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ABSTRACT: It is pointed out that the accidentally resonant charge-exchange reaction, O+ + H0 O0 + H+, is an important mechanism for causing the loss of singly charged oxygen ions from oxygen-contaminated hydrogen plasmas. Results of a Monte Carlo simulation are presented which show that the fraction of oxygen lost because of charge exchange exceeds 1/3 when the parameters ne ~ 1013 cm−3, nH0 ~ 1011 cm−3 and Te ~ 3 eV are attained. This process may be important in determining impurity losses from the edge plasmas of tokamaks.
Nuclear Fusion 01/2011; 18(1):5. · 4.09 Impact Factor
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ABSTRACT: Insufficient helium exhaust can seriously degrade the performance of any fusion reactor and has recently been identified as a potentially significant issue for the International Thermonuclear Experimental Reactor (ITER). The effects of variations of the ratios DHe/χi, DHe/DD and vHe/DHe and of the sawtooth period on helium exhaust in ITER have been studied with BALDUR code transport simulations. Recent measurements of DHe/χi in TFTR, of DHe/DD in TEXTOR and of vHe/DHe in TFTR, TEXTOR and JT-60 are found to be compatible with the requirements of sustained ignition and helium exhaust for ITER. Ignition is found to be critically sensitive to the ratio vHe/DHe, particularly at the plasma edge, and to a reduced sawtooth period. These critical dependences are moderated for reduced helium recycling at the separatrix and for hollow particle diffusivity profiles.
Nuclear Fusion 01/2011; 31(9):1689. · 4.09 Impact Factor
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J.D. Strachan,
N. Bretz,
E. Mazzucato,
C.W. Barnes,
D. Boyd, S.A. Cohen,
J. Hovey,
R. Kaita,
S.S. Medley,
G. Schmidt,
G. Tait,
D. Voss
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ABSTRACT: The evolution of the density profile in PLT during intense gas puffing is documented and analysed. Measurements of the spectrum of low-energy edge neutrals and of the change in central neutral density, indicate that charge-exchange processes alone cannot account for the central density rise. The transient density profile changes can be reproduced numerically by a diffusivity of ~ 104 cm2s−1 and a spatially averaged inward flow of 103 cms−1. These transport coefficients are 10—102 times larger than neoclassical. The ion energy confinement is reduced, the small-scale density fluctuations are increased, and runaway electron losses are increased during the density rise.
Nuclear Fusion 01/2011; 22(9):1145. · 4.09 Impact Factor
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04/2002;
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04/2002;
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R J Hawryluk,
V Arunasalam,
C W Barnes,
M Beer,
M Bell,
R Bell,
H Biglari,
M Bitter,
R Boivin,
N L Bretz, [......],
S von Goeler,
R Wieland,
M Williams,
J R Wilson,
K L Wong,
M Yamada,
S Yoshikawa,
K M Young,
M C Zarnstorff,
S J Zweben
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ABSTRACT: A review of TFTR plasma transport studies is presented. Parallel transport and the confinement of suprathermal ions are found to be relatively well described by theory. Cross-field transport of the thermal plasma, however, is anomalous with the momentum diffusivity being comparable to the ion thermal diffusivity and larger than the electron thermal diffusivity in neutral beam heated discharges. Perturbative experiments have studied nonlinear dependencies in the transport coefficients and examined the role of possible nonlocal phenomena. The underlying turbulence has been studied using microwave scattering, beam emission spectroscopy and microwave reflectometry over a much broader range in k perpendicular to than previously possible. Results indicate the existence of large-wavelength fluctuations correlated with enhanced transport.
Plasma Physics and Controlled Fusion 08/2000; 33(13):1509. · 2.42 Impact Factor
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ABSTRACT: This paper reports the buildup of helium ash has been studied in a series of simulations with the BALDUR transport code in the proposed International Thermonuclear Experimental Reactor (ITER) experiment at low density {bar n}{sub e} = 8.3 {times} 10{sup 19}/m{sup {minus}3}. Sustained ignition is found to be possible only for R{sub He} {lt} 0.5 {yields} 0.9, with lower values required at lower edge densities. Using radially dependent thermal diffusivities that were scaled from Joint European Torus (JET) values, the effects of particle transport coefficients and edge recycling on helium poisoning of ignition are studied. A sustained ignition is obtained when the exhaust of helium from the edge plasma is allowed to exceed 10% of the helium flux into the edge plasma from the core plasma, and the ratio of particle (helium ion) to thermal diffusivities. D/{chi}, is {gt}1/4. The simulations include the effects of sawtooth oscillations, radiative as well as conductive energy loss channels, and density profile variations.
Fusion Technology; (United States). 07/1991; 20:1.
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D.M. Meade,
V. Arunasalam,
M.G. Bell,
R. Bell,
M. Bitter,
R. Boivin,
N.L. Bretz,
R. Budny,
A. Cavallo,
C.Z. Cheng, [......],
G. Hammett,
R.J. Hawryluk,
K.W. Hill,
J.C. Hosea,
R.A. Hulse,
H. Hsuan,
A. Janos,
D.L. Jassby,
F.C. Jobes,
D.W Johnson
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ABSTRACT: TFTR experiments have emphasized the optimization of high performance plasmas as well as studies of transport in high temperature plasmas. The recent installation of carbon composite tiles on the main bumper limiter has allowed operation with up to 32 MW of neutral beam injection without degradation of plasma performance by large bursts of carbon impurities ( carbon blooms''). Plasma parameters have been extended to T{sub i}(0) {approximately} 35 keV, T{sub e}(0) {approximately} 12 keV, n{sub e}(0) {approximately}1.2 {times} 10{sup 20} m{sup {minus}3} producing D-D reaction rates of 8.8 {times} 10{sup 16} reactions per second. The fusion parameter n{sub e}(0){tau}{sub E}T{sub i}(0) in supershot plasmas is an increasing function of heating power up to an MHD stability limit, reaching values of {approximately}4.4 {times} 10{sup 20} m{sup {minus}3} sec keV. Peaked-density-profile hot-ion plasmas with the edge characteristics of the H-mode have been produced in a circular cross-section limiter configuration with n{sub e}(0){tau}{sub E}T{sub i}(0) values characteristic of supershots, namely up to four times those projected for standard H-modes with broad density profiles. Reduced transport is also observed in the core of high-density ICRF-heated plasmas when the density profile is peaked. At the highest performance, the central plasma pressure in TFTR reaches reactor level values of 6.5 atmospheres. In these regimes, MHD instabilities with m/n = 1/1, 2/1, 3/2 and 4/3 are often observed concurrent with a degradation in performance. High {beta}{sub p} plasmas with {var epsilon}{beta}{sub p} {approx} 1.6 and {beta}/(I/aB) {approx} 4.7 (%mT/MA) have demonstrated confinement enhancement over the low-mode confinement time with {tau}{sub E}/{tau}{sub L} {approximately} 3.5 and a bootstrap current of about 65% of the total plasma current.
06/1991;
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ABSTRACT: A new regime of enhanced energy confinement has been observed on TFTR with neutral beam injection at low plasma current. It is characterized by extremely peaked electron density profiles and broad electron temperature profiles. The electron temperature profile shapes violate the concept of profile consistency in which T/sub e/(O)//sub v/ is assumed to be a tightly constrained function of q/sub a/, but they are in good agreement with a form of profile consistency based on examining the temperature profile shape outside the plasma core. The enhanced confinement regime is only obtained with a highly degassed limiter; in discharges with gas-filled limiters convective losses are calculated to dominate the edge electron power balance. Consistent with the constraint of profile consistency, global confinement is degraded in these cases. The best heating results in the enhanced confinement regime are obtained with nearly balanced co- and counter-injection. Much of the difference between balanced and co-only injection can be explained on the basis of classically predicted effects associated with plasma rotation.
03/1987;
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ABSTRACT: The ability of the Tokamak Fusion Test Reactor (TFTR) to attain the largest fusion power gain depends critically on minimizing plasma contamination and controlling the densities of the reacting deuterium and tritium. Experiments on a number of tokamaks have demonstrated that gettering over an appreciable surface area (? 10%) of the vacuum vessel greatly facilitates both of these objectives. One particular problem in implementing a surface pumping system in TFTR, however, is a restriction on the maximum allowable tritium content of the getter. This restriction could require regeneration of the absorbed tritium after as few as 50 machine pulses. We have developed a scheme utilizing SAES Zr/Al getter modules which obviates the need for such frequent interruptions of machine operation by taking advantage of the pulsed operation of TFTR. With the Zr/Al getter at temperatures between 500°–600°C it is possible to achieve a quasisteady state in the tritium loading where the quantity of tritium desorbed between pulses is equal to the quantity which is absorbed during a pulse. Since frequent thermal cycling is not required, this scheme also reduces the possibility of Zr/Al getter material fatigue.
Journal of Vacuum Science and Technology 02/1980;
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ABSTRACT: An experiment measuring the scaling with changing plasma conditions of a parameter characteristic of the transport of aluminum injected into the ATC tokamak is discussed. This parameter is the time after injection at which the photon signal from Al XI reaches its maximum. It is found that the data are in agreement with the predictions of a computer code which uses neoclassical theory in the Pfirsch-Schluter regime. An approximate model describing impurity transport in ATC is also presented. This model is utilized to calculate the expected scaling with current of the aluminum transport assuming classical, neoclassical and pseudoclassical forms for the diffusion coefficient. The data are in agreement with both the neoclassical and pseudoclassical results from this model.
05/1976;
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ABSTRACT: A system for producing 300‐μsec bursts of 10<sup>17</sup> metal atoms with 3‐eV average energy is described. It is shown that this system can be successfully used to introduce impurities into CTR‐oriented tokamaks for transport and confinement studies.
Review of Scientific Instruments 10/1975; · 1.37 Impact Factor
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ABSTRACT: Using a combined electrostatic-calorimeter probe, a comprehensive survey of energy flux in the edge plasmas of low field ohmic and beam-heated PDX and PLT discharges has been made. Ions are shown to carry the majority of the power to floating probes. The intercepted energy is found to increase nearly linearly with applied heating power. The scrape-off distances, ranging from 0.5 cm for a 4 null diverted discharge to 2.5 cm for inside dee diverted discharge, are independent of heating power. Direct evidence is presented for the prompt loss to the walls and limiters in PDX of energetic beam ions.
Journal of Nuclear Materials.
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H.F. Dylla,
TFTR Team,
M.G. Bell,
W.R. Blanchard,
P.P Boody,
N. Bretz,
R. Budny,
C.E. Bush,
J.L. Cecchi, S.A. Cohen, [......],
G. Taylor,
M. Ulrickson,
S. Von Goeler,
W.R. Wampler,
K. Wilson,
M. Williams,
K.L. Wong,
K.M. Young,
M.C. Zarnstorff,
S. Zweben
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ABSTRACT: This paper presents a summary of plasma-material interactions which influence the operation of TFTR with high current (≤ 2.2 MA) ohmically heated, and high-power (∼ 10 MW) neutral-beam heated plasmas. The conditioning procedures which are applied routinely to the first-wall hardware are reviewed. Fueling characteristics during gas, pellet, and neutral-beam fueling are described. Recycling coefficients near unity are observed for most gas fueled discharges. Gas fueled discharges after helium discharge conditioning of the toroidal bumper limiter, and discharges fueled by neutral beams and pellets, show R<1. In the vicinity of the gas fueled density limit (at ne = 5–6 × 1019 m−3) values of Zeff are ≦1.5. Increases in Zeff of ≦1 have been observed with neutral beam heating of 10 MW. The primary low Z impurity is carbon with concentrations decreasing from ∼10% to <1% with increasing ne. Oxygen densities tend to increase with ne, and at the ohmic plasma density limit oxygen and carbon concentrations are comparable. Chromium getter experiments and He2+/D+ plasma comparisons indicate that the limiter is the primary source of carbon and that the vessel wall is a significant source of the oxygen impurity. Metallic impurities, consisting of the vacuum vessel metals (Ni, Fe, Cr) have significant (∼10−4 ne) concentrations only at low plasma densities (ne <1019 m−3). The primary source of metallic impurities is most likely ion sputtering from metals deposited on the carbon limiter surface.
Journal of Nuclear Materials.
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B.C. Stratton, S.A. Cohen,
F.P. Boody,
C.E. Bush,
R. Ellis III,
R.J. Fonck,
E. Fredrickson,
R.J. Groebner,
K.W. Hill,
R.A. Hulse,
E.S. Marmar,
A.T. Ramsey,
J.F. Schivell,
S.S. Sesnic,
J. Timberlake
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ABSTRACT: Impurity transport has been studied in ohmic and neutral-beam-heated TFTR discharges by observation and numerical modeling of time evolutions of emissions from germanium injected using the laser-blowoff method. VUV spectral lines, the total radiated power, and soft X-ray intensities are compared with code predictions in which the impurity flux is assumed to have diffusive and convective terms. In plasmas with Ip = 1.4 MA and , no difference in transport between ohmic and beam-heated discharges is observed. At Ip = 0.7 MA and , a three-fold increase in the diffusion coefficient is found during beam heating.
Journal of Nuclear Materials.
