstract] |
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PP8.00039: Reduced-order-model based feedback control of the Modified Hasegawa-Wakatani equations Imene Goumiri, Clarence Rowley, Zhanhua Ma, David Gates, Jeffrey Parker, John Krommes In this study, we demonstrate the development of model-based feedback control for stabilization of an unstable equilibrium obtained in the Modified Hasegawa-Wakatani (MHW) equations, a classic model in plasma turbulence. First, a balanced truncation is applied; a model reduction technique that has been proved successful in flow control design problems, to obtain a low dimensional model of the linearized MHW equation. A model-based feedback controller is then designed for the reduced order model using linear quadratic regulators (LQR) then a linear quadratic gaussian (LQG) control. The controllers are then applied on the original linearized and nonlinear MHW equations to stabilize the equilibrium and suppress the transition to drift-wave induced turbulences. [Preview Abstract] |
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PP8.00040: High Performance Field-Reversed Configuration Plasmas in the C-2 Device H. Gota, M. Tuszewski, A. Smirnov, H. Guo, M. Binderbauer, D. Barnes, T. Akhmetov, A. Ivanov A high temperature, stable, long-lived field-reversed configuration (FRC) plasma state has been produced in the C-2 device by dynamically colliding and merging two oppositely directed compact toroids, by biasing edge plasma near the FRC separatrix from a plasma-gun (PG) located at one end of the C-2 device, and by neutral-beam (NB) injection. The PG creates an inward radial electric field ($E_{r}<$0) which counters the usual FRC spin-up in the ion diamagnetic direction and mitigates the $n$ = 2 rotational instability without applying quadrupole magnetic fields. Better plasma centering is also obtained, presumably from line-tying to the gun electrodes. The PG produces $E\times B$ velocity shear in the FRC edge layer which may explain observations of improved transport properties The FRCs are nearly axisymmetric, which enables fast ion confinement. The combined effects of the PG and of NB injection yield a new High Performance FRC (HPF) regime with confinement times improved by factors 2 to 4 and FRC lifetimes extended from 1 to 3 ms. A second PG was newly installed at the other end of the C-2 device, and new experimental campaigns with 2 PGs have been explored. Characteristics of the HPF regime will be presented at the meeting as well as newly obtained results with 2 PGs and NBs. [Preview Abstract] |
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PP8.00041: High-Frequency Magnetic Fluctuations in Field-Reversed Configuration Plasmas with Neutral Beam Injection Matthew Thompson, Ryan Clary, Sergey Korepanov, Andrew Longman, Andy Sibley, Artem Smirnov, Michel Tuszewski The C-2 experiment [1] seeks to study the properties of field-reversed configuration (FRC) plasmas with significant super-thermal ion populations generated by neutral beam injection. Magnetic field fluctuations near the C-2 fast-ion cyclotron frequency ($\sim $ 650 kHz) are of great experimental interest since they are strongly coupled to the fast-ion population. The data acquisition system of the C-2 experiment's magnetic diagnostic suite [2] was recently upgraded to a sampling rate of 60 MS/s in order to study high-frequency oscillations in the magnetic field outside the FRC. Preliminary observations show significant field fluctuation amplitude near the fast-ion cyclotron frequency that varies in time and appears correlated to other signals indicative of the fast-ion population. \\[4pt] [1] M. Tuszewski et al., Phys. Rev. Lett. 108, 255008 (2012)\\[0pt] [2] M. C. Thompson et al., Rev. Sci. Instrum. 83, 10D709 (2012) [Preview Abstract] |
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PP8.00042: Interaction of Fast Ions with Global Plasma Modes in the C-2 Field Reversed Configuration Experiment Artem Smirnov, Sean Dettrick, Ryan Clary, Sergey Korepanov, Matthew Thompson, Erik Trask, Michel Tuszewski A high-confinement operating regime [1] with plasma lifetimes significantly exceeding past empirical scaling laws was recently obtained by combining plasma gun edge biasing and tangential Neutral Beam Injection (NBI) in the C-2 field-reversed configuration (FRC) experiment [2, 3]. We present experimental and computational results on the interaction of fast ions with the n=2 rotational and n=1 wobble modes in the C-2 FRC. It is found that the n=2 mode is similar to quadrupole magnetic fields in its detrimental effect on the fast ion transport due to symmetry breaking. The plasma gun generates an inward radial electric field, thus stabilizing the n=2 rotational instability without applying the quadrupole magnetic fields. The resultant FRCs are nearly axisymmetric, which enables fast ion confinement. The NBI further suppresses the n=2 mode, improves the plasma confinement characteristics, and increases the plasma configuration lifetime [4]. The n=1 wobble mode has relatively little effect on the fast ion transport, likely due to the approximate axisymmetry about the displaced plasma column. \\[4pt] [1] M. Tuszewski \textit{et al.}, Phys. Rev. Lett. 108, 255008 (2012).\\[0pt] [2] M. Binderbauer \textit{et al.}, Phys. Rev. Lett. 105, 045003 (2010).\\[0pt] [3] H.Y. Guo \textit{et al.}, Phys. Plasmas 18, 056110 (2011).\\[0pt] [4] M. Tuszewski \textit{et al.}, Phys. Plasmas 19, 056108 (2012) [Preview Abstract] |
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PP8.00043: Charge separation and transport of the n=2 instability in C-2 FRC plasmas Bihe Deng, Xuan Sun, Michel Tuszewski Charge separation is critical in the positive feedback loop for gravitational type instabilities to grow [1], such as in the case of the n=2 mode in the C-2 field reversed configuration (FRC) experiment [2]. A fast time response Langmuir probe with minimum perturbation to the plasma is inserted into the edge of the C-2 plasma to measure the plasma floating potential. With the combined plasma wobble motion and spin motion, 2-D scans of the plasma floating potential are obtained, and evidence of charge separation associated with the n=2 instability is observed. The transport due to charge separation is estimated. Charge neutralization can provide an alternative method to stabilize the n=2 mode. An experiment is proposed to test this method with two probes inserted into the plasma from two azimuthally separated ports and an external shorting circuit, to constantly neutralize the charge separation, thus suppress the growth of the n=2 mode. \\[4pt] [1] R.J. Goldston and P.H. Rutherford, \textit{Introduction to Plasma Physics} (Institute of Physics Publishing, Bristol, 2000).\\[0pt] [2]. M.W. Binderbauer \textit{et al}, Phys. Rev. Lett. \textbf{105,} 045003 (2010). [Preview Abstract] |
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PP8.00044: Impurity behavior in high performance FRC plasmas in the C-2 device Deepak Gupta, J. Douglass, H. Gota, A. Qerushi, M. Tuszewski, A. VanDrie In the C-2 FRC plasma, low impurities densities are essential to keep the radiation losses low as well as to achieve stable high performance FRC (HPF) plasma operation. In C-2, Titanium gettering, primarily used to reduce the wall recycling and neutrals, helps to significantly reduce impurities, namely carbon and oxygen. Therefore, these impurities provide a measure of wall conditioning, which is essential for HPF plasma operation. Time-dependent radiation measurements and modeling are needed for C-2 plasma, particularly to account for high-Z impurities including Titanium. Long-lived HPF plasmas with high confinement times also demand an exploration of poorly understood impurity transport in FRCs. Multiple diagnostics, related to impurities measurements, are available on C-2 (e.g., Survey spectrometers, Bolometer arrays, Bremsstrahlung arrays, various line emission monitors, fast-cameras with filters and Residual Gas Analyzers). Preliminary experimental results, based on Bolometer and Bremsstrahlung diagnostics, suggest an increase of impurity density with time during HPF plasma discharges. Experiments are also done with different gas species and impurity injections. Studies and developments are underway to further understand the role and behavior of impurities in C-2 FRC plasmas. [Preview Abstract] |
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PP8.00045: Upgrades to the Tri Alpha Energy Neutron Detector System [1] Eusebio Garate, Ian Allfrey, Travis Valentine, Vijay Patel, Artem Smirnov, Sergey Korepanov, Ryan Clary, Matthew Thompson We are implementing additional scintillator based neutron detectors on the Tri-Alpha Energy C2 system. The current He3 and scintillator detectors are used to estimate the total neutron emission from C2 and the corresponding ion temperature. The new detectors consist of BC-408 scintillator rod directly coupled to a Hamamatsu fine mesh PMT which can be placed in a magnetic field up to 1.5T with little loss in gain. The photocathode of the PMT is gated in order to avoid saturation effects due to high neutron flux during the collision of the compact toroids that form the C2 field reversed configuration. The detectors will be used to observe deuterium beam slowing down times in the C2 target plasma and to correlate neutron detection with C2's neutral particle analyzers, bolometers and fast magnetic probes. We will present a schematic of the detectors and preliminary results. \\[4pt] [1] E. Ruskov, et al., TAE neutron detectors and some physics results, \textit{Bull. Am. Phys. Soc.} \textbf{55}, 15, GP9.00103 (2010). [Preview Abstract] |
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PP8.00046: Simulations And Observations Of Radiation In the Electron Cyclotron Frequency Range On the C-2 Device E. Trask, E.P. Garate, R.W. Harvey, Yu. Petrov Several different antennas have recently been fielded on Tri Alpha Energy's C-2 experimental system to measure radiation emission in the range of 1-60 GHz, where the fundamental electron cyclotron frequency is approximately 2.5 GHz at the edge of the field-reversed configuration (FRC). Initial power measurements are correlated with changes in plasma parameters, and have magnitudes consistent with known electron temperatures and densities. Genray simulations of rays launched above the edge electron cyclotron frequency indicate that externally launched waves couple to the plasma in the OXB scheme and may damp inside the separatrix. [Preview Abstract] |
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PP8.00047: Flux-Coil Generated, Field-Reversed Configuration Frank Wessel, Thomas Roche, Nathan Bolte, Michl Binderbauer, Mark Morehouse, Norman Rostoker, Mikhail Slepchenkov We study two methods of forming a FRC: 1) pre-ionizing a static-gas fill, and 2) using a pre-injected plasma. A 0.6-m dia $\times$ 2-m pyrex vessel is configured with an external coil, to provide the bias-B$_z$ field, and a central, flux-coil to form the FRC diamagnetically. Diagnostics include: tomography, spectroscopy, interferometry, magnetic-probe arrays, particle time-of-flight, triple probe. The measured plasma parameters are: $n = 0.1-1 \times 10^{14}$ cm$^{-3}$, $T_i = 10-50$ eV, $B_{z0} \leq$ 20 mT and indicate that the FRC radial profile is a rigid-rotor,\footnote{N. Rostoker and A. Qerushi, Phys. Plasmas, 9(7):3057(2002)} $n(r) = n_0/ \cosh^2[(r^2-r_0^2)/r_0\Delta r]$, $B(r) = -B_0[1+\sqrt{\beta}\tanh[(r^2-r_0^2)/r_0\Delta r)]$, where $r_0$ is the magnetic-null radius, and $\Delta r$ is the FRC half-width. Data suggest that the sign of $E_r$ and the dominant-current carrying species are opposite to previous reports\footnote{W. S. Harris, E. Trask, T. Roche, et.al., PRL, 70(12):1818(2009)} and derive from different collisionality and magnetization regimes, resulting from the methods of formation. When $E_r$ points toward the magnetic field null, tomography indicates that the (ion) radial-diffusion coefficient is sub-classical and confining for ions. [Preview Abstract] |
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PP8.00048: FRC edge physics using NIMROD Ales Necas, Richard Milroy We have studied issues related to edge physics of the HPF regime FRC [1] using the NIMROD extended MHD code [2]. By modifying the end boundary conditions, we impose an electrostatic bias representing the experimentally applied bias voltage. The resulting self-consistent electric potential is a flux quantity ($i.e.$ follows field lines along the machine) as observed experimentally. Further, we observe that the open plasma rotation is controlled by the magnitude of the applied boundary potential. We also observe the penetration of this edge rotation into the closed field FRC by viscosity. Transport is affected by the resulting sheared rotation. Various rotationally related modes are observed and discussed. \\[4pt] [1] M. Tuszewski, \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{108}, 255008 (2012).\\[0pt] [2] C. R. Sovinec, \textit{et al.}, \textit{J. Comp. Phys.} \textbf{195}, 355 (2004). [Preview Abstract] |
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PP8.00049: ABSTRACT WITHDRAWN |
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PP8.00050: Rotational stability of a long, high-beta, field-reversed column D.C. Barnes, L.C. Steinhauer, J.P. Freidberg Rotationally driven modes are observed to be important in determining FRC stability and confinement. As a first model, we consider a long, rotating column. The $B$ is in the axial direction and the pressure gradient and centripetal acceleration are balanced by a strong radial gradient of $B$, which may pass through zero and reverse on axis, as in an FRC. The non-reversed version of this problem was considered earlier\footnote{J. P. Freidberg and L. D. Pearlstein, \textit{Phys. Fluids} \textbf{21}, 1207 (1978).} using an approximate solution of the ion kinetic equation. We simplify the present analysis by assuming incompressible motion of the plasma and including the gyro-viscous (GV) stress to construct an eigenvalue problem. This leads to a formally symmetric o.d.e. which contains the eigenfrequency in a complicated manner. Finite axial wavenlength is included to leading order. Regular solutions, which satisfy the outer boundary condition at a conducting wall, give normal modes, and are found by the shooting method. We show that this procedure reproduces the results of Ref. 1 and generalizes to more complicated equilibria having rotational shear and field reversal. In the case of field reversal, it is necessary to employ a GV form that is appropriate for small $B$. For this we use either the collisional form\footnote{N. Iwasawa, A. Ishida, L. C. Steinhauer, \textit{Phys. Fluids} \textbf{B8}, 1240 (2001).} or a new low-collisionality form which accounts for the de-magnetization of the ion orbits at small $B$. Results for FRC's with and without strong shear are presented. [Preview Abstract] |
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PP8.00051: First operation of the PFRC-2 device Samuel Cohen The PFRC-2 is a field-reversed-configuration device built, with ARRA funding, to investigate long-pulse plasma behavior and ion heating by rotating magnetic fields at a ratio of ion cyclotron frequency to RMF frequency near 1/5. Using high-temperature superconducting passive coils internal to the PFRC-2 vessel, we have produced stable plasma discharges over 100 ms in duration with a heating power of 15 kW. Characteristics of the discharges will be described. [Preview Abstract] |
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PP8.00052: Analysis and measurement of the 3D magnetic field in a rotating magnetic field driven FRC K.M. Velas, R.D. Milroy A translatable three-axis probe was installed on TCSU shortly before its shutdown. The probe has 90 windings that simultaneously measure B$_{r}$, B$_{\theta}$, and B$_{z}$ at 30 radial positions. Positioning the probe at multiple axial positions and taking multiple repeatable shots allows for a full r-z map of the magnetic field. Probe measurements are used to calculate the end-shorting torque and the rotating magnetic field (RMF) torque. The torque applied to the plasma is the RMF torque reduced by the shorting torque. An estimate of the plasma resistivity is made based on the steady state balance between the applied torque and the resistive torque. The steady state data from applying a 10 kHz low pass filter used in conjunction with data at the RMF frequency yields a map of the full 3D rotating field structure. Data from even- and odd-parity experiments will be presented. The NIMROD code has been adapted to simulate the TCSU experiment using boundary conditions adjusted to match both even- and odd-parity experimental conditions. A comparison of the n=0 components of the calculated fields to the 3-axis probe measurements shows agreement in the magnetic field structure of the FRC as well as in the jet region. [Preview Abstract] |
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PP8.00053: Improvements in FRC Stability and Confinement by Neutral Beam Injection in the TS-4 Device Toru Ii, Michiaki Inomoto, Keii Gi, Toshiyuki Umezawa, Taichi Ito, Yasuhiro Kaminou, Yasushi Ono The first experimental investigation of tangential neutral beam injection (NBI) application on oblate field-reversed configurations (FRCs) has been conducted in the TS-4 device. The low-$n$ modes, which cause destructive instabilities in FRCs produced from light gases, are stabilized by the NB fast ions to prolong the FRC lifetime significantly. FRCs produced from heavier gases such argon show better stability against the low-$n$ modes due to kinetic or two-fluid effects. The reduced total loss power of less than 5 MW indicates that 0.6 MW NBI not only heats FRC plasma but also changes the equilibrium and transport properties. The NBI also provides modifications not only on pressure profiles but also on current density ones, suggesting that the diamagnetic plasma current is spontaneously driven by the modified pressure profile in the NB-injected FRC. These results indicate that the utilization of NBI brings about improvement of FRC confinement by active control of pressure and current density profiles as well as electron heating. [Preview Abstract] |
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PP8.00054: Spectroscopic Measurement of Ion Flow During Merging Start-up of Field-Reversed Configuration Hirotaka Oka, Michiaki Inomoto, Hiroshi Tanabe, Masanobu Annoura, Yasushi Ono, Koshichi Nemoto The counter-helicity merging method [1] of field-reversed configuration (FRC) formation involves generation of bidirectional toroidal flow, known as a ``sling-shot.'' In two fluids regime, reconnection process is strongly affected by the Hall effect [2]. In this study, we have investigated the behavior of toroidal bidirectional flow generated by the counter-helicity merging in two-fluids regime. We use 2D Ion Doppler Spectroscopy to mesure toroidal ion flow during merging start-up of FRC from Ar gas. We defined two cases: one case with a radially pushed-in X line (case I) and the other case with a radially pushed-out X line(case O). The flow during the plasma merging shows radial asymmetry, as expected from the magnetic measurement, but finally relaxes to a unidirectional flow in plasma current direction in both cases. We observed larger toroidal flow in the plasma current direction in case I after FRC is formed, though the FRC in case O has larger magnetic flux. These results suggest that more ions are lost during merging start-up in case I. This selective ion loss might account for stability and confinement of FRCs probably maintained by high energy ions.\\[4pt] [1] Y. Ono, et al., Nucl. Fusion 39, pp. 2001-2008 (1999).\\[0pt] [2] M. Inomoto, et al., Phys. Rev. Lett., 97, 135002, (2006) [Preview Abstract] |
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PP8.00055: Microwave Emission Observations on the Maryland Centrifugal Experiment Remington Reid Microwave emission in the electron cyclotron range has been observed axially and radially on the Maryland Centrifugal Experiment. Microwave emission is measured in two bands, 8.5-12.5 GHz and 30.0-40.0 GHz using standard superheterodyne detectors with a noise temperature around 4 eV. If the plasma density is less than $10^{19}/m^3$ then the axial emission is consistent with cyclotron emission in the whistler mode. However there is evidence that the signal is contaminated by reflections. The radial emission is consistent with electron Bernstein emission, appearing at the second harmonic of the cyclotron frequency and is polarized perpendicular to the magnetic field. [Preview Abstract] |
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PP8.00056: Overview of HIT-SI Progress and Results B.S. Victor, C. Akcay, B.T. Doty, D.A. Ennis, R.P. Golingo, C.J. Hansen, A.C. Hossack, T.R. Jarboe, K.D. Morgan, B.A. Nelson, R.J. Smith, G.J. Marklin The HIT-SI program investigates the formation and sustainment of toroidal current in a simply-connected volume through the use of inductive helicity injectors. High current amplification shots, with toroidal currents up to 3 times the injector currents [B.S.~Victor \textit{et al.}, Phys.~Rev.~Lett. 107, 165005 (2011)], have led to new insights into the current drive process. The Imposed-Dynamo Current Drive (IDCD) model describes a process where the toroidal current is driven directly by injector driven fluctuations [T.R.~Jarboe \textit{et al.}, Nucl.~Fusion (2012)]. The IDCD model accurately predicts the toroidal current and $\lambda$ profile. Internal and surface magnetic field measurements show qualitative agreement with computational results from 2-fluid simulations using NIMROD. Simultaneous measurements of ion temperature and velocity data along two linear arrays of 36 fibers will be presented. Progress on HIT-SI3 and a new Thomson scattering system is given. Recent operations have increased the injector driving frequency from 14.5 to near 40 kHz. Higher frequency operations are analyzed with data from an FIR interferometer, IDS, magnetic probes, a high-speed imaging system and a Langmuir probe. [Preview Abstract] |
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PP8.00057: Measurements of Ion Temperature and Velocity in HIT-SI with Comparison to NIMROD Calculations, Development of Piezoelectric Valve A.C. Hossack, C. Akcay, T.R. Jarboe, J.A. Rogers, A.M. Kirkpatrick, R.J. Smith A one meter ion Doppler spectrometer has been upgraded to multichord capability using an image intensifier and high-speed camera. Two linear arrays of 36 fibers each simultaneously collect light across a toroidal and poloidal section of HIT-SI. Temperature and velocity data will be presented and compared with NIMROD calculation results. Additionally, a fast, piezoelectric valve has been developed which achieves a gas flow rates of over 325 Torr liters per second and response time of approximately 0.5 ms. Flow rate is proportional to voltage, so the valve will offer arbitrary fueling profile for HIT-SI3. Work supported by USDoE and ARRA. [Preview Abstract] |
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PP8.00058: Design of Thomson Scattering Diagnostic for HIT-SI Kyle Morgan, Taylor Fryett, Raymond Golingo, Tom Jarboe, Brian Victor Steady Inductive Helicity Injection (SIHI) is used to create a spheromak inside the HIT-SI machine. A multi-point Thomson scattering diagnostic has been designed and is under construction for the HIT-SI experiment. The system uses a 20J Ruby Laser with 20ns pulse length. The collection system allows for eight spatial measurement locations, with four being active at any time. Four polychromators are being used to spectrally resolve the scattered light. Present Langmuir probe measurements show an electron temperature of about 12eV, within the range the polychromators can resolve. Properties of system and expected measurement are given. [Preview Abstract] |
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PP8.00059: Extended MHD NIMROD Simulations of HIT-SI plasmas Cihan Akcay, Charlson Kim, Tom Jarboe, Brian Nelson We present results from zero $\beta$ two-fluid MHD (2fl-MHD) NIMROD calculations of the Helicity Injected Torus with Steady Inductive helicity injection (HIT-SI). HIT-SI uses two semi-toroidal helicity injectors oscillated out of phase to generate and sustain toroidal plasmas via steady inductive helicity injection (SIHI). All the plasma-facing walls of the experiment are coated with an insulating material to guarantee an inductive discharge. The helicity injectors are simulated as oscillating normal magnetic and parallel electric field boundary conditions with odd toroidal symmetry. A highly resistive edge-layer approximates the insulating walls. The Prandtl number ($Pm = 10$), and Lundquist number ($S = 10^3-10^4$) closely match the experimental values. 2fl-MHD calculations produce more toroidal current ($I_{tor}$) and faster growth rates than their resistive MHD (rMHD) counterparts. An energetics analysis indicates 2fl-MHD dynamo channels more energy into the axisymmetric mode than the MHD dynamo. The simulation results show good agreement with internal and surface magnetic measurements. Singular value decomposition indicates the calculations mostly capture the spatial eigenmode structure of the experiment. Simulation output is also comped with chord-averaged ion velocities. [Preview Abstract] |
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PP8.00060: Equilibrium and Stability Calculations in HIT-SI Chris Hansen, George Marklin, Thomas Jarboe The PSI-TET equilibrium code solves for solutions to the Ideal MHD equilibrium equation $\mu_0 j = \lambda B$ in arbitrary 3D geometry. A mimetic discretization on a tetrahedral mesh is employed, with up to 3rd order spatial representation. Geometric and polynomial multigrid along with a hybrid MPI/OpenMP parallelism model is used to provide solver scalability. Lambda is allowed to vary across flux surfaces but must be constant in stochastic regions. A scalar flux surface variable is computed by solving an artificial diffusion problem with a large ratio of parallel to perpendicular thermal conductivity. A fixed lambda profile, specified as a function of this flux surface variable, is defined. Equilibria in HIT-SI have been computed for the homogenous (spheromak) and inhomogeneous (injector) fields separately. Combined equilibria of interest with injector driving have also been computed for various lambda profiles. A linearized Ideal MHD module has been developed to evaluate the stability properties of computed equilbria. Equilibrium states and stability analysis will be presented for a range of lambda profiles. Results will also be presented comparing linear to high order Mimetic representations and Mimetic to standard nodal finite element representations. Work supported by DOE. [Preview Abstract] |
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PP8.00061: Progress on the HIT-SI3 Experiment R.P. Golingo, J.A. Rogers, D.A. Ennis, C.J. Hansen, A.C. Hossack, T.R. Jarboe, B.A. Nelson, G.J. Marklin, B.S. Victor The HIT-SI program at the University of Washington is investigating the formation and sustainment of toroidal current using steady inductive helicity injection (SIHI). The HIT-SI experiment consists of a bow tie shaped flux conserver with two helicity injectors on opposite sides of the machine. All plasma facing surfaces are coated with an insulting material to prevent arcing. The injectors are driven 90 degrees out of phase leading to a constant rate of helicity injection. Presently toroidal currents up to three times the injector current are created in the device. Discoveries made during the operation have guided the design of an improved injection geometry. Three injectors, which will operate with 120 degree phase differences, are presently being assembled to be mounted on one side of the device. These injectors will have the same preferred direction and produce a rotating structure with less $n$=1 than presently used. The injectors will operate at a higher frequency. The IDCD model predicts this will lead to a further reduction of the perturbations in the confinement volume. Additional diagnostic access has been gained by placing the injectors on one of the device. The present assembly status of the injectors and additional diagnostics for HIT-SI3 will be presented. [Preview Abstract] |
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PP8.00062: Imposed-Dynamo Current Drive (IDCD) Thomas Jarboe, Brian Victor, Brian Nelson, Chris Hansen, Cihan Akcay, David Ennis, Aaron Hossack, George Marklin, Roger Smith A mechanism for Steady Inductive Helicity Injection (SIHI) current drive has been discovered where the current driving fluctuations are not generated by the plasma but rather are imposed by the injectors.[T.R. Jarboe \textit{et al.}, accepted for publication in \textit{Nuclear Fusion}] Sheared flow of the electron fluid distorts the imposed fluctuations to drive current. The model accurately predicts the time dependent toroidal current, the injector impedance scaling, and the profile produced in the HIT-SI experiment. These results show that a stable equilibrium can be efficiently sustained with imposed fluctuations and the current profile can, in principle, be controlled. Both are large steps for controlled fusion. Some of the effects of the fluctuations on the confinement of tokamak and spheromak reactors are assessed and the degradation may be tolerable since the required fluctuations, in a reactor, are low (deltaB/B about 0.0001). A larger experiment (HIT-PoP) designed to test the confinement properties of a plasma sustained by IDCD is discussed. Since these very low fluctuation levels can provide current drive for the entire plasma the effect of random fluctuations on the plasma current profile is extremely important. The mechanism is also of interest to plasma self-organization, fast reconnection and plasma physics in general. [Preview Abstract] |
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PP8.00063: Extended MHD Simulations of Spheromaks E.C. Howell, C.R. Sovinec Nonlinear extended MHD simulations of a spheromak in a cylindrical flux conserver are performed using the NIMROD code (JCP 195, 2004). An idealized series of simulations starting from a Grad-Shafranov equilibrium and small non-axisymmetric perturbations are performed to model the sustained decay phase. The resulting confinement leads to steep resistivity gradients. Strong current gradients develop, driving tearing modes that dominate the evolution of the spheromak. Absent in these simulations are the remains of n=1 fluctuations created during the formation process. A second series of simulations start from vacuum fields and model the full spheromak evolution, including the formation process where the n=1 fluctuations dominate. To understand the role of pressure driven instabilities in the evolution of the spheromak, a numerical diagnostic is developed to calculate the Mercier stability criterion from the axisymmetric fields. [Preview Abstract] |
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PP8.00064: Measurements of dynamo electric field and momentum transport induced by fluctuations on HIST H. Hirono, T. Hanao, T. Hyobu, K. Ito, K. Matsumoto, T. Nakayama, Y. Kikuchi, N. Fukumoto, M. Nagata Coaxial Helicity injection (CHI) is an efficient current-drive method used in spheromak and spherical torus (ST) experiments. It is an important issue to investigate dynamo effect to explore CHI current drive mechanisms. To establish the dynamo model with two-fluid Hall effects, we verify the parallel mean-field Ohm's law balance. The spatial profiles of the MHD/Hall dynamo electric fields are measured by using Mach probe and Hall probe involving 3-axis magnetic pick-up coils. The MHD/Hall fluctuation-induced electromotive forces are large enough to sustain the mean toroidal current against the resistive decay. We have measured the electron temperature and the density with great accuracy by using a new electrostatic probe with voltage sweeping. The result shows that the electron temperature is high in the core region and low in the central open flux column (OFC), and the electron density is highest in the OFC region. The Hall dynamo becomes more dominant in a lower density region compared to the MHD dynamo. In addition, the fluctuation-induced Maxwell and Reynolds stresses are calculated to examine the fast radial transport of momentum from the OFC to the core region during the dynamo drive. [Preview Abstract] |
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PP8.00065: Propagation analysis of the helicity-drive Alfven wave in the HIST spherical torus plasmas T. Hyobu, T. Hanao, H. Hirono, K. Ito, K. Matsumoto, T. Nakayama, Y. Kikuchi, N. Fukumoto, M. Nagata Coaxial Helicity Injection is an efficient current-drive method used in spherical torus experiments. It is a key issue to investigate the dynamo mechanism required to maintain the plasmas. The behavior of a low frequency Alfven wave being possibly related to the dynamo current drive has been studied on HIST. The observed magnetic fluctuation with about 80 kHz propagates along the open flux column (OFC) region, spreading toward the core region. The parallel phase velocity is estimated at 321 km/s from the propagation velocity measured axially along the OFC. The parallel phase velocity agrees well to the Alfven velocity. The radial perpendicular propagation of the Alfven wave can be calculated by a theory based on cold or warm plasma approximation with the Hall term. The theoretical calculation indicates that there are two resonance points and is a cut-off point. These resonance and cut-off points agree well with the magnetic measurement. A part of fluctuation propagates slowly beyond the first resonance point. The wave polarization is left-handed near the resonance point and then converts to be nearly liner outside the resonance point. From these results, we speculate that the torsional Alfven wave evolves to the kinetic Alfven wave during the radial propagation. [Preview Abstract] |
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PP8.00066: Investigation of internal magnetic structures and comparison with two-fluid equilibrium configurations in the multi-pulsing CHI on HIST T. Nakayama, T. Hanao, H. Hirono, T. Hyobu, K. Ito, K. Matsumoto, Y. Kikuchi, N. Fukumoto, M. Nagata, T. Kanki Spherical torus (ST) plasmas have been successfully maintained by Muti-pulsing Coaxial Helicity Injection (M-CHI) on HIST. This research object is to clarify relations between plasma characteristics and magnetic flux amplifications, and to compare magnetic field structures measured in the plasma interior to a flowing equilibrium calculation. Two-dimensional magnetic probe array has been newly introduced nearby the gun muzzle. The initial result shows that the diverter configuration with a single X-point can be formed after a bubble burst process of the plasma. The closed magnetic flux is surrounded by the open magnetic field lines intersecting with the gun electrodes. To evaluate the sustained configurations, we use the two-fluid equilibrium code containing generalized Bernoulli and Grad-Shafranov equations which was developed by L.C. Steinhauer. The radial profiles of plasma flow, density and magnetic fields measured on the midplane of the FC are consistent to the calculation. We also found that the poloidal shear flow generation is attributed to ExB drift and ion diamagnetic drift. In addition, we will study temporal behaviors of impurity lines such as OV and OVI during the flux amplification by VUV spectroscopic measurements. [Preview Abstract] |
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PP8.00067: Measurements of Doppler-ion temperature and flow in the multi-pulsing CHI experiment on HIST T. Hanao, M. Ishihara, H. Hirono, T. Hyobu, K. Ito, K. Matsumoto, T. Nakayama, Y. Kikuchi, N. Fukumoto, M. Nagata The steady-state current sustainment of spherical torus (ST) configurations is expected to be achieved by Multi-pulsing Coaxial Helicity Injection (M-CHI) method. In the double-pulsing discharges, the plasma current can be sustained much longer against the resistive decay compared to the single CHI. The M-CHI has capabilities as a static ion heating method. Ion Doppler Spectrometer (IDS) measurements confirmed a significant increase in the ion temperature after the second CHI pulse. The ion heating mechanism is an important issue to be explored in the M-CHI experiments. It is considered due to the magnetic reconnection process of plasmoids and/or the damping of the Alfven wave. The ion heating becomes suppressed around the separatrix layer in the high field side where the amplitude of the magnetic fluctuations is minimized due to the poloidal flow shear. The shear flow generation is caused by ExB drift and ion diamagnetic drift. The contribution from the diamagnetic drift on the shear flow can be evaluated by measuring the flow velocity of hydrogen and impurity ions by using Mach probe and IDS. We will discuss the dependence of the ion heating characteristics on the variation of the density gradient by varying TF coil current. [Preview Abstract] |
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PP8.00068: Effects of the current boundary conditions at the plasma-gun gap on density in SSPX Roman Kolesnikov, L.L. LoDestro, W.H. Meyer The Sustained Spheromak Physics Experiment (SSPX) was a toroidal magnetic-confinement device without toroidal magnetic-field coils or a central transformer but which generated core-plasma currents by dynamo processes driven by coaxial plasma-gun injection into a flux-conserving vessel. Record electron temperatures in a spheromak ($T_e\sim500$eV) were achieved, and final results of the SSPX program were reported in [1]. Plasma density, which depended strongly on wall conditions, was an important parameter in SSPX. It was observed that density rises with $I_{\textrm{gun}}$ and that confinement improved as the density was lowered. Shortly after the last experiments, a new feature was added to the Corsica code's solver used to reconstruct SSPX equilibria. Motivated by $n=0$ fields observed in NIMROD simulations of SSPX, an insulating boundary condition was implemented at the plasma-gun gap. Using this option we will perform new reconstructions of SSPX equilibria and look for correlations between the location of the separatrix (which moves up the gun wall and onto the insulating gap as $I_{\textrm{gun}}$ increases) and plasma density and magnetic-flux amplification [2].\\[4pt] [1] H. S. McLean, APS, DPP, Dallas, TX, 2008.\\[0pt] [2] E. B. Hooper et al., Nucl. Fusion 47, 1064 (2007). [Preview Abstract] |
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PP8.00069: Effects of multi-pulsed coaxial helicity injection on dynamics of spherical torus T. Kanki, M. Nagata, Y. Kagei The mechanism to rebuild the magnetic fields and to amplify the currents in the high-$q$ spherical torus (ST) by the multi-pulsed coaxial helicity injection is investigated using the resistive nonlinear 3D-MHD simulations. During the driven phase, the dynamics is almost axisymmetric because the magnetic fluctuation level of $n$=0 mode compared with other higher modes is much larger. The toroidal current $I_{t}$ is effectively amplified due to the merging of plasmoid ejected from the gun region with the pre-existing ST in the confinement region. The poloidal flux is not significantly amplified because the current sheet generated by the merging process does not rapidly decay. The negative toroidal flow $v_{t}$ is then induced in the direction of $I_{t}$ around the central open flux column (OFC) region by inductive toroidal electric field $E_{t }$(=-$v_{z}B_{r})$ because of the plasmoid ejection. The strong poloidal flow $v_{z}$ (=$E_{r}B_{t})$ is also driven from the gun to confinement region due to the Lorentz force. As the result of $v_{z}$, the flow vortices associated with the dynamo effect are caused around the upper confinement region. During the decay phase, the closed field lines are regenerated due to the dissipation of magnetic fluctuations. The helical distortion of the OFC becomes small, and then ordered magnetic field structures without flows are built. Just after turning off the external electric field, the poloidal flow from the confinement to gun region is caused by the pressure gradients. The parallel current density \textit{$\lambda $} concentrated in the OFC diffuses to the core region, but does not relax in the direction of the Taylor state due to the pressure gradients. [Preview Abstract] |
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PP8.00070: Increase in compact toroid mass by accelerator-region ionization of high-Z noble gas on CTIX Robert D. Horton, David Q. Hwang, Fei Liu, Sean Hong, Ruth Klauser, Russell W. Evans, Dean A. Buchenauer A promising technique for runaway electron (RE) mitigation in large-tokamak disruptions is the injection of compact toroid (CT) plasmas of high atomic number. With sufficient kinetic energy density, high-Z CTs can reach the tokamak magnetic axis where RE effects are strongest. At CT velocities of 100 km/s or more, penetration to the axis occurs on a sub-millisecond time scale. In addition to reducing avalanche RE production by collisions, high-Z CTs can cool RE by bremsstrahung effects. From theoretical calculations, using Xe ions, bremsstrahlung cooling exceeds the effect of collisions at RE energy above about 10 MeV, a value expected to be well exceeded in large tokamaks. Past experiments on the CTIX compact-toroid injector have demonstrated increased CT mass using snowplow accretion of puffed noble gas by an initial hydrogenic CT. These experiments will be continued using a higher ratio of accreted high-Z plasma to H plasma, to maximize CT kinetic energy density. Results will be compared with a 1D model using external circuit effects, coaxial railgun kinetics, and ionization. The model will be used to predict performance of CT injectors of greater energy, suitable for RE suppression on mid-sized tokamaks. [Preview Abstract] |
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PP8.00071: Observation of Vertical Charge Separation Current in a Troidal ECR Plasma Kengoh Kuroda, Masaki Uchida, Hitoshi Tanaka, Takashi Maekawa In plasmas immersed in the toroidal field B$_{\phi }$, electrons drift downward while ions drift upward due to the field gradient and curvature (B$_{\phi }>$ 0 is assumed). The plasma is usually bounded at the top and bottom by the conducting vessel walls. The same amount of current must flow into and out from the walls at the top and bottom, respectively, to complete the current circulation via the vacuum vessel. In an ECR plasma in the LATE device radial profiles of vertical charge separation currents has been measured by radially aligned multi-electrodes fabricated at the top and bottom. Both the profiles at the top and bottom are nearly the same as the profile 2p$_{e}$/RB$_{\phi }$ in the plasma as theoretically predicted. Note that p$_{i}$ is usually much lower than p$_{e}$ in ECR plasmas. Current characteristics upon the sweep of external voltage onto the top ion collectors show that the current is due to inflow of ions with no secondary electron emission. Those results indicate that the current carrier is replaced from electrons to ions in the top boundary, where a steep down-slope of space potential develops toward the top wall, suggesting that the ions gain kinetic energy as they descend the potential slope by the charge separation vertical drift toward the top wall [Nishi et al., PPCF 52 125004 (2010) sections 4.4 and 4.5]. We are now investigating how the ion's kinetic energy increases on the potential down slope toward the top wall using an ion sensitive probe. [Preview Abstract] |
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PP8.00072: INTENSE LPI AND FI |
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PP8.00073: Spectral and angular distribution of photons via radiative damping in extreme ultra-intense laser-plasma interaction Rishi Pandit, Yasuhiko Sentoku Spectral and angular distribution of photons produced in the interaction of extremely intense laser ($>$ $10^{22}$\,W/cm$^2$) with dense plasma are studied with a help of a collisional particle-in-cell simulation, PICLS. In ultra-intense laser-plasma interaction, electrons are accelerated by the strong laser fields and emit $\gamma$-ray photons mainly via two processes, namely, Bremsstrahlung and radiative damping. We had developed numerical models of these processes in PICLS and study the spectrum and the angular distribution of $\gamma$-rays produced in the relativistic laser regime. Such relativistic $\gamma$-rays have wide range of frequencies and the angular distribution depends on the hot electron source. From the power loss calculation in PICLS we found that the Bremsstrahlung will get saturated at I $>$ $10^{22}$\,W/cm$^2$ while the radiative damping will continuously increase. Comparing the details of $\gamma$-rays from the Bremsstrahlung and the radiative damping in simulations, we will discuss the laser parameters and the target conditions (geometry and material) to distinguish the photons from each process and how to catch the signature of the radiative damping in future experiments. [Preview Abstract] |
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PP8.00074: Radiation transport in PIC modeling of laser interaction with high-Z targets Ioana Paraschiv, Yasuhiko Sentoku Laser-target interactions generate hot, dense, radiating plasmas and for high-Z target materials radiation effects are very important, becoming one of the dominating energy-exchange mechanisms. In order to take into account the cooling and heating effects due to the radiation field in a laser-produced plasma we are developing a radiation transport model coupled with a particle-in-cell code, PICLS. We have implemented a short-characteristics numerical scheme in order to solve the 1D steady-state radiation transport equation. In solving the equation of radiation transport it was assumed that opacities and emissivities were known in all the grid points. A database of emissivities and opacities as functions of photon frequency has been created for given densities and temperatures, using results computed by the 0-D code FLYCHK together with its postprocessor FLYSPECTRA [1]. FLYCHK calculates the ionization and population distributions for a plasma of given electron temperature and density using simplified population kinetics models. The results obtained from the implementation of the radiation transport model into the PICLS calculations will be reported in this presentation.\\[4pt] [1] H.-K. Chung, M.H. Chen, W.L. Morgan, Y. Ralchenko, HEDP 1, 3 (2005) [Preview Abstract] |
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PP8.00075: QED-PIC Codes for 10PW Laser-Plasma Simulation Christopher Ridgers, Christopher Brady, Roland Duclous, John Kirk, Keith Bennett, Tony Arber, Anthony Bell As lasers push to ever higher intensities an exciting new frontier will soon be reached in laser-plasma physics. 10PW lasers will create strong enough electromagnetic fields to access non-linear quantum electrodynamics (QED) processes. In contrast to other scenarios where such effects are typically seen, the fields in a 10PW laser's focus will directly access the non-linear QED processes to create a QED-plasma. Here the microscopic QED processes are inherently entwined with the full complexity of a macroscopic laser-plasma interaction and so neither the QED nor the classical plasma physics may be considered in isolation. As a result the standard particle-in-cell (PIC) simulation approach is inadequate for describing QED-plasmas and a new approach (QED-PIC) must be adopted. We will show that the inclusion of QED emission processes in a PIC code is considerably simplified if the electromagnetic fields in the plasma do not very significantly during the emission and are much less than the Schwinger field. In this case the relevant quantum processes can be included in a standard PIC code. We predict that the resulting QED-PIC algorithm will be essential to the future development of the field of high-intensity laser-matter interactions. [Preview Abstract] |
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PP8.00076: On the path to pair production: self-consistent PIC modeling of high energy photons in laser-plasma interaction Thomas Grismayer, Marija Vranic, Ricardo Fonseca, Christopher Harvey, Anton Ilderton, Mattias Marklund, Luis Oliveira Silva Electron-positron pair production, at the focus of an intense laser, is currently a topic of considerable interest due to the development of extreme light. Out of the possible mechanisms, pair production seeded by an electron is likely to be the most dominant. This process comes in two forms: a single step process, in which the intermediate photon is virtual (trident), and a two step process, in which non linear Compton scattering produces a real photon from the incoming electron and this real photon then goes on to create a pair via stimulated pair production (Breit-Wheeler). We describe our strategy to include theses processes in a massively parallel PIC code (using the Osiris 2.0 framework) in a self-consistent manner, also taking into account radiation reaction. [Preview Abstract] |
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PP8.00077: Simulation study of radiation enhancement through an interaction between periodically aligned carbon nanotubes and an intense laser Toshihiro Taguchi, Masahiko Inoue, Thomas Antonsen Interaction between ultra-intense laser and solid material has a lot of applications. One is an emission of electromagnetic wave in a wide range of wavelength, which is from THz to X-ray. In order to select the wavelength, one of interesting target materials is single-walled carbon nanotube. Huge number of carbon nanotubes can be grown vertically on a substrate and they can be aligned periodically on the substrate. Once an ultra-intense laser is irradiated on the nanotubes, the strong electric field of the laser forcedly oscillates electrons in each nanotube and they irradiate electromagnetic wave. Waves emitted from periodically aligned nanotubes are expected to interfere each other, and then the amplitude of the output radiation is to be enhanced. In order to analyze such radiation processes from the laser-matter interaction, we have been developing an electromagnetic particle-in-cell (PIC) code including collisional and ionization processes. We will present recent results about an interaction between strong laser and carbon nanotubes analyzed by the PIC code. In the presentation, we will show the radiation spectrum and its enhancement due to the periodic structure. [Preview Abstract] |
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PP8.00078: Ion Acceleration in a Solitary Wave by Laser Pulse with Ramping-up Amplitude Min-Qing He, Vipin Tripathi, Chuan-Sheng Liu, Xi Shao, Tung-Chang Liu, Jao-Jang Su, Zheng-Ming Sheng Recent work by Jung \textit{et al}. demonstrated experimentally the acceleration of mono-energetic ion beam by solitary waves generated and maintained by laser light with ramping-up amplitude.\footnote{D. Jung, L. Yin, B.J. Albright, D.C. Gautier, R. H\"orlein, D. Kiefer, A. Henig, R. Johnson, S. Letzring, S. Palaniyappan, R. Shah, T. Shimada, X.Q. Yan, K.J. Bowers, T. Tajima, J.C. Fern\'andez, D. Habs, and B.M. Hegelich, Phys. Rev. Lett. 107,115002(2011).} Theoretical model is developed in this work to study the formation of the solitary wave and effects of the radiation pressure force on a soliton in the accelerating plasma. 2D Particle-In-Cell (PIC) simulations are performed to compare and validate the theory. Differences in generating and maintaining solitary wave for laser with and without ramping-up amplitude are also investigated. We will also investigate effects of radiation pressure acceleration of plasma with near critical density. [Preview Abstract] |
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PP8.00079: Design of Laser Profile in Boosting the Energy of Monoenergetic Protons Accelerated by Radiation Pressure and Shielded Coulomb Repulsion Tung-Chang Liu, Xi Shao, Chuan-Sheng Liu, Minqing He, Bengt Eliasson, Vipin Tripathi, Jao-Jang Su, Jyhpyng Wang, Shih-Hung Chen Laser radiation pressure acceleration is considered as an effective method in obtaining monoenergetic ions with high energy. By irradiating a laser beam on a multi-species target made of carbon and hydrogen, the proton layer can be accelerated by radiation pressure and shielded Coulomb repulsion successively. The shielded Coulomb repulsion provided by the left-behind electron-carbon layer can not only further accelerate the proton layer, but also keep the proton layer stable for a long time. The acceleration time of quasi-monoenergetic protons by the combined mechanisms is extended over ten times longer compared to the case of applying single-species targets and using radiation pressure acceleration alone. 60 MeV of quasi-monoenergetic protons from a multi-species foil with input laser power of 70 TW is obtained, which is at least five times greater than the energy obtainable from pure hydrogen targets. In order to reach 100 MeV with the same laser power, we studies different designs of input laser profiles to further boost the energy outcome, and an improvement of at least 20\% energy enhancement is achieved. An analytical approach to interpret and optimize the results is also studied. [Preview Abstract] |
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PP8.00080: On the saturation of stimulated Raman scattering in laser amplification E.S. Dodd, J. Ren, T.J.T. Kwan, M.J. Schmitt The use of stimulated Raman scattering (SRS) in plasmas has been proposed as an alternative to the CPA technique for laser pulse amplification and compression [1]. Initial experiments demonstrated the amplification and compression of laser pulses in plasma to an unfocused intensity of $\sim $10$^{16}$ W/cm$^{2}$ [2], however the amplification was saturated at this level and was accompanied by deleterious spatial and temporal incoherence. The reasons for this incoherence have not been well understood. A physical picture has been developed with results from PIC simulations using the LSP code where spontaneous SRS in the pump modifies the plasma conditions, and which in turn significantly weakens the coupling strength for seed amplification. This led to the development of a novel experimental method to significantly increase the amplified power in the short-pulses via SRS.\\[4pt] [1] G. Shvets, N. J. Fisch, A. Pukhov, and J. Meyer-ter-Vehn, Phys. Rev. Lett. 81 4879 (1998).\\[0pt] [2] J. Ren, W.-F. Cheng, S.-L Li, and S. Suckewer, Nat. Phys. 3 732 (2007). LA-UR-12-22734 [Preview Abstract] |
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PP8.00081: Nonlinear saturation of laser backward Raman amplification in plasmas Vladimir Malkin, Zeev Toroker, Nathaniel Fisch Relativistic electron nonlinearity can limit the backward Raman amplification (BRA) of laser pulses in plasmas through the self-phase-modulation instability splitting the pumped spike into several spikes which no longer grow in amplitude. It is assumed usually that this scenario is not sensitive to a small group velocity dispersion (GVD) of the pumped spike in strongly undercritical plasmas. It was not known, however, how much undercritical should be the plasma for the GVD effect on the nonlinear BRA saturation to be negligible. This work examines this problem analytically and shows that the GVD effect becomes important already in tenuous, 1000 times undercritical plasmas. The analytical solution also shows how the naturally arising frequency chirp and other parameters of the BRA output pulses can be manipulated. [Preview Abstract] |
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PP8.00082: PIC Modeling of Relativistic Electron Transport Experiments on Omega EP Josh May, J. Tonge, W.B. Mori, F. Beg, C. McGuffey, M. Wei, R. Fonseca Recent experiments on the Omega EP laser system have used an intense laser ($I \sim 10^{19} W/{cm}^2$, $\tau \sim 8ps$) striking Au foil to generate a relativistic electron beam, which is subsequently transported through either CH plasma or room temperature CH foam, and then diagnosed with Cu $K \alpha$ from a Cu foil. An order of magnitude lower $K \alpha$ emission is seen in the plasma case compared to the cold case. We use the particle-in-cell code OSIRIS to model the experiment in the case of pre-formed plasma. Our 2D simulations show a similarly broad transverse profile as experiment. We also see a strong filamentary B-field in the CH region directly adjacent to the gold, with filaments similarly diverging from the laser spot. Increasing the CH density dampens these filaments, and leads to a more intense and more collimated electron spectrum in the Cu region, consistent with experiment. [Preview Abstract] |
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PP8.00083: Parametric amplification of laser-driven electron acceleration in underdense plasma Alexey Arefiev, Boris Breizman, Marius Schollmeier, Vladimir Khudik Electron heating in laser-irradiated targets is crucial for production of energetic ions and other applications, including x-ray generation and fast ignition. Electron quiver energy in a laser beam is of the order of the ponderomotive potential, which is typically below the energy range of interest. This poster presents a new collisionless mechanism that allows electrons to gain energies significantly higher than the ponderomotive potential. The mechanism involves an under-dense preplasma in front of a target. The laser beam creates a positively charged channel in this preplasma, so that an electron accelerated by the laser performs betatron oscillations across the channel while moving along with the beam. The betatron frequency is strongly modulated by the laser field in the ultrarelativistic limit. It has been previously overlooked that such modulation makes the oscillations parametrically unstable. The resulting amplification of the oscillations decreases electron dephasing from the laser and thereby significantly enhances the electron energy gain [Phys. Rev. Lett. 108, 145004 (2012)]. This work was supported by Sandia National Laboratories, U.S. DoE, and NNSA. [Preview Abstract] |
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PP8.00084: Investigation of high-intensity laser-plasma interaction and fast electron source characteristics from 1-10 ps pulse length A. Sorokovikova, B. Qiao, M.S. Wei, R.B. Stephens, P. Patel, H. Mclean, F.N. Beg Efficient conversion of laser energy to hot electrons is extremely important for the success of Fast Ignition (FI), where a drive laser pulse with duration of 10ps and energy 100's of kJ is required. Here we report the first theoretical and numerical study on the characteristics of laser-plasma interaction (LPI) and fast electron source production from 1-10 ps pulse drive. It is found that due to a significant hydrodynamic plasma expansion in picoseconds, the fast electron acceleration mechanism strongly relies on the laser leading edge depletion [1] in near critical plasma and the electrostatic potential [2] caused by low-density plasmas. Both the fast electron average temperature and the laser-electron conversion efficiency increase more than 2 times by extending the laser pulse length from 1ps to 10 ps. Their dependences on the preplasma scale length are also analyzed. \\[4pt] [1] A. P. L. Robinson et al., PPCF 53, 065019 (2011).\\[0pt] [2] B. S. Paradkar et al., PRE 83 046401 (2011). [Preview Abstract] |
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PP8.00085: The ePLAS code for Ignition Studies R.J. Faehl, R.J. Mason, R.C. Kirkpatrick The ePLAS code is a multi-fluid/PIC hybrid developing self-consistent \textit{E {\&} B-}fields by the Implicit Moment Method for stable calculations of high density plasma problems with voids on the electron Courant time scale. See: http://www.researchapplicationscorp.com. Here, we outline typical applications to: 1) short pulse driven electron transport along void (or high Z) insulated wires, and 2) the 2D development of shock ignition pressure peaks with $B-$fields. We outline the code's recent inclusion of SESAME EOS data, a DT/DD burn capability, a new option for K-alpha imaging of modeling output, and demonstrate a foil expansion tracked with either fluid or particle ions. Also, we describe a new super-hybrid extension of our implicit solver that permits full target dynamics studies on the ion Courant scale. Finally, we will touch on the very recent application of ePLAS to possible non-local/kinetic hydro effects NIF capsules. [Preview Abstract] |
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PP8.00086: Generation and Compression of Magnetic Field in Non-spherical cone-guided Implosion for Fast Ignition Hideo Nagatomo, Tomoyuki Johzaki, Atsushi Sunahara, Hitoshi Sakagami, Akio Nishiguchi, Kunioki Mima In the recent computational simulation research suggest that magnetic field plays an important role in Fast Ignition scheme, especially for high energy electron transport toward the compressed core plasma. Non-spherical implosion can produce the strong magnetic field due to the cross production of grad(Te) and grad(ne). In our previous work, 2-D simulation code for temporal evolution of magnetic field has been developed, and it has been coupled with radiation hydrodynamic code as a postprocessor. In the simulation, we have found that Nernst effect is important in amplifying the field. Finally compressed magnetic field reaches 60MG at maximum compression time. In such strong magnetic field we cannot ignore the effect to the hot electron transport and reduction of thermal conduction which is strongly related to implosion dynamics. Therefore, magnetic field transport code is solved with 2-D radiation hydrodynamic code simultaneously when we investigate the effect to the implosion dynamics. \\[4pt] [1] H. Nagatomo, et. al, Phys. Plasmas, 14 056303 (2007).\\[0pt] [2] Nishiguchi et al., Phys. Fluid 28, 3683, (1985). [Preview Abstract] |
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PP8.00087: Relativistic Particle Wakes and Their Impact on Electron Stopping Ian Ellis, Frank Graziani, David Strozzi, Frank Tsung, Viktor Decyk, Warren Mori A detailed understanding of electron stopping and scattering in plasmas with variable values for the number of particles within a Debye sphere is still not at hand. Presently, there is some disagreement in the literature concerning the proper description of these processes. Detailed theoretical models exist for the stopping power of a single relativistic electron in a plasma, including quantum mechanical effects. However, few theories take into account correlation effects, in which the wake produced by an electron modifies the dynamics and stopping power of the electrons that travel behind it. Some have performed simple studies of correlated stopping, but have neglected the tendency of electrons to move around inside the wake. Developing and validating proper descriptions requires studying the processes using first-principle plasma simulations. We are using the particle-in-cell (PIC) codes OSIRIS and QuickPIC to perform these simulations. As a starting point, we examine the wake of a particle passing through a plasma in 3D relativistic electromagnetic simulations using various cell sizes and compare the results with cold plasma theory. We also present some initial stopping power results. The relevance of the work to Fast Ignition is discussed. [Preview Abstract] |
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PP8.00088: Numerical and experimental investigations of fusion-relevant beam-plasma instabilities M. King, S.L. McConville, D.C. Speirs, R. Bryson, K.M. Gillespie, A.D.R. Phelps, A.W. Cross, C.G. Whyte, K. Ronald, R.A. Cairns, I. Vorgul, R. Bingham, R.M.G.M. Trines The growth and evolution of beam-plasma instabilities is of interest in a variety of fields of plasma physics. In fast-ignition inertial confinement fusion this instability may provide an additional ion heating mechanism. In this form of inertial confinement fusion, a deuterium-tritium fuel pellet is compressed by uniformly distributed intense laser radiation forming a high density plasma. A secondary short, high power laser pulse then interacts with the high density plasma, possibly via a conical gold insert, producing a highly relativistic electron beam that propagates through the plasma. During this propagation, the two-stream instability can occur. This instability takes the form of Langmuir waves which may parametrically decay to ion acoustic waves. These ion acoustic waves can then be damped by ion-ion collision providing a heating mechanism in addition to electron-ion collisions. To investigate this behaviour, numerical simulations have been conducted in a lower density and lower temperature regime utilising a two-dimensional particle-in-cell (PiC) code. The parameters used represent a laboratory experiment that is being designed and constructed to provide an experimental benchmark for the numerical predictions. [Preview Abstract] |
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PP8.00089: Simulation and Characterization of Gamma Ray Production by Petawatt Lasers Irradiating High-Z Solid Targets Alexander Henderson, Edison Liang, Pablo Yepes, Petr Chaguine, Nathan Riley, Gillis Dyer, Scott Pennington On interaction with a solid target an ultra-intense short pulse high-energy laser, such as the Texas Petawatt Laser (TPW), accelerates a sub-pico-second burst of electrons into the target at high energies. These electrons then undergo bremsstrahlung, producing a beam of high-energy gamma rays. Even for mm thick gold targets, most of the bremsstrahling gamma rays escape, while many hot electrons do not. Here we attempt to characterize the angular distribution, energy spectrum and total yield of these gamma rays as produced by the TPW irradiating mm thick gold targets. GEANT4 Monte-Carlo simulation results are then fitted to the data and used to extrapolate the results beyond the limits of the measurements. We will also discuss potential applications of such intense gamma-ray beams. [Preview Abstract] |
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PP8.00090: Preliminary characterization of ultra-short pulse laser-produced miniature hohlraum XUV sources A. Mckelvey, M. Vargas, L. Montier, J. Nees, B. Hou, A. Maksimchuk, A.G.R. Thomas, K. Krushelnick Experiments at the National Ignition Facility (NIF) allow the radiative properties of dense, high-temperature matter to be studied at previously unreachable regimes, but are limited by cost and system availability. A scaled down system using ultra-short laser pulses and delivering energy to a much smaller hohlraum could be capable of reaching comparable energy densities and depositing the energy before the wall material ablation closes the cavity. The Lambda Cubed laser system at University of Michigan--a high-power (0.3 TW), short pulse (30fs), 500 Hz repetition rate tabletop laser system-is used to machine 20-100 micron diameter cavities in copper targets. These cavities are machined with low laser powers, and then shot in situ with a single full power pulse. The emitted radiation is analyzed with an XUV spectrometer. This method may allow studies such as opacity measurements using plasma and radiation with the temperatures comparable to NIF type hohlraums, but with a significantly higher repetition rate and in a university scale system. [Preview Abstract] |
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PP8.00091: Paramount Deuteron Acceleration Using High-Intensity Short Laser Pulses F. Yu, A. Raymond, C. Zulick, L. Willingale, K. Krushelnick, A. Maksimchuk, G. Petrov, J. Davis It has long been a challenge to efficiently generate laser-driven ion beams having none-proton ions as the dominant species since protons are generally present as contamination layers on the target surface. During recent experiments at the University of Michigan, ion beams composed mainly of deuterons were produced with only a small relative number of protons and oxygen ions. The experiments were performed with the 400 fs, 20 TW T-cubed laser which has focused intensity up to 4*10$^{19}$ W/cm$^{2}$ at 1053 nm and ASE intensity contrast of 10$^{-7}$. The accelerated deuterons originate from liquid deuterium oxide deposited on both the front and rear surfaces of a cryogenically cooled Cu target (normally at -160C) by spraying $\sim $50 microliters of heavy water from 2 nozzles in the vicinity of the target's front and rear. The ion beams had a Maxwellian spectrum with maximum energy of 8 MeV for deuterons and 10 MeV for protons. Using a Thomson parabola ion spectrometer system combined with CR39 indicated that the forward-propagating deuteron beam had about 10$^{12}$ ions per steradian (integrated over spectrum). The FWHM of the beam was 20 degrees, ideal for applications involving neutron generation and isotope activation. [Preview Abstract] |
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PP8.00092: Directional, energetic neutron generation via high-intensity laser/plasma interactions at CUOS Anthony Raymond, Anatoly Maksimchuk, Vladimir Chvykov, Franklin Dollar, Louise Willingale, Victor Yanovsky, Fan Yu, Calvin Zulick, Karl Krushelnick, Jack Davis, George Petrov Pitcher- catcher arrangements were used for directional, energetic neutron (n) production by the interaction of a beam of deuterons (d's) with catcher materials via 7Li(p,n), d(d,n), and 7Li(d,n). The experiments were conducted at CUOS in Ann Arbor on the T-cubed and Hercules lasers. The utilized method of d-beam production involved depositing D2O onto various cryogenically cooled thin foils, which at optimal parameters produced via high-intensity laser/plasma interactions d-yields substantially greater than that observed in previous experiments [1] involving instead a pitcher target of deuterated-polystyrene (CD) coated 13$\mu $m Mylar, as the latter method was shown to be impaired by hydrocarbon contamination. We present results in which the more recent d-production technique is utilized, which for d-d reactions yielded on the T-cubed system n's up to nearly 2.5 MeV and on the Hercules system up to nearly 12 MeV, with forward yields on the order of 10E5 n/sr ($\sim $10x higher than that previously achieved using either a CD coated pitcher or a bulk target of deuterated polyethylene). The Office of Naval Research provided funding for this work. \\[4pt] [1] L. Willingale et al., Phys. Plasmas 18, 083106 (2011). [Preview Abstract] |
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PP8.00093: Laser driven hard x-ray sources for phase contrast imaging Zhen Zhao, Bixue Hou, Zhaohan He, John Nees, Alexander Thomas, Karl Krushelnick Phase contrast imaging using x-rays produced by laser-plasma interactions requires a very small source size. High quality images can be obtained by minimizing the source size as well as increasing the source-to-object distance. This experiment studies the properties of hard x-rays generated by the interaction of an ultra intense laser with various solid targets. A 10-mJ, 30-fs laser pulse operating at 0.5 kHz is focused onto a 1.5 micron spot, generating a focal intensity of $\sim $10$^{19}$W/cm$^{2}$. The targets used are 100-mm diameter, 10-mm thick Ni and Mo discs coated with a thin layer of SiO$_{2}$. The effects of the coating material and thickness on the x-ray source size are investigated. Phase contrast images are subsequently obtained using these sources. Spectroscopic measurements of these targets are also taken with and without the coating material. [Preview Abstract] |
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PP8.00094: Hollowing and filamentation of proton beams in high-intensity laser plasma interactions Yadab Paudel, N. Renard-Le Galloudec, V.L. Kantsyrev, A.S. Safronova, A.Ya. Faenov, I. Shrestha, G.C. Osborne, V.V. Shlyaptseva, Y. Sentoku Protons and multicharged ions accelerated from the rear surface of thin foil targets driven by a high-intensity laser have been studied in short (fs) and long (ns) pulse mode. The protons/ions beam features recorded on CR39 show a central hollow beam structure due to the effect of the self-generated magnetic field on the protons. This hollow structure surrounded by radial structures can be explained by the effect of the electrostatic fields on the shape of the sheath profile at the target rear-surface. [Preview Abstract] |
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PP8.00095: Measurement of heating beam injection time in Fast Ignition experiment with Gekko-XII and LFEX lasers Hiroyuki Shiraga, Takehiro Sogo, Shinsuke Fujioka, Hiroshi Azechi Fast Ignition integrated experiments have been performed with Gekko-XII laser for implosion of the shell target and LFEX laser for fast heating through the cone. Injection time of the heating beam relative to the core plasma formation was precisely observed with accuracy within 7 ps by using non-imaged hard x-ray signals on an streaked x-ray images. It was found that the time window of injection time for efficient neutron enhancement was only 50 ps around the peak compression of the imploded core plasma. [Preview Abstract] |
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PP8.00096: BEAMS AND COHERENT RADIATION |
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PP8.00097: A Possible Proton Beam Self Modulation Experiment at Fermilab C. Joshi, W. An, W. Mori, J. Thangaraj, C. Park, J.D. Lewis, P. Spentzouris Recently the proton beam driven PWFA has attracted much attention because the multi-TeV proton bunches can be used as the drive beam to accelerate leptons in a single PWFA stage to hundreds of GeV energy level. Unfortunately the existing beams all have a $\sim$10 cm pulse length, which is much longer than the ideal PWFA drive beam requirement ($\sim$50 $\mu$m). However such a long beam may be self modulated when propagating in a long plasma column, which will generate a string of micro bunches and excite a large amplitude plasma wave. Fermilab has a 120 GeV proton beam, which is available for demonstrating the beam self modulation in the plasma. The typical energy modulation of the protons after traversing a few meters of plasma is on the order a GeV and a high resolution spectrometer is needed to detect this beam modulation directly. Fortunately a portion of the Tevatron ring can be used as a spectrometer for detecting this energy modulation. The PIC code Osiris and QuickPIC are used for simulating the proton beam self modulation. The beam particle data can be also imported into another accelerator code for testing the spectrometer design. Preliminary simulation results show that 0.5 GeV energy modulation can be observed for reasonable beam parameters using a 2 meter long plasma. [Preview Abstract] |
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PP8.00098: Simulation Study of PWFA Experiments at FACET Weiming An, Navid Vafaei-Najafabadi, Ken Marsh, Chris Clayton, Warren Mori, Chan Joshi, Erik Adli, Sebastien Corde, Michael Litos, Selina Li, Spencer Gessner, Joel Frederico, Joel England, Dieter Walz, Mark Hogan, Patric Muggli, Jean P. Delahaye, Wei Lu Recent PWFA experiments at FACET use Rb gas ionized by the beam as the plasma source. The Rb has a lower ionization threshold than the Li, which was used in earlier experiments, consequently a smaller peak current beam can still produce a field ionized plasma. But the Rb vapor is confined by Ar and as a result it is possible to ionize both the first electron of Ar (I.P. 14eV) as well as the second electron of Rb (I.P.24 eV). This secondary ionization can lead to a source of dark current in a PWFA. In this work QuickPIC simulation results are presented for studying the influence by the ``unwanted'' ionization. In the simulation, both Ar and Rb vapor profiles are initialized as measured in the laboratory. We use different beam parameters (including different focal position) in the simulation. The ion density of the gas is a useful diagnostic showing the ionization level of the neutral gas in the simulation. Other simulation results related FACET experiments are also presented. [Preview Abstract] |
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PP8.00099: Experimental Study of the Current Filamentation Instability Brian Allen, Patric Muggli, Luis O. Silva, Joana Martins, Vitaly Yakimenko, Mikhail Fedurin, Karl Kusche, Marcus Babzien, Chengkun Huang, Warren Mori The Current Filamentation Instability (CFI) is of central importance for the propagation of relativistic electron beams in plasmas. CFI has potential relevance to astrophysics, afterglow of gamma ray bursts, inertial confinement fusion, energy transport in the fast-igniter concept, and places an upper limit on the plasma density and accelerating gradient in PWFA's. An experimental study at the Accelerator Test Facility at Brookhaven National Laboratory with the 60MeV e$^{-}$ beam and cm length plasma. The experiment included the systematic study and characterization of the instability as a function of the beam charge and plasma density. The transverse beam profile is measured directly at the plasma exit using OTR. Experimental results show the transition from plasma focusing to CFI near k$_{p}\sigma _{r}$=1 characterized by the appearance of multiple (1-5) beam filaments and scaling of the transverse filament size with the plasma skin depth. Suppression of the instability is seen by lowering the growth rate of the instability by reducing the beam charge. The experimental results are in excellent agreement with theory and simulations and we present and discuss simulation and experimental results. [Preview Abstract] |
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PP8.00100: Laser-seeded modulation instability within LHC proton beams Carl Siemon, Vladimir Khudik, Gennady Shvets A new method for seeding the modulation instability (MI) within an SPS-LHC proton beam using a laser pulse is presented. Using simulations, we show that a laser pulse placed ahead of a proton beam excites axially symmetric self-modulation modes within the proton beam and leads to peak accelerating fields that are comparable to previously proposed seeding methods. We then demonstrate that a plasma density ramp placed in the early stages of the laser-seeded MI leads to stabilization and sustained accelerating electric fields (of order several hundred MeV/m) over long propagation distances ({\_} 100 - 1000 meters). To directly compare the efficiencies of the laser and other seeding methods, simulation results for two scenarios are discussed: a) a laser is placed in the ``center'' of an infinitely long proton beam; b) the same proton beam is cut at the center (no laser pulse). Analytics are presented that determine the behavior of the strongly coupled, long beam regime for each of these cases. [Preview Abstract] |
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PP8.00101: Laser wakefield acceleration research by using a tapered capillary waveguide at GIST Minseok Kim, Donggyu Jang, Inhyuk Nam, Taehee Lee, Hyyong Suk The tapered plasma density in a gas-filled capillary waveguide can suppress the dephasing problem in laser wakefield acceleration (LWFA). As a result, the acceleration distance and the gained electron energy are expected to be increased significantly. For this purpose, we recently developed a tapered capillary waveguide, which can produce a plasma density of 10$^{18}$ cm$^{-3}$. This capillary plasma waveguide will be used for high-energy electron generation experiment together with a 20 TW/35 fs Ti:sapphire laser system that will be completed by this summer. In this presentation, the ongoing experiments will be reported. [Preview Abstract] |
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PP8.00102: Effects of asymmetric laser pulses in particle trapping and dephasing in the laser wakefield electron acceleration Myung-Hoon Cho, Min Sup Hur Electron acceleration in the plasma wakefield driven by asymmetric laser pulse is investigated with 2D PIC simulation. There exists an optimum length of the rising and falling time of the laser pulse for different plasma densities. In particular, for a fixed rising segments of the laser pulse, it is found that the particle trapping is influenced by the tail of the laser field. Furthermore different rising times of the pulse makes different range of the wakefield in which the particles are trapped. Consequently both rising and tail parts of the laser pulse influences the particle trapping yield in different ways. The trapping efficiency is shown as a function of the pulse shape along with particle trajectories from PIC simulations. In addition to that, effects of the pulse rising time on the dephasing is discussed. [Preview Abstract] |
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PP8.00103: Threshold for electron self-injection in a nonlinear laser-plasma accelerator Carlo Benedetti, Carl Schroeder, Eric Esarey, Wim Leemans The process of electron self-injection in the nonlinear bubble-wake generated by a short and intense laser pulse propagating in an uniform underdense plasma is investigated. A detailed analysis of particle orbit in the wakefield is performed by using reduced analytical models and numerical simulations carried out with the 2D cylindrical, envelope, ponderomotive, hybrid PIC/fluid code INF{\&}RNO. In particular, we consider a wake generated by a frozen (non-evolving) laser driver traveling with a prescribed velocity, which then sets the properties of the wake, so the injection dynamics is decoupled from driver evolution but a realistic structure for the wakefield is retained. We investigate the dependence of the injection threshold on laser intensity, plasma temperature and wake velocity for a range of parameters of interest for current and future laser plasma accelerators. The phase-space properties of the injected particle bunch will also be discussed. [Preview Abstract] |
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PP8.00104: Electron injection and emittance control by transverse colliding pulses in a laser-plasma accelerator Min Chen, Eric Esarey, Carl Schroeder, Cameron Geddes, Stepan Bulanov, Carlo Benedetti, Lule Yu, Sergei Rykovanov, Wim Leemans, Estelle Cormier, David Bruhwiler By using two colliding laser pulses propagating transversely to the wake, electron injection and emittance can be controlled in a laser plasma accelerator. A beam with extremely small emittance is obtained when position of the colliding pulses is close to the density peak of the wake. Electrons near the axis are accelerated by the colliding pulses and become trapped in the second bucket of the wake. Ionization is used to increase the final injection charge. Simulations show that the transverse momentum spread can be as small as 0.04mc, which is orders of magnitude smaller than typical laser injection schemes in plasma accelerators. For pulses with different frequencies, transverse beat waves can be used to generate asymmetric injection, which can increase the betatron radiation. Supported by DOE HEP DE-AC02-05CH11231, by DOE NNSA DNN, DE-SC0004441 and DE-FC02-07ER41499, and by the COMPASS SciDAC project. [Preview Abstract] |
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PP8.00105: Electron number control by laser frequency detuning in three colliding pulses injection Lule Yu, Eric Esarey, Carl Schroeder, Cameron Geddes, Min Chen, Carlo Benedetti, Estelle Cormier-Michel, David Bruhwiler, Wim Leemans Electron injection in laser-plasma accelerators can be achieved using colliding laser pulses [E. Esarey et al. Phys. Rev. Lett. 79, 2682 (1997)]. The background plasma electrons are heated and dephased by the slow phase velocity beat wave induced by two counterpropagating injection pulses, allowing some fraction to be trapped in the plasma wake excited by the pump pulse. It is found that the electron injection number can be controlled by tuning the frequencies of the injection pulses, which tunes the beat wave phase velocity.The optimal injection number is achieved at a negative beat wave phase velocity. With increasing laser intensities, the optimal beat wave phase velocity becomes larger and the range of the laser frequency difference for injection becomes larger. The local plasma wake suppression due to the axial electrostatic field induced by the beat wave is also studied. [Preview Abstract] |
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PP8.00106: Numerical Study of Self and Controlled Injection in 3D and 2D Wakefield Accelerators Asher Davidson, Ming Zeng, Wei Lu, Chang Joshi, Luis Silva, Joana Martins, Ricardo Fonseca, Warren Mori In plasma based accelerators (LWFA and PWFA), the methods of injecting high quality electron bunches into the accelerating wakefield is of utmost importance for various applications. We investigate the use of a two-stage ionization injected LWFA to achieve high quality monoenergetic beams through the use of 3D PIC simulations. The first stage constitutes the Injection Regime, which is 99.5\% He and 0.5\% N, while the second stage constitutes the Acceleration Regime, which is entirely composed of He. Two of the simulations model the parameters of the LWFA experiments for the LLNL Callisto laser, at laser powers of 90 and 100TW. energies as high as 660MeV were observed in the 90TW simulation, and those as high as 1.07MeV were observed in the 100TW simulation. The affect of the matching condition of the spot size in this LWFA is discussed. A third simulation at a much higher energy pulse (500TW) is being conducted, for further research. In addition, we investigate the use of higher order gaussean modes in LWFA as an alternative method to self-inject electrons in a pre-ionized plasma. [Preview Abstract] |
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PP8.00107: Beam dynamics in ionization injection Xinlu Xu, Wei Lu, Warren Mori We present new results from OSIRIS simulations and analytical theory on the beam dynamics of an electron beam formed via various ionization injection schemes. Two kinds of ionization injection scheme are carefully studied, a ``standard'' ionization injection scheme and the plasma photocathode scheme. Longitudinal mixing occurs as electrons created in the middle of the accelerating structure slip backwards to the rear of the bubble as they get trapped. The injection and trapping process induces emittance growth of the injection beam. We discuss thoroughly the influence of the injection distance, the acceleration distance, acceleration field and space charge force on the rms emittance of the injection beam. In the high charge case, the rms emittance is proportional to the spot size of the injection laser, $\epsilon \propto W_b^2$, where $W_b$ is defined as effective spot size. OSIRIS simulation is present to support our theory. [Preview Abstract] |
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PP8.00108: Study of trapping in nonlinear multi-dimensional wakes Ming Zeng, Asher Davidson, Wei Lu, Zhengming Sheng, Warren Mori Using the code OSIRIS we examine the wakes and particle trapping for non-evolving drivers. We examine how the trapping threshold depends on wake amplitude and wake phase velocity as well on the pseudo-potential. We concentrate on particle beam drivers and vary the driver strength (normalized current) driver shape, spot size, and velocity. We compare the phase velocity of the wake to that of the driver beam. We find that as one nears the onset of trapping the phase velocity of the wake becomes less stable and varies back through the wake. We compare the results with analytical predictions and with cases where the driver does evolve. [Preview Abstract] |
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PP8.00109: Modeling laser wakefield acceleration in a Lorentz boosted frame using OSIRIS and UPIC Peicheng Yu, Xinlu Xu, Viktor Decyk, Wei Lu, Frank Tsung, Warren Mori, Jorge Vieira, Ricardo Fonseca, Luis Silva We present recent results on the use of the Lorentz boosted frame to model laser wakefield acceleration using OSIRIS and UPIC framework. These include the modeling cases where there are no self-trapped electrons for gamma\_boost near gamma\_group where gamma\_group is the linear group velocity of the laser, and modeling the self-trapped regime. Detailed comparison between different gamma\_boost for the same lab frame parameters will be given. We also will discuss the observed short wavelength noise that is present for relatively high gamma\_boost, including detailed comparison between three FDTD solvers in OSIRIS and a spectral solver from the UPIC Framework. [Preview Abstract] |
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PP8.00110: Generalized SIMD algorithm for efficient EM-PIC simulations on modern CPUs Ricardo Fonseca, Viktor Decyk, Warren Mori, Luis Silva There are several relevant plasma physics scenarios where highly nonlinear and kinetic processes dominate. Further understanding of these scenarios is generally explored through relativistic particle-in-cell codes such as OSIRIS [1], but this algorithm is computationally intensive, and efficient use high end parallel HPC systems, exploring all levels of parallelism available, is required. In particular, most modern CPUs include a single-instruction-multiple-data (SIMD) vector unit that can significantly speed up the calculations. In this work we present a generalized PIC-SIMD algorithm that is shown to work efficiently with different CPU (AMD, Intel, IBM) and vector unit types (2-8 way, single/double). Details on the algorithm will be given, including the vectorization strategy and memory access. We will also present performance results for the various hardware variants analyzed, focusing on floating point efficiency. Finally, we will discuss the applicability of this type of algorithm for EM-PIC simulations on GPGPU architectures [2]. \\[4pt] [1] R. A. Fonseca et al., LNCS 2331, 342, (2002)\\[0pt] [2] V. K. Decyk, T. V. Singh; Comput. Phys. Commun. 182, 641-648 (2011) [Preview Abstract] |
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PP8.00111: Analytic model of electron self-injection in a plasma wakefield accelerator in the strongly nonlinear bubble regime Sunghwan Yi, Vladimir Khudik, Gennady Shvets We study self-injection into a plasma wakefield accelerator in the blowout (or bubble) regime, where the bubble evolves due to background density inhomogeneities. To explore trapping, we generalize an analytic model for the wakefields inside the bubble [1] to derive expressions for the fields outside. With this extended model, we show that a return current in the bubble sheath layer plays an important role in determining the trapped electron trajectories. We explore an injection mechanism where bubble growth due to a background density downramp causes reduction of the electron Hamiltonian in the co-moving frame, trapping the particle in the dynamically deepening potential well [2]. Model calculations agree quantitatively with PIC simulations on the bubble expansion rate required for trapping, as well as the range of impact parameters for which electrons are trapped. This is an improvement over our previous work [3] using a simplified spherical bubble model, which ignored the fields outside of the bubble and hence overestimated the expansion rate required for trapping. \\[4pt] [1] W. Lu et al., {\it Phys. Plasmas} {\bf 13}, 056709 (2006).\\[0pt] [2] S. Kalmykov et al., {\it Phys. Rev. Lett} {\bf 103}, 135004 (2009).\\[0pt] [3] S.A. Yi et al., {\it Plasma Phys. Contr. Fus.} {\bf 53}, 014012 (2011). [Preview Abstract] |
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PP8.00112: Modeling of electron self-injection and acceleration in the Texas Petawatt laser wakefield accelerator experiment Xi Zhang, Sunghwan Yi, Vladimir Khudik, Neil Fazel, Xiaoming Wang, Mike Downer, Gennady Shvets We present a numerical study of the electron self-injection and acceleration process for the parameters of the Texas Petawatt laser wakefield accelerator experiment, using the quasistatic particle-in-cell (PIC) code WAKE [1]. The laser parameters and plasma density are held fixed, while we vary the longitudinal location of the focal spot with respect to the plasma entrance. We also study the effect of varying the density upramp which extends a few millimeters outside of the main plasma body. These simulations are made possible even within a quasistatic PIC framework through the use of non-quasistatic test particles which can become self-injected, in contrast to macroparticles. We find that self-injection and multi-GeV acceleration occurs only when the laser focal point is located a few Rayleigh lengths outside of the plasma entrance, in agreement with experimental observations. The relationship between the location of the laser focal point and the bubble evolution which leads to electron self-injection [2] is presented. Quantitative agreement between simulated electron spectra and experimental results are achieved.\\[4pt] [1] P. Mora and T.M. Antonsen, Jr., Phys. Plasmas 4, 217 (1997).\\[0pt] [2] S. Kalmykov et al., Phys. Rev. Lett. 103, 135004 (2009). [Preview Abstract] |
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PP8.00113: Ultracompact quasi-monoenergetic GeV-scale laser plasma accelerator based on all-optical control of dark current in longitudinally tapered plasmas S.Y. Kalmykov, B.A. Shadwick, X. Davoine Negative chirp of an ultrabroad-bandwidth, sub-100-TW driving laser pulse, in combination with a plasma density taper and a plasma channel, prevents formation of an optical shock [1], reduces bubble expansion, and delays dephasing in the blowout regime of laser wakefield acceleration [2]. Precise compensation of the nonlinear frequency shift delays self-compression of the driving pulse into the optical shock; this compensation is achieved by using the specific shape of the frequency chirp extracted from reduced simulation models. In addition, plasma channel suppresses the diffraction of the pulse leading edge, further delaying formation of the optical shock, reducing longitudinal deformations of the pulse to a minimum. These features help suppress the continuous self-injection of electrons (a.k.a. the dark current), making possible to use the entire dephasing length to generate low-background, quasi-monoenergetic GeV electron beams from mm-scale dense plasmas (viz. $n_{e0}\ge5\times10^{18}$ cm$^{-3}$).\\[4pt] [1] S. Y. Kalmykov et al., Phys. Plasmas 18 (2011) 056704;\\[0pt] [2] S. Y. Kalmykov et al., New J. Phys. 14 (2012) 022025. [Preview Abstract] |
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PP8.00114: An unconditionally-stable numerical method for laser-plasma interactions Jonathan Reyes, B.A. Shadwick We have previously presented results surveying various numerical methods for solving the one-dimensional, cold Maxwell-fluid equations in a co-moving coordinate system [1]. Here we present an unconditionally-stable, second-order implicit method that permits a much larger time step than is allowed with a stability-constrained explicit method of the same order and accuracy. As one example, for a laser with frequency $\omega = 20\omega_p$, the implicit method allows a time step 80000 times larger than the largest permissible time step of the explicit method; the size of the time step scales linearly with the wave number. The extra cost in solving implicit equations is negligible compared to the gain in computational performance. Analysis based on the linearized case shows that the dynamics of a forwards propagating mode is faithfully reproduced, while a backwards propagating mode is poorly resolved. This latter mode is inconsequential for studying the physics of laser-plasma accelerators. We assess the accuracy of this method through detailed comparisons with an existing fully-explicit method. We discuss extending this method to higher dimensional problems. \\[4pt] [1] J. Paxon Reyes and B. A. Shadwick, AIP Conf. Proc. 1299, pp. 256-261 [Preview Abstract] |
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PP8.00115: Improved Particle Statistics for Laser-Plasma Self-Injection Simulations Benjamin Cowan, Serguei Kalmykov, Kyle Bunkers, John Cary, Brad Shadwick, Donald Umstadter Simulations of laser-plasma acceleration (LPA) play a key role in understanding the effect of initial conditions on injected beam parameters. Here we present a method for improving the accuracy of simulated particle beams from the LPA self-injection process. We recently demonstrated the ability to compute the collection volume of an injection process -- the range of initial locations of injected particles. We find that the collection volume consists of an annular region around the propagation axis. By loading this region with higher particle statistics than in other locations, we can significantly increase the number of macroparticles in the injected beam. We show that this technique captures much finer detail of particle phase space than does uniform loading, and results in lower noise. We demonstrate convergence of key beam parameters in 2D, and present results of full 3D simulations. In addition, we present results of a novel technique in which particles can deform and split if they expand, effectively self-generating statistics. We also discuss a perfect dispersion algorithm and its impact on self-injection results. [Preview Abstract] |
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PP8.00116: Low noise particle in cell simulations of laser plasma accelerator 10 GeV stages Estelle Cormier-Michel, David L. Bruhwiler, Eric J. Hallman, Benjamin M. Cowan, John R. Cary, Cameron G.R. Geddes, Jean-Luc Vay, Carl B. Schroeder, Eric Esarey, Wim P. Leemans Because of their ultra-high accelerating gradient, laser plasma based accelerators (LPA) are contemplated for the next generation of high-energy colliders and light sources. The upcoming BELLA project will explore acceleration of electron bunches to 10 GeV in a 1 meter long plasma, where a wakefield is driven by a PW-class laser. Particle-in-cell (PIC) simulations are used to design the upcoming experiments. As criteria on energy spread and beam emittance become more stringent, PIC simulations become more challenging as high frequency noise artificially increases those quantities. We show that calculating the beam self-fields using a static Poisson solve in the beam frame dramatically reduces particle noise, allowing for more accurate simulation of the beam evolution. Here, we will focus particularly on beam emittance evolution, where boosted frame simulations are used to model the full scale stages. [Preview Abstract] |
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PP8.00117: Tailored supersonic gas jets for laser plasma accelerators Robert Madden, Mahadevan Krishnan, Bastian Baudisch, Brian Bures, Kristi Wilson-Elliot, Philip Coleman Petawatt class lasers have been used to demonstrate acceleration of electron bunches to $\sim $1-3GeV energy over distances of $\sim $10-30mm, an accelerating gradient of $\sim $100GeV/m. Present gas jets have lengths of only 2-4 mm at densities of 10$^{19}$ cm$^{-3}$, sufficient for self trapping and electron acceleration to energies up to $\sim $150 MeV. Capillary structures 3 cm long have been used to accelerate beams up to 1 GeV. Several concepts have been suggested that use tailored gas density distributions to enhnace the laser plasma acceleration. Stepped profiles (high density followed by lower density) have been suggested in which the short and dense region acts as a nonlinear lens, followed by the lower density and long plateau in which background electrocn are trapped and accelerated by a nonlinear laser wakefield. Other profiles have been suggested to keep the electrons in phase with the wakefield and thereby increase energy and charge in the bunch. Such tailored gas profiles require innovative supersonic gas nozzles, the design of some of which are described. The nozzle flows are mapped using a laser interferometer. The non-axisymmetric density profiles demand multiple measurements at many angles around the azimuth and tomographic reconstruction techniques. S.Y. Kalmykov et al., Plasma Phys. Control. Fusion 53(2011). W. Rittershofer et al., PHYSICS OF PLASMAS 17, 063104, 2010. [Preview Abstract] |
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PP8.00118: Modified Corrugated Plasma Waveguide for Low Energy Electron Trapping in Direct Laser Acceleration Sung Jun Yoon, John Palastro, Daniel Gordon, Thomas Antonsen, Howard Milchberg A laser pulse propagating in a corrugated plasma channel is composed of spatial harmonics whose phase velocities can be subluminal. The subluminal spatial harmonics can be phase matched to relativistic electrons resulting in linear energy gain over the interaction length. However, phase matching over extended acceleration lengths requires large initial electron energies. With a density ramp in the plasma channel, the phase velocity of the spatial harmonic can be gradually increased to $c$ keeping low energy electrons in the accelerating phase over the entire interaction length. Here we examine the self-consistent interaction of the laser pulse and electron beam and low energy electron trapping in quasi-phase matched direct laser acceleration using particle-in-cell simulations. For low electron beam densities, we find that the ponderomotive force of the laser pulse pushes plasma channel electrons towards the propagation axis, causing a deflection of beam electrons. When the beam density is high, the space charge force of the beam drives the channel electrons off axis, providing collimation of the beam. In addition, by using a density ramp in the channel, the trapping energy for a normalized vector potential of $a_0 =0.1$ is reduced from a relativistic factor $\gamma _0 =170$ to $\gamma _0 =10$. [Preview Abstract] |
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PP8.00119: Plasma-based accelerator with magnetic compression Paul Schmit, Nathaniel J. Fisch A novel method is proposed to overcome dephasing and pump depletion in plasma-based accelerators, in which the modulation of a modest (few T) axial, uniform magnetic field in the acceleration channel leads to densification of the plasma through magnetic compression. This enables direct, time-resolved control of the plasma wave properties, including amplitude and phase velocity. The methodology is broadly applicable and can be optimized to improve the leading acceleration approaches, such as plasma beat-wave, plasma wakefield, and laser wakefield acceleration for relativistic electrons, as well as wave acceleration of nonrelativistic ions. In the case of wave-particle dephasing, many technical advantages exist compared to other proposed schemes to overcome dephasing. [Preview Abstract] |
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PP8.00120: Laser guiding due to transverse frequency chirp and plasma inhomogeneity: Relevance to laser wakefield acceleration Vishwa Bandhu Pathak, Jorge Vieira, Ricardo Fonseca, Luis Silva Multi-dimensional particle-in-cell (PIC) simulations using OSIRIS show that the transverse frequency chirp can induce pulse front tilt (PFT) in the laser as it propagates. The PFT leads to transverse inhomogeneity in the electron density at the laser front such that the laser drifts in the transverse direction followed by its wake and the injected/self-injected electron beam inside the blowout region. We further investigate the effect of the chirp and transverse plasma inhomogeneities (linear density gradient and parabolic plasma channel) on the transverse drift by developing an analytical model based on a variational principle approach. Theory and simulations predict a linear dependence of the frequency chirp on the transverse drift. In the presence of a linear density gradient the laser drifts towards the decreasing plasma density. We show that an appropriate transverse chirp can balance the drift, and can reduce/nullify the injected electron beam pointing angle. In extreme scenarios, dispersion effects due to transverse chirp can filament the laser generating multiple bubble in the same transverse plane. [Preview Abstract] |
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PP8.00121: A Basic Study of Plasma Waves/Electromagnetic Waves/Electron Beam Interactions Arnesto Bowman, Ronald Williams Plasma waves are capable of producing accelerating electric fields greater than what is capable by conventional radiofrequency accelerators. In order to understand plasma waves better non-intrusive diagnostics are needed. Intersecting a low energy electron beam perpendicular to a plasma wave has been suggested as a non-disturbing diagnostic. Simulations conveying an electron beam traversing a plasma wave will be presented. The plasma wave was created using the beat-wave technique; therefore, an electron beam traversing residual laser fields will be included the analysis. Inherent electron beam properties such spot size and beam energy will be examined. This will be accomplished by crossing a low energy Helium Neon laser with the previously mentioned electron beam. These simulations are used to imitate a laboratory experiment. The experiment employs a 5 Joule CO2 laser to create the plasma wave and 5-50 keV electron beam to diagnose the plasma wave. [Preview Abstract] |
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PP8.00122: CO$_{2}$ Laser Beat-Wave Experiment in an Unmagnetized Plasma Fei Liu, David Hwang, Robert Horton, Sean Hong, Russell Evans The ability to remotely generate plasma current in dense plasmas is a basic yet important investigation in experimental plasma physics and fusion energy research. It is even more advantageous if the wave penetration is independent of the electron acceleration process. Plasma current can be generated through beat-wave mixing process by launching two intense electromagnetic waves ($\omega >>\omega _{pe})$ into plasma. The beat wave formation process can be efficient if the difference frequency of the two pump waves is matched to a local resonant frequency of the medium, i.e. in this case the local plasma frequency. Beat wave can accelerate plasma electrons via quasi-linear Landau process, which has been demonstrated in a low-density plasma using microwaves.\footnote{Rogers, J. H. and Hwang, D. Q., Phys. Rev. Lett. v68 p3877 (1992).} The CO$_{2}$ lasers provide the high tunability for the wave-particle interaction experiment at a variety of plasma densities with plasma frequency in THz range. Two sections of Lumonics TEA CO$_{2}$ lasers have been modified to serve as the two pump wave sources with peak power over 100MW. The development of the tunable CO$_{2}$ lasers, a high-density plasma target source and diagnostics system will be presented. The initial results of unbalanced beat-wave experiment using one high-power pulsed and one low-power CW CO$_{2}$ lasers will be presented and discussed using the independent plasma source to control the $\omega _{pe}$ of the interaction region. This work is supported by U.S. DOE under Contract No. DE-FG02-10ER55083. [Preview Abstract] |
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PP8.00123: Laser Induced Autoresoance Acceleration of Charged Particle In Vacuum Vikram Sagar, Sudip Sengupta, Predhiman Kaw The exact relativistic dynamics of a charged particle acted upon the electromagnetic fields of a finite duration laser pulse of arbitrary pulse length, in the presence of a static axial magnetic field, is derived. From the solutions it is demonstrated that the particles with different energies can be obtained by tuning the cyclotron frequency of particle with characteristic frequencies determined by the frequency spectra of the laser pulse. The energy gain of the particle may be further improved by subjecting it to the focused field of the laser pulse in an external magnetic field. The focused field of the laser pulse is simply modeled by slow spatial modulation of the laser intensity along the direction of the laser propagation. [Preview Abstract] |
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PP8.00124: Laser Ion Acceleration from Shock Wave Generated Targets Michael Helle, Daniel Gordon, Dmitri Kaganovich, Antonio Ting Efficient acceleration of ions by means of high power laser radiation requires electron plasma densities at or in excess of the critical density. Traditionally, this has been achieved using solid targets. More recently, laser facilities at Brookhaven National Laboratory and the University of California in Los Angeles have achieved acceleration using Terawatt CO$_{2}$ interacting with gas jets. Gas targets are advantageous in that they are relatively simple and can be operated at high repetition rates; however, they typically operate at densities far below those required for optical wavelengths, where most of the world's terawatt lasers operate. To get around this and other issues, a new type of target, a ``gas foil,'' has been developed at the Naval Research Laboratory. The target is created by igniting an optically driven hydrodynamic shock into the flow of a gas jet in vacuum. Experiments have shown that a laser-ignited shock is capable of producing $<$10 $\mu$m gradients, thicknesses $\sim$100 $\mu$m, and peak densities $>$4 times ambient. These results have been incorporated into 3D PIC simulations. Results for a relatively compact and inexpensive 20 TW laser yielded protons with energies in excess of 5 MeV. Simulations as well as preliminary experimental results will be discus [Preview Abstract] |
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PP8.00125: Parametric study of ion acceleration in electrostatic shock waves Elisabetta Boella, Frederico Fi\'{u}za, Anne Stockem, Ricardo Fonseca, Luis Silva In past years, a considerable effort has been devoted to investigate the acceleration of ions by means of laser-plasma interactions. Recently, a new mechanism has been proposed to accelerate ions efficiently in laser-driven electrostatic shock waves. As experiments and numerical simulations demonstrated [1, 2], shocks with low Mach number can be generated in near critical density plasmas; such shocks are able to accelerate ions having an energy spectrum suitable for medical applications. We have developed a reduced electrostatic code [3] that captures all the relevant 1D physics of shock formation and particle acceleration; the code has been benchmarked with full-PIC OSIRIS calculations. In the poster, we will present results from a detailed parameter scan for different plasma temperature and density profiles inducing the shock formation; moreover, the optimal conditions for the generation of high energy and high quality ions beams will be illustrated.\\[4pt] [1] D. Haberberger \emph{et al.}, Nature Phys. {\bf 8}, 95 (2012)\\[0pt] [2] F. Fi\'uza \emph{et al.}, submitted to Phys. Rev. Lett. (2012)\\[0pt] [3] E. Boella \emph{et al}., Bull. Am. Phys. Soc. QRP1 (2011) [Preview Abstract] |
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PP8.00126: Laser-ion acceleration through controlled surface contamination Bixue Hou, Zhaohan He, John Nees, George Petrov, Jack Davis, Alexander Thomas, Karl Krushelnick Ion acceleration from interaction of intense laser pulses with solid targets was dominated by protons. This phenomenon is attributed to the presence of protons in the water and hydrocarbon residue on the target surface. Substitution of the contaminant layer on the target surface with a desired species can lead to selection of accelerated ion species and enhancement of acceleration. With a focal intensity of $\sim $4x10$^{18}$W/cm$^{2}$ at 0.5 kHz repetition rate, deuterons up to 75 keV are accelerated from a glass target simply by placing 1 mL of heavy water inside the experimental chamber prior to vacuum-pumping to generate a deuterated contamination layer on the target. Using the same technique with a deuterated-polystyrene-coated target also enhances deuteron yield by a factor of three to five while increasing the maximum energy of the generated deuterons to 140 keV. This technique was further developed by other researchers; the solid target is cryogenic-cooled to produce an ice layer of heavy water for deuteron acceleration. [Preview Abstract] |
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PP8.00127: Ion acceleration in the RPA regime by shaped pulses Young-Kuk Kim, Min Sup Hur Recently we presented a controllable pulse shaping by relativistic transparency in non-uniform, overdense plasmas [1]. In this shaping scheme, by tapering the density and thickness of an overdense plasma slab, the pulse front can be carved into various figures such as transversely flat or concave shape with longitudinally sharp pulse fronts. As an application of such a novel scheme of the pulse shaping, we studied the effects of the shaped pulse on ion beam energy, charge, and energy spread in the radiation pressure dominant regime. From the 2-dimensional PIC simulations, we observed that the flat pulse produces more energetic proton beam than a usual Gaussian beam, and concave pulse yields even more abundant proton beam. \\[4pt] [1] M.S. Hur et al., ``Versatile shaping of a relativistic laser pulse from a nonuniform overdense plasma,'' Phys. Plasmas, (accepted, to appear in 2012). [Preview Abstract] |
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PP8.00128: On the role of electrostatic potential well on electron acceleration in pre-plasma Booshan Paradkar, Sergei Krasheninnikov, Farhat Beg Recent experiments have shown that the presence of a pre-formed plasma in front of a solid target, produced due to the laser pre-pulse, results in a strong increase of both averaged and maximum energies of electron beam generated due to the interactions of main laser pulse with the target (e.g. see Ref.1). Moreover, these energies can greatly exceed corresponding ponderomotive scaling. This can be very beneficial from the point of view of the generation of energetic ($\sim $100 MeV) proton beams. However, until very recently the underlying physical mechanism of electron heating enhancement caused by pre-plasma was not clear. Numerical simulations reveal the formation of deep asymmetric electrostatic potential well in the pre-plasma region [2]. This potential well is formed due to strong electron heating caused by the synergistic effect of electron interactions with laser and potential well [3], which resembles the Fermi acceleration mechanism. In this work we describe the electron heating mechanism, physics of the formation of electrostatic potential well, and present the scaling for maximum electron energy, which can be gained due to synergistic effect of electron interactions with laser field and potential well in pre-plasma.\\[4pt][1] T. Yabuuchi, et al., PoP \textbf{17}, 060704 (2010); [2] B. S. Paradkar, et al., PRE \textbf{83}, 046401 (2011); [3] B. S. Paradkar, S. I. Krasheninnikov, and F. N. Beg, PoP \textbf{19}, 060703 (2012). [Preview Abstract] |
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PP8.00129: Generation of high-energy ($>$15 MeV) neutrons using short pulse lasers Jack Davis, George Petrov, Tzvetelina Petrova, Drew Higginson, Farhat Beg The production of high-energy ($>$15 MeV) neutrons has been demonstrated experimentally for the first time using the Titan laser as a driver of high-energy ion beams. Neutrons with energy of up to 18 MeV have been generated from $^{7}$Li(d,n)$^{8}$Be reactions driven by laser pulses with peak intensity 2$\times $10$^{19}$ W/cm$^{2}$, pulse duration of 9 ps and energy of 360 J. Three nuclear reactions, d(d,n)$^{3}$He, $^{7}$Li(d,n)$^{8}$Be, and $^{7}$Li(p,n)$^{7}$Be have been explored as potential candidates for high-energy neutron production using a 3D Monte Carlo simulation model. For each reaction the required driver ion beam energy and number have been determined. We found that for the $^{7}$Li(p,n)$^{7}$Be reaction 10$^{10}$ protons with energy $>$20 MeV are required to generate high-energy neutrons, while for the $^{7}$Li(d,n)$^{8}$Be reaction a comparable amount of deuterons with energy $>$5 MeV are needed. [Preview Abstract] |
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PP8.00130: MPI implementation of a generalized implicit algorithm for multi-dimensional PIC simulations George Petrov, Jack Davis The implicit 2D3V particle-in-cell (PIC) code developed to study the interaction of short pulse lasers with matter [G. M. Petrov and J. Davis, Computer Phys. Comm. 179, 868 (2008); Phys. Plasmas 18, 073102 (2011)] has been parallelized using MPI (Message Passing Interface). Performance evaluation has been made on a Linux cluster for two typical regimes of PIC operation: ``particle dominated,'' for which the bulk of the computation time is spent on pushing particles, and ``field dominated,'' for which computing the fields is prevalent. The MPI implementation of the code offers a significant numerical speedup, particularly in the ``particle dominated'' regime, which will allow extension to three dimensions and implementation of atomic physics. [Preview Abstract] |
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PP8.00131: Monoenergetic ion acceleration and Rayleigh-Taylor instability of the composite target irradiated by the laser pulse Vladimir Khudik, S. Austin Yi, Gennady Shvets Acceleration of ions in the two-specie composite target irradiated by a circularly polarized laser pulse is studied analytically and via particle-in-cell (PIC) simulations. A self-consistent analytical model of the composite target is developed. In this model, target parameters are stationary in the center of mass of the system: heavy and light ions are completely separated from each other and form two layers, while electrons are bouncing in the potential well formed by the laser ponderomotive and electrostatic potentials. They are distributed in the direction of acceleration by the Boltzmann law and over velocities by the Maxwell-Juttner law. The laser pulse interacts directly only with electrons in a thin sheath layer, and these electrons transfer the laser pressure to the target ions. In the fluid approximation it is shown, the composite target is still susceptible to the Rayleigh-Taylor instability [1]. Using PIC simulations we found the growth rate of initially seeded perturbations as a function of their wavenumber for different composite target parameters and compare it with analytical results. Useful scaling laws between this rate and laser pulse pressure and target parameters are discussed.\\[4pt] [1] T.P. Yu, A. Pukhov, G. Shvets, M. Chen, T. H. Ratliff, S. A. Yi, and V. Khudik, Phys. Plasmas, 18, 043110 (2011). [Preview Abstract] |
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PP8.00132: X-ray pulse generation via laser Compton scattering using quasi-monoenergetic electron beam driven by laser acceleration Eisuke Miura, Ryunosuke Kuroda, Hiroyuki Toyokawa We have demonstrated X-ray generation via laser Compton scattering using a laser-accelerated quasi-monoenergetic electron beam. X-rays were generated by scattering a femtosecond laser pulse (800 nm, 140 mJ, 100 fs) off a quasi-monoenergetic electron beam containing 70 pC electrons in the monoenergetic peak with an energy of 60 MeV produced by focusing an intense laser pulse (800 nm, 700 mJ, 40 fs) on a helium gas jet. A well-collimated X-ray beam with a divergence angle of approximately 5 mrad was generated. The number of X-ray photons was estimated to be $2\times 10^{7}$ per pulse. The characteristics of X-rays were also investigated using simulation. The spectrum of X-rays emitted within the scattered angle of 5 mrad had a quasi-monochromatic structure with a peak at 60 keV. The number of X-ray photons was $1.8 \times 10^{7}$, which was in a good agreement with the experimental result. The allowance delay range between the two laser pulses for X-ray generation was approximately 100 fs, and was nearly equal to the duration of the laser pulse scattered by the electron pulse. This suggests that the X-ray pulse duration was shorter than 100 fs. [Preview Abstract] |
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PP8.00133: ABSTRACT WITHDRAWN |
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PP8.00134: X-Ray Laser Driven Gold Planar Targets Tzvetelina Petrova, Kenneth Whitney, Jack Davis, George Petrov A non-equilibrium ionization model is assembled to investigate the subpicosecond ionization dynamics of the hole states, that are created and destroyed when an incident coherent high intensity x-ray laser pulse impinges on planar gold targets. There are two aspects to this modeling. One is the construction of simplified atomic models of gold and of its adjacent ions. Second is the study of the nonlinear optical dynamics and absorption physics of an x-ray pulse interacting with a gold target as a function of the x-ray wavelength, pulsewidth, and intensity. The ionization levels and excited state populations reached during an interaction provide important diagnostics of the x-ray pulse. Of interest are the $\sim $ 4.45 keV x-ray pulse generated in previous KrF experiments\footnote{Borisov \textit{et. al.} J. Phys B \textbf{41} 105602 (2008).} and extensively modeled\footnote{Petrova \textit{et. al.} J. Phys B \textbf{43} 025602 (2010), \textbf{44} 125601 (2011); HEDP \textbf{8} 209 (2012).}$^,$\footnote{Davis\textit{ et. al.} HEDP \textbf{8} 238 (2012).}$^,$\footnote{Whitney\textit{ et. al.}, PRA 003400 (2012).} as well as the x-ray pulses generated at SLAC. [Preview Abstract] |
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PP8.00135: Seed laser chirping for enhanced backward Raman amplification in plasmas Z. Toroker, V.M. Malkin, N.J. Fisch Backward Raman compression in plasma enables pulse compression to intensities not available using material gratings. Mediating the compression with higher density plasma (moderately undercritical plasmas) generally produces shorter and therefore more intense output pulses. However, very high density plasma, even if sufficiently tenuous to be transparent to the laser, also produces group velocity dispersion of the amplified pulse, deleteriously affecting the interaction. Nevertheless the seed dispersion can be used advantageously, by chirping the seed pulse, the length to reach the highest intensity is considerably reduced. The fact that the plasma length is reduced has considerable advantages: first, there is less distance over which deleterious instabilities can develop, such as forward Raman amplification; second, there is less radiative loss through inverse bremsstrahlung; and, third, less plasma is needed in the first place, requiring less input pump energy. [Preview Abstract] |
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PP8.00136: A two-color terawatt laser system for high-intensity laser-plasma experiments James Sanders, Rafal Zgadzaj, Michael Downer In some high-field laser-plasma experiments, it is advantageous to accompany the main high-energy ($\sim $1 J) laser with a second high-energy pulse ($\sim $0.1 J) which has been frequency-shifted by $\sim $10{\%}. Such a pulse-pair would have a low walk-off velocity while remaining spectrally distinct for use in two-color pump-probe experiments. Moreover, by shifting the second pulse by $\sim $plasma frequency, it is theoretically possible to enhance or suppress relativistic self-focusing, which is the first (uncontrolled) step in many laser-plasma experiments. We report a hybrid chirped pulse Raman amplifier (CPRA)/Ti-Sapphire amplifier ($>$200 mJ, 15-20 nm bandwidth (FWHM), $>$60 fs duration) that is capable of performing such two-color high-field experiments. When amplified and compressed, this beam's power exceeds 1 TW. This two-color capability can be added to any commercial terawatt laser system without compromising the energy, duration or beam quality of the main system. We will report progress with a two-color seeded relativistic self-phase modulation experiment. [Preview Abstract] |
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PP8.00137: Fluid theory of beam spiraling in high intensity cyclotrons Antoine Cerfon, Felix Parra, Jeffrey Freidberg Novel uses of cyclotrons for basic science, national security and medical therapy applications require the production of beams with ever higher intensities. At these high intensities, uncontrolled beam loss must be minimized. To satisfy the stringent beam loss criteria, it is crucial to have a detailed understanding of the beam dynamics in high intensity cyclotrons, and more specifically, an understanding of the effects of space charge on the dynamics. Using a two-dimensional fluid description, we investigate the nonlinear radial-longitudinal dynamics of intense beams in cyclotrons. With a multiscale analysis separating the time scale associated with the betatron motion and the slower time scale associated with space-charge effects, we show that the longitudinal-radial vortex motion can be understood in the frame moving with the charged beam as the nonlinear advection of the beam by the ExB velocity field, where E is the electric field due to the space charge and B is the external magnetic field. This interpretation provides simple explanations for the stability of round beams and for the development of spiral halos in elongated beams. By numerically solving the advection equation, we show that it is also in quantitative agreement with results obtained in PIC simulations. [Preview Abstract] |
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PP8.00138: A class of generalized Kapchinskij-Vladimirskij solutions and associated envelope equations for high-intensity charged particle beams Hong Qin, Ronald C. Davidson A class of generalized Kapchinskij-Vladimirskij (KV) solutions of the nonlinear Vlasov Maxwell equations and the associated envelope equations for high-intensity beams in a periodic lattice has been derived. It includes the classical Kapchinskij-Vladimirskij solution as a special case. In the classical KV solution, for a given focusing lattice waveform and a specified line density of the beam, the distribution function and associated envelope equations are specified by two free parameters, i.e., the transverse emittances in two transverse directions. In the generalized solutions described here, the distribution function and associated envelope equations are specified by eight free parameters, i.e., two transverse emittances and two 2x2 symmetric and positive definite matrices. The new class of solutions captures a wider range of dynamical envelope behavior for high-intensity beams, and thus provides a new theoretical tool to investigate the dynamics of high-intensity beams. [Preview Abstract] |
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PP8.00139: Compact energy selector for use with intense, short-pulse laser produced proton beams Andrew Hazi, Hui Chen, Frederic Perez, Edward Marley, Jaebum Park, Jackson Williams, Laura Vassura, Julien Fuchs, Sophia Chen, Ronnie Shepherd Irradiation of thin solid targets with short, intense laser pulses produces energetic charged particles. The proton and ion beams generated from such laser-plasma interactions have several attractive features, but usually exhibit a broad energy distribution extending up to tens of MeV. However for some applications, such as energy-loss measurements in plasmas or injection into high-energy accelerators, quasi-mono energetic beams are preferred [1]. We have designed, built and tested a small (9 x 7 x 5 cm$^{3})$ energy selector for use with laser-produced proton beams in beam-plasma interaction experiments that utilize multiple laser beams. The device uses permanent magnets in a dipole configuration, with a fixed entrance aperture and an adjustable exit slit to select a narrow portion of the broad energy distribution in the beam. The energy selector was tested in a recent experiment at the Titan laser at Livermore. Sample data from the experiment and simulations of the device's characteristics will be presented. \\[4pt] [1] T. Toncian, et al., ``Ultrafast Laser--Driven Microlens to Focus and Energy-Select Mega--Electron Volt Protons,'' Science, 312, 410 (2006). [Preview Abstract] |
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PP8.00140: Investigation of Nonlinear Collective Dynamics and Excitations in Intense Charged Particle Beams and Development of Laser-Induced-Fluorescence (LIF) Diagnostic Using the Paul Trap Simulator Experiment Hua Wang, Erik Gilson, Ronald Davidson, Philip Efthimion, Richard Majeski The Paul Trap Simulator Experiment (PTSX) is a compact Paul trap that simulates the nonlinear transverse dynamics of an intense charged particle beam propagating through an equivalent kilometers-long magnetic alternating-gradient (AG) focusing system. Understanding of the collective behavior and mechanisms of collective instabilities of intense charged particle beams is of critical importance to a wide range of accelerator applications. Collective modes can occur naturally or be excited by external perturbations in an intense charged particle beam. In the experiments presented here, different external perturbations including quadrupolar and dipolar perturbations are employed to excite the collective modes. The comparison between experimental results using different external perturbations is shown to describe the characteristics of each driving scheme. Finally, a laser-induced-fluorescence (LIF) technique is being developed to provide \textit{in situ} measurements of the radial density profile and, ultimately, the velocity distribution function of the intense charged particle beam. The new laser system and the new barium source will be described, and initial experimental results of the LIF technique will be presented. [Preview Abstract] |
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PP8.00141: Simulations of Ion Coupling Experiments on NDCX-II relevant to IFE J.J. Barnard, R.M. More, M. Terry The Neutralized Drift Compression Experiment II (NDCX-II) is an induction accelerator for which the construction project was completed at Lawrence Berkeley National Laboratory in March, 2012, and is presently being commissioned. The baseline design for NDCX-II will accelerate $\sim $0.03 $\mu $C of singly charged lithium ions to 1.2 MeV (with possible upgrades up to 3.1 MeV), delivered in sub-ns pulses with sub-mm rms beam radii. The purpose of NDCX-II is to carry out beam and target interaction experiments relevant to IFE. We have carried out detailed hydrodynamic simulations of planar targets having several configurations. In this poster we will focus on experiments that maximize shock strength by traveling wave deposition (i.e. by varying ion beam energy in a velocity chirp) and/or by varying intensity profile, and we will also explore methods to optimize shock strengths in composite materials where shocks can be formed at material boundaries and at end-of-range. These results will be discussed in the context of heavy ion fusion direct drive targets. [Preview Abstract] |
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PP8.00142: Alternate Operating Modes For NDCX-II W.M. Sharp, A. Friedman, D.P. Grote, R.H. Cohen, S.M. Lund, J.-L. Vay, W.L. Waldron NDCX-II is a newly completed accelerator facility at LBNL, built to study ion-heated warm dense matter and aspects of ion-driven targets for inertial-fusion energy. The baseline design calls for using twelve induction cells to accelerate 40 nC of Li+ ions to 1.2 MeV. During commissioning, though, we plan to extend the source lifetime by extracting less total charge. For operational flexibility, the option of using a helium plasma source is also being investigated. Over time, we expect that NDCX-II will be upgraded to substantially higher energies, necessitating the use of heavier ions to keep a suitable deposition range in targets. Each of these options requires development of an alternate acceleration schedule and the associated transverse focusing. The schedules here are first worked out with a fast-running 1-D particle-in-cell code ASP, then 2-D and 3-D Warp simulations are used to verify the 1-D results and to design transverse focusing. [Preview Abstract] |
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PP8.00143: Focusing Intense Charged Particle Beams with Achromatic Effects for Heavy Ion Fusion James Mitrani, Igor Kaganovich Final focusing systems designed to minimize the effects of chromatic aberrations in the Neutralized Drift Compression Experiment (NDCX-II) are described. NDCX-II is a linear induction accelerator, designed to accelerate short bunches at high current. Previous experiments showed that neutralized drift compression significantly compresses the beam longitudinally ($\sim$60x) in the z-direction, resulting in a narrow distribution in z-space, but a wide distribution in p$_z$-space. Using simple lenses (e.g., solenoids, quadrupoles) to focus beam bunches with wide distributions in p$_z$-space results in chromatic aberrations, leading to lower beam intensities (J/cm$^2$). Therefore, the final focusing system must be designed to compensate for chromatic aberrations. The paraxial ray equations and beam envelope equations are numerically solved for parameters appropriate to NDCX-II. Based on these results, conceptual designs for final focusing systems using a combination of solenoids and/or quadrupoles are optimized to compensate for chromatic aberrations. Lens aberrations and emittance growth will be investigated, and analytical results will be compared with results from numerical particle-in-cell (PIC) simulation codes. [Preview Abstract] |
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PP8.00144: Characterizing the Performance of the Princeton Advanced Test Stand Ion Source A. Stepanov, E.P. Gilson, L. Grisham, I. Kaganovich, R.C. Davidson The Princeton Advanced Test Stand (PATS) is a compact experimental facility for studying the physics of intense beam-plasma interactions relevant to the Neutralized Drift Compression Experiment - II (NDCX-II). The PATS facility consists of a multicusp RF ion source mounted on a 2 m-long vacuum chamber with numerous ports for diagnostic access. Ar+ beams are extracted from the source plasma with three-electrode (accel-decel) extraction optics. The RF power and extraction voltage (30 - 100 kV) are pulsed to produce 100 $\mu $sec duration beams at 0.5 Hz with excellent shot-to-shot repeatability. Diagnostics include Faraday cups, a double-slit emittance scanner, and scintillator imaging. This work reports measurements of beam parameters for a range of beam energies (30 - 50 keV) and currents to characterize the behavior of the ion source and extraction optics. Emittance scanner data is used to calculate the beam trace-space distribution and corresponding transverse emittance. If the plasma density is changing during a beam pulse, time-resolved emittance scanner data has been taken to study the corresponding evolution of the beam trace-space distribution. [Preview Abstract] |
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PP8.00145: Theory and Simulation of Passive Focusing of an Ion Beam Propagating Through Thin Foils S.M. Lund, R.H. Cohen, P. Ni, A. Yuen Ion beams (including protons) with low emittance and high space-charge intensity can be propagated with normal incidence through a sequence of thin metallic foils separated by vacuum gaps to collimate the beam or to focus it to a small transverse spot. The foils attenuate the beam's defocusing electrostatic field, allowing the focusing magnetic pinch force to dominate. We present envelope calculations and particle simulations of this effect, with particular application to near-term experiments using intense proton beams derived from laser-illuminated foils (TNSA process) and fusion drivers using accelerator-produced heavy ions. Several extensions have been added to earlier idealized calculations,\footnote{S.M. Lund, R.H. Cohen and P Ni, ``Envelope Model for Passive Magnetic Focusing of an Intense Ion or Proton Beam Propagating Through Thin Foils'', Phys. Rev. ST-AB, in press (2012)} including finite pulse length, electrons emitted from and ion scattering in foils, and realistic velocity-space distributions of the incident beam. [Preview Abstract] |
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PP8.00146: Scattering in Thin Foils Associated with Passive Proton and Ion Beam Focusing Albert Yuen, Steven Lund, John Barnard, Ronald Cohen Recent theory and simulations on passive focusing of intense proton and ion beams propagating through a stack of thin foils demonstrate the validity of the concept under several idealizations [1]. The beam was assumed to penetrate many (hundreds) of metallic foils with negligible beam attenuation or scattering. The foils must also be thick enough for mechanical strength and electron stopping for the scheme to work. Experiments with protons may use Al foils with $\sim$1 micron thickness. Here, we analyze the impact of finite scattering on the focusing concept. Analytical formulas characterizing the scattering effect are derived. The TRIM code [2], is applied for the case of protons in Al to extract a scaled fit to the analytical formula for the rms scattering angle of particles in the foil that scales as sqrt(t)/E where t is the foil thickness and E is the beam energy. The scattering strength is incorporated in a moment model to calculate the beam emittance growth through a single foil. Results are analyzed to estimate focusing limits due to foil scattering by incorporating the effect in an envelope model in Refs. [1] to characterize deviations of results with and without scattering.\\[4pt] [1] Lund, Cohen and Ni, PRSTAB, in press (2012).\\[0pt] [2] Ziegler and Biersack, NIMB 268, 1818 (2010). [Preview Abstract] |
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PP8.00147: Focusing and transport of high-energy protons in solid targets of different materials J. Kim, B. Qiao, C. McGuffey, D.C. Gautier, M.S. Wei, R.B. Stephens, E.M. Giraldez, M.E. Foord, M.H. Key, H.S. McLean, P.K. Patel, F.N. Beg Proton beams must transition into dense plasma for applications ranging from isochoric heating of plasma [1] to imaging implosion dynamics and magnetic fields [2, 3]. However, high-current proton beam interaction with plasma is complex and poorly understood. We present recent experimental and simulation results on the study of proton beam transport within solid density. The experiment was conducted on the TRIDENT laser (75 J, 0.6 ps) at LANL. Focusing proton beams produced from Au partial hemisphere targets heated a secondary solid transport foil with varied thickness and Z-material, specifically, Mylar, Al, Cu and Au. XUV emission from the rear of the transport foil indicated a clear dependence of proton beam transport on Z. Better focusing of the proton beam was achieved after transport through the higher Z foils. 2D PIC simulations using LSP helped to clarify the transport dynamics.The work was performed under the auspices of the U.S. DOE contract DE-SC0001265.\\[4pt] [1] P. K. Patel, et al., Phys. Rev. Lett. 91, 125004 (2003).\\[0pt] [2] M. Borghesi, et al., Plasma Physics and Controlled Fusion, 43, A267 (2001).\\[0pt] [3] C. K. Li, et al., Phys. Rev. Lett. 100, 225001 (2008). [Preview Abstract] |
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PP8.00148: Extended Underwater Plasma Generation Using Laser Filamentation T.G. Jones, M.H. Helle, D. Kaganovich, D. Gordon, A. Ting, J. Penano Techniques to trigger and guide underwater electrical discharges using a laser are currently being developed at NRL. This work may be useful for a variety of applications, including advanced micromachining and low-frequency laser acoustic generation. As part of this development we are studying underwater optical filamentation, which is the extended propagation of a small diameter high-power laser beam, and which typically includes a coincident plasma column. Our group recently made the first demonstration and characterization of ns underwater filaments over 50 cm in length (over 30 Rayleigh lengths). We are also developing a two-laser-pulse ionization and heating scheme to generate an extended underwater plasma and subsequent vapor channel for electrical discharge guiding. Initial extended underwater plasma generation results, as well as results from ongoing underwater laser filamentation experiments and laser plasma characterization experiments, will be presented. [Preview Abstract] |
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PP8.00149: 2D Numerical Model And Self-Consistent Particle-In-Cell Simulations Of Coherent Synchrotron Radiation Thomas Kwan, Chengkun Huang, Bruce Carlsten Understanding CSR effects in a bunch compressor requires accurate and self-consistent dynamical simulations accounting for the realistic beam shape and parameters, transient dynamics and possibly a material boundary. We first extend the well-known 1D CSR model into two dimensions and develop a simple numerical algorithm based on the Lienard-Wiechert formula for the electric field of a stiff beam. This numerical model includes the 2D spatial dependence of the field in the bending plane and is accurate for arbitrary beam energy. It also removes the singularity in space charge field presented in a 1D model. Good agreement is obtained with 1D CSR analytic [1] result for FEL related beam parameters but deviations are also found for low-energy or large spot size beams and off-axis fields. We also employ fully electromagnetic Particle-In-Cell (PIC) simulations for self-consistent CSR modeling. The relatively large numerical phase error and anisotropy in a standard PIC algorithm is improved with a high order Finite Difference Time Domain scheme. Detail self-consistent PIC simulations of the CSR fields and beam dynamics will be presented and discussed. [Preview Abstract] |
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PP8.00150: Theoretical and Experimental study of Electric field Screenings of Carbon Fiber Field Emitters Wilkin Tang, Don Shiffler, Matthew LaCour, Ken Golby, Tim Knowles Field emitter arrays have the potential to provide high current density, low voltage operation, and high pulse repetition for radar and communication. It is well known that packing density of the field emitter arrays significantly affect the emission current. Previous experiments were conducted with 1000s of field emitters which makes the analysis of electric field screening difficult. Here we describe experiments in a dual-cathode and four-cathode configuration. The experiments used different number of carbon fiber field emitters (two and four) with variable spacing to investigate the effect of electric field screening on current emission. Emission characteristic is compared for the case of two and four field emitters with different spacing. Analytic model and Particle-in-cell simulations are performed to compare with the experiments. [Preview Abstract] |
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PP8.00151: Theory and Experimental Characterization of Multipactor RF Window Breakdown Geoffrey Greening, Matthew Franzi, Peng Zhang, Y.Y. Lau, Adam Schutt, Ronald Gilgenbach Multipactor breakdown of RF windows is a potential defense against high-power microwaves. By applying a DC bias across a dielectric window in a vacuum-gas environment, the threshold for the onset of RF-initiated multipactor can be lowered. Recent Monte Carlo simulations of multipactor in a background gas with a DC bias have provided a theoretical baseline for comparison against experimental results [1]. Prior experimental work used a 1 kW CW, 2.45 GHz magnetron to direct RF at a Lucite vacuum window with embedded copper wires providing the DC bias. Results confirmed that a DC bias was effective at reducing the threshold for multipactor in air at sub-torr pressures, though with high variability. Continuing efforts include exploration of breakdown in argon at $>$5 torr to improve reproducibility of experimental results. Ongoing work is also combining the theories developed in [1] and [2] to characterize multipactor susceptibility in the presence of a background gas and a static magnetic field using Monte Carlo simulations. \\[4pt] [1] P. Zhang et al., Phys. Plasmas 18, 053508 (2011).\\[0pt] [2] A. Valfells et al., Phys. Plasmas 7, 750 (2000). [Preview Abstract] |
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PP8.00152: Rapid Formation of Distributed Plasma Discharges using X-Band Microwaves Xun Xiang, Brian Kupczyk, John Booske, John Scharer Observations of rapidly formed ($<$50-300 ns) distributed plasma discharges using X-band microwaves are presented. A cylindrical stainless steel chamber enclosed with polycarbonate windows is used to observe microwave breakdown in Ar and Ne gas from 10 to 760 torr. The chamber is illuminated by the output of 25 kW, 0.8 $\mu $s pulse-width, 9.382 GHz magnetron through an X-band waveguide pressed against the polycarbonate window. Measured incident, reflected, and transmitted microwave power to a moveable monopole antenna located beyond the discharge chamber are used to detect the discharge and attenuation characteristics as the pressure is varied. Observations of localized transmission spike measurements of -20 dB that occur within 50 ns caused by the plasma under certain conditions have been made. Additionally, an ICCD provides fast ($<$50 ns) time-scale optical images of the plasma, revealing the plasma formation and decay processes. Optical emission spectroscopy measurements provide plasma breakdown characteristics including gas and electron temperatures, plasma density and the plasma lifetime. Plasma modeling is also used to compare the experimental data with theoretical models. [Preview Abstract] |
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PP8.00153: Experimental Studies of sub-THz Gyrotron with Pulsed Solenoid for Air Breakdown Investigation Dmytro Kashyn, Gregory Nusinovich, John Rodgers, Carlos Romero-Talam\'as, Anatoly Shkvarunets The development of sub-THz gyrotron for air breakdown studies is one of the research tasks under the Center of Applied Electromagnetics program in University of Maryland. The goal is to remotely detect concealed radioactive materials as described by V. L. Granatstein and G. S. Nusinovich (J. Appl. Phys \textbf{108 }063304 (2010)). There it was proposed to focus high-power sub-THz radiation in a small volume where the wave field exceeds the breakdown threshold. The presence of the radioactive material in the vicinity ($\le $ 20-40m) of such volume significantly increases the probability of the air breakdown. The gyrotron can serve as a source of sub THz radiation required for this scheme. We report our experimental activities on the sub-THz gyrotron operating at 670 GHz at TE 31,8 mode with 28T pulsed magnetic field. This tube was developed in collaboration with Institute of Applied Physics of Russian Academy of Science. Our team was responsible for the design of major components while our colleagues manufactured the tube. We achieved 80 kW of output power in 10$\mu $s pulses which corresponds to 0.2 J of energy. We introduced several improvements to the original design addressing the issues with discharges and multipactoring that were impeding the performance of the tube. Unfortunately we had a catastrophic failure which ruined the existing device. We are now working on the design of another gyrotron that will operate at 220 GHz and can be capable of delivering 250-350 kW of RF power. [Preview Abstract] |
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PP8.00154: High-Gradient Photonic Bandgap (PBG) Accelerator Structure Breakdown Testing Brian Munroe, Michael Shapiro, Richard Temkin, Roark Marsh, Valery Dolgashev, Sami Tantawi, Anahid Yeremian Photonic bandagp (PBG) accelerator structures provide unique insight into the surface physics involved in vacuum breakdown in accelerator structures because of their unique surface E and H fields. Peak operating gradients in accelerator structures are limited by surface breakdowns. These breakdowns occur at regions of high surface E and H fields. The maximum surface H field can easily be varied relative to the peak surface E field and accelerating gradient in PBG structures, allowing new regimes of parameter space to be explored. A standing wave PBG structure utilizing elliptical rods at the highest field locations has been tested at SLAC to determine the breakdown properties of the structure under high-gradient operation. The structure achieved a maximum gradient of approximately $125$ MV/m at a breakdown probability of $3.6 *10^{-3}$ per pulse per meter for 150 ns pulses. This performance is comparable to conventional disc-loaded waveguide accelerator structures, but in a structure that provides intrinsic damping of wakefields. [Preview Abstract] |
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PP8.00155: A 250 GHz Photonic Band Gap Gyrotron Amplifier Emilio A. Nanni, Samantha M. Lewis, Michael A. Shapiro, Richard J. Temkin Initial results for a high power 250 GHz gyrotron traveling wave tube (gyro-TWT) amplifier will be presented. The amplifier uses a novel photonic band gap (PBG) interaction circuit that confines the TE$_{03}$-like mode for operation. Stability from oscillations in lower order modes is provided by the PBG circuit. At 26.6 kV and 0.25 A the gyro-TWT operates with peak small signal gain of 27.3 dB at 251 GHz. The instantaneous -3 dB bandwidth of the amplifier at peak gain is 0.4 GHz. The amplifier can be tuned for operation from 245-254 GHz. A peak output power of 7.5 W has been measured. Experimental results taken over a wide range of parameters, 15-30 kV and 0.25-0.5 A, show good agreement with a theoretical model in the small signal gain regime. The theoretical model incorporates cold test measurements for the transmission line, input coupler, PBG waveguide and mode converter. [Preview Abstract] |
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PP8.00156: 94 GHz Over-moded Traveling Wave Tube (TWT) Elizabeth J. Kowalski, Michael A. Shapiro, William C. Guss, Richard J. Temkin There is great interest in increasing the power and bandwidth of slow wave devices in order to meet developing requirements at W Band frequencies. One approach to W-Band amplifiers is the use of over-moded W-band TWTs. The design and cold test of an over-moded 94 GHz Coupled-Cavity Traveling Wave Tube (TWT) with operation in the rectangular TM$_{31}$ mode is discussed. Lower order modes in the over-moded TWT are suppressed with selective dielectric loading. This over-moded design allows for a larger cavity size, easier manufacturing, and larger beam tunnel than the equivalent fundamental-mode circuit; these factors lead to a large gain and high average power in the TWT at 94 GHz. Simulations with CST Particle Studio show 31 dB of gain with 300 W peak output power for the over-moded TWT design. Cold test results for a 9-cavity structure agree well with HFSS simulations. This design and experiment will be useful in expanding the use of TWTs to higher frequency ranges and impinging on the THz gap. The design also considers the practicality of experimentally testing this over-moded TWT. The over-moded TWT experiment is planned and is currently in set-up. [Preview Abstract] |
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PP8.00157: A Two Frequency 1.5 MW Gyrotron Experiment David Tax, William Guss, Michael Shapiro, Richard Temkin, Ben Rock, Ronald Vernon, Jeffrey Neilson Megawatt gyrotrons are an important microwave source for electron cyclotron heating and current drive (ECH/ECCD) in fusion plasmas due to their ability to produce megawatts of power at millimeter wave frequencies. The MIT gyrotron operates nominally at 96 kV and 40 A with 3 $\mu $s pulses and has previously demonstrated 1.5 MW of output power with $>$ 50 {\%} efficiency at 110 GHz with a depressed collector. A new cavity has been designed for 1.5 MW operation at two distinct frequencies: 110 GHz in the TE$_{22,6 }$mode and 124.5 GHz in the TE$_{24,7 }$mode. A new internal mode converter (IMC) consisting of a dimpled wall launcher and four smooth curved mirrors has also been designed and was optimized for both modes. Simulations of the IMC indicate that $>$ 98 {\%} Gaussian beam content could be achieved for each mode. Cold test results for the components will be presented as well as the current status of the hot test experiment. [Preview Abstract] |
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PP8.00158: Progress of a 140 GHz gyro-amplifier for DNP NMR Studies Alexander Soane, Emilio Nanni, Michael Shapiro, Richard Temkin We report on the experimental progress of a 140 GHz pulsed gyro-amplifier being developed at MIT for enhanced Dynamic Nuclear Polarization for NMR (DNP-NMR). A confocal geometry configuration is being studied as an approach to an overmoded interaction waveguide. The system has achieved 20 dB of gain and 150 watts of peak power over a 2 $\mu$s pulse. Additionally, computational work has been performed that investigates the effect of the cylindrically-asymmetric confocal geometry on predicted gain. The computational results show that while linear gain is equivalent to that of a circular interaction waveguide, saturated gain is lower by as much as 50\%. [Preview Abstract] |
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PP8.00159: ECH on the Madison Plasma Dynamo Experiment Jason Milhone, Mike Clark, Cami Collins, Chris Cooper, Noam Katz, Paul Nonn, John Wallace, Cary Forest The Madison Plasma Dynamo Experiment (MPDX) is a 3 meter diameter sphere consisting of 36 axisymmetric rings of samarium cobalt magnets in a ring-cusp configuration. Electrostatic electrodes on the edge will be used to spin the plasma. The purpose of MPDX is to study flow-driven magnetohydrodynamic instabilities. Electron cyclotron heating will be used for the ionization and heating of the plasma. A benefit of the ECH is the plasma will have hot electrons leading to good electrical conduction and high magnetic Reynolds number. In addition, direct heating of the electrons helps to obtain a large ionization fraction and a low neutral density. The ECH system on MPDX will consist of 5 separate lines distributed at various positions around the vacuum vessel. Each line will have a 20 kW magnetron operating in continuous wave mode at 2.45 GHz outputting in WR-340 waveguide. The power will be transferred to the vacuum vessel through WR-284 waveguide. Each line will contain a directional coupler for measuring reflected power. A manual 3-stub tuner will be used for impedance matching. The purpose of these elements is to optimize the efficiency of energy transfer to the plasma. [Preview Abstract] |
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PP8.00160: Computational and Experimental Development of a Linear Cross-Field Amplifier John Watrous, Jim Browning, Marcus Pearlman A linear crossed-field amplifier (CFA) is being developed by combining high-performance modeling, reduced physics models, and experiments. A prior CFA design showed no gain; models of the device suggested that the meander line produced strong standing and backward waves that frustrated the interaction with the electron beam. Calculations using the AFRL PIC code, ICEPIC, examined the meander line fields and their interaction with the electron beam, suggesting that increasing the distance between sole plate and meander line would reduce the influence of the standing and backward waves; calculations of a modified CFA showed improvements in behavior. Work focuses on analysis of the fields produced by the meander line. Reduced physics models point to regions in the parameter space defined by meander line and beam currents where useful interaction occurs. Promising regions will be explored in ICEPIC calculations. In the lab, the meander line circuit is being rebuilt to reduce the role of the standing and backward waves. Initial experiments will use field emitters in an injected beam configuration, but these will be replaced with a distributed array of controllable field emitters with the long-term goal of exploiting the control of the emission process to optimize the gain. [Preview Abstract] |
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PP8.00161: Faceted Magnetron Device Using Discrete Current Sources Sulmer Fernandez-Gutierrez, Jim Browning, David Smithe, Jack Watrous A faceted magnetron concept has a cathode structure comprised of field emitters instead of the traditional thermionic cathode. The faceted magnetron structure has been modeled using the 3D particle-in-cell codes VORPAL 5.2 and ICEPIC. The two-dimensional particle trajectory simulation Lorentz2E has been used to model the electron injection from gated field emitters in a slit type structure. The sensitivity of the electron injection into the device was studied using Lorentz2E with the variation in the operating voltages in the cathode structure -as well as the location of the pusher electrode. Volume electron charge was considered in the sensitivity analysis. Surface charge was not included. Together with this work a model of a ten cavity rising sun magnetron was developed using VORPAL 5.2. A cylindrical and pentagonal-shaped cathode was modeled to study the variation of results due to the cathode shape. The results of this model are used to compare with results obtained from ICEPIC. Discrete current sources were also modeled to come from each facet of the pentagon-shaped cathode to study its effect on the magnetron operation. It is planned to generate velocity distributions of the electrons for the VORPAL input by using results generated with Lorentz2E. [Preview Abstract] |
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PP8.00162: Microwave Triggered Laser Ionization of Air Ehsan Vadiee, Sarita Prasad, C. Jerald Buchenauer, Edl Schamiloglu The goal of this work is to study the evolution and dynamics of plasma expansion when a high power microwave (HPM) pulse is overlapped in time and space on a very small, localized region of plasma formed by a high energy laser pulse. The pulsed Nd:YAG laser (8 ns, 600mJ, repetition rate 10 Hz) is focused to generate plasma filaments in air with electron density of 10$^{17}$/cm$^3$. When irradiated with a high power microwave pulse these electrons would gain enough kinetic energy and further escalate avalanche ionization of air due to elastic electron-neutral collisions thereby causing an increased volumetric discharge region. An X-band relativistic backward wave oscillator(RBWO) at the Pulsed Power,Beams and Microwaves laboratory at UNM is constructed as the microwave source. The RBWO produces a microwave pulse of maximum power 400 MW, frequency of 10.1 GHz, and energy of 6.8 Joules. Special care is being given to synchronize the RBWO and the pulsed laser system in order to achieve a high degree of spatial and temporal overlap. A photodiode and a microwave waveguide detector will be used to ensure the overlap. Also, a new shadowgraph technique with a nanosecond time resolution will be used to detect changes in the shock wave fronts when the HPM signal overlaps the laser pulse in time and space. [Preview Abstract] |
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