| 1. |
- Sips, A. C. C., et al.
(author)
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An international database for the study of the formation of ITBs in tokamaks
- 2002
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In: Plasma Physics and Controlled Fusion. - 0741-3335 .- 1361-6587. ; 44, s. A391-A398
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Journal article (peer-reviewed)abstract
- For the first time, data from eight,different tokamaks have been combined in an international database for internal transport barriers (ITBs). An analysis of the data for the formation of an ITB with dominant ion heating shows a clear dependence of the threshold power on the minor radius and line-averaged electron density for the formation of ion ITBs. The dependence of ITB formation on the toroidal magnetic field is weak. For the formation of ITBs with dominant electron heating, the database is smaller, but for the threshold power a strong increase with plasma size and a weak toroidal field dependence could also be identified. Based on these results, an expression for the power required to form an ITB is given using global variables only. These results give a basis for the analysis of the database using local values (like magnetic shear) and a detailed comparison with theory-based models.
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| 2. |
- Hender, T. C., et al.
(author)
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MHD stability with strongly reversed magnetic shear in JET
- 2002
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In: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 0741-3335 .- 1361-6587. ; 44:7, s. 1143-1154
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Journal article (peer-reviewed)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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| 3. |
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| 4. |
- Bergkvist, Tommy, et al.
(author)
-
Nonlinear interaction between RF-heated high-energy ions and MHD-modes
- 2003
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In: RADIO FREQUENCY POWER IN PLASMAS. - 0735401586 ; , s. 459-462
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Conference paper (peer-reviewed)abstract
- Excitation of global Alfven eigenmodes by fast ions during ICRH is frequently observed in tokamaks. The importance of the phasing of the ICRH antennae for the excitation of these modes have been seen in experiments. The Alfven eigenmodes will drive the distribution function of the fast ions towards a state where the gradient in phase space is reduced. In general, the fast ions are displaced outwards, which can have a significant effect on the ICRH power deposition and lead to reduced heating efficiency. To calculate the effect on the heating profiles by the excitation of Alfven eigenmodes and the, effect on the resonating ions the Monte Carlo code FIDO, used for ICRH, has been upgraded to include particle interactions with MHD-waves. This allows self-consistent calculations of the mode amplitude and the distribution function during RF heating.
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| 5. |
- Laxåback, Martin, et al.
(author)
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Self-consistent simulations of ICRH in ITB plasmas
- 2001
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In: AIP Conf. Proc.. - : AIP. ; , s. 414-417
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Conference paper (other academic/artistic)abstract
- The RF power partition and power deposition on resonant ion species during ICRH depend strongly on the distribution functions of the heated ions. The distribution functions in turn depend on RF interactions and Coulomb collisions with the background plasma. It has previously been found that the finite ion orbit width, as well as the RF induced transport of resonant ions, are important for describing the distribution functions. This is particularly important in ITB plasmas, where low central current density results in broad orbits. To simulate ICRH in ITB plasmas the SELFO code has been upgraded to self-consistently calculate the wave field and the distribution functions of several ion species, including beam injected ions. Simulations of hydrogen minority heating of a deuterium, JET-like, ITB plasma have been made for different antenna phasings, with and without NBI. The distribution functions of both hydrogen and deuterium have been simulated. Finite orbit width and RF induced particle transport effects are shown to have a large impact on the partition of RF power absorption between hydrogen and deuterium, and thereby on the power transfer to the background.
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| 6. |
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| 7. |
- Litaudon, X., et al.
(author)
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Progress towards steady-state operation and real-time control of internal transport barriers in JET
- 2003
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In: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 43:7, s. 565-572
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Journal article (peer-reviewed)abstract
- In JET, advanced tokamak research mainly focuses on plasmas with internal transport barriers (ITBs) that are strongly influenced by the current density profile. A previously developed optimized shear regime with low magnetic shear in the plasma centre has been extended to deeply negative magnetic shear configurations. High fusion performance with wide ITBs has been obtained transiently with negative central magnetic shear configuration: H-IPB98(y,H-2) similar to 1.9, beta(N) = 2.4 at I-p = 2.5 MA. At somewhat reduced performance, electron and ion ITBs have been sustained in full current drive operation with 1 MA of bootstrap current: H-IPB98(y,H-2) similar to 1, beta(N) = 1.7 at I-p = 2.0 MA. The ITBs were maintained for up to 11 s for the latter case. This duration, much larger than the energy confinement time (37 times larger), is already approaching a current resistive time. New real-time measurements and feedback control algorithms have been developed and implemented in JET for successfully controlling the ITB dynamics and the current density profile in the highly non-inductive current regime.
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| 8. |
- Mantsinen, M. J, et al.
(author)
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Controlling the profile of ion-cyclotron-resonant ions in JET with the wave-induced pinch effect
- 2002
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 89:11, s. 5004-
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Journal article (peer-reviewed)abstract
- Experiments on the JET tokamak show that the wave-induced pinch in the presence of toroidally asymmetric waves can provide a tool for controlling the profile of ion-cyclotron-resonant He-3 ions. Direct evidence for the wave-induced pinch has been obtained from the measured gamma-ray emission profiles. Concurrent differences in the excitation of Alfven eigenmodes (AEs), sawtooth stabilization, electron temperatures, and fast-ion stored energies are observed. The measured location of the AEs and gamma-ray emission profiles are consistent with the fast-ion radial gradient providing the drive for AEs.
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| 9. |
- Mayoral, M L, et al.
(author)
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Sawtooth and neoclassical tearing mode seed island control by ICRF current drive on JET
- 2001
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In: RADIO FREQUENCY POWER IN PLASMAS. - 0735400385 ; , s. 106-109
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Conference paper (peer-reviewed)abstract
- In recent experiments at JET, ICRF waves were used to either stabilise or destabilise sawteeth, and consequently control the formation of the seed island responsible for the appearance of Neoclassical Tearing Modes (NTMs) known to limit the tohamak performance. Two scenarios were studied in deuterium plasmas with a hydrogen concentration of 5 to 10 %. Firstly, using a toroidal magnetic field B-0 of 2.5T, the fundamental hydrogen cyclotron resonance layer (omega=omega(cH)) was positioned at the High Field Side (HFS) at the sawtooth inversion radius (R(inv)similar toR(q=1)). With +90degrees phased wave, monster sawteeth were obtained that triggered NTMs. Secondly, ICRF waves at the 2(nd) harmonic hydrogen cyclotron resonance (omega=2omega(cH)) positioned at the Low Field Side (LFS) with B-0 between 1.2T and 1.6T, was used to reduce the sawtooth period and increase the NTM beta(n) onset level.
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| 10. |
- Noterdaeme, J. M., et al.
(author)
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Heating, current drive and energetic particle studies on JET in preparation of ITER operation
- 2003
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In: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 43:3, s. 202-209
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Journal article (peer-reviewed)abstract
- This paper summarizes the recent work on JET in the three areas of heating, current drive and energetic particles. The achievements have extended the possibilities of JET, have a direct connection to ITER operation and provide new and interesting physics. Toroidal rotation profiles of plasmas heated far off axis with little or no refuelling or momentum input are hollow with only small differences on whether the power deposition is located on the low field side or on the high field side. With LH current drive the magnetic shear was varied from slightly positive to negative. The improved coupling (through the use of plasma shaping and CD4) allowed up to 3.4 MW of PLH in internal transport barrier (ITB) plasmas with more than 15 MW of combined NBI and ICRF heating. The q-profile with negative magnetic shear and the ITB could be maintained for the duration of the high heating pulse (8 s). Fast ions have been produced in JET with ICRF to simulate alpha particles: by using third harmonic He-4 heating, beam injected He-4 at 120 kV were accelerated to energies above 2 MeV taking advantage of the unique capability of JET to use NBI with 4 He and to confine MeV class ions. ICRF heating was used to replicate the dynamics of alpha heating and the control of an equivalent Q = 10 `burn' was simulated.
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| 11. |
- Sauter, O., et al.
(author)
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Control of neoclassical tearing modes by sawtooth control
- 2002
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 88:10
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Journal article (peer-reviewed)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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| 12. |
- Van Poppel, H., et al.
(author)
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Precancerous lesions in the kidney
- 2000
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In: Scandinavian Journal of Urology and Nephrology, Supplementum. - Oslo, Norway : Taylor & Francis. - 0300-8886 .- 1651-2537 .- 0000-0000 .- 0036-5599. ; :205, s. 136-165
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Research review (peer-reviewed)abstract
- Renal cell carcinoma (RCC), although occurring less frequently than prostate and bladder cancer, is actually the most malignant urologic disease, killing >35% of affected patients. Therefore, investigation of the nature of premalignant lesions of the kidney is a relevant issue. Following the most recent histological classification RCC can be subdivided into four categories: conventional RCC; papillary RCC; chromophobe RCC; and collecting duct carcinoma. In contrast to many genitourinary malignancies, premalignant alterations in the kidney are scarcely described. Intratubular epithelial dysplasia has been recognized as the most common precursor of RCC. In analogy to prostatic intraepithelial neoplasia (PIN), the premalignant lesions of the kidney are described as high or low-grade renal intratubular neoplasia. In contrast, precancerous lesions have been described as part of the von Hippel-Lindau syndrome (VHL) where the evolution from a simple cyst to an atypical cyst with epithelial hyperplasia to cystic or solid conventional-type RCC is well documented. Finally, in the genesis of papillary RCC an adenoma-carcinoma sequence has been recognized with specific genetic changes. There are no data on the epidemiology of premalignant lesions of the kidney, but research into the etiology of RCC has been extended substantially. Familial and genetic factors are well documented in VHL disease, in hereditary papillary RCC, in the tuberous sclerosis complex and in familial RCC. Cigarette smoking and obesity are established risk factors for RCC. Hypertension or its medication has also been associated with an increased risk. Among dietary factors an inverse relation between risk and consumption of vegetables and fruit has been found. Occupational exposure to substances such as asbestos and solvents has been linked to an increased risk of RCC. Specific RCC variants have distinctive chromosome alterations and several genes have been implicated in the development of RCC. Loss of material from the 3p chromosome characterizes conventional RCC and the deletion of the VHL suppressor gene plays an important role in the genesis of this RCC variant. In contrast, numerical changes with trisomy of chromosomes 7 and 17 and loss of the sex chromosome are typical changes in papillary tumors, whereas papillary RCC have additional trisomies. Chromophobe RCC is characterized by loss of chromosomes with a combination of monosomies. Less consistent genetic alterations are associated with collecting duct carcinoma. The traditional treatment of RCC is surgery by radical or partial nephrectomy. The latter approach carries a risk of tumor recurrence as a result of unrecognized satellite lesions or premalignant lesions that might have been present at the time of surgery. However, the reported recurrence rates after partial nephrectomy are <1% and therefore the possible presence of premalignant disease does not alter the actual treatment strategy advocated. Although multifocality and bilateral occurrence of RCC are much more likely in cases of papillary RCC, biopsy of the renal remnant or contralateral kidney is not justified even in patients with this tumor type. Conversely, patients with RIN in a partial or radical nephrectomy specimen or in a renal biopsy taken for whatever reason should be subjected to closer follow-up with regularly repeated ultrasound. When an effective chemopreventive regimen becomes available it might be useful for patients with an inherited risk of RCC as well as in those who are at risk of tumor recurrence after intervention. Mass screening with the purpose of detecting RCC at its earliest stage is not recommended at the present time, but screening focused on certain risk groups can be advocated. Further research is needed to identify avoidable risks, develop effective chemoprevention and recognize patients at risk.
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| 13. |
- Westerhof, E., et al.
(author)
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Control of sawteeth and triggering of NTMs with ion cyclotron resonance frequency waves in JET
- 2002
-
In: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 42:11, s. 1324-1334
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Journal article (peer-reviewed)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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