Analysis of MHD activity in pellet enhanced performance (PEP) pulses is used to determine the position of rational surfaces associated with the safety factor q. This gives evidence for negative shear in the central region of the plasma. The plasma equilibrium calculated from the measured q values yields a Shafranov shift in reasonable agreement with the experimental value of about 0.2 m. The corresponding current profile has two large off-axis maxima in agreement with the bootstrap current calculated from the electron temperature and density measurements. A transport simulation shows that the bootstrap current is driven by the steep density gradient, which results from improved confinement in the plasma core where the shear is negative. During the PEP phase (m,n)=(1,1) fast MHD events are correlated with collapses in the neutron rate. The dominant mode preceding these events usually is n=3, whereas the mode following them is dominantly n=2. Toroidal linear MHD stability calculations assuming a non-monotonic q-profile with an off-axis minimum decreasing from above 1 to below 1 describe this sequence of modes (n=3,1,2), but always give a larger growth rate for the n=1 mode than for the n=2 mode. This large growth rate is due to the high central poloidal beta of 1.5 observed in the PEP pulses. Finally, a rotating (m,n)=(1,1) mode is observed as a hot spot with a ballooning character on the low field side. The hot spot has some of the properties of a 'hot' island consistent with the presence of a region of negative shear
This paper describes the contents of an international pellet ablation database (IPADBASE) that has been assembled to enable studies of pellet ablation theories that are used to describe the physics of an ablating fuel pellet in a tokamak plasma. The database represents an international effort to assemble data from several tokamaks of different magnetic configuration and auxiliary heating methods. In the initial configuration, data from JET, Tore Supra, DIII-D, FTU, TFTR, ASDEX-U, JIPP T-IIU, RTP, and T-10 have been included. The database contains measurements of deuterium and hydrogen pellet ablation, including pellet mass and speed, plasma electron density and temperature profiles, and pellet ablation light emission. A summary of the database contents and a scaling analysis of the data are presented.
The combination of two regimes of enhanced performance, the H-mode and the pellet enhanced performance (PEP) mode, has been achieved in JET. The strong enhancement of the central plasma parameters, obtained with pellet injection and subsequent auxiliary heating, is found to persist well into the H-mode phase. A characteristic of the PEP regime is that an improvement of the fusion reactivity over non-pellet discharges is obtained under the condition of nearly equal electron and ion temperatures. A maximum neutron production rate of 0.95 × 10l6 s−1 was obtained in a double-null X-point discharge with 2.5 MW of neutral beam heating and 9 MW of ion cyclotron resonance heating, with central ion and electron temperatures of about 10 keV and a central deuterium density of 8.0 × 1019 m−3. The corresponding fusion product nD(0)τETi(0) is between 7.0 and 8.6 × 1020 m−3·s·keV. The enhanced neutron production is predominantly of thermonuclear (Maxwellian) origin. The compatibility of these regimes is an important issue in the context of tokamak ignition strategies. Several technical developments on JET have played a role in the achievement of this result: (1) the use of low voltage plasma breakdown (0.15 V/m) to permit pellet injection in an X-point configuration before the formation of a q = 1 surface; (2) the elimination of ICRH specific impurities with antenna Faraday screens made of solid beryllium; (3) the use of a novel system of plasma radial position control that stabilizes the coupling resistance of the ion cyclotron heating system.
The problems of development of high-temperature ultrasonic transducers for modern science and technology applications are analysed. More than 10 piezoelectric materials suitable for operation at high temperatures are overviewed. It is shown that bismuth titanate based piezoelectric elements are most promisable. Bonding methods of piezoelectric elements to a protector and backing are discussed. Thermosonic gold-to-gold bonding is most modern and possesses unique features. Our achievements in this field are analysed in the context of world progress. In sol-gel and chemical vapour deposition technology bonding and sometimes coupling problems are avoided at all. Design peculiarities of the transducers are reviewed. Commercial sensors in the meanwhile are often characterized with poor performance in extreme conditions.In conclusions recommendations for high temperature ultrasonic transducers design are formulated.
The effective radial deposition profile of deuterium pellets injected into the Joint European Torus (JET) and the Tokamak Fusion Test Reactor (TFTR) has been determined from Thomson scattering diagnostic measurements of electron density taken immediately after pellet injection. The pellet ablation rate deduced from these measurements differs from that predicted by conventional pellet ablation theory. The possibility of enhanced radial transport during the ablation process has been examined to determine whether this can explain the difference between theory and experiment, but no evidence has been found to support such an explanation. The temporal evolution of the Balmer alpha light emitted during pellet ablation is found to be different from the effective pellet ablation rate determined from the density profile measurement. The authors conclude that the shielding mechanisms of conventional pellet ablation models have to be modified in order to predict and reproduce the observed effective ablation rate and penetration depth
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