| 1. |
- Van Oost, G., et al.
(författare)
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Turbulent transport reduction by E x B velocity shear during edge plasma biasing : recent experimental results
- 2003
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Ingår i: Plasma Physics and Controlled Fusion. - 0741-3335 .- 1361-6587. ; 45:5, s. 621-643
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Tidskriftsartikel (refereegranskat)abstract
- Experiments in the tokamaks TEXTOR, CASTOR, T-10 and ISTTOK, as well as in the reversed field pinch RFX have provided new and complementary evidence on the physics of the universal mechanism of E x B velocity shear stabilization of turbulence, concomitant transport barrier formation and radial conductivity by using various edge biasing techniques. In TEXTOR the causality between transport reduction and induced electric fields in the edge has been for the first time clearly demonstrated. The high electric field gradients have been identified as the cause for the quenching of turbulent cells. A quantitative analysis of the measured transport reduction is in good agreement with theoretical predictions. The scaling of plasma turbulence suppression with velocity shear has been established, revealing the density-potential cross-phase as a key element. Reduction in poloidal electric field, temperature, and density fluctuations across the shear layer lead to a reduction of the anomalous conducted and convected heat fluxes resulting in an energy transport barrier that is measured directly. In CASTOR the biasing electrode is placed at the separatrix in a non-intrusive configuration which has demonstrated strongly sheared electric fields and consequent improvement of the global particle confinement, as predicted by theory. The impact of sheared E x B flow on edge turbulent structures has been measured directly using a comprehensive set of electrostatic probe arrays as well as emissive probes. Measurements with a full poloidal Langmuir probe array have revealed quasi-coherent electrostatic waves in the SOL with a dominant mode number equal to the edge safety factor. In T-10 edge biasing is clearly improving the global performance of ECR heated discharges. Reflectometry and heavy ion beam probe measurements show the existence of a narrow plasma layer with strong suppression of turbulence. On ISTTOK, the influence of alternating positive and negative electrode and (non-intrusive) limiter biasing has been compared. Electrode biasing is found to be more efficient in modifying the radial electric field E, and confinement, limiter biasing acting mainly on the SOL. In the RFX reversed field pinch it has been demonstrated that also in RFPs biasing can increase the local E x B velocity shear in the edge region, and hence substantially reduce the local turbulence driven particle flux mainly due to a change in the relative phase between potential and density fluctuations.
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| 2. |
- Bohm, P., et al.
(författare)
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Edge Thomson scattering diagnostic on COMPASS tokamak : Installation, calibration, operation, improvements
- 2014
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Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 85:11, s. 11E431-
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Tidskriftsartikel (refereegranskat)abstract
- The core Thomson scattering diagnostic (TS) on the COMPASS tokamak was put in operation and reported earlier. Implementation of edge TS, with spatial resolution along the laser beam up to similar to 1/100 of the tokamak minor radius, is presented now. The procedure for spatial calibration and alignment of both core and edge systems is described. Several further upgrades of the TS system, like a triggering unit and piezo motor driven vacuum window shutter, are introduced as well. The edge TS system, together with the core TS, is now in routine operation and provides electron temperature and density profiles.
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| 3. |
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| 4. |
- Van Oost, G., et al.
(författare)
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The role of radial electric fields in the tokamaks TEXTOR-94, CASTOR, and T-10
- 2001
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Ingår i: Czechoslovak Journal of Physics. - 0011-4626 .- 1572-9486. ; 51:10, s. 957-975
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Tidskriftsartikel (refereegranskat)abstract
- Radial electric fields (E-r) and their role in the establishment of edge transport barriers and improved confinement have been studied in the tokamaks TEXTOR-94 and CASTOR, where E-r is externally applied to the plasma in a controlled way using a biased electrode, as well as in the tokamak T-10 where an edge transport barrier (H-mode) is obtained during electron-cyclotron resonance heating (ECRH) of the plasma. The physics of radial currents was studied and the radial conductivity in the edge of TEXTOR-94 (R = 1.75 m, a = 0.46 m) was found to be dominated by recycling (ion-neutral collisions) at the last closed flux surface (LCFS) and by parallel viscosity inside the LCFS. From a performance point of view (edge engineering), such electrode biasing was shown to induce a particle transport barrier, a reduction of particle transport, and a concomitant increase in energy confinement. An H-mode-like behaviour can be induced both with positive and negative electric fields. Positive as well as negative electric fields were shown to strongly affect the exhaust of hydrogen, helium, and impurities, not only in the H-mode-like regime. The impact of sheared radial electric fields on turbulent structures and flows at the plasma edge is investigated on the CASTOR tokamak (R = 0.4 m, a = 0.085 m). A non-intrusive biasing scheme that we call separatrix biasing is applied whereby the electrode is located in the scrape-off layer (SOL) with its tip just touching the LCFS. There is evidence of strongly sheared radial electric field and E x B flow, resulting in the formation of a transport barrier at the separatrix. Advanced probe diagnosis of the edge region has shown that the E x B shear rate that arises during separatrix biasing is larger than for standard edge plasma biasing. The plasma flows, especially the poloidal E x B drift velocity, are strongly modified in the sheared region, reaching Mach numbers as high as half the sound speed. The corresponding shear rates (approximate to5 x 10(6) s(-1)) derived from both the flow and electric field profiles are in excellent agreement and are at least an order of magnitude higher than the growth rate of unstable turbulent modes as estimated from fluctuation measurements. During ECRH in the tokamak T-10 (R = 1.5 m, a = 0.3 m), a regime of improved confinement is obtained with features resembling those in the H-mode in other tokamaks. Using a heavy ion beam probe, a narrow potential well is observed near the limiter together with the typical features of the L-H transition. The time evolution of the plasma profiles during L-H and H-L transitions is clearly correlated with that of the density profile and the formation of a transport barrier near the limiter, The edge electric field is initially positive after the onset of ECRH. It changes its sign during the L-H transition and grows till a steady condition is reached. Similar to the biasing experiments in TEXTOR-94 and CASTOR, the experimentally observed transport barrier is a barrier for particles.
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| 5. |
- Krlin, L., et al.
(författare)
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Anomalous impurity diffusion in models of tokamak edge plasma turbulence
- 2004
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Ingår i: Czechoslovak Journal of Physics. - 0011-4626 .- 1572-9486. ; 54:1-3, s. 157-163
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Tidskriftsartikel (refereegranskat)abstract
- Using a simple spatially periodical and stationary potential as a model of electrostatic tokamak edge plasma turbulence, we have found recently [1, 2] an anomalous impurity diffusion in this regime. In this contribution, we estimate this diffusion for a more general and realistic form of the potential, which is close to the Hasegawa-Wakatani (HW) potential. As an interesting consequence of the discussed dynamics, we present, possibility of radial electric field generation in the edge turbulence regime. This effect. might play a role in tokamak scenarios with transport barriers.
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| 6. |
- Krlin, L., et al.
(författare)
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Stochastic (E x B) diffusion of ions in a spatially periodical potential field
- 2000
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Ingår i: Physica scripta. T. - 0281-1847. ; T84, s. 221-224
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Tidskriftsartikel (refereegranskat)abstract
- In this contribution, the stochastic (E x B) motion of ions in a homogeneous magnetic field and in a spatially periodical electrostatic potential is discussed. This approach is used for the simulation of ion diffusion in potential structures formed at the boundary plasma in tokamaks. Nonintegrability of the system causes the spatial diffusion of ions. Depending on the choice of parameters, this diffusion has either the usual random-walk character, or appears, for sufficiently strong nonlinearity, in the form of Levy walks. The diffusion coefficient is sufficiently high for heavy impurities. Possible further applications are mentioned.
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| 7. |
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| 8. |
- Panek, R., et al.
(författare)
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Anomalous ion diffusion and radial-electric-field generation in a turbulent edge plasma potential weakly correlated in time and space
- 2005
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Ingår i: Physica Scripta. - 0031-8949 .- 1402-4896. ; 72:4, s. 327-332
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Tidskriftsartikel (refereegranskat)abstract
- Using a simple model of tokamak-edge-plasma turbulence in the form of a spatially periodic and time-independent electrostatic potential, we have found recently a new type of anomalous impurity diffusion in this regime [1], [2]. In the present contribution, we estimate this diffusion in the test-particle approach for a real turbulent potential obtained in an experiment. A significant difference in the impurity dynannics between the Hamiltonian and drift approaches is observed. As an interesting consequence of these dynamics, the possibility of radial-electric-field generation in the turbulence regimes appears. This effect can be of importance in scenarios related to transport barriers.
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| 9. |
- Tendler, Michael, et al.
(författare)
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Models for transitions into regimes with enhanced confinement
- 2002
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Ingår i: Comments on modern physics. - 1560-5892. ; 2:6, s. C203-C214
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Tidskriftsartikel (refereegranskat)abstract
- The model based upon the concept of E x B flow velocity shear suppression is explained. It is asserted that there are models enabling to understand the physics involved in L-H transitions. The issue is more whether one wishes to employ the theoretical results available in order to interpret the reality.
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| 10. |
- Tendler, Michael, et al.
(författare)
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Physics of transport barriers
- 2004
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Ingår i: Brazilian journal of physics. - 0103-9733 .- 1678-4448. ; 34:4B, s. 1822-1827
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Tidskriftsartikel (refereegranskat)abstract
- The concept of ExB flow velocity shear suppression is utterly fundamental in modem fusion research. It is asserted that there are models enabling to understand the physics involved in LH transitions. To improve the understanding of the mechanisms leading to the formation of Transport Barriers, especially the relation between Internal and Edge barriers it is necessary to invoke the issue of electric fields. Edge transport barriers are the feature of the H-mode, the baseline regime of ITER, whereas Internal Transport Barriers are used to develop regimes that might be employed for steady state operation of ITER, definitely beneficial for design and operation of fusion power plants in the future. Their synergy will be addressed. Plasma flows are closely connected to electric fields. Therefore, their role is crucial for understanding of tokamaks aimed at the achievement of fusion energy. This appears in the well known neoclassical theory as the most accomplished and selfconsistent basis for understanding of fusion plasmas. It pertains to the novel concept of zonal flows emerging from the recent development of gyro-kinetic transport codes. The equilibrium poloidal and toroidal flows are also crucial for the concept of the electric field shear suppression of plasma turbulence in tokamaks. Yet, this timely and topical issue has remained largely unaddressed experimentally because of great difficulties in measuring flows in plasmas.
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