R. Hulse

Princeton University, Princeton, NJ, United States

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Publications (91)163.21 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Quantitative spectroscopic measurements of Zeff, impurity densities and radiated power losses have been made for ohmically heated and neutral beam heated TFTR discharges at a plasma current of 2.2 MA and a toroidal field of 4.7 T. Variations in these quantities with line average plasma density (e) and beam power up to 5.6 MW are presented for discharges on a movable graphite limiter. A detailed discussion of the use of an impurity transport model to infer absolute impurity densities and radiative losses from line intensity and visible continuum measurements is given. These discharges were dominated by low-Z impurities, with carbon having a considerably higher density than oxygen, except in high e Ohmic discharges where the densities of carbon and oxygen were comparable. Metallic impurity concentrations and radiative losses were small, resulting in hollow radiated power profiles and fractions of the input power radiated being 30–50% for Ohmic heating and 30% or less for beam heating. Spectroscopic estimates of the radiated power were in good agreement with bolometrically measured values. Because of an increase in the carbon density, Zeff rose from 2.0 to 2.8 as the beam power increased from 0 to 5.6 MW, pointing to a potentially serious dilution of the neutron producing plasma ions with increasing beam power. Both the low-Z and the metallic impurity concentrations were approximately constant with minor radius, indicating no central impurity accumulation in these discharges.
    Nuclear Fusion 01/2011; 27(7):1147. · 2.73 Impact Factor
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    ABSTRACT: Impurity concentration profiles have been determined for H-mode discharges in the DIII-D tokamak from measured ne, Te, Zeff and radiated emissivity profiles. The central impurity levels in DIII-D high current H-modes, as modelled using this technique, remain below those seen in L-modes (fractional nickel concentrations ≤0.02%) throughout the neutral beam heating pulse. In contrast to some other experiments (ASDEX [1], JET [2], JFT-2M [3]), the H-mode does not terminate because of excessive radiation in DIII-D discharges heated with co-injected neutral beams. For increasing plasma current, the global impurity concentrations decrease and the profiles become more dominated by edge radiation. H-modes as obtained with electron cyclotron heating and co-injected neutral beams at similar heating powers also have low impurity levels, but the impurity distribution is significantly more hollow in the case of neutral beam heating.
    Nuclear Fusion 01/2011; 30(4):701. · 2.73 Impact Factor
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    ABSTRACT: Compression of low-temperature spheromak plasmas has been studied with the aid of a zero-dimensional two-fluid computer code. The major direct effect of compression is to raise the current and plasma densities, keeping their ratios constant. Above critical pre-compression values of the electron temperature, Te, and the product of electron density and particle confinement time, neτp, the rising plasma density shifts the impurity ions to higher ionization states, with correspondingly lower radiative losses. High post-compression temperatures may thus be achieved. In oxygen radiation-dominated plasmas with pre-compression values above the critical ones, and with constant τp, the electron temperature can be increased by up to a factor of seven for a compression of a factor of two. This can be compared with a factor-of-four temperature rise expected for adiabatic compression. If the energy balance is dominated by particle confinement losses rather than radiation losses, the effect of compression is to raise the temperature as Te ~ C6/5, for constant τp.
    Nuclear Fusion 01/2011; 26(2):235. · 2.73 Impact Factor
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    ABSTRACT: The H-mode transition can lead to a rapid increase in tokamak plasma confinement. A semiempirical transport model was derived from global OH and L-mode confinement scalings and then applied to simulation of H-mode discharges. The radial diffusivities in the model depend on local density and pressure gradients and satisfy an appropriate dimensional constraint. Examples are shown of the application of this model and of similar models to the detailed simulation of two discharges which exhibit an H-mode transition. The models reproduce essential features of plasma confinement in the Ohmic heating and the low- and highconfinement phases of these discharges. In particular, the evolution of plasma energy content through the H-mode transition can be reproduced without any sudden or ad hoc modification of the plasma transport formulation.
    Nuclear Fusion 01/2011; 25(11):1555. · 2.73 Impact Factor
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    ABSTRACT: The paper presents a study of impurity transport in ohmically heated TFTR plasmas by computer modelling of VUV line emissions from impurities injected using the laser blow-off technique. The results are sensitive to uncertainties in the ionization and recombination rates used in the modelling and, therefore, only a spatially averaged diffusion coefficient and parameterized convective velocity can be measured. Measurements of these transport parameters are presented for deuterium and helium discharges with Ip = 0.8−2.5 MA, e = (0.6-6.0) × 1019 m−3 and Zeff = 2-6. The diffusion coefficients are found to be in the range of 0.5-1.5 m2 s−1, considerably larger than neoclassical values. Non-zero inward convective velocities are necessary to fit the data in most cases. No dependence of the diffusion coefficient on injected element, working gas species or plasma current is found, but, at a given current, the diffusion coefficient in plasmas near the density limit is smaller by approximately a factor of two than in discharges with e < 3 × 1019 m−3.
    Nuclear Fusion 01/2011; 29(3):437. · 2.73 Impact Factor
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    ABSTRACT: Radial transport of medium- and high-Z ions during co- and counter-neutral-beam heating in the PLT tokamak is studied, using molybdenum and scandium ions as tracer elements. The time evolution of the radial profiles of several ionization stages of both elements, injected by laser blowoff during the neutral-beam heating, is measured under three significantly different beam-plasma combinations. No noticeable differences in the radial profiles attributable to the beam direction are observed. However, a given injected amount resulted in considerably larger interior concentrations of the tracer element in the counter-beam heating cases, suggesting larger penetration of the plasma periphery. Computer simulation with the MIST code suggests a net inward drift of the order 103 cms−1 superposed to a diffusion coefficient of the order 104 cm2s−1 for both scandium and molybdenum ions. Injection of larger amounts of the tracer element, sufficient to cause measurable central electron temperature changes, resulted in dramatic changes in ion-state distributions, making some appear peaked in the centre while others disappeared. This effect could be produced with both co- and counter-beam heating, but with lesser amounts in the latter case. It is interpreted as rearrangement of the ionization balance, rather than any preferential accumulation of the injected element.
    Nuclear Fusion 01/2011; 24(7):815. · 2.73 Impact Factor
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    ABSTRACT: This paper presents the results of a study of the increase in impurity line radiation in the 15-360 Å region during ICRF heating of PLT plasmas, with emphasis on metallic impurity (Ti and Fe) behaviour. Central titanium and iron densities are given for a variety of ICRF heating experiments; total central metallic impurity concentrations of up to about 0.3% of ne(0) are observed at the 2.0 MW RF power level. This study shows that the power radiated by these elements is a small (about 10%) fraction of the total input power to the plasma for the present heating efficiency at 2.0 MW RF power. The impurity line brightnesses scale approximately linearly with RF power up to 2.8 MW. The antenna Faraday shields are shown to be the primary source of metallic impurities during both ICRF heating and Ohmic heating only. The impurity content of discharges heated using a single half-turn antenna and a pair of centre-fed antennas (having the same total surface area but half the poloidal extent of the half-turn antenna) is the same at a relatively low RF power of 350 kW, indicating that the impurity influx does not depend on the poloidal length of the antennas (or that the plasma interacts only with a localized area on the Faraday shields).
    Nuclear Fusion 01/2011; 24(6):767. · 2.73 Impact Factor
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    ABSTRACT: Introduction of large amounts of neon into Ohmically heated deuterium discharges in the PLT tokamakl results in higher central electron temperature (Te(0) 3 keV) and values of electron energy containment time that are larger than in regular discharges at the same electron density (τEe = 44 ms at e = 2 × 1019 m−3). For steady-state discharges with high effective Z (~ 5–8) the conductance is larger than that predicted by neoclassical theory by as much as a factor of two. Transport rates of hydrogen-like and helium-like ions can be fairly well approximated by assuming a diffusion constant of between 0.4 and 1 m2s−1. Within experimental uncertainties the diffusion of hydrogen-like neon is the same for co- and counter-directed high-power neutral beams.
    Nuclear Fusion 01/2011; 24(1):3. · 2.73 Impact Factor
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    ABSTRACT: The large increase in radiation during neutral injection in DITE is identified as being primarily due to the charge-exchange recombination of the impurities on the fast neutrals. The time dependence of the increase in line eion and its dependence on injection power and plasma density is shown to fit the theory. – The line emission and the total radiated power are found to be toroidally asymmetric, being larger in the region of beam deposition. A theoretical explanation for this asymmetry is given. The recombination of impurities by thermal neutrals is found to be important in low-density Ohmic discharges, and the implications of this are discussed.
    Nuclear Fusion 01/2011; 22(3):333. · 2.73 Impact Factor
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    ABSTRACT: In H-mode plasmas in DIII-D, large modulations in spectroscopically measured impurity densities have been observed during shots with giant edge localized modes (ELMs). These spectral modulations have been analysed with the MIST impurity transport code. This analysis indicates that impurities are alternately flowing towards the plasma centre and then away from it. This alternating flow is correlated with ELM produced changes in the electron density. The electron density oscillations are extreme, causing the density profile to switch from hollow (just before an ELM) to centrally peaked (just after an ELM). Neoclassical convection, dependent on ion density gradients, causes impurities to concentrate most heavily where the electron density is largest and can explain the modulating impurity behaviour. Anomalous diffusion, D 1.0 × 104 cm2/s, reduces the degree of impurity peaking. As the plasma current increases, the increase in hollowness of electron density profiles can account for the observed decrease in central impurity accumulation. Transport of cobalt, injected by laser ablation, has also been studied; cobalt transport variations are consistent with the ELM induced changes seen in intrinsic impurity transport. The transport results may be consistent with neoclassical impurity convective fluxes and suggest that impurity accumulation in tokamaks will occur unless the electron density profile is flat or particle confinement is low.
    Nuclear Fusion 01/2011; 31(10):1859. · 2.73 Impact Factor
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    ABSTRACT: Intrinsic impurity behaviour and transport properties in neutral beam heated L- and H-mode PBX tokamak plasmas were studied with a variety of impurity diagnostics. Central impurity accumulation was most often observed in H-mode discharges; sometimes it resulted in a thermal collapse due to high central metallic radiation (~1.5 W • cm−3). The impurity accumulation was evident from peaked Zeff and radiated power profiles and was further substantiated by specific vacuum ultraviolet and X-ray spectroscopy measurements. It is shown that impurity accumulation was neither unique nor inevitable in H-mode discharges and it could be suppressed by sufficient gas puffing. Central impurity accumulation was also seen in L-mode plasmas even with co-injected neutral beams. This usually occurred at high beam power and relatively low density. While there was no significant difference in the degree of accumulation between L-mode and H-mode discharges, the Zeff profile itself was more peaked in the H-mode because the electron density profiles are flatter in H-mode plasmas than in L-mode plasmas. The degree of accumulation increased as Zeff(0) itself increased, which suggests that neoclassical convection and diffusion driven by both impurity-impurity and impurity-plasma ion collisions contribute to the central plasma particle transport.
    Nuclear Fusion 01/2011; 29(2):231. · 2.73 Impact Factor
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    ABSTRACT: Transport of iron in TFTR discharges heated by 6.7 MW of co-injected neutral beam power has been studied by spatially resolved charge exchange recombination spectroscopy in the visible region of the spectrum. The time evolutions of the densities of Fe24+ and Fe23+ ions following injection of iron are modelled by the neoclassical flux plus a moderately hollow diffusivity, increasing linearly from 1.3 m2/s on axis to 2.4 m2/s at the plasma edge. For r/a > 0.5, the iron diffusivity is significantly smaller than the helium diffusivity measured in identical discharges, while it is larger in the region immediately surrounding the plasma axis.
    Nuclear Fusion 01/2011; 31(1):171. · 2.73 Impact Factor
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    ABSTRACT: A zero-dimensional time-dependent energy balance model is used to explore the energy loss mechanisms of the CTX spheromak experiment at Los Alamos National Laboratory. A coupled set of model equations representing electron, ion, neutral, and impurity particle balance, electron and ion temperature, and magnetic field decay, are solved from initial values and the results compared to the time behaviour of experimentally measured average densities, temperature, and magnetic field. The energy balance model considers all the major atomic physics processes, especially the effects of radiation from a non-equilibrium distribution of impurity charge states evolving in time. The model includes the effects of a strong neutral-particle source which replaces by ionization the plasma being lost by a short particle confinement time. The neutral source is required in experiments to prevent the sudden termination of the discharge associated with low densities. A major new conclusion is that all the data from resistively decaying spheromaks can be effectively modelled, when the flux loss effects of a resistive flux conserver are also included, using plasma resistivity increased by a factor of 3.2 ± 0.6 over the Spitzer-Härm value evaluated with the volume-average temperature. This factor appears to be constant for all discharges at all times. The analysis has determined that the power density associated with the particle replacement is the most important loss in the warm, non-radiation-dominated CTX spheromaks.
    Nuclear Fusion 01/2011; 25(11):1657. · 2.73 Impact Factor
  • D. Post, R. Hulse, D. Stotler
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    ABSTRACT: The present concepts for power and particle control in the next generation of tokamak experiments rely upon atomic processes to transfer the power and momentum from the divertor plasma to the divertor chamber walls. These concepts are being evaluated with sophisticated one- and two-dimensional divertor models. We present a survey of some of the candidate atomic processes of impurity radiation, hydrogen radiation and atomic physics and molecular effects to aid the development of the models.
    Beiträge aus der Plasmaphysik 02/2006; 34(2‐3):300 - 305.
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    ABSTRACT: A new method for obtaining a transient ('pulse') electron heat diffusivity (χep) in the radial region 0.38 < r/a < 0.56 in TFTR L mode discharges is presented. Small electron temperature perturbations were caused by single bursts of injected impurities which radiated and cooled the plasma edge. A case of iron injection by laser ablation was found to be more definitive than a supporting helium gas puff case. In this new 'cold pulse' method, the authors concentrate on modelling just the electron temperature perturbations, tracked with electron cyclotron emission diagnostics, and on being able to justify separation of the perturbations in space and time from the cooling source. This χep is obtained for these two cases to be χep = (6.0 m2/s ± 35%) ~ 4χe (power balance), which is consistent with but more definitive than results from other studies that are more susceptible to ambiguities in the source profile
    Nuclear Fusion 11/2002; 34(3):349. · 2.73 Impact Factor
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    ABSTRACT: Sequential injection of multiple deuterium pellets combined with high power neutral beam heating in TFTR has produced stable plasmas with very high central densities, up to 5×1020 m-3. The line averaged densities achieved in these experiments correspond to Murakami parameters neR/BT up to 12×1019 m-2.T-1. This value, which does not appear to represent an intrinsic density limit, surpasses the values previously obtained in TFTR and exceeds, by more than a factor of two, the density limit predicted by the scaling of Greenwald et al. (Nuclear Fusion 28 (1988) 2199). The highest densities obtained with both pellet and gas fuelling have been achieved since boronization was applied to the first wall in TFTR. The characteristics and energy balance of the highest density plasma are discussed
    Nuclear Fusion 10/2002; 32(9):1585. · 2.73 Impact Factor
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    ABSTRACT: New long-pulse ion sources have been employed to extend the neutral beam pulse on TFTR from 0.5 sec to 2.0 sec. This made it possible to study the long-term evolution of supershots at constant current and to perform experiments in which the plasma current was ramped up during the heating pulse. Experiments were conducted with co and counter injection as well as with nearly balanced injection of deuterium beams up to a total power of 20 MW. The best results, i.e., central ion temperatures Tio > 25 keV and neo τE Tio values of 3 × 1020 keV sec m-3, were obtained with nearly balanced injection. The central toroidal plasma rotation velocity scales in a linear-offset fashion with beam power and density. The scaling of the inferred global momentum confinement time with plasma parameters is inconsistent with the predictions of the neoclassical theory of gyroviscous damping. An interesting plasma regime with properties similar to the H-mode has been observed for limiter plasmas with edge qa just above 3 and 2.5.
    Plasma Physics and Controlled Fusion 08/2002; 29(10A):1235. · 2.37 Impact Factor
  • Source
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    ABSTRACT: During the 1987 run, TFTR reached record values of QDD, neutron source strength, and Ti(0). Good confinement together with intense auxiliary heating has resulted in a plasma pressure greater than 3*105 Pascals on axis, which is at the ballooning stability boundary. At the same time improved diagnostics, especially ion temperature profile measurements, have led to increased understanding of tokamak confinement physics. Ion temperature profiles are much more peaked than previously thought, implying that ion thermal diffusivity, even in high ion temperature supershot plasmas, is greater than electron thermal diffusivity. Based on studies of the effect of beam orientation on plasma performance, one of the four neutral beamlines has been re-oriented from injecting co-parallel to counter parallel, which will increase the available balanced neutral injection power from 14 MW to 27 MW. With this increase in balanced beam power, and the addition of 7 MW of ICRF power it is planned to increase the present equivalant QDT of 0.25 to close to break-even conditions in the coming run.
    Plasma Physics and Controlled Fusion 12/2000; 30(11):1391. · 2.37 Impact Factor
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    ABSTRACT: The TFTR tokamak has reached its original machine design specifications (Ip=2.5 MA and BT=5.2 T). Recently, the D degrees neutral beam heating power has been increased to 6.3 MW. By operating at low plasma current (Ip approximately=0.8 MA) and low density (ne approximately=1*1019 m-3), high ion temperatures (9+or-2 keV) and rotation speeds (7*105 m/s) have been achieved during injection. At the opposite extreme, pellet injection into high current plasmas has been used to increase the line-average density to 8*1019 m-3 and the central density to 1.6*1020 m-3. This wide range of operating conditions has enabled the authors to conduct scaling studies of the global energy confinement time in both ohmically and beam heated discharges as well as more detailed transport studies of the profile dependence.
    Plasma Physics and Controlled Fusion 12/2000; 28(1A):17. · 2.37 Impact Factor
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    ABSTRACT: The characteristics of plasma operation on the axisymmetric inner toroidal limiter in TFTR are described. After conditioning, plasmas with low metal content and low zeff are obtained with this limiter. There is no substantial increase in zeff with total input power during neutral beam injection. Compared to operation on the outer blade limiter, additional gas is required to fuel plasmas on the inner limiter. Injection of D pellets increased the plasma density substantially and produced energy confinement times up to 0.5 s in ohmically heated plasmas. The four neutral beam lines have injected up to 13.5 MW total power into the plasma for 0.5 s with up to 90 kV accelerating voltage. The scaling of the plasma stored energy was studied as a function of the input power, plasma current and plasma density. In the range 1.4 to 2.2 MA, the overall and incremental confinement times for both the total and thermal stored energies increase with plasma current at fixed density. There appears to be a weak negative scaling of the total stored energy with density at high injection power.
    Plasma Physics and Controlled Fusion 12/2000; 28(9A):1329. · 2.37 Impact Factor