| 1. |
- Ongena, J., et al.
(författare)
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Recent progress on JET towards the ITER reference mode of operation at high density
- 2001
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Ingår i: Plasma Physics and Controlled Fusion. - 0741-3335 .- 1361-6587. ; 43, s. A11-A30
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Tidskriftsartikel (refereegranskat)abstract
- Recent progress towards obtaining high density and high confinement in JET as required for the ITER reference scenario at Q = 10 is summarized. Plasmas with simultaneous confinement H-98(y.2) = 1 and densities up to n/n(Gw) similar to 1 are now routinely obtained. This has been possible (i) by using plasmas at high (delta similar to 0.5) and medium (delta similar to 0.3-0.4) triangularity with sufficient heating power to maintain Type I ELMs, (ii) with impurity seeded plasmas at high (delta similar to 0.5) and low (delta less than or equal to 0.2) triangularity, (iii) with an optimized pellet injection sequence, maintaining the energy confinement and raising the density, and (iv) by carefully tuning the gas puff rate leading to plasmas with peaked density profiles and good confinement at long time scales. These high performance discharges exhibit Type I ELMs, with a new and more favourable behaviour observed at high densities, requiring further studies. Techniques for a possible mitigation of these ELMs are discussed, and first promising results are obtained with impurity seeding in discharges at high triangularity. Scaling studies using the new data of this year show a strong dependence of confinement on upper triangularity, density and proximity to the Greenwald limit. Observed MHD instabilities and methods to avoid these in high density and high confinement plasmas are discussed.
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| 2. |
- McDonald, C., et al.
(författare)
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ELMy H-modes in JET helium-4 plasmas
- 2004
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 0741-3335 .- 1361-6587. ; 46:3, s. 519-534
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Tidskriftsartikel (refereegranskat)abstract
- ELMy H-modes in helium-4 plasmas provide valuable information on ELMy H-mode physics as well as a possible early low activation operational phase for next-step tokamaks, such as ITER. With this in mind, a series of helium-4 H-mode experiments were performed on JET with pure helium-4 NBI auxiliary heating (up to 12 MW). A set of ELMy H-mode plasmas were produced, in both the Type I ELM regime and a second regime, which showed characteristics similar to the deuterium Type III regime, but with a reverse ELM frequency dependence on power. Sawteeth were also observed, and had similar behaviour to those seen in deuterium. Compared with deuterium plasmas, Type I ELMy H-mode confinement is seen to be 28 +/- 6% poorer in helium-4 plasmas and the L-H power threshold 42 +/- 10% larger. This is the opposite of the behaviour predicted by experimental isotope mass scalings from hydrogenic plasmas.Comparison with a wider hydrogenic database, enables the effects of isotopic charge and mass to be studied independently.
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| 3. |
- Valovic, M., et al.
(författare)
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Density peaking in low collisionality ELMy H-mode in JET
- 2004
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 0741-3335 .- 1361-6587. ; 46:12, s. 1877-1889
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Tidskriftsartikel (refereegranskat)abstract
- Low collisionality, low particle source. ELMy H-modes (type-III) with sawteeth are produced in JET in order to address the question of density profile evolution in the reference q(95) = 3 ITER scenario. The paper focuses on particle transport in the core zone around the mid-radius. The pedestal region including ELMs and the region affected by sawteeth are not considered. By replacing a significant part of the neutral beam heating by RF power the beam particle flux at mid-radius has been reduced to Gamma(Beam)/n(e) = 0.07 m s(-1). The additional flux due to wall neutrals is estimated as Gamma(Wall)/n(e) = 0.12 m s(-1). Density profiles are found to be modestly peaked under these conditions with a relative density difference of Deltan/(n) = 0.23 across the zone not affected by sawteeth and ELMs. In a region around the mid-radius the ratio of effective particle diffusivity to electron thermal diffusivity is found to be D-e,D-eff/chi(e) approximate to 0.2, which might indicate an anomalous pinch provided the particle diffusivity D-e is sufficiently lame. The measured values of D-e,D-eff/chi(e) are at the lower end of the range used in ITER models.
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| 4. |
- Vennila, R S, et al.
(författare)
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High pressure electrical resistivity of composition controlled Nd-123 based bulk material
- 2004
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Ingår i: Modern physics letters B. - 0217-9849. ; 18:1, s. 27-33
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Tidskriftsartikel (refereegranskat)abstract
- High pressure electrical resistivity studies were carried out on the high temperature superconductor Nd1-xCaxBa2Cu3O7-delta for various calcium concentrations x = 0.00, 0.03, 0.06 and 0.10 obtained by the solid-state reaction method. The electrical resistivity study was performed using the four-probe technique with a Bridgman opposed anvil device. All four samples show an initial drastic fall in electrical resistivity up to a pressure of around 3 GPa that remains almost constant up to 8 GPa.
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| 5. |
- Ghorbani, S. R., et al.
(författare)
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An anomalous dip in thermoelectric power of Nd1-xPrxBa2Cu3O7-delta
- 2003
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Ingår i: Physica. C, Superconductivity. - 0921-4534 .- 1873-2143. ; 388, s. 351-352
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Tidskriftsartikel (refereegranskat)abstract
- The thermoelectric power, S, has been studied for sintered samples of Nd1-xPrxBa2Cu3O7-delta with 0 less than or equal to x less than or equal to 0.30 in the temperature range from the superconducting T-c to room temperature. S increases with decreasing temperature,and has a broad maximum at T-max in the region around 120 K before decreasing strongly when T-e is approached. Several properties indicate a decrease of charge concentration with increasing doping, x, from S(x, 290 K), T-max, and the resistivity rho(x, 290 K). An anomaly has been observed in S(T) for x greater than or equal to 0.20 in the form of a dip at 78 K of order 15% of S. The origin of this feature is not known.
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| 6. |
- Ghorbani, S. R., et al.
(författare)
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Thermoelectric power of charge-neutral Nd1-2xCaxMxBa2Cu3O7-delta (M = Th and Pr) : Evidence for different types of localization
- 2002
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 66:10
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Tidskriftsartikel (refereegranskat)abstract
- The superconducting T-c of Nd1-2xCaxMxBa2Cu3O7-delta (M=Th or Pr) is strongly depressed with increasing doping concentration in spite of the fact that these dopings are nominally charge neutral for small x. We have inquired into the reasons for this behavior by studies of the thermoelectric power S in sintered samples with x up to 0.1 and for temperatures from T-c up to room temperature. S(x,T) was analyzed in terms of two different semiempirical models, which assume the existence of a narrow electron band. In both models and for both dopings the bands broaden with increasing x and the tendency for localization increases. For Ca-Th doping this can be explained by weak electronic disorder. For Ca-Pr doping, the increase of electronic disorder is weaker, and the results indicate that charge localization in addition is important.
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| 7. |
- Han, Shunhui, et al.
(författare)
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Fluctuation conductivity at high temperatures in polycrystalline Hg, Tl-1223 - Is there ID fluctuation behavior?
- 2004
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Ingår i: Physica. C, Superconductivity. - : Elsevier BV. - 0921-4534 .- 1873-2143. ; 408, s. 679-680
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Tidskriftsartikel (refereegranskat)abstract
- Detailed resistivity measurements were made for polycrystalline Hg1-xTlxBa2Ca2Cu3O8+delta (x = 0.2) in zero field and in the temperature region from zero resistance up to 300 K. The fluctuation conductivity Deltasigma was analyzed as a function of is an element of equivalent to In(T/Tc-mf) in the range -5.5 < Inc < -0.5. 1D behavior Of fluctuation conductivity was found at high temperatures (above the 2D regime) with a crossover temperature T-1*, above which the critical exponent was found to be -1.5.
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| 8. |
- Lundqvist, P., et al.
(författare)
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mu SR study of charge neutrally doped Nd1-2xCaxThxBa2Cu3O7-delta
- 2000
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Ingår i: Physica. C, Superconductivity. - 0921-4534 .- 1873-2143. ; 338:4, s. 263-268
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Tidskriftsartikel (refereegranskat)abstract
- Equal amounts of Ca and Th were doped into the Nd1-2xCaxThxBa2Cu3O7-delta superconductor with 0 less than or equal to x less than or equal to 0.10. X-ray diffraction showed a single orthorhombic phase for all samples except at x = 0.10, suggesting a solubility limit below x = 0.10. For increasing doping concentration x the superconducting T-c and the electrical resistivity, rho at 275 K both vary strongly, with a linear decrease of T-c and an increase of rho. The muon spin relaxation rate, sigma, follows the Uemura line for large x, and deviates in the direction of increasing sigma-values for decreasing x. The approach of the data to the Uemura Line was compared between different alloy systems by plotting the normalized distance to the Uemura line as a function of doping concentration. Ca-Th doped samples were found to approach the Uemura line about three times faster than for Ca-Pr doping. The same replacements in Y-123 confirm this result. The strong doping concentration dependence of sigma in Ca-Th substituted samples is explained in terms of electronic disorder effects.
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| 9. |
- Lundqvist, P, et al.
(författare)
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mu SR study of charge neutrally doped Nd1-2xCaxThxBa2Cu3O7-delta
- 2000
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Ingår i: PHYSICA C. - : ELSEVIER SCIENCE BV. - 0921-4534. ; 338:4, s. 263-268
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Tidskriftsartikel (refereegranskat)abstract
- Equal amounts of Ca and Th were doped into the Nd1-2xCaxThxBa2Cu3O7-delta superconductor with 0 less than or equal to x less than or equal to 0.10. X-ray diffraction showed a single orthorhombic phase for all samples except at x = 0.10, suggesting a solub
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| 10. |
- Maddison, G. P., et al.
(författare)
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Impurity-seeded plasma experiments on JET
- 2003
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 43:1, s. 49-62
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Tidskriftsartikel (refereegranskat)abstract
- Scaling to larger tokamaks of high confinement plasmas with radiating edges, induced by impurities, is being studied through internationally collaborative experiments on JET. In campaigns till the end of 2000, three different regimes have been explored. A small number of limiter L-mode discharges seeded with neon have most closely repeated the approach used on TEXTOR-94, but different collisionality and particle transport in JET impede central peaking of the density associated with improved confinement. Divertor L-modes at intermediate density, again with neon injection, have pursued transiently enhanced states found on DIII-D. Confinement up to H-mode quality, together with radiation fractions of approximate to40%, have briefly been obtained, though central Z(eff) quickly increases. Most effectively, neon and argon seeding of higher density ELMy H-modes formed mainly at low triangularity on the septum of the MkIIGB divertor, resembling a pumped-limiter arrangement, have been examined. Good confinement has been sustained at densities close to the Greenwald level in 'afterpuff' (AP) phases following the end of main gas fuelling, for little change of central Z(eff) but up to approximate to60% radiation. Outstanding normalized properties up to H-97 = 0.99 at f(Gwd) = 0.94 have thus been achieved, above the conventional H-mode density limit for diverted plasmas. Stationarity of states has also been extended to many energy confinement times by including low, extra gas inputs in the 'AP', suggestive of an optimized fuelling scheme. Further development in 2001 is reported separately in [1]. Accompanying ELMs are generally reduced in frequency though not evidently in size, electron pedestal pressure being almost unchanged from unseeded behaviour. There are indications of the most favourable impurity species scaling with plasma parameters, performance, radiation and its concentration within a mantle all increasing with argon compared to neon in JET. These benefits in terms of integrated properties are just as required for long burning pulses in ITER, supporting its use of a radiating mantle to assist not only power exhaust but performance too. Impurity-seeded H-modes can therefore contribute directly to next-step scenario development.
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| 11. |
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| 12. |
- Philipps, V., et al.
(författare)
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Comparison of tokamak behaviour with tungsten and low-Z plasma facing materials
- 2000
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Ingår i: Plasma Physics and Controlled Fusion. - 0741-3335 .- 1361-6587. ; 42, s. B293-B310
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Tidskriftsartikel (refereegranskat)abstract
- Graphite wall materials are used in present day fusion devices in order to optimize plasma core performance and to enable access to a large operational space. A large physics database exists for operation with these plasma facing materials, which also indicate their use in future devices with extended burn times. The radiation from carbon impurities in the edge and divertor regions strongly helps to reduce the peak power loads on the strike areas, but carbon radiation also supports the formation of MARFE instabilities which can hinder access to high densities. The main concerns with graphite are associated with its strong chemical affinity to hydrogen, which leads to chemical erosion and to the formation of hydrogen-rich carbon layers. These layers can store a significant fraction of the total tritium fuel, which might prevent the use of these materials in future tritium devices. High-Z plasma facing materials are much more advantageous in this sense, but these advantages compete with the strong poisoning of the plasma if they enter the plasma core. New promising experiences have been obtained with high-Z wall materials in several devices, about which a survey is given in this paper and which also addresses open questions for future research and development work.
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| 13. |
- Sauter, O., et al.
(författare)
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Control of neoclassical tearing modes by sawtooth control
- 2002
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 88:10
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Tidskriftsartikel (refereegranskat)abstract
- The onset of a neoclassical tearing mode (NTM) depends on the existence of a large enough seed island. It is shown in the Joint European Torus that NTMs can be readily destabilized by long-period sawteeth, such as obtained by sawtooth stabilization from ion-cyclotron heating or current drive. This has important implications for burning plasma scenarios, as alpha particles strongly stabilize the sawteeth. It is also shown that, by adding heating and current drive just outside the inversion radius, sawteeth are destabilized, resulting in shorter sawtooth periods and larger beta values being obtained without NTMs.
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| 14. |
- Srinivas, V., et al.
(författare)
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Electronic transport properties of high-resistivity icosahedral AlPdRe below 1 K
- 2002
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 65:9
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Tidskriftsartikel (refereegranskat)abstract
- The temperature dependence of the conductivity, sigma(T), and the magnetoresistance up to B = 6 T have been studied in the temperature range 0.01 Kless than or equal toTless than or equal to1 K for two differently prepared i-AlPdRe samples on the insulating side of the metal-insulator transition. sigma(T) for both samples was observed to saturate in zero magnetic field below about 20 mK. With increasing magnetic field the temperature dependence of the conductivity increased, and at 6 T, or(T,B = 6 T) was found to decrease linearly with decreasing temperature down to the lowest measurement temperature of 10 mK. The estimated value of the conductivity at T=0 K, sigma(0 K), was found to decrease strongly with increasing resistance ratio R [ = rho(4 K)/rho(295 K)] over a wide range of R values and for differently prepared samples, suggesting that a finite sigma(0,R) is a characteristic property of the icosahedral phase of high-resistivity i-AlPdRe. The magnetoresistance is positive below 1 K. The relation Deltarho(B)/rhosimilar toB(2) is followed at larger fields by a B-2/3 dependence. With decreasing temperature these characteristic field regions are successively depressed towards smaller magnetic fields and eventually become unobservable. A scaling of the magnetoresistance is found for data in the region Tless than or equal to1 K, Bless than or equal to6 T of the form [Deltarho(B)/rho]/T-alpha=f(B/T) for each sample, with alpha about 0.3 for both samples.
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| 15. |
- Westerhof, E., et al.
(författare)
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Control of sawteeth and triggering of NTMs with ion cyclotron resonance frequency waves in JET
- 2002
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 42:11, s. 1324-1334
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Tidskriftsartikel (refereegranskat)abstract
- A new scenario to delay or prevent neoclassical tearing mode (NTM) onset is presented. By active sawtooth destabilization, short period and low amplitude sawteeth are generated, such that the sawtooth produced NTM seed island is reduced and the threshold normalized plasma pressure for triggering of NTMs, beta(Nonset), is increased. The scenario has been explored experimentally in the Joint European Torus (JET). Ion cyclotron resonance frequency (ICRF) waves tuned to the 2nd harmonic H-minority resonance have been used for sawtooth control. Whereas ICRF waves generally induce sawtooth stabilization, favouring the triggering of NTMs and reducing beta(Nonset), the present experiments show that by toroidally directed waves, ion cyclotron current drive is produced, and that sawteeth can be destabilized by careful positioning of the 2nd harmonic H resonance layer with respect to the sawtooth inversion radius. As a result, NTM onset is delayed and beta(Nonset) is increased above its value obtained in discharges with additional heating from neutral beam injection alone.
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