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  • 1
    Electronic Resource
    Electronic Resource
    Collinear reactive scattering of state-selected H+2+He→HeH++H: A method for probing dynamical resonances (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 8615-8617 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report on an experimental geometry for studying dynamical resonances in bimolecular collisions. The method is sensitive to atom–diatom reactions at low angular momentum and low impact parameter corresponding to the collinear coordinate. State-selected reaction probabilities for H+2+He→HeH++H were measured at c.m. scattering angles θ=0° and 180°; the collision energy was scanned from 0.35–1.90 eV.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460718
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  • 2
    Electronic Resource
    Electronic Resource
    State-selected reactive scattering. II. He+H+2→HeH++H (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 4894-4904 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The endoergic reaction He+H+2→HeH++H is investigated in a molecular-beam experiment as a function of H+2 vibrational energy at c.m. collision energies between 0.3 and 1.9 eV. Reactant ions generated by resonantly enhanced four-photon ionization are impulsively accelerated to collide with a beam of He. Time-of-flight velocity distributions of HeH+, measured at one laboratory angle, yield the differential cross section at c.m. angles θ=0° and 180°. A shift from "He rebound'' to "H+ stripping'' behavior accompanies the enhancement in the cross section as the H+2 vibrational energy increases, which matches previous studies at higher collision energy. Small-impact-parameter events produce HeH+ with less recoil velocity (more internal energy) than those at large impact parameters. Within the limits of sensitivity and resolution (ΔE≈0.15 eV), definitive resonance features in the collision energy dependence of dσ/dω are not evident. Improvements in the technique to enable such observations are suggested.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461705
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  • 3
    Electronic Resource
    Electronic Resource
    State-selected reactive scattering. I. H+2+H2→H+3+H (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 4877-4893 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Cross sections for the reaction H+2+H2→H+3+H, differential in scattering angle and recoil energy, are measured in a molecular-beam experiment at c.m. collision energies of 1.5, 2.3, 3.5, and 5.3 eV. Resonantly enhanced four-photon ionization prepares H+2 in selected vibrational-state distributions, allowing a systematic exploration of the effects of reactant energy on the product angular and energy distributions. Angular data are interpreted on the basis of competition between H+3 formation and collision-induced dissociation. The nominal atom-transfer (AT) and proton-transfer (PT) processes are identified respectively with forward and backward scattered H+3. Effects of reactant energy on AT and PT cross sections in H+2+H2 are compared with previous observations on D+2+H2 and H+2+D2. The fraction of the available energy appearing as H+3+H recoil ranges from 26% to 39% depending on reactant conditions. Previous surface-hopping trajectory calculations successfully predict most of the observed trends. Evidence is given for the existence of long-lived H+3 with internal energy in excess of the dissociation limit.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461704
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  • 4
    Electronic Resource
    Electronic Resource
    Isotopic scaling of confinement in deuterium–tritium plasmas (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 2299-2307 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The confinement and heating of supershot plasmas are significantly enhanced with tritium beam injection relative to deuterium injection in the Tokamak Fusion Test Reactor [Plasma Phys. Controlled Fusion 26, 11 (1984)]. The global energy confinement and local thermal transport are analyzed for deuterium and tritium fueled plasmas to quantify their dependence on the average mass of the hydrogenic ions. Radial profiles of the deuterium and tritium densities are determined from the D–T fusion neutron emission profile. The inferred scalings with average isotopic mass are quite strong, with τE∝〈A〉0.85±0.20, τEthermal∝〈A〉0.89±0.20, χitot∝〈A〉−2.6±0.5, and De∝〈A〉−1.4±0.2 at fixed Pinj. For fixed local plasma parameters χitot∝〈A〉−1.8±0.4 is obtained. The quoted 2σ uncertainties include contributions from both diagnostic errors and shot irreproducibility, and are conservatively constructed to attribute the entire scatter in the regressed parameters to uncertainties in the exponent on plasma mass. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871253
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  • 5
    Electronic Resource
    Electronic Resource
    Sustained rotational stabilization of DIII-D plasmas above the no-wall beta limit : Review,Tutorial and Invited Papers from the 43rd Annual Meeting of the APS Division of Plasma Physics (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 1997-2005 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Sustained stabilization of the n=1 kink mode by plasma rotation at beta approaching twice the stability limit calculated without a wall has been achieved in DIII-D by a combination of error field reduction and sufficient rotation drive. Previous experiments have transiently exceeded the no-wall beta limit. However, demonstration of sustained rotational stabilization has remained elusive because the rotation has been found to decay whenever the plasma is wall stabilized. Recent theory [Boozer, Phys. Rev. Lett. 86, 5059 (2001)] predicts a resonant response to error fields in a plasma approaching marginal stability to a low-n kink mode. Enhancement of magnetic nonaxisymmetry in the plasma leads to strong damping of the toroidal rotation, precisely in the high-beta regime where it is needed for stabilization. This resonant response, or "error field amplification" is demonstrated in DIII-D experiments: applied n=1 radial fields cause enhanced plasma response and strong rotation damping at beta above the no wall limit but have little effect at lower beta. The discovery of an error field amplification has led to sustained operation above the no-wall limit through improved magnetic field symmetrization using an external coil set. The required symmetrization is determined both by optimizing the external currents with respect to the plasma rotation and by use of feedback to detect and minimize the plasma response to nonaxisymmetric fields as beta increases. Ideal stability analysis and rotation braking experiments at different beta values show that beta is maintained 50% higher than the no wall stability limit for durations greater than 1 s, and approaches beta twice the no-wall limit in several cases, with steady-state rotation levels. The results suggest that improved magnetic-field symmetry could allow plasmas to be maintained well above no-wall beta limit for as long as sufficient torque is provided. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1446036
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  • 6
    Electronic Resource
    Electronic Resource
    Active feedback stabilization of the resistive wall mode on the DIII-D device : The 42nd annual meeting of the division of plasma physics of the American Physical Society and the 10th international congress on plasma physics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 2071-2082 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A proof of principle magnetic feedback stabilization experiment has been carried out to suppress the resistive wall mode (RWM), a branch of the ideal magnetohydrodynamic (MHD) kink mode under the influence of a stabilizing resistive wall, on the DIII-D tokamak device [Plasma Phys. Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1986), p. 159; Phys. Plasmas 1, 1415 (1994)]. The RWM was successfully suppressed and the high beta duration above the no-wall limit was extended to more than 50 times the resistive wall flux diffusion time. It was observed that the mode structure was well preserved during the time of the feedback application. Several lumped parameter formulations were used to study the feedback process. The observed feedback characteristics are in good qualitative agreement with the analysis. These results provide encouragement to future efforts towards optimizing the RWM feedback methodology in parallel to what has been successfully developed for the n=0 vertical positional control. Newly developed MHD codes have been extremely useful in guiding the experiments and in providing possible paths for the next step. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1351823
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  • 7
    Electronic Resource
    Electronic Resource
    Enhancement of Tokamak Fusion Test Reactor performance by lithium conditioning (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1892-1897 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Wall conditioning in the Tokamak Fusion Test Reactor (TFTR) [K. M. McGuire et al., Phys. Plasmas 2, 2176 (1995)] by injection of lithium pellets into the plasma has resulted in large improvements in deuterium–tritium fusion power production (up to 10.7 MW), the Lawson triple product (up to 1021 m−3 s keV), and energy confinement time (up to 330 ms). The maximum plasma current for access to high-performance supershots has been increased from 1.9 to 2.7 MA, leading to stable operation at plasma stored energy values greater than 5 MJ. The amount of lithium on the limiter and the effectiveness of its action are maximized through (1) distributing the Li over the limiter surface by injection of four Li pellets into Ohmic plasmas of increasing major and minor radius, and (2) injection of four Li pellets into the Ohmic phase of supershot discharges before neutral-beam heating is begun. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871984
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  • 8
    Electronic Resource
    Electronic Resource
    Anomalous losses of deuterium–deuterium fusion products in the Tokamak Fusion Test Reactorf| (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1469-1478 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In the Tokamak Fusion Test Reactor (TFTR) [International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Wurzburg, Paper No. A-2-2 (International Atomic Energy Agency, Vienna, 1993)] there have been at least three types of anomalous loss of alpha-like deuterium–deuterium (D–D) fusion products: (1) a magnetohydrodynamic (MHD)-induced loss of D–D fusion products correlated with Mirnov and fishbone-type oscillations and sawtooth crashes, (2) a slow "delayed'' loss of partially thermalized D–D fusion products occurring without large MHD activity, and (3) ion cyclotron resonance heating (ICRH)-induced loss of D–D fusion products ions observed during direct electron heating experiments, and possibly also during 3He minority heating. In this paper each of these will be reviewed, concentrating on those due to MHD activity, which are the largest of these anomalous losses. The experimental results are compared with numerical models of various fusion product transport mechanisms.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870697
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  • 9
    Electronic Resource
    Electronic Resource
    Validation of spatial profile measurements of neutron emission in TFTR plasmas (invited) : Proceedings of the 9th topical conference on high temperature plasma diagnostics (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 4517-4522 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The TFTR multichannel neutron collimator provides spatially localized information on nuclear reaction rates in fusion plasmas. Absolutely calibrated NE 451 ZnS scintillators detect chord-integrated neutron emission along ten vertical sightlines, which are defined by a massive shielding structure in the facility basement. If such observations are to be of value, virgin neutrons along each sightline must be distinguishable from a background of gamma rays and scattered neutrons, and the measurements must be sufficiently precise, over a wide range of plasma conditions, to allow meaningful comparison with results of transport analysis computer codes. As a useful check, the volume-integrated neutron emission must agree with independent measurements of the global neutron source strength. Although NE 451 detectors are insensitive to gamma rays and to neutrons with energy less than ∼2 MeV, the scintillation pulse height distribution depends on both the incident neutron energy spectrum and the physical parameters of the individual detector. In order to establish the validity of our measurements, we have carried out a detailed characterization of our detectors, including computer modeling of scintillation pulse height spectra and of temporal pulse pileup effects in the counting circuitry. Satisfactory operation of the system has been demonstrated for all deuterium plasmas encountered in TFTR. The detector modeling, together with further laboratory measurements, will enable us to prepare for deuterium–tritium experiments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143708
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  • 10
    Electronic Resource
    Electronic Resource
    Control of the neutron detector count rate by optical imaging : Proceedings of the 9th topical conference on high temperature plasma diagnostics (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 4539-4541 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The signal processing electronics used for the NE451 detectors on the TFTR multichannel neutron collimator are presently showing saturation effects at high counting rates equivalent to neutron yields of ∼1016 n/s. While nonlinearity due to pulse pileup can be corrected for in most present TFTR experiments, additional steps are required for neutron source strengths above ∼3×1016 n/s. These pulse pileup effects could be reduced by inserting sleeves in the collimator shielding to reduce the neutron flux in the vicinity of the detectors or by reducing the volume of detector exposed to the flux. We describe a novel method of avoiding saturation by optically controlling the number neutron events processed by the detector electronics. Because of the optical opacity of the ZnS-plastic detectors such as NE451, photons from a proton-recoil scintillation arise from a spatially localized area of the detector. By imaging a selected portion of the detector onto a photomultiplier, we reduce the effective volume of the detector in a controllable, reversible way. A prototype system, consisting of a focusing lens, a field lens, and a variable aperture, has been constructed. Results of laboratory feasibility tests are presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143713
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