| 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. |
- Ismail, AA, et al.
(författare)
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Incidence of limb fracture across Europe: Results from the European Prospective Osteoporosis Study (EPOS)
- 2002
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Ingår i: Osteoporosis International. - : Springer Science and Business Media LLC. - 1433-2965 .- 0937-941X. ; 13:7, s. 565-571
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this population-based prospective study was to determine the incidence of limb fracture by site and gender in different regions of Europe. Men and women aged 50-79 years were recruited from population registers in 31 European centers. Subjects were invited to attend for an interviewer-administered questionnaire and lateral spinal radiographs. Subjects were subsequently followed up using an annual postal questionnaire which included questions concerning the occurrence of new fractures. Self-reported fractures were confirmed where possible by radiograph, attending physician or subject interview. There were 6451 men and 6936 women followed for a median of 3.0 years. During this time there were 140 incident limb fractures in men and 391 in women. The age-adjusted incidence of any limb fracture was 7,3/1000 person-years [pyrs] in men and 19 per 1000 pyrs in women, equivalent to a 2,5 times excess in women. Among women, the incidence of hip, humerus and distal forearm fracture, though not 'other' limb fracture, increased with age, while in men only the incidence of hip and humerus fracture increased with age. Among women, there was evidence of significant variation in the occurrence of hip, distal forearm and humerus fractures across Europe, with incidence rates higher in Scandinavia than in other European regions. though for distal forearm fracture the incidence in east Europe was similar to that observed in Scandinavia. Among men, there was no evidence of significant geographic variation in the occurrence of these fractures. This is the first large population-based study to characterize the incidence of limb fracture in men and women over 50 years of age across Europe. There are substantial differences in the descriptive epidemiology of limb fracture by region and gender.
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| 3. |
- O'Neill, TW, et al.
(författare)
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Back pain, disability, and radiographic vertebral fracture in European women: a prospective study
- 2004
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Ingår i: Osteoporosis International. - : Springer Science and Business Media LLC. - 1433-2965 .- 0937-941X. ; 15:9, s. 760-765
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Tidskriftsartikel (refereegranskat)abstract
- Vertebral fractures are associated with back pain and disability. There are, however, few prospective data looking at back pain and disability following identification of radiographic vertebral fracture. The aim of this analysis was to determine the impact of radiographically identified vertebral fracture on the subsequent occurrence of back pain and disability. Women aged 50 years and over were recruited from population registers in 18 European centers for participation in the European Prospective Osteoporosis Study. Participants completed an interviewer-administered questionnaire which included questions about back pain in the past year and various activities of daily living, and they had lateral spine radiographs performed. Participants in these centers were followed prospectively and had repeat spine radiographs performed a mean of 3.7 years later. In addition they completed a questionnaire with the same baseline questions concerning back pain and activities of daily living. The presence of prevalent and incident vertebral fracture was defined using established morphometric criteria. The data were analyzed using logistic regression with back pain or disability (present or absent) at follow-up as the outcome variable with adjustment made for the baseline value of the variable. The study included 2,260 women, mean age 62.2 years. The mean time between baseline and follow-up survey was 5.0 years. Two hundred and forty participants had prevalent fractures at the baseline survey, and 85 developed incident fractures during follow-up. After adjustment for age, center, and the baseline level of disability, compared with those without baseline prevalent fracture, those with a prevalent fracture (odds ratio [OR]=1.4; 95% confidence interval [CI] 1.0 to 2.0) or an incident fracture (OR=1.7; 95% CI, 0.9 to 3.2) were more likely to report disability at follow-up, though the confidence intervals embraced unity. Those with both a prevalent and incident fracture, however, were significantly more likely to report disability at follow-up (OR=3.1; 95% CI, 1.4 to 7.0). After adjustment for age, center, and frequency of back pain at baseline, compared with those without baseline vertebral fracture, those with a prevalent fracture were no more likely to report back pain at follow-up (OR=1.2; 95%CI, 0.8 to 1.7). There was a small increased risk among those with a preexisting fracture who had sustained an incident fracture during follow-up (OR=1.6; 95%CI, 0.6 to 4.1) though the confidence intervals embraced unity. In conclusion, although there was no significant increase in the level of back pain an average of 5 years following identification of radiographic vertebral fracture, women who suffered a further fracture during follow-up experienced substantial levels of disability with impairment in key physical functions of independent living.
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| 4. |
- Mantsinen, M. J., et al.
(författare)
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Analysis of ion cyclotron heating and current drive at omega approximate to 2 omega(cH) for sawtooth control in JET plasmas
- 2002
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 0741-3335 .- 1361-6587. ; 44:8, s. 1521-1542
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Tidskriftsartikel (refereegranskat)abstract
- Ion cyclotron heating and current drive at omega approximate to 2omega(cH) in JET deuterium plasmas with a hydrogen concentration n(H)/(n(D)+n(H)) in the range of 5-15% are analysed, comparing results of numerical computer modelling with experiments. Second harmonic hydrogen damping is found to be maximized by placing the resonance on the, low-field side (LFS) of the torus, which minimizes competing direct electron damping and parasitic high-harmonic D damping in the presence of D beams. The shape of the calculated current perturbation and the radial localization of the heating power density for the LFS resonance are consistent with the experimentally observed evolution of the sawtooth period when the resonance layer moves near the q = 1 surface. Since the calculated driven current is dominated by a current of diamagnetic type caused by finite orbit widths of trapped resonating ions, it is not too sensitive to the ICRF phasing. Control of sawteeth with ion cyclotron current drive using the LFS omega approximate to 2omega(cH) resonance in the present experimental conditions can thus be best obtained by varying the resonance location rather than the ICRF phasing. Due to differences in fast ion orbits, collisional electron heating and fast ion pressure profiles are significantly more peaked for a LFS resonance than for a high-field side (HFS) resonance. For the HFS omega approximate to 2omega(cH) resonance, an enhanced neutron rate is observed in the presence of D beam ions, which is consistent with parasitic D damping at the omega approximate to 2omega(cD) resonance in the plasma centre.
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| 5. |
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| 6. |
- Hender, T. C., et al.
(författare)
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MHD stability with strongly reversed magnetic shear in JET
- 2002
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 0741-3335 .- 1361-6587. ; 44:7, s. 1143-1154
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Tidskriftsartikel (refereegranskat)abstract
- Recent operation of JET with centrally strongly reversed magnetic shear, produced with the help of lower hybrid current drive, has extended the domain in which internal transport barriers (ITBs) can be formed in JET. Performance is frequently limited by magnetohydrodynamic (MHD) instabilities in these reversed shear regimes. The most severe limit is a pressure driven kink mode which leads to a disruption. This disruptive limit is essentially the same in ITB plasmas with low or strongly reversed shear. Unique to the reversed shear regime is a dominantly n = 1 mode, which has multiple harmonics. This mode is a seemingly common limit to performance, in the highest performance plasmas. Also unique to the reversed shear regime are q > 1 sawteeth events, which can in turn trigger n = 1 post-cursor oscillations. In general, these post-cursor oscillations are benign but do provide valuable information on the q-profile. Other instabilities, including 'snakes' at the outer q = 3 surface, are also observed to limit the performance of reversed magnetic shear ITB regimes.
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| 7. |
- Pamela, J., et al.
(författare)
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Overview of results and possibilities for fast particle research on JET
- 2002
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 42:8, s. 1014-1028
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Tidskriftsartikel (refereegranskat)abstract
- The large physical size of the JET tokamak, its heating systems and diagnostics, and its capability to operate with full deuterium-tritium (D-T) plasmas, including high-power tritium neutral beam injection (NBI), give it unique possibilities in fast particle research in fusion plasmas. These have already been used to generate significant (2-3 MW level) power in fusion a-particles in the 1997 D-T campaign. Recent JET experiments have concentrated on two important scenarios of relevance to next-step tokamak devices: the ELMy H-mode plasmas and plasmas with strong internal transport barriers (ITBs). The achieved progress will help in preparation for a possible second D-T experiment on JET. Fast particle studies have also been carried out recently using ion cyclotron resonance heating (ICRH)-accelerated particles and external-excitation methods to study Alfven eigenmodes (AEs). Looking towards the future, the capability of JET will be enhanced by upgrades to the NBI system, ICRH system and various diagnostics. Results of the first JET D-T experiment (DTE1) form a basis on which to elaborate a second D-T experiment (DTE2) which could be proposed after these enhancements. The alpha-physics part of this programme would be divided between the investigation of alpha-particle confinement, heating and loss processes in the 'integrated scenarios' (where the discharge is as close as possible to an ITER-relevant scenario), and dedicated 'alpha-physics' experiments, with specially prepared plasmas. In ELMy H-mode plasmas the fusion performance could roach Q(=P-fusion/P-input) of similar to0.33 at the highest combined heating powers, corresponding to similar to 6x10(-4), allowing a test of the margins of TAE stability in quasi-steady-state conditions. The integrated-scenario fast particle programme could concentrate on the instabilities and heating in plasma regimes with strong steady-state ITBs, with expected Q values similar to0.58 and similar to2x10(-3), demonstrating the compatibility of these operating scenarios with alpha-effects. Excitation of TAEs by alpha-particles in the plasma core could also be studied in such integrated scenarios. An issue which will receive attention is the confinement of MeV energy ions in the centre of ITB plasmas with strongly reversed shear, where the low current density in the centre may lead to the alpha-particles entering loss orbits. In preparation for a D-T campaign, studies of triton burn-up in deuterium ITB plasmas will begin in the 2002 experimental campaigns. Special 'afterglow' experiments to measure TAEs after the termination of the (stabilizing) NBI have already been explored in JET deuterium ITB scenarios and would be planned for DTE2. It is intended to develop special versions of ITB plasmas with dominant ion heating which would maximize the sensitivity to degradation of alpha-heating effects.
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| 8. |
- 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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