| 1. |
- Dumont, R. J., et al.
(författare)
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Multi-megawatt, gigajoule plasma operation in Tore Supra
- 2014
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 56:7
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Tidskriftsartikel (refereegranskat)abstract
- Integrating several important technological elements required for long pulse operation in magnetic fusion devices, the Tore Supra tokamak routinely addresses the physics and technology issues related to this endeavor and, as a result, contributes essential information on critical issues for ITER. During the last experimental campaign, components of the radiofrequency system including an ITER relevant launcher (passive active multijunction (PAM)) and continuous wave/3.7 GHz klystrons, have been extensively qualified, and then used to develop steady state scenarios in which the lower hybrid (LH), ion cyclotron (IC) and electron cyclotron (EC) systems have been combined in fully stationary shots (duration similar to 150 s, injected power up to similar to 8MW, injected/extracted energy up to similar to 1 GJ). Injection of LH power in the 5.0-6.0MW range has extended the domain of accessible plasma parameters to higher densities and non-inductive currents. These discharges exhibit steady electron internal transport barriers (ITBs). We report here on various issues relevant to the steady state operation of future devices, ranging from operational aspects and limitations related to the achievement of long pulses in a fully actively cooled fusion device (e. g. overheating due to fast particle losses), to more fundamental plasma physics topics. The latter include a beneficial influence of IC resonance heating on the magnetohydrodynamic (MHD) stability in these discharges, which has been studied in detail. Another interesting observation is the appearance of oscillations of the central temperature with typical periods of the order of one to several seconds, caused by a nonlinear interplay between LH deposition, MHD activity and bootstrap current in the presence of an ITB.
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| 2. |
- Ekedahl, Annika, 1964
(författare)
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Coupling and Absorption of Lower Hybrid Waves in Current Profile Control Experiments in the JET Tokamak
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Current drive with radio frequency waves in the lower hybrid frequency range is an important method of current profile control via non-inductive current drive in tokamaks. This thesis is concerned with profile control experiments, carried out with the lower hybrid current drive system in the Joint European Torus (JET) tokamak, with the aim to explore scenarios for operation in high confinement regimes with peaked pressure profiles. Good coupling of the lower hybrid waves from the antenna to the plasma is needed for efficiency. The power reflection coefficient depends on electron density in front of the waveguides, which leads to a dependence on the plasma-antenna distance. For a large antenna, as used in JET, the plasma configuration plays a role. This is linked to the connection length of the magnetic field lines in front of the antenna. The effect of gas injection near the antenna is investigated. At a given plasma-antenna distance, the reflection coefficient can be decreased, but if large gas flow is used the density of fast electrons is decreased. The profile of the lower hybrid driven current in the JET plasmas is usually peaked off-axis, which is advantageous for current profile control in order to suppress magnetohydrodynamic instabilities and improve confinement. The broad profiles can be explained by an increase in parallel wave number of the launched wave, experienced during propagation in plasmas with large elongation. At high electron density and low magnetic field, the wave penetration is limited. The current drive efficiency of the lower hybrid waves is analysed in conditions with high power and off-axis deposition. At high electron temperature and low electron density, local overdriving of the plasma current can take place in the region where the lower hybrid driven current exceeds inductive current. The effect of a negative toroidal electric field must then be taken into account. Finally, applications of current profile control with lower hybrid waves in JET are demonstrated.
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| 3. |
- Goniche, M., et al.
(författare)
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Lower hybrid current drive at high density on Tore Supra
- 2013
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 53:3
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Tidskriftsartikel (refereegranskat)abstract
- Lower hybrid current drive (LHCD) experiments with line-averaged density varying between 1.5 x 1019 and 6 x 10(19) m(-3) are performed on the Tore Supra tokamak under quasi-steady-state conditions with respect to the fast electron dynamics. The LHCD efficiency is analysed from the fast electron bremsstrahlung (FEB) and electron cyclotron emission (ECE). The effect of plasma equilibrium and particle fuelling is documented. It is concluded that the fast decay of FEB with plasma density could be consistent with simple scaling of the current drive efficiency and FEB. Plasma edge measurements are presented looking for the effect on fast electron emission. In a specific case of particle fuelling, an anomalous decay of the hard x-ray and ECE signals suggests deleterious interaction of the wave with edge plasma.
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| 4. |
- Hillairet, J., et al.
(författare)
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Recent progress on lower hybrid current drive and implications for ITER
- 2013
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 53:7
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Tidskriftsartikel (refereegranskat)abstract
- The sustainment of steady-state plasmas in tokamaks requires efficient current drive systems. Lower hybrid current drive is currently the most efficient method to generate a continuous additional off-axis toroidal plasma current and to reduce the poloidal flux consumption during the plasma current ramp-up phase. The operation of the Tore Supra ITER-like lower hybrid (LH) launcher has demonstrated the capability to couple LH power at ITER-like power densities with very low reflected power during long pulses. In addition, the installation of eight 700 kW/CW klystrons at the LH transmitter has allowed increasing the total LH power in long-pulse scenarios. However, in order to achieve pure stationary LH-sustained plasmas, some R&D is needed to increase the reliability of all the systems and codes, from radio-frequency (RF) sources to plasma scenario prediction. The CEA/IRFM is addressing some of these issues by leading a R&D programme towards an ITER LH system and by the validation of an integrated LH modelling suite of codes. In 2011, the RF design of a mode converter was validated at a low power. A 500 kW/5 s RF window is currently under manufacture and will be tested at a high power in 2012 in collaboration with the National Fusion Research Institute. All of this work aims to reduce the operational risks associated with the ITER steady-state operations.
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| 5. |
- Nilsson, Emelie, 1985, et al.
(författare)
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Comparative modelling of lower hybrid current drive with two launcher designs in the Tore Supra tokamak
- 2013
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 53:8
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Tidskriftsartikel (refereegranskat)abstract
- Fully non-inductive operation with lower hybrid current drive (LHCD) in the Tore Supra tokamak is achieved using either a fully active multijunction (FAM) launcher or a more recent ITER-relevant passive active multijunction (PAM) launcher, or both launchers simultaneously. While both antennas show comparable experimental efficiencies, the analysis of stability properties in long discharges suggest different current profiles. We present comparative modelling of LHCD with the two different launchers to characterize the effect of the respective antenna spectra on the driven current profile. The interpretative modelling of LHCD is carried out using a chain of codes calculating, respectively, the global discharge evolution (tokamak simulator METIS), the spectrum at the antenna mouth (LH coupling code ALOHA), the LH wave propagation (ray-tracing code C3PO), and the distribution function (3D Fokker-Planck code LUKE). Essential aspects of the fast electron dynamics in time, space and energy are obtained from hard x-ray measurements of fast electron bremsstrahlung emission using a dedicated tomographic system. LHCD simulations are validated by systematic comparisons between these experimental measurements and the reconstructed signal calculated by the code R5X2 from the LUKE electron distribution. An excellent agreement is obtained in the presence of strong Landau damping (found under low density and high-power conditions in Tore Supra) for which the ray-tracing model is valid for modelling the LH wave propagation. Two aspects of the antenna spectra are found to have a significant effect on LHCD. First, the driven current is found to be proportional to the directivity, which depends upon the respective weight of the main positive and main negative lobes and is particularly sensitive to the density in front of the antenna. Second, the position of the main negative lobe in the spectrum is different for the two launchers. As this lobe drives a counter-current, the resulting driven current profile is also different for the FAM and PAM launchers.
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