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07/2011;
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D.L. Correll,
S.L. Allen,
T.A. Capser,
J.F. Clauser,
P. Coakley,
F.H. Coensgen,
W. Condit,
W.F. Cummins,
J.C. Davis,
R.P. Drake, [......],
R.S. Hornady,
A.L. Hunt,
C.V. Karmendy, A.W. Molvik,
W.E. Nexsen,
W.L. Pickles,
P. Poulsen,
T.C. Simonen,
B.W. Stallard,
O.T. Strand
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ABSTRACT: TMX experimental data on ambipolar potential control and on the accompanying electrostatic confinement are reported. In the radial core of the central cell, measurements of electrostatic potentials of 150 V which augment axial ion confinement are in agreement with predictions using the Maxwell-Boltzmann result. Central-cell ion confinement was observed to scale according to electrostatic potential theory up to average enhancement factors of eight times over mirror confinement alone.
Nuclear Fusion 01/2011; 22(2):223. · 4.09 Impact Factor
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ABSTRACT: Five microwave interferometers are used to measure radial and longitudinal electron density profiles versus time in the 2XII minimum-B mirror containment experiment. The mean plasma radius determined from a radially offset interferometer agrees with electrostatic probe and laser measurements. The longitudinal profile is consistent with a collisional ion angular distribution with a density at the mirrors of approximately 5% of the central density. After compression the plasma shape remains approximately constant. Plasma densities beyond the mirrors were less than 5% of the central density.
Nuclear Fusion 01/2011; 14(1):61. · 4.09 Impact Factor
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ABSTRACT: Measurements of impurity ion concentrations and energy distributions in the 2XII mirror device are desaibed. The measurements were made using a fast-pulsed gas valve and a charge-exchange analyser that employs both electrostatic and magnetic separation to obtain energy spectra of separate particle species. The predominant high-Z material detected wascarbon, having a concentration of ~2% and a mean energy of 3-6 keV. Smaller amounts of oxygen were the only materialother than hydrogen detected. These energetic impurity levels are determined to be contained against self-scattering into themirror loss cones and to have only minor influences on plasma life-time and on mean ion and electron energy.
Nuclear Fusion 01/2011; 15(5):813. · 4.09 Impact Factor
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ABSTRACT: High density (non-thermal-barrier) operation in the Tandem Mirror Experiment Upgrade (TMX-U) is found to be restricted by a stability limit. This limit is observed in the ratio of the neutral beam sustained central cell beta βc to the hot ion beta βih in the minimum-B anchor cells at both ends of the central cell, qualitatively consistent with a flute interchange stability limit. The ratio is βc/βih = 5, over the range of 0.03 ≤ βc ≤ 0.22, with no apparent reduction due to ballooning at high βc. This is a factor of six below the standard magnetohydrodynamic (MHD) m = 1 stability theory prediction of βc/βih = 33 at βc ≤ 0.1, where ballooning corrections to flute modes are small. The discrepancy could be due to approximations in the theory; however, experimental data indicate that the stability limit is due to drift wave turbulence or to large-m MHD flute or ballooning modes. The experimental beta limit is nearly independent of the hot electron beta in the anchor cells, which is compatible with theoretical predictions that the hot electron beta will decouple from MHD activity.
Nuclear Fusion 01/2011; 30(6):1061. · 4.09 Impact Factor
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ABSTRACT: Ion cyclotron heating (ICH) is applied to TMX-U to improve the thermal barrier performance by reducing the passing ion collisionality. During its development, measurements of the antenna loading resistance, Rp, and the absorption efficiency, η, were compared with calculations with the antenna design code ANTENA over a wide range of densities and frequencies. Good agreement in Rp was obtained in the short wavelength slow wave regime but not for long wavelength fast waves because the experimental magnetic field gradients are not modelled in ANTENA. Similarly, η is much larger experimentally (40%) than in ANTENA (10%) due to the magnetic beach in TMX-U. In its application, ICH successfully decreased the passing ion collisionality tenfold but did not extend thermal barrier plugging to higher density, indicating that collisional barrier filling is not currently limiting TMX-U performance.
Nuclear Fusion 01/2011; 27(12):1959. · 4.09 Impact Factor
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ABSTRACT: A quiescent plasma is sustained for 80 energy confinement times by only gas fuelling and neutral beam heating in an axisymmetric region of the Tandem Mirror Experiment Upgrade (TMX-U). This plasma should be unstable because of the bad magnetic curvature and the absence of ion cyclotron heating which previously provided ponderomotive stabilization to sustain plasmas in bad-curvature regions of other axisymmetric mirror experiments. The TMX-U data are consistent with stabilization by a symbiosis between two mechanisms – line tying, which reduces the growth rate, and finite Larmor radius edge stabilization, which can result in quiescent operation.
Nuclear Fusion 01/2011; 30(5):815. · 4.09 Impact Factor
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B G Logan,
J J Barnard,
F M Bieniosek,
R H Cohen,
J E Coleman,
R C Davidson,
P C Efthimion,
A Friedman,
E P Gilson,
W G Greenway, [......],
A B Sefkow,
P A Seidl,
W Sharp,
E A Startsev,
J-L Vay,
W L Waldron,
J S Wurtele,
D Welch,
G A Westenskow,
S S Yu
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ABSTRACT: During the past two years, the U.S. heavy ion fusion science program has made significant experimental and theoretical progress in simultaneous transverse and longitudinal beam compression, ion-beam-driven warm dense matter targets, high brightness beam transport, advanced theory and numerical simulations, and heavy ion target designs for fusion. First experiments combining radial and longitudinal compression of intense ion beams propagating through background plasma resulted in on-axis beam densities increased by 700X at the focal plane. With further improvements planned in 2007, these results will enable initial ion beam target experiments in warm dense matter to begin next year at LBNL. We are assessing how these new techniques apply to low-cost modular fusion drivers and higher-gain direct-drive targets for inertial fusion energy.
Journal of Physics Conference Series 06/2008; 112(3):032029.
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ABSTRACT: Contamination from electrons is a concern for solenoid-focused ion accelerators being developed for experiments in high-energy-density physics. These electrons, produced directly by beam ions hitting lattice elements or indirectly by ionization of desorbed neutral gas, can potentially alter the beam dynamics, leading to a time-varying focal spot, increased emittance, halo, and possibly electron-ion instabilities. The electrostatic particle-in-cell code WARP is used to simulate electron-cloud studies on the solenoid-transport experiment (STX) at Lawrence Berkeley National Laboratory. We present self-consistent simulations of several STX configurations and compare the results with experimental data in order to calibrate physics parameters in the model.
Particle Accelerator Conference, 2007. PAC. IEEE; 07/2007
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M.K. Covo, A.W. Molvik,
R.H. Cohen,
A. Friedman,
P.A. Seidl,
G. Logan,
F. Bieniosek,
D. Baca,
J.-L. Vay,
E. Orlando,
J.L. Vujic
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ABSTRACT: Beam interaction with background gas and walls produces ubiquitous clouds of stray electrons that frequently limit the performance of particle accelerator and storage rings. Counterintuitively we obtained the electron cloud accumulation by measuring the expelled ions that are originated from the beam-background gas interaction, rather than by measuring electrons that reach the walls. The kinetic ion energy measured with a retarding field analyzer (RFA) maps the depressed beam space-charge potential and provides the dynamic electron cloud density. Clearing electrode current measurements give the static electron cloud background that complements and corroborates with the RFA measurements, providing an absolute measurement of electron cloud density during a 5 mus duration beam pulse in a drift region of the magnetic transport section of the High-Current Experiment (HCX) at LBNL.
Particle Accelerator Conference, 2007. PAC. IEEE; 07/2007
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S.S. Yu,
B.G. Logan,
J.J. Barnard,
F.M. Bieniosek,
R.J. Briggs,
R.H. Cohen,
J.E. Coleman,
R.C. Davidson,
A. Friedman,
E.P. Gilson, [......], A.W. Molvik,
C.L. Olson,
H. Qin,
P.K. Roy,
A. Sefkow,
P.A. Seidl,
E.A. Startsev,
J-L. Vay,
W.L. Waldron,
D.R. Welch
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ABSTRACT: Over the past two years noteworthy experimental and theoretical progress has been made towards the top-level scientific question for the US programme on heavy-ion-fusion-science and high energy density physics: 'How can heavy-ion beams be compressed to the high intensity required to create high energy density matter and fusion conditions?' New results in transverse and longitudinal beam compression, high-brightness transport and beam acceleration will be reported. Central to this campaign is final beam compression. With a neutralizing plasma, we demonstrated transverse beam compression by an areal factor of over 100 and longitudinal compression by a factor of > 50. We also report on the first demonstration of simultaneous transverse and longitudinal beam compression in plasma. High beam brightness is key to high intensity on target, and detailed experimental and theoretical studies on the effect of secondary electrons on beam brightness degradation are reported. A new accelerator concept for near-term low-cost target heating experiments was invented, and the predicted beam dynamics validated experimentally. We show how these scientific campaigns have created new opportunities for interesting target experiments in the warm dense matter regime. Finally, we summarize progress towards heavy-ion fusion, including the demonstration of a compact driver-size high-brightness ion injector. For all components of our high intensity campaign, the new results have been obtained via tightly coupled efforts in experiments, simulations and theory.
Nuclear Fusion 07/2007; 47(8):721. · 4.09 Impact Factor
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A W Molvik,
H Kollmus,
E Mahner,
M Kireeff Covo,
M C Bellachioma,
M Bender,
F M Bieniosek,
E Hedlund,
A Krämer,
J Kwan,
O B Malyshev,
L Prost,
P A Seidl,
G Westenskow,
L Westerberg
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ABSTRACT: During heavy-ion operation in several particle accelerators worldwide, dynamic pressure rises of orders of magnitude were triggered by lost beam ions that bombarded the vacuum chamber walls. This ion-induced molecular desorption, observed at CERN, GSI, and BNL, can seriously limit the ion beam lifetime and intensity of the accelerator. From dedicated test stand experiments we have discovered that heavy-ion-induced gas desorption scales with the electronic energy loss (dE_{e}/dx) of the ions slowing down in matter; but it varies only little with the ion impact angle, unlike electronic sputtering.
Physical Review Letters 03/2007; 98(6):064801. · 7.37 Impact Factor
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P A Seidl,
J Armijo,
D Baca,
F M Bieniosek,
J Coleman,
R C Davidson,
P C Efthimion,
A Friedman,
E P Gilson,
D Grote, [......],
B G Logan, A W Molvik,
D V Rose,
P K Roy,
A B Sefkow,
W M Sharp,
J L Vay,
W L Waldron,
D R Welch,
S S Yu
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ABSTRACT: This paper presents plans for neutralized drift compression experiments, precursors to future target heating experiments. The target-physics objective is to study warm dense matter (WDM) using short-duration ($1 ns) ion beams that enter the targets at energies just above that at which dE/dx is maximal. High intensity on target is to be achieved by a combination of longitudinal compression and transverse focusing. This work will build upon recent success in longitudinal compression, where the ion beam was compressed lengthwise by a factor of more than 50 by first applying a linear head-to-tail velocity tilt to the beam, and then allowing the beam to drift through a dense, neutralizing background plasma. Studies on a novel pulse line ion accelerator were also carried out. It is planned to demonstrate simultaneous transverse focusing and longitudinal compression in a series of future experiments, thereby achieving conditions suitable for future WDM target experiments. Future experiments may use solenoids for transverse focusing of un-neutralized ion beams during acceleration. Recent results are reported in the transport of a high-perveance heavy ion beam in a solenoid transport channel. The principal objectives of this solenoid transport experiment are to match and transport a space-charge-dominated ion beam, and to study associated electron-cloud and gas effects that may limit the beam quality in a solenoid transport system. Ideally, the beam will establish a Brillouin-flow condition (rotation at one-half the cyclotron frequency). Other mechanisms that potentially degrade beam quality are being studied, such as focusing-field aberrations, beam halo, and separation of lattice focusing elements.
Nuclear Instruments and Methods in Physics Research A. 01/2007; 57775(52).
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ABSTRACT: A Retarding Field Analyzer (RFA) was inserted in a drift region of the magnetic transport section of the High-Current Experiment (HCX), that is at high-vacuum, to measure ions and electrons resulting from beam interaction with background gas and walls. The ions are expelled during the beam pulse by the space–charge potential and the electrons are expelled mainly at the end of the beam, when the beam potential decays. The ion energy distribution shows the beam potential of $2100 V and the beam–background gas total cross-section of 3:1 Â 10 À19 m 2 . The electron energy distribution reveals that the expelled electrons are mainly desorbed from the walls and gain $22 eV from the beam potential decaying with time before entering the RFA. Details of the RFA design and of the measured energy distributions are presented and discussed.
Nuclear Instruments and Methods in Physics Research A. 01/2007; 577507520(79).
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ABSTRACT: A primary concern for high current ion accelerators is contaminant electrons. These electrons can interfere with the beam ions, causing emittance growth and beam loss. Numerical simulation is a main tool for understanding the interaction of the ion beam with the contaminant electrons, but these simulations then require accurate models of electron generation. These models include ion-induced electron emission from ions hitting the beam pipe walls or diagnostics. However, major codes for modeling ion beam transport are written in different programming languages and used on different computing platforms. For electron generation models to be maximally useful, researchers should be able to use them easily from many languages and platforms. A model of ion-induced electrons including the electron energy distribution is presented here, including a discussion of how to use the Babel software tool to make these models available in multiple languages and how to use the GNU Autotools to make them available on multiple platforms. An application to simulation of the end region of the High Current Experiment is shown. These simulations show formation of a virtual cathode with a potential energy well of amplitude 12.0 eV, approximately six times the most probable energy of the ion-induced electrons. Oscillations of the virtual cathode could lead to possible longitudinal and transverse modulation of the density of the electrons moving out of the virtual cathode.
Physics of Plasmas 05/2006; 13(5):056702-056702-6. · 2.15 Impact Factor
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ABSTRACT: A Retarding Field Analyzer (RFA) was designed and inserted in a drift region of a magnetic transport section of the High Current Experiment (HCX). It measures ions or electrons resulting from the beam interaction with the background gas and walls. The ions are expelled during the beam by the space-charge beam potential, and the electrons are expelled mainly at the end of the beam, when the beam potential decays. The measured electrons have a Maxwellian energy distribution and the measured ions have an energy distribution that gives the information of the beam profile, details will be presented and discussed.
03/2006
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M. Kireeff Covo, A. W. Molvik,
A. Friedman,
G. Westenskow,
J. J. Barnard,
R. Cohen,
P. A. Seidl,
J. W. Kwan,
G. Logan,
D. Baca,
F. Bieniosek,
C. M. Celata,
J.-L. Vay,
J. L. Vujic
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ABSTRACT: The cost of accelerators for heavy-ion inertial fusion energy (HIF) can be reduced by using the smallest possible clearance between the beam and the wall from the beamline. This increases beam loss to the walls, generating ion-induced electrons that could be trapped by beam space charge potential into an "electron cloud", which can cause degradation or loss of the ion beam. In order to test the physical mechanism model of ion- induced electrons production we have measured the impact of K+ ions with energies up to 400 KeV on stainless steel surfaces near grazing incidence, using the ion source test stand (STS-500) at LLNL. The electron yield will be discussed and compared with experimental
measurements from 1 MeV K+ ions in the High-Current Experiment at LBNL.
Physical Review Special Topics - Accelerators and Beams 01/2006; · 1.52 Impact Factor
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A.W. Molvik,
M.K. Covo,
A. Friedman,
R. Cohen,
S.M. Lund,
J.J. Barnard,
F. Bieniosek,
P. Seidl,
D. Baca,
J.-L. Vay,
C.M. Celata,
W.L. Waldron,
J.L. Vujic
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ABSTRACT: Electron clouds and gas pressure rise limit the performance of many major accelerator rings. We are studying these issues experimentally with ∼ 1 MeV heavy-ion beams, coordinated with significant efforts in self-consistent simulation and theory. The experiments use multiple diagnostics, within and between quadrupole magnets, to measure the sources and accumulation of electrons and gas. In support of these studies, we have measured gas desorption and electron emission coefficients for potassium ions impinging on stainless steel targets at angles near grazing incidence. Our goal is to measure the electron particle balance for each source – ionization of gas, emission from beam tubes, and emission from an end wall – determine the electron effects on the ion beam and apply the increased understanding to mitigation. We describe progress towards that goal.
Particle Accelerator Conference, 2005. PAC 2005. Proceedings of the; 06/2005
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J.-L. Vay,
M.A. Furman,
P.A. Seidl,
R.H. Cohen,
A. Friedman,
D.P. Grote,
M.K. Covo, A.W. Molvik,
P.H. Stoltz,
S. Veitzer,
J.P. Verboncoeur
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ABSTRACT: Electron clouds and gas pressure rise limit the performance of many major accelerators. A multi-laboratory effort to understand the underlying physics via the combined application of experiment, theory, and simulation is underway. We present here the status of the simulation capability development, based on a merge of the three-dimensional parallel Particle-In-Cell (PIC) accelerator code WARP and the electron cloud code POSINST, with additional functionalities. The development of the new capability follows a “roadmap” describing the different functional modules, and their inter-relationships, that are ultimately needed to reach self-consistency. Newly developed functionalities include a novel particle mover bridging the time scales between electron and ion motion, a module to generate neutrals desorbed by beam ion impacts at the wall, and a module to track impact ionization of the gas by beam ions or electrons. Example applications of the new capability to the modeling of electron effects in the High Current Experiment (HCX) are given.
Particle Accelerator Conference, 2005. PAC 2005. Proceedings of the; 06/2005
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ABSTRACT: Heavy‐ion induction linacs for inertial fusion energy and high‐energy density physics have an economic incentive to minimize the clearance between the beam edge and the aperture wall. This increases the risk from electron clouds and gas desorbed from walls. We have measured electron and gas emission from 1 MeV K+ incident on surfaces near grazing incidence on the High‐Current Experiment (HCX) at LBNL. Electron emission coefficients reach values >100, whereas gas desorption coefficients are near 104. Mitigation techniques are being studied: A bead‐blasted rough surface reduces electron emission by a factor of 10 and gas desorption by a factor of 2. We also discuss the results of beam transport (of 0.03–0.18 A K+) through four pulsed room‐temperature magnetic quadrupoles in the HCX at LBNL. Diagnostics are installed on HCX, between and within quadrupole magnets, to measure the beam halo loss, net charge and expelled ions, from which we infer gas density, electron trapping, and the effects of mitigation techniques. A coordinated theory and computational effort has made significant progress towards a self‐consistent model of positive‐ion beam and electron dynamics. We are beginning to compare experimental and theoretical results. © 2005 American Institute of Physics
AIP Conference Proceedings. 06/2005; 773(1):226-228.
